Zener Diode Resistor Calculator

Work out zener diode resistor instantly with clear inputs, formula shown and shareable results.

About Zener Diode Resistor Calculator

The series resistor must pass the load current plus a minimum zener current: R = (Vin - Vz)/(IL + IZmin). The worst case for the zener is no load, when it carries the entire current, so its power rating must cover Vz times the full current.

Formula

Zener shunt regulator

R = (Vin - Vz) / (Iload + Izmin); Pzener(worst case) = Vz x (Vin - Vz) / R

Worked example

With the default values below, Zener Diode Resistor Calculator returns Series resistor of 276.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Input voltage: 12 V
Zener voltage: 5.1 V
Load current: 20 mA
Minimum zener current: 5 mA

Results

Series resistor: 276.00 Ω
Resistor dissipation: 172.50 mW
Zener dissipation at no load: 127.50 mW
Nearest E24 resistor: 270.00 Ω

Step by step

1Values used
2Zener shunt regulator
3Series resistor= 276.00 Ω
4Resistor dissipation= 172.50 mW
5Zener dissipation at no load= 127.50 mW
6Nearest E24 resistor= 270.00 Ω

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

Why is no load the worst case?

Because the resistor current is fixed by the input voltage, so with the load disconnected all of it flows through the zener.

When should I use a proper regulator instead?

Whenever load current varies much or efficiency matters. A shunt zener wastes the full current continuously and regulates poorly.