Zener Diode Regulator Calculator
Enter the input range, Zener voltage and load range to size the series resistor and check Zener power.
Step-by-step
- Enter valid values to begin.
Model: Vz is the datasheet value at its test current and the Zener is treated as Vz with a small series dynamic resistance rz (about 7–10 Ω for 1N47xx parts at 5–12 V, tens of ohms for low-current or high-voltage parts: read Zzt from the datasheet). The Zener limits in the 1N47xx / 1N5xxx families are specified at 25–50 °C and must be derated at higher ambient temperature, and Vz also drifts with temperature and tolerance (typically ±5% for an “A” suffix). Resistor values are chosen from the IEC 60063 E12 series. Load and line regulation are small-signal linear estimates.
Zener Shunt Regulator: Sizing the Series Resistor for the Worst Case
Core Engineering Principles
A Zener regulator is just a resistor feeding a diode that holds roughly Vz across itself while it conducts reverse current. The series resistor Rs sees whatever the input does, and the Zener absorbs the difference between that current and the load current. So we design at two corners. At minimum input and maximum load the Zener must still carry its minimum current, or regulation collapses. At maximum input and minimum load the Zener carries nearly everything, and that’s where it overheats.
The Zener is not a perfect source, either. Its dynamic impedance rz, a few ohms for a 12 V part at tens of milliamps and much higher near the knee, turns every milliamp of load change into a small voltage change, and it adds a fraction of the input ripple. We also derate: the 1 W 1N47xx family is rated at 50 °C lead temperature, and an enclosure at 70 °C gives you far less than the headline watt. Efficiency is poor by nature, and above about 25 mA of load a pass transistor or LDO is a better design.
Pz = Vz × Iz(max) • PRs = (Vin(max) − Vz)² / Rs • ΔVout ≈ (rz ∥ Rs) × ΔIL
NEC & Standard References
The JEDEC registered 1N47xx series (1N4728A to 1N4764A, 1 W), the 1N52xx series (0.5 W) and the 1N53xx series (5 W) define Vz at a test current Izt, the dynamic impedance Zzt, the knee impedance, the power rating and the derating slope. IEC 60747 is the family of standards for discrete semiconductor devices. IEC 60063 defines the E12 resistor values used here, and IEC 60115-1 is the generic specification for fixed resistors, whose power rating assumes a specified ambient temperature.
1. Rs ≤ (15 − 12) / (20 mA + 5 mA) = 120 Ω, which is an E12 value.
2. Iz(max) = (20 − 12) / 120 − 5 mA = 61.7 mA.
3. Pz = 12 × 61.7 mA = 0.74 W, 74% of the 1 W rating.
4. PRs = 8² / 120 = 0.533 W, so we use a 2 W resistor for 2× headroom.
5. Load regulation ≈ (9 ∥ 120) × 15 mA = 126 mV, about 1.05%.
6. Efficiency at 18 V and 20 mA load is only 26.7%.
- Check the no-load case. When the load disconnects, all the current goes through the Zener at the highest input; that is the power that kills them.
- Remember temperature. The Zener rating falls with lead temperature, and Vz itself drifts, so don’t use it as a precision reference.
- Lighter loads need a bigger knee margin. Keep Iz(min) well above the datasheet knee current, or the output sags and gets noisy.
- Hot resistors. Rs dissipates more than the Zener in many designs, so rate it generously and space it from other parts.