Heat Sinks

Most of our competitors mount their heat sinks on the top of their controllers while we mount ours on the side. And when I get a chance to ask ‘Why’ the answer is always something along the lines of ‘Because heat rises’. We think they are wrong, not about heat rising, but about why most of them have top mounted heat sinks. We think they all have top mounted heat sinks for no other reason than one did it and the rest just copied without asking ‘why’!

Actually heat doesn’t rise, hot air rises as it’s being pushed up by sinking cooler and heavier air. Everything else being equal heat will radiate the same in all directions. From a cooling point of view your SSR does not care if the heat sink is on the top or the side or the bottom. Heat will transfer equally well between your SSR and heat sink regardless of where it’s mounted.

What really counts is how heat is removed from your heat sink. Heat sinks are cooled by radiation and by convection. Radiation is the heat you feel off the eye of an electric stove – it radiates away from the eye. Convection is heat you would feel coming out of a heat register, it’s heat that’s carried away by circulating air.

We mount our heat sinks on the side because mounting on the side improves convection cooling. The warm air rising off the top helps pull cooler air in from the bottom our heat sink. Both a top mount and side mount heat sink radiate the same amount of heat but a side mounted heat sink does a far better job of convection cooling than a top mounted heat sink does.

To show the difference between the two we drilled a hole into one of our heat sinks and put a sensor inside. Then we ran a cooling test with far more heat than the heat sink would see from one of our SSRs. We applied about 36 Watts of heat to the back side of one of our heat sinks. And for each test we let the heat sink heat-up for 1.5 hours to reach its maximum temperature.

We mounted the test housing and heat sink in the hallway in our shop. Then we tested with the heat sink mounted on the side as in the picture then we let everything cool of and rotated the housing 90 degrees to move the heat sink to the top.

Heat sink and housing mounted for testing

You can see convection cooling in action in this FLIR picture of the side mounted heat sink. The top right corner is the hottest and a large area of the wall above the heat sink and housing are warmed by the rising air.

The side mounted heat sink leveled off at 125F.

FLIR picture of side mounted heat sink reaching 125F

You can see less convection cooling effect for the top mounted heat sink in this FLIR picture. A smaller area of the wall above the heat sink and housing are warmed by the rising air because convection cooling is not as good across a top mounted heat sink.

The top mounted heat sink leveled off at 129F,
4 degrees warmer than the side mounted heat sink!

FLIR picture of top mounted heat sink reaching 129F