Question

What is the use of parallel resistance in a zener diode voltage regulator circuit?

Answer 1

1.zener has a fixed power dissipation Pz max, like say 470 mWatt. So, suppose its Vz is 4.7V, its Iz max is 100mA. Taking factor of safety as 0.5, we can pass only 100X0.5 = 50 mA current through the zener. So, set this current using the series resistance.

2. When the input voltage is varying, the variations appear across this series resistance.Since Vin = VR +Vz,, Vz remaining constant, the variations appear across Rs as VR according to KVL.