thermocouple signal amplification for use with USB-6009

[tenacious84]

I'm a newb, so I hope I'm posting this in the correct place. 

I'm trying to record thermocouple outputs that are mounted on a neighborhood electric vehicle.  The problem with simply hooking the thermocouples up to the usb-6009 is the 72v electrical system on the vehicle distorts the thermocouple signal.  An advisor on the project recommended using an op amp and that worked just fine for temperatures down to 40 degrees F.  However, I am looking to measure temperature down to -10 degrees F and was looking for suggestions on how to amplify the signal when the thermocouple voltage is near 0 and/or goes negative.

Thanks for any help,

Chris

[Wilfried_71]

Hi,

You must texplain us how you plug your thermocouple sensor on the 6009 ... because I don't see how it can work without conditonning ...

[Paul_C.]

Hi Chris,

First, I just want to mention that the USB-6009 isn't rated for use in a -10 degree F enviroment if that is the case.  In the detailed specifications here, you can see that the 6009 is rated for use indoors within 0 to 55 degrees C.  I'd also like to point out that the USB-6009 can read voltages between -10 and 10V so I'm a little confused by these lower voltages / negative voltages are causing a problem.  In regards to amplification, the USB-6009 has a gain that is set according to the voltage range you specify.  By setting the proper voltage range for the signal range you intend to receive, the gain on the card will be increased / decreased appropriately.  If you need additional amplifications, you would need an external amplification circuit.

I hope this helps,
Paul C.



Message Edited by Paul C. on 01-17-2008 01:11 PM

[tenacious84]

the project box housing the 6009 will never see temps below specifications.  as for the temp readings, i had the thermocouple reading correctly when originally connected directly to the 6009.  however, once the vehicle was turned on, the 72v electrical system caused the thermocouple reading to become unusable.  after a suggestion from an advisor, i wired the thermocouple to an op amp circuit (to amplify the signal high enough for the vehicles electrical system to have no effect on the reading) and then into the 6009.  I was able to calibrate the output voltage so i knew what temperature it was reading.  once i started lowering the temperature however, the output voltage from the operation amplifier circuit stopped going down at 0.003v.  obviously this is the limit for use of this type of circuitry, i was looking for any suggestions to for reading lower type with similar circuitry.  the op amps i am using are LT1077.

[Paul_C.]

Hi Chris,

I believe what is happening is that your op-amp is uni-polar (the positive connection to your power supply and the negative to ground).  This clips the output value to between this range.  You might want to consider using another op amp to shift the output of the thermocouple to only positive values.  Depending on the type of thermocouple, I'd shift the output such that it will make all the temperature outputs a positive voltage before you amplify it with the other op amp.  Another method might be using a bi-polar power supply to drive your op amp (you might be able to supply the negative voltage with the analog output of the 6009).  If you need more information about op amps, I'd recommend taking at this great op amp article I found here.

I hope this helps,
Paul C.