How many 'c/W

Leolabs · 23rd September 2006, 03:03 PM

For a LM3875 running at +/-25V into 4 ohms,is a 2 'c/W heatsink enough??

SpittinLLama · 25th September 2006, 05:15 PM

Depends.

What is the ambient air temperature?

Is that using the non-isolated T package or isolated TF package?

Is there a mica washer if using the T package?

What is the min and maximum supply voltage rails?

Or you can just download the Overture Design Guide from National, put all the info in yourseld and see what it tells you about thermals.

http://www.national.com/appinfo/audio/

Middle column about 1/2 way down the page.

-SL

Leolabs · 26th September 2006, 01:47 AM

Quote:

What is the ambient air temperature?

Preferably not more than 50'c.

Quote:

Is that using the non-isolated T package or isolated TF package?

T package.

Quote:

Is there a mica washer if using the T package?

Yes.

Quote:

What is the min and maximum supply voltage rails?

Stable 25V.

Nordic · 26th September 2006, 10:19 AM

My Gainclone works fine on 24V rails an heatsinks about the size of my hand, but a bit shorter, about 3 inches... gets to about 50 degrees, but seems quite happy... Someday I will integrate everything into a new case and get larger sinks.... but noone died in the meantime.

Leolabs · 27th September 2006, 02:06 AM

Quote:

Originally posted by Nordic
My Gainclone works fine on 24V rails an heatsinks about the size of my hand, but a bit shorter, about 3 inches... gets to about 50 degrees, but seems quite happy... Someday I will integrate everything into a new case and get larger sinks.... but noone died in the meantime.

What is the exact size of your heatsink???

Nordic · 27th September 2006, 08:05 AM

speaking from memory.. was a while since I made the case.... about 8 x 5 x 1.5cm.... i.e a little small... idealy you need something just a bit bigger than this....

dietcoke · 27th September 2006, 04:42 PM

Could one get away with using a smaller than recommended heatsink if used with a small quiet fan? +/-18 with small 8ohm bookshelf speakers.

Nordic · 27th September 2006, 06:43 PM

Fans make a big difirence to the efficiency of a heatsink.... unplug you PC's CPU fan to see what I mean (just jokeing - please don't)... I would give it a test but sit close and make sure those sinks are not getting too hot....

Fans introduce a host of issues of their own from the noise created by the motor to (on badly designed systems) microphonics due to induced vibration...

23rd September 2006, 03:03 PM	#1
Leolabs diyAudio Member Join Date: Dec 2004 Location: Bukit Mertajam	How many 'c/W For a LM3875 running at +/-25V into 4 ohms,is a 2 'c/W heatsink enough??

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25th September 2006, 05:15 PM	#2
SpittinLLama diyAudio Member Join Date: Apr 2005 Location: In the Wild, Wild West	Depends. What is the ambient air temperature? Is that using the non-isolated T package or isolated TF package? Is there a mica washer if using the T package? What is the min and maximum supply voltage rails? Or you can just download the Overture Design Guide from National, put all the info in yourseld and see what it tells you about thermals. http://www.national.com/appinfo/audio/ Middle column about 1/2 way down the page. -SL

26th September 2006, 10:19 AM	#4
Nordic diyAudio Member Join Date: Sep 2005	My Gainclone works fine on 24V rails an heatsinks about the size of my hand, but a bit shorter, about 3 inches... gets to about 50 degrees, but seems quite happy... Someday I will integrate everything into a new case and get larger sinks.... but noone died in the meantime.

27th September 2006, 08:05 AM	#6
Nordic diyAudio Member Join Date: Sep 2005	speaking from memory.. was a while since I made the case.... about 8 x 5 x 1.5cm.... i.e a little small... idealy you need something just a bit bigger than this....

27th September 2006, 04:42 PM	#7
dietcoke diyAudio Member Join Date: Apr 2006 Location: United States NY	Could one get away with using a smaller than recommended heatsink if used with a small quiet fan? +/-18 with small 8ohm bookshelf speakers.

27th September 2006, 06:43 PM	#8
Nordic diyAudio Member Join Date: Sep 2005	Fans make a big difirence to the efficiency of a heatsink.... unplug you PC's CPU fan to see what I mean (just jokeing - please don't)... I would give it a test but sit close and make sure those sinks are not getting too hot.... Fans introduce a host of issues of their own from the noise created by the motor to (on badly designed systems) microphonics due to induced vibration...

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