Zener Diode Resistor Calculator
Series resistor for a zener shunt regulator from the supply range, zener voltage, minimum zener current and load, with worst-case zener and resistor power.
The largest series resistor that keeps the zener regulating at the lowest supply and full load, and the heat the zener and resistor must survive at the highest supply with the load removed.
Example: An 11–13 V supply, 5.1 V zener needing 5 mA and a 20 mA load: R_s ≤ 236 Ω, so 220 Ω (E24). At 13 V with no load the zener carries 35.9 mA (0.18 W) and the resistor dissipates 0.28 W.
Size the resistor for the worst case.
Then check the other worst case.
Why the resistor is set at minimum supply and full load, why the power is checked at maximum supply and no load, and what the zener datasheet contributes.
The series resistor
The resistor drops the difference between the supply and the zener voltage, and the current through it splits between the load and the zener. The zener only regulates while it carries at least its minimum current (I_z,min, from the datasheet — the knee of its curve). The hardest moment is the lowest supply with the largest load, so R_s must be no larger than (V_in,min − V_z) ÷ (I_z,min + I_load). The page reports that limit and the largest E24 value under it as a starting point; you may enter your own resistor and see whether it still regulates.
Power at the other extreme
With the resistor fixed, the most current flows at the highest supply, and if the load is disconnected all of it goes through the zener: P_z = V_z × (V_in,max − V_z) ÷ R_s. That figure is compared with the zener's power rating you enter, and the resistor sees (V_in,max − V_z)² ÷ R_s at the same moment. A wide supply range or a large load makes a shunt regulator wasteful — the efficiency figure shows how much of the supply power reaches the load — and points to a series regulator instead.
What the datasheet still owns
Zener voltage is specified at a test current and moves with current through the dynamic resistance, and with temperature (positive coefficient above about 5.6 V, negative below); ratings derate with temperature. None of those curves are on this page — enter V_z, I_z,min and the rating from the part you use and allow for the spread. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- Zener shunt regulator design relations — standard electronics (e.g. Horowitz & Hill, The Art of Electronics, §1.2.6 and §9.x on zener references); datasheet values entered by the user
- IEC 60063:2015, Preferred number series — E24 (for the suggested resistor only)
Last reviewed 19 September 2026. How results are checked: How we verify.