Electronics · Free Calculator
Zener Diode Series Resistor Calculator
Design a simple zener shunt regulator: the series resistor value and the worst case power in the zener and the resistor.
Series resistorZener powerWorst caseShunt regulator
V
V
V
Calculated resistor236 Ω
Use E24 value220 Ω
Zener power (no load, max input)183.1 mW
Suggested zener rating0.5 W
Resistor power (max input)283.7 mW
Maximum zener current35.91 mA
How this result was calculated
- R = (Vin,min − Vz) ÷ (IL,max + Iz,min) = (11 − 5.1) ÷ 25 mA = 236 Ω
- Rounded down to 220 Ω so regulation holds at minimum input
- Worst case zener current with no load: (13 − 5.1) ÷ 220 Ω = 35.91 mA
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Zener Regulator Design
R = (Vin,min − Vz) ÷ (IL,max + Iz,min)
Pz,max = Vz × (Vin,max − Vz) ÷ R
PR = (Vin,max − Vz)² ÷ R
Pz,max = Vz × (Vin,max − Vz) ÷ R
PR = (Vin,max − Vz)² ÷ R
Worked example
Input 11 to 13 V, 5.1 V zener, 20 mA load, 5 mA minimum zener current: R = 5.9 ÷ 0.025 = 236 Ω, use 220 Ω. With no load at 13 V the zener carries 35.9 mA and dissipates 0.18 W.
Key insight: the zener must survive the worst case: maximum input with the load disconnected, when all the current flows through it.
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Frequently Asked Questions
How do I calculate a zener series resistor?
R = (Vin min − Vz) ÷ (I load max + Iz min).
What minimum zener current should I use?
About 5 mA for small zeners, or the test current Izt from the data sheet for best voltage accuracy.
When should I use a regulator IC instead?
When load current is above a few tens of mA or efficiency matters; a zener regulator wastes power continuously.
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