Zener Diode with Series Resistor
The series resistor R sets the current through the Zener diode. The Zener clamps at Vz, and the resistor drops Vin − Vz. Simple, two-component circuit.
Zener Diode Resistor Calculator
One question: what series resistor do I need for my Zener diode? Enter input voltage, Zener voltage, desired current, and power rating. The calculator returns the resistor value (E24), actual Zener current, power dissipation in both components, and the safe input voltage range. Two components, one current path, one formula.
The Formula
Vin minus Vz gives the voltage dropped across the resistor. Divide by the desired Zener current to get the resistance. The underlying relationship is Ohm’s law applied to the series resistor.
Calculator Inputs
Vin — DC supply voltage. Must be higher than Vz.
Vz — Zener diode rated voltage from the data sheet. Common values: 3.3 V, 5.1 V, 6.2 V, 9.1 V, 12 V.
Iz — desired Zener current. Typical range: 5–20 mA for standard Zeners. Higher current = more stable voltage but more power wasted. Use the data sheet’s test current for the most accurate Vz.
Pz(max) — maximum Zener power rating. Common values: 250 mW, 500 mW, 1 W, 5 W. The calculator checks actual power against this limit.
Why the Resistor Rounds Down
The E24 value is rounded down, not up. A lower resistor pushes more current through the Zener, keeping it reliably in breakdown. Rounding up reduces current and risks the Zener dropping out of regulation. This is the opposite of LED resistor sizing, where rounding up protects the LED from overcurrent.
Worked Example — 5.1 V Zener on 12 V Supply
Vin = 12 V, Vz = 5.1 V, Iz = 10 mA, Pz(max) = 500 mW.
Nearest E24 (rounded down): 680 Ω
Iz(actual) = 6.9 / 680 = 10.15 mA
Pz = 5.1 × 0.01015 = 51.8 mW — 10.4% of 500 mW rating
PR = 6.9 × 0.01015 = 70.0 mW
Classic microcontroller voltage reference. The Zener runs at 10% of its power rating — plenty of thermal margin.
Safe Input Voltage Range
Vin(max) = Vz + (Pz(max) / Vz) × R = 5.1 + (0.5 / 5.1) × 680 = 71.7 V
With the 680 Ω resistor, the circuit works from ~5.8 V to ~71.7 V input. That is an enormous safe range — the 500 mW Zener can handle the current even at very high input voltages because the resistor limits how much gets through.
Worked Example — 3.3 V Zener on 5 V Supply
Vin = 5 V, Vz = 3.3 V, Iz = 5 mA, Pz(max) = 250 mW.
Nearest E24 (rounded down): 330 Ω
Iz(actual) = 1.7 / 330 = 5.15 mA
Pz = 3.3 × 0.00515 = 17.0 mW — 6.8% of 250 mW rating
Vin(min) = 3.3 + 0.001 × 330 = 3.63 V
Vin(max) = 3.3 + (0.25 / 3.3) × 330 = 28.3 V
Common level-shifting or sensor supply reference. Low headroom (1.7 V) means Vin cannot drop much below 5 V before the Zener loses regulation.
Worked Example — 12 V Zener on 24 V Supply (1 W)
Vin = 24 V, Vz = 12 V, Iz = 20 mA, Pz(max) = 1 W.
Nearest E24 (rounded down): 560 Ω
Iz(actual) = 12 / 560 = 21.4 mA
Pz = 12 × 0.0214 = 257 mW — 25.7% of 1 W rating
PR = 12 × 0.0214 = 257 mW
Vin(min) = 12 + 0.001 × 560 = 12.56 V
Vin(max) = 12 + (1.0 / 12) × 560 = 58.7 V
Higher current (20 mA) gives a more stable 12 V reference at the cost of more power. The 1 W Zener has ample headroom at 26% utilisation. Safe up to nearly 59 V input.
Safety Status Indicators
Yellow — Power above 80% of rating. Circuit works but has little thermal margin. Consider a higher-wattage Zener.
Red — Power exceeds rating. The Zener will overheat. Use a higher-wattage Zener or increase the series resistor (which reduces current).
Choosing the Zener Current
Low Current (5 mA)
Minimum for stable regulation. Lowest power waste. Use for voltage references feeding high-impedance inputs (op-amp non-inverting input, ADC reference, comparator threshold). Vz accuracy is slightly worse than at the data sheet test current.
Medium Current (10–20 mA)
Standard operating point. Vz is closest to the data sheet value because most Zeners are tested at 10–20 mA. Good stability, moderate power. The default choice for most applications.
High Current (>20 mA)
Best voltage stability but high power waste. Only justified when the Zener must absorb significant current variation or when the data sheet specifies a high test current. Check Pz carefully at high currents.
Common Applications
Voltage reference — stable Vz feeds the non-inverting input of an op-amp or the reference pin of an ADC. No load current through the Zener — this calculator is the right tool.
Overvoltage clamping — Zener clamps a signal line to Vz, protecting downstream components from voltage spikes. The series resistor limits the current during a clamp event.
Bias supply — Zener provides a fixed gate-drive voltage for a MOSFET or a bias point for a transistor circuit. Low current, minimal load.
Level shifting — 3.3 V Zener on a 5 V line creates a 3.3 V reference for logic-level conversion.
This Calculator vs. Voltage Divider with Zener Calculator
Use this calculator when there is no load (or negligible load) on the Zener — voltage references, clamping, biasing. Four inputs, one answer.
Use the Voltage Divider with Zener Calculator when the Zener supplies current to a load — regulated power for a small circuit. It adds load current, maximum load analysis, and worst-case safety checks.
Frequently Asked Questions
Last updated: March 2026