Zener diode as voltage regulator

A zener diode when working in breakdown region, can be used as a voltage regulator because it maintains a constant output voltage even when the current through it changes. The circuit employing a zener diode as a voltage regulator is shown in figure. It consists of a R connected in the series with the unregulated dc input voltage Vin and a zener diode of zener break down voltage Vz connected in parallel with the load RL across which a constant dc output voltage is desired. The zener diode is reverse biased with the input voltage and operates in break down region so long as the input voltage does not fall below Vz. Once operating in the break down region, the zener diode holds the load voltage constant (equal to Vz) even though the input voltage Vz or load resistance RL may vary.

Case I: Let us consider the case in which RL is fixed and input voltage Vin fluctuates. If Vin increases, the total current I will increases. This increase in I will pass through the zener diode without affecting the load current IL. The increase in Vin will be dropped across Rs, thereby keeping Vout constant. Conversely if Vin falls, the zener diode will draw small current and voltage drop across Rs will be reduced, thus again keeping Vout constant. Hence, when Vin changes I and IR drop change in such a way as to keep Vout (=Vz) constant.

Case 2: Let us consider the case in which Vin is fixed and RL fluctuates. If RL decreases, the load current IL will increases then zener current Iz with decrease, thereby keeping I = Iz+IL and hence IRs drop constant. In this way Vout remains constant because Vout= Vin - IRs= Vin -(Iz+IL)R.

Consensually, if RL increases, IL will decreased and Iz will increase in order to keep I and hence IR drop constant. In this way, again Vout remains unaffected.

Thus, we see that a zener diode as a voltage regulator holds the output voltage constant in spite of changes input voltage and load resistance.

This note is taken from MSC physics, Nepal.