Variable Voltage Power Supply

Continuing on from our previous article about converting an ATX PSU to a bench power supply, the question has been asked if it is possible to use this ATX PSU to produce different voltage outputs other than the fixed voltage power supply of +5 or +12 volts. Yes we can do that either as a fixed DC output voltage of say +6V or +9V or as a variable output voltage from zero to some maximum value.With the aid of a small bit of additional circuitry added to the output of the PSU we can have a bench power supply capable of a range of fixed or variable voltages either positive or negative in nature. In fact this is more simple than you may think as the transformer, rectification and smoothing has already been done by the PSU beforehand all we need to do is connect our additional circuit to the +12 volt yellow wire output. But firstly, lets consider a fixed voltage output.

Fixed 9v Power Supply

There are a wide variety of 3-terminal voltage regulators available in a standard TO-220 package with the most popular fixed voltage regulator being the 78.. series positive regulators which range from the very common 7805, +5V regulator to the 7824, +24V regulator. There is also a 79.. series of negative regulators which produce a complementary negative voltage from -5 to -24 volts but in this tutorial we will only use the positive 78.. types.

The fixed 3-terminal regulator is useful in applications were an adjustable output is not required making the output power supply simple, but very flexible as the voltage it outputs is dependant only upon the chosen regulator. They are called 3-terminal voltage regulators because they only have three terminals to connect to and these are the input, common andoutput respectively. The input voltage which will be the +12v yellow wire from the PSU is connected between the input and common terminals, with the stabilised voltage taken across the output and common as shown.Voltage Regulator Circuit

So suppose we want an output voltage of +9 volts from our PSU bench power supply, then all we have to do is connect a +9v voltage regulator to the +12V yellow wire. As the PSU has already done the rectification and smoothing to the +12v output, the only additional components required are a capacitor across the input and another across the output. These capacitors aid in the stability of the regulator and can be anywhere between 100nF and 330nF. The additional 100uF output capacitor helps smooth out the supply giving it a good transient response.

These 78.. series regulators give a maximum output current of about 1.5 amps at fixed voltages of 5, 6, 8, 9, 12, 15, 18 and 24V respectively. But what if we wanted an output voltage of +9V but only had a 7805, +5V regulator?. The +5V output of the 7805 is referenced to the “ground, Gnd” or “0v” terminal. If we increased this pin-2 terminal voltage from 0V to 4V then the output would also rise by an additional 4 volts providing there was sufficient input voltage. Then by placing a small 4 volt (nearest preferred value of 4.3V) Zener diode between pin-2 of the regulator and ground, we can make a 7805 5V regulator produce a +9 volts output voltage as shown.

Increased Output Voltage

So how does it work, the 4.3V Zener diode requires a reverse bias current of around 5mA to maintain an output with the regulator taking about 0.5mA. This total current of 5.5mA is supplied via resistor “R1″ from the output pin-3. So the value of the resistor required for a 7805 regulator will be R = 5V/5.5mA = 910 Ohm. The feedback diode, D1 connected across the input to output terminals is for protection and prevents the regulator from being reverse biased when the input supply voltage is switched OFF while the output supply remains ON or active for a short period of time due to a large inductive load such as a solenoid or motor.Then we can use 3-terminal voltage regulators and a suitable Zener diode to produce a variety of fixed output voltages from our previous bench power supply ranging from +5V up to +12V. But we can improve on this design by replacing the fixed voltage regulator with a variable voltage regulator such as the LM317T.

Variable Voltage Power Supply

The LM317T is an adjustable 3-terminal positive voltage regulator capable of supplying 1.5 amps with an output voltage range of 1.25 to 30 volts just by using the ratio of two resistances, one of a fixed value and the other variable used to set the output voltage to the desired level with an input voltage of between 3 and 40 volts. The LM317 device also has built in current limiting and thermal shutdown which makes it short-circuit proof, ideal for our homemade bench power supply.The output voltage of the LM317T is determined by ratio of the two feedback resistors R1 andR2 which form a potential divider network across the output terminal as shown below.Adjustable Regulator

The voltage across the feedback resistor R1 is a constant 1.25V reference voltage, Vrefproduced between the Output and Adjustment terminal. The adjustment terminal current is a constant current of 100uA. Since the reference voltage across resistor R1 is constant, a constant current i will flow through the other resistor R2, resulting in an output voltage of:

Then whatever current flows through resistor R1 also flows through resistor R2 (ignoring the very small adjustment terminal current), with the sum of the voltage drops across R1 and R2being equal to the output voltage, Vout. The input voltage, Vin must be at least 2.5V greater than the required output voltage. Also, the LM317T has very good load regulation providing that the minimum load current is greater than 10mA. So to maintain a constant reference voltage of 1.25V, the minimum value of feedback resistor R1 needs to be 1.25V/10mA = 120 Ohm and this value can range anywhere from 120 ohms to 1,000 ohms with typical values ofR1 being 220 or 240 ohms for good stability.

If we know the value of the required output voltage, Vout and the feedback resistor R1 is say 240 ohms, then we can calculate the value of resistor R2 from the above equation. For example, our original output voltage of 9V would give a resistive value for R2 of: R1.((Vout/1.25)-1) = 240.((9/1.25)-1) = 1,488 Ohms or 1,500 ohms to the nearest preferred value, or 1k5.Of course in practice, resistors R1 and R2 would normally be replaced by a potentiometer so as to produce a variable voltage power supply, or by several switched preset resistances if several fixed output voltages are required. But in order to reduce the math’s required in calculating the value of resistor R2 everytime we want a particular voltage we can use resistance tables as shown below which give the regulators output voltage for different ratios of R1 and R2 using E24 resistance values.Ratio of Resistances R1 to R2

R2Value

Resistor R1 Value

150 180 220 240 270 330 370 390 470

100 2.08 1.94 1.82 1.77 1.71 1.63 1.59 1.57 1.52

120 2.25 2.08 1.93 1.88 1.81 1.70 1.66 1.63 1.57

150 2.50 2.29 2.10 2.03 1.94 1.82 1.76 1.73 1.65

180 2.75 2.50 2.27 2.19 2.08 1.93 1.86 1.83 1.73

220 3.08 2.78 2.50 2.40 2.27 2.08 1.99 1.96 1.84

240 3.25 2.92 2.61 2.50 2.36 2.16 2.06 2.02 1.89

270 3.50 3.13 2.78 2.66 2.50 2.27 2.16 2.12 1.97

330 4.00 3.54 3.13 2.97 2.78 2.50 2.36 2.31 2.13

370 4.33 3.82 3.35 3.18 2.96 2.65 2.50 2.44 2.23

390 4.50 3.96 3.47 3.28 3.06 2.73 2.57 2.50 2.29

470 5.17 4.51 3.92 3.70 3.43 3.03 2.84 2.76 2.50

560 5.92 5.14 4.43 4.17 3.84 3.37 3.14 3.04 2.74

680 6.92 5.97 5.11 4.79 4.40 3.83 3.55 3.43 3.06

820 8.08 6.94 5.91 5.52 5.05 4.36 4.02 3.88 3.43

1000 9.58 8.19 6.93 6.46 5.88 5.04 4.63 4.46 3.91

1200 11.25 9.58 8.07 7.50 6.81 5.80 5.30 5.10 4.44

1500 13.75 11.67 9.77 9.06 8.19 6.93 6.32 6.06 5.24By changing resistor R2 for a 2k ohm potentiometer we can control the output voltage range of our PSU bench power supply from about 1.25 volts to a maximum output voltage of 10.75 (12-1.25) volts. Then our final modified variable power supply circuit is shown below.Variable Voltage Power Supply

We can improve on our basic voltage regulator circuit by connecting an Ammeter and a Voltmeter to the output terminals. These instruments will give a visual indication of both the current and voltage output from the regulator. A fast-acting fuse can also be incorporated if desired in the design to provide additional short circuit protection as shown.

Disadvantages of the LM317T

One of the main disadvantages of using the LM317T to regulate a voltage is that as much as 2.5 volts is dropped or lost as heat in the regulator. So for example, if the required output voltage is to be +9 volts, then the input voltage will need to be as much as 12 volts or more if the output voltage is to remain stable under maximum load conditions. This voltage drop across the regulator is called “dropout”. Also due to this dropout voltage some form of heatsinking is required.

