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Old 23rd May 2013, 02:24 PM   #1
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Default Another heat sink size question

Sorry for yet another heat sink question.

Initially I bought the larger heat sink for a LM4780 and the smaller one for LM3886TF.

Click the image to open in full size.

As size comparison LM3886 and its board are included in the picture. The exact heat sink dimensions are:

Larger: 300x100x26mm
Smaller: 150x92x40mm

However, I wonder if I can get by with both chips on the larger heat sink? The heat sink is going to be mounted externally on top of the chassis.

Transformer voltage: 2x18V
Load for LM3886: 1x4Ω speaker
Load for LM4780: 2x4Ω speaker (in stereo mode)
Ambient temperature: Lets assume ~28-30˚C

Thanks.
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Old 23rd May 2013, 02:32 PM   #2
Mihkus is offline Mihkus  Estonia
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You sure can!
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Old 23rd May 2013, 02:57 PM   #3
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The heatsink is best mounted so that the fins are orientated so that air freely circulates up the fins due to natural convection. With a huge heatsink that shouldn't be too much of an issue but I would "birds nest" run the amps on their heatsinks to see how warm they get before mounting everything into a case.
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Old 23rd May 2013, 03:05 PM   #4
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Thanks guys.

Note: the bigger heat sink has a shorter fin length.

By "birdnest" I assume you mean the fins on top and the flat side underneath?

I would really prefer a heat sink that has its fins oriented the other way. But since it would be obscenely tall if I mount it standing, the only other way would be to mount it "birdnest" style on top of the chassis.

Last edited by Navyblue; 23rd May 2013 at 03:08 PM.
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Old 23rd May 2013, 03:21 PM   #5
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Quote:
Originally Posted by Mihkus View Post
You sure can!
You sure can not !

A heatsink like that is efficient up to 100-150mm length max.
Beyond that length, it brings absolutely nothing to the table, expect in the order of 1.5 C/W at best. (need a reference ? )

Mounted flat on top of the case, thermal efficiency turns a lot worse.
Especially this one, due to few ribs, with only 4/5th of an inch rib height.
Raises the number to above 2C/W.

18V transformer => ~25V rails (assume 25C ambient, numbers will be worse for 30C )
LM3886 datasheet => 3C/W heatsink
LM4780 datasheet => < 1C/W heatsink (for 2 channels)

1/3 + 1/1 = 4/3 W/C => 0.75 C/W Heatsink !
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Last edited by jacco vermeulen; 23rd May 2013 at 03:25 PM.
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Old 23rd May 2013, 03:35 PM   #6
AndrewT is offline AndrewT  Scotland
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Vertical finned heatsinks are designed and specified for vertical fin orientation !!!!!!!
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Old 23rd May 2013, 03:36 PM   #7
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26mm is more than an inch?

And would that calculation stands with more than 1 heat sources? If the heat sink has a rating of 2˚C/W with heat radiating from a single point, surely the rating can not be the same with the same amount of heat radiating from opposite ends especially with such a long dimension (you said efficiency tops at 100-150mm)?
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Old 23rd May 2013, 03:46 PM   #8
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Quote:
Originally Posted by Navyblue View Post
Thanks guys.

Note: the bigger heat sink has a shorter fin length.

By "birdnest" I assume you mean the fins on top and the flat side underneath?

I would really prefer a heat sink that has its fins oriented the other way. But since it would be obscenely tall if I mount it standing, the only other way would be to mount it "birdnest" style on top of the chassis.
Birdnest means just connect it up as a mass of wires and see how it works best.
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Old 23rd May 2013, 03:47 PM   #9
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Quote:
Originally Posted by AndrewT View Post
Vertical finned heatsinks are designed and specified for vertical fin orientation !!!!!!!
Andrew, I phrased my post to try and prevent confusion. Vertical fins can be read as lying on its back. The fins need to be, as the OP suggested, in the obscenely high orientation.
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Old 23rd May 2013, 03:51 PM   #10
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Quote:
Originally Posted by AndrewT View Post
Vertical finned heatsinks are designed and specified for vertical fin orientation !!!!!!!
When the fins are pointing up, does it count as "vertical fin orientation"?
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