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Old 5th June 2011, 07:17 AM   #1
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Default Heatsink Question

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During an Aleph 4 build it was necessary to reduce the number of heatsinks on the rear side to one instead of the two on the other sides - 7 in total.

Question is, why is the heat not transferred from the back panel to the heatsinks on the sides ?

All alluminium plate and angles are polished on their adjoining surfaces and smeared with a very light coating of heat transfer compound.

The heatsink at the back gets warm but the one on the left remains stubbornly cool.

How could I improve the sharing of the heatsinks ?
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Old 5th June 2011, 11:28 AM   #2
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Any advice available on the use of heatpipes. I've never tried using them.

All I want to do is to share 7 heatsinks amongst the 24 MOS-FETs that are spread around the F4.
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Old 5th June 2011, 11:32 AM   #3
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One would assume - Obviously incorrectly.

That the MOS-FETs on the rear plate would heat up the 10mm alluminium rear plate. SO FAR SO GOOD, the rear plate gets HOT.

That the LEFT HAND alluminium bracket would act like a normal heatsink bracket.
Even though this is tightly secured against the back panel and is thermally tied to it, as it is to the left hand panel, it doesnt seem to transfer any heat to the Left Hand panel.

??????????????

Would a heat pipe help in any way ??
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Old 5th June 2011, 11:48 AM   #4
AndrewT is offline AndrewT  Scotland
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The standard rule I have applied is that the backplate of the heatsink should be ~ 1/10 the maximum radius from hot device to extreme corner of effective heatsinking area.

Your single heatsink could be 200mm wide by 120mm high by 8mm thick backplate with 40mm by 4mm thick fins.
The fins have a 10:1 ratio for good heatspreading ability.
The 8mm thick backplate will have good heatspreading ability if the largest radius from hot device to extreme corner is <=80mm.

If you add on, via a perfect interface, but increase the spreading distance to 160mm then the ratio has moved to 20:1. If the add on heatsink is 200mm wide then the total spreading distance from the nearest hot device is ~250mm. The ratio is now ~30:1.

The distant heatsink can never be an effective radiating area. It is simply too cold due to too much thermal resistance.
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Old 5th June 2011, 12:41 PM   #5
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So my total backplane thickness, Heatsink + Alluminium Plate is 20mm thick. I would have expected some heat transfer through the corner bracket to the adjoining heatsink.

Again my question. Would a heatpipe help to conduct the heat away from the poorer heatsink on the rear panel. If YES, how are they best employed ?
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Last edited by Andy5112405; 5th June 2011 at 12:57 PM.
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Old 5th June 2011, 01:04 PM   #6
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Quote:
Originally Posted by Andy5112405 View Post
So my total backplane thickness, Heatsink + Alluminium Plate is 20mm thick. I would have expected some heat transfer through the corner bracket to the adjoining heatsink.

Again my question. Would a heatpipe help to conduct the heat away from the poorer heatsink on the rear panel. If YES, how are they best employed ?
Thermogen does not flow good through narrow objects...
It's highly ulikely to get proper mating between the parts. Somewhat 3-5um roughness and 30um per 100mm straightness/warpage. If you insist on perpendicularity of parts, try T connection, i.e. shortening the path for thermogen
You may search for hockey puck diodes/triac heatsinks which are probably the best shot without going beserk
Sure heatpipe will tranfer the heat if (again) properly implemented. Have not seen them on avail, though... besides proc. coolers
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Old 5th June 2011, 01:11 PM   #7
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I cant afford to replace the heatsinks. I just want to spread the heat through them as best as possible. As they stand they are not getting too hot. The three DOUBLE heatsinks are actually quite cool, I just wnt to reduce the heat of the back panel with its single heatsink.
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Old 5th June 2011, 04:39 PM   #8
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Quote:
Originally Posted by Andy5112405 View Post
I cant afford to replace the heatsinks. I just want to spread the heat through them as best as possible. As they stand they are not getting too hot. The three DOUBLE heatsinks are actually quite cool, I just wnt to reduce the heat of the back panel with its single heatsink.
If you have an access to milling shop, I would suggest to make an additional heatsink of yours u-shaped to increase mating surface. I mean bend the plate and mill the mating surfaces.
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Old 5th June 2011, 09:18 PM   #9
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You could make some heat pipes yourself. All you need is some copper tubing to make the heat pipes ans access to machine shop to machine some channels in the heat-sinks and plates.
Hers is guy who made some of his own heat pipes.
Benchtest.Com - Heat Pipe 1

Let us know how it turns out.
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Old 6th June 2011, 04:00 PM   #10
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Solution.

I'm not a great favourite of fans because of the noise they make. However 120cm PABT 24V fan running at 12V is almost silent.

With transistor bank dissipating 250W the heatsink is sitting at a happy 45 Degrees on the transistor side and cool to the touch on the outside.
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