A zener diode reverse-biased into its breakdown region holds a nearly constant voltage across itself. Put a resistor in series from a higher supply and you get a simple shunt regulator: the resistor carries the total current and drops the surplus voltage, while the zener absorbs whatever current the load doesn't take, keeping its voltage fixed. Designing it means choosing the series resistor so the zener always keeps a minimum holding current, and rating both parts for their worst-case power.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: Wikipedia: Zener diode and standard analog-design texts.
The zener regulator formulas
Sized at maximum load and minimum input, this guarantees the zener still keeps its holding current Iz,min. The total current through Rs is roughly constant, so the worst case for the zener is when the load is removed:
Pick a standard resistor value at or just below Rs (so the zener current never drops below its minimum), and choose zener and resistor power ratings with roughly a 2× margin over these worst-case figures.
Worked example — 5.1 V reference from 12 V
Scenario: Vin = 12 V, a 5.1 V zener, load 20 mA, minimum zener current 5 mA.
So a 270 Ω quarter-watt resistor and a 5.1 V half-watt zener make a robust 5.1 V reference. For higher load currents, a linear regulator wastes far less power than a zener.
Frequently Asked Questions
Rs = (Vin − Vz) ÷ (Iload + Iz,min). For 12 V in, 5.1 V zener, 20 mA load, 5 mA min: Rs = 6.9 ÷ 0.025 = 276 Ω → use 270 Ω.
P = (Vin − Vz)² ÷ Rs. For 270 Ω: 6.9² ÷ 276 ≈ 0.17 W, so a 0.25 W resistor is fine.
It happens at no load, when all the current flows through the zener: Iz,max = (Vin − Vz) ÷ Rs, Pz = Vz × Iz,max. Choose a zener rated above this with ~2× margin.
Below a few mA the zener leaves the sharp breakdown knee and stops regulating. Keep a holding current — about 5–10% of the maximum zener current, or roughly 5 mA for small zeners — flowing even at full load.
Use a zener for low-current references and simple bias rails. For higher current or better regulation and efficiency, a linear regulator IC or switching regulator is better — a zener wastes the surplus current as heat all the time.