What's a "temperature comparator" called?

Discussion in 'General Electronics Chat' started by DMahalko, Jul 11, 2014.

  1. DMahalko

    Thread Starter Senior Member

    Oct 5, 2008
    I'm looking for a seemingly simple electronic commercial product but I don't know what it's called.

    I want to compare the temperature of two thermal sensors, and if the temperature of one rises 1-2 degrees F higher than the other, trigger a relay to start a motor. If the high temp then drops to equal or lower of the other, turn off the relay.

    Optionally: "But if the overall temperature is less than 120 F, ignore any temperature difference and don't do anything."


    This is for a PEX hydronic heating system using a heat exchanger, to save energy. If the supply-side temp of the heat exchanger rises above the secondary loop by 1-2 degrees F, turn on the secondary loop circulator pump.

    If the supply-side heat source turns off, the supply-side temp will drop until it is equal or lower than the secondary loop, and the supply-side is now robbing heat from the secondary loop. So turn off the secondary loop circulator pump.


    Since the supply-side heat source is outdoors, it can warm and cool in the heat of the day and has a circulator that runs all the time even if it's not actually heating, so it would be best to ignore any supply-side temperature variations that are below the point that useful heat is really entering the exchanger, say 120F.
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    For that i would use a microcontroller such as a pic, or your using an op-amp comparator using the lower temp as a reference.
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    I'd think they might have temperature controllers that can do that.. something with 2 thermocouple inputs.. but maybe not.. it does require some "logic" besides the if greater than setpoint turn on which is how most temperature controllers work.

    I'd suspect that such an "off the shelf" device is out there as PEX heating systems are fairly common now.
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    How about a simple bridge circuit with two identical thermistors (placed as needed) connected to a comparator? Bias the comparator such that it trips when the one side of the bridge is offset from the other...
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
    Add an op amp integrator and differentiator to your op amp and you suddenly have a PID cintroller - no Microcontroller needed.