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Old 1st May 2003, 10:17 PM   #11
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cocolino,

I think from analyzing the convection aspect of the problem you're right.

But in this problem, there's conduction, convection, and radiation. And, at these temperatures, the heat transfer coefficients for each will be in that order: conduction, convection, and radiation. So, the simple analysis (not always right) is that you get the biggest improvement when you help the biggest contributor. In this case, that is the conduction from device to heatsink. If we improve that the most, then that is the best solution.
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Old 1st May 2003, 10:17 PM   #12
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Thanks so much...I'll keep the transistors at the jxn of the upper and middle 1/3's. I think that makes the most sense. Thanks for the advice guys

-Matthew K. Olson
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Old 1st May 2003, 10:48 PM   #13
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After thinking about it, disregard my last post. The overall heat transfer gets better when you help a *weaker* contributor. Put 'em at the top!
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Old 1st May 2003, 11:57 PM   #14
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Quote:
But in this problem, there's conduction, convection, and radiation. And, at these temperatures, the heat transfer coefficients for each will be in that order: conduction, convection, and radiation. So, the simple analysis (not always right) is that you get the biggest improvement when you help the biggest contributor. In this case, that is the conduction from device to heatsink. If we improve that the most, then that is the best solution.
Hi leadbelly,

the conduction of heat from the device through the insulation pad into the heatsink and from there into every part of the sink is one aspect. Without convection the conduction could be as good as it wants - it wouldn`t help much to cool the devices (assumed taking aside radiation effect).
Imagine a transistor/insulation/heatsink arrangement with a thermal conductivity of infinity (of course not possible in the real world) but with zero thermal conductivity from heatsink to air the heatsinks temperature would saturate on the transistor temperature after a while - but it would not cool.

Therefore I would assume that the position of mounting the transistors does not negatively effect conductivity (at least not for steady state power dissipation) but maybe could help convection.

So the abilty of a heatsink to actually cool the devices not only depends on conductivity but it is at least as important how good the heat can be transfered to air and in turn this air away from the heatsink, respectively to bring as much amount as possible of "fresh"-air in contact with the heatsink surface - and this is what convection does.
We all know forced convection (fans) can increase the max. power dissipation ability by a very large extend and it would be great to find a way to increase convection (even only a slight increase might bring a big improvement in the max. dissipation) by mechanical arrangement of heatsinks or transistor mounting only, without using fans (Im not a fan of fans ).
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Old 2nd May 2003, 12:11 AM   #15
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After thinking about it, disregard my last post. The overall heat transfer gets better when you help a *weaker* contributor. Put 'em at the top!
Overlooked this one while busy writing (always need so much time )

Ahh - so we agree on this one ....unless that according what I said in the previous post I still believe convection is not *weaker* but equivalent (important).
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Old 2nd May 2003, 04:39 AM   #16
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If not sure about transistor placement on a heatsink, why not populate the whole heatsink area with transistors, like Krell does?

PS. I love that pic and I bring it up whenever I have a chance
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Old 2nd May 2003, 05:12 AM   #17
grataku is offline grataku  United States
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The heatsinks do look a lot like a pair of r-theta like the ones we bought put side by side.
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Old 2nd May 2003, 05:44 AM   #18
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Speaking of Krell, don't the amps
on their site look cheesy and weak?

Trendy ?
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Old 3rd May 2003, 07:01 AM   #19
sajti is offline sajti  Hungary
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Hi,

As You can see, I live in Hungary, here we have only one company which makes heatsinks. I know some guys from there, and they gave me some documents about the usage of the heatsinks.
There are some interesting facts.
If You use only one transistor, the best placing is the middle of the heatsink.
If You use two of them, the placing is more interesting, from the top of the heastsink:
Top - 15% - first tr. - 70% - second tr. -15% - Bottom (Ihope it's clear)
For three devices it's more complicated. If anybody interesting i will scan it, and put up to my homepage.

Sajti
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Old 3rd May 2003, 07:35 AM   #20
Netlist is offline Netlist  Belgium
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Quote:
Originally posted by sajti
Top - 15% - first tr. - 70% - second tr. -15% - Bottom
This is the way I positioned the transistors for my AlephX.
It's test running for days now, and they don't get too hot.
I used Sil-Pad2000 from Bergquist, which is fairly expensive ( 1.85 each) but has good thermal ratings.
It will look a bit like the Peter_Daniel_prototype_setup_and_wiring.
Does the company also have a valid explanation why to put them like they explain? Could you check that? In building Aleph's the wiring would become much longer then I prefer. Thats why I made a 'group' on the top part of the sink.

/Hugo
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