Cannot measure temperature

[Hernar]

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

[Brian_Beal]

Can you clarify if you are applying the 13 volts to the t/c input or not?  Measuring the temperature of a transistor that has 13 volts going through it different than applying that 13 volts to the input.  Clearly, any directly applied or induced voltage on the t/c input will cause inaccurate readings.

[Hernar]

The transistor has 13 volts going through, and I´m measuring the temperature in the transistor. If the transistor has no voltage I can obtain the temperature readings, if I supply 13 volts in the transistor I get negative temperature values (-53.01 °C).

If I use the MAX Test Panels, there is an option in which I can select the input voltage range: +/-.625mv, +/- 1.25, +/- 2.5, +/- 3.75, +/- 7.5, +/- 15 volts. But in my LabVIEW code I cannot find how to change this input range, I think that is the problem.

[johnsold]

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 

[Hernar]

Thanks, Lynn & Brian for your replies. I didnt want to hear that but I guess I will have to do some kind of isolation.

I have another question, I want to use the example "NI 435x Thermocouple.vi " to measure my thermocouples, but whenever I select the build in CJC the program shows a "NaN". I Highlighted the VI, and I obtain a NaN after the VI "(Sub) 4350 Remove offset and get CJC voltage.vi".

If I use the CJC in MAX or with the Plug and Play driver it works, but with this VI it doesnt.