Zener Diode Calculator (Series Resistor)
Compute the series resistor a zener regulator needs, plus the power dissipated in the resistor and the zener.
Worst case for the zener is no load — all current flows through it. Choose a zener rated above this power.
Disclaimer: This calculator is provided for general informational and educational purposes only, on an “as is” basis and without any warranty of accuracy or fitness for a particular purpose. Results may contain errors — always verify independently before relying on them in real designs. PartAndStock accepts no liability for any loss or damage arising from use of this tool, including when embedded on third-party sites.
How to use
- 1Enter the input voltage (Vin) and zener voltage (Vz).
- 2Enter the load current to supply and the minimum zener current (typically 5 mA).
- 3The series resistor, nearest standard value and power dissipations are computed instantly.
How it works
Enter the input voltage, zener voltage and load current to find the series resistor and power dissipation for a simple zener voltage regulator.
How a zener regulator works
A zener diode conducts in reverse at a set voltage (Vz) and holds that voltage roughly constant. With a series resistor (Rs), the excess of the input voltage is dropped across Rs while the zener clamps the output at Vz. If the load draws little current, the surplus flows through the zener; if it draws more, less flows through the zener — the total current stays the same. Rs = (Vin − Vz)/(Iz + Iload).
Power, worst case and limits
The critical point is power dissipation. The worst case for the zener is when the load is removed entirely: all current then flows through the zener and its dissipation rises to Vz·I_total. The zener you choose must be rated above that. This simple regulator suits only low, fairly constant currents; for high or varying loads it's inefficient — use a regulator like the LM317 instead.
Worked examples
- Vin=12 V, Vz=5.1 V, Iload=10 mA, Iz=5 mA → Rs = 6.9/0.015 = 460 Ω (std 470 Ω)
- Same circuit: P_Rs = 6.9·0.015 ≈ 104 mW
- Load removed: zener power = 5.1·0.015 ≈ 77 mW (worst case)
Common Zener Voltages
| Vz | Typical use |
|---|---|
| 3.3 V | Logic reference |
| 5.1 V | USB/logic level |
| 5.6 V | General purpose |
| 9.1 V | Pre-regulation |
| 12 V | Relay/analog |