Explain the use of a Zener diode as a voltage regulator by drawing a circuit diagram.

(N/A) When the $AC$ input voltage of a rectifier fluctuates,its rectified output also fluctuates.
To obtain a constant $DC$ voltage from the unregulated $DC$ output of a rectifier,a Zener diode can be used.
The circuit diagram of a $DC$ voltage regulator using a Zener diode is shown in the figure.
The unregulated $DC$ voltage (filtered output of a rectifier) is connected to the Zener diode through a series resistance $R_S$ such that the Zener diode is reverse-biased.
If the input voltage increases,the current through $R_S$ and the Zener diode also increases. This increases the voltage drop across $R_S$ without any change in the voltage across the Zener diode. This is because,in the breakdown region,the Zener voltage remains constant even though the current through the Zener diode changes.
Similarly,if the input voltage decreases,the current through $R_S$ and the Zener diode also decreases. The voltage drop across $R_S$ decreases without any change in the voltage across the Zener diode. Thus,any increase or decrease in the input voltage results in an increase or decrease of the voltage drop across $R_S$ without any change in the voltage across the Zener diode.
Thus,the Zener diode acts as a voltage regulator. To maintain a constant voltage across the Zener diode,we must select the Zener diode according to the required output voltage and choose the series resistance $R_S$ accordingly.