Also called zener diode. Using the reverse breakdown state of PN junction, its current can change in a large range and the voltage is basically unchanged, a diode with voltage stabilizing effect is made. This diode is a semiconductor device with high resistance until the critical reverse breakdown voltage. At this critical breakdown point, the reverse resistance decreases to a very small value. In this low resistance region, the current increases and the voltage remains constant. The zener diode is divided according to the breakdown voltage. Because of this characteristic, the zener tube is mainly used as a voltage regulator or voltage reference element. Zener diodes can be connected in series for use at higher voltage, and higher stable voltage can be obtained by connecting them in series.

The working conditions of the zener diode: the voltage applied reversely to the zener diode should be greater than the regulated value of the zener tube. The current passing through the regulator should meet its working conditions.

Selection of current limiting resistance of Zener diode circuit: if R is too large, IR is very small. When IL increases, the current of Zener tube may decrease below the critical value and lose the effect of voltage stabilization; If R is too small, IR is large. When RL is large or open circuit, IR flows to the regulator, which may exceed its allowable quota and cause damage.