Fortunately low dropout variable voltage regulators are available such as the National Semiconductor LM2941T Low Dropout variable voltage regulator which has a low dropout voltage of just 0.9 volts at maximum load. This low dropout comes at a cost as this device is only capable of delivering 1.0 amp with a variable voltage output from 5 to 20 volts. However, we can use this device to give an output voltage of about 11.1V, just a little lower than the input voltage.So to summarise, our bench power supply that we made from an old PC power supply unit in the previous article can be converted to a variable voltage power supply by using a LM317T to regulate the voltage. By connecting the input of this device across the +12V yellow output wire of the PSU we can have both fixed +5V, +12V and a variable output voltage reanging from about 2 to 10

volts at a maximum output current of 1.5A, enjoy 

Optocoupler Introduction

Previously we saw that we can use transformers to not only provide higher or lower voltage differences between their primary and secondary windings, but to also provide “electrical isolation” between the higher voltages on the primary side and the lower voltage on the secondary side. In other words, transformers isolate the primary input voltage from the secondary output voltage using electromagnetic coupling by means of a magnetic flux circulating within the iron laminated core. But we can also provide electrical isolation between an input source and an output load using just light by using a very common and valuable electronic component called an optocoupler.

An Optocoupler, also known as an Opto-isolator or Photo-coupler, are electronic components that interconnect two electrical circuits by means of an optical interface. The basic design of an optocoupler consists of an LED that produces infra-red light and a semiconductor photo-sensitive device that is used to detect this emitted infra-red light. Both the LED and photo-sensitive device are enclosed in a light-tight body or package with metal legs for the electrical connections as shown.An optocoupler or opto-isolator consists of a light emitter, the LED and a light sensitive receiver which can be a single photo-diode, photo-transistor, photo-resistor, photo-SCR, or a photo-TRIAC and the basic operation of an optocoupler is very simple to understand.

Photo-transistor Optocoupler

Assume a photo-transistor device as shown. Current from the source signal passes through the input LED which emits an infra-red light whose intensity is proportional to the electrical signal. This emitted light falls upon the base of the photo-transistor, causing it to switch-ON and conduct in a similar way to a normal bipolar transistor. The base connection of the photo-transistor can be left open for maximum sensitivity or connected to ground via a suitable external resistor to control the switching sensitivity making it more stable.

When the current flowing through the LED is interrupted, the infra-red emitted light is cut-off, causing the photo-transistor to cease conducting. The photo-transistor can be used to switch current in the output circuit. The spectral response of the LED and the photo-sensitive device are closely matched being separated by a transparent medium such as glass, plastic or air. Since there is no direct electrical connection between the input and output of an optocoupler, electrical isolation up to 10kV is achieved.

Optocouplers are available in four general types, each one having an infra-red LED source but with different photo-sensitive devices. The four optocouplers are: photo-transistor, photo-darlington, photo-SCR and photo-triac as shown below.Optocoupler Types

The photo-transistor and photo-darlington devices are mainly for use in DC circuits while the photo-SCR and photo-triac allow AC powered circuits to be controlled. There are many other kinds of source-sensor combinations, such as LED-photodiode, LED-LASER, lamp-photoresistor pairs, reflective and slotted optocouplers.

Simple homemade optocouplers can be constructed by using individual components. An Led and a photo-transistor are inserted into a rigid plastic tube or encased in heat-shrinkable tubing as shown. The tubing can be of any length.

Optocoupler Applications

Optocouplers and opto-isolators can be used on their own, or to switch a range of other larger electronic devices such as transistors and triacs providing the required electrical isolation between a lower voltage control signal and the higher voltage or current output signal. Common applications for optocouplers include microprocessor input/output switching, DC and AC power control, PC communications, signal isolation and power supply regulation which suffer from current ground loops, etc. The electrical signal being transmitted can be either analogue (linear) or digital (pulses).

In this application, the optocoupler is used to detect the operation of the switch or another type of digital input signal. This is useful if the switch or signal being detected is within an electrically noisy environment. The output can be used to operate an external circuit, light or as an input to a PC or microprocessor.

Optocoupler Switch

As well as detecting DC signals and data, Opto-triac isolators are also available which allow AC powered equipment and mains lamps to be controlled. Opto-coupled triacs such as the MOC 3020, have voltage ratings of about 400 volts making them ideal for direct mains connection and a maximum current of about 100mA. For higher powered loads, the opto-triac may be used to provide the gate pulse to another larger triac via a current limiting resistor as shown.

Optocoupler Triac Control

This type of optocoupler configuration forms the basis of a very simple solid state relay application which can be used to control any AC mains powered load such as lamps and motors. Also unlike a thyristor (SCR), a triac is capable of conducting in both halves of the mains AC cycle with zero-crossing detection.

Optocouplers and Opto-isolators are great electronic devices that allow devices such as transistors and triacs to be controlled from an output port, switch or low voltage data signal. Their main advantage is the high electrical isolation between the input and output. Optocouplers utilizing a SCR or triac as the photo-detector are designed for AC power-control applications. The main advantage in this configuration is the complete isolation from any noise or voltage spikes present on the AC line as well as zero-crossing detection which reduces the generation of electrical noise.

The Darlington Transistor

The Darlington Transistor named after its inventor Sidney Darlington, is a special arrangement of two standard NPN or PNP bipolar junction transistors (BJT) connected together to produce a more sensitive transistor with a larger current gain. As we saw in our transistor as a switch tutorial, the bipolar transistor can be made to operate as an on/off switch as shown.Transistor as a Switch

The ratio of collector current to base current ( β ) is known as the current gain of the transistor. A typical value of β for a standard bipolar transistor may be in the range of 50 to 200 and varies even between transistors of the same part number. In some cases where the current gain of a single transistor is too low to directly drive a load, a Darlington transistor is used.A Darlington transistor, also known as a Darlington pair or super-alpha circuit, consist of two NPN or PNP transistors connected so that the emitter current of the first transistor TR1becomes the base current of the second transistor TR2. Then transistor TR1 is connected as an emitter follower and TR2 as a common emitter amplifier as shown below. Also in this configuration, the collector current of the slave transistor TR1 is “in-phase” with that of the master transistor TR2.Darlington Transistor Configuration

The base-emitter resistor is optional and can be use to improve turn-off time. This resistance must be small in the range of 1Ω to 1kΩ to ensure that any leakage current from TR1 does not activate transistor TR2.For a base current ib, we assume that the collector current is β.ib where the current gain is greater than one, unity. Then the current gain of TR1 and TR2 are both approximately equal to β and thus the overall current gain of a Darlington transistor circuit is given as:Darlington transistor Gain

 

This means that the overall current gain is given by the gain of the first transistor multiplied by the gain of the second transistor as the current gains of the two transistors multiply. If two identical bipolar transistors are used to make a single Darlington device then the overall current gain will be given as β2. In other words, a pair of bipolar transistors combined together to make a Darlington transistor circuit that is nearly identical to a single transistor except that the current gain is β2 instead of β acting like a big transistor, and Darlington transistors with current gains of more than a thousand with maximum collector currents of several amperes are available.The advantage of using an arrangement such as this, is that the switching transistor is much more sensitive as only a tiny base current is required to switch a a much larger load as the typical gain of a Darlington configuration is around 1000 whereas normally a single transistor stage produces a gain of about 100. Then with a gain of 1,000:1 an input current of 1mA will produce an output current of 1 ampere in the collector-emitter circuit making them ideal for interfacing with relays, lamps and motors to low power microcontroller, computer or logic controllers.

However, the disadvantage of a Darlington transistor pair is the minimum voltage between the base and emitter when fully saturated. Unlike a single transistor which has a saturated voltage drop of between 0.3v and 0.7v when fully-ON, a Darlington device has twice the base-emitter voltage drop (1.2 V instead of 0.6 V) as the base-emitter voltage drop is the sum of the base-emitter diode drops of the two individual transistors which can be 0.6v to 1.5v depending on the current through the transistor. This higher voltage drop means that the transistor will heat up more and therefore requires good heat sinking. Also, Darlington transistors have slower response times as it takes longer for the slave transistor TR1 to turn the master transistor TR1 either “ON” or “OFF”.To overcome the slow response, increased voltage drop and thermal disadvantages of a standard Darlington Transistor device, complementary NPN/PNP transistors can be used in the same cascaded arrangement to produce another type of Darlington transistor called aSziklai Configuration.

