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On TO-220 heat sinks, there are usually two sides, on which the transistor fits easily. One side is more narrow, surrounding the transistor. The other side is more open. Which of these sides is thermodynamically best option to put transistor on?

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  • \$\begingroup\$ The one on the right, you put your flat side against the board/mounting surface. The fins go in the air so I guess you would say you put the transistor on the "narrow side". As for the other, my hunch is also the narrow side but I've never used one like that so others will need to chime in. \$\endgroup\$ – scld Nov 25 '13 at 11:53
  • \$\begingroup\$ The one on the left will fit a number of devices, it's up to you how you fit it to the one you use. It will make little difference. If your pushing the edge of the devices thermal ratings I would go take a measurement. What ever you do. Make sure the tab is flat against the heatsink. I have seen 2 devices mounted together back to back but I would only do that if I needed thermal coupling. \$\endgroup\$ – Spoon Nov 25 '13 at 12:15
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The second one is often screwed flat on a PCB, with the transistor's legs at 90 degree angles, and then of course you put the transistor on the narrow side.

But otherwise it doesn't matter. The heatsink will receive its heat at the same place and the fins don't really care whether the heat entered from the left or the right.

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  • \$\begingroup\$ Even when the second case is not screwed flat to the PCB, it's preferential to use the narrow side for the slight space savings. \$\endgroup\$ – Charlie Feb 5 '17 at 6:34

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