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Zener Diode Calculator

Design a shunt Zener regulator from the input and load extremes, select a practical series resistor, and check its Zener and resistor dissipation.

Design inputs

Input-voltage range

V
V

Zener model

V
Shunt-regulator circuit
Zener shunt regulator with an input source, series resistor, Zener diode and parallel load

Equations

RS,max=Vin,min−VZIL,max+IZ,minR_{S,max}=\dfrac{V_{in,min}-V_Z}{I_{L,max}+I_{Z,min}}
RS,min=Vin,max−VZIL,min+PZ,rated/VZR_{S,min}=\dfrac{V_{in,max}-V_Z}{I_{L,min}+P_{Z,rated}/V_Z}
IZ=Vin−VZRS−ILI_Z=\dfrac{V_{in}-V_Z}{R_S}-I_L
Load-current envelope
Preferred resistor values

Series resistor under test

Worst-case sizing result
Safe nominal resistor window106.295 Ω – 262.857 Ω
Suggested preferred resistor240 Ω
Selected series resistor240 Ω
Zener current at regulation corner7.381 mA
Peak Zener dissipation221.447 mW
Recommended resistor rating1 W
Regulation checkRegulation maintained

The low-input, full-load corner still leaves the requested Zener current with the resistor at its positive tolerance.

Zener thermal checkWithin rated power

The high-input, minimum-load Zener dissipation stays within the entered rating with the resistor at its negative tolerance.

Selected-resistor checkInside the safe window

This selected value satisfies the two DC worst-case checks in this model.

Operating-corner analysis

The first row uses the positive resistor tolerance to test regulation at low input and full load. The second uses the negative tolerance to test Zener dissipation at high input and minimum load.

Configuration ready
ConditionVinILRSIRIZPZPRVout
Regulation: low input / full load12 V20 mA252 Ω27.381 mA7.381 mA37.643 mW188.929 mW5.1 V
Thermal: high input / minimum load15 V0 nA228 Ω43.421 mA43.421 mA221.447 mW429.868 mW5.1 V

This is an ideal DC shunt-regulator calculation. Before release, check the Zener tolerance, dynamic impedance, temperature coefficient, package derating, resistor working voltage, startup and surge current, and fault conditions in the chosen datasheets.

Shunt-regulator guide

Size a Zener diode against the two real worst cases

A Zener diode and series resistor can provide a reference or a small regulated supply. Selecting it is not just a voltage division: maximum load can collapse Zener current, while a light load can concentrate too much dissipation in the diode.

Method and assumptions

The tool models a Zener connected as a shunt after a series resistor. At minimum Vin and maximum IL, it checks that remaining Zener current reaches the entered knee current with the resistor at positive tolerance. At maximum Vin and minimum IL, it checks that excess current at negative resistor tolerance stays within the diode rating. The proposed value is the largest E-series resistor inside that nominal window: it limits wasted current without giving up worst-case regulation.

Inputs to verify

Input-voltage range

Use the limits the circuit actually sees, including supply tolerance, battery state, slow ripple, and any relevant sustained overvoltage. Nominal voltage alone is not enough.

Zener and load currents

Minimum Zener current should come from the datasheet curve or regulation region, not only its test current. For a load that can disconnect or sleep, set minimum IL to zero.

Power rating and package

Enter the power allowed in the real thermal conditions. A diode’s headline rating is often specified for a particular copper area, ambient temperature, or pulse duration.

Recommended workflow

  1. 1Enter minimum/maximum input and load limits, nominal VZ, minimum Zener current, and the component’s power rating.
  2. 2Choose an E series and apply the proposed resistor, or test the value actually available in your BOM.
  3. 3Read both worst-case rows, then confirm tolerances, heating, surges, and faults against the chosen datasheets.

Checkable example

With Vin from 12 to 15 V, a 5.1 V 0.5 W Zener, IZ min = 5 mA, a 0 to 20 mA load, and a 5% resistor, the safe nominal window is about 106 to 263 Ω. The E24 value 240 Ω leaves about 7.4 mA in the Zener at 12 V / 20 mA with 252 Ω, and dissipates about 221 mW in it at 15 V / 0 mA with 228 Ω.

Limits to keep in mind

This is a DC model that assumes VZ is constant once the minimum current is reached. It does not model dynamic impedance, VZ spread, temperature, noise, capacitive stability, pulses, inrush, or output-short behavior. A shunt Zener is chiefly suited to low power; a series regulator or converter is often more efficient when load varies widely.

Frequently asked questions

Why is minimum load so important?

When the load consumes less, nearly all current through the series resistor can flow in the Zener. This is usually the condition that sets its maximum dissipation.

Can I use nominal VZ as a precise output?

Not without analysing the datasheet. VZ is specified at a test current and changes with current, temperature, tolerance, and noise. For a precise reference, prefer a voltage reference or regulator designed for that role.

References to consult

  • Method references: chosen Zener’s datasheet (I-V curve, VZ across temperature, power and derating), IEC 60063 preferred values, and the series resistor’s overload characteristics.

Original educational content, reviewed for technical clarity on 20 August 2026. Always verify datasheets, applicable standards, and your design before power-up or manufacture.