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.
Worked example: 5.6 V from 12 V at 20 mA
- Series resistor: Rs = (Vs − Vz)/(IL + Iz) = (12 V − 5.6 V)/(20 mA + 5 mA) = 256 Ω → next lower standard value 240 Ω
- Total current 26.7 mA, resistor power 171 mW → a ½ W resistor
- With no load the zener takes 149 mW, so use a zener rated above that (e.g. 1 W). The minimum load is (Vz)/(It − Iz) = 258 Ω.
Series resistor by zener voltage (12 V, 20 mA)
| Vz | Rs | E24 | Total current | Resistor rating | Zener power, no load | Min. load |
|---|---|---|---|---|---|---|
| Vz3.3 V | Rs348 Ω | E24330 Ω | Total current26.4 mA | Resistor rating½ W | Zener power, no load87 mW | Min. load154 Ω |
| Vz5.1 V | Rs276 Ω | E24270 Ω | Total current25.6 mA | Resistor rating½ W | Zener power, no load130 mW | Min. load248 Ω |
| Vz5.6 V | Rs256 Ω | E24240 Ω | Total current26.7 mA | Resistor rating½ W | Zener power, no load149 mW | Min. load258 Ω |
| Vz6.2 V | Rs232 Ω | E24220 Ω | Total current26.4 mA | Resistor rating½ W | Zener power, no load163 mW | Min. load290 Ω |
| Vz9.1 V | Rs116 Ω | E24110 Ω | Total current26.4 mA | Resistor rating¼ W | Zener power, no load240 mW | Min. load426 Ω |
For a 12 V supply, 20 mA load and 5 mA minimum zener current. What happens below the minimum load was checked on the bench: with 100 Ω in series on 10 V the minimum load works out at about 127 Ω, and a 100 Ω load pulled the output down to 4.964 V (see the zener regulator guide).
Checked on our bench: the minimum load is real
The numbers below are computed from values the site owner recorded in lab classes. Measured values are left exactly as recorded.
A regulator from a lab class: 10 V supply, 100 Ω series resistor, 5.6 V zener, with the load changed step by step. The voltage across the series resistor and the output were measured; the resistor current (VR ÷ R) minus the load current (Vo ÷ RL) gives the zener current. By this calculator's formulas, a 100 Ω series resistor passes 44 mA in total, so the "minimum load resistance" it reports is Vz ÷ 44 mA = 127 Ω.
| Load RL | Output (bench) | IL | IZ (difference) | Formula predicts |
|---|---|---|---|---|
| Load RL100 Ω | Output (bench)4.964 V | IL49.6 mA | IZ (difference)460 µA | Formula predictsout of regulation |
| Load RL470 Ω | Output (bench)5.617 V | IL12 mA | IZ (difference)31.4 mA | Formula predictsregulating |
| Load RL1 kΩ | Output (bench)5.605 V | IL5.61 mA | IZ (difference)37.6 mA | Formula predictsregulating |
| Load RL3.3 kΩ | Output (bench)5.672 V | IL1.72 mA | IZ (difference)41.3 mA | Formula predictsregulating |
| Load RLnone | Output (bench)5.668 V | IL0 | IZ (difference)43 mA | Formula predictsregulating |
- 100 Ω, below the minimum load: the output fell to 4.964 V and the zener current came out at 460 µA, within measurement error, so the zener was effectively off and the output was close to a plain two-resistor divider (5.00 V). That is what the calculator warns about.
- 470 Ω and up: across a sevenfold change in load the output stayed within 67 mV, between 5.605 and 5.672 V.
- No load: the zener carried 43 mA, or 0.244 W, close to the calculator's "zener power with no load" (0.246 W). That figure is the one to size the zener's rating by.
The full load sweep and the PSpice comparison are in the zener regulator guide.
Designing a zener regulator
From load current to a series resistor
- Enter the supply voltage (use the lowest it will be), the zener voltage and the load current.
- Read the series resistor and the zener current it gives.
- Check the resistor and zener power, especially for the case where the load is disconnected.
Zener designs that fail
- Sizing the zener for the loaded case only. When the load is removed, all the resistor current flows through the zener. Its power rating must cover that worst case.
- Letting the zener current fall too low. Below its knee current a zener stops regulating and the voltage drops. Keep several milliamps flowing at the maximum load and the minimum supply.
- Using it as a power supply. A zener regulator wastes power all the time and handles only small loads. For more than a few tens of milliamps, use a linear regulator IC.
Tolerance, noise and better references
Zener voltages have a tolerance (often ±5%) and change with current and temperature; low-voltage zeners around 3 to 4 V have particularly soft knees. A 100 nF capacitor across the zener reduces noise. For a precise reference, a shunt reference such as the TL431 does the same job far better. Our zener regulator guide shows with numbers why a heavy load makes the output collapse.
Worked example: a supply that varies
A 5.1 V zener (1N4733A) runs from a supply that moves between 11 V and 13 V, and the load draws anywhere from 0 to 10 mA. Design for the worst case first: lowest supply, highest load, with 5 mA left for the zener. Rs = (11 V − 5.1 V)/(10 mA + 5 mA) = 393 Ω, so use 390 Ω. At 11 V and full load that leaves 5.1 mA in the zener.
Then check the other corner: highest supply, no load. The resistor now carries (13 − 5.1) ÷ 390 = 20.3 mA, all through the zener, which dissipates 5.1 V × 20.3 mA = 103 mW, well inside a 1 W rating. The resistor dissipates 160 mW, so a half-watt part keeps it cool.
How much the output moves
A zener is not a perfect voltage source; above its knee it behaves like a voltage in series with a small dynamic resistance, about 7 Ω for the 1N4733A at its 49 mA test current and higher at the few milliamps used here. Across the corners above the zener current changes from 5.1 mA to 20.3 mA, so the output moves by at least 15.2 mA × 7 Ω ≈ 0.11 V. That is fine for a reference that feeds a comparator, but not for an ADC reference. If 0.1 V is too much, a shunt reference such as the TL431 has a dynamic resistance well under 1 Ω and fits the same circuit.
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.