Cannot measure temperature

Im having troubles measuring temperature with thermocouples. When I measure the temperature of the air or places which are not with potential the card works well. But if I want to measure temperature for example in a transistor that is under a voltage of 13 volts the card shows wrong readings. If I´m not mistaken this card has an input voltage range of +/- 15 volts, so the voltage in the transistor is within the input voltage range.
I read in the manual that you can configure the input voltage for +-.625mv, +- 1.25, +- 2.5, +- 3.75, +- 7.5, +- 15 volts, but I cannot find the tool or the way to do this.
If the common mode voltage is < 2,5 volts I can measure the temperature, but if its bigger I get wrong readings from the card.
I already post this question in another forum, but nobody answered, I guess its more a software question! Im using the plug and play driver for the 435x devices.
Do you have any suggestion of what should I modify to make it work? I attached my code.
Thanks in advance,
Hernar
LabVIEW 8.2
PXI 4351
Attachments:
Temp_Messung.vi ‏140 KB

Hernar,
Thermocouples are made of electrically conductive wires.  If you connect them to an electrically energized point, you will measure a value which is determined by the voltage and impedance of the energized point as well as the thermocouple voltage and impedance.  Since the thermocouple voltages are usually quite small (mV), the voltage from the other circuit (your 13 V transistor) is likely to be much larger.
You need to use an electrically isolated thermocouple to measure the temperature of your energized transistor.  Or connect the thermocouple to the transistor through an electrically insulating, thermally conducting material such as the insulators used to mount power transistors to heatsinks.
Lynn 

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