Re: Cooling a TO-220 7812 regulator

On 16/09/2024 1:48 am, Pimpom wrote:

On 15-09-2024 06:11 pm, Martin Brown wrote:

On 15/09/2024 12:28, Pimpom wrote:

I'm going to use a standard 7812 TO-220 regulator on a single-sided 1 oz board with the dissipation varying as below:
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- Idle at 120-150mW
- 0.5W for 1-5 minutes at irregular intervals
- Up to 4W for 1 second at the start of each 0.5W phase
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The interval between the 1-5-minute periods is variable and may be 1 to 10 minutes.
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The ground pin is soldered to ~2 sq.in. of copper on the PCB.
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Options:
1) As described. No extra heatsinking.
2) Mount it on a small Al heatsink.
3) Place the regulator horizontally on the underside and bolt it to the copper.
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Tamb = ~35ºC max
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Do you think option 1) is enough? Or should I go for 2) or 3)?

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I suspect that under those load conditions you will get away with it #1. I'm pretty sure the package is well able to handle that dissipation and if it can't it will fold back to thermally protect itself (in which case you will have to add a bit more heatsinking).
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I once built a 78xx based PSU that was in a bright bare metal aluminium extruded box expecting it to be more than adequate heatsinking. It wasn't! The bright metal was a terrible radiator and it got hot enough to go into thermal foldback. A bit of matt black paint fixed that.
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Thanks. I'm inclined to think that way too. Calculating temerature rise and heatsink requirement for steady-state dissipation is simple enough and it's easy to make a good guesstimate from experience. But transient cases like this are different.

It's not that difficult to weigh a chunk of material and calculate it's heat capacity. We did it back in 1993.
Sloman A.W., Buggs P., Molloy J., and Stewart D. “A microcontroller-based driver to stabilise the temperature of an optical stage to 1mK in the range 4C to 38C, using a Peltier heat pump and a thermistor sensor” Measurement Science and Technology, 7 1653-64 (1996)
and measured the step response of the system as well, with three different Peltier junctions between the point we were stabilising and the heat sink.
It's discussed in the third page of the paper. It does get a bit messy, but it is do-able.
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Bill Sloman, Sydney