Table of Contents
Zener diode:
A junction diode specially designed to operate only in the reverse breakdown region continuously (without getting damaged) is called a Zener diode. Zener diodes with different breakdown voltages can be obtained by changing the doping concentrations of p- and n-sides which, in turn, change the width of depletion layer and also the barrier field across the junction. The symbol of a Zener diode is shown in Fig. 14.31.
Zener diode as a voltage regulator:
Principle:
When a Zener diode is operated in the reverse breakdown region, the voltage across it remains practically constant (equal to the breakdown voltage Vz) for a large change in the reverse current. The use of Zener diode as a d.c. voltage regulator is based on this fact.
Working:
Figure given below shows the circuit for using Zener diode as a voltage regulator. Here the Zener diode is connected in reverse bias to a source of fluctuating d.c. (e.g., the output from a rectifier) through a dropping resistor Rs. Thus, the voltage gets divided between Rs and zener diode. The output is obtained across the load resistance RL’ connected in parallel with the zener diode.
If the input voltage increases, the current through Rs and Zener diode also increases. This increases the voltage drop across Rs without any change in the voltage across the Zener diode. This is because in the breakdown region, Zener voltage remains constant even though the current through the Zener diode changes. Similarly, if the input voltage decreases, the voltage across Rs decreases without any change in the voltage across the Zener diode. Thus, any increase/decrease of the input voltage results in, increase/decrease of the voltage drop across Rs without any change in voltage across Zener diode. Hence the Zener diode acts as a voltage regulator.
Figure given below shows the graph of output voltage VQ versus input voltage V. for a Zener diode. Clearly, the output voltage remains constant after the reverse breakdown voltage Vz.
