Zener diode voltage regulator: choosing the series resistor
A zener diode run backwards holds a fixed voltage across itself, so a single resistor and a zener make the simplest regulator there is. The resistor drops the difference between the supply and the zener voltage; the zener takes whatever current the load does not. Get the resistor too big and the zener starves when the load is heavy; too small and the zener cooks when the load is light. This calculator designs Rs = (Vin − Vz) / (IL + Iz,min), or checks the resistor you have, and shows the currents, the heat in both parts and the limits.
How to use the zener regulator calculator
- Type the supply (and, if it varies, its highest value), the zener voltage (standard values are offered) and the load current in milliamps; leave the load blank for a bare reference.
- Leave Rs blank and it is designed for you, rounded to the E24 value at or below the exact one; or type a resistor you have and it is checked instead.
- Set the minimum zener current (5 mA is the usual choice for small zeners) and the zener's power rating from its package.
- The drawing shows the currents on each part and the budget panel where the series current goes, the worst case with the load off, and both parts' heat against their ratings. Click a part for its own calculation; the Supply sweep tab shows what happens as the supply varies.
How a zener regulates
Reverse-biased past its breakdown voltage a zener conducts freely, and its voltage then changes only a little with current: a few ohms of dynamic resistance for mid-range parts. Put it across the output and feed the node through a resistor from a higher supply, and the resistor passes a nearly constant current Is = (Vin − Vz) / Rs. The load takes what it needs and the zener absorbs the rest. As long as the rest stays above the zener's minimum current, the output sits at Vz; as the load rises, the zener's share falls, until at IL = Is − Iz,min the regulator runs out and the output drops.
Designing the series resistor
Rs = (Vin,min − Vz) / (IL,max + Iz,min)
Design at the hardest moment: the lowest supply and the heaviest load. The resistor must still pass the load current plus the zener's minimum. Round the result down to a standard value, never up: a slightly smaller resistor passes a little more current, which the zener simply absorbs. Iz,min is typically 5 mA for 0.5 W and 1 W zeners, 1–2 mA for low-power references, and about 10% of the zener's maximum current for larger parts. Below it the diode sits on its knee, the voltage sags and supply ripple leaks through.
Heat and ratings
Check at the opposite extreme: the highest supply with the load disconnected, when the zener takes the whole series current. Its dissipation is Vz × Iz,max and must sit under the package rating, ideally under 70% of it: 0.4–0.5 W for small glass parts (BZX55, BZX79, 1N52xx), 1 W for 1N47xx, 1.3 W for BZX85, 5 W for 1N53xx. The resistor dissipates (Vin − Vz)² / Rs regardless of the load; pick a rating with a factor of two in hand. A hot zener drifts: parts above 6 V rise about 0.1% per °C, parts below 5 V fall; 5.1 V and 5.6 V are the most stable.
Limits of a shunt regulator
The regulator draws its full current all the time, so the efficiency is Vz IL / Vin Is and is poor at light load and zero with no load. The output moves with load by the zener's dynamic resistance times the load swing, and with supply by rz / (Rs + rz) of the input ripple. Both are far worse than a 78xx or an LDO, which is why zener regulators are for references and loads of a few tens of milliamps, or for feeding a transistor whose base needs a steady voltage.
When to use one
| Need | Choice |
|---|---|
| A reference voltage, under 10 mA | Zener and resistor, or a TL431 for precision |
| Clamping a signal or protecting an input | Zener (no resistor design needed beyond limiting current) |
| 10–100 mA with a steady supply | Zener plus an emitter follower, or a 78Lxx |
| Over 100 mA, or a supply that varies a lot | Linear regulator (7805, LM317, LDO) or a buck converter |
| Battery powered, efficiency matters | LDO or switching regulator; a zener wastes the battery |
Your regulator, step by step
- Resistor: 270 Ω drops 6.9 V and passes 25.56 mA at 12 V.
- Split: load 20 mA, zener 5.556 mA (minimum 5 mA): regulating.
- Worst case: 12 V in with the load off puts 25.6 mA through the zener, 130 mW of its 500 mW rating.
- Resistor heat: 176 mW → a 1/2 W part. Load up to 20.6 mA; dropout below 11.8 V; efficiency 33%.
Worked example: 12 V to 5.1 V for a 20 mA load
With a 5 mA minimum zener current the resistor must pass 25 mA while dropping 12 − 5.1 = 6.9 V: Rs = 6.9 / 0.025 = 276 Ω, rounded down to 270 Ω. That resistor passes 25.6 mA, of which the load takes 20 mA and the zener 5.56 mA. Unplug the load and the zener takes all 25.6 mA: 130 mW, which a 500 mW part handles at 26% of its rating. The resistor dissipates 176 mW, so use a 1/2 W one. The load can grow to 20.6 mA and the supply can fall to 11.8 V before regulation is lost; the efficiency at the nominal load is 33%.
Questions
Why round the resistor down and not to the nearest value?
Because the two failure modes are not symmetrical. A smaller resistor only adds a little zener current and heat, which you check; a larger one lets the zener current fall below its minimum at full load, and the output quietly sags.
Can I leave the load off?
Only if the zener's worst-case power allows it, which the calculator checks. A regulator designed for a heavy load may need a 1 W or 5 W zener purely for the moment the load is unplugged.
What does the minimum zener current do?
It keeps the diode past its knee, where the voltage is flat. Starved, a zener still conducts, but its voltage drops with current and it passes supply ripple almost unattenuated.
How accurate is the output?
Zener voltages come in 5% (BZX55C) or 2% (BZX55B) grades and drift with temperature and current. Expect a few percent unless you use a TL431 or a proper reference.