Sziklai Transistor Pair

The Sziklai device, named after its Hungarian inventor George Sziklai, which is also known as a complementary or compound Darlington device and consists of separate NPN and PNP complementary transistors connected together as shown. This cascaded combination of NPN and PNP transistors has the advantage that the Sziklai pair performs the same basic function of a Darlington pair except that it only requires 0.6v for it to turn-ON and like the standard Darlington configuration, the current gain is equal to β2 for equally matched transistors or is given by the product of the two current gains for unmatched individual transistors.Sziklai Transistor Configuration

We can see that the base-emitter voltage drop of the Sziklai device is equal to the diode drop of a single transistor in the signal path. However, the Sziklai configuration can not saturate to less than one whole diode drop, i.e. 0.7v instead of the usual 0.2v. Also, as with the Darlington pair, the Sziklai pair have slower response times than a single transistor. Sziklai pair complementary transistors are commonly used in push-pull and class AB audio amplifier output stages allowing for one polarity of output transistor only. Both the Darlington and Sziklai transistor pairs are available in both NPN and PNP configurations.

Darlington Transistor Applications

In most electronics applications it is sufficient for the controlling circuit to switch a DC output voltage or current “ON” or “OFF” directly as some output devices such as LED’s or displays only require a few milliamps to operate at low DC voltages and can therefore be driven directly by the output of a standard logic gate. However, generally more power is required to operate the output device such as a DC motor than can be supplied by an ordinary logic gate. If the digital logic device cannot supply sufficient current then additional circuitry will be required to drive the device and the Darlington Transistor is well suited for this type of application.

Transistor Amplifier – Emitter Resistance

I have received a few emails lately asking about the need for an “emitter resistance” and an “emitter capacitance” in a common emitter amplifier circuit, so I hope this explains it.

The aim of an AC signal amplifier is to stabilise the DC biased input to the amplifier and thus only amplify the AC signal. This stabilisation is achieved by the use of an Emitter Resistance to provide the required automatic biasing of the common emitter amplifier. Consider the following basic amplifier circuit.Basic Common Emitter Amplifier Circuit

The common emitter amplifier circuit shown uses a voltage divider biasing network which itself is a very popular way of designing bipolar transistor amplifier circuits, because this type of amplifier design counteracts the effects of the uncertainty of beta, ( β). The voltage at the junction of the two biasing resistors, R1 and R2, holds the transistors base voltage at a constant voltage. This “class-A” amplifier circuit is always designed so that the base current (Ib) is less than 10% of the current flowing through the biasing resistor R2.Since the voltage divider formed by R1 and R2 is lightly loaded, the base voltage, Vb can be easily calculated by using the simple voltage divider formula. However, with this type of biasing arrangement, the voltage divider network is not loaded by the base current so if there is any change in the supply voltage Vcc, then the voltage level on the base will also change. Then some form of stabilisation of the bias point is required.

Emitter Resistance Stabilisation

The amplifiers bias voltage can be stabilised by placing a single resistor, RE in the transistors emitter circuit as shown. This resistance is known as the Emitter Resistance. Now if the supply voltage Vcc increases, the transistors collector current Ic also increases for a given load resistance. If the collector current increases, the emitter current must also increase causing the voltage drop across RE to increase.Since the base is held constant by the divider resistorsR1 and R1, the DC voltage on the base relative to the emitter Vbe is lowered. In other words, the forward bias of the base-emitter junction causes the base current to reduce, which in turn reduces the collector current and so the circuit tends to be stabilised at a fixed level.The addition of this emitter resistance, RE means that the transistors emitter terminal is no longer at earth or zero volt potential but sits at a small potential above it given by the equation: VE = IE.RE where IE is the actual emitter current. Since part of the supply is dropped across RE, its value should be as small as possible so that the largest possible voltage can be developed across the load resistance, RL. However, its value cannot be too small or instability of the circuit will suffer.As a general rule of thumb, the voltage drop across this emitter resistance is generally taken to be one-tenth (1/10th) the value of the supply voltage, Vcc. A common figure is between 1 to 2 volts. The value of the emitter resistance, RE can also be found from the voltage gain as now the AC voltage gain is equal to: RL / RE

Emitter By-pass Capacitor

In the basic series feedback circuit above, the emitter resistor (RE) performs two functions: DC negative feedback for stable biasing and AC negative feedback for signal transconductance and voltage gain specification. But as the emitter resistance RE is a feedback resistor, it will reduce the amplifiers gain due to fluctuations in the emitter current IE owing to the AC input signal.This is overcome by placing a large “by-pass capacitor”CE across the emitter resistance as shown. Over the operating range of frequencies, the capacitors reactance will be extremely low giving little AC feedback effect but good AC gain. The value of this by-pass capacitor CE is generally chosen to provide a capacitive reactance of, at most one-tenth (1/10th) of emitter resistor RE at the lowest signal frequency. Below the operating range of frequencies the reactance will increase, producing a negative feedback effect, reducing the amplifiers gain.Then to summarise, the bias of a common emitter class-A type amplifier circuit on the transistor input can be stabilised against changes in supply voltage by the addition of an Emitter Resistance, RE. The voltage drop across this resistance is usually given as between 1 to 2 volts. In order to overcome any loss of gain with AC signals, a by-pass capacitor, CE is connected in parallel with the emitter resistor. The value of this capacitor is determined from its capacitive reactance (XC) value at the lowest signal frequency.

Voltage Multiplier Circuit

A Voltage Multiplier Circuit, is a special type of rectifier circuit which produces a DC output voltage which is many times greater than its AC input voltage. Although it is usual to use a transformer to increase the voltage, sometimes a suitable step-up transformer or a specially insulated transformer required for high voltage applications may not be available. One alternative approach is to use a voltage multiplier circuit.Voltage multipliers are AC-to-DC voltage converters used in electrical and electronic circuit applications such as in microwave ovens, cathode-ray tube (CRT) field coils, electrostatic and high voltage test equipment, etc, where it is necessary to have a very high DC voltage generated from a relatively low AC voltage. The high voltage produced by a voltage multiplier circuit is in theory unlimited, but due to their relatively poor voltage regulation and low current capability most multiplier circuits produce voltages in the range of about 10kV to 30kV with a low current of less than ten milliamperes.Voltage multiplier circuits are constructed from series combinations of rectifier diodes and capacitors that give a DC output equal to some multiple of the peak voltage value of the AC input voltage. By adding a second diode and capacitor to the output of the simple half-wave rectifier, we can increase its output voltage. The most commonly used type of voltage multiplier circuit is the Half Wave Series Multiplier also known as a “cascade voltage doubler”.

Voltage Doubler

The Voltage Doubler circuit shown consists of only two diodes, two capacitors and an oscillating input voltage. This simple diode-capacitor pump circuit gives a DC output voltage equal to the peak-to-peak value of the input. In other words, double the peak voltage value because the diodes and the capacitors function together to effectively double the voltage.During the negative half of the sinusoidal input waveform, diode D1 is forward biased and conducts charging up the pump capacitor, C1 to the peak value of the input voltage, (Vp). Capacitor C1 now acts as a battery in series with the supply. At the same time diode D2conducts via D1 charging up capacitor, C2. During the positive half cycle, diode D2 is forward biased and diode D1 is reverse biased, adding the peak AC input voltage to the voltage Vpacross capacitor C1 and transferring this summed voltage to capacitor, C2 through diode D2as shown.DC Voltage Doubler Circuit

The voltage across capacitor, C2 is equal to the sum of the peak supply voltage and the voltage across input capacitor, C1. Then a half wave voltage doubler’s output voltage can be calculated as: Vout = 2Vp, where Vp is the peak value of the input voltage.The advantage of “Voltage Multiplier Circuits” is that it allows higher voltages to be created from a low voltage power source without a need for an expensive high voltage transformer as the voltage doubler circuit makes it possible to use a transformer with a lower step up ratio than would be need if an ordinary full wave supply were used. However, while voltage multipliers can boost the voltage, they can only supply low currents to a high-resistance (+100kΩ) load because the generated output voltage quickly drops-off as load current increases.

By connecting the output of one multiplying circuit onto the input of the next (cascading), we can continue to increase the DC output voltage to produce voltage triplers, or voltage quadruplers circuits, etc, as shown.

DC Voltage Tripler Circuit

A “voltage tripler circuit” consists of one and a half voltage doubler stages. This voltage multiplier circuit gives a DC output equal to three times the peak voltage value (3Vp) of the sinusoidal input signal. Note: the real output voltage will be 3(Vp – Von).

