Probably a simple op amp / thermocouple question

Discussion in 'General Electronics Chat' started by kutalinelucas, Apr 30, 2013.

  1. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
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    Hello

    I'm trying to put together an op-amp circuit to read temps from a K-type thermocouple into the adc of my PIC18F. To save time, I ordered the LTK001 op amp and cold-junction compensation IC's, ...but I'm in a bit of a pickle! (http://www.linear.com/product/LTK001)

    The thermocouple produces up to 40mV in a linear fashion up to 100c when measured across its terminals, but when hooked up as shown in the diagram on page one, heating the thermocouple has no effect on the op amp output.

    I'm currently reading the voltage through a C++ app, and everything must be working OK because I get sensible readings when I connect my bench supply to the op amp + input with a common ground. Although I have a cheap bench supply which is difficult to set at low voltages, the output of the ADRESH register of the PIC seems to correlate to the voltage I apply to the op amp input.

    So..what I'm asking really is can anybody see why this circuit will not work as it is, and should the + and - terminal of the thermocouple connected in series as shown in the diagram?

    Thanks for reading...
     
  2. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
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    0
    Hello again

    I hate to post again to bump the post further to the top, but I'm sure there's something simple I'm doing wrong, and was hoping there would be someone that could shed some light on it...
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Can you post your circuit so we can assist you further?
     
  4. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
    98
    0
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    744
    I dont see a -Ve supply line as per the datasheet, it uses a split supply +v/-V gnd ,, you have just a single supply,also the inverting inputs of the first opamps are connected to the +ve supply maybe that is why the thermocouple is not giving out the voltage?
     
    Last edited: May 1, 2013
  6. kutalinelucas

    Thread Starter Active Member

    Nov 20, 2007
    98
    0
    I have never come across a split voltage supply before and brushed over the v- by grounding it...thank you so much for this
     
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