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Thanks, yes we appreciate that. The thing is , if the tip of the thermocouple is actually touching the drain tab, and is glued to it, then static glue immediately surrounds the thermocouple end, and therefore any air currents are not absolutely local to the thermocouple end, and therefore there is less chance of getting a falsely too_cool measurement result. Do you agree?About air currents: they are present with and without the thermocouple bead touching the tab. The disturbance caused by the thermocouple wire may be small.
Thanks, yes we appreciate that. The thing is , if the tip of the thermocouple is actually touching the drain tab, and is glued to it, then static glue immediately surrounds the thermocouple end, and therefore any air currents are not absolutely local to the thermocouple end, and therefore there is less chance of getting a falsely too_cool measurement result. Do you agree?
Thanks, do you know of such a glue (Manufac P/N)? Every glue on farnell thats for 200degC plus and cures within 6 hours costs >£100.Thermoset glues (epoxy class) do not melt or get soft at higher temp.
Thanks, will a normal battery thermocouple reader be OK?If you place the thermo couple on the drain tab, you usually need high voltage isolation. Or a temperature meter that can accept superimposed switching voltage.
It will work, maybe the shown values are not that stable.Thanks, will a normal battery thermocouple reader be OK?
Thanks, do you know of such a glue (Manufac P/N)? Every glue on farnell thats for 200degC plus and cures within 6 hours costs >£100.
Also, previously we've always put a bit of kapton tape over the metal of the drain tab and then fixed the thermocouple to that for isolation purposes.
Thanks, i was wondering at what temperature it decomposes for Loctite 382?(not cyanoacrylate - it decomposes at high temperature and releases cyanide!)
Thanks, though we need it to be thermally conductive.Most common two component glues are epoxy based; they are readily available at the neighborhood shopping complex.
Thanks JB Weld doesnt say what is the thermal conductivity...I think someone already mentioned JBweld. I've used it for gluing heatsinks since it has decent thermal conductivity, though I don't think I've taken it past 100C.
Thanks, and I assume that to achieve this we should perhaps smother heat paste round the thermocouple end as it lies against the metal drain tab?…then we wont have this situation of the end of the thermocouple having different temperatures.Unless you need high accuracy (< ~0.25C) all you have to do is ensure there is no 'escape route' for the heat that could cause a thermal gradient at the measurement point.
Thanks, though we need it to be thermally conductive.
As well as thermocouples, we would liek to stick a thermistor to the fet drain tab and see how hot that gets.....any thermal glue recomendations for this much appreciated.
What I meant was to thermally insulate the back of the thermocouple to prevent heat loss but to fit a small (~4mm) pad of silicone heat pad on the front of it. The pads I mean are the ones used under T0220 and similar packages instead of the old mica washers. The pad material is designed for good heat conduction AND good electrical insulation, the two properties you are needing. The bonus is it is slightly compressible so you can apply pressure to it and it will mold itself to eliminate air and give good thermal contact. The settling time from first touch to stable temperature reading should be no more than about a second or so. Remember it is only the spot weld between wires that generates the voltage so it's mass is very, very tiny.Thanks, and I assume that to achieve this we should perhaps smother heat paste round the thermocouple end as it lies against the metal drain tab?…then we wont have this situation of the end of the thermocouple having different temperatures.
JBweld does this, with steel particles.I wonder if any common glue is thermally conductive but it may be mixed with some metal oxides (like ZnO, or Al2O3, TiO2 etc) to increase the thermal conductivity.
Thanks yes, this is why we've also put a thermocouple on the plastic case of the D2PAK FET, and will also do a thermistor reading for comparison, as having microvolt measurements riding atop the voltage thats fluctuating over 100's of volts always sounds like a recipe for inaccuracies to creep in to the temperature measurement.Also take note of the previous warnings about isolation if you bond directly to the tab of the transistor. You will be measuring microvolts of DC against potentially several hundreds of volts of background DC or AC so even if you trusted a sliver of glue as an insulator, bear in mind the whole thermometer circuit could be very 'live'.
I accidentally glued the thermocouple end directly on to the metal drain for one of the FETs, and the tecpel 316 thermocouple reader would not read it due to the noise...(the unit has to be powered off in order to read the thermocouple temperature or else the reading comes out ridiculously low).
Not too surprising.the tecpel 316 thermocouple reader would not read it due to the noise
Most people will prefer an isolated thermo couple mount.
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