Electronics > Projects, Designs, and Technical Stuff

DC load using a CPU cooler

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metacollin:
@microbug: I use a 1300W heat gun.  But anyway, you've got things under control and in the way you want thermally, sorry to keep talking about much earlier parts of the project.  Can't wait to see more of this project though!

@eneuro and the other guy talking about welding/brazing:

I would discourage anyone from spot welding copper for thermal purposes, and even more so actual IC packages to copper.  CPU coolers have heat pipes soldered to a thin copper plate which is intended to have heat from some watt nugget conducted into it.  It then spreads the heat and distributes it into the heat pipes. Then, very thin copper sheets will be soldered to the heat pipes, and sometimes additionally the copper sheets/fins are skived together to make the outer edges of the fins a rigid structure that is more diffidult to bend.  That is all that is holding together the CPU cooler in your computer, laptop, GPU, etc, and its more than enough.

But its soldered using Sn42Bi58, which is used extensively in thermoelectric applications, mainframes (for its tin whisker retardant properties), and niche industrial applications (sometimes used for large BGA packages, fro example).  It is very strong mechanically compared to electrical solders and has excellent wetting, is eutectic, very low thermal expansion, and fairly importantly in the context of thermal performance, it expands slightly as it solidifies, filling in any gaps and making a very secure thermal and mechanical bond between two surfaces.

You are using heat sink/heat pipe assemblies inappropriately if you require them to be stronger than what the thermoelectric soldering used in every commercial passive or forced air CPU and GPU cooler on the market can provide.  Heat conduction is a function of cross section, spot welding is, uh, not known for the cross section of the welds, and any weld will only penetrate so far.  You could not weld a semiconductor package such that the entire back surface was bonded without destroying the device. And beyond that...why? What benefit does welding or brazing heatsinks to each other offer?  Why use silver filler? Surely not for its thermal conductivity.  If you want heat to move from one place to another you don't wait for it to heat up a bunch of mass in between, watching it spread on timescales measured in _minutes_ through solid material.  If you want to move heat out of and quickly away from something, ore move it from one spot to another spot, you  use a heat pipe. Heat pipes transfer almost all heat energy to the 'cold' end in a couple seconds.  The thermal conductivity of heat pipes used for CPU cooling is in excess of 100 times that of copper. 


Anyway, I'm done rambling about heat.  and using a CPU (or GPU for even phatter watts) for a dummy load is a great idea, and the heat pipes they use will keep the actual MOSFET junctions at much lower temperatures as long as your airflow is removing heat as fast as they make it.  And, surplus server coolers, for xeons and the like, which usually need to shed 125W+, are both abundant and cheap on ebay.  I'm eager to see how things develop with the digital control and DAC. 

macboy:

--- Quote from: metacollin on October 02, 2014, 07:39:10 am --- Heat conduction is a function of cross section, spot welding is, uh, not known for the cross section of the welds, and any weld will only penetrate so far.

--- End quote ---
I agree and tried to make this point earlier. Spot welding a device to heatsink is a non-starter idea for many reasons, especially the one above.

microbug:
@metacollin, I'm glad you're interested!

I'm using an ADG715 for the analog mux rather than the PSoC 4; the PSoC 4's 400 ohm resistance makes it unsuitable (the ADG715 has a resistance of 2.5 ohms).

EDIT: Any ideas for removing the op-amp / DAC offset? I'm using the REF2041 to bias the op-amp at 2.048V, and the DAC outputs from 2.048V to 4.096V.

Jeroen3:
Socket 775 is relatively small compared to the newer ones. If you get some old Pentium 4 heatsinks for servers with heatpipes you'll also have somewhat compact model.
Like these: 66h080000-046.
The 3 heatpipes can all serve 1 TO-220, with a total of 3, or 2 and an LM35 in the middle, not sure if there is need for copper filling plates. Awaiting dimension spec from oem.

Does anyone know of temperature sensor ic's in TO220 that do not have the tab connected to anything?

microbug:
@Jeroen3, unless I'm wrongly estimating the size, that heatsink won't get close to 300W.

SeanB said this:

--- Quote ---Those copper rods are going to make poor contact, so you probably want the 3mm copper plate, machine it smooth on the faces and use the thermal epoxy to hold it and the devices together. You will not get good contact onto those flattened pipes without thermal epoxy or a good thermpad.
--- End quote ---

I think he probably knows what he's talking about!

EDIT: For the MCU, I'm going to use a PSoC 4 chip (not the PSoC 4 dev board). I can use the dev board to program the PSoC 4 with a bootloader that will allow firmware updates over USB (there's going to be a USB to UART chip too).

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