The Zener shunt regulator
A Zener diode conducts in reverse once the voltage reaches its Zener voltage Vz, and then holds that voltage almost constant. With a series resistor, it makes the simplest voltage regulator: the resistor drops the excess voltage, and the Zener absorbs whatever current the load does not take.
Sizing the resistor
The resistor must pass the load current plus a minimum Zener current, even at the lowest input voltage:
R = (Vin min − Vz) ÷ (I load + I Zener min)
The minimum Zener current keeps the diode in its regulation region; 5 mA, or about 10% of the maximum current, is a common choice.
Example
From 12 V to 5.1 V, for a 20 mA load with 5 mA minimum Zener current:
- R = (12 − 5.1) ÷ 0.025 = 276 Ω; choose the next lower standard value, 270 Ω;
- resistor power: 6.9² ÷ 276 ≈ 0.17 W, so use a 0.25 W or better 0.5 W resistor;
- worst case for the Zener, with the load disconnected: all 25 mA flow through it, so it dissipates 5.1 × 0.025 ≈ 0.13 W, well under a 0.5 W rating.
Check the worst cases
- Lowest input, highest load: the Zener current must stay above its minimum, or the output sags.
- Highest input, no load: the Zener current and dissipation peak. Compute it with the maximum input voltage.
- Derate the Zener’s power rating at high temperature, often to half.
Limits of a Zener regulator
- It wastes power: the full current flows through the resistor all the time.
- The output varies a little with current and temperature (the Zener’s dynamic resistance and temperature coefficient).
- For currents above a few tens of milliamps, a linear regulator IC (such as a 78xx or LDO) or a switching regulator is more efficient and more accurate.
Zener shunts remain useful for references, clamps, overvoltage protection and biasing low-current circuits.