Thermocouple meter, measuring MOSFET temperatures

leadcoma · 22nd February 2011, 12:19 AM

What's everyone using for measuring the temps of their SMPS components?

I bought a cheap thermocouple meter from China and almost fell off my chair when the temperature of my MOSFET tab appeared to go from 20 to 100 Deg C in 1 second with heatsink attached. Of course this higher unlikely so it appears as though the meter is very susceptible to EMI.

My meter is a dual channel but the K type thermocouples aren't electrically isolated so no simultaneous measurements of the HS and LS MOSFETS.

I want to buy a better meter but I don't know what meters are not influenced by EMI and electrically isolated between channels. Any suggestions are appreciated!

mag · 22nd February 2011, 08:49 PM

Hi leadcoma,
isolate all your thermocouples with a thin film of adhesive kapton tape to avoid insulation problems. In this case you can measure at the same time all the temperatures you want.
If you put the thermocouple directly on the mosfet tab without insulation it is very likely to occur noise pickup that disturbs the measure.
Don't rely on the insulation between the channels of the instrument.
If noise is still picked up with insulated thermocouples you can simpliy heat up your smps with all the thermocouples in place and then switch it off to take the temperature measurement. Temperature shuould not change significantly in a second or two, especially if your mosfets are attached to an heatsink.

Ciao

-marco

jrockhead · 22nd February 2011, 09:25 PM

I bought an EXTECH Easyview 11A meter which is a single channel meter. Works great and I have had no problems with funny readings as I do with the 15 dollar Chinkanese meter I originally bought.

Extech also make a dual channel meter and I should of gotten that one instead, oh well.

nigelwright7557 · 22nd February 2011, 11:18 PM

I use a handheld infra red thermometer.
Works very well and was about £20.

leadcoma · 23rd February 2011, 01:56 PM

thanks for your responses

Marco, good tip, looks like that works

Nigel, I too have an IR type but mine can't differentiate between the heatsink and MOSFET tab

AndrewT · 23rd February 2011, 02:52 PM

Quote:

Originally Posted by mag

Temperature should not change significantly in a second or two, especially if your mosfets are attached to an heatsink.

some of the thermal time constants are in the tens of milliseconds.

22nd February 2011, 12:19 AM	#1
leadcoma diyAudio Member Join Date: Jan 2009	Thermocouple meter, measuring MOSFET temperatures What's everyone using for measuring the temps of their SMPS components? I bought a cheap thermocouple meter from China and almost fell off my chair when the temperature of my MOSFET tab appeared to go from 20 to 100 Deg C in 1 second with heatsink attached. Of course this higher unlikely so it appears as though the meter is very susceptible to EMI. My meter is a dual channel but the K type thermocouples aren't electrically isolated so no simultaneous measurements of the HS and LS MOSFETS. I want to buy a better meter but I don't know what meters are not influenced by EMI and electrically isolated between channels. Any suggestions are appreciated!

22nd February 2011, 11:18 PM	#4
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	I use a handheld infra red thermometer. Works very well and was about £20. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.

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22nd February 2011, 08:49 PM	#2
mag diyAudio Member Join Date: Jan 2008 Location: Chiasso	Hi leadcoma, isolate all your thermocouples with a thin film of adhesive kapton tape to avoid insulation problems. In this case you can measure at the same time all the temperatures you want. If you put the thermocouple directly on the mosfet tab without insulation it is very likely to occur noise pickup that disturbs the measure. Don't rely on the insulation between the channels of the instrument. If noise is still picked up with insulated thermocouples you can simpliy heat up your smps with all the thermocouples in place and then switch it off to take the temperature measurement. Temperature shuould not change significantly in a second or two, especially if your mosfets are attached to an heatsink. Ciao -marco

22nd February 2011, 09:25 PM	#3
jrockhead diyAudio Member Join Date: Dec 2006 Location: Overtaxed Long Island, NY	I bought an EXTECH Easyview 11A meter which is a single channel meter. Works great and I have had no problems with funny readings as I do with the 15 dollar Chinkanese meter I originally bought. Extech also make a dual channel meter and I should of gotten that one instead, oh well.

23rd February 2011, 01:56 PM	#5
leadcoma diyAudio Member Join Date: Jan 2009	thanks for your responses Marco, good tip, looks like that works Nigel, I too have an IR type but mine can't differentiate between the heatsink and MOSFET tab

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