Zener Diode Regulator Calculator
Series resistor, power dissipation and safe load range for a zener shunt regulator.
Example: Regulating 12 V down to 5.1 V at 20 mA needs a 276 Ω series resistor — use 270 Ω with a zener rated above 130.3 mW.
Formula
R_s = (V_in − V_z) / (I_load + I_z,min) P_z,max = V_z × (V_in − V_z) / R_sWorked example
Regulating 12 V down to 5.1 V at 20 mA needs a 276 Ω series resistor — use 270 Ω with a zener rated above 130.3 mW.
R_s = (V_in − V_z) / (I_load + I_z,min)
(12 V − 5.1 V) / (20 mA + 5 mA)
276 Ω
The 5 mA keeps the zener in its knee so it actually regulates.
P_z,max = V_z × (V_in − V_z) / R_s
5.1 V × 6.9 V / 270 Ω
130.3 mW
This is the no-load case, where the zener carries everything.
Frequently asked questions
How do I size the resistor for a zener regulator?
Divide the voltage across the resistor by the total current it must pass — load current plus about 5 mA to keep the zener regulating. For 12 V down to 5.1 V at 20 mA, that is 6.9 V / 25 mA = 276 Ω, so 270 Ω from E24.
Why does the zener get hottest with no load?
Because the series resistor passes a roughly fixed current regardless of the load. Whatever the load does not take, the zener shunts to ground. Disconnect the load entirely and the diode absorbs all of it — that is the case you must size for.
How much current can a zener regulator supply?
Not much, practically. Beyond about 50–100 mA the resistor and diode dissipation get unreasonable and regulation is poor because the zener has real dynamic resistance. Use a proper regulator above that — it will be smaller and cooler.
Why is a zener regulator so inefficient?
It burns the difference between input and output as heat, and it draws full current whether the load needs it or not. Even at full load most of the input power ends up as heat, and at no load all of it does.
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