Zener Controlled Transistor Voltage Regulators
The efficiency of Zener regulated power supply becomes very low when the load current is high. Under such conditions a Zener controlled transistor is always used for maintaining output voltage constant. Basically there are two types of Zener controlled transistor voltage regulators. They are
- Series Voltage Regulators
- Shunt Voltage Regulators.
Transistor SeriesVoltage Regulator or Emitter Follower Voltage Regulator
A simple series voltage regulator using an NPN transistor and a Zener diode is shown in the figure. This circuit is called a series regulator because collector and emitter terminals of the transistor are in series with the load, as illustrated in the figure. This circuit is also called an emitter follower voltage regulator because transistor Q is connected in emitter follower configuration. Here, the transistor Q is termed a series-pass transistor. The unregulated dc supply (or filtered output from the rectifier) is fed to the input terminals and regulated output voltage Vout is obtained across the load resistor RL. Zener diode provides the reference voltage and the transistor acts as a variable resistor, whose resistance varies with the operating conditions (base current IB). The principle of operation of such a regulator is based on the fact that a large proportion of the change in supply (or input) voltage appears across the transistor and, therefore output voltage tends to remain constant.
Keeping in mind the polarities of different voltages we have
Vout = Vz – VBE
The base voltage of the transistor remains almost constant being equal to that across the Zener diode, Vz.
(i) Let the supply (or input) voltage increase which will cause the output voltage Vout to increase. An increase in output voltage Vout will result in decrease of VBE because Vz is fixed and decrease in VBE will reduce the level of conduction. This will lead to increase m the collector-emitter resistance of the transistor causing an increase in collector to emitter voltage and as a result the output voltage will be reduced. Thus output voltage will remain constant. Similar explanation can be given for decrease in supply voltage.
(ii) Now let us consider the effect of change in load on the output voltage — say current is increased by decrease in RL. Under such a situation the output voltage Vout tends to fall and, therefore, VBE tends to increase. As a result the conduction level of the transistor will increase leading to decrease in the collector-emitter resistance. The decrease in the collector-emitter resistance of the transistor will cause the slight increase in input current to compensate for the decrease in RL. Thus the output voltage being equal to IL RL remains almost constant. Similar explanation will hold true for increase in RL.
The advantage of such a circuit is that the changes in Zener current are reduced by a factor β and thus the effect of Zener effect is greatly reduced and much more stabilized output is obtained.
Output voltage from a series regulator, Vout = (Vz – VBE), and maximum load current IL(max) can be the maximum emitter current that the transistor Q is capable of passing. For a 2N 3055 transistor load current IL could be 15 A. When load current IL is zero, the current drawn from the supply is approximately (Iz + IC(min). The Zener regulator (resistor R and Zener diode form a simple Zener regulator) has to supply only the base current of the transistor. The emitter follower voltage regulator is, therefore, much more efficient than a simple Zener regulator.
- The output voltage cannot be maintained absolutely constant because both VBE and Vz decrease with the increase in room temperature. Further, VBE increases slightly with the increase in load.
- The output voltage cannot be changed as there is no provision for it in the circuit.
- It cannot provide good regulation at high currents because of small amplification provided by one transistor.
- It has poor regulation and ripple suppression with respect to input variations as compared to other regulators.
- The power dissipation of a pass transistor is large because it is equal to Vcc Ic and almost all variation appears at VCE and the load current is approximately equal to collector current. Thus for heavy load currents pass transistor has to dissipate a lot of power and, therefore, becoming hot.
Because of above limitations application of this regulator is limited to low output voltages.
Transistor Shunt Voltage Regulator
A shunt voltage regulator using an NPN transistor and a Zener diode is shown in the figure. A series resistance RSE is connected in series with the unregulated (or input), supply. Zener diode is connected across the base and collector terminals of the NPN transistor and the transistor is connected across the output, as shown in the figure.
Unregulated voltage is reduced, due to voltage drop in series resistance RSE, by an amount that depends on the current supplied to the load RL. The voltage across the load is fixed by the Zener diode and transistor base-emitter voltage VBBE
Output voltage is given as Vout = Vz + VBE = Vin – I RSE
Since both Vz and VBE remain nearly constant so output voltage Vout remains nearly constant. This is explained below :
If the input (or supply) voltage increases, it causes increase in Vout and VBE resulting in increase in base current IB and therefore, increase in collector current Ic (Ic = β IB). Thus with the increase in supply voltage, supply current I increases causing more voltage drop in series resistance RSE and thereby reducing the output voltage. This decrease in output voltage is enough to compensate the initial increase in output voltage. Thus output voltage remains almost constant. Reverse happens should the supply voltage decrease.
If the load resistance RL decreases, output current IL increases and this increase in output current is supplied by decrease in base and collector currents IB and Ic. Thus the input current I remains almost constant causing no change in voltage drop across series resistance RSE. Thus output voltage Vout being the difference of supply voltage (fixed) and series resistor drop VR (fixed) remains constant. Reverse happens should the load resistance increase.
- There is considerable power loss in series resistor RSE.
- A large proportion of supply current I flows through the transistor rather than to load.
- There are problems of over-voltage protection in the circuit.
For the above reasons, a series voltage regulator is preferred over the shunt voltage regulator.