S NAGA MALLESWARA REDDY

Bypass Diodes which in electronics we know as free-wheeling diodes, are wired in parallel with individual solar cells or panels, to provide a current path around them in the event that a cell or panel becomes faulty or open-circuited. This allows a series (called a string) of connected cells or panels to continue supplying power at a reduced voltage rather than no power at all.

Photovoltaic Solar Cell Construction

Diodes in Photovoltaic Arrays

Bypass Diodes in Photovoltaic Arrays

The Diode Clipper, also known as a Diode Limiter, is a wave shaping circuit that takes an input waveform and clips or cuts off its top half, bottom half or both halves together to produce an output waveform that resembles a flattened version of the input. For example, the half-wave rectifier is a clipper circuit, since all voltages below zero are eliminated.

But Diode Clipping Circuits can be used a variety of applications to modify an input waveform using signal and Schottky diodes or to provide over-voltage protection using Zener Diodes to ensure that the output voltage never exceeds a certain level protecting the circuit from high voltage spikes. Then diode clipping circuits can be used in voltage limiting applications.

We saw in the Signal Diodes tutorial that when a diode is forward biased it allows current to pass through itself clamping the voltage. When the diode is reverse biased, no current flows through it and the voltage across its terminals is unaffected, and this is the basic operation of the diode clipping circuit.

Although the input voltage to diode clipping circuits can have any waveform shape, we will assume here that the input voltage is sinusoidal. Consider the circuits below.

Positive Diode Clipping Circuits

In this diode clipping circuit, the diode is forward biased (anode more positive than cathode) during the positive half cycle of the sinusoidal input waveform. For the diode to become forward biased, it must have the input voltage magnitude greater than +0.7 volts (0.3 volts for a germanium diode).

When this happens the diodes begins to conduct and holds the voltage across itself constant at 0.7V until the sinusoidal waveform falls below this value. Thus the output voltage which is taken across the diode can never exceed 0.7 volts during the positive half cycle.

During the negative half cycle, the diode is reverse biased (cathode more positive than anode) blocking current flow through itself and as a result has no effect on the negative half of the sinusoidal voltage which passes to the load unaltered. Then the diode limits the positive half of the input waveform and is known as a positive clipper circuit.

Negative Diode Clipping Circuits

Here the reverse is true. The diode is forward biased during the negative half cycle of the sinusoidal waveform and limits or clips it to -0.7 volts while allowing the positive half cycle to pass unaltered when reverse biased. As the diode limits the negative half cycle of the input voltage it is therefore called a negative clipper circuit.

Clipping of Both Half Cycles

If we connected two diodes in inverse parallel as shown, then both the positive and negative half cycles would be clipped as diode D1 clips the positive half cycle of the sinusoidal input waveform while diode D2 clips the negative half cycle. Then diode clipping circuits can be used to clip the positive half cycle, the negative half cycle or both.

For ideal diodes the output waveform above would be zero. However, due to the forward bias voltage drop across the diodes the actual clipping point occurs at +0.7 volts and -0.7 volts respectively. But we can increase this ±0.7V threshold to any value we want up to the maximum value, (VPEAK) of the sinusoidal waveform either by connecting together more diodes in series creating multiples of 0.7 volts, or by adding a voltage bias to the diodes.

Biased Diode Clipping Circuits

To produce diode clipping circuits for voltage waveforms at different levels, a bias voltage, VBIAS is added in series with the diode as shown. The voltage across the series combination must be greater than VBIAS + 0.7V before the diode becomes sufficiently forward biased to conduct. For example, if the VBIAS level is set at 4.0 volts, then the sinusoidal voltage at the diode's anode terminal must be greater than 4.0 + 0.7 = 4.7 volts for it to become forward biased. Any anode voltage levels above this bias point are clipped off.

Positive Bias Diode Clipping

Likewise, by reversing the diode and the battery bias voltage, when a diode conducts the negative half cycle of the output waveform is held to a level -VBIAS - 0.7V as shown.

