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Electronic load - getting rid of heat

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Harvs:
I'm putting together a DIY electronic load for testing car amp SMPSs that I'm building (just as a hobby to keep my brain active.)

Really I need something that can dissipate at least 150W continuous to put these SMPSs under any real load.

I'm thinking of using a 2U rack enclosure since I already have a couple of other instruments in this size that will stack neatly in the lab.  Or another alternative would be a PC case, but that wont be as attractive.

Anyway, I'm trying to think out of the box a bit when it comes to heat sinking.  The local retailers (Jaycar & Altronics) have a number of traditional large slab heat sinks, a couple of those with the addition of several 80mm fans might do the trick.

Another thought was putting something together out of CPU coolers, with something like the thermaltake Contac16 at only $20AUD there's potentially a fair value per watt (however there's no actual figure on how much heat they can get rid of, only that their good for 100W CPUs.)  But they're going to be fairly complicated to attach devices to.  Anyone tried putting TO220 devices on something like this before?

Any other ideas for me?

Thanks

amspire:
At $20 each they are a bargain. I am sure each heatsink is easily good for at least 80 watts, and $40 of heatsink to get rid of 150W is not bad at all. There are definitely coolers that can easily do the 150W.

It is really hard for non-heat tube heatsinks to match the performance of a heat tube based heatsink. It is amazing how much heat drop you get across a large slab heatsink and a heat drop means wasted efficiency, so you do have to spread the heat out with mulltiple FETs to get the heatsink to work efficiently. I suspect $40 of fan+slab heatsink will see you well short of 150W, and it will take up a lot more space, and weight.

For mounting a FET, I would drill and tap a hole in the base between the heat-tubes or even a pair of devices. Not hard to do if you have any kind of drill press. Drilling all the way through metal means a single tapping operation rather then two or three with different tapered taps. Definitely add a temperature sensor, as if there is a fan problem, the heat will rise very quickly. Best if you can get away with a metal to metal contact from the fet to the heatsink. Then a minute amount of silver or diamond loaded heatsink compound and you are set. An insulated thermal pad will lower the power, but at least you do not have a live heatsink.

If you need to get rid of all 150W in one heatsink, I think you will be looking at $45-$50 coolers.

Richard.

Harvs:
Thanks Amspire, if I can really just put a drill down between the heatpipes then it should be easy as pie. Looking at the pics in the product page it looks like there may be as much as 5-6mm between the pipes.  I think there should be enough room to get a couple of TO220s on there somehow.

http://www.thermaltake.com/products-model.aspx?id=C_00001805

To put it into perspective, $20 isn't some price off eBay that I'd have to wait 4 weeks to receive either.  I can go pick them up at that price from the local bricks and morter msy computer store.

With the CPU coolers I think it will be easy to isolate them in the case.  It's never going to see more than about 60VDC.  The large slab/fin heat sinks however would be rather difficult to isolate from the outside and provide adiquite airflow (certainly not impossible, but just more of a PITA.

I don't plan on putting more than about 30W per FET.  FETs are cheap as chips, and I've got a bag of IRF830's kicking around from a previous project so they're effectively free.  Even if I could get 2x FETs per CPU cooler, and use 3 coolers they would still easily fit in the 400mm+ width of the 2U rack case.

SeanB:
The CPU coolers work well, and are pretty cheap for what you get. Just buy a few of the cheapest ones that are available at your local computer shop, most likely LGA775 units ( cheap but capable of dissipating 100W while keeping the heatsink at 70c) and mount them in a 3U case, such that they draw air in from the front ( you will need a 120mm hole with guard on the front for each, so you will lose basically 2/3 of the front panel to the air inlets) and blow it out the back through a similar guarded hole. The Intel ones have a small solid centre for the device, and the airflow can be radial through the device with no turning corners so giving max airflow. You will only really fit a single TO220 or TO3P on each one, and it will be good to add a NTC thermistor  in a drilled hole to sense heatsink temperature. The fans will be run at full speed off 12V, and the sensor wires can be used as a input to a micro ( or a simple frequency discriminator, a 555 arranged as a retriggerable monostable will work as well) to detect a fan failure, the NTC thermistors can be used with 2 cheap panel mount thermometers to show heatsink temp. You could go up to 200W with 2 fans and 2 power transistors, using the outlet air to cool the 2 emitter resistors ( you can make them high enough in value to dissipate 50W of the max power as well, making the transistor control easier). Mounting will require making 2 brackets to hold into the fan screws, as these will no longer be mountable as designed, as there will be no board to screw through.

Harvs:
Interesting idea, I know the sort you're talking about because I've got a computer with that style of cooler.  But the list of coolers that the local computer store has (which is actually quite extensive) are all heat pipe style except for the intel cooler (pic attached @ $19.) So they're all like the TT one attached

Then there's the issue of trying to find a tap small enough for the tiny fine thread screws...

I might just pick up one of the Thermaltake ones tomorrow and play around fitting it in a case.  Since it will never actually be fitted in a rack as such, just stacked with other instruments, maybe mounting two either side drawing air in from the sides and then venting at the back?

Just for giggles I just mounted a couple of FETs on an old video card cooler I had lying around.  Worked surprisingly well! With 60W it maintained a device temp of about 63 degrees...

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