DC Voltage Quadrupler Circuit

If a voltage tripler circuit can be made by cascading together one and a half voltage multipliers, then a “voltage quadrupler circuit” can be constructed by cascading together two full voltage doubler circuits as shown. The first stage doubles the peak input voltage and the second stage doubles it again, giving a DC output equal to four times the peak voltage value (4Vp) of the sinusoidal input signal. Also, use large value capacitors to reduce the ripple voltage.

By cascading together individual half and full multiplier stages in series, any desired amount of voltage multiplication can be obtained and a cascade of “N” doublers, would produce an output voltage of 2N.Vp volts. For example, a 10-stage voltage multiplier circuit with a peak input voltage of 100 volts would give a DC output voltage of about 1,000 volts or 1kV, assuming no losses. However, the diodes and capacitors used in all multiplication circuits need to have a minimum reverse breakdown voltage rating of at least twice the peak voltage across them. One word of caution!. Multi-stage voltage multiplication circuits can produce very high voltages, so take care.The Voltage Multiplication Circuits shown above, are all designed to give a positive DC output voltage. But they can also be designed to give negative voltage outputs by simply reversing the polarities of all the multiplier diodes and capacitors to produce a negative voltage doubler.

The Potential Divider

Potential Divider Circuits are basically resistors connected together in series. The potential divider circuit, also known as a Voltage Divider, is commonly used in Electronics to produce a fixed voltage output. For example, we may want to generate a voltage level of six volts DC (6VDC) from a battery voltage source of nine volts (9V), then a potential divider circuit could be used. But first let us define the laws relating to a single resistance.

The circuit shows the familiar fixed value resistance, R connected to a voltage supply. The relationship between resistance and voltage causes a current (i) to flow around the circuit. The current flows in the circuit due to the electrical force (the emf) or voltage applied. The amount of current flowing through this circuit is limited by the value of the resistance in Ohms and as we know, the relationship between the three electrical properties of Voltage, Current and Ohms is defined by Ohms Law.If we know two of the three values relating to our resistor circuit, we can use Ohms Law to find the value of the missing third value. As current is proportional to voltage but inversely proportional to resistance, we can use Ohms Law to find the voltage drop across the resistor, (VR). Of course in this simple single resistor circuit the voltage drop across the resistor would be equal to the supply voltage, Vs but what if we added a second resistor to our circuit in series with the first.

Resistors in Series

The circuit shows two resistors, R1 and R2 connected together in series across the supply voltage, Vs. The resistance of the circuit will now be equal to the sum of the two individual resistances added together, that is R(total) = R1 + R2. This new value of resistance, R(total) will cause a different value of current to flow around the circuit compared to the single resistive circuit above. Since the current in a series resistive path has only one path to follow, as it cannot be diverted in any other direction, the circuit current, i is therefore common to both resistors.The voltage applied to a series circuit will be dropped across all the resistors in proportion to the magnitude of the individual resistors and the amount of current flow. By using Ohms Law, we can find the value of the circuit current, as we know the value of the supply voltage and the two resistances.

Assuming that the supply voltage is 12V, and R1 = 20Ω and R2 = 10Ω, then the voltage drop across each individual resistor will be as follows:Voltage Division

As you can see, the 10Ω resistor drops 4V, while the 20Ω resistor drops 8V. From this example, you will notice that the larger value resistor has the largest voltage drop as the value of the resistance and therefore the voltage drop across it are proportional to one another.

If a single resistor in the series circuit is very large compared to the other series resistors, the voltage across that resistor will be nearly equal to the source voltage, and if the resistor is very small, the voltage across it will be extremely small, essentially zero.

Potential Divider Circuit

This type of series connected circuit is often referred to as a Potential Divider circuit, because the source voltage, Vs is divided or dropped proportionally across each resistor in the series chain. We have seen that the amount of voltage dropped across a resistor is proportional to the value of the resistance, and so a larger resistance will have a larger voltage drop, while a smaller resistance will have a smaller voltage drop.The voltage dropped across a resistor may be determined by the voltage across any other resistor, or combination of resistors by using the Potential Divider Rule. This potential divider formula allows us to calculate the voltage drop across “ANY” resistor in the series chain without having to first calculate the current flowing through the circuit. This potential divider formula is given as:Potential Divider Formula

Where, VRx is the required voltage, Rx is the particular resistor value and RT is the total series resistance.This formula can be used to find the voltage, V across any resistor, R if two or more resistors are connected together in series. So by using our series circuit with the values given above, we can calculate the voltage drop across resistor, R2 using the potential divider formula as:

Which is the same eight volts (8V) as calculated before using the circuit current method.

Then to summarise, we can use the “Potential Divider Formula” to find the voltage drop across any resistor or group of resistors in a series circuit. The voltage drop across a single resistor is equal to the ratio of that resistance, Rx to the total series resistance, RT, multiplied by the source voltage Vs.

Thyristor Basics

In many ways the Silicon Controlled Rectifier, (SCR) or the Thyristor as it is more commonly known, is similar to the transistor. It is a multi-layer semiconductor device, hence the “silicon” part of its name. It requires a gate signal to turn it “ON”, the “controlled” part of the name and once “ON” it behaves like a rectifying diode, the “rectifier” part of the name. In fact the circuit symbol for the thyristor suggests that this device acts like a controlled rectifying diode.

Thyristor SymbolThe Thyristor is a 4-layer (p-n-p-n) semiconductor device that contains three PN junctions in series, and is represented by the symbol as shown. Like a diode, the thyristor will conduct current in one direction only, but can be made to operate as either an open-circuit switch or as a rectifying diode depending upon how the thyristors Gate terminal is used. The thyristor is one of several semiconductor devices along with Triacs, Diacs and Sidacs that are all capable of acting like very fast solid state switches for controlling large AC currents.The operation of a “thyristor” can be explained by assuming it to be made up of two transistors connected back-to-back as a pair of complementary regenerative switches as shown.

A Thyristors Two Transistor Analogy

The two transistor equivalent circuit shows that the collector current of the NPN transistor TR2feeds directly into the base of the PNP transistor TR1, while the collector current of TR1 feeds into the base of TR2. These two inter-connected transistors rely upon each other for conduction as each transistor gets its base-emitter current from the other’s collector-emitter current. So until one of the transistors is given some base current nothing can happen even if a Anode-to-Cathode voltage is present.When the thyristors Anode terminal is negative with respect to the Cathode, the centre N-Pjunction is forward biased, but the two outer P-N junctions are reversed biased and it behaves very much like an ordinary diode. Therefore a thyristor blocks the flow of reverse current until at some high voltage level the breakdown voltage point of the two outer junctions is exceeded and the thyristor conducts without the application of a Gate signal. This is an important negative characteristic of the thyristor, as Thyristors can be unintentionally triggered into conduction by an overvoltage as well as high temperature or a rapidly rising dv/dt voltage such as a spike.If the Anode terminal is positive with respect to the Cathode, the two outer P-N junctions are forward biased but the centre N-P junction is reverse biased. Therefore forward current is also blocked. If now a positive current is injected into the base of the NPN

transistor TR2, the resulting collector current flows in the base of transistorTR1. This in turn causes a collector current to flow in the PNP transistor, TR1 which increases the base current of TR2 and so on.

Typical ThyristorVery rapidly the two transistors force each other to conduct to saturation as they are connected in a regenerative feedback loop. Once triggered into conduction, the current flowing through the device between the Anode and the Cathode is limited only by the resistance of the external circuit. Then a thyristor can be turned “ON” and made to act like a normal rectifying diode by the application of a positive current to the base of transistor, TR2 which for a silicon controlled rectifier is called the “Gate” terminal.Once the thyristor has been turned “ON” and is conducting in the forward direction (anode positive), the gate signal looses control due to the regenerative latching action of the two internal transistors. Then applying a momentary Gate pulse to the device is enough to cause it to conduct and will remain permanently “ON” even if the gate signal is completely removed. Then the thyristor canalso be thought of as a Bistable Switch having two states “ON” or “OFF”, as once it is triggered “ON” it cannot be turned “OFF” again by its Gate.So how do we turn “OFF” the thyristor?. Once the thyristor has self-latched into its “ON” state, it can only be turned “OFF” again by removing the supply voltage and therefore the Anode current completely or by reducing the Anode to Cathode current by external means to below a value commonly called the “minimum holding current”, (IH). The Anode current must therefore be reduced below this minimum holding level long enough for the thyristors internalP-N junctions to recover their blocking state before a forward voltage is again applied to the device without conduction.Since the thyristors turns “OFF” whenever the Anode current is reduced below this minimum value, it follows then that the device automatically turns “OFF” in AC circuits near to the crossing point at each half cycle and as we now know, remains “OFF” until the application of a Gate trigger pulse. Since an AC sinusoidal voltage continually reverses in polarity every half-cycle allowing the thyristor to turn “OFF”, this effect is known as “natural commutation” and is a very important characteristic of the silicon controlled rectifier.

