Why zener diode as voltage regulator
Because a Zener diode is used as a voltage stabiliser or voltage regulator to provides a constant voltage from a source whose voltage may fluctuate over a wide range. The circuit diagram of a Zener diode as a voltage stabiliser is shown in Fig.
This is the “circuit diagram of zener diode as voltage regulator experiment“.
Derivation of how to use zener diode as voltage regulator
The source voltage Vi and series resistance Rs are chosen in such a way that the diode operates in the breakdown region.
When Vi is less than Vz, then the zener diode does not conduct and current through the diode is zero. In this case, I = IL. Therefore, current I is given by
When Vi increases, the output voltage will increase as output voltage becomes equal to VZ, the zener diode begins to conduct and voltage across it becomes constant. In this case,
This is the minimum value of input voltage below which voltage regulation is not done by the zener regulator circuit.
When Vi becomes greater than the value given by egn. (2), voltage across output becomes constant and equal to VZ. Therefore, the current through load resistance is given by
The excess input voltage is dropped across Rs so that the sum of the voltage drop across Rs and across zener diode is equal to the supply voltage Vi
The voltage Vz is constant and therefore the load current IL is also constant. Therefore, when supply voltage Vi increases, the current Iz increases and the voltage drop across the series resistor Rs increases but the voltage across RL is maintained constant.
The power dissipated across zener diode is given by
Thus, power dissipated across zener diode as Iz increases.
Maximum power dissipated is given by
Put this value in eqn. (3), we get the maximum permitted supply voltage upto which zener diode can act as a voltage stabilizer safely,
This is the eqn. that shows how to use zener diode as voltage regulator.