Zener Regulator
Series resistor, zener current, and power ratings for a shunt regulator.
Rounded up to the nearest E12 value, which keeps the zener above its minimum current at full load.
Rounding the resistor up pushed the zener below its minimum current — step down to the previous E12 value or raise the input voltage.
The zener dissipates 160 mW at no load and the resistor 216 mW continuously, both with 2× headroom applied.
A zener shunt regulator wastes power continuously — the resistor carries full current whether the load needs it or not, so efficiency is poor and it suits reference voltages and light loads rather than powering a circuit. For anything above roughly 100 mA a linear regulator IC is both cheaper and cooler.
Size the series resistor for the worst case — full load current plus the zener's minimum. From 12 V to 5.1 V with a 30 mA load and 5 mA minimum, Rs = 6.9 V ÷ 35 mA = 197 Ω, rounded up to a standard 220 Ω.
A regulator that works by wasting current
A zener diode conducts backwards once the voltage across it reaches its rated value, and then holds that voltage nearly constant regardless of how much current flows through it. Put a resistor in series with an unregulated supply and a zener across the output, and you have a voltage regulator with two components.
The mechanism is worth understanding because it explains the design's weakness. The resistor sets a roughly fixed total current, and the zener absorbs whatever the load does not take. When the load draws nothing, the zener takes it all; when the load draws its maximum, the zener takes almost none. The supply current is therefore constant whether the circuit is working hard or idling.
Design for the worst case at both ends
Two opposite conditions constrain the design, which is what makes it easy to get wrong.
The resistor must be small enough that at full load there is still enough current left over to keep the zener in regulation — usually 5 to 10 mA. Below that the zener falls off its knee and the output sags. But it must also be large enough that at no load the zener does not exceed its power rating, since it is then dissipating everything.
Compute Rs at the lowest expected input voltage and the highest load, then check the zener's dissipation at the highest input and no load.
Worked example: 12 V to 5.1 V at 30 mA
Size for full load, then check the no-load case:
- 1 Find the voltage across the resistor. 12 V − 5.1 V = 6.9 V. This is what Rs has to drop.
- 2 Add the worst-case currents. 30 mA load + 5 mA minimum zener = 35 mA total.
- 3 Divide for the resistance. 6.9 V ÷ 0.035 A = 197 Ω.
- 4 Round up to a standard value. 220 Ω is the next E12 size. Rounding up slightly reduces the zener current, so check it is still above the minimum.
- 5 Check the no-load case. At 220 Ω the total is 6.9 ÷ 220 = 31.4 mA, all of it through the zener if the load disconnects — 5.1 V × 31.4 mA = 160 mW.
- 6 Specify both ratings. A 400 mW zener covers that with headroom, and the resistor dissipates 6.9 V × 31.4 mA = 217 mW, so a 1/2 W part.
Series resistor for common supplies
Assuming a 5 mA minimum zener current, rounded up to the nearest E12 value.
| Vin | Vz | Load | Rs (calculated → E12) |
|---|---|---|---|
| 9 V | 5.1 V | 10 mA | 260 Ω → 270 Ω |
| 12 V | 5.1 V | 30 mA | 197 Ω → 220 Ω |
| 12 V | 9.1 V | 20 mA | 116 Ω → 120 Ω |
| 15 V | 12 V | 10 mA | 200 Ω → 220 Ω |
| 24 V | 12 V | 50 mA | 218 Ω → 220 Ω |
| 5 V | 3.3 V | 20 mA | 68 Ω → 68 Ω |
When to use something else
The efficiency is poor by design. In the worked example the circuit draws about 31 mA continuously from a 12 V supply — 377 mW — to deliver at most 153 mW to the load. More than half the energy becomes heat even when everything is working correctly, and all of it is wasted when the load is idle.
That is acceptable for a voltage reference, a bias network, or a light load of a few milliamps. Beyond roughly 100 mA the resistor and zener both become uncomfortably hot, and a three-terminal linear regulator such as a 78xx is cheaper, more accurate, and handles load changes far better. Above a volt or two of drop at real current, a switching regulator saves the heat entirely.
Two accuracy notes. Zener voltages are specified at a particular test current and drift with both current and temperature, so the output is not precisely the marked value — for a reference that matters, a dedicated reference IC is far tighter. And below about 5 V the mechanism is actually Zener breakdown while above it is avalanche breakdown; the two have opposite temperature coefficients, which is why 5.6 V devices are unusually stable and often chosen for references.