Thyristor Phase Control

Thyristors used in circuits fed from DC supplies, this natural commutation condition cannot occur as the DC supply voltage is continuous so some other way to turn “OFF” the thyristor must be provided at the appropriate time. However in AC sinusoidal circuits natural commutation occurs every half cycle. Then during the positive half cycle of an AC sinusoidal waveform, the thyristor is forward biased (anode positive) and a can be triggered “ON” using a Gate signal or pulse. During the negative half cycle, the Anode becomes negative while the Cathode is positive. The thyristor is reverse biased by this voltage and cannot conduct even if a Gate signal is present.

So by applying a Gate signal at the appropriate time during the positive half of an AC waveform, the thyristor can be triggered into conduction until the end of the half cycle. Thus phase control (as it is called) can be used to trigger the thyristor at any point along the positive half of the AC waveform and one of the many uses of a Silicon Controlled Rectifier is in the power control of AC systems.Thus far we have seen that a thyristor is essentially a half-wave device and conducts in only the positive half of the cycle when the Anode is positive and blocks current flow like a diode when the Anode is negative, irrespective of the Gate signal. But there are more semiconductor devices available which come under the banner of “Thyristor” that can conduct in both directions, full-wave devices, or can be turned “OFF” by the Gate signal. Such devices include “Gate Turn-OFF Thyristors” (GTO), “Static Induction Thyristors” (SITH), “MOS Controlled Thyristors” (MCT), “Silicon Controlled Switch” (SCS), “Triode Thyristors” (TRIAC) and “Light

Activated Thyristors” (LASCR) to name a few, with all these devices available in a variety of voltage and current ratings making them attractive for use in applications at very high power levels.

Thyristor Summary

Silicon Controlled Rectifiers known commonly as Thyristors are three-junction PNPN semiconductor devices which can be regarded as two inter-connected transistors that can be used in the switching of heavy electrical loads. They can be latched-”ON” by a single pulse of positive current applied to their Gate terminal and will remain “ON” indefinitely until the Anode to Cathode current falls below their minimum latching level.Static Characteristics of a Thyristor

1. Conducts current only when forward biased and there is triggering current applied to the gate.2. The thyristor acts like a diode once it is “ON”.3. Blocks current flow when reverse biased, no matter if there is gate current applied.4. Once triggered “ON”, will be latched “ON” conducting even when the gate current is no longer applied.

Thyristors are high speed switches that can be used to replace electromechanical relays in many circuits as they have no moving parts, no contact arcing or suffer from corrosion or dirt. But in addition to simply switching large currents “ON” and “OFF”, thyristors can be made to control the mean value of an AC load current without dissipating large amounts of power. A good example of thyristor power control is in the control of electric lighting, heaters and motor speed.

In the next part of this tutorial we will look at some basic Thyristor circuits and applications using both AC and DC supplies.

Thyristor as a Switch

In the previous tutorial we looked at the basic construction and operation of the Silicon Controlled Rectifier more commonly known as a Thyristor. This time we will look at how we can use the thyristor as a switch to control larger loads such as lamps, motors, or heaters etc.We said previously that in order to get the Thyristor to turn-”ON” we need to inject a pulse of current into the Gate, (G) when the thyristor is in the forward direction, that is the Anode, (A) is positive with respect to the Cathode, (K). Generally, this pulse need only be a few micro-seconds in duration but the longer the Gate pulse is applied the faster the internal avalanche breakdown occurs and the faster the turn-”ON” time of the thyristor, but the maximum Gate current must not be exceeded.Once fully conducting, the voltage drop across the thyristor is reasonably constant at about 1.0V for all values of Anode current up to its rated value. But remember though that once a thyristor starts to conduct it continues to conduct until the Anode current decreases below the devices holding current, (IH) and below this value it automatically turns-”OFF”. Therefore in DC and some highly inductive AC circuits the current has to be artificially reduced by a separate switch or turn off circuit.

DC Thyristor Circuit

When connected to a direct current DC supply, the thyristor can be used as a DC switch to control larger DC currents and loads. When using the Thyristor as a switch it behaves like an electronic latch because once activated it remains in the “ON” state until manually reset. Consider the DC thyristor circuit below.

DC Thyristor Circuit

The circuit shows a simple DC on-off circuit which uses the thyristor as a switch and could be used as an on-off control circuit for a lamp, motor, or some other DC load. The thyristor is forward biased and is triggered into conduction by briefly closing the normally-open “ON” pushbutton, S1 which connects the Gate terminal to the DC supply via the Gate resistor, RGallowing current to flow into the Gate. If RG is set too high with respect to the supply voltage, the thyristor may not trigger. Once the circuit has been turned-”ON”, it self latches and stays “ON” even when the pushbutton is released. Additional operations of pushbutton, S1 will have no effect on the circuits state as once “latched” the Gate looses all control.

The thyristor is now turned fully “ON” allowing full load circuit current to flow through the device in the forward direction and back to the battery supply. One of the main advantages of using a thyristor as a switch in a DC circuit is that it has a very high current gain. The thyristor is a current operated device because a small Gate current can control a much larger Anode current. The Gate-cathode resistor RGK is generally included to reduce the Gate’s sensitivity and increase its dv/dt capability thus preventing false triggering of the device.As the thyristor has self latched into the “ON” state, the circuit can only be reset by interrupting the power supply and reducing the Anode current to below the thyristors minimum holding current (IH) value. Opening the normally-closed “OFF” pushbutton, S2 breaks the circuit, reducing the circuit current flowing through the Thyristor to zero, thus forcing it to turn “OFF” until the application again of another Gate signal.However, one of the disadvantages of this DC thyristor circuit design is that the mechanical normally-closed “OFF” switch S2 needs to be big enough to handle the circuit power flowing through both the thyristor and the lamp when the contacts are opened. Then we could just replace the thyristor with the one simple mechanical switch. One way to overcome this problem and reduce the need for a larger more robust “OFF” switch is to connect the switch in parallel with the thyristor as shown.Alternative DC Thyristor Circuit

Here the thyristor switch receives the required terminal voltage and Gate pulse signal as before but the larger normally-closed switch of the previous circuit has be replaced by a smaller normally-open switch in parallel with the thyristor. Activation of switch S2 momentarily applies a short circuit between the thyristors Anode and Cathode stopping the device from conducting by reducing the holding current to below its minimum value.

AC Thyristor Circuit

When connected to an alternating current AC supply, the thyristor behaves differently from the previous DC connected circuit. Because AC power reverses polarity periodically, any thyristor used in an AC circuit will automatically be reverse-biased causing it to turn-”OFF” during one-half of each cycle. Consider the AC thyristor circuit below.

