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Old 2nd July 2002, 06:43 AM   #11
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Default heatsink question

Yes, mostly bigger is better. But how much bigger do you need to be better? So, even if you don't wont any stinkin calculation, you need at least some intelligent reasonning to get to the ballpark, right?

Treat heatsinks as resistors for heat current, and dissipation as heat voltage.

Example: Your output stage dissipates 100 watts, and you want to limit the max temp to 90 degrees at 40 degrees ambient, this is a temp rise of 50 degrees, right? You need a heatsing that has at most 0.5 degrees per watt "resistance". Using a larger heatsink of say 0.25 degrees per watt resistance, multiply by the dissipation gives 0.25 times 100 is 25 degrees temp rise above ambient.

If you use a bracket between the output transistors and the heatsing, that add "heat resistance" in series to the heatsink.

There is also the "heat resistance" of transistor junction to case, say 0.15 degrees per watt (don't know how realistic this is, look at the data sheet).

Example: use the 0.25 degrees per watt heatsink with a bracket that adds say 0.1 degrees per watt, and a transistor with a 0.15 degrees per watt juntion to case heat resistance, total 0.5 plus 0.1 plus 0.15 = 0.5 degrees per watt. With your 100 watt dissipation that gives 100 times 0.5 equals 50 degrees temp rise. With 40 degrees ambient that gives a max temp of the output transistor junction of 90 degrees.

Easy. It's surprising how far you can yet in amplifier design with just some secondary school math....


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Old 2nd July 2002, 11:11 AM   #12
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Hi Circlotron,



Quote:
Originally posted by Circlotron


If you have the choice between a tall skinny heatsink and a short broad one with a same area, the short broad one will win every time. Only the first couple of inches seems to do anything much, after that the rising air has been warmed enough as it rises through the fins that it cannot cool any further fins higher up.

GP.
to reduce this effect you must mount the transistors as high as possible so the temperature difference between air and sink keeps as high as possible. In most amps I see people mount the transistors as low as possible for some mysterious reason.

love fans too! I can get away with 4x0.65K/W for an Aleph5 (250 watts dissipation) and a bit over 50C heatsink temp.

Apogee,

its not so difficult to calculate for paralleled heatsinks. If you use 4 x 0.65K/W you get 0.16..K/W. You must just add the thermal resistance for the connection from aluminium to heatsink wich is quite low (measured almost <1C difference between them)


william
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Old 2nd July 2002, 03:44 PM   #13
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I don't calculate either.
I always use Fischerelektronik LA1-10A with Papst 4184NXH and i never bother about noise because there is no noise.

Nick
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Old 3rd July 2002, 12:47 AM   #14
Apogee is offline Apogee  United States
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Thumbs up Thanks all for the help!!!

I appreciate it!!!!
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Old 3rd July 2002, 12:50 AM   #15
jleaman is offline jleaman  Belgium
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What was the link to the heat sink store i got ther catoouge but i cant find it in my mess ..
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Old 3rd July 2002, 01:38 AM   #16
Apogee is offline Apogee  United States
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JasonL:

Go here:

http://www.diyaudio.com/forums/showt...tals#post22723

Good luck!
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Old 3rd July 2002, 03:19 AM   #17
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Quote:
Originally posted by Nelson Pass
Calculations? We don't need no stinkin calculations! We just
make them really big!
I love this philosophy too...
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Old 3rd July 2002, 05:07 AM   #18
jleaman is offline jleaman  Belgium
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Default HUmm

Im thinking about using these for my aleph 5 amps.. any one comment on this..
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Old 3rd July 2002, 05:08 AM   #19
jleaman is offline jleaman  Belgium
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or i could use 2 of these witch are verry heavy and thick and came off come old radio equipment from my dads work..
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Old 3rd July 2002, 12:51 PM   #20
PedroPO is offline PedroPO  Portugal
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Default Heatsink fashion '02

Check this drawing of my future SOZ

There you can see a cross section of the profile I'll use.

It has 0.30m of heigth.

(all the measures are in meters).

the weigth of each one is 12kg.

Later i'll post a real picture
!
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