PT100 without op-amp?

Discussion in 'The Projects Forum' started by urb-nurd, Sep 8, 2014.

  1. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    Hey guys, im going to be using a pt100 with my pic to control a heater.
    I am aware of the wheatstone bridge configuration however i have seen a-lot of layouts with op-amps.

    The only op amp i have is a TI opa2132a which i have used previously for audio applications many years ago.
    I wasn't sure this op-amp would be suitable for boosting the voltage from the wheatstone bridge.

    Is it possible to get good result without amplification?

    Also, could someone please explain how i would set up my bridge to measure a temperature between 0-250c and output a voltage between 0-5V for my pics ADC?

    Thanks for reading!
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    You can use a constant current source across the pt100 with a constant current of 26mA, the resistance change is 0.384 ohms per deg C, this will give a 10mV change for 1 degC temp, so 100 degC will be 1V, 200 degC will be 2V, 250 degC will give you 2.5Volts output. Pic.png

    or use 2.6mA and get 1mV per deg C, so 250C will give you 250mV.
     
    Last edited: Sep 8, 2014
  3. bertus

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    Apr 5, 2008
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    #12 likes this.
  4. #12

    Expert

    Nov 30, 2010
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    In summary, that OPA won't work because of its nature to not accept inputs within 2 volts of the supply voltage or ground, and outputs don't miss as badly, but still significant. You need to keep the driving current to 1 ma or less, so you are going to buy a different amplifier to get this done correctly.
     
  5. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    thanks for all the replies guys.
    One last clarification.
    What is the constant current source of 26 mA pictured?
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The current source uses a LM317.
    The circuit is taken from the attached datasheet.

    [​IMG]

    Please keep in mind that the PT100 will have a maximum current allowed.
    The value you should be able to find in the datasheet of the used PT100.

    Bertus
     
    Last edited: Sep 9, 2014
  7. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    i see, so there is no need for an op-amp in this configuration?

    Thanks again, i really do appreciate the help i get on here!
     
  8. crutschow

    Expert

    Mar 14, 2008
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    The only problem with that is if you want less than about 6mA for the constant current output (which may be the case here) since that is the minimum operating current of the LM117 (12mA for the LM317).
     
  9. crutschow

    Expert

    Mar 14, 2008
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    The signal across the PT100 will be quite low at the 1mA or so typical operating current so you will still need an op amp to amplify that to the desired signal range for your micro.
     
  10. urb-nurd

    Thread Starter Member

    Jul 9, 2014
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    Thanks, what rough specs should i look for in this op amp?
     
  11. crutschow

    Expert

    Mar 14, 2008
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    You want one with low input offset voltage. If operating from a single supply voltage then you also want it to be a rail-rail type
     
  12. bertus

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  13. crutschow

    Expert

    Mar 14, 2008
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  14. urb-nurd

    Thread Starter Member

    Jul 9, 2014
    269
    3
    i think i need to study these replies.
    Thanks for this.
    Ill do some reading and maybe come back to make sure im ordering the correct components
     
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