AC Thyristor Circuit

The above circuit is similar in design to the DC SCR circuit except for the omission of an additional “OFF” switch and the inclusion of diode D1 which prevents reverse bias being applied to the Gate. During the positive half-cycle of the sinusoidal waveform, the device is forward biased but with switch S1 open, zero gate current is applied to the thyristor and it remains “OFF”. On the negative half-cycle, the device is reverse biased and will remain “OFF” regardless of the condition of switch S1.If switch S1 is closed, at the beginning of each positive half-cycle the thyristor is fully “OFF” but shortly after there will be sufficient positive trigger voltage and therefore current present at the Gate to turn the thyristor and the lamp “ON”. The thyristor is now

latched-”ON” for the duration of the positive half-cycle and will automatically turn “OFF” again when the positive half-cycle ends and the Anode current falls below the holding current value. During the next negative half-cycle the device is fully “OFF” anyway until the following positive half-cycle when the process repeats itself and the thyristor conducts again as long as the switch is closed. Then in this condition the lamp will receive only half of the available power from the AC source as the thyristor acts like a rectifying diode, and conducts current only during the positive half-cycles when it is forward biased. The thyristor continues to supply half power to the lamp until the switch is opened.If it were possible to rapidly turn switch S1 ON and OFF, so that the thyristor received its Gate signal at the “peak” (90o) point of each positive half-cycle, the device would only conduct for one half of the positive half-cycle. In other words, conduction would only take place during one-half of one-half of a sine wave and this condition would cause the lamp to receive “one-fourth” or a quarter of the total power available from the AC source. By accurately varying the timing relationship between the Gate pulse and the positive half-cycle, the Thyristor could be made to supply any percentage of power desired to the load, between 0% and 50%. Obviously, using this circuit configuration it cannot supply more than 50% power to the lamp, because it cannot conduct during the negative half-cycles when it is reverse biased.Thyristor Phase Control

Phase control is the most common form of thyristor AC power control and a basic phase-control circuit can be constructed as shown above. Here the thyristors Gate voltage is derived from the RC charging circuit via the trigger diode, D1.During the positive half-cycle when the thyristor is forward biased, capacitor, C charges up via resistor R1 following the AC supply voltage. The Gate is activated only when the voltage at point A has risen enough to cause the trigger diode D1, to conduct and the capacitor discharges into the Gate of the thyristor turning it “ON”. The time duration in the positive half of the cycle at which conduction starts is controlled by RC time constant set by the variable resistor, R1.Increasing the value of R1 has the effect of delaying the triggering voltage and current supplied to the thyristors Gate which in turn causes a lag in the devices conduction time. As a result, the fraction of the cycle over which the device conducts can be controlled between 0 and 180o, which means that the average power dissipated by lamp can be adjusted. However, the thyristor is a unidirectional device so only a maximum of 50% power can be supplied.There are a variety of ways to achieve 100% full-wave AC control using “thyristors”. One way is to include a single thyristor within a diode bridge rectifier circuit which converts AC to a unidirectional current through the thyristor while the more common method is to use two thyristors connected in inverse parallel. A more practical approach is to use a single Triac as this device can be triggered in both directions, therefore making them suitable for AC switching applications.

Triac Tutorial and Basic Principals

In the previous tutorial we looked at the construction and operation of the Silicon Controlled Rectifier more commonly known as a Thyristor, which can be used as a switch to control lamps, motors, or heaters etc. However, one of the problems of using a thyristor for controlling such circuits is that the “thyristor” is a unidirectional device, meaning that they pass current in one direction only, from Anode to Cathode.For DC switching circuits this “one-way” switching characteristic may be acceptable as once triggered all the DC power is delivered straight to the load. But on sinusoidal AC switching circuits this undirectional switching may be a problem as they only conduct during one half of the cycle when the Anode is positive irrespective of whatever the Gate signal is doing. Then for AC operation only half the power is delivered to the load.

For full wave power control we could connect a single thyristor inside a full-wave bridge rectifier or connect two thyristors together in inverse parallel as shown below but this increases both the complexity and number of components used in the switching circuit.

Thyristor Configurations

There is however, another type of semiconductor device called a “Triode AC Switch” or Triacfor short which is also a member of the thyristor family, can be used as a solid state power switch and more importantly is a “bidirectional” device. In other words, a Triac can be triggered into conduction by both postive and negative voltages applied to its Anode and with both positive and negative trigger pulses applied to its Gate terminal making it a two-quadrant switching Gate controlled device.The triac behaves just like two conventional thyristors connected together in inverse parallel with respect to each other and are so arranged that the two thyristors share a common Gate terminal all within a single three-terminal package. Because the triac conducts in both directions, the concept of an Anode terminal and a Cathode terminal used to identify the main power terminals of a thyristor are replaced with identifications; MT1, for Main Terminal 1 andMT2 for Main Terminal 2 with the Gate terminal G referenced the same. In most applications, the triac gate terminal is associated with the MT1 terminal, similar to the gate-cathode relationship of the thyristor. The construction, P-N doping and schematic symbol used to represent a Triac is given below.Triac Symbol and Construction

We now know that a “triac” is a 5-layer, PNPN in the positive direction and a NPNP in the negative direction, three-terminal bidirectional device that blocks current in its “OFF” state acting like an open-circuit switch, but unlike a conventional thyristor, the triac can conduct current in either direction when triggered by a single gate pulse. Then a triac has four possible triggering modes of operation as follows.

1. Ι +  Mode = MT2 current positive (+ve), Gate current positive (+ve)2. Ι –  Mode = MT2 current positive (+ve), Gate current negative (-ve)3. ΙΙΙ +  Mode = MT2 current negative (-ve), Gate current positive (+ve)4. ΙΙΙ –  Mode = MT2 current negative (-ve), Gate current negative (-ve)

Even though the two thyristors are combined into a single triac device, they still exhibit individual electrical characteristics such as different breakdown voltages, holding currents and trigger voltage levels the same as we would expect from a single SCR.

Triac Applications

The Triac is most commonly used for switching and power control of AC systems as the triac can be switched “ON” by either a positive or negative Gate pulse, regardless of the polarity of the AC supply at that time. This makes the triac ideal to control a lamp or motor load with a very basic triac switching circuit given below.Triac Switching Circuit

The circuit above shows a simple DC triggered triac power switching circuit. With switch SW1open, no current flows into the Gate of the triac and the lamp is therefore “OFF”. When SW1 is closed, Gate current is applied to the triac from the battery supply VG via resistor R and the triac is driven into full conduction acting like a closed switch and full power is drawn by the lamp.As the battery supplies a positive Gate current to the triac whenever switch SW1 is closed, the triac is therefore continually gated in modes Ι + and ΙΙΙ + regardless of the polarity of terminalMT2.Of course, the problem with this simple triac switching circuit is that we would require an additional positive or negative Gate supply to trigger the triac into conduction. But we can also trigger the triac using the actual AC supply voltage itself as the gate trigger voltage. Consider the circuit below.

Triac Switching Circuit

The circuit shows a triac used as a simple static AC power switch providing an “ON”-”OFF” function similar in operation to the previous DC circuit. When switch SW1 is open, the triac acts as an open switch and the lamp passes zero current. When SW1 is closed the triac is gated “ON” via current limiting resistor R and self-latches shortly after the start of each half-cycle, thus switching full power to the lamp load.As the supply is sinusoidal AC, the triac automatically unlatches at the end of each AC half-cycle as the instantaneous supply voltage and thus the load current briefly falls to zero but re-latches again using the opposite thyristor half on the next half cycle as long as the switch remains closed. This type of switching control is generally called full-wave control due to the fact that both halves of the sine wave are being controlled.

Electrical AC power control using a Triac is extremely effective when used properly to control resistive type loads such as incandescent lamps, heaters or small universal motors commonly found in portable power tools and small appliances. But please remember that these devices can be attached directly to the AC power source so circuit testing should be done when the power control device is disconnected from the mains power supply. Remember safety first.

Biasing of a Transistor

Biasing a Transistor amplifier is the process of setting the DC operating voltage and current to the correct level so that any AC input signal can be amplified correctly by the transistor. The correct bias point of the transistor is generally somewhere between the two extremes of operation with respect to it being either “fully-ON” or “fully-OFF” along the load line. This central operating point is called the “Quiescent Operating Point”, or Q-point for short.

When a bipolar transistor is biased so that the Q-point is near the middle of its operating range, that is approximately halfway between cutoff and saturation, it is said to be operating as a Class-A amplifier. This mode of operation allows the output current to increase and decrease around the amplifiers Q-point without distortion as the input signal swings through a complete cycle. In other words, the output current flows for the full 360o of the input cycle.So how do we set this Q-point biasing of a transistor? – The correct biasing of the transistor is achieved using a process know commonly as Base Bias. The function of the “DC Bias level” or “no input signal level” is to correctly set the transistors Q-point by setting its Collector current ( IC ) to a constant and steady state value without an input signal applied to the transistors Base. This steady-state or DC operating point is set by the values of the circuits DC supply voltage ( Vcc ) and the value of the biasing resistors connected the transistors Base terminal.Since the transistors Base bias currents are steady-state DC currents, the appropriate use of coupling and bypass capacitors will help block bias current setup for one transistor stage affecting the bias conditions of the next. Base bias networks can be used for Common-base (CB), common-collector (CC) or common-emitter (CE) transistor configurations. In this simple transistor biasing tutorial we will look at the different biasing arrangements available for a Common Emitter Amplifier.

