Zener diode as voltage regulator

When operating in the breakdown area, a zener diode can be utilized as a voltage regulator since it keeps the output voltage constant even when the current flowing through it fluctuates. Figure depicts the circuit that uses a zener diode as a voltage regulator. It is made up of a R linked in series with an unregulated dc input voltage Vin and a zener diode with a desired constant dc output voltage Vz connected in parallel with the load RL. As long as the input voltage does not drop below Vz, the zener diode functions in the break-down zone while being reverse biased by the input voltage. The zener diode holds the load once it enters the break-down area.

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.

This note is a part of the Physics Repository.