Unlike (almost) legacy filament bulbs, LEDs (and fluorescent bulbs) need a special circuit to make them work. They are simply called as LED Drivers (or a ballast in case of fluorescent bulbs).

Since LEDs are inevitable in our lives, it is a good idea for the interested folks (engineers, driver designers etc.) to get to know the Light Emitting Diode Basics. This article is composed as a brief understanding guide to LED, which includes a brief introduction, the electrical symbol of LED, types, construction, characteristics, LED Drivers and many.

NOTE: There is a simpler version of this article “LED – Light Emitting Diode“, which gives an overview of an LED in a more simpler manner without going into the technical details.

Introduction

The two most significant semiconductor light emitting sources extensively used in various applications are LASER diodes and LED’s. The principle operation of LASER diodes is based on stimulated emission, whereas LED is based on spontaneous emission.

Light Emitting Diodes are the most common eminent source of light available in electronic components. For instance, they are widely used for displaying the time and many other types of data on screens in certain display devices. LEDs are the opto-semiconductor devices, which easily converts electric current into illumination (or light). Area of the LED is usually very less and many integrated optical components may be used in designing its radiation pattern. It has the major advantage of low manufacturing cost and renders longer life than the laser diode.

A light emitting diode consists of two principal elements of semiconductor. They are positively charged P-type holes and negatively charged N-type electrons.

When the positive P side of the diode is connected to power supply and N side to the ground, then the connection is said to be in forward bias, which allows the electric current to flow through the diode. The majority and minority charge carriers of P side and N side combine with each other and neutralize the charge carriers in the depletion layer at the PN junction.

The migration of electrons and holes in turn releases some amount of photons, which discharges energy in the form of monochromatic light at a constant wavelength usually in nm, which resembles the color of an LED. The color spectrum of LED emission is typically extremely narrow.

In general, it can be specified as a certain specific range of wavelengths in the electromagnetic spectrum. The selection of emission of color from the LED is fairly limited due to the nature of semiconductor used in the manufacture. Commonly available colors of LED are red, green, blue, yellow, amber and white.

The light from red, blue and green colors can be easily combined to produce white light with limited brightness. The working voltage of red, green, amber and yellow colors is around 1.8 volts. The actual range of working voltage of a light emitting diode can be determined by the breakdown voltage of semiconductor material involve in the construction of LED.  The color of the light emitted in LED is determined by the semiconductor materials that form the diode’s PN junction.

It is due to the differences in the energy gap band structure of semiconductor materials and so different number of photons is emitted with varying frequencies. However the wavelength of light depends on the band gap of the semiconductor materials at the junction and the intensity of light depends on the amount of power or energy applied through the diode. The output wavelength can be maintained by using compound semiconductors, so that required color can be observed, providing the output within the visible range.

Light can be produced and controlled by electronic means in a number of ways. In light emitting diodes, light is produced through the concept of electroluminescence which is a solid state process. Under certain specific conditions of producing the light, solid state procedures can produce a coherent light, similarly as in laser diodes.

Types of LEDs

Light emitting diodes can be broadly classified as two major categories of LEDs. They are

•  Visible LEDs
•  Invisible LEDs

Visible LEDs are primarily used for switches, optical displays and for illumination purposes without the use of any photo sensors. Invisible LEDs are used in applications including optical switches, analysis and optical communications, etc., with the use of photo sensors.

Efficacy

The rating of light emitting diodes is determined in terms of its luminous efficacy. It is defined as the ratio of luminous flux to the electrical input power supplied to the diode and it can be expressed in lumens per watts. Luminous flux represents the response of the eye to different wavelengths of light.

LED Construction

The structure and construction of Light Emitting Diodes are much different from that of a regular semiconductor signal diode. Light will be emitted from the LED when its PN junction is forward biased. The PN junction is covered by a transparent solid and plastic epoxy resin hemispherical shaped shell body which protects the LED from atmospheric disturbances, vibrations and thermal shock. The PN junction is formed using the lowest band gap materials like Gallium Arsenide, Gallium Arsenide Phosphide, Gallium Phosphide, Gallium Indium Nitride, Aluminum Gallium Nitride, Silicon Carbide etc.

Actually an LED junction does not emit much amount of light so that the epoxy resin body is built in such a way that the photons of light emitted by the junction are reflected away from the surrounding substrate base and are focused through the domed top of the LED, which itself acts as a lens concentrating the larger amount of light.

It is the reason why the emitted light appears to be brightest at the top of LED.

Usually, Light Emitting Diodes which emits red colored light are built on Gallium Arsenide substrate and the diodes which emit green/yellow/orange colored lights are fictitious on the Gallium Phosphide substrate. For red color emission, the N – type layer is doped with Tellurium  (Te) and the P – type layer is doped with Zinc. Contact layers are formed using Aluminum on P – side and Aluminum Tin on N – side respectively.

The LEDs are designed to make sure that the most of the recombination of charge carriers takes place on the surface of the PN junction by the following ways.

• By increasing the substrate doping concentration, the additional minority charge carriers electrons move to the top of the structure, recombine and emit light at the surface of LED.
• By increasing the diffusion length of charge carriers, i.e., L = √ Dτ, where D is the coefficient of diffusion and τ is the charge carrier life time. When increased beyond the critical value there will be a chance of re-absorption of the released photons into the device.

When the diode is connected in forward bias, the charge carriers acquire sufficient amount of energy to surmount the barrier potential existing at the PN junction. Whenever the forward bias is applied, the minority charge carriers on both P – type and N – type are injected across the junction and recombine with the majority carriers. This recombination of majority and minority charge carriers may be either radiative or nonradiative. Radiative recombination emits light and Nonradiative recombination produce heat.

Organic Light Emitting Diodes (OLED)

In organic Light Emitting Diodes the compound semiconductor material used in designing the LED is organic in nature. The organic semiconductor material is electrically conductive in some part or the entire molecule due to the conjugated electron; as a result it is an organic semiconductor. The material may be in crystalline phase or polymeric molecules. It has the advantage of thin structure, less cost, low voltage for driving, excellent radiation pattern, high radiance, maximum contrast and intensity.

Light Emitting Diode Colors

In contrast to the normal semiconductor, signal diodes that are used for switching circuits, rectifiers and power electronics circuits made from either silicon or germanium semiconductor materials, the Light Emitting Diodes are manufactured from compound semiconductor materials such as Gallium Arsenide, Gallium Arsenide Phosphide, Silicon Carbide and Gallium Indium Nitride are all mixed together in different ratios to produce a unique distinctive wavelength of color.

Different semiconductor compounds emit light in definite regions of the visible light spectrum and therefore they produce different intensity levels of light. The choice of the semiconductor material used in manufacturing the LED will determine the wavelength of the photon emissions and the resulting color of the emitted light.

Radiation Pattern

It is defined as the angle of the light emission with respect to the emitting surface. The maximum amount of power, intensity or energy will be obtained in the perpendicular direction with the surface emitting. The angle of light emission depends on the color being emitted and it usually varies between around 80° to 110°.

The color of the light emitted by an LED is not determined by the color of the plastic body enclosing the LED. The enclosing is used to both enhance the light emission and to indicate its color when it’s not driven by an electrical supply. In the recent years, blue and white LEDs are also available, but these are more expensive than the normal standard color LEDs due to the production costs of mixing two or more complementary colors in an exact ratio within the semiconductor compound.

General Characteristics of Light Sources

Drive Current Vs Light Output

For high values of forward drive current the temperature of the PN junction of semiconductor increases due to considerable power dissipation. This type of rise in temperature at the junction results in decrease in the efficiency of radiative recombination. As a result, the density of current is further increased; internal series resistance will tend to reduce the light emitting efficiency of any light source.

Quantum Efficiency

The Quantum efficiency of any light source is defined as the ratio of the radiative recombination rate, which emits light to total recombination rate and it is given as:

 η=Rr/Rt 

Switching Speed

The switching speed of a light source resembles how fast a light source can turn on and off by an applied electrical supply to produce a corresponding pattern of optical output. LEDs have slow switching speed than usual LASER diodes.

Spectral Wavelength

The peak spectral wavelength is defined as the wavelength at which the maximum intensity of light is generated. It is determined by the energy band gap of the semiconductor material used in LED manufacturing.

Spectral Width

The spectral width of a light source is defined as the range of wavelengths over which a light source emits light. The light source has to emit light within the narrower spectral width.

LED I-V Characteristics

Before emitting light from any light emitting diode, it needs to have current to flow across it, since LED is a current dependent device with its output light intensity being directly proportional to the forward current passing through the LED.

Light emitting diode has to be connected in a forward bias combination across the power supply and it should be current limited by using a resistor connected in series to protect it from the excess current flow. LED should not be connected directly to the battery or power supply because excess amounts of current will flow through it and LED may damage.

Each LED has its own individual forward voltage drop along the PN junction and this parameter has been determined by the semiconductor material used in manufacturing of LED for a specified amount of forward conduction current, usually for a forward current of about 20mA.

At low forward voltages, the driving current of the diode is dominated by the non-radiative recombination current due to recombination of charge carriers across the length of the LED chip. At higher forward voltages, the diode driving current is dominated by the radiative diffusion current.

Even at larger voltages than the usual, the diode current is limited by the series resistance. The diode should never reach to reverse breakdown voltage for a short duration of time since permanent damage of the diode may occur. The below figure shows the I-V characteristics of the different color LEDs.

LED Series Resistance Calculation

Light emitting diode functions well when it is connected in series with the resistance, as a result the forward current required by the LED is provided by supply voltage across the combination. The resistance value of the series resistor can be calculated using the below formula. Usually the forward current of a normal LED is considered as 20mA.

Multi – Color Light Emitting Diode

There are large numbers of LEDs available in the market with varying shapes and sizes, different colors and different light output intensities. Gallium Arsenide Phosphide red colored Led with the diameter of 5 mm is the most commonly used LED and it is very cheap to produce. Light emitting diodes with multiple color emission are being manufactured nowadays and they are available in many packages, most of them are two to three LEDs within a single package.

Bi-Color Light Emitting Diodes

The bi-color light emitting diodes are a type of LEDs similar to single color LEDs just with additional one more LED chip enclosed in the package. The bi-color LEDs may have either two or three leads for connecting; it depends on the method used. In general the two LED leads are connected in inverse parallel combination. The anode of one LED is connected to the cathode of another LED and vice versa. When the supply is given to either of the anode only one LED will glow. We can also turn on both LEDs at same time with dynamic switching at high speed.

Tri Colored Light Emitting Diode

Usually three lead LED have common cathode lead in which both the other two LED chips are connected internally. Either one or two LEDs have to be turned on, it is necessary to connect the common cathode to ground. The current limiting resistors are connected to the both anodes for controlling the current individually.

For single or bi-color LED illumination it is necessary to connect power supply to either of the anodes individually or at the same time. These tricolored LEDs comprises of single RED and GREEN LED chips connected to the same cathode. This type of diodes generates additional shades of the primary colors by switching ON the two LEDs in different ratios of forward current.

LED Driver Circuits

Integrated circuits either the combinational circuits or sequential circuits can be used to drive the light emitting diodes. The light emitting diodes can be switched on or off using integrated circuits. The output stages of TTL or CMOS logic gates can be used to drive the light emitting diodes as switches in two modes of configuration. They are source and sink modes of configuration.

The output current given by integrated circuits in sink mode configuration can be about 50 mA and in source mode configuration the forward current can be about 30 mA. However the current driven by the light emitting diode should be limited by the resistor connected in series.

Driving an LED Using Transistor

Instead of using Integrated circuits, the LEDs can be driven by using discrete components such as bipolar PNP and NPN transistors. The discrete components can be used in driving more than one LED as in large LED array structures.

Fewer applications use solely a single LED in their functioning. Junction transistors are used to drive current across the multiple light emitting diodes in such a way that the forward current driven by LED is about 10 – 20 mA. If NPN transistor is used in driving the LED then the series resistor acts as a current source. If PNP transistor is used in driving LEDs then the series resistor acts as a current sink.

Applications such as backlighting array of screen, street lights or as a replacement for fluorescent lamp or incandescent lamp, most of the applications require more than one LED. Generally, driving a number of single LEDs in parallel causes non uniform current sharing among the LEDs; even then all the LEDs are rated for the same forward voltage drop.

If single LED fails in driving the LEDs in series can be overcome by providing parallel Zener diodes or silicon controlled rectifiers (SCRs) across each single LED in series. SCRs are the smart choice because they dissipate less power if they have to carry out around the failed LED.

In the case of a parallel combination including a separate driver for each string is more expensive than using a few drivers with appropriate output capacity.

Controlling of LED Light Intensity using PWM

The intensity of light emitted by the LED is controlled by the current flowing through it. As the current across it varies, the brightness of the light can be controlled. If a large amount of current is allowed to pass through the diode, LED light glows much better than the usual.

If the current exceeds its maximum value, the intensity of light increases further and cause the LED to dissipate heat. The forward current limit set for designing LED ranging about 10 to 40 mA. When the current required is very less there may be chances of turning off the LED.

