Zener Diode:
In reverse biasing arrangement of P-N junction diode we saw that in the state of breakdown voltage the reverse current of the diode is not less but increases rapidly; but the potential at the ends of the diode remains approximately equal to the breakdown voltage meaning remains constant.
For any diode; breakdown voltage depends on the quantity of impurities mixed. Therefore, according to the quantity of the impure element any special breakdown voltage diode’s fabrication can be done. When any diode is so formed for any special breakdown voltage that in reverse biasing also at breakdown voltage it works without being damaged; then such type of diode is called zener diode. With the help of various quantities of impurity mixed today the diodes are available of less than 1 volt and in hundreds volt also. The sign of zener diode is shown in the figure.
‘When the current flowing through the breakdown point from the zener diode is increased highly still the potential difference at both its ends is constant. This property makes its use for voltage stabilization and voltage regulation. We know that in an ordinary dc power supply with the use of rectifier and equalizer filter, ac is changed to dc. In such type of power supplies when there is change in load current, the value of dc voltage also changes. Therefore, in such situations where current is to be obtained at constant dc voltage such power supply cannot be used. With the help of a zener diode the output voltage of a power supply can be kept constant.
Figure shows a power supply whose output voltage is assumed to be Vi and a resistance Rs and a zener diode are connected at its output terminals. A load resistor RL is parallel to the zener diode. The zener diode is so selected whose breakdown voltage Vz is equal to the constant value of that dc voltage which is to be obtained at load resistor RL. Zener diode is so placed in the circuit that it is in reverse biasing state. If the value of Vi is more than Vz then the diode comes in breakdown region and the potential Vz remains at its ends. Since RL is parallel to the diode, hence on it also there would be Vz potential. If the value of RL is changed then also it will have Vz potential. In this way the output voltage at both the ends of a zener diode is stable. The resistance Rs is so selected that the diode is not damaged by the thermal energy due to the increase in the zener current. This value of current is acknowledged by the data sheet given by the zener diode manufacturing company.
