A Zener diode can act as a voltage regulator when it is operated in its reverse breakdown mode. When a Zener diode is reverse biased, it experiences a phenomenon called the Zener breakdown, which allows it to maintain a constant voltage across its terminals even when the input voltage varies. This makes the Zener diode useful as a voltage regulator.
To act as a voltage regulator, a Zener diode is connected in parallel with the load that needs to be regulated, and the diode is biased in reverse using a resistor. The Zener diode maintains a constant voltage across its terminals, which is then applied to the load. As the input voltage increases, the excess voltage is dropped across the resistor, and the voltage across the load remains constant.
The Zener diode can act as a regulator as long as the current through the diode and the load does not exceed the maximum current rating of the diode. Also, the Zener voltage of the diode should be chosen appropriately to match the voltage requirement of the load.
In summary, a Zener diode acts as a regulator when it is operated in its reverse breakdown mode, and it is used to maintain a constant voltage across a load by dropping any excess voltage across a resistor.