Circuit Analysis – Purpose of R3 in a Circuit

circuit analysis

What is the purpose of R3 on the shunt regulator circuit below? I simulated it but even without the R3, the circuit still works:
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Best Answer

R3 provides enough current to thr Zener diode to ensure that it is in a stable condition past the reverse breakdown knee voltage. Without it I suspect that you will find the regulation is not so good over a range of input voltages.

Try running the simulation with a DC sweep on the input and graph that and the output.