Zener diode is a special type of diode which, like ordinary diode, not only conducts current in the forward direction , but also in the opposite direction if the voltage exceeds the level of breakdown voltage. Zener diodes are used in voltage stabilizers. Because we need to get the same amount of current from the voltage stabilizer all the time. Although the voltage at the input is different, the same amount of voltage is required at the output all the time. And this is what the Zener diode does. Whatever the input voltage is more or less the Zener diode gives the output voltage the same all the time. As a result, the equipment can run properly. This means that the Zener diode is used to get the same output voltage from the voltage stabilizer. The main function of the Zener diode is to use the shunt regulator and the voltage reference as the voltage of the circuit.