There is an excellent article in Wikipedia on this subject.

The Zener Diode is a silicon diode designed with a characteristic that makes it ideal for use as a voltage regulating or voltage reference device.

The Zener diode is the heart of many voltage regulator circuits.

When forward biased, the zener diode behaves in exactly the same manner as a normal silicon diode; that is, it will conduct when the barrier potential of 0.6 v is overcome. The forward resistance is very low value and decreases as the current increases, providing a forward voltage drop that is substantially constant.

It is the reverse characteristic that is quite different. When the reverse voltage reaches a particular value, the Zener diode will conduct and the resistance decrease to a low value. Again the the voltage drop is substantially constant as the current varies.

Applications:

Zener Role in Power Supplies

The zener diode is widely used as a voltage regulator because of its capacity to maintain a constant voltage over a sizeable range of currents. It can be used as a single component across the output of a rectifier or incorporated into one of the variety of one-chip regulators.

Zener Regulator

The constant reverse voltage of the zener diode makes it a valuable component for the regulation of the output voltage against both variations in the input voltage from an unregulated power supply or variations in the load resistance. The current through the zener will change to keep the voltage at within the limits of the threshold of zener action and the maximum power it can dissipate.

ETC

Thanx for your detailed information.

Hari

Your welcome Hari