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Old 26th June 2005, 07:19 AM   #1
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Default heatsinks, heatsinks.....

Attached is drawing of 2 heatsinks. The total fin length in A is (h/s+1)*l while in B is (l/s+1)*h. The difference will be l-h. But I believe that B is a better dissipater because of the convection of hot air... right?

I had a set up as in B, the width of the circuit board and thus the bracket is about 170mm. I had two piece of 165mm (l) x 100mm (h) side by side and bolted to the bracket. I had about 80mm running off the board on either side. After I use it for some time (I didn't remember for how long). I found that the 2 heatsink was only heated up in the centre where they are in contact with the bracket. The temp. on either ends are just a bit over ambient. If I am trying to get better dissipation, I would have to go for 'taller' h rather than 'longer ' l ?

If I am using a bracket that is wider than 170mm, do you think it would do better?. (But the O/Ps are still concentrated within the 170mm ie, width of the circuit board.)

I don't know what might happen if I am using a A setup. Would the heat be still concentrating in the middle section. Since the fins are running horizontally, I guess that won't happen. So which one is a better dissipater? Can somebody shed some light on this?

I am now blowing it with a 8cm fan, just to play safe.
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Old 26th June 2005, 07:21 AM   #2
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Oops, forget the attachment
Attached Files
File Type: pdf heat_comp.pdf (7.3 KB, 93 views)
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Old 26th June 2005, 07:54 AM   #3
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All other things equal, the vertical arrangement will have more flow and more dissipation for same temp rise.

Jan Didden
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Old 26th June 2005, 08:40 AM   #4
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Hi.

The conduction of heat away from the contact point is related to the thickness of the back of the heatsink, not the fin arrangement, the thermal efficiency of the fins is highest with them vertical.

To aid the spread of heat, a 6mm plate of aluminium (the same size as the two heatsinks) could be screwed between the backs of the heatsinks and the bracket, this will spread the heat more evenly to the fins.

Remember to use heatsink compound.

Your local metal merchant may well have small offcuts of plate that they will let you have cheap.

Chris
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Old 26th June 2005, 08:42 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
what thickness is your heatsink?
To work efficiently you need the heat to dissipate from the whole surface area and this requires that the main sink is roughly the same temperature across the whole back surface.
The thickness to width ratio to achieve this is normally in the range 5 to 12. Assuming 10 then your sink needs to be about 8.5mm thick (85mm from centre to edge).

B layout with the fins vertical is much better. A will have a heat dissipation capacity about 64% of B (80% due to horizontal fins and 80% due to long fins) These percentages are just guesstimates but you can measure this in situ.

A blown sink usually has close fin spacing of 2mm to 3 mm.
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