For proper operation, the input voltage Vi must be greater than the Zener voltage Vz. This ensures that the Zener diode operates in the reverse breakdown condition. The unregulated input voltage Vi is applied to the Zener diode. Suppose this input voltage exceeds the Zener voltage. This voltage operates the Zener diode in reverse breakdown region and maintains a constant voltage, i.e. Vz = Vo across the load inspite of input AC voltage fluctuations or load current variations. The input current is given by,
We know that the input current IS is the sum of Zener current Iz and load current IL
As the load current increase, the Zener current decreases so that the input current remains constant.
According to Kirchhoff’s voltage law, the output voltage is given by,
As the input current is constant, the output voltage remains constant (i.e. unaltered or unchanged). The reverse would be true, if theload current decreases. This circuit is also correct for the changes in input voltage. As the input voltage increases, more Zener current will flow through the Zener diode. This increases the input voltage Is, and also the voltage drop across the resistor Rs, but the load voltage Vo would remain constant. The reverse would be true, if the decrease in input voltage is not below Zener voltage. Thus, a Zener diode acts as a voltage regulator and the fixed voltage is maintained across the load resistor RL