I have a 100W LED module that I believe is overheating due to the heatsink being too small. I use a piece of anodised aluminium from an old fanless PSU. It's only 300g so far too small for 100W led so I added 120mm fan. I still feel it gets far too hot. I have an IR thermometer and it shows temperatures over 120C after a few minutes when measuring the led itself and 40C for the metal part of the led. Thermal grease in between but the led is not firmly fixed to the heatsink.

I am planning on using arduino with a basic temperature sensor to control cooling fan based on temperature and dim the led if it overheats. What is the correct way of measuring the temperature of the led and at what part of the led?

Here's the LED:

enter image description here

  • \$\begingroup\$ can you put a penny or something next to the image? ...cant tell the size. \$\endgroup\$ – hassan789 Jan 25 '14 at 2:38
  • \$\begingroup\$ 2 euro coin fits in the yellow part snugly. \$\endgroup\$ – DominicM Feb 9 '14 at 21:38
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    \$\begingroup\$ Yea, realized what you have there, we in the LED industry call it a "fried egg" \$\endgroup\$ – hassan789 Feb 9 '14 at 22:23
  • \$\begingroup\$ That's funny, didn't notice it before :) \$\endgroup\$ – DominicM Feb 10 '14 at 2:35

Best way of measuring the junction temperature of an LED is by placing a thermocouple at the heat pad (or if the LED doesn't have a heat pad pin, get as close as you can to the anode pin). Secure the thermocouple using some type of thermally conductive epoxy/glue. Light up the LED and wait...

Read this and this for more information.

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  • \$\begingroup\$ By heat pad do you mean the integrated heatsink made from aluminium? See edit for image. \$\endgroup\$ – DominicM Jan 24 '14 at 19:36
  • \$\begingroup\$ from the looks of it, what u have there in not an led. its an led module. The thermal pad here seems to be connected to a large aluminum piece (that has the thermal grease on it). There is no way you can get to the thermal pad in this piece. Best bet is to just GET AS CLOSE AS YOU CAN TO THE CENTER of the module and read the temperature of the aluminum piece. you will have to cut the white plastic surrounding the module. \$\endgroup\$ – hassan789 Jan 25 '14 at 2:35
  • \$\begingroup\$ I don't need very accurate temp, if I can establish the difference between temperature at the core/hottest place and where the temp sensor is I can just add the difference. I could do that by measuring the center of the led (yellow area) but is Infrared thermometer accurate during led operation, what's my best bet of getting accurate once off reading of the temps? \$\endgroup\$ – DominicM Jan 25 '14 at 4:40
  • \$\begingroup\$ didnt understand the last part of your question... but IR is VERY good measure of relative junction temperature (visible LEDs emit virtually zero IR). Just power it on and wait... wait.. until the temp stop rising and has reached a steady-state. \$\endgroup\$ – hassan789 Jan 25 '14 at 5:52
  • \$\begingroup\$ Not sure how accurate IR actually is because I am getting 160C after just 10 seconds of operation with 120mm fan and a room temp heatsink. What part of the module should I be measuring? Now I am pointing the IR sensor at the very center. \$\endgroup\$ – DominicM Feb 9 '14 at 21:22

There is only one temperature that matters, and that is the junction temperature.

You could use a small thermocouple and try to put it under the base, perhaps by drilling a small blind hole into the heatsink under the LED base. (from the other side). Lots of inexpensive multimeters have type-K (Chromel-Alumel) thermocouple inputs that are probably good enough for your purposes provided you check that they're reading correct room temperature when cold.

Another method would be to pass a small constant current such as 1mA through the LED junction and vary the temperature in an oven from room temperature up to, say, 100°C. Plot that graph, fit a curve to it, or whatever, so you can find the inverse (temperature from voltage at 1mA). Now if you run the lamp at a known ambient temperature, allow it to stabilize and then very quickly switch from the "on" current of 600mA or whatever to the 1mA current you can get a true reading of the actual junction temperature before it cools too much.

The above method is similar to the standard method of checking temperature rise in transformers- using coil resistance (though we can skip the oven step because we know pretty much how the copper wire will behave with temperature, we just need one resistance reading at one known temperature).

As an approximation, with your IR thermometer, you could paint the back of the heatsink black (if it isn't already) to make the emissivity closer to 1, measure the temperature and add a fudge factor such such as 3-3.5°C/watt of dissipation to account for the junction-package and package-heatsink thermal resistance (use the actual number for the junction-to-case \$R_{\theta JA}\$ if you have a datasheet for the LED module. There will be a bit of gradient through the thickness of the heatsink but if you can get right under it with just some copper or aluminum between, the number should be close enough.

Current LEDs are good enough that it's a bit pessimistic to assume that all the electrical power that goes in is converted to heat- a significant amount is emitted as light, so does not contribute to heating of the LED.

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  • \$\begingroup\$ Measuring the back of the module is of no use as then I would not know the operating temperature with heatsink and fan not to mention LED would burn out in seconds with no heatsink. \$\endgroup\$ – DominicM Jan 24 '14 at 21:36
  • \$\begingroup\$ Yes, if you can't get to it under normal operating conditions then it's not a useful suggestion. The thermocouple by the side method (hassan789) is not as accurate, but might actually be feasible for you. \$\endgroup\$ – Spehro Pefhany Jan 24 '14 at 21:37
  • \$\begingroup\$ I dont have a thermocouple, would a normal temperature sensor work? I have a few with max temperature of 125C, this way I could dim the led and or increase the fan speed with arduino. \$\endgroup\$ – DominicM Jan 24 '14 at 21:41
  • \$\begingroup\$ It will work if it measures temperature of itself, but what it measures might not represent the heatsink temperature as accurately or respond as fast. \$\endgroup\$ – Spehro Pefhany Jan 24 '14 at 22:13
  • \$\begingroup\$ That's true but it is bare(no casing) so it should respond quite fast. \$\endgroup\$ – DominicM Jan 24 '14 at 22:19

The module should specify a maximum temperature, and these are usually either specified at a specific location (they have to tell you where) or it's the "hotspot" temperature, which means exactly what it sounds like - the hottest spot you can find on the module. Without more info on the tolerances of the part I don't think you're going to be able to collect meaningful information or know how close to spec you are. ;(

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  • \$\begingroup\$ These modules seem to have 80C max temperature but no datasheet. \$\endgroup\$ – DominicM Jan 24 '14 at 21:42

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