Explaining how a Zener diode can be used as voltage regulator.

Zener diode is specially designed silicon diode which is optimized to operate in the breakdown region. 

Zener diode as a Voltage Regulator:

  It provides a constant voltage to the load from a source whose voltage may vary over sufficient range. 

Fig. (a) shows the circuit arrangement.  The Zener diode of Zener voltage Vz is reverse connected across the load RL across which constant voltage (Vo = Vz) is desired.  

A resistor R is connected in series with the circuit which absorbs the input voltage fluctuations so as to maintain constant voltage (Vo) across the load. Let a variable voltage Vin be applied to the regulator. 

When the value of Vin is less than Zener voltage Vz of the Zener diode, no current flows through it and the same voltage appears across the load.  When the input voltage Vin is more than VZ, this will cause the Zener diode to conduct a large current IZ.

Consequently, more current flows through series resistor R which increases the voltage drop across it.  Thus, the input voltage excess of Vz (i.e. Vin – Vz_) is absorbed by the series resistor.  Hence a constant voltage V0 (=Vz) is maintained across the load RL. When the load resistance RL decreases for constant input voltage Vin, load current IL increases. 

This additional current is not supplied from the source of supply but the demand of additional load current is met by decrease in Zener current Iz.   This keeps the voltage drop across series resistance R constant and also the output voltage Vout. 

The diode current Iz attains its maximum Izmax when load current IL becomes zero.  Thus the value of series resistor R can be determined from the equation