Zener Diode Resistor Calculator
Find the series resistor and power ratings for a simple zener voltage reference or regulator.
Take the minimum zener current as roughly 10–25% of the datasheet test current (Izt).
Formula and cautions
Rs = (Vs − Vz) / (IL + Iz). Use the next standard value below the result so the current never falls short. With the load disconnected, all the current flows through the zener, so size its power rating for the no-load case.
Frequently asked questions
Why is my zener voltage lower than expected?
If too little current flows through the zener it falls out of regulation. Make sure the series resistor leaves enough zener current at the maximum load, and use the next lower standard resistor value.
Is a zener regulator efficient?
No. The series resistor and zener waste the difference between the supply and output, even with no load. Zeners suit references and small currents; for more than a few tens of milliamps use a linear regulator or a switching converter.