Zener Diode Regulator Calculator
Zener Diode Shunt Regulator Calculator
Size the series resistor for a zener shunt regulator over its whole input and load range, and see what it really costs: the worst-case zener dissipation, the resistor wattage you need, and the output swing the zener’s own impedance causes.
Zener shunt regulator
10 V 1 W zener (1N4740A, Zz 7 Ω at 25 mA) on a 12 V ±5% rail feeding 0–20 mA, 5 mA minimum zener current, E24
Sizing a zener shunt regulator
- Rs
- the series resistor; it carries the load current and the zener current together, so it is rounded DOWN to a standard value
- Iz,min
- the smallest zener current you will accept — below the knee the zener stops regulating
- Zz
- the zener’s dynamic impedance at the test current, from the data sheet; it is why the output moves when the current does
- Vout
- Vz + Zz·(Iz − Izt), using the data sheet’s own test current as the reference point
Worked example
10 V 1 W zener (1N4740A, Zz 7 Ω at 25 mA) on a 12 V ±5% rail feeding 0–20 mA, 5 mA minimum zener current, E24
Rs ≤ (11.4 − 10) ÷ (20 mA + 5 mA) = 56.0 Ω → 56 Ω (E24, rounded down)
Worst corner: (11.4 − 10) ÷ 56 − 20 mA = 5 mA of zener current — exactly the minimum asked for
Other corner: (12.6 − 10) ÷ 56 − 0 = 46.43 mA, so the zener dissipates 464.3 mW, 46% of its 1 W rating
Resistor: (12.6 − 10)² ÷ 56 = 120.7 mW, so fit a 250 mW part
Regulation: ΔI = 41.43 mA, so ΔV = 7 Ω × ΔI = 0.290 V — the output moves from 9.860 V to 10.150 V, 2.9% of nominal
Where the power goes at the nominal input and full load
| Power | Share | |
|---|---|---|
| Into the load (10 V × 20 mA) | 200 mW | 46.7% |
| Wasted in the series resistor | 71.43 mW | 16.7% |
| Wasted in the zener | 157.1 mW | 36.7% |
| Drawn from the source | 428.6 mW | 100.0% |
What a zener shunt regulator can and cannot do
A zener shunt regulator is the simplest voltage regulator there is: one resistor and one diode. The resistor drops whatever the input has above the zener voltage; the zener sits across the output in reverse and conducts whatever current the load does not take, holding the voltage roughly at its breakdown value. Roughly is the word. Everything that is wrong with the circuit follows from the fact that the resistor carries the load current and the zener current together, so the two are locked to each other.
Size it at the worst corner, and round the resistor down. The hard case is the lowest input voltage with the heaviest load, because that is when there is least current to go round: Rs ≤ (Vin,min − Vz) ÷ (IL,max + Iz,min). In the example that is 56.0 Ω, so the next standard value down — 56 Ω — is the right choice. Rounding up would pass less current and starve the zener at exactly the moment it is needed; rounding down passes a little more, and the zener simply absorbs the surplus. This page always rounds down for that reason.
Then check the opposite corner. With the input at its highest and the load disconnected, every milliamp goes through the zener: 46.43 mA here, against 5 mA at the other end — a swing of more than nine to one. That is 464.3 mW in the diode, 46% of a 1 W part’s rating, and 120.7 mW in the resistor, which wants a 250 mW part for a comfortable twice-the-dissipation margin. If your load can be unplugged, design for no load at all.
The regulation nobody quotes. A zener is not an ideal voltage source; it has a dynamic impedance Zz, and the output moves by Zz × ΔIz as the current through it changes. The 1N4740A’s data sheet gives 7 Ω at its 25 mA test current, so the 41.43 mA swing above moves the output by 0.290 V — from 9.860 V to 10.150 V, or 2.9% of nominal. That is before the ±5% tolerance on Vz itself and its temperature coefficient (positive above about 5 V, negative below). Near the knee it is far worse: the same data sheet gives Zzk = 700 Ω at 0.25 mA, a hundred times the test-current figure, which is why you keep a few milliamps flowing.
And the waste. The source current is (Vin − Vz) ÷ Rs whatever the load is doing, so the circuit draws the same power idle as loaded. At the nominal input the table above shows 200 mW reaching the load out of 428.6 mW taken from the supply — 46.7% efficiency at full load, and zero with the load off. A three-terminal regulator doing the same job wastes 102.5 mW and reaches 66.1%, and it regulates properly: see the LM317 calculator and the LDO loss and thermal calculator. A switching regulator does better still — the buck converter designer.
So what is it for? Small, roughly constant loads where simplicity matters more than accuracy: a reference for a comparator, a clamp on a signal line, biasing the base of a pass transistor, protecting a gate. It is a poor choice for anything whose current varies much, for battery-powered equipment, or for more than a few tens of milliamps. If you only need to limit current rather than fix a voltage, the LED series resistor calculator and the Ohm’s law calculator cover that.
Frequently asked questions
How do you calculate the series resistor for a zener diode?
Rs = (Vin,min − Vz) ÷ (IL,max + Iz,min), then round DOWN to a standard value. A 10 V zener on an 11.4 V worst-case input feeding 20 mA with 5 mA of zener current needs 56.0 Ω, so use 56 Ω.
How much power does the zener dissipate?
Vz times the worst-case zener current, which occurs at the highest input with the lightest load: 46.43 mA × 10 V = 464.3 mW in the example, 46% of a 1 W part.
Why does my zener regulator output drift with load?
Because the zener has a dynamic impedance. The output changes by Zz × ΔIz, and in a shunt regulator the zener current changes by the full load swing. Seven ohms and a 41 mA swing is 0.290 V of movement.
Should I round the zener series resistor up or down?
Down. A smaller resistor passes more current; the zener takes the extra and the output still regulates. A larger one can leave the zener below its minimum current at the worst corner, where the output collapses toward a plain voltage divider.
How much current can a zener regulator supply?
Only as much as the resistor can pass at the lowest input, minus the zener’s minimum current. Beyond a few tens of milliamps the wasted power becomes absurd; use a three-terminal regulator or add a pass transistor with the zener as its reference.
Is a zener regulator efficient?
No. It draws the same current whatever the load takes. The example reaches 46.7% at full load and zero when the load is off; an LM317 doing the same job reaches 66.1%.
Related calculators
References
- Diodes Incorporated. 1N4728A – 1N4761A: 1.0 W zener diodes, DS18007 Rev. 20. 1N4740A: VZ 10 V at IZT 25 mA, ZZT 7 Ω, ZZK 700 Ω at 0.25 mA; 1 W derated 6.67 mW/°C above 50 °C.
- Horowitz P, Hill W. The Art of Electronics, 3rd ed. Cambridge University Press, 2015. Chapter 1 (resistors in series and parallel, reactance, resonance, zener regulators) and Chapter 9 (voltage regulators).
- IEC 60063:2015. Preferred number series for resistors and capacitors (the E6, E12, E24, E48, E96 and E192 series). International Electrotechnical Commission.