Zener Diode Resistor Calculator

Q: Why is the Zener power calculated with no load?

The worst case for Zener power dissipation is when the load is disconnected and all of the series current flows through the Zener. Sizing the Zener for this condition guarantees it survives a load fault or hot-swap.

Q: When should I use a 1/2 W or higher resistor?

Standard 1/4 W (0.25 W) resistors should typically be derated to around 60% of rated power for reliability. If calculated dissipation exceeds about 0.25 W, step up to 1/2 W or 1 W. The calculator flags this automatically.

Calculate the correct series resistor for a Zener diode voltage regulator. Enter the supply voltage, Zener voltage, load current, and Zener keep-alive current to get the resistor value, both power dissipations, and the nearest E24 standard value.

Input Voltage (V_in) V

Zener Voltage (V_z) V

Load Current (I_L)

Zener Keep-Alive Current (I_z)

Minimum current to keep the Zener in the breakdown region (from datasheet).

Total Series Current (I_s) —

Series Resistor (R_s) —

Nearest E24 Standard Value —

Resistor Power Dissipation (P_Rs) —

Zener Power Dissipation, No Load (P_z) —

💡 Sizing for worst case: the series resistor must supply both the load and the Zener keep-alive current at once, but the Zener itself dissipates the most power when the load is disconnected and the full series current flows through it. The calculator handles both cases.

For educational and reference use only. Always verify results before use in real-world designs or safety-critical applications. For more information, see Calculation Assumptions and Disclaimer.

How to Use This Calculator

Enter the input supply voltage (V_in) feeding the circuit and the Zener diode's nominal voltage (V_z). Then enter the load current (I_L) your circuit draws and the Zener's keep-alive current (I_z) — the minimum current required to hold the diode in reverse breakdown, taken from the Zener's datasheet.

Currents can be entered in milliamps (mA) or microamps (µA) using the dropdown next to each field. Results update as you type.

You will get the series resistor value, the nearest E24 standard resistor, the power the resistor must dissipate, and the worst-case power the Zener must handle (calculated for the no-load condition). If V_in is not higher than V_z, or if the resistor dissipation exceeds 0.25 W, a warning will appear.

Formula

Total current through the series resistor:

I_s = I_z + I_L

Series resistor value:

R_s = (V_in − V_z) / I_s

Resistor power dissipation:

P_Rs = (V_in − V_z) × I_s

Worst-case Zener power dissipation (load disconnected):

P_z = V_z × I_s

Where: V_in = source supply voltage (V), V_z = Zener breakdown voltage (V), I_L = load current (A), I_z = Zener keep-alive current (A), I_s = total series current (A), R_s = series resistor value (Ω), P_Rs = resistor power dissipation (W), P_z = Zener power dissipation (W).

Example

Given: V_in = 12 V, V_z = 5.1 V, I_L = 20 mA, I_z = 10 mA

Total current: I_s = 0.010 + 0.020 = 0.030 A (30 mA)

Resistor value: R_s = (12 − 5.1) / 0.030 = 6.9 / 0.030 = 230 Ω

Resistor power: P_Rs = 6.9 × 0.030 = 0.207 W

Zener power (no load): P_z = 5.1 × 0.030 = 0.153 W

Use a 220 Ω (nearest E24) resistor rated 1/4 W or higher, with a Zener rated at least 0.25 W to give margin above the 0.153 W worst case.

Frequently Asked Questions

How does a Zener diode voltage regulator work?
A Zener diode is reverse-biased so it conducts at a known breakdown voltage. A series resistor drops the difference between the supply voltage and the Zener voltage, while the Zener clamps the output to its rated voltage. Together they form a simple shunt regulator that's cheap and reliable for low-current loads.

Why does the calculator use I_z + I_L for the resistor current?
The series resistor carries the total current flowing into the regulator node. That current splits between the Zener (I_z keep-alive) and the load (I_L). Sizing the resistor for I_z + I_L ensures the Zener stays in regulation even when the load is drawing its maximum current.

What is the E24 standard resistor series?
E24 is a standardised set of 24 preferred resistor values per decade: 1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1. Picking the nearest E24 value makes sourcing far easier than using an arbitrary resistance.

Why is the Zener power calculated with no load?
The worst case for Zener dissipation is when the load is disconnected and all of the series current flows through the Zener instead. Sizing the Zener for this condition guarantees it survives a load fault, hot-swap, or transient disconnection.

When should I use a 1/2 W or higher resistor?
Standard 1/4 W (0.25 W) resistors should usually be derated to around 60% of their rated power for long-term reliability. If the calculated dissipation exceeds about 0.25 W, step up to 1/2 W or 1 W. The calculator flags this automatically with a warning.