The circuit diagram of Zener voltage regulator is shown below
A zener diode of breakdown voltage VZ is connected in reverse biased condition across the load RL such that it operates in breakdown region. Any fluctuations in the current are absorbed by the series resistance Rs. The Zener will maintain a constant voltage VZ
( equal to Vo) across the load unless the input voltage does not fall below the zener breakdown voltage VZ.
Case(i) When input voltage Vin varies and RL is constant
If the input voltage increases, the Zener diode which is in the breakdown region is equivalent to a battery VZ as shown in figure. The output voltage remains constant at VZ (equal to Vo) and the excess voltage is dropped across the series resistance RS. We know that for a zener diode under breakdown region large change in current produces very small change in voltage, thereby the output voltage remains constant.
Case (ii) When Vin is constant and RL varies.
If there is a decrease in the load resistance RL and the input voltage remains constant then there is a increase in load current.
Since Vin is constant the current cannot come from the source. This addition load current is driven from the battery VZ and we know that even for a large decrease in current the Zener output voltage Vz remains same. Hence the output voltage across the load is also constant..