Electronic load - getting rid of heat - Page 1
Forum section: electronics / projects

author: Harvs
author_rank: Super Contributor
timestamp: April 29, 2012, 07:53:45 am
content: 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

author: amspire
author_rank: Super Contributor
timestamp: April 29, 2012, 08:23:52 am
content: 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.

author: Harvs
author_rank: Super Contributor
timestamp: April 29, 2012, 09:24:32 am
content: 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_00001805To 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.

author: SeanB
author_rank: Super Contributor
timestamp: April 29, 2012, 09:49:56 am
content: 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.

author: Harvs
author_rank: Super Contributor
timestamp: April 29, 2012, 10:59:05 am
content: 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 attachedThen 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...

author: SeanB
author_rank: Super Contributor
timestamp: April 29, 2012, 11:38:12 am
content: Tap sizes I have tried for insulated cases are M3 or 1/8 BSF, with M4 working for non insulated cased transistors where you have the tab at ground ( 7805, or insulated heatsink).The heatpipes are nice, but beware of drilling into the active area, as often there is a network of channels to increase the heat transfer from the face into the active fluid, and they are difficult to refill with R134A after you have let it out. The Intel ones fit nicely into a piece of 75mm pipe which makes a nice duct to direct the exhaust airflow to prevent it just recirculating the same hot air.

author: Psi
author_rank: Super Contributor
timestamp: April 29, 2012, 11:46:29 am
content: You can significantly boost a heatsinks ability to dissipate heat if you put some of it underwater.The water will thermally couple to the metal and it will take some time to boil the water if it even gets that far.

author: SeanB
author_rank: Super Contributor
timestamp: April 29, 2012, 11:59:31 am
content: I used to have a sacrificial 2N3055 with 3 leads soldered on that was dropped into a cup of water and used with a resistance decade box as a dummy load to test power delivery on the odd occasions I worked on that power supply. It did make a nice cup of tea, and I rarely had to replace it from overload unless all the water boiled off.I am using an Intel LGA775 heatsink without the fan as a replacement for a halogen lamp in my desk lamp. It has a 1W Led puck screwed to it. The die runs at most 5c above ambient, even though it is enclosed with minimal airflow

author: M. András
author_rank: Super Contributor
timestamp: April 29, 2012, 12:25:31 pm
content: http://www.arctic.ac/en/p/cooling/cpu/365/freezer-13-pro.html?c=2181this one can dissipate 300 wats according to the manufacturer claim, but would be tricky to drill the baseplate of this, i think a holding clamp would be better otherwise you damage the pipes

author: SeanB
author_rank: Super Contributor
timestamp: April 29, 2012, 12:37:15 pm
content: With that one I would recommend making a spreader bar that clamps the transistors by the plastic cases without bowing. A thick ( 20mm or so ) square bar would be ideal, and can be used as well as a holder for some tag strips to handle the lead wiring, to make an assembly that is easy to test before installation for any problems. It will have to screw into the existing holes for the sheet steel mounts after removing them

author: Harvs
author_rank: Super Contributor
timestamp: May 01, 2012, 09:43:43 am
content: So I've had some success, I got a couple of Thermaltake Contac 21's since that's what the computer store had in stock. Apparently good for 140W CPUs.They come with a bunch of mounting hardware that with a little bit of creativity you can use as a clamp. In the pic I have 4x TO220 mosfets over the 4 heat pipes.I tryed pumping 120W into it, since that was the limit of my bench supply. Worked like a charm, heatsink temp stabalised at about 48 degrees C. The devices were just a couple of degrees above that. So clearly the 140W rating is quite realistic for this application... So with the two of them, I should be good for close to 300W.Edit: Pic attached

author: M. András
author_rank: Super Contributor
timestamp: May 01, 2012, 03:22:13 pm
content: with or without fan? put 2 arctic 12cm fans on both sides, those are silent and have 135m3/h airflow, and i mean silent you can sleep next to them

author: T4P
author_rank: Super Contributor
timestamp: May 01, 2012, 05:33:53 pm
content: Quote from: M. András on May 01, 2012, 03:22:13 pmSometimes if you just need CFM and not noise reductions , it's okay to go noisier ...I have no problems with loud fans , it's on a bench anyway ( Hopefully not screeching like the Nvidia GTX480/580 )