In such cases to control the brightness of light and the current required by LED, a process known as pulse width modulation is used for repeatedly turning the LED ON and OFF depending on the intensity of light required. Linear control devices dissipate the excess energy in the form of heat, as a result to deliver the required amount of power, PWM drivers are used because they do not deliver the power at all.

First of all to inject PWM pulses to the LED circuits, a PWM oscillator is first required. There are different numbers of PWM generators.

LED Displays

Single color, bicolor, multicolor and several other Light emitting diodes are combined as a single package. They can be used as back lightening, strips and bar graphs. One essential requirement of digital display devices is visual numeric display. The common example of such single package of several LEDs is seen in seven segment displays.

A seven segment display, as the name suggests it consists of seven LEDs within the single display package. It can be used for displaying the information.

The display information may be in the digital data form of numbers, letters, characters and also alphanumeric characters. The seven segment display usually has eight combinations of input connections, one for each LED and the remaining one is a common connection point for all the internal LEDs.

If the cathodes of all the LEDs are connected together and by applying a logic HIGH signal, then the individual segments are illuminated. In the same manner if anodes of all the LEDs are connected together and by applying a logic LOW signal, then the individual segments are illuminated.

LED Advantages, Disadvantages and Applications

Advantages

• Small Chip size and low cost
• Long life time
• High energy efficiency
• Low temperature
• Flexibility in design
• Many colors
• Eco friendly
• High switching speed
• High luminous intensity
• Designed to focus its light in a particular direction
• Less affected by damages
• Less radiated heat
• More resistant to thermal shock and vibrations
• No presence of UV Rays

Disadvantages

• Ambient temperature dependence of radiant output power and wavelength of the LED.
• Sensitivity to damages by excess voltage and/or excess current.
• Theoretical overall efficiency is achieved only in special cool or pulsed conditions.

Applications

• In motor vehicles and bicycle lights
• In traffic light Indicators, signs and signals
• In data displaying boards
• In medical applications and toys
• Non visual applications
• In light bulbs and many more
• Remote controls

The post Light Emitting Diode Basics appeared first on ElectronicsHub.

Introduction

We know that a Diode allows the current flow only in one direction and hence it acts as a one-way switch. Diode is made of P and N type materials and has two terminals namely anode and cathode. This device can be operated by controlling the voltage applied to these terminals.

When the voltage applied to the anode is positive with respect to the cathode, the diode is said to be in Forward Bias. If the voltage applied to the diode is greater than the threshold level (generally, it is of ≈0.6V for Silicon Diodes), then diode acts as a short circuit and allows the current flow.

If the polarity of the voltage is changed i.e., the cathode is made positive with respect to anode, then it is said to be in Reverse Bias and acts as open circuit. As a result, no current flows through it.

The application areas of diodes include communication systems as limiters, clippers, gates; computer systems as logic gates, clampers; power supply systems as rectifiers and inverters; television systems as phase detectors, limiters, clampers; radar circuits as gain control circuits, parameter amplifiers, etc. The following description describes the various applications of diodes briefly.

Some Common Applications of Diodes

Before taking a look at various applications of diodes, let us quickly take a peek at a small list of common applications of diodes.

• Rectifiers
• Clipper Circuits
• Clamping Circuits
• Reverse Current Protection Circuits
• In Logic Gates
• Voltage Multipliers

and many more. Now let us understand each of these applications of diodes in more detail.

Diode as a Rectifier

The most common and important application of a diode is the rectification of AC power to DC power. Using diodes, we can construct different types of rectifier circuits. The basic types of these rectifier circuits are half wave, full wave center tapped and full bridge rectifiers. A single or combination of four diodes is used in most of the power conversion applications. Below figure shows diode operation in a rectifier.

• During the positive half cycle of the input supply, anode is made positive with respect to cathode. So, the diode gets forward biased. This results in the current to flow to the load. Since the load is resistive, the voltage across the load resistor will be same as the supply voltage i.e., the input sinusoidal voltage will appear at the load (only the positive cycle). And the load current flow is proportional to the voltage applied.
• During the negative half-cycle of the input sinusoidal wave, anode is made negative with respect to cathode. So, the diode gets reverse biased. Hence, no current flows to the load. The circuit becomes open circuit and no voltage appears across the load.
• Both voltage and current at the load side are of one polarity means the output voltage is pulsating DC. Often, this rectification circuit has a capacitor that is connected across the load to produce steady and continuous DC currents without any ripples.

Diodes in Clipping Circuits

Clipping Circuits are used in FM transmitters, where noise peaks are limited to a particular value so that excessive peaks are removed from them. The clipper circuit is used to put off the voltage beyond the preset value without disturbing the remaining part of the input waveform.

Based on the diode configuration in the circuit, these clippers are divided into two types:

• Series Clipper
• Shunt Clipper

Further, these are again classified into different types.

The above figure shows the positive series and shunt clippers. And using these clipper circuits, positive half cycles of the input voltage waveform will be removed. In positive series clipper, during the positive cycle of the input, the diode is reverse-biased so the voltage at the output is zero.

Hence, the positive half-cycle is clipped off at the output. During the negative half cycle of the input, the diode is forward-biased and the negative half cycle appears across the output.

In positive shunt clipper, the diode is forward-biased during the positive half cycle so the output voltage is zero as diode acts as a closed switch. And during the negative half cycle, the diode is reverse-biased and acts as open switch so the full input voltage appears across the output. With the above two diode clippers positive half-cycle of the input is clipped at the output.

Diodes in Clamping Circuits

A clamper circuit is used to shift or alter either positive or negative peak of an input signal to a desired level. This circuit is also called as Level Shifter or DC restorer. These clamping circuits can be positive or negative depending on the diode configuration.

In positive clamping circuit, negative peaks are raised upwards so the negative peaks fall on the zero level. In case of the negative clamping circuit, positive peaks are clamped so that it pushes downwards such that the positive peaks fall on the zero level.

Look at the below diagram for understanding the diode application in clamping circuits. During the positive half-cycle of the input, diode is reverse-biased so the output voltage is equal to the sum of input voltage and capacitor voltage (considering the capacitor is initially charged). During the negative half-cycle of the input, diode is forward-biased and behaves as a closed switch so the capacitor charges to a peak value of the input signal.

Diodes in Logic Gates

Diodes can also perform digital logic operations. Low and high impedance states of logic switch are analogous to the forward and reverse-biased conditions of the diode respectively. Thus, the diode can perform logic operations such as AND, OR, etc. Although diode logic is an earlier method with some limitations, these are used in some applications. Most of the modern logic gates are MOSFET based.

The below figure shows the OR gate logic implemented using a pair of diodes and a resistor.

In the above circuit, input voltage is applied at V and by controlling the switches we get the OR logic at the output. Here logic 1 means high voltage and logic 0 means zero voltage. When both switches are in open state, both the diodes are in reverse-biased condition and hence the voltage at the output Y is zero. When any one of the switches is closed, the diode becomes forward-bias and as a result the output is high.

Diodes in Voltage Multiplier Circuits

Voltage multiplier consist of two or more diode rectifier circuits, which are cascaded to produce a DC output voltage equal to the multiple of the applied input voltage. These multiplier circuits are of different types like voltage doubler, tripler, quadrupler, etc. By the usage of diodes in combination with capacitors, we get the odd or even multiple of the input peak voltage at the output.

Above figure shows a half-wave voltage doubler circuit whose DC output voltage is twice that of peak input AC voltage. During the positive half-cycle of the AC input, diode D1 is forward-biased and D2 is reverse-biased. So, the capacitor C1 charges up to peak voltage Vm of the input through the diode D1. During the negative half-cycle of the AC input, D1 is reverse-biased and D2 is forward-biased. So, capacitor C2 starts charging thorough D2 and C1. Thus, the total voltage across the C2 is equal to the 2Vm.

During next positive half-cycle, the diode D2 is reverse-biased so the capacitor C2 will discharge through the load. Likewise, by cascading the rectifier circuits we will get the multiple values of input voltage at the output.

Diodes in Reverse Polarity Protection

The reverse polarity or current protection is necessary to avoid the damage that occurs due to connecting the battery in a wrong way or reversing the polarities of the DC supply. This accidental connection of supply causes to flow a large amount current thorough the circuit components, which might result in their failure or in a worst case, their explosion.

Therefore, a protective or blocking diode is connected in series with the positive side of the input to avoid the reverse connection problem.

Above figure shows the reverse current protection circuit, where diode is connected in series with the load at the positive side of the battery supply. In case of the correct polarity connection, diode gets forward-biased and load current flows through it. But, in case of wrong connection, the diode is reverse-biased and that doesn’t allow any current to flow to the load. Hence, the load is protected against the reverse polarity.

Diodes in Voltage Spike Suppression

In case of an inductor or inductive loads, sudden removal of supply source produces a higher voltage due to its stored magnetic field energy. These unexpected spikes in the voltage can cause the considerable damage to the rest of the circuit components.

Hence, a diode is connected across the inductor or inductive loads to limit the large voltage spikes. These diodes are also called by different names in different circuits such as Snubber diode, Flyback diode, Suppression diode, Freewheeling diode and so on.

In the above figure, the freewheeling diode is connected across the inductive load for suppressing of voltage spikes in the inductor. When the switch is suddenly opened, a voltage spike is created in the inductor. Therefore, the freewheeling diode makes a safe path for the flow of current to discharge the voltage offered by the spike.

Diodes in Solar Panels

The diodes which are used for protection of solar panels are called as bypass diodes. If the solar panel is faulty or damaged or shaded by fallen leaves, snow and other obstructions, the overall output power decreases and arise hot spot damage because the current of the rest of the cells must flow through this faulty or shaded cell and causes overheating. The main function of the bypass diode is to protect the solar cells against this hot spot heating problem.

The above figure shows the connection of bypass diodes in solar cells. These diodes are connected in parallel with the solar cells thereby, limiting the voltage across the bad solar cell and allows the current from good solar cells to the external circuit. Thus, reduces the overheating problem by limiting the current flow through the bad solar cell.

Conclusion

We have some of the important Applications of Diodes. These include Rectifiers, Clippers, Clampers, Voltage Multipliers, Logic Gates, Solar Panels, Reverse Polarity Protection and Voltage Spike Suppression.

The post Applications of Diodes | Rectifier, Clipper, Reverse Current Protection appeared first on ElectronicsHub.

Introduction

Diodes are two-terminal electronic devices / components that functions as a one-way switch i.e., they allow current to flow only in one direction. These diodes are manufactured using semiconductor materials like Silicon, Germanium and Gallium Arsenide.

The two terminals of the diode are known as Anode and Cathode. Based on the potential difference between these two terminals, the operation of diode can be classified in two ways:

• If anode has higher potential than cathode, then the diode is said to be in Forward Bias and it allows current to flow.
• If cathode has higher potential than anode, then the diode is said to be in Reverse Bias and it doesn’t allow current to flow.

Different types of diodes have different voltage requirements. For Silicon Diodes, the forward voltage is 0.7V and for Germanium diodes, it is 0.3V. Usually, in Silicon Diodes, the dark band on one end of the diode indicates the Cathode terminal and the other terminal is anode.

One of the main application of Diodes is Rectification i.e., to convert AC to DC. Since diodes allow current to flow only in one direction and blocks current flow in the other direction, diodes are used in reverse polarity protector and transient protector applications.

There are many Different Types of diodes and some of them are listed below.

Different Types of Diodes

Let us now briefly see about few common types of diodes.

1. Small Signal Diode

It is a small device with disproportional characteristics whose applications are mainly involved at high frequency and very low current applications such as radios and televisions etc. To protect the diode from contamination it is enveloped with a glass so it is also named as Glass Passivated Diode. One of the popular diodes of this type is the 1N4148.

Appearance wise, signal diodes are very small when compared with power diodes. To indicate the cathode terminal, one edge is marked with black or red color. For applications at high frequencies, the performance of the small signal diode is very effective.

With respect to the other functionalities, the signal diodes usually have a small current carrying capability and power dissipation. Usually, these are in the range of 150mA and 500mW respectively.

The Small Signal Diode can be made of either Silicon or Germanium type semiconductor material, but the characteristics of the diode varies depending up on the doping material.

Small Signal Diodes are used in general purpose diode applications, high speed switching, parametric amplifiers and many other applications. Some important characteristics of Small Signal Diode are:

• Peak Reverse Voltage (V_PR) – It is the maximum reverse voltage that can be applied to the diode before it breaks down.
• Reverse Current (I_R) – The current (very small value) that flows when it is reverse biased.
• Maximum Forward Voltage at Peak Forward Current (V_F at I_F)
• Reverse Recovery Time – The time required for reverse current to fall down from forward current to I_R.

2. Large Signal Diode

These diodes have large PN junction layer. Thus, they are usually used in rectification i.e., converting AC to DC. The large PN Junction also increases the forward current carrying capacity and reverse blocking voltage of the diode. The large signal diodes are not suitable for high frequency applications.

The main applications of these diodes are in Power Supplies (rectifiers, converter, inverters, battery charging devices, etc.). In these diodes, the value of forward resistance is few Ohms and the value of reverse blocking resistance is in Mega Ohms.