Negative Bias Diode Clipping

A variable diode clipping or diode limiting level can be achieved by varying the bias voltage of the diodes. If both the positive and the negative half cycles are to be clipped, then two biased clipping diodes are used. But for both positive and negative diode clipping, the bias voltage need not be the same. The positive bias voltage could be at one level, for example 4 volts, and the negative bias voltage at another, for example 6 volts as shown.

Diode Clipping of Different Bias levels

When the voltage of the positive half cycle reaches +4.7 V, diode D1 conducts and limits the waveform at +4.7 V. Diode D2 does not conduct until the voltage reaches –6.7 V. Therefore, all positive voltages above +4.7 V and negative voltages below –6.7 V are automatically clipped.

If the diode clipping levels are set too low or the input waveform is too great then the elimination of both waveform peaks could end up with a square-wave shaped waveform.

The advantage of biased diode clipping circuits is that it prevents the output signal from exceeding preset voltage limits for both half cycles of the input waveform, which could be an input from a noisy sensor or the positive and negative supply rails of a power supply.

Zener Diode Clipping Circuits

The use of a bias voltage means that the amount of the voltage waveform that is clipped off can be accurately controlled. But one of the main disadvantages of using voltage biased diode clipping circuits, is that they need an additional emf battery source which may or may not be a problem.

One easy way of creating biased diode clipping circuits without the need for an additional emf supply is to use Zener Diodes.

As we know, the zener diode is a another type of diode that has been specially manufactured to operate in its reverse biased breakdown region and as such can be used for voltage regulation or zener diode clipping applications. In the forward region, the zener acts just like an ordinary silicon diode with a forward voltage drop of 0.7V (700mV) when conducting, the same as above.

However, in the reverse bias region, the voltage is blocked until the zener diodes breakdown voltage is reached. At this point, the reverse current through the zener increases sharply but the zener voltage, VZ across the device remains constant even if the zener current, IZ varies.

Then we can put this zener action to good effect by using them for clipping a waveform as shown.

Zener Diode Clipping

The zener diode is acting like a biased diode clipping circuit with the bias voltage being equal to the zener breakdown voltage. In this circuit during the positive half of the waveform the zener diode is reverse biased so the waveform is clipped at the zener voltage, VZD1. During the negative half cycle the zener acts like a normal diode with its usual 0.7V junction value.

We can develop this idea further by using the zener diodes reverse-voltage characteristics to clip both halves of a waveform using series connected back-to-back zener diodes as shown.

Full-wave Zener Diode Clipping

The output waveform from full wave zener diode clipping circuits resembles that of the previous voltage biased diode clipping circuit. The output waveform will be clipped at the zener voltage plus the 0.7V forward volt drop of the other diode. So for example, the positive half cycle will be clipped at the sum of zener diode, ZD1 plus 0.7V from ZD2 and vice versa for the negative half cycle.

Zener diodes are manufactured with a wide range of voltages and can be used to give different voltage references on each half cycle, the same as above. Zener diodes are available with zener breakdown voltages, VZ ranging from 2.4 to 33 volts, with a typical tolerance of 1 or 5%. Note that once conducting in the reverse breakdown region, full current will flow through the zener diode so a suitable current limiting resistor, R1 must be chosen.

Diode Clipping Summary

As well as being used as rectifiers, diodes can also be used to clip the top, or bottom, or both of a waveform at a particular dc level and pass it to the output without distortion,. In or examples above we have assumed that the waveform is sinusoidal but in theory any shaped input waveform can be used.

Diode Clipping Circuits are used to eliminate amplitude noise or voltage spikes, voltage regulation or to produce new waveforms from an existing signal such as squaring off the peaks of a sinusoidal waveform to obtain a rectangular waveform as seen above.

The most common application of a "diode clipping" is as a flywheel or free-wheeling diode connected in parallel across an inductive load to protect the switching transistor form reverse voltage transients.

