need help with PT100

Discussion in 'The Projects Forum' started by 466576266, Mar 29, 2010.

  1. 466576266

    Thread Starter New Member

    Mar 17, 2010
    hello,my friends.

    Recently, i am designing a circuit with PT100. what the requirements of the circuit are as follows:

    Temperature Range: 0-100°C.
    output range :1.0000V-1.9999V.
    resolution:0.01 °C.

    i have tried a lot of circuits, but thay all donnt work.

    could you plz me? thanks for your suggestion.
  2. JDT

    Well-Known Member

    Feb 12, 2009
    There are many examples of circuits that will do this on the web.

    But, if you want to go back to basics: The PT100 temperature sensor has a resistance at 0 deg C of 100 ohms. At 100 deg C it's resistance is 138.51 ohms. Approximately linear in between - not quite actually.

    So let's have a standing current of 1mA passing through it. You need to keep current this as low as possible to reduce self-heating. Therefore the voltage across it will be 0.1V at 0deg C and 0.1385V at 100deg C.

    A voltage change of 0.0385V. You want a voltage change of 0.9999V on the output of your circuit. Call it 1.000V.

    So the voltage gain of your circuit will be 1/0.0385 = 25.974

    You can construct a typical non-inverting amplifier as in the attached circuit. The formula for gain is shown.

    To get the correct output voltage at 0deg C you will need a fixed offset.

    I would make the offset voltage and gain slightly adjustable and set it up on test. As I say, this is a very basic circuit and there are many things I have not taken into account: Offset voltage, temperature drift, input bias currents, reference voltages, etc. But it's a start!

    To make an accurate constant current of 1mA requires another op-amp but the simple way is to use a fixed resistor from, say, an accurate 12V supply. The voltage offset can be a voltage divider from the -12V supply.

    Attached: Simple circuit. Spreadsheet to play around with circuit values (sorry had to zip this).
    466576266 likes this.
  3. 466576266

    Thread Starter New Member

    Mar 17, 2010
    thanks for your suggestion. but i think it's very hard to design the circuit you recommended. As i know,the -0.0615V power is very hard to design, coz the voltage is too small.
    and could you tell me how you calculate the REF voltage -0.0615V?
    in fact, the resolution of the temperature is ok with 0.1C . in this case, could you give some suggestions? thankz very much!
  4. SgtWookie


    Jul 17, 2007
    JDT already gave you the information you need to build one of those circuits.

    What were you expecting; for someone to built it for you and ship it to you?

    These forums don't work that way.