Since it has high current and voltage performance, these can be used in electrical devices which are used to suppress high peak voltages.

3. Zener Diode

It is a passive element which works under the principle of ‘Zener Breakdown’. First produced by Clarence Zener in 1934, it is similar to normal diode in forward bias condition i.e., it allows current to flow.

But in reverse bias condition, the diode conducts only when the applied voltage reaches the breakdown voltage, known as Zener Breakdown. It is designed to prevent the other semiconductor devices from momentary voltage pulses. It acts as voltage regulator.

4. Light Emitting Diode (LED)

These diodes convert the electrical energy in to light energy. First production started in 1968. It undergoes electroluminescence process in which holes and electrons are recombined to produce energy in the form of light in forward bias condition.

In the early days, LEDs are very costly and used only in special application. But over the years, the cost of the LEDs has comedown significantly. This and the fact they are extremely power efficient, makes LEDs as the main source of lighting in homes, offices, streets (for street lighting as well as traffic lights), automobiles, mobile phones.

5. Constant Current Diodes

It is also known as Current-Regulating Diode or Current-Limiting Diode or Diode-Connected Transistor. The function of the diode is to regulate the voltage at a particular current.

It functions as a two terminal current limiter. In this, JFET acts as current limiter to achieve high output impedance. The constant current diode symbol is shown below.

6. Schottky Diode

In this type of diode, the junction is formed by contacting the semiconductor material with metal. Due to this, the forward voltage drop is decreased to a minimum. The semiconductor material is N-type silicon, which acts as an anode and metals such as Chromium, Platinum, Tungsten etc. acts as cathode.

Due to the metal junction, these diodes have high current conducting capability and hence the switching time is reduced. So, Schottky Diode has greater use in switching applications. Mainly because of the metal – semiconductor junction, the voltage drop is low, which in turn increases the diode performance and reduces power loss. So, these are used in high frequency rectifier applications. The symbol of Schottky diode is as shown below.

7. Shockley Diode

It was one of the first semiconductor devices to be invented. Shockley Diode has four layers. It is also called as PNPN diode. It is equal to a thyristor without a gate terminal, which means the gate terminal is disconnected. As there is no trigger input, the only way the diode can conduct is by providing forward voltage.

It stays ON once it turned “ON” and stays OFF once it turned “OFF”. The diode has two operating states conducting and non-conducting. In non-conducting state the diode conducts with less voltage.

The symbol of the Shockley diode is as follows:

Shockley Diode Applications

• Trigger switches for SCR.
• Acts as relaxation oscillator.

8. Step Recovery Diodes

It is also called as snap-off diode or charge-storage diode. These are the special type of diodes which stores the charge from positive pulse and uses in the negative pulse of the sinusoidal signals. The rise time of the current pulse is equal to the snap time. Due to this phenomenon, it has speed recovery pulses.

The applications of these diodes are in higher order multipliers and in pulse shaper circuits. The cut-off frequency of these diodes is very high which are nearly at Giga hertz order.

As multiplier, this diode has the cut-off frequency range of 200 to 300 GHz. In the operations which are performing at 10 GHz range, these diodes play a vital role. The efficiency is high for lower order multipliers. The symbol for this diode is as shown below.

9. Tunnel Diode

It is used as high-speed switch, with switching speed in the order of few nano-seconds. Due to tunneling effect it has very fast operation in microwave frequency region. It is a two-terminal device in which concentration of dopants is too high.

The transient response is being limited by junction capacitance plus stray wiring capacitance. Mostly used in microwave oscillators and amplifiers. It acts as most negative conductance device. Tunnel diodes can be tuned both mechanically and electrically. The symbol of tunnel diode is as shown below.

Tunnel Diode Applications

• Oscillatory circuits.
• Microwave circuits.
• Resistant to nuclear radiation.

10. Varactor Diode

These are also known as Varicap diodes. It acts like the variable capacitor. Operations are performed mainly at reverse bias state only. These diodes are very famous due to its capability of changing the capacitance ranges within the circuit in the presence of constant voltage flow.

They can be able to vary capacitance up to high values. In varactor diode, we can decrease or increase the depletion layer by changing the reverse bias voltage. These diodes have many applications as voltage-controlled oscillator for cell phones, satellite pre-filters etc. The symbol of varactor diode is given below.

Varactor Diode Applications

• Voltage-controlled capacitors
• Voltage-controlled oscillators
• Parametric amplifiers
• Frequency multipliers
• FM transmitters and Phase locked loops in radio, television sets and cellular phone

11. Laser Diode

Similar to LED in which active region is formed by p-n junction. Electrically laser diode is P-I-N diode in which the active region is in intrinsic region. Used in fiber optic communications, barcode readers, laser pointers, CD/DVD/Blu-ray reading and recording, Laser printing.

Laser Diode Types:

• Double Heterostructure Laser: Free electrons and holes available simultaneously in the region.
• Quantum Well Lasers: lasers having more than one quantum well are called multi quantum well lasers.
• Quantum Cascade Lasers: These are heterojunction lasers which enables laser action at relatively long wavelengths.
• Separate Confinement Heterostructure Lasers: To compensate the thin layer problem in quantum lasers we go for separate confinement heterostructure lasers.
• Distributed Bragg Reflector Lasers: It can be edge emitting lasers or VCSELS.

The symbol of the Laser Diode is as shown:

12. Transient Voltage Suppression Diode

In semiconductor devices, transients will occur due to the sudden change in the state voltage. They will damage the device’s output response. To overcome this problem, Voltage Suppression Diodes are used. The operation of voltage suppression diode is similar to Zener diode operation.

The operation of these diodes is normal as p-n junction diodes but at the time of transient voltage its operation changes. In normal condition, the impedance of the diode is high.  When any transient voltage occurs in the circuit, the diode enters in to the avalanche breakdown region in which a low impedance is provided.

It is very spontaneously because the avalanche breakdown duration ranges in Pico seconds. Transient voltage suppression diode will clamp the voltage to the fixed levels, mostly its clamping voltage is in minimum range.

These are having applications in the telecommunication fields, medical, microprocessors and signal processing. It responds to over voltages faster than Varistors or gas discharge tubes.

The symbol for Transient voltage suppression diode is as shown below.

The diode is characterized by:

• Leakage current
• Maximum reverse stand-off voltage
• Breakdown voltage
• Clamping voltage
• Parasitic capacitance
• Parasitic inductance
• Amount of energy it can absorb

13. Gold Doped Diodes

In these diodes, Gold is used as a dopant. These diodes are faster than other diodes. In these diodes, the leakage current in reverse bias condition is also less. Even at the higher voltage drop it allows the diode to operate in signal frequencies. In these diodes, Gold helps for the faster recombination of minority carriers.

14. Super Barrier Diodes

It is a rectifier diode having low forward voltage drop as Schottky diode with surge handling capability and low reverse leakage current as P – N junction diode. It was designed for high power, fast switching and low-loss applications. Super barrier rectifiers are the next generation rectifiers with low forward voltage than Schottky diode.

15. Peltier Diode

In this type of diode, it generates heat at the two-material junction of a semiconductor, which flows from one terminal to another terminal. This flow is done in only single direction which is same as the direction of current flow.

This heat is produced due to electric charge produced by the recombination of minority charge carriers. This is mainly used in cooling and heating applications. This type of diodes used as sensor and heat engine for thermo electric cooling.

16. Crystal Diode

This is also known as Cat’s whisker, which is a type of point contact diode. Its operation depends on the pressure of contact between semiconductor crystal and the point.

In this, a metal wire is present, which is pressed against the semiconductor crystal. In this, the semiconductor crystal acts as cathode and metal wire acts as anode. These diodes are obsolete in nature. Mainly used in microwave receivers and detectors.

Crystal Diode Applications

• Crystal diode rectifier
• Crystal diode detector
• Crystal radio receiver

17. Avalanche Diode

This is passive element works under principle of Avalanche Breakdown. It works in reverse bias condition. It results in a large current due to the ionization produced by P – N junction during reverse bias condition.

These diodes are specially designed to undergo breakdown at specific reverse voltage to prevent the damage. The symbol of the avalanche diode is as shown below:

Avalanche Diode Uses

• RF Noise Generation: It acts as source of RF for antenna analyzer bridges and also as white noise generators.
• Used in radio equipment and also in hardware random number generators.
• Microwave Frequency Generation: In this the diode acts as negative resistance device.
• Single Photon Avalanche Detector: These are high gain photon detectors used in light level applications.

18. Silicon Controlled Rectifier

It consists of three terminals they are anode, cathode and a gate. It is nearly equal to the Shockley diode. As its name indicates it is mainly used for the control purpose when small voltages are applied in the circuit. The symbol of the Silicon Controlled Rectifier is as shown below:

Modes of Operation:

1. Forward blocking mode (off state): In this J1 and J3 forward biased and J2 is reverse biased. It offers high resistance below breakover voltage and hence it is said to be off state.
2. Forward conduction mode (on state): By increasing the voltage at anode and cathode or by applying positive pulse at the gate we can turn ON. To turn off the only way is to decrease the current flowing through it.
3. Reverse blocking mode (off state): SCR blocking the reverse voltage is named as asymmetrical SCR. Mostly used in current source inverters.

19. Vacuum Diodes

Vacuum diodes consist of two electrodes which will acts as an anode and the cathode. Cathode is made up of Tungsten, which emits the electrons in the direction of anode. Always electron flow will be from cathode to anode only. So, it acts like a switch.

If the cathode is coated with oxide material, then the electrons emission capability is high. Anode is a bit long in size and in some cases their surface is rough to reduce the temperatures developing in the diode. The diode will conduct only in one case that is when the anode is positive with respect to cathode terminal. The symbol is as shown in figure:

20. PIN Diode

The improved version of the normal P-N junction diode gives the PIN diode. In PIN diode doping is not necessary. The intrinsic material i.e., the material which has no charge carriers, is inserted between the P and N regions, which increase the area of depletion layer.

When we apply forward bias voltage, the holes and electrons will be pushed into the intrinsic layer. At some point due to this high injection level, the electric field will conduct through the intrinsic material also. This field makes the carriers to flow from two regions. The symbol of PIN diode is as shown below:

PIN Diode Applications:

• RF Switches: PIN diode is used for both signal and component selection. For example, PIN diodes acts as range-switch inductors in low phase noise oscillators.
• Attenuators: it is used as bridge and shunt resistance in bridge-T attenuator.
• Photo Detectors: it detects x-ray and gamma ray photons.

21. Point Contact Devices

A gold or tungsten wire is used to act as the point contact to produce a PN junction region by passing a high electric current through it. A small region of PN junction is produced around the edge of the wire which is connected to the metal plate which is as shown in the figure.

In forward direction, its operation is quite similar but in reverse bias condition the wire acts like an insulator. Since this insulator is between the plates, the diode acts as a capacitor. In general, the capacitor blocks the DC currents but the AC currents can flow in the circuit at high frequencies. So, these are used to detect the high frequency signals.

22. Gunn Diode

Gunn diode is fabricated with n-type semiconductor material only. The depletion region of two N-type materials is very thin. When voltage increases in the circuit, the current also increases. After certain level of voltage, the current will exponentially decrease, thus this exhibits the negative differential resistance.

It has two electrodes with Gallium Arsenide and Indium Phosphide. Due to this, it has negative differential resistance. It is also termed as transferred electron device. It produces micro wave RF signals so it is mainly used in Microwave RF devices. It can also use as an amplifier. The symbol of Gunn diode is shown below:

The post Types of Diodes | Small Signal, LED, Schottky, Zener appeared first on ElectronicsHub.

Diode Clippers

Most of the electronic circuits like amplifiers, modulators and many others have a particular range of voltages at which they have to accept the input signals. Any of the signals that have an amplitude greater than this particular range may cause distortions in the output of the electronic circuits and may even lead to damage of the circuit components.

As most of the electronic devices work on a single positive supply, the input voltage range would also be on the positive side. Since the natural signals like audio signals, sinusoidal waveforms and many others contain both positive and negative cycles with varying amplitude in their duration.

These waveforms and other signals have to be modified in such a way that the single supply electronic circuits can be able to operate on them.

The clipping of a waveform is the most common technique that applies to the input signals to adapt them so that they may lie within the operating range of the electronic circuits. The clipping of waveforms can be done by eliminating the portions of the waveform which crosses the input range of the circuit.

Clippers can be broadly classified into two basic types of circuits. They are:

• Series Clippers
• Shunt or Parallel Clippers

Series clipper circuit contains a power diode in series with the load connected at the end of the circuit. The shunt clipper contains a diode in parallel with the resistive load.

The half – wave rectifier circuit is similar to a series clipper circuit. If the diode in a series clipper circuit is in forward bias condition, then the output waveform at the load follows the input waveform. When the diode is in reverse bias and it is unable to conduct current, the output of the circuit is nearly zero volts.

The direction in which the diode is connected determines the polarity of the clipped output waveform. In case of Series Clipper Circuits, if the diode is reverse biased i.e., Cathode is connected to the positive terminal of the supply and Anode to the load, the circuit will be a Positive Series Clipper, as it clips off the positive half cycle of the input sinusoidal waveform.

