About the Zener Diode Calculator
This zener diode calculator designs the classic shunt regulator: a resistor in series from the supply and a zener diode across the load. Enter the minimum and maximum input voltage, the zener voltage, the range of load current and the minimum current the zener needs to stay in regulation, and it returns the exact series resistance, the nearest standard resistor below it, and the worst-case power in both the zener and the resistor with a suggested wattage for each.
It is meant for hobbyists and engineers building simple reference voltages, clamping circuits, bias supplies and low-current rails where a linear regulator chip would be overkill. The worst cases matter: the resistor must still pass enough current at the lowest input voltage and full load, while the zener must survive the highest input voltage when the load draws the least — often nothing at all.
The model treats the zener as an ideal voltage source at Vz once it carries at least its minimum current (often quoted as Izk or a test current Izt of a few mA). Real zeners have a dynamic resistance, so the output rises slightly with current; check the datasheet when precision matters.
With the default inputs, the calculated series resistor is 196 Ω. Change any value above to recalculate instantly.
How to use the zener diode calculator
- 1Enter the lowest and highest voltage your supply can reach.
- 2Enter the zener voltage you need at the output.
- 3Enter the maximum load current and the minimum (0 if the load can be removed).
- 4Set the minimum zener current from the datasheet, or leave 5 mA.
- 5Fit the standard resistor shown and choose zener and resistor wattages at or above the suggestions.
Formula and method
The series resistor drops the difference between the input and the zener voltage and carries the sum of the load and zener currents. It is sized at the worst case for regulation — lowest input voltage and highest load — so that the zener still receives its minimum current. The calculator then rounds down to the next standard value so that this minimum is still met.
The zener’s worst case is the opposite corner: the highest input voltage with the lightest load, when almost all the resistor current is shunted through the diode. Its dissipation is Vz × Iz. The resistor’s worst-case dissipation is (Vin,max − Vz)²/Rs. A 2× safety margin is applied when suggesting standard wattage ratings, because zener power ratings assume ideal heat-sinking at 25 °C.
- Rs
- Series resistor (Ω)
- Vin
- Unregulated input voltage (min and max)
- Vz
- Zener breakdown voltage
- IL
- Load current (min and max)
- Iz,min
- Minimum zener current for regulation
- Pz, PR
- Power dissipated in the zener and resistor
Worked examples
5.1 V rail from a 10–14 V supply, 20 mA load
Rs = (10 − 5.1) ÷ (20 + 5 mA) = 196 Ω, so the next lower E24 value is 180 Ω. At 14 V with no load the zener takes (14 − 5.1)/180 = 49.4 mA and dissipates 0.252 W — just over half of a 0.5 W part, so a 1 W zener (such as a 1N4733A) keeps a 2× margin. The resistor dissipates 0.44 W, so use a 1 W resistor too.
12 V zener from 24 V, 50–100 mA load
Rs = (22 − 12) ÷ 0.110 A = 90.9 Ω; the next lower E24 value is 82 Ω. At 26 V the resistor carries 170.7 mA; with the load at 50 mA the zener takes 120.7 mA and 1.45 W. That needs a 3 W zener, and the resistor burns 2.39 W, so it needs a 5 W rating.
3.3 V reference from 9 V, 2 mA load, E12 resistor
Rs = (8 − 3.3) ÷ 5 mA = 940 Ω, rounded down to 820 Ω in the E12 series. The zener then sees 3.73–5.56 mA and dissipates only 18 mW, so a standard 0.25 W or 0.5 W zener is plenty.
Frequently asked questions
How do I calculate the resistor for a zener diode?+
Use Rs = (Vin,min − Vz) / (IL,max + Iz,min). Take the lowest input voltage, subtract the zener voltage, and divide by the full load current plus the minimum zener current. Then pick the next lower standard resistor value.
What happens if the series resistor is too large?+
At low input voltage or high load the resistor cannot supply enough current, the zener drops out of breakdown and the output voltage sags below Vz. Smaller resistors keep regulation but increase wasted power in the zener at light load.
What wattage zener diode do I need?+
Calculate the worst case Pz = Vz × Iz at maximum input voltage and minimum load, then choose a zener rated at least twice that. Common ratings are 0.5 W (DO-35), 1 W/1.3 W (DO-41) and 3–5 W packages.
What is the minimum zener current?+
Below a knee current (Izk, often 0.25–1 mA for small zeners) the diode does not regulate well. Datasheets specify the nominal voltage at a test current Izt — about 5 mA for small 500 mW glass zeners and 20–75 mA for 1 W 1N47xx parts. Running well below Izt still regulates, but the voltage sits a little under the nominal value.
Is a zener regulator efficient?+
No. The resistor and zener always draw the full design current from the supply, even at light load, so efficiency is often under 50%. For loads above a few tens of milliamps a linear regulator (such as an LM317) or a buck converter is usually better.