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Old 1st July 2002, 10:45 PM   #1
Apogee is offline Apogee  United States
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Question Heatsink question

Yes, I know this topic has been beaten to death, but I wasn't able to find an answer to this question....

Yes, H.H - I did search...

Take just about any standard flatback heatsink. Now imagine taking 3 of them and laying them next to each other like this:


If I take a piece of 1/4" aluminum flat plate and cut it to the correct dimension and then bolt the heatsinks to it (using grease), does this raise or lower the thermal efficiency? I'm wondering if I can buy smaller extrusion and ultimately achieve the same thermal efficiency as using a larger one...

This is what I'm envisioning (the vertical lines would actually be one continuous piece of metal):


I ask this question because I've seen many TO3 style amps that have been designed using a "carrier" to mount the TO3's which is then bolted to the heatsinks... It seems to me that this would significantly raise the junction temps but it doesn't seem to be a problem... The carriers often look like this (only the heatsink is actually one large extrusion):


How would one go about calculating the thermal sinking of the combination of the two pieces of metal? Just add their dissipations together and subtract the joint loss? Or, is it not so simple?

Any help would be appreciated!

"If I had my life to live over again, I would have made a rule to read some poetry and listen to some music at least once a week." - Charles Darwin
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Old 1st July 2002, 11:22 PM   #2
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I do this sort of thing all the time, just look at Threshold
and X products.

Calculations? We don't need no stinkin calculations! We just
make them really big!
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Old 1st July 2002, 11:25 PM   #3
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I once tried to make a large heat sink out of many small ones. It was for Zen amp. But since there were only 2 transistors attached to the sink the heat distribution wasn't even and some spots were much warmer than the others even after many hours of operation (I used thermal grase).
I liked the looks anyway.
I plan to build Aleph 5 on that and with 6 transistors per side it should work really fine. I usually don't do calculations as well. It either runs hot or not at all.
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Old 1st July 2002, 11:26 PM   #4
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very sympatic way of engineering!

i love this philosophy, thats the way i could handle and understand building your amps.
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Old 1st July 2002, 11:45 PM   #5
Jean is offline Jean  United States
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Is there a good source for just various thickness aluminum in sheets ?
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Old 2nd July 2002, 12:08 AM   #6
PedroPO is offline PedroPO  Portugal
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Default Heatsink Calculation

Last time I tried to calculate a heatsink, I put my amp at 100șC...''

I guess you can allways try to bolt a single power resistor (with aluminium housing), apply a voltage until the heatsink get the temperature you want (say 50șC), then read the tension and P=R^2/R. You get the maximum power dissipation of the heatsink before you drill all the dissipators to put the transistors and / or resistors.

Pedro Oliveira
PS: I agree with Nelson Pass: Bigger is better at least in heatsinks.
Pedro Oliveira
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Old 2nd July 2002, 12:35 AM   #7
herm is offline herm  United States
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Old 2nd July 2002, 12:53 AM   #8
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Bigger is better with heat sinks and bias current

If you want to use multiple heatsinks, you are best to mount the transistors on each individual heatsink and then use strap or angle pieces to mechanically join each heat sink around the edges. Just keep the number of devices per heatsink equal.
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Old 2nd July 2002, 02:06 AM   #9
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Thumbs up nice link, herm

I checked out some of the sheet goods and related prices. They compete well with prices at my local metal yard. A 3'x4'x1/16" piece of aluminum cost roughly $2.55/pound. New aluminum is $3.00/pound here in Portland, Oregon. Excellent lead! It's still way cheaper to buy in bulk and make your own cuts, though. They don't give their labor away.

John Inlow
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Old 2nd July 2002, 03:59 AM   #10
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Originally posted by Nelson Pass
Calculations? We don't need no stinkin calculations! We just
make them really big!
Harley Davidsons are designed using a similar philosophy I think.

If you have the choice between a tall skinny heatsink and a short broad one with a same area, the short broad one will win every time. Only the first couple of inches seems to do anything much, after that the rising air has been warmed enough as it rises through the fins that it cannot cool any further fins higher up.

I love fans! I did a little test with a h/s 110mm x 95 with 11 fins 25 mm deep and 1.5mm thick, both on it's lonesome and with a 12v fan covering most of its finned side and a 25w metal clad resistor attached to the flat side with thermal paste. Here are the figures:

Heatsink vertical both sides exposed
24.25 watts
44.40 deg C rise
1.83 deg C per w

Heatsink vertical with 2.8 watt Sunon fan @ 5v
24.25 watts
10.20 deg C rise
0.42 deg C per w 4.36 times improvement

Heatsink vertical with 2.8 watt Sunon fan @ 12v
24.25 watts
7.70 deg C rise
0.32 deg C per w 5.77 times improvement

Heatsink vertical with 2.8 watt Sunon fan @ 15v
24.25 watts
7.50 deg C rise
0.31 deg C per w

Heatsink vertical with 2.8 watt Sunon fan @ 20v (12v fan, mind you!)
24.25 watts
7.50 deg C rise
0.31 deg C per w

At 5v the fan is inaudible.

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