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Old 12th June 2004, 07:42 AM   #1
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Default Heat sink schematics

Hi,

My first post in this forum!
I'm thinking of building an aleph5, and is pondering about the heatsinks. If I have the math correct, I need heatsinks with Rth = 0,133C/W for a total of 300W. Assuming ambient 20C and keeping temps below 60C. (60C-20C)/300W
Please correct me if I'm wrong as I'm an noob at this.
I've found an company in Sweden that supplies sinks that looks interesting, www.webra.se, but I'm not sure I understand the graphs for their sinks.
Looking at the following:
http://www.webra.se/html/products/w-150-44-11-22.htm
As far as I understand Rth is 0,08 for 157x200, but surely this can't be? I recon I would need alot more cooling than one of those!?
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Old 12th June 2004, 02:30 PM   #2
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Those figures could be correct, notice that they used fans: at the bottom of the graph you can see how much air flow is needed to get to a certain C/W.
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Old 12th June 2004, 02:42 PM   #3
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Thanks for the reply. I understand they used fans. I assume 0 in that graph yields normal and not forced convection? If thats true then an Rth of 0,08 will result in only 24C temp rise if 300W of heat is applied. This sounds a bit to incredible. I'm just waiting to say doh! and smack my head somewhere here.
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Old 12th June 2004, 03:11 PM   #4
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Default Re: Heat sink schematics

Quote:
Originally posted by Llafriel

I need heatsinks with Rth = 0,133C/W for a total of 300W.

Hi there, well you need one with 0,133C/W or two (one for each channel) with about 0,25C/W

I think

/Tobias
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Old 12th June 2004, 04:12 PM   #5
JEKYLL is offline JEKYLL  France
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Heat sinking for A5: total 0.08 deg/watt
http://camars.kaist.ac.kr/~cgkim/audio/amp/zen/www.passlabs.com/a5lit.htm
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Old 12th June 2004, 08:57 PM   #6
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Correct me if I am wrong but during active operation the Aleph 5 consumes 300 watts. Since the Aleph 5 is a stereo amplifier each channel comsumes half of that. For a single channel 60 watts of the 150watts is passed to the load and the other 90 watts is dissipated as heat. To be conservative lets say we need to dissapate 150 watts of heat. with a heat sink rated a .25/watt we have 20+37.5 or 57.5 degrees C. Of course you could pick a better heatsink than .25/watt but that is up to you.
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Old 12th June 2004, 10:14 PM   #7
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Default Welcome Llafriel!

I think the lowest curve on the graph is if the device covers the whole heatsink surface (not likely). The upper two curves are for smaller devices (2*52 mm and 2*25 mm ). Remember to include the Rth of the devices (internally) and isolation pads (if any) in the calculation.
I'm no expert myself, but those look like good sinks to me!

Btw, thanks for the link, i had never heard of Webra before.
Might be useful for some future project.

regards
/Andreas
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Old 13th June 2004, 02:22 AM   #8
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Personally, I recommend trying to keep the sinks down
around 50 deg, so that nobody loses any skin accidentally.

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Old 15th June 2004, 11:08 AM   #9
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The 0.13*C/W spec on that sink is with a distributed load and in 20 lineal feet per minute air flow.

Not stated is that sink manufactureres state the thermal resistance with a difference in temperature of 100*C (unless otherwise stated).

This is why the forced convection chart does not start at zero, in natural convection with a 100* delta the airflow from convection will be quite high.

Since Nelson likes to see a delta of 25* to keep the sink under 50* in a 25* ambient, the 0.13* spec is bogus unless you add a fan.

How bogus?

The tight spacing on those fins indicates this sink is designed for forced convection.

Without a fan and with only a 25* delta I would bet it is no better than 0.5*C/W, in other words, about 4X worse.
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Old 17th June 2004, 10:56 PM   #10
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Sorry for the late reply, and thanks for all the answers. I've done a bit of searching on this forum and found www.conradheatsinks.com. So I'm gonna order heatsinks from them. Good prices, even when freight is added. And I think I'll get the temperatures below 50C so even mr. Pass' fingers are kept unharmed. (How great is it not to get input from the man himself?)
I'm planning to use a 400VA 2x30V transformator from www.trafoshop24.de, and 8 22000uF/63V Aerovox caps from www.reichelt.com pr channel. Are these good components?
I'm uncertain however where to get transistors, as those at reichelt.de seems to be unnamed?
Anyone who knows where to get some ok transistors or if those at reichelt are good enough?
Thanks again for the help.
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