Re: Cooling a TO-220 7812 regulator

On Sun, 15 Sep 2024 23:03:09 +0530, Pimpom <Pimpom@invalid.invalid>
wrote:

On 15-09-2024 09:46 pm, john larkin wrote:

On Sun, 15 Sep 2024 16:58:01 +0530, Pimpom <Pimpom@invalid.invalid>
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:
>
- 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
>
The interval between the 1-5-minute periods is variable and may be 1 to
10 minutes.
>
The ground pin is soldered to ~2 sq.in. of copper on the PCB.
>
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.
>
Tamb = ~35ºC max
>
Do you think option 1) is enough? Or should I go for 2) or 3)?

 
Option 1 sounds fine to me. It will barely get warm.
 
You don't need it, but internal plane layers help spread out heat too.
 
https://www.dropbox.com/scl/fo/l0pfn2qtj9pylri0kxnbs/ADjThVe5WV989K8DgotdKVI?rlkey=z4gtiaxmnybem099s6l9yz20m&dl=0
 
https://www.dropbox.com/scl/fo/dcua00tr8fgxtudnqzhzr/APMIydE2mZFz5e2aFRd-2TY?rlkey=2y16ic2watsnfnthzcf7zbt0i&dl=0
 

Thanks for the input. Calculating heatsink requirement for continuous
dissipation is one thing. Short bursts are another.

You can estimate the mass and specific heat of a fet and do the math.
It's even easier to test one. You could do that quickly.

Given a TO-220 gadget, it would be nice to know the temp rise per
joule dumped into it. Maybe I'll measure that.

First google hit:

https://www.avrfreaks.net/s/topic/a5C3l000000UNfNEAW/t104767

"A TO-220 package has a thermal capacitance of 0.54(J/C) and thermal
resistance of about 62(C/W) without a heatsink."

Based on that, 4 watts for 1 second works out to about 8C rise. The
PCB will help a little.