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Electrical & Electronics

Zener Diode & Series Resistor Calculator

Choose or check a Zener regulator resistor across supply and load extremes, including tolerance and power.

01

Your inputs

Fixed-voltage DC estimate; power assumes regulation. Use derated ratings and datasheet minimum current. Check voltage tolerance and temperature effects.

The rating is at least twice the calculated dissipation. Apply the manufacturer’s temperature derating.

Vishay 84117

02

Your results

Resistance
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Recommended resistor
Allowed nominal resistance range
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Minimum Zener current estimate
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Maximum Zener current estimate
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Worst-case Zener dissipation
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Resistor dissipation
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Suggested resistor rating
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Formula—

Size a Zener resistor across supply and load extremes

Find a preferred series resistor or analyse an existing value. Check minimum Zener current and maximum dissipation using supply, load and resistor-tolerance extremes. The model assumes constant Zener voltage while regulated; it does not predict output after regulation fails.

Step by step

  1. Choose design or analyse mode and enter supply bounds, Zener voltage and load-current bounds.
  2. Enter required minimum Zener current, its allowed power and resistor tolerance. Choose a preferred series or existing resistance.
  3. Review the feasible resistance interval, current limits, Zener power and resistor rating together.

Settings and limits

Worst-case corners
Low supply with high load checks regulation. High supply with low load checks Zener heating; resistor tolerance changes both.
Preferred resistance
Design selects the largest supported value inside the feasible interval. An empty interval or missing series value gives no recommendation.
Power rating
The suggested resistor rating allows headroom above calculated loss. Verify ambient derating and the component's actual ratings.

Worked example

Use the design example below. The selected resistor meets the entered current and power bounds despite tolerance.

Example input

Vin,min = 10 V; Vin,max = 14 V; Vz = 5.1 V
Iload,min = 5 mA; Iload,max = 20 mA
Iz,min = 5 mA; Pz,max = 500 mW
t = 5%; E24
Expected result
R / I / P≈
Rlow90.920927 Ω
Rhigh186.666667 Ω
R180 Ω
Iz,min5.925926 mA
Iz,max47.046784 mA
Pz,max239.938596 mW
PR,max463.216374 mW
PR,rated1 W

Increase the maximum load and recalculate. The feasible interval can disappear even though the nominal Zener voltage is unchanged.

Questions and troubleshooting

Why is there no recommended resistor?

The constraints may conflict or the selected series may contain no value in the interval. Recheck limits and compare supported series.

Is the output exactly the entered Zener voltage?

No. Real voltage varies with current, tolerance and temperature; check the device characteristics.

Can I ignore a low-current warning?

No. The assumed regulated voltage is then unreliable. Use a suitable model or measurements to determine the actual output.