Op-Amp InstrumTemperature Control System

Discussion in 'The Projects Forum' started by Pete102, Oct 15, 2013.

  1. Pete102

    Thread Starter New Member

    Oct 15, 2013
    Hi guys,

    Im hoping some of you might be able to offer some advice for me with regards to a project I'm on with at college.

    My design brief is to build a relatively simple temperature control system (on/off, no PID) using basic electronic components where possible.

    I have split the system into three distinct elements, input - logic - output. I have been working on the input element and so far decided upon use of a three-wire RTD (PT100), bridge circuit and 741 differential amplifier to take the mV signal into something more usable.

    My question (or rather concern) at the moment is how to go about scaling the differential op-amp output into a 1-5V range such that I can pop in a 0.25 ohm resistor and obtain a 4-20mA signal (part of the brief).

    For those that are interested the plan is to then feed the conditioned output signal into a comparitor, (where the comparitor reference signal sets the output point) to then drive a relay coil or other component for integration into a wider circuit.

    Many thanks for any replies, as you can probably gather Im very much a newbie when it comes to electronics, but I am willing to learn and open to other options.

    I also have access to Multisim to simulate the project as I go along.

  2. #12


    Nov 30, 2010
    Who's design? Your design or the teachers design? What "brief"?
    Why would you make it so complicated?
    And, why use the oldest possible, least convenient, amplifier chip?
  3. wayneh


    Sep 9, 2010
    An LM35 thermometer, LM339 comparator, and a MOSFET switch appropriate for your DC load. Maybe a relay or SSR if your load is AC. Done.

    Why all the other stuff?
  4. zeb59

    New Member

    Oct 6, 2013

    The resistor would be 0.25 K-ohm to convert the o/p to a 4-20 mA signal

    Use the 741 in a difference Amp configuration to amplify the signal from the PT100 bridge. This will require designing the 4 feedback resistors which is a very simple task. Look at the 741 datasheet and you should spot the circuit.

    However if your brief allows you consider using an INSTRUMENTATION AMPLIFIER such as the AD620 in place of the 741. You can set the gain of the amplifier by changing only one resistor. This helps tremendously.

    I suggest you start with the PT100 bridge first by drawing up an MS-EXCEL sheet for your temp. range. You can then try simulating the bridge in Multitism to see if the results match the EXCEL sheet.

    Last edited: Oct 17, 2013