Base Biasing a Common Emitter Amplifier

One of the most frequently used biasing circuits for a transistor circuit is with the self-bias of the emitter-bias circuit where one or more biasing resistors are used to set up the initial DC values of transistor currents, ( IB ), ( IC ) and ( IE ). This is achieved either using a feed back resistor or by using a simple voltage divider circuit to provide biasing voltage. The following are five examples of transistor Base bias configurations from a single supply ( Vcc ).Current Biasing a Transistor

A two resistor biasing network is used to establish the initial operating region of transistor with a “fixed current bias”. The Emitter diode of the transistor is forward biased by applying the required positive Base bias voltage via the current limiting resistor RB.Assuming a standard bipolar transistor, the forward Base-emitter voltage drop will be 0.7V. Then the value of IB is simply: (VCC – VBE)/IB. With this type of biasing method the biasing voltages and currents do not remain stable during transistor operation. Feedback Biasing a Transistor

This transistor biasing configuration again requires only two resistors. The Collector feedback configuration ensures that the transistor is always biased in the active region regardless of the value of Beta (β). The Base bias is derived from the Collector voltage.If the Collector current increases, the Collector voltage drops, reducing the Base drive and thereby automatically reducing the Collector current. The transistors stability using this type of bias network is generally good for most amplifier designs.  Dual Feedback Biasing a Transistor

 Adding an additional resistor to the Base bias network of the previous configuration improves stability even more with respect to variations in Beta, (β) by increasing the current flowing through the Base bias resistors. The current flowing through RB1 is generally set at a value equal to about 10% of Collector current, IC.One of the advantages of this type of biasing configuration is that the resistors provide both biasing and Rf feedback at the same time.    Voltage Divider Biasing a Transistor with Compensation

The common Emitter transistor is biased using a voltage divider network. The name ofthis biasing configuration comes from the fact that the two resistors RB1and RB2 are connected to the transistors Base terminal across the supply. This voltage divider configuration is the most widely used biasing method, as the Emitter diode of the transistor is forward biased by the voltage dropped across resistor RB2.To calculate the voltage developed across resistor RB2 and therefore the voltage applied to the Base terminal we simply use the voltage divider formula. The current flowing through resistor RB2 is generally set at 10 times the value of the required Base current IB so that it has no effect on the voltage divider current.Dual Feedback Biasing a Transistor with Emitter Compensation

 This type of biasing configuration uses both Emitter and Collector-base feedback to stabilize the Collector current. Resistor values are generally set so that the voltage drop across Emitter resistor RE is approximately 10% of VCC and the current flowing through resistor RB1 is 10% of the Collector current IC.This type of transistor biasing configuration works best at relatively low power supply voltages.    The goal of biasing a transistor is to establish a known Q-point in order to work efficiently and produce an undistorted output signal. Correct biasing of the transistor also establishes its initial AC operating region with practical biasing circuits using either a two or four-resistor bias network. In bipolar transistor circuits, the Q-point is represented by (VCE, IC) for the NPN transistors or (VEC, IC) for PNP transistors. The stability of the Base bias network and therefore the Q-point is generally assessed by considering the Collector current as a function of both Beta (β) and temperature.Here we have looked briefly at five different configurations for “biasing a transistor” using resistive networks. But we can also bias a transistor using either silicon diodes, zener diodes or active networks all connected to the Base terminal of the transistor or by biasing it from a dual power supply.

I-V Characteristic Curves

The I-V Characteristics Curves, which is short for Current-Voltage Characteristics Curves or simply I-V curves of an electrical device or component, are a set of graphical curves which define its operation. These I-V characteristics curves show the relationship between the current flowing through an electrical or electronic device and the applied voltage across its terminals. I-V characteristics curves are generally used as a tool to determine and understand the basic parameters of a device and can also be used to mathematically model its behavior in an electrical circuit.For example, the “current-voltage characteristics” of an ideal resistor are reasonably constant within certain ranges of current, voltage and power as it is a linear or ohmic device. There are however, resistive elements (such as LDR’s, thermistors, varistors, etc) whose I-V characteristic curves are not straight or linear lines but instead are curved or shaped and are therefore called nonlinear devices because their resistances are nonlinear resistances. Consider the circuit below.

I-V Characteristic Curve of an Ideal Resistor

Suppose above that a current i in amperes flows through the resistance R when an electrical supply with an electromotive force of V volts is connected across its terminals. This current can be characterised as: I = V/R, and is one of Ohm’s Law equations which most people are familiar with. We know from Ohm’s Law that as the voltage across the resistor increases so too does the current flowing through it.

If the voltage and current are positive in nature the I-V characteristics curve will be positive in quadrant Ι, if the voltage and therefore the current are negative in nature then the curve will be displayed in quadrant ΙΙΙ as shown.In a pure resistance this relation is linear and constant at a constant temperature, such that the current ( i ) is proportional to the potential difference V times the constant of proportionality 1/R giving i = (1/R) x V. Then the current through the resistor is a function of the applied voltage and we can demonstrate this visually using an I-V characteristics curve.In this simple example, the current i against the potential difference V, is a straight line with constant slope 1/R as the relation is linear and ohmic. But there are many electronic components and devices which have non-linear characteristics, that is their V/I ratio is not constant. So, while the resistance at a particular voltage and current can be obtained using Ohm’s Law, it will vary as the voltage and current vary.

I-V Characteristic Curves of Semiconductors

Semiconductor devices such as diodes and transistors are constructed using semiconductor P-N junctions. For example, the primary function of a semiconductor diode is rectification. When a diode is forward biased (the higher potential is connected to its Anode), it will pass current. When the diode is reverse biased (the higher potential is connected to its Cathode), the current is blocked. Then a P-N junction needs a bias voltage of a certain polarity and amplitude to control the flow of electrons through it. This bias voltage controls the resistance of the junction and therefore the flow of current through it. Consider the diode circuit below.

I-V Characteristic Curve of a Diode

When the diode is forward biased, anode positive with respect to the cathode, a forward or positive current passes through the diode and operates in the top right quadrant of its I-V characteristics curves as shown. Starting at the zero intersection, the curve increases gradually into the forward quadrant but the forward current and voltage are extremely small. When the forward voltage exceeds the diodes P-N junctions internal barrier voltage, which for silicon is about 0.7 volts, avalanche occurs and the forward current increases rapidly for a very small increase in voltage producing a non-linear curve. The “knee” point on the forward curve.

Likewise, when the diode is reversed biased, cathode positive with respect to the anode, the diode blocks current except for an extremely small leakage current, and operates in the lower left quadrant of its I-V characteristics curves. The diode continues to block current flow through it until the reverse voltage across the diode becomes greater than its breakdown voltage point resulting in a sudden increase in reverse current producing a fairly straight line downward curve as the voltage losses control. This reverse breakdown point is used to good effect in zener diodes.

Then we can see that the I-V Characteristic Curves for a silicon diode are non-linear and very different to that of the previous resistors linear I-V curves as their electrical characteristics are different. Current-Voltage characteristics curves can be used to plot the operation of any electrical or electronic component from resistors, to amplifiers, to semiconductors and solar cells. They are a useful tool in determining the operating characteristics of a device by showing its possible combinations of current and voltage, and as a graphical aid can help visually understand better what is happening.

Bypass Diodes in Solar Panels

Received and interesting question asking “why are there bypass diodes in solar panels and arrays”. But before we can answer this question in more detail we need to understand exactly what a “solar panel” is and how it works.

A “solar panel” is constructed using individual solar cells, and solar cells are made from layers of silicon semiconductor materials. One layer of silicon is treated with a substance to create an excess of electrons. This becomes the negative or N-type layer. The other layer is treated to create a deficiency of electrons, and becomes the positive or P-type layer similar to transistors and diodes. When assembled together with conductors, this silicon arrangement becomes a light-sensitive PN-junction semiconductor. In fact photovoltaic solar cells or PV’s as they are more commonly called, are no more than big, flat diodes.Photovoltaic solar cells convert the photon light around the PN-junction directly into electricity without any moving or mechanical parts. PV cells produce energy from sunlight, not from heat. In fact, they are most efficient when they are cold!. When exposed to sunlight (or other intense light source), the voltage produced by a single solar cell is about 0.58 volts DC, with the current flow (amps) being proportional to the light energy (photons). In most photovoltaic cells, the voltage is nearly constant, and the current is proportional to the size of the cell and the intensity of the light.

