Zener Diode Regulator Calculator

Analyze a shunt regulator or design the series resistor — with exact Zener current, load current, power dissipation, and wattage recommendations.
Circuit Analysis
Resistor Design

Analyze a Zener Shunt Regulator

Series (unregulated) circuit → Zener holds Vout = VZ:
IS = (Vin − VZ) / RS  •  IL = VZ / RL  •  IZ = IS − IL
PZ = VZ × IZ  •  PRS = (Vin − VZ)² / RS
12V → 5.1V
9V → 3.3V
15V → 12V
24V → 5.1V
V
V
Ω
Ω
W
Enter values and press Calculate.

Design the Series Resistor

Worst case = lowest input & full load must still keep the Zener above IZ(min):
RS(max) = (Vin(min) − VZ) / (IZ(min) + IL)
Worst-case Zener power at highest input, no load: PZ(max) = VZ × (Vin(max) − VZ) / RS
12V rail → 5.1V, 20mA
9V rail → 3.3V, 10mA
24V rail → 12V, 30mA
V
V
V
mA
mA
Enter values and press Design.

How a Zener Diode Voltage Regulator Works

A Zener diode connected in reverse bias holds a nearly constant voltage (its Zener voltage VZ) once the reverse voltage reaches breakdown. Placed in parallel with the load and fed through a series resistor RS, it forms a simple shunt regulator: the resistor drops the difference between the supply and VZ, while the diode "shunts" any excess current to ground so the output stays fixed.

Key relationships

QuantityFormulaNotes
Series currentIS = (Vin − VZ) / RSTotal current through RS
Load currentIL = VZ / RLCurrent drawn by the load
Zener currentIZ = IS − ILMust stay > IZ(min) to regulate
Zener powerPZ = VZ × IZMust stay below the diode's rating
Resistor powerPRS = (Vin − VZ)² / RSSize RS wattage accordingly

Regulation condition: the circuit only regulates while IZ > 0 (ideally above IZ(min), typically 5–10 mA). If the load draws so much current that IZ would fall below zero, the Zener stops conducting and the output drops below VZ.

Real-World Applications

Worked example

A 12 V car supply must feed a 5 V, 10 mA sensor. Using a 5.1 V Zener with IZ(min) = 5 mA:
RS = (12 − 5.1) / (5 mA + 10 mA) = 6.9 / 0.015 ≈ 460 Ω → use 470 Ω.
With the load disconnected the Zener carries (12 − 5.1)/470 ≈ 14.7 mA, so PZ = 5.1 × 14.7 mA ≈ 75 mW — a 0.5 W Zener is safe. Try these numbers in the tabs above.

Frequently Asked Questions

How is a Zener different from a normal diode?

Both block reverse current at low voltage, but a Zener is built to conduct safely in reverse once its rated breakdown (Zener) voltage is reached, and to hold that voltage steady. A normal diode would be damaged in that region.

Can a Zener regulator supply a lot of current?

No — it suits a few mA up to roughly 100 mA. For more, add a series-pass transistor (a "Zener + transistor" regulator), or use an IC regulator or buck converter.

How tight is the Zener voltage?

Typical Zeners are ±5% and drift slightly with current and temperature (a few mV/°C). For precision use a dedicated voltage-reference IC. Zeners near 5–6 V have the best temperature stability.

What Zener voltage should I pick?

Choose the diode whose rated VZ equals your target output. Common values: 3.3, 5.1, 6.2, 9.1 and 12 V.

What is the series resistor for?

It limits the current and drops the voltage difference between the supply and the Zener voltage. Without it, the diode would draw unlimited current and be destroyed.

How do I pick the Zener wattage?

Use the highest input voltage with the load disconnected — that is when the diode carries the most current. The design tab computes this worst-case PZ(max) and suggests the next standard wattage (0.25, 0.5, 1, 1.3, 2, 5 W) with margin.

Why did my output drop below VZ?

The load is drawing too much current, so IZ fell to zero and the diode left breakdown. Lower RS or reduce the load. The analysis tab warns you when this happens.

Is a Zener regulator efficient?

No — it wastes power in RS and the diode, so it suits low-current references and light loads. For higher power, use a series pass transistor or a switching regulator (see our buck/boost calculators).