Zener Diode Biasing

Size the series resistor and verify currents and dissipation for a simple zener shunt regulator.

How to Use

  1. Enter your input supply voltage and desired zener voltage.
  2. Enter load current (or load resistance) and zener minimum current.
  3. Review the computed resistor, currents, and power dissipation.
  4. Open “Show Work” to see formulas and steps in base units.
Regulator Snapshot
Series resistor + zener shunt + optional load.
Rseries
—
Iseries
—
Izener
—
Pzener
—
Status: —
Vin Rseries Vout ≈ Vz Zener Load Visual heat scales with dissipation (Rseries and Zener).
Inputs & Settings
Enter your voltages and load. Tool computes resistor, currents, and dissipation.
Examples: 5V, 9V, 12V, 24V
Common: 3.3V, 5.1V, 6.2V, 9.1V, 12V
In “Design Rs” mode this is computed. In “Analyze” you can enter your chosen resistor.
If you know load resistance instead, you can switch to “Load as R” below.
Minimum current to keep the zener in regulation (from datasheet or rule-of-thumb).
Common: 500mW, 1W, 5W (varies by part/package).
Common: 250mW (1/4W), 500mW (1/2W), 1W+
If using RL: IL = Vz / RL. Keep RL > 0.
Show Work (step-by-step)
—
Work is shown in base units (V, A, Ω, W) for clarity.

Reference

Core relationships for a shunt zener regulator:

  • Series current: Iseries = (Vin − Vz) / Rseries
  • Zener current: Iz = Iseries − Iload
  • Zener power: Pz = Vz × Iz
  • Resistor power: Pr = (Vin − Vz) × Iseries = Iseries² × Rseries
  • Regulation check: ensure Iz ≥ Iz(min) across conditions
Note: Real zeners have dynamic resistance; this tool uses the common idealized bias model for sizing.

FAQ

Why does zener current increase at no load?

With little/no load, most of the series current flows through the zener, so Iz rises and Pz can become the limiting case.

What happens if Vin is too close to Vz?

You lose headroom. Series current may be too low to keep Iz ≥ Iz(min), so regulation collapses under load.

How do I pick Iz(min) if I don’t have the datasheet?

Use the datasheet when possible. If you must estimate, keep a small margin (a few mA) for small-signal zeners, but always verify against your part’s rating and temperature.

Is a zener regulator efficient?

Not usually. It burns extra power in the series resistor and zener. For higher currents or efficiency, use a linear regulator or switching regulator.

Tool Info

Last updated:

Updates may include UI improvements, unit support, and edge-case handling.