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Zener shunt regulator

Diodes and rectifiers · Beginner · about 10 minutes · part of Zen’s Electronics Design Mastery

A Zener diode is built to break down in reverse at a precise voltage. With a series resistor it holds the output almost constant: the resistor drops the excess, and the Zener soaks up whatever current the load does not take.

Zener shunt regulator: the circuit on the simulator's bench

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How it works

IR = (Vin − VZ) / Rs = (12 − 4.88) / 220 Ω = 32.37 mA

IZ = IR − IL = 32.37 mA − 10.38 mA = 21.99 mA

The 1N4733A has 7 Ω of dynamic resistance, so Vout moves only a little as IZ changes. It stops regulating if the load needs more than IR: RL must stay above VZ·Rs/(Vin − VZ) = 163 Ω.

Try it on the bench

  1. Change V_in from 9 V to 15 V: the output moves only a few tenths of a volt.
  2. Open the load switch: all of I_R now flows through the Zener, and its power rises.
  3. Set R_L to 100 Ω: the load wants more than R_s can pass, so the output drops out of regulation.

Parts on the bench

In the course

Zen’s Electronics Design Mastery, module “Diodes & Power Supplies”.

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