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Old 1st June 2007, 06:41 AM   #1
DonoMan is offline DonoMan  United States
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Default Small idea about cooling LM4780

I don't claim this to be some sort of breakthrough idea, and I'm probably not the first to think of it, but...

I'm building a 6 channel bridged LM4780 amp and I'm a little worried about heat, and at the same time, I don't want my main heatsinks connected to -V. So what if I connected each 4780 to a piece of aluminum maybe 2-3" square (that is width and length each, not square inches) and then used a thermal pad between that and the main heatsinks? Would that help much?
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Old 1st June 2007, 08:05 AM   #2
DonoMan is offline DonoMan  United States
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What's the difference in thermal resistance between just thermal grease and Bergquist K10 or mica + grease?
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Old 1st June 2007, 08:31 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
a thick lump of aluminium (or better still copper) to spread the heat will help dissipation. The increased area across the isolator will reduce the thermal resistance.

This is exactly what happens inside the chip, to improve heat flow.

Very thin mica is very good. Only a few "plastic" isolators can match 0.001" mica. But most mica you can buy is 0.002 to 0.006" and these are less effective.

If you go for the "live" heat spreader then the quality of the isolator will have slightly less effect in reducing heat flow into the sink.
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Old 1st June 2007, 12:54 PM   #4
YFW is offline YFW  United States
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The Hi-End brand 'Cello' by Mark Levinson use the same tricks.
According to the 'Cello Book',all the power transistors are mounted on the heatsink directly without mica to improve the heat dissipation efficiency.
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Old 1st June 2007, 06:01 PM   #5
DonoMan is offline DonoMan  United States
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OK, I will get some copper to use. Thanks guys.
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Old 31st July 2007, 12:14 AM   #6
bm0rg is offline bm0rg  United Kingdom
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Surface area is very important the more the better. I've just bought an ingot of aluminum 35mm wide 75mm high and 1300mm long to make 4 300mm long heatsinks for my bridge and paralleled LM4780 amps for a pair of monoblocks im make for A level. Hopefully should keep the two chips cool.
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Old 31st July 2007, 07:55 AM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by bm0rg
.........I've just bought an ingot of aluminum 35mm wide 75mm high and 1300mm long to make 4 300mm long heatsinks for my bridge and paralleled LM4780 amps for a pair of monoblocks im make for A level. Hopefully should keep the two chips cool.
so you've bought the aluminium to transfer the heat to the heatsink, but you've not given us any details of the heat dissipator yet.
Quote:
Originally posted by bm0rg
Surface area is very important the more the better.
is exactly what is needed. This is usually achieved by using fins that have a high surface to volume ratio.
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Old 31st July 2007, 12:11 PM   #8
bm0rg is offline bm0rg  United Kingdom
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No andy that is the alu for the heatsink. I have to mill it out yet. The idea is to get 22 fins over a length of 300mm. This equates to the fins being 4mm thick and 10mm apart. I don't want to make the fins to fin over wise it becomes difficult to machine. But do you think I should make the gaps smaller than 10mm so i can get more fins in?

Surely you only need to use a bit of 4mmx15mm flat section alu to actually clamp an secure the chips to the heatsink. I say this because I don't have any room at front or the chip to mount a lump of alu of a heatsink for that mater due to the pcb layout.

Regards, Ben
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Old 31st July 2007, 12:21 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
if your fins are 26mm high (9mm backplate) then you could thin them down to 3mm thick without losing too much efficiency.
A gap of 6 or 7mm works for a passive sink.
That works out at 9mm (6+3) to 11mm (7+4) centres. 30 fins @3 to 3.5mm will cool better than 22 fins @ 4mm.

A slightly tapered fin will do no harm. 4mm at the root down to 2.5mm at the tip.

I presume you have the facilities. Lucky person.

4mm flat bar for a clamp might be just a little on the thin side. It will tend to deform as the clamping force increases. 5mm or 6mm would be better, but whichever, use a bolt on every side of the chips, don't leave a free end waving in the wind. I don't have any but Belleville washer under the bolt heads is ideal for removing some of the thermally induced stress/strain.

A level, what subject? Electronics related or production/craft or?
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Old 11th August 2007, 01:31 PM   #10
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Default cpu cooling fans

i did some experimenting with 12 volt cpu cooling fans and found that if you use a seperate wall wart transformer for the fan supply and regulate it using a 7812 you get no fan noise.
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