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Zener diode resistor calculator

A Zener diode holds a steady voltage across a load, as long as a resistor in series feeds it enough current. Too big a resistor and the output sags; too small and the Zener overheats. Type your supply, the Zener's voltage and what the load draws, and this gives the resistor to fit.

Its highest, if it varies.

For a battery that runs down. Leave empty if it's steady.

The output you want, from the Zener's part number.

What the load draws at the Zener voltage.

Below about 5 mA a Zener goes soft and stops holding.

Series resistor, rounded down

270 Ω
Exact value
276 Ω
Current through it
25.6 mA
Left for the Zener
5.56 mA
Zener's power, no load
0.13 W
Zener rating to buy
½ W or more
Resistor's power
0.176 W
Resistor rating to buy
½ W or more
Holds down to a supply of
10.5 V

276 Ω isn't a value you can buy, so this rounds down to the next standard (E12) value. Unlike an LED's resistor, rounding down here keeps the Zener fed.

The formula

The resistor feeds the load and the Zener. Below a few milliamps a Zener goes soft, so give it at least IZ,min, about 5 mA:

R = Vin − VZ over Iload + IZ,min

Then check the worst case: the load unplugged, so the Zener takes all the current.

PZener = VZ × IR

A worked example

12 V in, a 5.1 V Zener and a 20 mA load:

R = 12 V − 5.1 V over 0.020 A + 0.005 A = 6.9 V over 0.025 A = 276 Ω

Round down to 270 Ω. Then 6.9 ÷ 270 ≈ 25.6 mA flows: 20 mA to the load and 5.6 mA to the Zener. With no load, the Zener takes all 25.6 mA: 5.1 × 0.0256 ≈ 0.13 W, so a 0.5 W Zener runs cool. The resistor burns 6.9 × 0.0256 ≈ 0.18 W, so give it ½ W.

The output holds as long as the resistor can still pass the load's 20 mA. That's down to a supply of 5.1 + 0.02 × 270 = 10.5 V. Below that, the Zener goes quiet and the output falls with the supply.

When to use something else

A Zener regulator passes a fixed current all the time, and the Zener burns whatever the load doesn't use. That's fine for a few tens of milliamps: a voltage reference, a sensor, or a clamp on a chip's input. For bigger loads, a three-pin regulator such as the 7805 wastes far less, because it takes only the current the load needs.

Free lesson

Now build it yourself

Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.

Try it yourself: free lesson

“Zener regulators” is lesson 3 of 6 in Silicon Ridge, Electronics World 5. Free with an account. Lessons unlock in order, so you’ll do the 30 before it first.

Or start from the very first lesson, no account needed

The ideas behind it

Questions people ask

How do I calculate the resistor for a Zener diode?

R = (Vin − VZ) ÷ (I_load + I_Z,min). With 12 V in, a 5.1 V Zener, a 20 mA load and 5 mA for the Zener: 6.9 ÷ 0.025 = 276 Ω. Round down to 270 Ω.

Why round the Zener resistor down, not up?

A bigger resistor passes less current. If it can't feed the load and still leave a few milliamps for the Zener, the Zener stops holding and the output sags. Rounding down keeps it fed.

What power rating does the Zener need?

Work it out with the load unplugged, when the Zener takes all the resistor's current: P = VZ × I. In the example that's 5.1 × 0.0256 ≈ 0.13 W, so a common 0.5 W Zener has plenty of headroom.

What if my supply voltage changes?

Design the resistor at the lowest supply, so the Zener is still fed when the battery runs down, and check the power at the highest. Put both into the calculator.

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