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Zener Diode Calculator

Size the series resistor for a Zener shunt regulator and check resistor and Zener power dissipation, including the worst case with no load.

SI prefixes accepted: 20m = 20 mA, 500m = 0.5 W. Minimum Zener current is typically 5–10 mA to stay in regulation.
Series resistor R_s–
Resistor dissipation–
Zener current, no load (worst case)–
Zener dissipation, no load–
Against the Zener's rating–
V_inR_sV_outV_zLoad
Zener shunt regulator — R_s sets the current, the Zener holds V_out

Zener shunt regulation

A Zener diode holds a roughly constant voltage when reverse-biased beyond its breakdown point. Put a resistor in series with the supply and the Zener across the load, and the Zener "shunts" whatever current the load doesn't take, holding the output near V_z.

R_s = (V_in − V_z) / (I_zmin + I_load(max))

Worst-case Zener current (no load): I_z = (V_in − V_z) / R_s

The resistor is sized for the heaviest load, but the Zener's worst case is the lightest load — with the load disconnected, all the current flows through the diode. That's the condition that destroys Zeners, so it's the one this calculator checks against the power rating.

When to use something else

Shunt regulators waste current continuously and suit only small, steady loads (references, bias). For anything meaningful, alinear regulator is more efficient, and a buck converterfar more so.

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