Measuring COB LED temperature

[made2hack]

Hi all,

So I've been playing with a LED COB for a project and wanted to gauge the temperature it reaches when run at various currents. The COB LED in question has an exposed test point on the surface which gives you the case temperature.

The way I've been measuring it is to simply kapton the tip of my thermocouple to the point and hope to get a reasonably measured temperature. Has anyone had experience with measuring temperatures this way? What is the proper way of doing it? Is it a fixed jig using pogo pins? Could I perhaps solder the tip of the thermocouple to the pad? Will this kill the thermocouple?

Thanks,

[Ian.M]

You've got poor thermal contact, and far too much heat loss up the thermistor wires.  The best option would probably be a SOT23 temperature sensor IC, with hair-thin magnet wire (40AWG ?) for the connections, pressed into a small block of expanded polystyrene, with a tiny dab of heatsink compound on it, flipped over onto the test spot and held in place by a spring finger. 

[Gregg]

If you are interested only in relative temperature changes with different LED controls what you are doing is on the right track.  As Ian stated, there will be heat conduction into the thermocouple wires which can be reduced by using the smallest available thermocouple wires and making sure there is the best possible thermal connection with the relatively tiny measurement location. 
Some good thermal compound that is not electrically conductive at the measuring point will help; but if it is electrically conductive, it will throw off the thermocouple readings.  I can’t tell from your picture, but you may already have thought of this.
Do not solder the thermocouple.  Thermocouples generate a tiny voltage at the junction of two dissimilar metals and solder would in effect short out the output at the source.
I see kapton tape covering a lot of the heatsink; if you are attempting to test the LED and heatsink combination, it would be better to make a little dog clamp out of non-conductive material (plastic, fiberglass, etc.) that would hold the tip of the thermocouple in place and hold the LED module against the heatsink at the same time.