RTD td-1000 circuit

Discussion in 'The Projects Forum' started by TimothyBlue, Dec 31, 2011.

  1. TimothyBlue

    Thread Starter Member

    Jan 4, 2009
    11
    0
    Hello,

    As much as I hate to admit it, I am not very good with analog circuits. And I always find myself very frustrated when I require one.

    A few years ago (I got interrupted) I started working on a project that was using the analog input on a PIC 18F4550 to read a 1000 Ohm 2 wire RTD sensor. I eventually gave up after trying many different avenues.

    I am back at it finally. I have traded in my PIC for a netduino for the time being because the netduino is much easier to use, and I figure this will remove the problem of errors in the program so that I can concentrate on the circuit.

    That being said, I have some statements that will lead to a couple of questions.

    It appears that one option is to use a constant current circuit to drive the RTD and then read the voltage coming from the RTD. Because the current is known then I should be able to derive the resistance if I know the voltage?

    I also saw some references to some circuits that would cause 0 voltage at 0 degrees and higher voltage when the temperature rises. I believe that these circuits were using a comparator to compare the RTD to a known value resistor.

    I have also seen some circuits that linearize the output of the RTD so that it does not have to be done in code.

    This seems like it should be a simple problem, but I just don't seem to be getting it.

    I am hoping that someone can point me in the correct direction. I don't want to have to use super accurate components if I can help it (i.e. precision resistors).

    I would appreciate it if someone could post a circuit or give me some information about the best combination of simple and correct. The sensor only needs to be accurate to 1 degree Fahrenheit (2 would probably do, but 1 would be great).

    My original post which contains my first circuit is in this thread:

    I didn't really like this circuit because it does require precision resistors to operate correctly, and I never really got a good result in the end, but that could have been due to many factors.


    Hopefully I am not asking too much here. Maybe simple and good don't exist for this situation? Any info would be helpful, but please remember that I am a newbie when it comes to analog so explanations will probably be necessary.

    Thanks!!
    ~Tim
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Do you have a schematic of your current circuit?

    Are you using precision resistors for a divider from the thermometer?

    How accurate do you need the measurement to be? ±1 °F or less? If more accuracy is desired, it gets more complicated, an external ADC may be what you end up with for enough bits to get the resolution you need.
     
  3. TimothyBlue

    Thread Starter Member

    Jan 4, 2009
    11
    0
  4. xylon89del

    New Member

    Dec 28, 2011
    17
    0
  5. TimothyBlue

    Thread Starter Member

    Jan 4, 2009
    11
    0
    Thanks for the link, I have studied this page for some time. The Wheatstone bridge circuit given there is an interesting idea, but it would seem that the resistors in this circuit would have to be very precise.

    Would you say that the bridge circuit is the most common way of reading an RTD?

    ~Tim
     
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