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 excess power — P = (Vin − Vz) × Iz — burns up as heat in both the resistor and the diode, 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.