If the diode is forward biased i.e., Anode is connected to positive of power supply and Cathode to load, then the circuit will be a Negative Series Clipper, as it clips off the negative half cycle of the input sinusoidal waveform.

The series clipper diode has an output voltage of V_OUT = V_IN, when the diode is conducting and when it is not conducting, the input voltage applied by the supply will be dropped and has an output voltage of V_OUT = 0 V.

In contrast to the Series Clipper Circuit, a Parallel Clipper circuit provides the output when the diode is connected in reverse bias and when it is not conducting. When the diode is non–conducting, the shunt combination diode acts as an open circuit and both the series resistor and load resistor acts as a voltage divider. The output voltage will be calculated as:

 V_OUT = V_IN [R_LOAD / (R_LOAD + R_SERIES)] 

When the diode is conducting, it acts as a short circuit and the output voltage across the load will be V_OUT = 0 V. The series limiting resistor is connected in series with the supply to prevent the diode from short circuits.

In this case, the output voltage of the circuit should be ±0.7 volts. It depends on the polarity of the shunt clipper which is determined by the direction of diode connection.

Above circuit is a shunt clipper circuit which uses the DC supply voltage to bias the diode. It is the biasing voltage at which the diode starts conducting. The diode in the shunt clipper circuit starts to conduct when it reaches the biasing voltage.

Clipper circuits are used in a variety of systems to perform one of the two functions:

1. Altering the waveform shapes
2. Protecting the circuits from transients

The first application is commonly noticed in the operation of half-wave rectifiers that changes an alternating voltage into an output pulsating DC waveform. A transient is defined as an abrupt change in current or voltage with extremely short duration. Clipper circuits can be used to protect the sensitive circuits from transient effects.

Types of Clipper Circuits

Series Negative Clipper

This is the basic clipper circuit using a diode and it nothing but a Half Wave Rectifier. From the following circuit, it is clear that the diode is forward biased during the positive cycle and it acts as a closed switch. Hence, the output voltage is equal to the positive half of the input voltage.

During the negative cycle, the diode is reverse biased and acts as an open switch. As a result, the output voltage is zero.

•  Output voltage (V_OUT) during Positive Half Cycle = (V_IN – V_D) Volts
•  Output voltage (V_OUT) during Negative Half Cycle = 0 Volts

where, V_D is the Threshold Voltage of the Diode. If the diode is ideal, then V_D = 0 V.

Series Positive Clipper

Simply by reversing the diode, we can obtain a positive clipper configuration as show in the following circuit.

Cathode is connected to the positive of the power supply and anode is connected to the load, which makes diode reverse biased for positive cycle and forward biased for negative cycle.

• During Positive Half Cycle: Output voltage (V_OUT) = 0 V
• During Negative Half Cycle: Output voltage (V_OUT) = (V_IN + V_D) Volts

Where, V_D is the Diode Threshold Voltage.

Shunt Positive Clipper

Anode is connected to the the power supply through a resistor R and the cathode is at ground potential.

• During Positive Half Cycle: Output voltage (VO) = Vd Volts
• During Negative Half Cycle: Output voltage (VO) = Vin Volts

Shunt Negative Clipper

Cathode is connected to the power supply through a resistor R and anode is maintained at ground potential.

• During Positive Half Cycle: Output voltage (VO) = Vin Volts
• During Negative Half Cycle: Output voltage (VO) = – Vd Volts

Series Positive Clipper with Positive Bias Voltage

Positive Half Cycle: Cathode is connected to the positive supply and the anode is maintained at positive bias potential.

• When Vin < Vd + Vdc, Output Voltage (VO) = (Vin + Vd) Volts
• When Vin > Vd + Vdc, Output Voltage (VO) = + Vdc Volts

Negative Half Cycle: Cathode is connected to the negative supply and anode is maintained at positive bias potential.

• Output voltage (VO) = (Vin + Vd)

Series Positive Clipper with Positive Bias Voltage Connected in Series

Positive Half Cycle: Anode is maintained at ground potential and cathode is connected to a positive voltage. The diode is reverse biased during the whole positive half cycle.

• Output Voltage (VO) = 0 Volts

Negative Half Cycle: Anode is maintained at ground potential and cathode is connected to a negative supply.

• When Vin < Vd + Vdc, Output voltage (VO) = 0 Volts
• When Vin > Vd + Vdc, Output voltage (VO) = (Vin +Vdc +Vd) Volts

Series Positive Clipper with Negative Bias Voltage

Positive Half Cycle: Cathode is connected to the positive supply and the anode is maintained at negative bias potential.

• Output Voltage (VO) = -Vdc Volts

Negative Half Cycle: Cathode is connected to the negative supply and anode is maintained at negative bias potential.

• When Vin < Vd + Vdc, Output Voltage (VO) = – Vdc Volts.
• When Vin > Vd + Vdc, Output Voltage (VO) = (Vin + Vd) Volts.

Series Positive Clipper with Negative Bias Voltage Connected in Series

Positive Half Cycle: Anode is maintained at ground potential and cathode observes a variable voltage. The diode is forward biased during the whole positive half cycle.

• When Vin < Vdc – Vd, Output voltage (VO) = (Vin –Vdc +Vd) Volts
• When Vin > Vd + Vdc, Output voltage (VO) = 0 Volts

Negative Half Cycle: Anode is maintained at ground potential and cathode observes variable negative voltage. The diode will be forward biased during the negative cycle.

• Output Voltage (VO) = (Vin –Vdc +Vd) Volts

Series Negative Clipper with Positive Bias Voltage

Positive Half Cycle: In this case the anode is connected to the positive supply and the cathode is maintained at positive bias potential.

• When Vin < Vd + Vdc, Output Voltage (VO) = Vdc Volts
• When Vin > Vd + Vdc, Output Voltage (VO) = (Vin – Vd) Volts

Negative Half Cycle: In this case the anode is connected to the negative supply and the cathode is maintained at positive bias potential.

• Output Voltage (VO) = + Vdc Volts

Series Negative Clipper with Positive Bias Voltage Connected in Series

Positive Half Cycle: Cathode is maintained at negative potential, anode observes a variable voltage. The diode is forward biased during the whole positive half cycle.

• When Vin < Vd + Vdc, Output Voltage (VO) = (Vin +Vdc – Vd) Volts
• When Vin > Vd + Vdc, Output voltage (VO) = 0 Volts

Negative Half Cycle: Cathode is maintained at negative potential and anode observes variable negative voltage.

• When Vin < Vdc – Vd, Output voltage (VO) = (Vin +Vdc –Vd) Volts
• When Vin > Vdc – Vd, Output voltage (VO) = 0 Volts

Series Negative Clipper with Negative Bias Voltage Connected in Parallel

Positive Half Cycle: In this circuit the anode is connected to the positive supply and the cathode is maintained at negative bias potential.

• When Vin < Vd + Vdc, Output Voltage (VO) = (Vin + Vd) Volts
• When Vin > Vd + Vdc, Output Voltage (VO) = + Vdc Volts

Negative Half Cycle: In this circuit the anode is connected to the negative supply and the cathode is maintained at negative bias potential.

• Output Voltage (VO) = (Vin + Vd) Volts

Series Negative Clipper with Negative Bias Voltage Connected in Series

Positive Half Cycle: Cathode is maintained at Vdc and anode observes a variable voltage.

• When Vin < Vd + Vdc, Output Voltage (VO) = 0 Volts
• When Vin > Vd + Vdc, Output voltage (VO) = (Vin –Vdc –Vd) Volts

Negative Half Cycle: Cathode is maintained at Vdc and anode observes a variable negative voltage.  The diode will be reverse biased during the negative cycle.

• Output voltage (VO) = 0 Volts

Shunt Positive Clipper with Positive Shunt Bias Voltage

Positive Half Cycle: In this circuit, anode is connected to the positive supply and the cathode is maintained at positive bias potential.

• When Vin < Vd + Vdc, Output Voltage (VO) = Vin Volts
• When Vin > Vd + Vdc, Output Voltage (VO) = (Vd + Vdc) Volts

Negative Half Cycle: In this circuit, anode is connected to the negative supply and the cathode is maintained at positive bias potential.

• Output voltage (VO) = Vin Volts

Shunt Positive Clipper with Negative Shunt Bias Voltage

Positive Half Cycle: In this circuit, anode node is connected to the positive supply and the cathode is maintained at negative bias potential.

• Output voltage (VO) = (-Vdc + Vd) Volts

Negative Half Cycle: In this circuit, anode is connected to the negative supply and the cathode is maintained at negative bias potential.

• When Vin < Vdc, Output voltage (VO) = (-Vdc + Vd) Volts
• When Vin > Vdc, Output voltage (VO) = Vin Volts

Shunt Negative Clipper with Positive Bias Voltage

Positive Half Cycle: Cathode is connected to the positive supply and the anode is maintained at positive bias potential.

• When Vin < Vdc – Vd, Output voltage (VO) = (Vdc – Vd) Volts
• When Vin > Vdc – Vd, Output voltage (VO) = Vin Volts

Negative Half Cycle: Cathode is connected to the negative supply and anode is maintained at positive bias potential.

• Output voltage (VO) = (Vdc – Vd) Volts

Clipping Both Half Wave Cycles

Positive Half Cycle: In this cycle, cathode of first diode D1 is maintained at +Vdc1 and its anode observes a variable positive voltage. Similarly anode of diode D2 is maintained at -Vdc2 and its cathode observes a variable positive voltage. The diode D2 will be completely reverse biased during the whole positive half cycle.

• When Vin < Vdc1 + Vd1 – Diodes D1 &D2 are reverse biased, Output voltage (VO) = Vin Volts.
• When Vin > Vdc1 + Vd1 – Diode D1 will be forward biased and D2 will be reverse biased, Output voltage (VO) = (Vdc1 + Vd1) Volts

Negative Half Cycle: In this cycle, cathode of diode D1 is maintained at +Vdc1 and its anode observes a variable negative voltage. Similarly anode of diode D2 is maintained at -Vdc2 and its cathode observes a variable negative voltage. The diode D1 will be completely reverse biased during the whole negative half cycle.

• When Vin < Vdc2 + Vd2 – Diodes D1 &D2 are reverse biased, Output voltage (VO) = Vin Volts.
• When Vin > Vdc2 + Vd2 – Diode D2 will be forward biased and D1 will be reverse biased, Output voltage (VO) = (-Vdc2 – Vd2) Volts

In this two side clipping circuit, both the positive and negative clipping levels can be varied independently. This type of circuit is called as Parallel based Clipper. It uses two diodes and two voltage sources connected in opposite directions.

Diode Clampers

Clampers can also be referred as DC restorers. Clamping circuits are designed to shift the input waveform either above or below a DC reference level without altering the shape of the waveform. This shifting of the waveform results in a change in the DC average voltage of the input waveform. The levels of peaks in the signal can be shifted using the clamper circuit, hence clampers can also be referred as level shifters.

Clampers can be broadly classified into two types. They are:

• Positive Clampers
• Negative Clampers

Positive Clamper: This type of clamping circuit shifts the input waveform in a positive direction, as a result the waveform lies above a DC reference voltage.

Negative Clamper: This type of clamping circuit shifts the input waveform in a negative direction, as a result the waveform lies below a DC reference voltage.

The direction of the diode in the clamping circuit determines the type of clamper circuit. The operation of a clamping circuit is mainly based on the switching time constants of a capacitor, which charges through the diode and discharges through the load.

Types of Clamper Circuits

Positive Clamper

The circuit of a Positive Clamper is shown in the following circuit. Here, the circuit consists of three main elements:

• Capacitor
• Diode
• Load

The diode is connected in parallel to the load in such a way that the cathode of the diode is connected to the capacitor and anode to the ground.

Let us analyze the circuit starting with a negative cycle after startup. During the first negative cycle, the diode is forward biased and acts as a closed switch. As a result, the capacitor charges to the peak input voltage, which we will call as V_C.

In the next positive and negative cycles, the capacitor doesn’t lose much charge due to the RC Time Constant. As a result, the diode pretty much stays reverse biased.

Hence, the output voltage is the sum of applied input voltage and the charge stored at capacitor.

 V_OUT = V_IN + V_C 

where V_C is the peak of the input voltage.

From the above equation, it is clear that the above circuit adds a positive DC Shift to the input voltage.

Positive Clamper with Positive Reference Voltage

A positive reference voltage is connected in series with the diode in the positive clamper circuit such that the positive terminal of the reference voltage is connected in series with the anode of the diode. The operation is similar to the above circuit except that the capacitor charges to peak of the input voltage plus the DC Voltage.

If V_P is the peak voltage and V_DC is the DC Reference, then the capacitor voltage V_C is V_P + V_DC and output voltage V_OUT is V_IN + V_C.

Positive Clamper with Negative Reference Voltage

During the negative half cycle, the diode starts conducting and the capacitor charges to V_P – V_DC.

During the consecutive positive and negative half cycles of the input waveform, the diode pretty much does not conduct and as a result, the output is equal to sum of voltage stored in the capacitor and applied input voltage.

