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Old 12th February 2009, 02:04 AM   #2871
labjr is offline labjr  United States
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Quote:
Originally posted by udailey
So copper dissipates more heat. Makes sense since it will spread out through the sink faster. But it looks like vertical mounting, for instance like Peter mounted his sinks, actually dissipates more heat. This doesnt make sense to me. You would think you would want the large surface area of the sink facing upward. Well, I would think that anyway
Uriah
Interesting discussion

Copper conducts heat faster but stores it longer. Aluminum dissipates heat faster. Therefore if you have two output devices which run very hot on a large surface and you want to mount heatsink horizontal with the fins straight up it would be better if it had a copper heat spreader since the heat will want to rise straight up and out the other side without spreading out and may have two hotspots.
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Old 12th February 2009, 02:26 AM   #2872
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Default Nomograph

Jackinnj, I love your nomograph, but the scales must have been set by a Class AB amp guy....we need thermal resistance numbers more like 0.1 to 1.0 !! Do you have the next page of this thing ??

I went through Conrad's spec tables and came up with thermal resistance (C/W) ~ 0.1 / (heat sink fin area in sq. meters), for typical side-mounted black aluminum heat sinks with 30-50 vertical fins. For instance, a Conrad MF35-151.5 has (35) 40mm x 151.5mm fins with a total surface area of .424 sq. meters, (not counting the base plate) and a spec. thermal resistance of .21 C/W for an 80 degree C rise.

There are, of course a million things to screw this up, like air blockage, surface emissivity, (i.e. how truly "black" the &^%$ thing is in the infrared), the thermal impedance between the MOSFETs and the sink, and placement of the device on the sink, (remember the heat sink temperature is a means to an end - the real thing you need to worry about is the device junction temperature, so you want to minimize the heat sink temperature AT the device location, not the average sink temperature.) In any case, I still think 0.1 / sq. meter C/W will get you pretty darn close in most cases.

Lesson-learned - Class A amps need more heat sink area than an old Porsche 911 air-cooled flat six, and are (almost) as much fun to work on !!!
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Old 12th February 2009, 02:42 AM   #2873
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Default Re: Nomograph

Quote:
Originally posted by dcbingaman
Jackinnj, I love your nomograph, but the scales must have been set by a Class AB amp guy....we need thermal resistance numbers more like 0.1 to 1.0 !! Do you have the next page of this thing ??
The magazine is dead, mort, tot, guasto and only had one nomo -- (how's that for onomatopaeia?) there's a thermal impedance tool at Aavid-Thermalloy's website fwiw.

You can demonstrate all this stuff for yourself with a DC power supply, a mountable resistor like a Dale or Caddock and a decent digital thermometer from Williams Sonoma and a little patience. You just cook the heat sink and measure the delta temp given the dissipation of the resistor. I let the heatsink cook for an hour as I usually have nothing else to do but match JFET's.
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Old 12th February 2009, 02:58 AM   #2874
pooge is offline pooge  United States
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Quote:
Originally posted by labjr


Copper conducts heat faster but stores it longer. Aluminum dissipates heat faster. Therefore if you have two output devices which run very hot on a large surface and you want to mount heatsink horizontal with the fins straight up it would be better if it had a copper heat spreader since the heat will want to rise straight up and out the other side without spreading out and may have two hotspots.
Copper does not store it longer and aluminum does not dissipate it faster.

Copper is denser so it has more thermal mass. That mass will collect more heat into it than the same volume of aluminum. Copper is more conductive, so heat will move (conduct) more easily through it. Both will be low in emissivity (their ability to radiate heat away from) when shiney. Color them black or another dark color, and they will radiate the collected heat better. That's why heat sinks are annodized black.

The vertical reference is directed to how the air can move across it when heated. Heated air rises, so if a plate it oriented vertically, the air will wash the heat out better as it rises along the plate. If the plate is horizontal, the air will rise off the surface and not be as efficient at removing heat.

However, if fins are very long, the resistance to air movement between the fins gets higher, and the air will move out of (bypass) the fins and move away from the heat sink, rendering the air movement less effective at removing heat. For a given volume and mass of heat sink, it is better to go wider than taller.
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Old 12th February 2009, 03:23 AM   #2875
ichiban is offline ichiban  United States
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Default Re: F5 Heat Sink Dissipation

Quote:
Originally posted by dcbingaman
Ichiban -
Three questions - 1) where did the temperature settle out at the FET's after a couple hours, 2) did the bias stabilize and at what current,
After four hours the temps were 56-57c along the top of the sink. Ambient room temp 68f. Temp at tab on mosfet was 88c.

