Power calculator
Linear regulator heat
A linear regulator is a smart resistor: the extra volts become heat.
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The example, worked out
Given
- Input
- 12 V
- Output
- 5 V
- Load current
- 300 mA
- Quiescent current
- 5 mA
- Thermal resistance to air
- 60 °C/W
- Air temperature
- 25 °C
Working
- the volts dropped times the current, plus what it eats itself
- useful power over power drawn
- heat flows like a current through a thermal resistance
Results
The formulas
- P = (Vin − Vout) × Iout + Vin × Iqthe volts dropped times the current
- η = Vout × Iout / (Vin × (Iout + Iq))roughly Vout / Vin
- Tj = Ta + P × RθJAheat flows like a current
The usual traps
- 12 V to 5 V at 1 A: 7 W of heat, a heatsink or a buck.
- Too little headroom: a 7805 needs 2 V, an LDO a few hundred millivolts.
Questions
How hot does a linear regulator get?
P = (Vin − Vout) × Iout + Vin × Iq: the volts dropped times the current. η = Vout × Iout / (Vin × (Iout + Iq)): roughly Vout / Vin. Tj = Ta + P × RθJA: heat flows like a current.
What does it give for typical values?
2.16 W of heat, 41 %, Tj 155 °C (12 V → 5 V at 300 mA). Heat in the regulator: 2.16 W; Junction temperature: 154.6 °C.
What are the usual mistakes?
12 V to 5 V at 1 A: 7 W of heat, a heatsink or a buck. Too little headroom: a 7805 needs 2 V, an LDO a few hundred millivolts.
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
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Open the calculatorOpens in the app on 1 December, with Circuit Notebook 1.0
Also in Power
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- Boost converter Duty cycle, inductor, capacitor, and the current the input must give.
- Flyback converter Turns ratio, duty, primary inductance and peak currents from the reflected voltage.
- Heatsink & junction temperature From the air to the junction through the thermal resistances; the heatsink you need.
- Zener regulator The series resistor, the Zener's heat, the current it wastes.
- Bulk capacitor The capacitor after a rectifier for a given ripple, or for a hold-up time.