Zener Diode Calculator
Sizes the series resistor for a Zener shunt regulator and gives the worst-case dissipation in both parts.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
R = (V_in − V_z) ÷ (I_z,min + I_load) P_zener(worst case) = V_z × (V_in − V_z) ÷ R
The resistor sets the total current. The Zener takes whatever the load does not, which means the diode is most stressed when the load takes nothing at all — so the power rating has to be chosen for the no-load case rather than the normal one.
What to watch out for
Three things sink Zener regulators:
- The diode is hottest with no load. The resistor passes a roughly fixed current; whatever the load declines, the Zener absorbs. Rate it for the full current.
- Regulation is poor. A Zener has real dynamic resistance, typically tens of ohms, so the output moves with both input voltage and load current. It is a reference, not a power supply.
- Efficiency is bad by construction. The circuit always draws the full current whether the load needs it or not. For anything past a few tens of milliamps, use a proper regulator.
Frequently Asked Questions
How do I calculate the resistor for a Zener diode?
Subtract the Zener voltage from the input, then divide by the sum of the load current and the minimum Zener current. 12 V into a 5.1 V Zener feeding 20 mA, with 5 mA of bias, needs about 276 Ω — so a 270 Ω part.
What power rating does the Zener need?
Enough for the no-load case, where the diode carries the entire current the resistor passes. That is usually well above its dissipation under normal load, and it is where most Zener failures come from.
When should I not use a Zener regulator?
Whenever the load is more than a few tens of milliamps, whenever the input varies widely, or whenever the output has to be accurate. A linear regulator costs about the same, regulates far better and wastes less.