Power calculator
Zener regulator
A Zener is a relief valve: it burns what the load does not take.
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The example, worked out
Given
- Supply, lowest
- 11 V
- Supply, highest
- 13 V
- Zener voltage
- 5.1 V
- Load current, most
- 20 mA
- Load current, least
- 0 A
- Zener knee current
- 5 mA
- Resistor from
- E12 (10 %)
Working
- keep the Zener on at the worst moment
- what it swallows with the load gone
- the heat, in the Zener and in the resistor
Results
The formulas
- R ≤ (Vin min − Vz) / (Iz min + IL max)keep the Zener on at the worst moment
- Iz max = (Vin max − Vz) / R − IL minwhat it swallows with the load gone
- Pz = Vz × Iz maxthe heat it must take
The usual traps
- Size the resistor at the lowest supply and the biggest load, the Zener at the highest supply and no load.
- A Zener needs a few milliamps to regulate: below its knee the voltage sags.
- The shunt draws the full current all the time: fine for milliamps, wasteful above.
Questions
How do I choose a Zener diode's series resistor?
R ≤ (Vin min − Vz) / (Iz min + IL max): keep the Zener on at the worst moment. Iz max = (Vin max − Vz) / R − IL min: what it swallows with the load gone. Pz = Vz × Iz max: the heat it must take.
What does it give for typical values?
220 Ω for 5.1 V from 11 V to 13 V, up to 20 mA: 183 mW in the Zener. Series resistor: 220 Ω; Zener power, worst case: 183 mW.
What are the usual mistakes?
Size the resistor at the lowest supply and the biggest load, the Zener at the highest supply and no load. A Zener needs a few milliamps to regulate: below its knee the voltage sags. The shunt draws the full current all the time: fine for milliamps, wasteful above.
In the app
Your circuit, drawn live
Change a value: the schematic redraws with your numbers on the parts.
The part you can buy
Results rounded to standard values (E12, E24, E96) and power ratings.
The working, shown
Every step written out. Try it first by hand, then check.
Into your notebook
One click puts the circuit on a sheet of your project.
Open the calculatorOpens in the app on 1 December, with Circuit Notebook 1.0
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