I'm thinking when the sinks are attached to top/bottom/front/back some heat would transfer to them but probably running two amps/balanced on one Conrad 151 sink would not bode well for fet longevity. If bias current was lowered it
would be possible. As one poster has indicated distortion would go up.

Bias was set so that .740v was across all source resistances and after four plus hours that was where it remained. All resistors measured .58 ohms before installing on pcbs. When I measured them after this evenings session, with amp off, they measured .59 .65 .65 .55 ? Don't know why, maybe heat changed values ?

Also, I did a range check on the cen-tech meter and it read out -1/-2 on an ice cube and 98/99c at the top of boiling water. So, it looks to be off 1-2c and the hs temps are maybe 58-59c and 89-90c at tabs on mosfets.
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Old 12th February 2009, 03:32 AM   #2876
amp_guy is offline amp_guy  United States
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The best of both materials can be used effectively, quite a few cpu coolers have a copper plug pressed into the sink so that it rests on top of the cpu package. I have used some cpu coolers that are entirely made of copper and they worked very well , however the fins were quite thin and they had more of them than would be normal for an alu construction. the idea of a copper spreader bar I think is sound and worth the effort.

I would like to try an F5 but I can't justify another project right now , especially one that is not capable of driving my speakers (maggies).
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Old 12th February 2009, 04:23 AM   #2877
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Default MOSFET Junction Temperatures

Ichiban - If you look at the FQA19N20C datasheet, the maximum junction temperature is 150 degrees C, before you destroy the little devil. The device can safely dissipate 180 watts at 25 degrees C, but this goes down by 1.45 watts per degree C case temperature rise, (the P-channel device is a little less sensitive at 1.2 watts per degree). At 90 degrees C, the device can still safely dissipate ~ 85 watts.

At 1.3A bias, the device must safely dissipate ~ 31W so it looks like you still have some margin to spare for four MOSFETs on a Conrad MF35-151.5 - they are pretty tough little "F"er's. I would still mount them lower, use Kapton or Mica insulators and possibly use copper mounting plates or some of those surplus 17W Wakefield heatsinks on the BACKSIDE of the Conrad sink at the MOSFET location to lower the local heat gradiant below the ~ 20 degrees C you are seeing.

I don't know what these temperatures would do for the rest of the amp though.....I think Peter Daniel's open case design is a pretty good idea, if you can keep your pet cat, (or Scottish Terriers in my case), from sticking his nose in the thing and getting the thrill of his young life. (Mine like to "mark" everything that is new, but we won't even go there - urine is VERY electrically conductive - I know from personal experience.)

In my case, I will probably look into a very porous top cover to keep the inside power supply components, etc. as cool as possible. Papa likes to keep everything below 50 degrees C for a lot of good reasons - the MOSFETs are designed to get hot, but a lot of the other stuff, (caps etc.) isn't.
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Old 12th February 2009, 04:25 AM   #2878
Variac is offline Variac  United States
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Papa also mentions in his manuals that if your baby or pet urinates on the amp- unplug it ..
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Old 12th February 2009, 07:35 AM   #2879
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Default passive cooling

What about just drilling holes in the bottom and the top. As the interior heats up and the air rises it will draw in cooler air from the bottom, forcing the hot air out through the top. Or wouldn't this be enough?
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Old 12th February 2009, 10:00 AM   #2880
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Quote:
Originally posted by amp-guy
The best of both materials can be used effectively, quite a few cpu coolers have a copper plug pressed into the sink so that it rests on top of the cpu package. I have used some cpu coolers that are entirely made of copper and they worked very well , however the fins were quite thin and they had more of them than would be normal for an alu construction. the idea of a copper spreader bar I think is sound and worth the effort.

I would like to try an F5 but I can't justify another project right now , especially one that is not capable of driving my speakers (maggies).

Ichiban, I have a bunch of surplus little CPU like heatsinks you can glue onto the smooth side of a heatsink if you want. It will give you some more dissapation. I did this on my F4 to help bias a little higher.
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