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Old 28th July 2007, 08:22 PM   #11
Nordic is offline Nordic  South Africa
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When you have it flat, the air has to fill in from the side almost like water, filling a valley, which is much less efficient then when it just speeds up travelling upwards...

Cutting it in two, will allow you to use it as the sides of a case + give you the benifit of vertical convection...

A tablesaw also warks nicely for cutting these square..., just make sure you have the sink solidly attached to a large plank.

I preffer LM3875 as it is very easy to knock an amp together just useing stripboard, snipping off unused legs, and bending other legs to fit.
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Old 28th July 2007, 08:58 PM   #12
pchw is offline pchw  United States
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Ah, the magic is the ability to use the HS as the sides and align the fins vertically. Thanks!!
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Old 28th July 2007, 10:29 PM   #13
svbear is offline svbear  Canada
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Amazing as you. You guyz know your stuff!

If I have to cut the heatsink, I might just instead get the 10inch long ones that the guy offers and make them upright thin amps.

I figure I can put 4 amps on one 10inch long heatsink upright with no problems.

I think I will get the pcbs from brian and do some pin leg manipulation to aline the correct pins the the holes on the pcb.

At $3 per pcb, i think it's a pretty good deal, nice quality.

My origianl plan was to use heatsinks from some PII chips that I was tossing away, but I am scared that they are too small.

Is it better to use 220uf or 100uf on the rails on the chips pcb board close to the amp chip?

Thanks,

adrian
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Old 29th July 2007, 03:30 AM   #14
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svbear,

I could use a couple of those heatsinks myself. Is this someone online selling these or someone local to you? If it's an online source could you please post a link? Thanks!

-claiborne
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Old 29th July 2007, 04:04 AM   #15
imix500 is offline imix500  United States
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He's on ebay...

http://cgi.ebay.com/NEW-Rockford-Fos...QQcmdZViewItem
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Old 29th July 2007, 09:04 AM   #16
AndrewT is online now AndrewT  Scotland
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Quote:
I figure I can put 4 amps on one 10inch long heatsink upright with no problems.
two chips on a 5inch high sink will run cooler.

one chip on a 2.5inch high sink will run cooler than either of the suggestions above.

I assumed that all combinations had the sink standing up and external to the case.

The cooling capacity of a particular vertical sink is roughly proportinal to the square root of the height. Double the height and one gets about 1.4Times the dissipation capacity. Quadruple the height and one gets about double the dissipation capacity.

If each chip is putting out 3W quiescent power and fitted to a single heatsink with Rth s-a of about 2C/W then the sink operates at about 6C degrees above ambient temperature.
Put 4 chips on on a sink 4times taller with Rth s-a of about 1C/W and the sink will operate at about 12C degrees above ambient.
You have just increased the Tc of the chips by 6degC.

Now put some power through the chips, let's assume each chip is putting out just 2W of signal output. Do the numbers.

BTW,
if you lie the sink flat with the fins pointing up, it's dissipation capacity drops significantly. i.e. your chip heating gets even worse. A very wide and very long sink relative to the fin heght suffers more than a sink with tall fins in this position.
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Old 29th July 2007, 11:07 PM   #17
svbear is offline svbear  Canada
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Yup, that's the guy selling them on ebay. He also has the newer rockford heatsinks which are powder coated texture for a bit more.

Andrew, thanks again. I had a feeling the using a wide foot print would not make up for short fins. That heatsink is actually fan cooled with a shroud from rockford.

One of the helpful members here forwarded me a good spotting of surplus heatsinks at A1 electronics. It might be easier for me to go local.

But If I have to cut the heatsink down, any recommened methods without buying a new chopsaw. I do have table saw, which blade do I need?

Thanks,

Adrian
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Old 29th July 2007, 11:23 PM   #18
OzMikeH is offline OzMikeH  Australia
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A fine Tungsten carbide tipped blade will cut aluminium well. It will leave a few little burrs but these can be filed off easily.
Take it slowly, use a blade with lots of teeth, NOT a ripping blade.
Use a bit of wax to lube it,

Do NOT stand in line with the blade - if the blade grabs any fine offcuts it will throw them at you at high velocity, wear a face shield and safety glasses because metal will be going everywhere.
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Old 30th July 2007, 04:15 AM   #19
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Thanks for the link to the heatsinks. I think I have a project or two that these will work with.

-claiborne
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Old 30th July 2007, 01:36 PM   #20
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svbear,

I use PII (2.5w*5l*2h) heatsinks for my lm3886 powered computer speakers. I've not had any problem with this configuration, but I have a low voltage supply (18-0-18 at rails). Not a demanding application my any means, but just an example for you!

added: I love the blade advice given above. That is experience speaking at it's finest! Please do be careful if you try to cut the heatsinks. I have actually seen the carbide tips fly off a blade while cutting aluminum and chip a cinder block wall. Please heed the advice given above.
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