I want to measure the temp of a transistor heat sink which is at +20 volt potential. How do I isolate the thermistor from being affected by the transistor bias voltage?

mc-irf · ‎07-08-2003

I'm using a 4351 with a thermistor. If I isolate the thermistor with thermal tape I get a correct reading but this won't work for all transistor packages. I've tried the set up grounded and ungrounded but both results give me a wrong reading (- temps and really high temps)

AVR · ‎07-08-2003

If you wire the input using differential mode, the 20 volt bias voltage will be eliminated because of the "commom-mode rejection" of the amplifier circuit interface; however, be VERY CAREFUL to determine the maximum rated voltage of your DAQ Card. Read the manual to determine the maximum allowable input voltage (it may be as high as 25 VDC).

You must realize when you use differential input mode in this manner in order to remove a bias voltage, you risk exposing the DAQ system to the entire voltage signal. When you input signals to your DAQ system, be VERY CAREFUL about the wire hook-ups -- if you "inadvertantly" establish another path to ground for the 20volt signal, you will saturate the A/D amplifier, your signal will be "clipped at +10VDC, and you could dama
ge the entire DAQ system.

Multifunction DAQ

I want to measure the temp of a transistor heat sink which is at +20 volt potential. How do I isolate the thermistor from being affected by the transistor bias voltage?

I want to measure the temp of a transistor heat sink which is at +20 volt potential. How do I isolate the thermistor from being affected by the transistor bias voltage?

Re: I want to measure the temp of a transistor heat sink which is at +20 volt potential. How do I isolate the thermistor from being affected by the transistor bias voltage?