Zener Resistor Calculator

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.