Component / heatsink positioning

[upl8]

I am installing this phase angle controller
Web page: http://www.united-automation.com/psr-25/
Datasheet: http://www.united-automation.com/content/X10205%20-%20PSR-25.pdf

On this heatsink
http://www.altronics.com.au/p/h0582-128-x-100mm-extra-heavy-duty-heatsink/



The load will be drawing about 13 amps, which requires about 3°C/W according to the datasheet. The heatsink is rated to 1.3°C/W. There is a 120 X 38 fan mounted directly below the heatsink, but it's mounted on the same wall as the heatsink. So air won't be blowing directly over the heatsink from the fan. I haven't measured the enclosure temperature yet, but it's going to be warmer than the ambient.

Should I install the controller at the bottom of the heatsink, or in the middle for best cooling? I initially thought the middle so the heat could conduct evenly in all directions, but the bottom might be better if the heat will rise towards the top of the heatsink.

[T3sl4co1l]

It won't make much difference.  Put it in the middle so it's most evenly distributed.

If you have thermal problems, consider adding a little deflector, just in front of the fan, so air is forced over and along the heatsink fins.  Should be much better than convection.

Tim