Explain the use Zener diode as a voltage regulator.

Explanation

The circuit connections are made as shown in the figure. The circuit contains a zener diode, a unregulated voltage source, a series resistance `R_s` and a load resistance `R_L`.

If the input D.C voltage increases, the current through zener diode increases. But the voltage drop across the zener diode is unaltered.
If the input DC voltage decreases, the current through `R_s`, voltage drop across `R_s` and the current through zener diode decreases. But the voltage drop across the zener diode is unaltered.
Any change(`"increase"//"decrease"`) in the input voltage results in change (`"increase"//"decrease"`) of voltage drop across `R_s` without any change in voltage across the zener diode. Thus the zener diode acts as voltage regulator.