Light Emitting Diodes or LED´s, are among the most widely used of all the different types of semiconductor diodes available today. They are the most visible type of diode, that emit a fairly narrow bandwidth of either visible light at different coloured wavelengths, invisible infra-red light for remote controls or laser type light when a forward current is passed through them.

A "Light Emitting Diode" or LED as it is more commonly called, is basically just a specialised type of PN junction diode, made from a very thin layer of fairly heavily doped semiconductor material.

When the diode is forward biased, electrons from the semiconductors conduction band recombine with holes from the valence band releasing sufficient energy to produce photons which emit a monochromatic (single colour) of light. Because of this thin layer a reasonable number of these photons can leave the junction and radiate away producing a coloured light output.

LED Construction

Then we can say that when operated in a forward biased direction Light Emitting Diodes are semiconductor devices that convert electrical energy into light energy.

The construction of a Light Emitting Diode is very different from that of a normal signal diode. The PN junction of an LED is surrounded by a transparent, hard plastic epoxy resin hemispherical shaped shell or body which protects the LED from both vibration and shock.

Surprisingly, an LED junction does not actually emit that much light so the epoxy resin body is constructed in such a way that the photons of light emitted by the junction are reflected away from the surrounding substrate base to which the diode is attached and are focused upwards through the domed top of the LED, which itself acts like a lens concentrating the amount of light. This is why the emitted light appears to be brightest at the top of the LED.

However, not all LEDs are made with a hemispherical shaped dome for their epoxy shell. Some indication LEDs have a rectangular or cylindrical shaped construction that has a flat surface on top or their body is shaped into a bar or arrow. Also, nearly all LEDs have their cathode, ( K ) terminal identified by either a notch or flat spot on the body, or by one of the leads being shorter than the other, ( the Anode, A ).

Unlike normal incandescent lamps and bulbs which generate large amounts of heat when illuminated, the light emitting diode produces a "cold" generation of light which leads to high efficiencies than the normal "light bulb" because most of the generated energy radiates away within the visible spectrum. Because LEDs are solid-state devices, they can be extremely small and durable and provide much longer lamp life than normal light sources.

Light Emitting Diode Colours

So how does a light emitting diode get its colour. Unlike normal signal diodes which are made for detection or power rectification, and which are made from either Germanium or Silicon semiconductor materials, Light Emitting Diodes are made from exotic semiconductor compounds such as Gallium Arsenide (GaAs), Gallium Phosphide (GaP), Gallium Arsenide Phosphide (GaAsP), Silicon Carbide (SiC) or Gallium Indium Nitride (GaInN) all mixed together at different ratios to produce a distinct wavelength of colour.

Different LED compounds emit light in specific regions of the visible light spectrum and therefore produce different intensity levels. The exact choice of the semiconductor material used will determine the overall wavelength of the photon light emissions and therefore the resulting colour of the light emitted.

Light Emitting Diode Colours

Thus, the actual colour of a light emitting diode is determined by the wavelength of the light emitted, which in turn is determined by the actual semiconductor compound used in forming the PN junction during manufacture.

Therefore the colour of the light emitted by an LED is NOT determined by the colouring of the LED's plastic body although these are slightly coloured to both enhance the light output and to indicate its colour when its not being illuminated by an electrical supply.

Light emitting diodes are available in a wide range of colours with the most common being RED,AMBER,  YELLOW  and GREEN and are thus widely used as visual indicators and as moving light displays.

Recently developed blue and white coloured LEDs are also available but these tend to be much more expensive than the normal standard colours due to the production costs of mixing together two or more complementary colours at an exact ratio within the semiconductor compound and also by injecting nitrogen atoms into the crystal structure during the doping process.

From the table above we can see that the main P-type dopant used in the manufacture of Light Emitting Diodes is Gallium (Ga, atomic number 31) and that the main N-type dopant used is Arsenic (As, atomic number 33) giving the resulting compound of Gallium Arsenide (GaAs) crystalline structure.

