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Heatsink C/W rating question
Heatsink C/W rating question
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Old 16th October 2017, 11:49 PM   #1
brian92fs is offline brian92fs  United States
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Heatsink C/W rating question
Default Heatsink C/W rating question

I see that the C/W ratings for various size heatsinks are indicated in the store. Is there any information as what temperature rise this is spec'd at?
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Old 17th October 2017, 12:20 AM   #2
rayma is online now rayma  United States
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Originally Posted by brian92fs View Post
I see that the C/W ratings for various size heatsinks are indicated in the store.
Is there any information as what temperature rise this is spec'd at?
Normally it's from ambient.
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Old 17th October 2017, 01:00 AM   #3
brian92fs is offline brian92fs  United States
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Heatsink C/W rating question
My question is for the amount of rise. It's my understanding that the rating is based on a amount of rise that the manufacturer chooses when testing (i.e. 50C rise from ambient of 25C). Or am I mistaken? So if you wanted your heatsinks to run cooler than the spec'd rise, you'd need to take this into account.

As an example, this manufacturer specs at a 70c rise and provides a correction factor for lower rises.

Last edited by brian92fs; 17th October 2017 at 01:09 AM.
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Old 17th October 2017, 01:55 AM   #4
rayma is online now rayma  United States
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Originally Posted by brian92fs View Post
the rating is based on a amount of rise that the manufacturer chooses when testing (i.e. 50C rise from ambient of 25C).
Or am I mistaken? So if you wanted your heatsinks to run cooler than the spec'd rise, you'd need to take this into account.
You can push things as far as the device junction temperature will allow,
to minimize the derating factor, but a sink temperature of around 60C is as high as most here would go.
Also remember that the Rth is based on a uniform thermal loading of the entire useable sink surface.
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Old 17th October 2017, 02:10 AM   #5
brian92fs is offline brian92fs  United States
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Heatsink C/W rating question
Don't we still need the temp rise the heatsinks are spec'd at? If I make my calcs for a max temp of 60c and assume ambient of 25c, I need c/w spec'd for 35c rise. If it's spec'd at 70c rise, then I need to adjust. Without knowing this, how do I select a chassis and it's related heatsinks other than trial and error?
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Old 17th October 2017, 02:22 AM   #6
Ron E is offline Ron E  United States
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76125 - Extruded Heat Sinks
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Old 17th October 2017, 02:30 AM   #7
rayma is online now rayma  United States
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Quote:
Originally Posted by brian92fs View Post
Don't we still need the temp rise the heatsinks are spec'd at? If I make my calcs
for a max temp of 60c and assume ambient of 25c, I need c/w spec'd for 35c rise.
Aavid gives a correction factor, but it's still one of a number of variables, like the device thermal interface,
how flat the mounting surfaces are, torquing of the mounting bolts, usage factor of sink surface, etc.
The temperature will vary significantly over the sink surface, even with a long stabilization time.

Last edited by rayma; 17th October 2017 at 02:39 AM.
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Old 17th October 2017, 02:52 AM   #8
brian92fs is offline brian92fs  United States
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Heatsink C/W rating question
Sorry, this is the DIY Audio Store forum. Didn't mean to get sidetracked into methodology for calculating heatsink requirements.

But I would still like to know if more detailed info is available for the heatsinks offered.
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