Voltage regulation

Most diodes have avalanche breakdown voltages much higher than the reverse bias ever gets. The value of the avalanche voltage depends on how a diode is manufactured. Zener diodes are made to have well-defined, constant avalanche voltages.
Suppose a certain Zener diode has an avalanche voltage, also called the Zener voltage, of 50 V. If a reverse bias is applied to the P-N junction, the diode acts as an open circuit below 50 V. When the voltage reaches 50 V, the diode starts to conduct. The more the reverse bias tries to increase, the more current flows through the P-N junction. This effectively prevents the reverse voltage from exceeding 50 V. The current through a Zener diode, as a function of the voltage, is shown in Fig. 1. The Zener voltage is indicated by the abrupt rise in reverse current as the reverse bias increases. A typical Zener-diode voltage-limiting circuit is shown in Fig. 2.
There are other ways to get voltage regulation besides the use of Zener diodes, but Zener diodes often provide the simplest and least expensive alternative. Zener diodes are available with a wide variety of voltage and power-handling ratings. Power supplies for solid-state equipment commonly employ Zener diode regulators.

Fig. 1: Current through a Zener diode, as a function of the bias voltage.

Fig. 2: Connection of Zener diode for voltage regulation.