As V_C = V_P – V_DC, the output voltage is V_IN + V_C = V_IN + (V_P – V_DC). The shift is still positive but less than the peak by V_DC.

Negative Clamper

The Negative Clamping circuit consists of a diode connected in parallel with the load. The capacitor used in the clamping circuit can be chosen such that it must charge very quickly and it should not discharge very drastically. The anode of the diode is connected to the capacitor and cathode to the ground.

During the first positive half cycle of the input, the diode is in forward bias and as the diode conducts the capacitor charges very quickly to the peak of the input V_P.

During the subsequent negative and positive half cycles of the input, the diode will be in reverse bias and the diode will not conduct. The output voltage will be equal to the sum of the applied input voltage and the charge stored in the capacitor. The output waveform is same as input waveform, but shifted below 0 volts by V_P.

Negative Clamper with Positive Reference Voltage

The circuit arrangement is very similar to the Negative clamper circuit, but a DC reference supply is connected in series with the diode. The output waveform is also similar to the Negative clamper output waveform, but it is shifted towards the positive direction by an amount equal to the reference voltage at the diode.

Negative Clamper with Negative Reference Voltage

If the reference voltage directions in the above case are reversed and connected to the diode in series, then during the positive half cycle the diode starts conducting current before applying input voltage. Since the cathode has a very small negative reference voltage less than zero volts, the waveform is shifted away from the 0 volts towards the negative direction by an amount of the reference voltage.

Applications of Clippers

1. Used for generating new waveforms and/or shaping the existing older waveforms.
2. Clippers can be used as freewheeling diodes in protecting the transistors from transient effects by connecting the diodes in parallel with the inductive load.
3. Commonly used in power supplies.
4. In the separation of synchronizing signals existing from the composite color picture signals.
5. Frequently used in FM transmitters for removing the excess ripples in the signals above a certain noise level.

Applications of Clampers

1. Clampers can be frequently used in removing the distortions and identification of polarity of the circuits.
2. For improving the reverse recovery time, clampers are used.
3. Clamping circuits can be used as voltage doublers and for modelling the existing waveforms to a required shape and range.
4. Clampers are widely used in test equipment and other sonar systems.

The post Diode Clippers and Clampers appeared first on ElectronicsHub.

Introduction

If you are starting to develop your own electronics project or if you want to troubleshoot any electronic circuit or project, then you must have a sound knowledge on basic electronic components and their working. You don’t have to understand its construction and internal working but at least some basic knowledge on how a component works, how to test a component and see whether the component is working properly or not.

Learn more about the basics of Semiconductor Diodes.

Having knowledge on how to test a component and assessing whether it is good or not is a very good electronic circuits troubleshooting skill.

To avoid getting undesired results, it is advisable to test all the basic components like a resistors, diode, LED etc., for their normal working or operation before assembling the components in a circuit (PCB). In the worst case scenario, if we do not perform any tests before assembly and if the output is not as expected, then it is very difficult to identify the source of the problem and we have test all the components (which is very difficult after assembling).

Let us focus on Testing Diodes in this tutorial. As mentioned earlier, diodes are one of the important components in electronic circuits, especially in Power Supplies (and there are many other applications of diodes).

How to Test a Diode?

The diode is a two terminal semiconductor device that allows current to flow only in one direction. These are found in different applications like rectifiers, clampers, clippers and so on.

When the anode terminal of the diode is made positive with respect to cathode, the diode is said to be forward-biased. The forward-biased diode voltage drop is typically 0.7V for Silicon diodes. This is the minimum potential difference between Anode and Cathode of the Diode to become forward biased.

Before testing a diode, we have to first identify the terminals of the diode i.e., its Anode and Cathode. Most of the PN Junction diodes have a white band on its body and the terminal near this white band is the cathode. And the remaining one is anode. Both through-hole and surface mount Diodes have this marking.

Some diodes may have a different color band (for example, some Zener Diodes have a Black marking on its Red / Orange body), but the terminal near this colored mark is almost always the cathode.

The testing of a diode can be carried in different ways, however here we have given some basic testing procedures of the diode.

NOTE: The below mentioned testing procedures are only for normal PN diode.

NOTE: If the diode you want to test is already in a circuit (on the PCB), then you can perform the following mentioned tests by removing / de-soldering only one lead of the diode.

How to Test a Diode using a Digital Multimeter?

The diode testing using a Digital Multimeter (DMM) can be carried in two ways because there are two modes available in DMM to check the diode. These modes are:

• Diode Mode
• Ohmmeter Mode (or Resistance Mode)

The Diode Test Mode is the best way to test a diode as it relies on the characteristics of the Diode. In this method, the diode is put in forward bias and the voltage drop across the diode is measured, using a Multimeter. A normally working diode will allow current to flow in forward bias and must have voltage drop.

In the Resistance Mode Test of the diode, both the forward and reverse bias resistances of the diode are measured. For a good diode, the forward bias resistance should be few hundreds of Ohms to few Kilo Ohms and the reverse bias resistance should be very high (usually indicated as OL – open loop in a multimeter).

Diode Mode Testing Procedure

• Identify the anode and cathode terminals of the diode.
• Keep the Digital Multimeter (DMM) in diode checking mode by rotating the central knob to the position where the diode symbol is indicated. In this mode, the multimeter is capable to supply a current of approximately 2mA between the test leads.
• Connect the red probe of the multimeter to the anode and black probe to the cathode. This means the diode is forward-biased.
• Observe the reading on multimeter’s display. If the displayed voltage value is in between 0.6 to 0.7 (for a Silicon Diode), then the diode is healthy and perfect. For Germanium Diodes, this value is in between 0.25 to 0.3.
• Now, reverse the terminals of the meter i.e., connect the red probe to cathode and black to anode. This is the reverse biased condition of the diode where no current flows through it. Hence, the meter should read OL or 1 (which is equivalent to open circuit) if the diode is healthy.

If the meter shows irrelevant values to the above two conditions, then the diode is defective. The defect in the diode can be either open or short.

Open diode means the diode behaves as an open switch in both reverse and forward biased conditions. So, no current flows through the diode in either bias condition. Therefore, the meter will indicate OL (or 1) in both reverse and forward-biased conditions.

Shorted diode means diode behaves as a closed switch, so the current flows through it irrespective of the bias and the voltage drop across the diode will be between 0V to 0.4V. Therefore, the multimeter will indicate zero voltage value, but in some cases it will display a very little voltage as the voltage drop across the diode.

Ohmmeter (Resistance) Mode Testing Procedure

Similar to the Diode Test method, the Resistance Mode is also a simple method to check the diode whether it is good, short or open.

• Identify the terminals of the diode i.e., anode and cathode.
• Keep the Digital multimeter (DMM) in resistance or ohmmeter mode by rotating the central knob or selector to the place where ohm symbol or resistor values are indicated. Keep the selector in low resistance (may be 1K ohm) mode for forward-bias and keep it in high resistance mode (100K ohm) for the reverse bias testing procedure.
• Connect the red probe to the anode and black probe to the cathode. This means diode is forward-biased. When the diode is forward-biased, the resistance of the diode is so small.

If the meter displays a moderately low value on the meter display i.e., a few tens of ohms, then the diode is not good. But if the resistance reading is few hundred ohms to few kilo ohms, then the diode is good and working properly.

• Now reverse the terminals of the multimeter such that anode is connected to black probe and cathode to red probe. So the diode is reverse biased.
• If the meter shows a very high resistance value or OL on meter display, then the diode is good and functions properly. Since in reverse biased condition diode offers a very high resistance.

From the above it is clear that for proper working of the diode, DMM should read some low resistance in the forward-biased condition and a very high resistance or OL in reverse-biased condition.

If the meter indicates a very high resistance or OL in both forward and reverse-biased conditions, then the diode is said to be opened. In other hand, if the meter reads a very low resistance in both directions, then the diode is said to be shorted.

How to Test a Diode using Analog Multimeter?

Most analog multimeters usually do not have a dedicated Diode Test Mode. So, we will be using the Resistance Mode in Analog Multimeter, which is similar to the testing of diode using DMM ohmmeter mode.

• Keep the multimeter selector switch in low resistance value
• Connect the diode in the forward-biased condition by connecting the positive terminal to anode and negative to the cathode.
• If the meter indicates a low resistance value, then it says that the diode is healthy.
• Now put the selector in high resistance position and reverse the terminals of the meter by connecting positive to the cathode and negative to anode. In this case, the diode is said to be in reverse bias.
• If the meter indicates OL or a very high resistance, then it refers to the perfect condition of the diode.
• If the meter fails to show above readings, then the diode is said to be defective or bad.

This is about simple PN diode-testing using digital and analog multimeters. These testing procedure may not be applicable for all types of diodes. So, now let us see how to test an LED and a Zener diode.

How to Test LED (Light Emitting Diode)?

As discussed above, before testing any diode we must know its pins (terminals). The terminals of the LED can be identified by the length of the leads. Longer one is anode and the shorter one is the cathode. Also, another method is using the surface structure wherein a flat surface indicates the cathode and other one is the anode.

Let us now see how to test an LED using a digital multimeter.

• Identify the anode and cathode terminals of the LED.
• Place the multimeter selector / knob in diode mode.
• Connect the probes of the meter to LED such that it is forward-biased.
• If the LED is working properly, then it glows otherwise the LED is defective.
• Reverse-biased testing cannot be possible with LED since it doesn’t work in reverse-biased condition.

How to Test a Zener Diode?

When compared to the testing a normal diode, testing a Zener diode needs some extra circuitry. Because, the Zener diode conducts in reverse-biased condition and only if the applied reverse voltage is more than the Zener breakdown voltage.

• Identify the terminals anode and cathode of the Zener diode and its identification process is similar to the normal PN diode (using a mark).
• Connect the test circuit as shown in the above figure.
• Place the multimeter knob in voltage mode.
• Connect the meter probes across the Zener diode as shown in figure.
• Gradually increase the input supply to the diode, and observe the voltage on the meter display. This reading on the meter must be such that as we increase the variable supply, meter output should increase until the breakdown voltage of the diode. And beyond this point meter should show a constant value of voltage irrespective of any increase of the input variable supply. If it so, then Zener diode is healthy, otherwise defective.

Suppose, if we apply a 12V to the Zener diode (with a breakdown voltage is 6V) from the battery through a resistor, then multimeter must show a reading which is approximately equal to the 6V, if the Zener diode is healthy.

Conclusion

A complete beginner’s guide on how to test a diode. Learn how to identify the terminals of a Diode, test a diode with Digital Multimeter (DMM), Analog Multimeter, test LEDs and Zener Diodes.

The post How to Test a Diode? Using Analog and Digital Multimeter (DMM) appeared first on ElectronicsHub.

Commonly used Diode Characteristics

Some of the frequently used diode characteristics are given below.

• Current Equation
• DC Resistance
• AC Resistance
• Transition Capacitance
• Diffusion Capacitance
• Storage Time
• Transition Time
• Recovery Time

Now, we will see a little bit more about these diode characteristics in brief. 

Diode Current Equation

PN junction diode is widely known for passing the electric current solely in one direction. The amount of current flowing through the PN junction diode greatly depends on the type of material used and also depends on the concentration of doping in the fabrication of PN diode.

The main reason for the flow of current is due to the generation or recombination of majority charge carriers in the structure of the PN junction diode.

We will have three regions responsible for the flow of majority charge carrier current. These regions are namely quasi neutral P – region, depletion region, quasi neutral N – region. The region of the quasi neutral P – type is the separation between the edge of the depletion region and the edge of the diode on the P – side.

The region of the quasi neutral N – type is the separation between the edge of the depletion region and the edge of the diode on the N – side. For assumption, this separation distance is infinity. There will be no variation in the concentration of charge carriers as we move towards the boundaries of the diode. The electric field will not present in the quasi neutral region.

Δn_p(x → -∞) = 0

Δp_n(x → +∞) = 0

The diode current in the forward bias is due to the recombination of majority charge carriers. The charge carrier recombination takes place either in the P – type or N – type quasi neutral regions, in the depletion region or at the ohmic contacts i.e., at the contact of metal and semiconductor.

The current flow in the reverse bias is due to generation of charge carriers. This type of charge carrier generation process further increases the current flow in forward and also in reverse bias condition.

The flow of current in the PN junction diode is determined by the charge carrier density, the electric field throughout the structure of PN junction diode and the quasi Fermi level energies of the P – type and N – type. The carrier density and the electric field are used for determining the drift current and diffusion current of the PN diode.

The quasi Fermi level energies of the electrons and holes within the depletion region and that of in the N – type and P – type quasi neutral regions are assumed to be approximately equal in obtaining an analytical solution.

If the Fermi energy levels are assumed to be constant in the depletion region, the minority charge carrier density at the boundary of the depletion region would be as follows,

When there is no external voltage applied, thermal equilibrium state is reached at the above stated equations. The separation between the Fermi levels increases with the external applied voltage. This external voltage is multiplied by the charge of the electron.