The problem with using Gallium Arsenide on its own as the semiconductor compound is that it radiates large amounts of low brightness infra-red radiation (850nm-940nm approx.) from its junction when a forward current is flowing through it.

The amount of infra-red light it produces is okay for television remote controls but not very useful if we want to use the LED as an indicating light. But by adding Phosphorus (P, atomic number 15), as a third dopant the overall wavelength of the emitted radiation is reduced to below 680nm giving visible red light to the human eye. Further refinements in the doping process of the PN junction have resulted in a range of colours spanning the spectrum of visible light as we have seen above as well as infra-red and ultra-violet wavelengths.

By mixing together a variety of semiconductor, metal and gas compounds the following list of LEDs can be produced.

Types of Light Emitting Diode

• • Gallium Arsenide (GaAs) - infra-red
• • Gallium Arsenide Phosphide (GaAsP) - red to infra-red, orange
• • Aluminium Gallium Arsenide Phosphide (AlGaAsP) - high-brightness red, orange-red, orange, and yellow
• • Gallium Phosphide (GaP) - red, yellow and green
• • Aluminium Gallium Phosphide (AlGaP) - green
• • Gallium Nitride (GaN) - green, emerald green
• • Gallium Indium Nitride (GaInN) - near ultraviolet, bluish-green and blue
• • Silicon Carbide (SiC) - blue as a substrate
• • Zinc Selenide (ZnSe) - blue
• • Aluminium Gallium Nitride (AlGaN) - ultraviolet

Like conventional PN junction diodes, light emitting diodes are current-dependent devices with its forward voltage drop VF, depending on the semiconductor compound (its light colour) and on the forward biased LED current. The point where conduction begins and light is produced is about 1.2V for a standard red LED to about 3.6V for a blue LED.

The exact voltage drop will of course depend on the manufacturer because of the different dopant materials and wavelengths used. The voltage drop across the LED at a particular current value, for example 20mA, will also depend on the initial conduction VF point. As an LED is effectively a diode, its forward current to voltage characteristics curves can be plotted for each diode colour as shown below.

Light Emitting Diodes I-V Characteristics.

Light Emitting Diode (LED) Schematic symbol and I-V Characteristics Curves
showing the different colours available.

Before a light emitting diode can "emit" any form of light it needs a current to flow through it, as it is a current dependant device with their light output intensity being directly proportional to the forward current flowing through the LED.

As the LED is to be connected in a forward bias condition across a power supply it should becurrent limited using a series resistor to protect it from excessive current flow. Never connect an LED directly to a battery or power supply as it will be destroyed almost instantly because too much current will pass through and burn it out.

From the table above we can see that each LED has its own forward voltage drop across the PN junction and this parameter which is determined by the semiconductor material used, is the forward voltage drop for a specified amount of forward conduction current, typically for a forward current of 20mA.

In most cases LEDs are operated from a low voltage DC supply, with a series resistor, RS used to limit the forward current to a safe value from say 5mA for a simple LED indicator to 30mA or more where a high brightness light output is needed.

LED Series Resistance.

The series resistor value RS is calculated by simply using Ohm´s Law, by knowing the required forward current IF of the LED, the supply voltage VS across the combination and the expected forward voltage drop of the LED, VF at the required current level, the current limiting resistor is calculated as:

LED Series Resistor Circuit

Light Emitting Diode Example No1

An amber coloured LED with a forward volt drop of 2 volts is to be connected to a 5.0v stabilised DC power supply. Using the circuit above calculate the value of the series resistor required to limit the forward current to less than 10mA. Also calculate the current flowing through the diode if a 100Ω series resistor is used instead of the calculated first.

1). series resistor required at 10mA.

2). with a 100Ω series resistor.

