The Zener diode makes its use as a voltage regulator due to the following property:
When a Zener diode is operated in the breakdown region, the voltage across it remains practically constant for a large change in the current.
A simple circuit of a voltage regulator using a Zener diode is shown in Figure 1.
The Zener diode is connected across load such that it is reverse biased.
The series resistance R absorbs the output voltage fluctuations so as to maintain a constant voltage across the load.
If the input dc voltage increases, the current through resistance R and Zener diode also increases. So, the voltage drop across R increases, without any change in the voltage across the Zener diode.
I-V Characteristics of Zener diode
figure 2 shows an I-V Characteristics of the Zener diode.