author: M. András
author_rank: Super Contributor
timestamp: May 01, 2012, 06:14:55 pm
content: Quote from: Dave.S on May 01, 2012, 05:33:53 pmtry the old ati cards 3xxx series at max speed fanits ok to have high cfm but does it worth the 60dB noise? when you can put 2 of these arctic fans in push pull config to a big heatsink and stil lwell under 25dB noise. other fans ive looked at with high cfm are those huge axial exhaust fans meant to be wall or window mounted few of them can do over 2k m3/h airflow while under 40dB noise, even those sunon and the other big fan company have too much noise to their airflow levels

author: T4P
author_rank: Super Contributor
timestamp: May 01, 2012, 07:02:26 pm
content: Quote from: M. András on May 01, 2012, 06:14:55 pmI would prefer them . The small and cheap ones are usually loud because of their need for high speed and low power .The expensive ones are better designed, for me it's worth the 60dB noise if it's in my lab . Commercial equipment ? No .3xxx were loud ? i remember i joined the geek-dustry back during the 3xxx era and now they still think the 480/580 is jet-takeoff compared to the 3xxx (those were the days) It's about the pitch . 60dB at 200Hz does not annoy me as much as 4000Hz annoys me

author: Harvs
author_rank: Super Contributor
timestamp: May 02, 2012, 04:13:47 am
content: Yeah with one fan. I just had the fan removed (they unclip) to get all the screws in place.The Tt fans aren't too noisy, but as a colleage put it, they are pretty ugly. Not that I care, but his thoughts were I should be using a perspex top on the rack case and illuminating with LEDs like some people do with their computer cases...These coolers have a second set of clips to put a second fan on in push-pull arrangement, any 90mm fan should fit. The 120mm fans would just overhang in all directions. Once I've got the PCBs made and proper temp monitoring permanantly installed I'll do some tests to see if it makes much difference. Each heatsink is going to have it's own small uC setting current, monitoring voltage, temps, adjusting fan speed, etc.

author: T4P
author_rank: Super Contributor
timestamp: May 02, 2012, 06:50:48 pm
content: Quote from: Harvs on May 02, 2012, 04:13:47 amI really can't understand what PC modders are thinking ... perspex case ?

author: M. András
author_rank: Super Contributor
timestamp: May 02, 2012, 09:42:15 pm
content: http://www.arctic.ac/en/p/cooling/case-fans.htmlcheck their fans, i prefer them for pc cooling too quiet and powerfull,btw the later ati card series has a poorly designed air exhaust, i bet if they would make bigger exhaust area the whole thing would be much better with the stock cooler that small hole makes the whole thing noisy the actual fan in it its silent compared to the air noise, btw i hate noisy fans the high pitch noise hurts my ears, even the "old" crt televisions and monitors when they have no input signal just the black screen it gives off a very high pitch noise which i hear from even 10m away, same with the fans

author: T4P
author_rank: Super Contributor
timestamp: May 02, 2012, 10:05:44 pm
content: Quote from: M. András on May 02, 2012, 09:42:15 pmWhich series ? The 5xxx and 6xxx and now 7xxx sounds pretty nice to me .It's the Nvidia cards that tick me off with their every now and then

author: M. András
author_rank: Super Contributor
timestamp: May 03, 2012, 08:27:52 am
content: Quote from: Dave.S on May 02, 2012, 10:05:44 pma 6970 sits in my desktop and thats why i said the exhaust area is poorly designed its like compressed air comes out of the card at max fan speed, and it gives the noise

author: T4P
author_rank: Super Contributor
timestamp: May 03, 2012, 02:50:05 pm
content: Quote from: M. András on May 03, 2012, 08:27:52 amIt came out somewhere after the 480, it was pretty quiet for a high end GFX. But you must not forget that it's only 54C rise from ambient and that it draws more then 190 watts at full power and that's just the dissipation , of course you need a noisier fan for the same package but at the same time if i wanted a better fan i would have gone for the Asus DirectCU II or MSI Twin Frozr II or XFX's Dual Dissipation/XXX Edition massive fans.Ahh the days of when i used the HP tx2028AU nvidia chipset's fan ... VERY EAR PIERCING ( 12000Hz ) AT 60dB !Oh wait , the 480 isn't actually a graphic card , it's a Rolls Royce Trent 900.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 06, 2012, 02:35:39 pm
content: What about cheap space heaters? A 120V, 1500W heater run at 48V would dissipate 240W. If you add taps to the element, you can use up to the maximum rating of the element, and you can use PWM to fine tune the load level.