Introduction : Voltage regulator circuit is required to maintain the constant DC output voltage across load terminals in spite of the variations :
- Variation in input mains voltage
- Change in Load current
- Change in Temperature
For the purpose of regulating the voltage, zener diode is always operated in reverse biased condition.
Zener diode is operated in breakdown region and it is used to regulate the voltage across a load when there are variations in the supply voltage or load current.
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| Fig. 1 Zener Shunt Regulator |
The input current ( i.e. current through the limiting resistor ) is given by the relation,
Is = Vs - Vz / Rs ------- (1)
Where Vs = The DC input voltage to the regulator circuit, and
Vz = The zener voltage.
The ideal zener diode acts as a constant voltage source of voltage ( Vz ). However, a practical zener diode has a finite value of resistance called zener resistance (rz). Because of the zener resistance, there is a voltage drop across it which is Iz.rz. Therefore the voltage across the terminals of the zener diode.
VL = Vz + Iz.rz
if rz is negligible,
VL = Vz
The input current is the sum of zener current and load current i.e.
Is = Iz + IL i.e. -------- (2)
Iz = Is - IL
From equation 1 and 2
Vs - Vz / Rs = Iz + IL ------ (3)
Is = Vs - Vz / Rs --------- (4)
Regulation with a varying input voltage ( Line Regulation ) :
It is defined as the change in regulated voltage with respect to variation in line voltage. In this, input voltage (Vs) varies but load resistance (RL) remain constant hence, the load current (IL) remains same. As the input voltage increases , from equation (4) Is varies accordingly. Therefore zener current Iz will increase. This increases the voltage drop across series resistance, thereby keeping the load voltage as constant.
On other hand, if the input voltage is decreased, the input current also decreases. As a result of this, the current through zener (Iz)will also decrease. Consequently, the voltage drop across series resistance will be reduced. Thus the load voltage (VL) and load current (IL) remains constant.
Regulation with a varying load ( Load Regulation ) :
Here the input voltage ( Vs ) is kept fixed and load resistance ( RL ) varies. The variation of load resistance changes the current ( IL ) through it, thereby changing voltage ( VL ) across it. When load resistance decreases , the load current increases. This causes zener current to decrease. As a result of this, the input current and voltage drop across series resistance remains constant. Thus the load load voltage (VL ) is also kept constant. On other hand, if the load resistance increases, the load current decreases. As a result of this, the zener current increases. This again keeps the values of input current and voltage drop across series resistance as constant. This way the load voltage remains constant.
Disadvantages of zener diode Shunt regulator :
1. The maximum load current , which can be supplied to load resistor ( RL ) is limited to Iz(max) and Iz(min).
2. A large amount of power is wasted in the zener-diode and the series resistance ( Rs ) in comparison with load power.