About this calculator
A zener diode conducts backwards once the voltage across it reaches its breakdown value, and then holds that voltage steady over a wide range of current. Put a resistor in front of one and you have the simplest possible voltage regulator.
The whole design reduces to choosing that resistor. Too large and the load starves at low input voltage; too small and the zener cooks at high input voltage with no load. This calculator sizes it against both worst cases at once and checks the power ratings that result.
How it works
The resistor has to pass enough current to feed the load and keep at least a few milliamps flowing through the zener to hold it in breakdown. The tightest condition for that is minimum input voltage with maximum load, which gives Rs = (Vin_min − Vz) / (I_load_max + Iz_min).
Having fixed the resistor, the opposite extreme decides whether the zener survives. At maximum input with the load disconnected, every bit of current the resistor passes goes through the zener instead. That is where its dissipation peaks, and it is the case people forget.
The calculator deliberately rounds the resistor down to a standard value rather than up. Rounding down passes a little more current than strictly needed, which keeps the load supplied at low input; rounding up risks the output sagging exactly when the input is weakest.
Zeners are a shunt regulator, which means they always draw full current whether the load needs it or not. That makes them wasteful for anything beyond a few tens of milliamps, but excellent as a voltage reference or a clamp.
Worked example
A 5.1 V reference from an automotive supply that ranges 12–15 V, feeding a load that draws up to 20 mA and can be switched off entirely.
- Rs = (12 − 5.1) / (0.020 + 0.005) = 6.9 / 0.025 = 276 Ω
- Round down to the nearest E24 value: 270 Ω
- Worst case for the zener: (15 − 5.1) / 270 = 36.7 mA, all of it through the diode
- Pz = 5.1 × 36.7 mA = 187 mW → a 400 mW zener is marginal, use 1 W
- P_Rs = (15 − 5.1)² / 270 = 363 mW → use a ½ W resistor
270 Ω with a 1 W zener. Note that the zener dissipates most when the load is switched off — sizing it for the loaded case would destroy it the first time the load disconnects.
Practical notes
- Always check the no-load case. A zener regulator is at maximum stress when the thing it is regulating for is switched off.
- Keep Iz_min at 5–10 mA, or about 10% of the load current. Below the knee the zener's regulation becomes soft and the output voltage drifts with current.
- Derate zeners hard — aim for half the rated dissipation or less. Their ratings assume specific lead lengths and PCB copper.
- Zeners around 5.6 V have the flattest temperature coefficient. Below 5 V the mechanism is Zener breakdown with a negative coefficient; above 6 V it is avalanche with a positive one.
- For more than a few tens of milliamps, add an emitter follower: the zener sets the base voltage of a transistor that carries the load current, and the output becomes
Vz − 0.7 V. Far more efficient. - Zener tolerance is typically ±5%, so a 5.1 V part may sit anywhere from 4.85 V to 5.36 V. For a genuine precision reference, use a TL431 or a dedicated reference IC.
Frequently asked questions
How do I calculate the series resistor for a zener?
Use the worst case for the load: Rs = (Vin_min − Vz) / (I_load_max + Iz_min), with Iz_min typically 5 mA. Then check the zener's dissipation at maximum input with the load removed.
Why does my zener get hot?
Almost always because the load draws less than you assumed. All the current the resistor passes has to go somewhere, and with no load it all goes through the zener. Recalculate the diode's dissipation with the load fully disconnected.
Can I use a zener to power a circuit?
Only a very light one — a reference, a bias point, a few milliamps of logic. Beyond that the constant current draw is wasteful and the resistor and diode both run hot. Use a proper regulator instead.
What is Iz min and why does it matter?
The minimum current that keeps the diode in solid breakdown. Below it the zener operates on the soft knee of its curve, where the voltage varies substantially with current and regulation effectively stops.
What zener voltage should I choose for a 5 V rail?
5.1 V is the standard part and the nearest in the E24 series. If temperature stability matters more than the exact figure, 5.6 V zeners have the lowest temperature coefficient of any value.