The excess charge carriers present in either of the quasi region recombines straight away when they reach the metal – semiconductor contact. The process of recombination takes place rapidly at the ohmic contact and it further increases by the presence of metal. Therefore valid boundary conditions can be stated as follows,

p_n (x = w_n) = p_n0

n_p (x = -w_p) = n_p0

Consider the diffusion current equation for both the quasi neutral regions of N – type and P – type, the expression for the current of the ideal diode will be obtained  by the using the boundary conditions to the considered diffusion current equation.

Converting the above equations in terms of hyperbolic functions, rewriting the above equations as

p_n (x≥x_n) = p_n0 + A cosh {(x-x_n)/L_p} + B sinh {(x-x_n)/L_p}

n_p (x ≤ -x_p) = n_p0 + C cosh {(x+x_p)/L_n} + D sinh {(x+x_p)/L_n}

Here A, B, C and D are the constant values to be determined. If the boundary conditions are applied to the above hyperbolic equations, then we will have

Where the widths of the quasi neutral region of N – type and P – type are given as

w´_n = w_n – x_n

w´_p = w_p – x_p

The charge carrier current density in each of the quasi neutral region is calculated from the diffusion current equation as

The amount of electric current flowing throughout the entire structure of the PN junction diode always should be constant, because no charge can disappear or accumulate in entire structure of diode.

Hence, total current through the diode is equal to the sum of the maximum hole current in the n-region, the maximum electron current in the p-region and the current because of the recombination of charge carriers in depletion region. The maximum currents in the quasi neutral regions occur at the sides of the depletion region.

DC or Static Resistance

Static resistance or DC resistance of a PN junction diode defines the diode’s resistive nature when a DC source is connected to it. If an external DC voltage is given to the circuit in which the semiconductor diode is a part of it, results in a Q-point or operating point on the PN junction diode characteristic curve that does not alter with time.

The static resistance at the knee of the curve and below of it will be much greater than the resistance values of the vertical rise section of the characteristic curve. Minimum is the current passing through a diode maximum is the level of DC resistance.

R_DC = V_DC / I_DC

AC or Dynamic Resistance

Dynamic resistance is derived from Shockley’s Diode Equation. It defines the diode resistive nature when an AC source which depends on the DC polarisation of the PN junction diode is connected to it.

If an external sinusoidal signal is given to the circuit consisting of a diode, the altering input will shift the instantaneous Q – point slightly from the current position in the characteristics and therefore it defines a definite change in voltage and current.

When no external alternating signal is applied, the operating point will be the Q – point (or quiescent point) which is determined by the applied DC signal levels. The AC resistance of the diode is increased by lowering the Q-point of operation. In short, it is equivalent to slope of voltage – current of the PN diode.

r_d = ΔV_d / ΔI_d

Average AC Resistance

If the input signal is sufficient enough to produce a large swing, then the resistance related to the diode for this region is called as AC average resistance. It is determined by the straight line that is drawn linking the intersection of the minimum and maximum values of external input voltage.

R_avg = ( ΔV_d / ΔI_d ) _{pt to pt}

Transition Capacitance

Transition capacitance can also be termed as depletion layer capacitance or space charge capacitance. It is mainly observed in a reverse biased configuration where the P – type and N – type regions have lower resistances and the depletion layer may act like a dielectric medium.

This type of capacitance is due to the variations in the external voltage where the immobile charges get vary at the edges of the layer of the depletion region. It depends upon the dielectric constant and the width of the depletion layer. If the depletion layer width increases the transition capacitance decreases.

C_T = ε_s / w = √{[qε_s / 2(ϕ_i – V_D)][N_aN_d / (N_a + N_d)]}

Diffusion Capacitance

Diffusion capacitance can also be termed as storage capacitance mainly observed in forward biased configuration. It is the capacitance caused by the transport of charge carriers between the two terminals of a diode i.e., from anode to cathode in the forward biased configuration of a PN junction diode.

If the electric current is allowed to pass through the semiconductor device, there will be some charge created across the device at some point of time. In case if the applied external voltage and current changes to a different value, there will be a different amount of charge created in the transit.

The ratio of transiting charge created to the differential change in voltage will be the diffusion capacitance. If the level of current is increased then the diffusion capacitance levels automatically increases.

The increased levels in current will result in reduced levels of associated resistance and also the time constant, that is important in very high-speed applications. The diffusion capacitance value is much greater than the value of transition capacitance and it is directly proportional to the value of direct current.

C_diff = dQ/dV = [dI(V)/dV]Γ_F

Storage Time

The PN junction diode acts like a perfect conductor in forward biased configuration and acts like a perfect insulator in reverse biased configuration. During the switching time from forward to reverse biased condition the current flow switches and remains constant at the same level. This time duration over which the current reverses and maintains constant level is called as storage time (T_s).

The time taken by the electrons to move from P – type back to N- type and holes to move from N – type back to P – type is the storage time. This value can be determined by the geometry of the PN junction. During this storage time the diode behaves as a short circuit.

Transition Time

The time for the current to decrease to a reverse leakage current value after it remains at a constant level is called as a transition time. It is denoted as  the transition time value is determined by the geometry of the PN junction and concentration of doping levels of the P – type and N – type materials.

Reverse Recovery Time

The sum of the storage time and transition time is termed as reverse recovery time. It is the time taken by the diode to raise the applied current signal to 10% of the constant state value from the reverse leakage current. The reverse recovery time value for PN junction diode is usually of the order of microseconds.

Its value for a widely used small signal diode rectifier 1N4148 is usually 4 ns and for general purpose rectifier diode it is 2 μs. The fast switching speeds can be achieved by the high value of reverse leakage currents and high forward voltage drops. It is denoted by T_rr.

Datasheet Analysis

Summary

A brief view on different Diode Characteristics is presented here.

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Need for Bypass Diodes in Solar Panels

It is necessary to add the additional components to bypass or circumvent the shaded or damaged parts of PV (photovoltaic) cells, to continue the producing of power usually. These additional components which allow the flow of current through PV cells when the cells are not able to produce power can be termed as bypass diodes.  

These diodes are necessary because a small damage or any disturbance in the PV module may affect the output current substantially. The effect in output current may be due to the cells in the module which are connected in series fashion, a single PV cell with some shade and due to the modules in a string can stop producing the power.

Bypass diodes are quite similar to the diodes that are used in the solar cells where the bypass diodes allow greater amount of current to pass through them with a very little amount of losses in them. In general, bypass diodes are arranged in reverse bias between the positive and negative output terminals of the solar cells and has no effect on its output.

Preferably there will be one bypass diode for each and every solar cell, but this is more expensive, so that there is one diode per small group of series connected solar cells. They are normally connected along with the several solar cells where no current is allowed to pass through them in the case when all the cells are in use without any shading.

The bypass diodes are helpful in the special cases when the cells are unable to pass the current through them. This type of bypass diode connection prevents the loss of power which allows the solar group to handle the real – world problems more efficiently.

Consider the above connection, if one of the connected panels is shaded for some reason. The panel will not produce any amount of significant power and the panel will also have a higher resistance which blocks the power flowing of the unshaded panel. Then the bypass diodes came into existence as shown in the diagram.

Consider if one of the panels is shaded in the above diagram, then the current of the unshaded panel flows through the bypass diode to avoid the higher resistance and current blocking of the shaded panel. Bypass diodes are useless, unless the panels are connected in a series fashion to produce high voltage.

Recently, some solar panels are being manufactured by the cells divided into groups with a built in bypass diode in that group. Solar modules with bypass diodes are manufactured because of two reasons. Primarily, the bypass diode improves the overall system performance of the solar module. The second reason is that they can provide a greater amount of product safety.

Under standard test conditions solar modules consistently can produce a maximum voltage of nearly 0.5 Vdc. The standard cell configuration of a solar module has 72 cells connected in a series fashion to produce an operating voltage somewhere nearly around 36 Vdc. Typically, a bypass diode is connected in parallel with every 24 cells in a 72 – cell solar module.

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Effects of Partial Shading

A solar cell that is shaded will not able to pass current and/or voltage to an unshaded cell through them, which causes the maximum power rating of the shaded cell to drop as a result of shading. More the cell shading more will be the drop in power. A cell with 75% shading would be more worsen than the three cells with 25% shading.

The unshaded cells develop a negative voltage and draws power than the shaded cells since they try to pass more amount of current than the shaded cells. When the power output of the shaded string reduces, the power output of the remaining panels in the string reduces as well. The inverter circuit will try to reduce the power output, and also eventually the output voltage of the string also drops out of the operating window of the inverter.

In such a case, under shaded conditions, a string of cells that are connected in a series fashion may produce a voltage drop of 12 Vdc.

When a bypass diode is connected in parallel to the string of cells that are connected in series, produces a voltage drop of around 0.7 Vdc. As the electricity flows through the least resistance path, here the current flows through the diode and bypasses the shaded cells.

However, if the bypass diodes were not present in the circuit, the effect of the shading would be even greater as the shaded solar cells draw about 12 Vdc, so that the solar module’s voltage may be reduced to 24 Vdc.

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Solar Panels Safety

The most horrible condition that can be imaginable with a solar module that has no bypass diodes is that it may cause fire and the by-product certainly will be the heat. This is improbable but possible under certain possible conditions. After a few days of operating under the shaded conditions, the additional amount of heat produced and multiple temperature cycles may cause the solar cell joints weak.

If the joints get more weaken and disconnect, there might be a possibility of producing an electrical arc. The high temperature penetrating from the electric arc may cause the glass to explode by allowing oxygen into the lamination of glass which holds the cells in the solar panel. In such a case the high and flammable EVA which holds the glass laminate and solar cell together may catch the fire.

This condition has to be avoided at all costs. Hence, bypass diodes are therefore needed in all solar electric modules/panels.

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Photovoltaic Solar Cell Construction

A photovoltaic cell is created when a positively charged P – type semiconductor and a negatively charged N – type semiconductor placed in opposite directions to each other which forms a diode. In practice, this semiconductor sandwich is combined with supporting materials otherwise can be called as doping materials to make the diode.

This diode is connected in a circuit by means of metal conductors on both the top and bottom of the silicon sandwich to make panels, where they can be arranged in an arrayed fashion to provide different amounts of electricity.

Actually the PV cell includes an anti‐reflective coating to accept the most amount of sunlight into the silicon sandwich. This anti reflecting sheet tries to reduce the amount of sunlight reflecting from the glass by allowing the most amount of sunlight to hit the photovoltaic cell and increases the solar panel’s efficiency.

The photovoltaic cell is the vital element in a whole photovoltaic system, the photovoltaic panel is used to make a cell or a group of cells make usable. In photovoltaic panel, photovoltaics may be used alone or in a group of panels to power the large number of different electrical loads. Various types of photovoltaics vary in their size and structure.

• A single cell or multiple numbers of cells are the core part of the photovoltaic panel.
• A glass lamination is placed over the photovoltaic cell to protect it from the outside elements by allowing the sunlight to pass through to the photovoltaic cell.
• An additional plastic anti‐reflecting sheet is frequently used to improve the effect of the glass laminated cover and anti-reflective coating of the photovoltaic cell to block the reflection.
• A panel backing that is usually plastic and a frame will usually complete the photovoltaic panel by holding all the pieces together and thereby protecting it from damage during the process of installation.

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Photovoltaic Array Connections

A simple photovoltaic array in the above diagram consists of four photovoltaic modules by producing two branches that are in parallel fashion where there will be two PV panels which are electrically connected together to produce a series fashioned circuit.

Therefore, the output voltage from the solar cell array may be equal to the sum of the voltages of PV panels that are in series connection. From the above circuit, the output voltage is V_out = 12V + 12V = 24 Volts. The output current of a photovoltaic array is equal to the overall sum of the parallel branch currents.

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Photovoltaic Array Characteristics

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Short circuit current (Isc) and/or open circuit voltage (Voc)

The solar cells or photovoltaic panel can be typically characterized by the short circuit current represented as I_sc and the open circuit voltage represented as V_oc. The short circuit current of the solar panel can be termed as the current generated by the solar cell or panel if the output voltage is set to zero volts.

I_L = I_SC + I_SC.(R_S/R_P) + I_O.[exp[(q/kT).I_SC.R_S)-1]

I_SC ~ I_L

R_S / R_P and R_S are negligible, the short circuit current of the solar cell or panel is close to the photocurrent I_L that is generated by the cell and it is the maximum possible amount of current generated by the cell for a fixed amount of illumination.

I_L = I_O.{exp[(q/kT).V_OC]-1}-(V_OC/R_P)

The open circuit voltage represented as V_oc is the output voltage which is measured at zero solar current. The photocurrent is equal to the loss of current in the intrinsic element of the solar cell and open circuit voltage V_oc is equal to the forward voltage of intrinsic diode V_d.

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Maximum Output Power Point

It is defined as the point at which the maximum amount of power is produced by the solar panel that is associated with the batteries and/or inverter load. The maximum output power point of the photovoltaic solar array panel can be usually measured in Peak Watts or Watts. It can be given as follows.

P_max = (I_max) x (V_max)

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V_oc and I_sc Variations with Relevant Ambient Temperature

The open circuit voltage of the solar cell or panel can be linked with the forward voltage of the parasitic diode. Therefore, the open circuit voltage V_oc is temperature dependent with negative temperature coefficient. The maximum value for open circuit voltage is the value at a minimum temperature at the junction that is specified in the panel data sheet.

