Help with a PT1000 Circuit

Discussion in 'The Projects Forum' started by nicmon, Jun 21, 2011.

  1. nicmon

    Thread Starter Member

    Mar 18, 2011
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    I would like to measure temperatures from 0 to 450 oC and send the results to PC. I drawn this circuit after months of research. As I'm a real noob in electronic, would somebody take a look at my circuit to check if I have no error in it? Thank you.

    PT1000 3.png
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. nicmon

    Thread Starter Member

    Mar 18, 2011
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    I thought that I linearise the values in ATMEGA that I have, do you prefer using ADUC834 instead (as in your pdf link)?

    as I said, I'm also not sure about whole system that I have, PT1000 interface, MAX232 to communicate with RS232, ATMEGA and it components (e.g. do I using the correct quartz there)

    Do I need 1 Powerline for each of 3 PT1000s or it's enough to have one for all?
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Using the ATMEGA for calculating the linearization is fine.
    The PDF I showed you has some formulas in it that might help.

    I am only worried about the current source.
    The parameters for jfets can be quite off, so the currents may be different on the three channels you want to make.
    This will result in different readings from the different channels.

    Bertus
     
  5. nicmon

    Thread Starter Member

    Mar 18, 2011
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  6. bertus

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    Apr 5, 2008
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    Hello,

    Here is a page from aQD-TEC with some examples of current sources:
    http://www.4qdtec.com/csm.html

    I would go for circuit 4.
    If you want to keep the PT1000 to ground, make the circuit "up side down" and use PNP transistors.

    Bertus
     
  7. nicmon

    Thread Starter Member

    Mar 18, 2011
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    I thought you worry about dropping the current on 2nd or 3rd instance of PT1000.
    this is from Circuit 4:




    EDIT:
    I got what you meant. I put that BF256 exactly for this purpose. see here: http://www.eleccircuit.com/fet-current-source-by-bf256/
     
    Last edited: Jun 21, 2011
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    If you want three current sources, take a look on how to create them as in this post by Bill_Marsden:
    http://forum.allaboutcircuits.com/showpost.php?p=117638&postcount=3


    [​IMG]


    The first drawing in figure 2.4 will suit you. (just add a third transistor and limiting resistor).

    The current will be about 0.7 / R .

    Take care not to take a to high current for the PT1000, as self heating will disturb the measurement.

    Bertus
     
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  9. DickCappels

    Moderator

    Aug 21, 2008
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    Please check the datasheet for your thermistor. 450°C is beyond the range of all thermistors I have seen. You are in thermocouple territory now.
     
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