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Old 1st May 2003, 07:37 PM   #1
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Question transistor placement on a heatsink

Ok...as far as placement on a heatsink goes...where is the best place to put transistors? Say the heatsink is upright, fins perpendicular to the floor ...is the best place in the middle..height wise? lowish? or highish of center? I would think lowish of center would work best but i've heard highish works the best. Highish being on the jxn of the middle and top 1/3 heightwise of the heatsink. Thanks ya'll! Hope I was clear enough

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Old 1st May 2003, 08:03 PM   #2
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Hi Matthew,

transistor placement on a heatsink has been discussed before on this forum. One of the members gave a clear argument that a transistor should be placed at top of the sink, while noting that most designs he had seen to date had placed the devices in the centre or lower part of the sink. Try a search on the Pass or solid state forums. You may be able to find the original and not have to rely on my dodgy recollections...

James
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Old 1st May 2003, 08:05 PM   #3
Jeff R is offline Jeff R  United States
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I think right in the middle is best. If we assume the thermal resistance of the heat sink and the mounting pad is nil, then placement at the center allows for the most even distribution of heat, which will therefore allow for the best cooling via radiation.

I can see why some would think mounting a little low might be better, as heat tends to rise. If the thermal resistance of the heat sink is not close to 0, then this might make sense. But, most of us DIY'ers use massive heat sinks so I think that the ideal location would be very close to the center anyway. I suppose one could experiment with both locations, but I think that for a transistor mounted very much off center, the heat distribution will not be so uniform, and this equates hot areas.

I am just guessing here - so others in the know might have some real data to provide.
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Old 1st May 2003, 08:05 PM   #4
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I have searched actually...but didn't find anything...but thanks for the reply. A friend of mine said placing them towards the top was a good idea. I will do just that. Thanks

-Matthew K. Olson
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Old 1st May 2003, 08:08 PM   #5
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Quote:
Originally posted by Mattyo5
I have searched actually...but didn't find anything...but thanks for the reply. A friend of mine said placing them towards the top was a good idea. I will do just that. Thanks

-Matthew K. Olson
Hi Matthew,

found it -

http://www.diyaudio.com/forums/showt...0578#post40578

You may want to ask wuffwaff for clarification.

James
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Old 1st May 2003, 08:11 PM   #6
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heatsinks use both radiation and convection to get rid of heat. (well they also use conduction of course...). i'm guessing the reason placing a transistor toward the top is favored is because it helps with the chimney effect (convection). but i don't know any of the formulae involved, so maybe there's a tradeoff between convection and radiation there.

/andrew
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Old 1st May 2003, 08:27 PM   #7
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I think mounting to geometric center is best. That way you would get the best conduction of heat from the device to the sink. As for convection, I doubt moving the device around affects the convection heat transfer coefficient very much. And radiation is minor at the temperatures we are talking about and not worth considering.
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Old 1st May 2003, 09:11 PM   #8
tiroth is offline tiroth  United States
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Actually, I was surprised, but check out ESP's heatsink calculator. At temperatures around 60C there is a large contribution from radiation.
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Old 1st May 2003, 09:35 PM   #9
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If you examine quality car audio amplifiers, their
transistor placement is not in the center, if any
thing, it's located on the the edge of the pcb,
but quality car amplifiers are pretty robust and
take major abuse, plus ambient temperatures
due to climate pre-heat the fins, in any case,
these abnormal placement of transistors don't
seem to bother these amplifiers, even kilowatt
ones

If your heatsinks are high in mass, then I wouldn't
trip out on location.

Do the propane torch test.

Take a piece of metal. fire up your propane torch and
heat the edge of the metal. Place finger on metal 1 inch below
where the flame hits, tell me if your finger burns

Do the same by heating up the center of the metal,
does your finger burn ?

But if I had to choose, I choose center just because .... lol ....
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Old 1st May 2003, 09:42 PM   #10
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Also I believe it can`t be very wrong placing transistors into the geometrical middle of a heatsink but after reading and rethinking that idea about placing them rather in the "upper" half of a sink, this makes more and more sense to me.

Think about this way: the surface of a heatsink around the transistors is always somewhat hotter than the rest. When the transistors are mounted in the lower half, the upper half of the heatsink is cooler so the heated (and hotter) air in proximity of the "lower" heatsink parts want to move upwards whereas the cooler air around the "upper" area is blocking (reducing) that convection as it rather wants to move down (because it`s cooler).

At least theoretically this makes really sense to me and speaks for mounting transistors in the "upper" area actually.
If at all and how far this accounts for in the real world would be very interesting to know.
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