The short circuit current of the cell or panel increases slightly with the junction temperature. The below figure shows the variations in the V_oc and I_sc with the temperature.

There will be a substantial bound to the amount of maximum current of a particular photovoltaic solar cell. The value of I_max of a PV solar cell or panel greatly depends on the size and structure of the cell/panel, the total quantity of sunlight directly hitting the panel/cell, its effectiveness in converting the direct sunlight power into the current and the semiconductor material type where the solar cell is fabricated from the semiconductor material either cadmium Telluride, cadmium sulphide, gallium arsenide and/or silicon and germanium etc.

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Bypass Diodes in Solar Panels (Photovoltaic Arrays)

For example, assume that the output of solar panel is connected to a DC battery. So when there is light, solar panel produces the voltage and if this voltage is greater than the battery voltage battery charges. If no light incidents on the solar panel, then the battery discharges through the solar panel.

Hence, in order to avoid the battery discharge when the solar panel is in the dark we use a diode in series with the solar panel, this diode is called is blocking diode. In the above circuit the diodes which are in series with the solar panels are the blocking diodes.

In the above circuit the diodes which are connected in parallel with solar panels are called as bypass diodes. These diodes provide the separate path for the current to flow when the solar panels are shaded or damaged.

The blocking diodes and bypass diodes are physically same, but their functionality is different. Blocking diodes are also called as series diodes or isolation diodes. For each parallel brach of solar panels we will use a single blocking diode. Type and size of the blocking diode depend on photovoltaic array type.

Generally two types of diodes are used as a bypass diode in solar arrays. They are normal PN junction Si diode and Schottky diode. Both types of diode have wide range of current ratings. Schottky diode is preferable as a bypass diode than the normal PN silicon diode because it has less voltage drop of about 0.4V, where as normal Si diode has a voltage drop of 0.7V.

In recent days, most of the solar panel manufacturers include both blocking and bypass diodes in their solar panel design.

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Introduction

Today the markets of digital communication signal processing equipment and other portable devices has the need of developing a large number of small size electronic components. The integration of multiple functions and miniaturization of electronic components on the boards due to constraints on the board space provides the requirement of other packaging technologies.

Signal diodes are used in switching operations, snubbing circuits on which short duration waveforms are concentrated, high speed data lines and other I/O parallel connected ports. These signal diodes have enormous applications in signal processing and digital communications.

There is a series of signal diodes available in the market. Of them, 1N4148 series signal diode is widely used in a large number of electronic circuits due to its small size, power requirements and other useful parameters. In the following diagram pin 5 is used for ground.

To combat the over space requirement on digital circuit boards, signal diodes are connected in parallel configuration forming an array called as small signal diode arrays. They are enclosed in a plastic case or glass case in single line or dual line packages, features from 4 to 14 diodes to provide common cathode or common anode configurations.

The space saving signal diode array configurations provide electro static discharge protection, heat control and over voltage transients. These advantages make the diode arrays, ideal to be placed in the circuits on PCBs.

When the signal diodes are connected in series with the respective power supply terminals, then the data lines that are connected at the junction between the two signal diodes are protected from unnecessary transients and thereby the data will continue to pass along the data lines.

If the signal diodes are connected in 6-fold, then the array can guard all the 6 data signal lines in a single inline package. Signal diode arrays can be used for regulating the voltage that is applied to circuit on the PCBs. If the voltage applied exceeds the maximum voltage rating, then the excess energy provided will be penetrated as heat which may damage the device.

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Signal Diode Arrays –  In Series

In order to protect the board from excess voltages the signal diodes can be connected either in series or in parallel configurations for providing the fixed known voltage within the limit. When the signal diodes are connected in series the maximum current required by the diodes in the array is same and the maximum voltage drop in the array will be the sum of all the forward voltage drops in the array of signal diodes.

In the array of signal diodes in series configuration, the output voltage will be constant in spite of variations in the current in the load connected or in the variations of the input voltage applied. Hence supply of constant voltage is provided by the signal diode series combination.

Since the forward voltage drop of silicon diode is 0.7 V and the current through the silicon diode alters by a fairly large number, the signal diode connected in forward bias will make a circuit of the simple voltage regulator.

The individual forward voltage drop of each signal diode in series combination is thus subtracted from the input voltage applied to depart a certain amount of voltage across the load resistor connected at the end of the circuit. This is due to ON resistance of each diode in addition to the load resistance R.

With the addition of a number of signal diodes in series, a large amount of declination in voltage will takes place. Furthermore the signal diodes connected in series, in parallel with the load resistor R act as a voltage regulator circuit.

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Freewheel Diodes

Freewheel diodes also termed as fly back diodes, suppression diodes or clamp diodes. Free wheel diodes or suppressor diodes are a combination of small signal diodes that are connected in the parallel combination across the  inductive load to suppress the sudden voltage spikes when either of the supply voltage or connected inductive  load is turned off, thereby freewheel diodes prevents the switching circuit from damage.

These types of diodes provide the smooth current to the load connected, thereby eliminating the negative voltage at the load. It is mostly seen in rectifiers and probably useful in power electronics. One universal example of freewheel diode is 1N4007.

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Principle of Working

Whenever a voltage source is connected to a switch and an inductor load, then there will be a possibility of two steady states. In the first case when the switch is closed, the inductor connected at the end acting as a load will get the full energy from the input voltage applied and thus all the current in the circuit will pass from the positive terminal to negative terminal through the inductor.

In the second case when the switch is opened, the inductor load experiences a sudden drop in the current and will defend against it with its accumulated magnetic field energy.

When a positive potential is applied there will be a large negative potential concentrated and when a negative potential is applied there will be a large positive potential concentrated. Since no physical connection is made for passing the current, the charge carriers will cross over the band gap of the switch or transistor.

A freewheel diode prevents the crossover problem of the charge carriers with inductor load  by allowing the inductor to draw current till the energy is penetrated through the diode and wire in a continuous loop.

The freewheel diode acts as a forward biased diode with respect to inductor when the switch is opened, allowing the inductor to conduct electric current from the positive terminal to negative terminal in a continuous manner. The voltage present at the inductor load will be a certain function of the forward voltage drop of the freewheel diode and the total power dissipation time of the diode will be usually few milliseconds.

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Selection of Freewheeling Diodes

Depending on the application required one would choose an ideal freewheel diode based on the parameters like maximum forward current capacity, least forward voltage drop and appropriate reverse breakdown voltage that are best suited for voltage at the inductor.

Schottky diodes are best preferred in free wheel diode applications in switching converters. The prevailing series diodes 1N4001, 1M5400 are best used for dissipating the energy in the inductive load.

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Freewheel Diode Applications

Free wheel diodes are used for switching applications when the inductive loads are switched off in relay driving circuits, H-bridge motor driving circuits.

In the past years, the functioning speed of the many semiconductor switching type devices, either the transistor or any FETs have been reduced by the addition of a freewheel diode at the inductive load instead of using Schottky,  Zener and other types of diodes in certain applications.

But in the recent years, free wheel diodes are used most frequently due to their improved fast reverse-recovery times and the use of ultra fast semiconductor materials capable of operating at higher switching frequencies.

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Introduction

PN junction is a significant building block and it is one among the indispensable structures offered by the semiconductor technology in electronics. Electronic components such as bipolar junction transistors, junction FETs and MOSFETs, or diodes such as light-emitting diodes (LEDs), and analog or digital integrated circuits (ICs) are all supported in semiconductor technology.

The exciting property of semiconductor diode is facilitating the electrons to flow exclusively in one direction across it; as a result it acts as a rectifier of Alternating Current. The indispensable operation in semiconductor diode is the basis for understanding of all the semiconductor diodes.

The diode can be observed as a straightforward bipolar semiconductor device. The characteristics of diode look to be a graph of current that a diode produces when the voltage applied to it. A perfect diode can be absolutely distinguished by its current and voltage curve.

It permits the current to flow solely in forward direction and effectively blocks the current in the reverse direction. It is vital to recognize that the semiconductor is entirely a single-crystal material, made from two separate blocks of semiconductor opposite kind.

One block is doped with trivalent impurity atoms to create the P region that acts as acceptors with holes as majority charge carriers and the adjacent block is doped with pentavalent impurity atoms to create the N region that acts as donors with electrons as majority charge carriers.

The boundary splitting the n and p region is referred to as the metaphysical junction. The concentration of doping is same all over in every block and there will be an abrupt modification in doping at the junction. When the two blocks are placed nearer to each other, the electrons and holes diffuse towards the region of lower concentration from the region of higher concentration.

In the process of diffusion, electrons from N region diffuse towards the P region whereas holes from P region diffuse towards the N region. Once holes enter the N region, they will recombine with donor atoms. At the same time, donor atoms admit additional holes and become positively charged stationary donor atoms.

The electrons spreading from N region to P region recombine with the acceptor atoms in P region. At the same time, acceptor atoms admit additional electrons and become negatively charged immobile acceptor atoms.

As a result, a large number of positively charged ions are produced at the junction on the N side and a large number of negatively charged ions are produced at the junction on P side.

The net positively and negatively charged ions within the N and P regions induce an electric field in the space near to the metaphysical junction. Merging these two regions wherever the electric field is small and wherever the free carrier density is equivalent to the net doping density can be named as the space charge region.

It can also be referred as a quasi neutral region. Fundamentally, all electrons and holes are swept out of the free space charge region by the electric field. The tapered region in which depletion of free mobile charge carriers takes place is called as Depletion Region.

It is assumed that the depletion region around the metallurgical junction has well-defined edges. It conjointly assumes that the transition between the depletion region and the free space charge region is abrupt.

Depletion region contains preset positive ions on the N-side and preset negative ions on the P-side. The width of the depletion layer is inversely proportional to the concentration of dopants present in each region.

The electric field within the depletion region creates an opposing force that opposes the electrons and holes from diffusing attributable to the impact of charged ions within the depletion region. This opposing force can be often cited as potential barrier voltage. The typical value of potential barrier for silicon is 0.72V and for germanium is 0.3V.

When the electric field and barrier potential are balanced with one another, then the state of equilibrium is reached that result in potential difference Vo connecting the two sides of the depletion layer. The net contact potential difference depends on the type of material and it is high for n-type than the p-type.

In the state of thermal equilibrium, barrier potential provides low potential energy for the electrons on N-side than P-side. Energy bands bend in the free space charge region, since conduction and valence band positions with respect to the Fermi energy levels changes between P and N regions.

No conduction of current takes place in this equilibrium state and the current due to diffusion and drift current cancel for both the electrons and holes. The built-in barrier potential maintains balance between majority charge carriers in the N region and minority charge carriers in the P region as well as between majority charge carriers in the P region and minority charge carriers in the N region.

The built-in potential barrier can also be estimated as the distinction between the intrinsic Fermi energy levels in P and N regions.

PN junction diode is a diode which can be used as a rectifier, logic gate, voltage stabiliser, switching device, voltage dependent capacitor and in optoelectronics as a photodiode, light-emitting diode (LED), laser diode, photo detector, or solar cell in electronics.

Working of PN Junction Diode

If an external potential is applied to the terminals of PN junction, it will alter the potential between the P and N-regions. This potential difference can alter the flow of majority carriers, so that the PN junction can be used as an opportunity for the diffusion of electrons and holes.

If the voltage applied decreases the width of the depletion layer, then the diode is assumed to be in forward bias and if the applied voltage increases the depletion layer width then the diode is assumed to be in reverse bias. If the width of depletion layer do not alters then it is in the zero bias state.

• Forward Bias: External voltage decreases the built-in potential barrier.
• Reverse Bias: External voltage increases the built-in potential barrier.
• Zero Bias: No external voltage is applied.

PN Junction Diode When No External Voltage is Applied

In zero bias or thermal equilibrium state junction potential provides higher potential energy to the holes on the P-side than the N-side. If the terminals of junction diode are shorted, few majority charge carriers (holes) in the P side with sufficient energy to surmount the potential barrier travel across the depletion region.

Therefore, with the help of holes, current starts to flow in the diode and it is referred to as forward current. In the similar manner, holes in the N side move across the depletion region in reverse direction and the current generated in this fashion is referred to as reverse current.

Potential barrier opposes the migration of electrons and holes across the junction and allow the minority charge carriers to drift across the PN junction. As a result of it, a state of equilibrium is established when the majority charge carriers are equal in concentration on either side of the junction and when minority charge carriers are moving in opposite directions.

A net zero current flows in the circuit and the junction is said to be in dynamic equilibrium. By increasing the temperature of semiconductors, minority charge carriers have been continuously generated and thereby leakage current starts to rise. In general no conduction of electric current takes place because no external source is connected to the PN junction.

Forward Biased Pn Junction Diode

With the externally applied voltage, a potential difference is altered between the P and N regions.When positive terminal of the source is connected to the P side and the negative terminal is connected to N side then the junction diode is said to be connected in forward bias condition. Forward bias lowers the potential across the PN junction.

