Zener Diode Regulator Calculator

How it works

R ≤ (Vin min − Vz) ÷ (I load + Iz min); P_R = (Vin max − Vz)² ÷ R; P_Z (no load) = Vz × (Vin max − Vz) ÷ R

A Zener regulator is the simplest voltage reference: a resistor drops the excess input voltage and the Zener diode shunts whatever current the load does not take, holding the output at its breakdown voltage. The resistor must be small enough that, at the lowest input and highest load, the Zener still gets its minimum current to stay in breakdown — that fixes the maximum resistance. The penalty is that at the highest input with the load disconnected, all that current flows through the Zener, setting its power rating; the resistor sees the full current whenever the supply is on. The circuit is therefore inefficient and only suitable for small, fairly constant loads; for anything beyond a few tens of milliamps a three-terminal regulator does better.

Worked example

Input 11–14 V, 5.1 V Zener, 20 mA load, 5 mA minimum: R ≤ 5.9 / 0.025 = 236 Ω → 220 Ω. At 14 V the resistor passes 40 mA (0.36 W → 1/2 W part) and the unloaded Zener dissipates 206 mW — a 500 mW Zener suffices.

Frequently asked questions