The equivalent circuit of a PV, shown on the left, is that of a battery with a series internal resistance, RINTERNAL, similar to any other conventional battery. However, due to variations in internal resistance, the cell voltage and therefore available current will vary between photovoltaic cells of equivalent size and structure, connected to the same load, and under the same light source so this must be accounted for in the solar panel assemblies you buy.The silicon wafer of the photovoltaic solar cell that faces the sunlight consist of the electrical contacts and is coated with an anti-reflective coating that helps absorb the sunlight more efficiently. Electrical contacts provide the connection between the semiconductor material and the external electrical load, such as a light bulb or battery.

When sunlight shines on a photovoltaic cell, photons of light strike the surface of the semiconductor material and liberate electrons from their atomic bonds. During manufacture certain doping chemicals are added to the semiconductors composition to help to establish a path for the freed electrons. These paths creates a flow of electrons forming an electrical current which starts to flow over the surface of the photovoltaic solar cell.

Metallic strips are placed across the surface of a photovoltaic cell to collect the electrons which form the positive (+) connection of the cell. The back of the cell, the side away from the incoming sunlight consists of a layer of aluminum or molybdenum metal which forms the negative (-) connection to the cell. Then a photovoltaic solar cell has two electrical connections, one positive, on the top, and one negative, at the bottom as shown.Photovoltaic Solar Cell Construction

The type of solar power produced by a photovoltaic solar cell is DC the same as from a battery. Most photovoltaic solar cells produce a “no load” open circuit voltage of about 0.5 to 0.6 volts when there is no external circuit connected. This output voltage ( VOUT ) depends very much on the load current ( I ) demands of the PV cell.For example on very cloudy or dull day the current demand would be low and so the cell could provide the full output voltage, but at a reduced output current. But as the current demand of the load increases a brighter light (solar radiation) is needed at the junction to maintain a full output voltage, VOUT.However, there is a physical limit to the maximum current that a single photovoltaic solar cell can provide no matter how intense or bright the suns radiation is. This is called the maximum deliverable current and is symbolised as IMAX. The IMAX value of a single photovoltaic solar cell depends upon the size or surface area of the cell (especially the PN-junction), the amount of direct sunlight hitting the cell, its efficiency of converting this solar power into a current and of course the type of semiconductor material that the cell is manufactured from either silicon, gallium arsenide, cadmium sulfide or cadmium telluride etc.So when selecting blocking diodes or bypass diodes to connect to solar cells or panels, this maximum current value, IMAX needs to be taken into account.

Diodes in Photovoltaic Arrays

The PN-junction diode acts like solid state one way electrical valve that only allows electrical current to flow through themselves in one direction only. The advantage of this is that diodes can be used to block the flow of electric current from other parts of an electrical solar circuit. When used with a photovoltaic solar panel, these types of silicon diodes are generally referred to as Blocking Diodes.Bypass Diodes are used in parallel with either a single or a number of photovoltaic solar cells to prevent the current(s) flowing from good, well-exposed to sunlight solar cells overheating and burning out weaker or partially shaded solar cells by providing a current path around the bad cell. Blocking diodes are used differently than bypass diodes.Bypass diodes in solar panels are connected in “parallel” with a photovoltaic cell or panel to shunt the current around it, whereas blocking diodes are connected in “series” with the PV panels to prevent current flowing back into them. Blocking diodes are therefore different then bypass diodes although in most cases the diode is physically the same, but they are installed differently and serve a different purpose. Consider our photovoltaic solar array below.

Bypass Diodes in Photovoltaic Arrays

As we said earlier, diodes are devices that allow current to flow in one direction only. The diodes coloured green above are “bypass diodes”, one in parallel with each solar panel to provide a low resistance path. Bypass diodes in solar panels and arrays need to be

able to safely carry this short circuit current. The two diodes coloured red are referred to as the “blocking diodes”, one in series with each series branch. These blocking diodes ensure that the electrical current only flows in one direction “OUT” of the series array to the external load, controller or batteries.The reason for this is to prevent the current generated by the other parallel connected PV panels in the same array flowing back through a weaker (shaded) network and also to prevent the fully charged batteries from discharging or draining back through the array at night. So when multiple solar panels are connected in parallel, blocking diodes should be used in each parallel connected branch.

Generally speaking, blocking diodes are used in PV arrays when there are two or more parallel branches or there is a possibility that some of the array will become partially shaded during the day as the sun moves across the sky. The size and type of blocking diode used depends upon the type of photovoltaic array.

Two types of diodes are available as bypass diodes in solar panels and arrays: the PN-junction silicon diode and the Schottky barrier diode. Both are available with a wide range of current ratings. The Schottky barrier diode has a much lower forward voltage drop of about 0.4 volts as opposed to the PN diodes 0.7 volt drop for a silicon device. This lower voltage drop allows a savings of one full PV cell in each series branch of the solar array therefore, the array is more efficient since less power is dissipated in the blocking diode.

Most manufacturers include blocking diodes and bypass diodes in their solar panels simplifying the design. 

The Insulated Gate Bipolar Transistor, IGBT

The Insulated Gate Bipolar Transistor also called an IGBT for short, is a cross between a conventional Bipolar Junction Transistor, or “BJT” and a Field Effect Transistor, or “FET”. The IGBT transistor takes the best parts of these two types of transistors and combines them together to produce another type of transistor switching device that is capable of handling large collector-emitter currents with virtually zero gate current drive.Typical IGBT

The insulated gate bipolar transistor, (IGBT) uses the insulated gate (hence the first part of its name) technology of the MOSFET with the output performance characteristics of a conventional bipolar transistor, (hence the second part of its name). The result of this combination is that the IGBT transistor has the output switching and conduction characteristics of a bipolar transistor but is voltage-controlled like a MOSFET.

The advantage gained by the IGBT is that it offers greater power gain than the bipolar type together with the higher voltage operation and lower input losses of the MOSFET. In effect it is an FET integrated with a bipolar transistor in a form of Darlington configuration as shown.

Insulated Gate Bipolar Transistor

We can see that the insulated gate bipolar transistor is a three terminal, transconductance device with two of the terminals (C-E) associated with a conductance path and the third terminal (G) associated with its control. The amount of amplification achieved by the insulated gate bipolar transistor is a ratio between its output signal and its input signal.For a conventional bipolar junction transistor, the amount of gain is approximately equal to the ratio of the output current to the input current, called Beta. For a metal oxide semiconductor field effect transistor or MOSFET, there is no input current as the gate is

isolated from the main current carrying channel. Therefore, an FET’s gain is equal to the ratio of output current change to input voltage change, making it a transconductance device and this is also true of the IGBT.

The Insulated Gate Bipolar Transistor can be used in small signal amplifier circuits in much the same way as the BJT or MOSFET type transistors. But as the IGBT combines the low conduction loss of a BJT with the high switching speed of a power MOSFET an optimal solid state switch exists which is ideal for use in power electronics applications.When used as static controlled switch, the insulated gate bipolar transistor has voltage and current ratings similar to that of the bipolar transistor. However, the presence of an isolated gate in an IGBT makes it a lot simpler to drive than the BJT. The IGBT is turned “ON” or “OFF” by activating and deactivating its Gate terminal. A constant positive voltage input signal across the Gate and the Emitter will keep the device in its “ON” state, while removal of the input signal will cause it to turn “OFF” in much the same way as a bipolar transistor or MOSFET.

In other words, because the IGBT is a voltage-controlled device, it only requires a small voltage on the Gate to maintain conduction through the device unlike BJT’s which require that the Base current is continuously supplied in a sufficient enough quantity to maintain saturation. Also the IGBT is a unidirectional device, meaning it can only switch current in the “forward direction”, that is from Collector to Emitter unlike MOSFET’s which have bi-directional current switching capabilities (controlled in the forward direction and uncontrolled in the reverse direction).

The principal of operation and Gate drive circuits for the insulated gate bipolar transistor are very similar to that of the N-channel power MOSFET. The basic difference is that the resistance offered by the main conducting channel when current flows through the device in its “ON” state is very much smaller in the IGBT. Because of this, the current ratings are much higher when compared with an equivalent power MOSFET.

The main advantages of using the Insulated Gate Bipolar Transistor over other types of transistor devices are its high voltage capability, low ON-resistance, ease of drive, relatively fast switching speeds and combined with zero gate drive current makes it a good choice for moderate speed, high voltage applications such as in pulse-width modulated (PWM), variable speed control, switch-mode power supplies or solar powered DC-AC inverter and frequency converter applications operating in the hundreds of kilohertz range.