Current through a Zener diode as a function of the bias voltage.
Most diodes have an avalanche breakdown voltage that is much higher than the reverse bias ever gets. The value of the avalanche voltage depends on how a diode is manufactured. Zener diodes are specially made so they exhibit well-defined, constant avalanche voltages.
Suppose a certain Zener diode has an avalanche voltage, also called the Zener voltage, of 50 V. If reverse bias is applied to the P-N junction, the diode acts as an open circuit as long as the bias is less than 50 V. But if the reverse-bias voltage reaches 50 V—even for a brief instant of time—the diode conducts. This effectively prevents the reverse-bias voltage from exceeding 50 V.
The current through a Zener diode, as a function of the voltage, is shown in above figure. The Zener voltage is indicated by the abrupt rise in reverse current as the reverse-bias voltage increases. A simple Zener-diode voltage-limiting circuit is shown in following figure. Note the polarity of the diode: the cathode is connected to the positive pole, and the anode is connected to the negative pole.
Connection of a Zener diode for voltage regulation.