Zener Regulator Formulas: Series Resistor

Zener diode shunt regulator formulas: series resistor, zener current with and without the load, power in the zener and in the resistor, with a worked example.

The formulas

Vz is the zener voltage, IL the load current and Iz min the smallest zener current that keeps the voltage regulated (take it from the data sheet, often 5 mA or more). Design at the lowest input voltage for the resistor and check the zener power at the highest input voltage with the load removed, which is the worst case. The zener voltage also changes a little with current and temperature.

Worked example: 12 V to 5.1 V at 20 mA

The minimum zener current is 5 mA.

1. Rs = (12 − 5.1) / (0.020 + 0.005) = 276 Ω
2. Is = (12 − 5.1) / 276 = 25 mA
3. With the load removed the zener carries all of it: Pz = 5.1 × 0.025 = 0.1275 W
4. The resistor dissipates 0.025² × 276 = 0.1725 W

Series resistor for 12 V to 5.1 V

Frequently asked questions

Why is the zener power worst with no load?

The whole supply current then flows through the zener, so its power is Vz × Is. With a load, the load takes part of the current.

Is a zener a good regulator for large currents?

No. It wastes the full supply current all the time. For more than a few tens of milliamperes use a series regulator or a switching converter.

What if the input voltage varies?

Calculate Rs for the lowest input and the highest load, then check the zener current and power at the highest input and the lowest load.

Does the load voltage stay exactly at Vz?

No. It moves with the zener current through the dynamic impedance of the diode, a few ohms for a 5 V part, and with temperature.

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