Use this zener diode calculator to find the series resistor for a simple zener voltage regulator, and what power rating the resistor and the zener need.
A zener diode holds a steady voltage across it, as long as enough current flows through it. The series resistor has to let through the current for your load, plus a little extra for the zener, even when the input voltage is at its lowest:
- Series resistor value
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- Nearest standard resistor (E12, next lower)
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- Power in the resistor (at the highest input)
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- Resistor power rating to use
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- Power in the zener (no load, highest input)
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- Zener power rating to use
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How to Use the Result
The calculator picks the nearest standard resistor that is lower, so there’s always enough current.
The power ratings are for the worst case. The resistor gets hottest at the highest input voltage. The zener gets hottest when there’s no load, because then all the current goes through it. Both ratings include a 2× margin.
A zener regulator is fine for small currents, like a reference voltage or a sensor. For more than a few tens of milliamps, a voltage regulator wastes a lot less power.