The majority charge carriers in N and P regions are attracted towards the PN junction and the width of the depletion layer decreases with diffusion of the majority charge carriers. The external biasing causes a departure from the state of equilibrium and a misalignment of Fermi levels in the P and N regions, and also in the depletion layer.

So an electric field is induced in a direction converse to that of the incorporated field. The presence of two different Fermi levels in the depletion layer represents a state of quasi-equilibrium. The amount of charge Q stored in the diode is proportional to the current I flowing in the diode.

With the increase in forward bias greater than the built in potential, at a particular value the depletion region becomes very much thinner so that a large number of majority charge carriers can cross the PN junction and conducts an electric current. The current flowing up to built in potential is called as ZERO current or KNEE current.

Forward Biased Diode Characteristics

With the increase in applied external forward bias, the width of the depletion layer becomes thin and forward current in a PN junction diode starts to increase abruptly after the KNEE point of forward I-V characteristic curve.

Firstly, a small amount of current called as reverse saturation current exists due to the presence of the contact potential and the related electric field. While the electrons and holes are freely crossing the junction and causes diffusion current that flows in the opposite direction to the reverse saturation current.

The net result of applying forward bias is to reduce the height of the potential barrier by an amount of eV. The majority carrier current in the PN junction diode increases by an exponential factor of eV/kT. As result the total amount of current becomes I = I_{s *} exp(eV/kT), where I_s is constant.

The excess free majority charge carrier holes and electrons that enter the N and P regions respectively, acts as a minority carriers and recombine with the local majority carriers in N and P regions. This concentration consequently decreases with the distance from the PN junction and this process is named as minority carrier injection.

The forward characteristic of a PN junction diode is non linear, i.e., not a straight line. This type of forward characteristic shows that resistance is not constant during the operation of the PN junction. The slope of the forward characteristic of a PN junction diode will become very steep quickly.

This shows that resistance is very low in forward bias of the junction diode. The value of forward current is directly proportional to the external power supply and inversely proportional to the internal resistance of the junction diode.

Applying forward bias to the PN junction diode causes a low impedance path for the junction diode, allows for conducting a large amount of current known as infinite current. This large amount current starts to flow above the KNEE point in the forward characteristic with the application of a small amount of external potential.

The potential difference across the junction or at the two N and P regions is maintained constant by the action of depletion layer. The maximum amount of current to be conducted is kept limited by the load resistor, because when the diode conducts more current than the usual specifications of the diode, the excess current results in the dissipation of heat and also leads to severe damage of the device.

Reverse Biased PN Junction Diode

When positive terminal of the source is connected to the N side and the negative terminal is connected to P side, then the junction diode is said to be connected in reverse bias condition. In this type of connection majority charge carriers are attracted away from the depletion layer by their respective battery terminals connected to PN junction.

The Fermi level on N side is lower than the Fermi level on P side. Positive terminal attracts the electrons away from the junction in N side and negative terminal attracts the holes away from the junction in P side. As a result of it, the width of the potential barrier increases that impedes the flow of majority carriers in N side and P side.

The width of the free space charge layer increases, thereby electric field at the PN junction increases and the PN junction diode acts as a resistor. But the time of diode acting as a resistor is very low. There will be no recombination of majority carriers taken place at the PN junction; thus, no conduction of electric current.

The current that flows in a PN junction diode is the small leakage current, due to minority carriers generated at the depletion layer or minority carriers which drift across the PN junction. Finally, the result is that the growth in the width of the depletion layer presents a high impedance path which acts as an insulator.

In reverse bias condition, no current flows through the PN junction diode with increase in the amount of applied external voltage. However, leakage current due to minority charge carriers flows in the PN junction diode that can be measured in micro amperes.

As the reverse bias potential to the PN junction diode increases ultimately leads to PN junction reverse voltage breakdown and the diode current is controlled by external circuit. Reverse breakdown depends on the doping levels of the P and N regions.

With the increase in reverse bias further, PN junction diode become short circuited due to overheat in the circuit and maximum circuit current flows in the PN junction diode.

Reverse Biased Diode Characteristics

V-I Characteristics of PN Junction Diode

In the current–voltage characteristics of junction diode, from the first quadrant in the figure current in the forward bias is incredibly low if the input voltage applied to the diode is lower than the threshold voltage (Vr). The threshold voltage is additionally referred to as cut-in voltage.

Once the forward bias input voltage surpasses the cut-in voltage (0.3 V for germanium diode, 0.6-0.7 V for silicon diode), the current spectacularly increases, as a result the diode functions as short-circuit.

The reverse bias characteristic curve of diode is shown in the fourth quadrant of the figure above. The current in the reverse bias is low till breakdown is reached and therefore the diode looks like as open circuit. When the reverse bias input voltage has reached the breakdown voltage, reverse current increases spectacularly.

PN Diode Ideal and Real Characteristics

For ideal characteristics, the total current in the PN junction diode is constant throughout the entire junction diode. The individual electron and hole currents are continuous functions and are constant throughout the junction diode.

The real characteristics of PN Junction diode varies with the applied external potential to the junction that changes the properties of junction diode. The junction diode acts as short circuit in forward bias and acts as open circuit in reverse bias.

Summary

• Semiconductors contain the properties in middle of conductors and insulators.
• Commonly used material for semiconductor is silicon.
• Semiconductors contain electrons and holes as charge carriers.
• The charge carriers in semiconductors are free to move throughout the device, so they are called as mobile charge carriers.
• Holes are positively charged particles and electrons are negatively charged particles.
• Charge carriers are responsible for conducting electric current.
• Semiconductors are of two types namely intrinsic and extrinsic semiconductors.
• Intrinsic semiconductors are purest semiconductors as they don’t have any impurities in it.
• Extrinsic semiconductors contain impurities called as dopants that change the electrical properties of semiconductors.
• Extrinsic semiconductors are classified into two types. They are N-type and P-type.
• N-type impurities are called as donors because they contain electrons as majority chare carriers.
• P-type impurities are called as acceptors because they contain holes as majority charge carriers.
• PN junction is formed in a single crystal by joining two N-type and P-type semiconductors.
• PN junction diode is a two terminal device, the characteristics of diode depends on the polarity of the external potential applied to the PN junction diode.
• The junction of N and P semiconductors is free of charge carriers; hence the region is called as depletion region.
• The width of depletion region alters with the external applied potential.
• When no external potential is applied to PN junction, the condition is called as zero bias. The junction potential for silicon diodes is 0.6V – 0.7V and for germanium diodes is 0.3V.
• When the junction is biased in the forward direction, the majority carriers are attracted towards the junction and get replenished at the junction. In this condition, width of the depletion region decreases and with the increase in external potential diode acts as short circuit that allows the maximum amount of current to flow through it.
• When the junction diode is biased in the reverse direction, the majority charge carriers are attracted by the respective terminals away from the PN junction, thus avoiding the diffusion of electrons and holes at the junction. There will be a small amount of current called as leakage current due to minority charge carriers at the junction. This small current is called as drift current. When the reverse bias potential is increased further the diode acts as open circuit, thereby blocking the current to flow through it.

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Introduction

Diodes are often used as straightforward rectifiers, as mixers in compounding the signals and as switches to open or close a circuit. Diodes in the mixers are utilized for detecting the signals and these diodes are typically referred as signal diodes. The simple and conventional application of signal diodes is that it acts as a basic diode switch.

A signal diode could be a non linear semiconductor device available in recent days that form a kind of small elements of electrical and electronic circuits, made from the semiconductor crystals.

Signal diodes are widely acknowledged because they are frequently found in electronic circuits such as in televisions, radios, some other digital logic circuits and they are designed to drive very little or no power although high frequency currents exclusively in one direction.

The PN junction signal diode is usually fabricated in glass case or plastic case and generally has a black or a red band at the cathode end of the terminal.

Signal diodes have an advantage of quick recovery time and it has a huge variety of applications in signal processing. Signal diodes can be used for clocking functions in digital devices, conjointly serves to prevent the reverse signal from damaging of the microcontroller. Signal diodes will be employed in switching and clipping applications wherever short duration pulse waveforms are typically clipped off.

Signal diodes enable the current capability up to 100 milliamps and they are known for processing the information found in electrical signals sent from electrical transmitter. Germanium diodes have a forward voltage drop about 0.2 volts are used as detecting circuits in radios.

In electronic circuits that do not require the accuracy of germanium diodes, silicon semiconductor, signal diodes are normally employed owing to their lower value of resistance and their vulnerability to heat.

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Types of Signal Diodes

Here is the list of some of the signal diodes which are frequently used.

• 1N4973
• 1N4148
• 1N34A (Germanium Diode)
• 1N4454

Small signal diodes have low power and current ratings, around 500mW & 150mA almost compared to traditional rectifier diodes. The characteristics of a signal diode are completely different for germanium signal and silicon signal diodes. They are given as follows:

Germanium Signal Diodes – These diodes have very little amount of reverse bias resistance values resulting in a low forward voltage  drop across the PN junction, usually about 0.2 V- 0.3 V, however it has a high forward bias resistance value attributable to the small PN junction area.

Silicon Signal Diodes – These diodes have a terribly high reverse bias resistance values leading to a forward voltage drop about 0.6 – 0.7V across the PN junction. They have moderately low values of forward bias resistance resulting in higher values of forward current and reverse bias voltage.

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V-I Characteristics of a Signal Diode

If both the positive and negative terminals of external power supply are connected to the respective terminals of the PN junction, then the signal diode is said to be in Forward Bias. The voltage supplied to the diode in forward bias generates a forward current denoted by I_F.

The value of forward current is directly proportional to the external voltage applied and reciprocally proportional to the inbuilt resistance of the diode. The electrostatic force that prohibits the electrons and holes passing away from the junction due to the effect of charge ions within the depletion layer is named as barrier voltage

The typical values of barrier voltage at the p-n junction of germanium diode are 0.2 V ~ 0.3 V, whereas it is 0.6 V~ 0.7 V for silicon diode.

If the positive terminal of the supply is connected to cathode of the signal diode and negative terminal is connected to anode of the diode, then the signal diode is said to be in reverse bias. When an external voltage is applied to the diode in reverse bias, a small amount of current known as leakage current exists due to the minority charge carriers crossing the depletion layer and moving away from it.

This leakage current is also termed as Reverse Saturation Current denoted by  or which is independent of external voltage applied, but depends on the temperature of the device.

If the applied reverse bias voltage is very high, the minority charge carriers acquire enough energy to collide and split up the covalent bonds to generate a significant number of electron – hole pairs.

The phenomenon of electron – hole pair generation is called as breakdown. The maximum reverse voltage applied to the diode before its breakdown condition can be referred as peak reverse voltage or peak inverse voltage.

In forward bias, signal diode acts as closed switch and thus short circuited for driving current solely in one direction (from the positive terminal to the negative terminal). In reverse bias, silicon diode acts as open switch and thus open circuited for blocking the current flowing in the diode.

Silicon signal diodes act as rectifiers, switching circuits, limiting circuits and in clipping circuits for clipping the short duration waveforms.

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Signal Diode Characteristics

The parameter details of the signal diode characteristics and specifications for signal diodes are given below.

Peak Inverse Voltage (PIV)

Peak Inverse Voltage parameter is defined as the maximum amount of voltage that can be applied to the diode in reverse direction. This peak voltage should not be exceeded because the voltage greater than this peak voltage may cause the device failure. It is also referred to as maximum reverse voltage and it is less than the avalanche breakdown condition of the diode in reverse bias characteristic.

Typical values of peak inverse voltage may vary from a few volts to thousands of volts. In rectifier circuits with regards to amplitude, the peak inverse voltage is termed as the utmost negative value of the sine-wave surrounded by a cycle’s negative alternation.

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Power Dissipation (P_D­)

Total power dissipation is defined as the maximum amount of power that will be dissipated at the PN junction signal diode during the conduction of current. The excess power will be dissipated in the form of heat. The forward resistance of signal diode is a dynamic property, it is very small and sometimes it is varied.

In that condition the total power dissipated will be measured by multiplying the voltage applied to the diode and forward current flowing through the signal diode.

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Forward Current (I_F)

The forward current rating parameter of a signal diode is defined as the maximum amount of anode current that a signal diode can handle easily without damaging the device. If the current exceeds the forward current rating value, then the signal diode may get damaged at the junction due to thermal overload.

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Operating Temperature (T)

The maximum operating temperature parameter of a signal diode is more often related to the total power dissipation and also related to the temperature of the PN junction. It is defined as the maximum temperature of the device at which maximum forward current is reached.

Beyond this temperature value the device gets damaged and leads to failure of the device. The PN junction signal diode should be maintained at a temperature wherein the maximum forward current is achieved before it gets deteriorates.

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Specifications of 1N4148 Signal Diode

Here are some specifications of 1N4148 signal diode. They are as follows.

• Maximum repetitive reverse voltage = 100 V
• Average rectified forward current = 200 mA 
• Maximum direct forward current = 300 mA
• Maximum forward voltage drop = 1.0 V at 10 mA.
• Non-repetitive peak forward surge current = 1.0 A (pulse width = 1 s)
• Total power dissipation = 500 mW
• Reverse recovery time < 4 ns

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