Zener Voltage Regulator Circuit
The series resistor R limits the total current from Vin. The Zener diode clamps the output at Vz, absorbing any excess current not taken by the load. As load current increases, Zener current decreases — regulation is maintained as long as Iz stays above its minimum.
Voltage Divider with Zener Calculator
A Zener diode conducts in reverse at a precise voltage (Vz), clamping the output regardless of input fluctuations or moderate load changes. A series resistor limits the current. This calculator sizes that resistor, checks whether the Zener stays within safe operating limits, and shows the maximum load current before regulation is lost.
The Formula
Itotal = Iz + IL — total current through the series resistor
Pz = Vz × Iz — power dissipated in the Zener
PR = (Vin − Vz) × Itotal — power dissipated in the resistor
IL(max) = (Vin − Vz) / R − Iz(min) — maximum load current
The series resistor drops the difference between Vin and Vz. The current through it splits: part goes through the load (IL), the rest through the Zener (Iz). The Zener must always carry at least Iz(min) — typically 5 mA — to maintain regulation. If the load draws so much current that Iz drops below this minimum, the output voltage collapses. The underlying relationship is Ohm’s law applied to the series resistor.
Calculator Inputs
Vin — unregulated supply voltage. Must be higher than Vz.
Vz — Zener voltage (regulated output). Common values: 3.3 V, 5.1 V, 6.2 V, 9.1 V, 12 V. From the Zener data sheet.
Iz(min) — minimum Zener current for regulation, typically 5 mA. Below this, the Zener voltage becomes poorly defined and the output drifts.
IL — load current drawn from the regulated output.
Pz(max) — maximum Zener power rating from the data sheet. Common values: 250 mW, 500 mW, 1 W, 5 W.
Why the Resistor Is Rounded Down
Unlike LED calculators (which round up for safety), the Zener calculator rounds the E24 resistor value down. A lower resistance pushes more current through the circuit, keeping the Zener comfortably above its minimum current. Rounding up would reduce current and risk losing regulation under load.
Worked Example — 5.1 V from 12 V
Vin = 12 V, Vz = 5.1 V, Iz(min) = 5 mA, IL = 20 mA, Pz(max) = 500 mW.
Nearest E24 (rounded down): 270 Ω
Itotal = 6.9 / 270 = 25.6 mA
Iz = 25.6 − 20.0 = 5.6 mA — above 5 mA minimum
Pz = 5.1 × 0.0056 = 28.4 mW
PR = 6.9 × 0.0256 = 176 mW
Worst Case — No Load
When the load disconnects, all current flows through the Zener:
Pz(no load) = 5.1 × 0.0256 = 130 mW
130 mW is well within the 500 mW Zener rating. Safe.
Maximum Load Current
Above 20.6 mA, the Zener current drops below 5 mA and regulation is lost. This is the practical limit of the circuit.
Worked Example — 3.3 V from 5 V
Vin = 5 V, Vz = 3.3 V, Iz(min) = 5 mA, IL = 10 mA, Pz(max) = 250 mW.
Nearest E24 (rounded down): 110 Ω
Itotal = 1.7 / 110 = 15.5 mA
Iz = 15.5 − 10.0 = 5.5 mA
Pz(no load) = 3.3 × 0.0155 = 51 mW — safe for 250 mW Zener
IL(max) = 15.5 − 5.0 = 10.5 mA
Only 10.5 mA available for the load. The small voltage headroom (1.7 V) limits current. If you need more load current from a 5 V → 3.3 V conversion, use a linear regulator (LDO) instead.
Worked Example — 6.2 V Precision Reference (No Load)
Vin = 12 V, Vz = 6.2 V, Iz(min) = 5 mA, IL = 0 mA (reference only), Pz(max) = 500 mW.
Nearest E24 (rounded down): 1.1 kΩ
Iz = 5.8 / 1100 = 5.27 mA
Pz = 6.2 × 0.00527 = 32.7 mW — well within rating
With no load, the Zener acts as a stable voltage source for an op-amp or ADC reference input. The high resistor value (1.1 kΩ) minimises wasted current while keeping the Zener just above its minimum operating point.
Safety Status Indicators
Yellow — Zener power above 80% of rating. Consider derating or using a higher-wattage Zener for reliability margin.
Red — Zener power exceeds rating at no load, or Zener current has fallen below Iz(min). The circuit will not regulate properly. Increase the Zener power rating or reduce the load current.
Limitations of Zener Regulators
Low Efficiency
The series resistor and Zener both dissipate power continuously. Efficiency is typically 20–50%. For anything drawing more than ~50 mA, a linear regulator (LDO) or switching regulator is more practical.
Poor Load Regulation
As load current increases, Zener current decreases. The Zener’s voltage shifts slightly because its V-I characteristic is not perfectly vertical. Regulation accuracy is ±5% at best for standard Zeners. For tighter regulation, use a voltage reference IC (e.g. LM4040, REF3033).
Poor Line Regulation
If Vin changes, the current through the resistor changes, and the Zener current shifts — which slightly changes Vz. A varying input supply means a slightly varying output. Dedicated regulators include feedback loops that eliminate this drift. For a simple resistive voltage divider without regulation, see the Voltage Divider Calculator.
When to Use a Zener Regulator
Low-current loads under ~50 mA. Simple voltage references for op-amps, ADCs, or comparators. Level shifting between logic families. Quick prototyping where efficiency does not matter. Anywhere you need a fixed voltage with minimal component count (one resistor, one Zener, two components total).
Frequently Asked Questions
Last updated: March 2026