Different temperature on both side of same heatsink?  - Page 1
Forum section: electronics / projects

author: fivefish
author_rank: Frequent Contributor
timestamp: September 11, 2015, 09:50:14 pm
content: So I have (2) power transistors mounted on a heatsink (fin type).Transistors are bolted on the flat side (let's call this component side), with the fins naturally on the other side.I measured the temperature using those laser thermometer (eBay/china). On the component side of the heatsink, between the 2 transistors, I measure 58*C.On the fin side, measuring the same point at the opposite side, I'm measuring 36C.That got me wondering why the difference, when it's the same piece of aluminum, same spot (just opposite sides), will there be a big temperature difference like this.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 11, 2015, 09:59:00 pm
content: heatsinks can be fairly shiny, even if black anodised, so you are probably seeing reflected IR.

author: electr_peter
author_rank: Unknown
timestamp: September 11, 2015, 10:07:19 pm
content: Quote from: fivefish on September 11, 2015, 09:50:14 pmIR thermometers show accurate results only in ideal conditions, do not trust actual temperature reading too much. Asmikeselectricstuffmentioned, temperature reading depends of reflectivity/emissivity of material as well as surrounding environment temperature. Also, distance to object matters, as it "averages" the temperature of wide spot.Use more robust measurement equipment to be sure - try doing it with your fingers or thermocouple. I would think there is at most few degrees of difference on the heatsink.

author: Andy Watson
author_rank: Super Contributor
timestamp: September 11, 2015, 10:11:20 pm
content: Wouldn't you expect the side where the heat is dissipated to be cooler then the heat source?

author: fivefish
author_rank: Frequent Contributor
timestamp: September 11, 2015, 10:15:16 pm
content: Thanks mike and peter... you're right. I'm using the IR thermomenter without knowing it's limitations.I grabbed my Fluke thermocouple and did the same measurement, now fluke meter reports approx. 52C on both sides (+/- 1C variation) of heatsink.How do IR thermometer works anyway? It's a bit voodoo for me.

author: ez24
author_rank: Super Contributor
timestamp: September 11, 2015, 10:15:25 pm
content: Quote from: mikeselectricstuff on September 11, 2015, 09:59:00 pmie it is hard using these IR meters accurately. If important I think you can paint on some flat black muffler paint and take measurements off of that, maybe fingernail polish. Maybe roughen up the heatsink with sandpaper to reduce reflections. But anyway physics trumps these IR meters.

author: fivefish
author_rank: Frequent Contributor
timestamp: September 11, 2015, 10:17:03 pm
content: Quote from: Andy Watson on September 11, 2015, 10:11:20 pmYeah, that was my initial reasoning too... but as mike & peter said, it could be the IR thermometer giving me the wrong readings. And theyre right.Using a proper thermocouple showed equal temp readings on both sides.

author: SeanB
author_rank: Super Contributor
timestamp: September 12, 2015, 07:08:51 am
content: Best is to first make a black mark using a dry wipe marker on the measuring spot. That way the emissivity of the spot is the same all round. I noticed that simply by moving the sense spot on a heater from the edge, where it is well anodised aluminium plate, to the top where it is a coating of a PTFE lining, firmly bonded to the plate. Around a 5C difference in 1cm of movement. Even a difference between putting the pocket probe in the hole and using the IR on the black oxide surface of the brass heater near it.

author: ez24
author_rank: Super Contributor
timestamp: September 12, 2015, 07:15:12 am
content: Quotethanks

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 12, 2015, 07:22:38 am
content: Quote from: fivefish on September 11, 2015, 09:50:14 pmClassic case of not trusting readings that are clearly suspect and need a reality check - a quick prod with a finger would have revealed the misleading figures.