MAXScanner electronics calculator
Updated 2026-09-16
Zener Resistor Calculator — Series Resistor, Current & Power
Size a basic Zener shunt-regulator series resistor from supply, Zener voltage, load current and target Zener current, then estimate resistor and diode power.
Inputs
Results
Series resistor 276 Ω
Resistor power 0.1725 W
Zener power 0.0255 W
Total current 25 mA
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Your calculation, step by step
- 1.Required current: (20 + 5) mA = 0.025 A.
- 2.Series resistor: R = (Vs−Vz)/I = (12−5.1)/0.025 = 276 Ω.
Formula, example and limits
R=(Vs−Vz)/(IL+Iz); PR=(Vs−Vz)(IL+Iz); PZ=Vz×Iz.
12 V to 5.1 V at 20 mA load plus 5 mA Zener current needs about 276 Ω.
- Simple DC shunt regulator with constant entered load current.
- Real design must check tolerances, dynamic resistance, temperature and source variation.
Frequently asked questions
Why include Zener current?
A Zener needs current in addition to load current to operate in its regulation region.
Is the calculated resistor a final design value?
No; select a standard value and re-check worst-case currents and power against the diode datasheet.