What's possible with these heatsinks ? - Page 1
Forum section: electronics / beginners

author: MathWizard
author_rank: Super Contributor
timestamp: June 28, 2025, 06:53:04 am
content: I have some heatsinks from a plasma TV, if the fins were pointed down, they look like long benches, w/ a back and 5 legs/fins. I worked out a mass of 123.9g of say 6060 Aluminum alloy, my scales say 122g.I have 4 like this, except 1 has no back11111111 FETS33333333333333333 PCB11 1 1 1 1 PCB11 1 1 1 1 PCB1 1 1 1 1 PCB1 1 1 1 111 PCBThe front and back legs are a bit different and the fins don't have equal spacing, but average 6mm on center.The length into the page is 29.9cm ~11 3/4 inchThe seat thickness is 3.05mm, it's full extent front to back is 30.3mm, it's exposed area is 124cm^2, with a volume of 20.3cm^3The back is 2.05mm thick, height is 15.2mm, with an area of 75cm^2 and volume of 7.1cm^3The most legs are 1.2mm thick, have a height of 12.2mm, and with extra details, the area is 304cm^2, volume of 18.4cm^3Total area is ~500cm^2, w/o mosfets covering itTotal volume is ~46cm^3total mass 122gSo for an electronic load, with fans, maybe ducting or channelling, in a metal box like a DigitalCableTV box, what's a guessimate of the thermal resistivity of the heatsinks, and how many MOSFET's might a heatsink handle? How hard would it be to keep the case of an average MOSFET, under 50C ? (without turning it off)What do you think is possible ?

author: Andy Chee
author_rank: Super Contributor
timestamp: June 28, 2025, 07:00:16 am
content: My normal approach to this problem is to scour Digikey and element14 websites for heat sinks of similar dimensions, and base the thermal design using the specifications I find.For a salvaged part project, such data extrapolation guesstimation should be good enough me thinks.

author: MathWizard
author_rank: Super Contributor
timestamp: June 28, 2025, 07:11:57 am
content: Does maybe something like thesehttps://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/2444/Extrusions%20R15_0121.pdf

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 28, 2025, 10:44:10 am
content: Guesstimating from a similar sized heatsink as Andy Chee suggests is a good option.Another option is to just measure it.1. Take a beefy NPN BJT (TO220 or TO247 package is the easiest)2. Put an 1 Ohm resistor on it's emitter, and connect it to "GND".3. Apply 21 Volts to the collector.4. Apply approx 1.6V to the base, adjust until you have 1A from the 21V power supply.You have now created a rudimentary current sink that dissipates 20W in the BJT (and 1W in the shunt).You can stick it on the heatsink and then measure it's temperature after a while. You can also do some experiments with different fan options. Airflow has a huge impact on the thermal resistance of a heatsink. With a fan the thermal resistance can be reduced to about 1/6th of the thermal resistance you find in a datasheet. And of course, when air flow is hindered (such as in a closed enclosure) the thermal resistance becomes higher, approaching infinite if the box is insulated really well. IMHO the effect of air flow on a heatsink is so big that the datasheet values are no more then a rough guideline, and you always have to do tests to verify whethether it's performance is adequate in your situation.

author: MathWizard
author_rank: Super Contributor
timestamp: June 28, 2025, 03:01:24 pm
content: Yeah I already have a little TO-220 MOSFET E-load, on another heatsink. I'll have to hook it up. I hate trying to measure temps tho, with a DMM and temp probe. And I can try some CPU TIM paste, but I never know what values to really use for that stuff either, or how much to use.I'll have to get more side tracked into some of the thermal calculations, and airflow stuff for fans. I know these are pretty small heatsinks, but I want to start building a modular E-load, that I can scale up a bit.

author: MathWizard
author_rank: Super Contributor
timestamp: June 29, 2025, 12:44:44 pm
content: With 2x IRFZ44N, with metal tabs and thermal paste right on the heatsink, they are about centered and seperated by 2.5"With the HS fins pointing up, and the whole thing on a 45deg angle (better than laying it on the desk), it starts getting too hot to touch w/ only 7.3W total on the FETs. So that would be only ~30W for the 4 HS's.I guess I'll have to cut up a cereal box and try hooking up a 12V computer fan.

author: MathWizard
author_rank: Super Contributor
timestamp: June 29, 2025, 04:26:23 pm
content: I put a block of foam in a cereal box, it's about the same width and lenght, and it fills say 2/3 or 3/4 of the box. And with the heatsink all the way in the box, and a hole in the far end of the box, not much bigger than the heatsink.And with a 120mm 12V fan, about half the fan, has a clear path into the box. The fan already has plastic covering it's top 1/2. A lot of air doesn't make it into the box, but it's doing great.With the fan running nice and quiet off 5V, I could do ~14.9W on the heatsink, without it being too hot to touch.Now with 12V on the fan, I'm up to 32.5W and it's just about getting uncomfortable to touch, at the far end of the heatsink where the air exits the hole in the box.So with 4 of these heatsinks, and proper air channels, that would be at least 130W, so thats not bad at all. I'd use 2 fans, at 1/2 the voltage, but with the ability for 1 to take over at full voltage, if 1 breaks. I have lots of fans, I can use 3-4, then it would be very quiet too.I've been looking at the Array Electronic loads, and I have most of the main parts to make something like that, and eventually, make it digital too.I'll have to actually measure some temperatures, but wow this worked way better than I expected, compared to no forced convection.According to my rough calculations, if 50C is too hot to touch, then the heatsink has an Rth of 1.81 w/o the fan, and 0.71 with it. And the junction temp would be about 60C w/o the fan, and at 38.3W total, the max I tried so far, so 19W per mosfet, the junction temp should be around 77C.

author: planet12
author_rank: Regular Contributor
timestamp: June 30, 2025, 10:32:11 pm
content: I have the following post bookmarked for a physics-based method to determine thermal resistance of unknown heat sinks (method is in reply #8):https://www.eevblog.com/forum/beginners/thermal-resistance-of-unknown-heatsink/

author: MathWizard
author_rank: Super Contributor
timestamp: June 30, 2025, 11:17:33 pm
content: Yeah I did the node eqn's for the basic thermal circuit, , and I got near 89C for the junction when there's 19W on a MOSFET, not 77C. But yeah with moving air that's heating up, and measuring the temp by hand, that's alright.I'm just trying to find what's possible, with the Array E-load I'm using as a base, to make a lower power version of. All the wire and connector resistances are having a big effect on the output currents. IDK how much they might be in real life, with my cheap parts.I'll have to try and measure the inductance of my 0.1R 10W resistors too.