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Zener Resistor Calculator

Find the series resistor for a Zener regulator.

How it is calculated

The tool sizes the series resistor from Ohm's law applied to the voltage left over after the diode: R = (Vin − Vz) ÷ Iz in ohms and P = (Vin − Vz) × Iz in watts. Vin and Vz are in volts and Iz is in amps (0.02 means 20 mA). Vin must be above Vz, Vz cannot be negative and Iz must be above zero, otherwise nothing is returned. The power shown is the heat in the resistor only; the diode itself dissipates Vz × Iz, which you work out separately. The calculation assumes the whole current flows through the diode, so it ignores any load current taken from the regulated output.

Worked example

A 12 V supply, a 5.1 V Zener and a target diode current of 20 mA = 0.02 A. Voltage across the resistor = 12 − 5.1 = 6.9 V. R = 6.9 ÷ 0.02 = 345.00 Ω. Power in the resistor = 6.9 × 0.02 = 0.1380 W, so a 1/4 W resistor would run close to its limit and a 1/2 W part gives a safer margin. The diode itself dissipates 5.1 × 0.02 = 0.102 W. The closest standard value is then chosen near 345 Ω.

Try this example →

Common mistakes

  • Entering the current in milliamps. The field is amps, so 20 is read as 20 A, giving a resistor of 0.35 Ω and 138 W; type 0.02 for 20 mA.
  • Ignoring the load. Any current drawn from the output flows through the same resistor, so the diode current Iz must be what remains after the load takes its share; size the resistor for the maximum supply and load together.
  • Choosing a resistor rated exactly at the computed power. Use a rating with a margin, such as double, because it will run hot at that dissipation.
  • Forgetting the diode's own power limit. The diode dissipates Vz × Iz, and if the load disconnects the whole current goes through it.
  • Using Vin below or equal to Vz. The supply must exceed the Zener voltage with some headroom, otherwise there is no resistor value that works and the tool gives no result.

About this calculator

A Zener diode voltage regulator holds a fixed output voltage by letting the diode conduct in reverse breakdown, but it needs a series resistor to limit current to a safe level; that resistor is sized by R = (Vin − Vz) / Iz, where Vin is the supply voltage, Vz is the diode's rated Zener voltage, and Iz is the target regulating current through the diode.

Getting R right matters on both sides: too large and the diode can't draw enough current to regulate under load, too small and too much current flows: the resistor dissipates P = (Vin − Vz) × Iz and the diode Vz × Iz as heat, risking thermal failure.

Electronics hobbyists building simple voltage references, technicians repairing power supplies, and electrical engineering students prototyping their first regulator circuits all use this calculation before reaching for a resistor from the parts bin.

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