We remember from the Resistors tutorials, that resistors come in standard preferred values. Our first calculation above shows that to limit the current flowing through the LED to 10mA exactly, we would require a 300Ω resistor. In the E12 series of resistors there is no 300Ω resistor so we would need to choose the next highest value, which is 330Ω. A quick re-calculation shows the new forward current value is now 9.1mA, and this is ok.

Connecting LEDs Together in Series

We can connect LED's together in series to increase the number required or to increase the light level when used in displays. As with series resistors, LED's connected in series all have the same forward current, IF flowing through them as just one. As all the LEDs connected in series pass the same current it is generally best if they are all of the same colour or type.

LED's in Series

Although the LED series chain has the same current flowing through it, the series voltage drop across them needs to be considered when calculating the required resistance of the current limiting resistor, RS. If we assume that each LED has a voltage drop across it when illuminated of 1.2 volts, then the voltage drop across all three will be 3 x 1.2v = 3.6 volts.

If we also assume that the three LEDs are to be illuminated from the same 5 volt logic device or supply with a forward current of about 10mA, the same as above. Then the voltage drop across the resistor, RS and its resistance value will be calculated as:

Again, in the E12 (10% tolerance) series of resistors there is no 140Ω resistor so we would need to choose the next highest value, which is 150Ω.

LED Driver Circuits

Now that we know what is an LED, we need some way of controlling it by switching it "ON" and "OFF". The output stages of both TTL and CMOS logic gates can both source and sink useful amounts of current therefore can be used to drive an LED. Normal integrated circuits (ICs) have an output drive current of up to 50mA in the sink mode configuration, but have an internally limited output current of about 30mA in the source mode configuration.

Either way the LED current must be limited to a safe value using a series resistor as we have already seen. Below are some examples of driving light emitting diodes using inverting ICs but the idea is the same for any type of integrated circuit output whether combinational or sequential.

IC Driver Circuit

If more than one LED requires driving at the same time, such as in large LED arrays, or the load current is to high for the integrated circuit or we may just want to use discrete components instead of ICs, then an alternative way of driving the LEDs using either bipolar NPN or PNP transistors as switches is given below. Again as before, a series resistor, RS is required to limit the LED current.

Transistor Driver Circuit

The brightness of a light emitting diode cannot be controlled by simply varying the current flowing through it. Allowing more current to flow through the LED will make it glow brighter but will also cause it to dissipate more heat. LEDs are designed to produce a set amount of light operating at a specific forward current ranging from about 10 to 20mA.

In situations where power savings are important, less current may be possible. However, reducing the current to below say 5mA may dim its light output too much or even turn the LED "OFF" completely. A much better way to control the brightness of LEDs is to use a control process known as "Pulse Width Modulation" or PWM, in which the LED is repeatedly turned "ON" and "OFF" at varying frequencies depending upon the required light intensity of the LED.

LED Light Intensity using PWM

When higher light outputs are required, a pulse width modulated current with a fairly short duty cycle ("ON-OFF" Ratio) allows the diode current and therefore the output light intensity to be increased significantly during the actual pulses, while still keeping the LEDs "average current level" and power dissipation within safe limits.

This "ON-OFF" flashing condition does not affect what is seen by the human eye as it "fills" in the gaps between the "ON" and "OFF" light pulses, providing the pulse frequency is high enough, making it appear as a continuous light output. So pulses at a frequency of 100Hz or more actually appear brighter to the eye than a continuous light of the same average intensity.

Multi-coloured Light Emitting Diode

LED's are also available in various "packages" arranged to produce both letters and numbers with the most common being that of the "seven segment display" arrangement.LEDs are available in a wide range of shapes, colours and various sizes with different light output intensities available, with the most common (and cheapest to produce) being the standard 5mm Red Gallium Arsenide Phosphide (GaAsP) LED.

Nowadays, full colour flat screen LED displays, hand held devices and TV's are available which use a vast number of multicoloured LED's all been driven directly by their own dedicated IC.

