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  Uses of Transistors in Stabilized Power Supplies, Physics tutorial

Solid state voltage regulators:

In solid state voltage regulation, diode or the active element like a transistor is utilized as one half of potential divider to control output voltage of regulator.

Where the active circuit element is utilized, a feedback circuit can be made to compare output voltage to the reference voltage to adjust input to active circuit element. This maintains the constant voltage at output of regulator. Linear solid state voltage regulators are inefficient: As active element serves like the resistor and as such dissipates electrical energy through conversion to heat.

Series regulators are more common form of linear regulators. They are called series regulators because series regulator functions by giving a path from supply voltage to load through the variable resistance in series with load. This way, it serves as variable voltage divider and power dissipated by regulating device is equal to product of power supply output current and voltage drop in regulating device.

The shunt regulator on other hand operates by giving the path from supply voltage through the variable resistance that bypasses load. Current through shunt regulator is diverted away from load and in effect is wasted. This renders this form of regulation even less proficient than series regulator, though it has simpler topology and can be as simple as comprising of just voltage-reference diode. This minimal form though is utilized in very low-powered circuits where wasted current is considered insignificant. It is common in voltage reference circuits. All linear regulators need input voltage that is higher than desired output voltage. Minimum value that this voltage difference can have is known as dropout voltage. Output voltage for common 7805 regulator is 5 Volts but regulator will only maintain output voltage if input voltage remains above approx 7 Volts as any value below 7V will result in the output below rated 5V. Dropout voltage in this case is 2 Volts. In the unregulated direct current power supply, low ripple voltage can be attained at output by using LC filter network but when inductors become expensive and bulky or degree of stability given by LC circuits described above is inadequate, voltage stabilizer circuit is needed.

The simplest regulator is shunt regulator comprising the shunt Zener diode, and this circuit is frequently utilized as the reference voltage to apply to base of the active device like a transistor, or to input of an operational amplifier to get higher output current.

Simplest example of the series regulator includes the transistor that is joined as the voltage follower and where output voltage is approx 600 - 700mV lower than Zener voltage because of emitter-base junction. Resistor R biases Zener diode and base current is supplied to base of Q1. This regulator can be restricted by load requiring that base current be higher than that which flows through resistor R, in which case, circuit illustrated below which is called as Darlington may be utilized in place of single transistor.

912_Darlington Buffered Series Voltage Regulator.jpg

When even better regulation performance is needed operational amplifier is suggested. As open loop gain of the operational amplifier is very high, very good regulation is achievable and high output current is possible when output current buffer devices are comprised.

Stabilized power supplies has been simplified dramatically by voltage regulator Integrated Circuits like L78xx and L79xx series that are three terminal regulators which give the very stable output, comprise current limiter and has thermal protection functions.

Operational amplifier voltage regulator:

Stability of output voltage of the unregulated direct current supply can be considerably increased by using the operational amplifier. Unregulated direct current input is applied to circuit and at output; regulated direct current with very low ripple content is available. Input voltage should be higher than desired output voltage level by the enough margins to get proper regulation at output. Zener diode Vz serves as voltage reference for circuit, and is fed in non-inverting input of operational amplifier. Voltage divider formed by R1 and RF determines voltage level of inverting input of operational amplifier while NPN transistor is utilized to boost output current of circuit.

800_Operational Amplifier Voltage Regulator.jpg

Voltage at non-inverting input of operational amplifier is set to Zener voltage while voltage at inverting input is always the fraction of output voltage as stated by RF and R1. When output exceeds set level, inverting input voltage exceeds that of non-inverting input, causing reduction of output voltage of operational amplifier. This in turn decreases emitter current of NPN transistor that causes reduction in output voltage. Conversely, when output voltage falls below level set by reference network, operational amplifier's output increases and causes NPN transistor to raise emitter current. This results in the increase in output voltage of regulator. This is continuous procedure with circuit reacting immediately to deviations in output voltage whereas resistor RF is utilized to set desired output voltage of circuit.

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