When the ac input voltage of a rectifier fluctuates, its rectified output also fluctuates.

To get a constant dc voltage from the dc unregulated 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 figure.

The unregulated dc voltage (filtered output of a rectifier) is connected to the zener diode which through a series resistance $\mathrm{R}_{\mathrm{S}}$ such that the zener diode is reverse biased.

If the input voltage increases, the current through $\mathrm{R}_{\mathrm{S}}$ and zener diode also increases. This increases the voltage drop across $\mathrm{R}_{\mathrm{S}}$ without any change in the voltage across the zener diode. This is because in the breakdown region, zener voltage remains constant even though the current through the zener diode changes.

Similarly, if the input voltage decreases the current through $\mathrm{R}_{\mathrm{S}}$ and zener diode also decreases. The voltage drop across $\mathrm{R}_{\mathrm{S}}$ decreases without any change in the voltage across the zener diode. Thus, any increaseldecrease in the input voltage results in increaseldecrease of the voltage drop across $\mathrm{R}_{\mathrm{S}}$ without any change in voltage across the zener diode.

Thus, the zener diode acts as a change in voltage across the zener diode we have to select the zener diode according to the required output voltage and accordingly the series resistance $\mathrm{R}_{\mathrm{S}}$.