Zener diode : Zener diode is a heavily doped germanium (or) silicon p-n junction diode. It works on reverse bias break down region.
The circuit symbol of zener diode is shown in figure.
Zener diode can be used as a voltage regulator. In general zener diode is connected in reverse bias in the circuits.
(i) The zener diode is connected to a battery, through a resistance R. The battery reverse biases the zener diode.
(ii) The load resistance RL is connected across the terminals of the zener diode.
(iii) The value of R is selected in such away that in the absence of load RL maximum safe current flows in the diode.
(iv) Now consider that load is connected across the diode. The load draws a current.
(v) The current through the diode falls by the same amount but the voltage drops across the load remains constant.
(vi) The series resistance R absorbs the output voltage fluctuations, so as to maintain constant voltage across the load.
(vii) The voltage across the zener diode remains constant even if the load RL varies. Thus, zener diode works as voltage regulator.
(viii) If I is the input current, IZ and IL are zener and load currents.
I = IZ + IL; Vin = IR + VZ
But Vout = VZ
∴ Vout = Vin – IR