Most light emitting diodes produce just a single output of coloured light however, multi-coloured LEDs are now available that can produce a range of different colours from within a single device. Most of these are actually two or three LEDs fabricated within a single package.

Bicolour Light Emitting Diodes

A bicolour light emitting diode has two LEDs chips connected together in "inverse parallel" (one forwards, one backwards) combined in one single package. Bicolour LEDs can produce any one of three colours for example, a red colour is emitted when the device is connected with current flowing in one direction and a green colour is emitted when it is biased in the other direction.

This type of bi-directional arrangement is useful for giving polarity indication, for example, the correct connection of batteries or power supplies etc. Also, a bi-directional current produces both colours mixed together as the two LEDs would take it in turn to illuminate if the device was connected (via a suitable resistor) to a low voltage, low frequency AC supply.

A Bicolour LED

LED Selected Terminal A AC + – LED 1 ON OFF ON LED 2 OFF ON ON Colour Green Red Yellow

Tricoloured Light Emitting Diode

The most popular type of tricolour light emitting diode comprises of a single Red and a Green LED combined in one package with their cathode terminals connected together producing a three terminal device. They are called tricolour LEDs because they can give out a single red or a green colour by turning "ON" only one LED at a time.

These tricoloured LED's can also generate additional shades of their primary colours (the third colour) such as Orange or Yellow by turning "ON" the two LEDs in different ratios of forward current as shown in the table thereby generating 4 different colours from just two diode junctions.

A Multi or Tricoloured LED

Output Colour Red Orange Yellow Green LED 1 Current 0 5mA 9.5mA 15mA LED 2 Current 10mA 6.5mA 3.5mA 0

LED Displays

As well as individual colour or multi-colour LEDs, several light emitting diodes can be combined together within a single package to produce displays such as bargraphs, strips, arrays and seven segment displays.

A 7-segment LED display provides a very convenient way when decoded properly of displaying information or digital data in the form of numbers, letters or even alpha-numerical characters and as their name suggests, they consist of seven individual LEDs (the segments), within one single display package.

In order to produce the required numbers or characters from 0 to 9 and A to F respectively, on the display the correct combination of LED segments need to be illuminated. A standard seven segment LED display generally has eight input connections, one for each LED segment and one that acts as a common terminal or connection for all the internal segments.

• The Common Cathode Display (CCD) - In the common cathode display, all the cathode connections of the LEDs are joined together and the individual segments are illuminated by application of a HIGH, logic "1" signal.
• The Common Anode Display (CAD) - In the common anode display, all the anode connections of the LEDs are joined together and the individual segments are illuminated by connecting the terminals to a LOW, logic "0" signal.

A Typical Seven Segment LED Display

Opto-coupler

Finally, another useful application of light emitting diodes is Opto-coupling. An opto-coupler or opto-isolator as it is also called, is a single electronic device that consists of a light emitting diode combined with either a photo-diode, photo-transistor or photo-triac to provide an optical signal path between an input connection and an output connection while maintaining electrical isolation between two circuits.

An opto-isolator consists of a light proof plastic body that has a typical breakdown voltages between the input (photo-diode) and the output (photo-transistor) circuit of up to 5000 volts. This electrical isolation is especially useful where the signal from a low voltage circuit such as a battery powered circuit, computer or microcontroller, is required to operate or control another external circuit operating at a potentially dangerous mains voltage.

Photo-diode and Photo-transistor Opto-couplers

The two components used in an opto-isolator, an optical transmitter such as an infra-red emitting Gallium Arsenide LED and an optical receiver such as a photo-transistor are closely optically coupled and use light to send signals and/or information between its input and output. This allows information to be transferred between circuits without an electrical connection or common ground potential.

Opto-isolators are digital or switching devices, so they transfer either "ON-OFF" control signals or digital data. Analogue signals can be transferred by means of frequency or pulse-width modulation.