How to drive 3wired pt100?

Discussion in 'The Projects Forum' started by eric_s88, Nov 10, 2013.

  1. eric_s88

    Thread Starter Member

    Apr 20, 2011
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    Hi everyone!

    I posted a new topic a few days ago about driving 2wire pt100. but now I have to use an industrial 3 wire pt100 sensor.
    this is the picture of sensor :

    [​IMG]

    this is my first experience with pt100 and now almost nothing about driving issue. all I know is that we should a constant current of 1ma to produce voltage drop over sensing element.:confused:

    any circuit suggestion ? and any other useful information about this :)

    thank you
     
  2. eric_s88

    Thread Starter Member

    Apr 20, 2011
    157
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    in addition, I want to use uC adc , with 10bit resolution :)
     
  3. bertus

    Administrator

    Apr 5, 2008
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    eric_s88 likes this.
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Look at post 2 in this thread.

    Two of the wires are tied to one end of the sensor. You can find which ones with an Ohmmeter. Using two wires vs three wires is covered in the app note. If using the typical 10bit AtoD, the two wire connection works fine.
     
    eric_s88 likes this.
  5. eric_s88

    Thread Starter Member

    Apr 20, 2011
    157
    1
    thank you bertus and MikeML for your replies ;-)


    MikeML thanks again for posting in this topic again. I watched your suggested circuit. would you explain a little ?

    is this circuit converts the bridge output to 0-10v ??

    in this schematic the reference voltage is 5v for a typical pic uC, but I'm going to use arm7 family uC with 3.3V voltage ref. is that a matter ??

    thanks again and sorry for poor english
     
  6. eric_s88

    Thread Starter Member

    Apr 20, 2011
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    and is that useful for negative temperatures ? my temp. range is -20 ~ 120 deg.
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    A circuit here for pt100 temp range from -50C to +125C
     
  8. eric_s88

    Thread Starter Member

    Apr 20, 2011
    157
    1
    thank you dear bertus , Dodgydave and MikeML for circuits. would you explain the way of the circuits operation ? I get that they transmit pt100 signal to a larger signal in order to uC adc can read it.
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The original circuit is on another computer. I will tweak the gain/offset/bridge balance to go span -20 to +120 degC from 0V to 3.3V tomorrow...
     
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  10. eric_s88

    Thread Starter Member

    Apr 20, 2011
    157
    1
    thank you for your full help :) would explain a little about the operation that the circuit do too ?

    thanks
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I tweaked the circuit for your temp. range and to make a full-span voltage range of 0V to 3.3V. The circuit is ratiometric to the 5.00V supply, so if you have a different reference voltage, that will require changes to R3 and R6.

    As to how it works. First read this application note. It describes how the resistance of the PT100 is related to temperature. R1 is the simulation model I created for the PT100. The resistance is varied by the independent variable T (temperature) for the simulation. The red (upper) trace shows the voltage V(noninv) which is a voltage divider consisting of R2 and R1. Note that V(noninv) goes from 46mV to 72mV as temperature goes from -20degC to 120degC. The blue trace is the resistance of R1 created by plotting the expression V(noninv)/I(R1)

    The opamp is in the non-inverting gain configuration. The gain of the opamp is primarily set by R6. The offset is set by the voltage divider R3 and R4. The goal is to fit the line V(adin) by changing gain and offset so that the green trace (lower) goes from 0V at -20degC to 3.3V at 120degC. Tweaking R3 and R6 to the standard 1% values shown comes pretty close.
     
    Last edited: Nov 13, 2013
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