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Heatsink & junction temperature

Tj 96 °C for 8 W through 7 °C/W at 40 °C

Heat flows like current: watts are amps, degrees are volts, °C/W are ohms.

Opens in the app on 1 December, with Circuit Notebook 1.0 See it worked out

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The example, worked out

Junction96 °C

Case84 °C

Heatsink80 °C

Total resistance7 °C/W

Margin to the limit29 °C

Sink for 125 °C≤ 8.63 °C/W

Do I need a heatsink, and how big?

Tj = Ta + P × (RθJC + RθCS + RθSA): the resistances add in series. RθSA ≤ (Tj,max − Ta) / P − RθJC − RθCS: the heatsink that just does it.

What does it give for typical values?

Tj 96 °C for 8 W through 7 °C/W at 40 °C. Junction: 96 °C.

What are the usual mistakes?

Thick paste or a loose screw adds as much as a worse heatsink. The air inside a box is warmer than the room.

Change a value: the schematic redraws with your numbers on the parts.

Results rounded to standard values (E12, E24, E96) and power ratings.

Every step written out. Try it first by hand, then check.

One click puts the circuit on a sheet of your project.

Opens in the app on 1 December, with Circuit Notebook 1.0

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