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Old 11th February 2009, 08:41 PM   #2851
udailey is offline udailey  United States
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Andrew is right about the heatsink placement.
That hot air isnt staying stationary, its moving up and quickly. The heat doesnt spread DOWN the heatsink very far so mid sink placement is a waste of sink. Also all these heatsinks with the fins facing anything but straight up are being used inefficiently, but they still will work if they are big enough to make up for the fact that the heat isnt getting out as quickly as it would like.
A good thing to consider is a nice 1/4" thick piece of copper as a heat spreader and your heatsink will be used even more efficiently.
Uriah
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Old 11th February 2009, 08:47 PM   #2852
sekess is offline sekess  United States
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Default heat spreader

Hey Uriah,
Could you be more specific as to how to use a 1/4" pice of copper plate as a heat spreader? How do you best make the connection betwen the heatsink and the copper plate?

Thanks,
Steve
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Old 11th February 2009, 08:58 PM   #2853
udailey is offline udailey  United States
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You could use thermal paste to thermally connect the spreader and the sink. Connect the device to the spreader the same as you would to the heatsink. The copper will diffuse the heat quicker and more evenly than the aluminum and then the aluminum will take it from there.
So if you could get a hold of a piece of copper, shine it up really nice so it makes great contact with both other surfaces, or dont, if its really large probably not having perfect contact will still be fine. Since a spreader isnt necessary but is an advantage you can go with any size and I think that something about 2" by 3" would be fine. Ebay probably has metal vendors that will offer to cut it for you. You could go larger, but going larger gets into problems when the copper or aluminum is a bit warped and may not be making contact over the whole surface but you wont probably see that as it will be between the copper and aluminum. You will have to drill both the copper and the aluminum and bolt/screw it into place. Use lock washers so it never backs out.
Uriah
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Old 11th February 2009, 09:02 PM   #2854
sekess is offline sekess  United States
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Default heat spreader

Oh I see -- just 2" to 3" pieces per MOSFET. I thought you meant covering the entire aluminum surface the same size sheet of copper. I gottcha.

Thanks
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Old 11th February 2009, 09:30 PM   #2855
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Quote:
Originally posted by Peter Daniel
It would look like this: http://www.diyaudio.com/forums/showt...67#post1724767
Peter,

I took a peek at this and the parts count is almost like an SET amplifier (lower than most actually). Can you comment on the use of the Vishay/Caddock and which positions you used them in? I think it might be interesting to do the entire thing in nude Vishays as the parts count is so low.

Chris
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Old 11th February 2009, 09:33 PM   #2856
pooge is offline pooge  United States
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Aren't copper and alumimum together a no-no as far as corrosion.

Regarding my earlier comments about transistor placement on a heat sink, I found a blurb on ESP site. Don't know if it's the info I recall.

ESP: Heat sinks

See section 14.

Not saying that this is the last word. No rational is given. I'd like to find a good explanation.
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Old 11th February 2009, 09:48 PM   #2857
pooge is offline pooge  United States
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Quote:
Originally posted by udailey
Andrew is right about the heatsink placement.
That hot air isnt staying stationary, its moving up and quickly. The heat doesnt spread DOWN the heatsink very far so mid sink placement is a waste of sink. Uriah
That must be because the temperature differential between the air and heat sink are is greater below. It would be nice to see a good work on this issue, like they show for fin density and pressure drop, etc. I couldn't find anything. Looking back, Elliot's point about not having them mounted on the bottom is not really inconsistant with Andrew's. He may just be saying don't mount them on the VERY bottom.
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Old 11th February 2009, 09:50 PM   #2858
udailey is offline udailey  United States
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I would think that if you do not allow air inbetween them they would not corrode. I didnt know that btw but I think corrosion needs oxygen.
I would say Rod Elliot is the end all be all answer on most questions, however Andrew's suggestion of 40% from the bottom is not nearly the first time I have heard this.
While it may be true that heat doesnt care which direction it spreads in a solid it will care if it is trapped under that solid once it hits air. It will also make a difference if those fins are not facing directly up since once the heat hits the air convection is now in play and if the only surface facing straight up is 2" by 5" rather than lets say the face of the sink at about 12" by 5" by a hugely larger area since the fins have surface area as well, yes it will matter a lot. The heat will get to the top of the sink that is standing on end and not be able to get OUT of the sink whereas it will be able to dissipate much quicker with a few hundred square inches facing upward like when the fins are pointing up.
Uriah
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Old 11th February 2009, 09:52 PM   #2859
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Heatsinks are usually anodized, at least for external use, so copper/aluminum corrosion shouldn't be a problem.

I used copper spreader in F3 with good results:

Click the image to open in full size.

Quote:
Originally posted by chrismercurio
I took a peek at this and the parts count is almost like an SET amplifier (lower than most actually). Can you comment on the use of the Vishay/Caddock and which positions you used them in? I think it might be interesting to do the entire thing in nude Vishays as the parts count is so low.
In this amp, I used what I already had in stock, but my choice was also biased by previous experience with other amps.

So I used Vishay S102 for R9, R13 and R14.
Caddock MK132 for R10 R5-R8.
Riken for R1-R4 but only because I had that particular value only with Rikens.
Mills for R11/R12.
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Old 11th February 2009, 10:19 PM   #2860
pooge is offline pooge  United States
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Quote:
Originally posted by Peter Daniel
Heatsinks are usually anodized, at least for external use, so copper/aluminum corrosion shouldn't be a problem.

True, and probably also needs moisture. However, some of the Conrad heat sinks I've seen in group buys have a milled bright surface. Also, use of compound should probably be a great help.
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