Voltage regulator in line diode (also known as Zener diode) Its circuit symbol is: this diode is a semiconductor device with high resistance until the critical reverse breakdown voltage. At this critical breakdown point, the reverse resistance is reduced to a very small value, in this low resistance region, the current is added and the voltage is stable, the voltage regulator diode is graded according to the breakdown voltage, because of this characteristic, the voltage regulator is mainly used as a regulator or voltage reference element. The volt-ampere characteristic voltage regulator diodes can be connected in series for use at higher voltages, and more stable voltages can be obtained through series.