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  1. #1
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    20k NTC repeater/signal conditioner?

    Hello controls gurus,

    I have a thermostat that takes a 20k NTC input for temp sensing. I want to be able to feed the thermistor into some kind of a box that will allow me to read the temperature directly via a separate output, and also provide the temperature to the stat. How might I be able to do this?

    The sort of magic box I'm looking for would have a 20k NTC input for my thermistor, a 20k NTC output for my stat, and a separate 2-10v or 4-20ma output for my remote display.

  2. #2
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    Not possible? No one makes something?

    The closest I have found is the Honeywell WRECVR which is the wireless receiver that works with the TR2x series wireless temperature sensors. It connects to a T7351's temp inputs, which usually take a 20k thermistor. So I know that a 20k signal can be provided by an electronic module, but I don't know of anywhere that makes a device like I described in the first post.

  3. #3
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    The 20K ohm sensor is a resistive device. It doesn't send out a signal. Instead, a controller sources a voltage through it to figure out its resistance by measuring the voltage drop. A programmable controller with a free AO can get your output as a 2-10V or 4-20mA out, but I'm not sure about a resistive output. It'd be easier to install 2 sensors on the wall.

  4. #4
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    Quote Originally Posted by digo View Post
    The 20K ohm sensor is a resistive device. It doesn't send out a signal. Instead, a controller sources a voltage through it to figure out its resistance by measuring the voltage drop. A programmable controller with a free AO can get your output as a 2-10V or 4-20mA out, but I'm not sure about a resistive output. It'd be easier to install 2 sensors on the wall.
    Yep I got that, but as you mentioned the resistive output seems to be the sticking point - can't seem to find such a thing on any controller. Putting 2 sensors in each location is more of a problem when I am trying to read the temperature off of an existing averaging network of multiple sensors.

  5. #5
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    Quote Originally Posted by rjk_cmh View Post
    Yep I got that, but as you mentioned the resistive output seems to be the sticking point - can't seem to find such a thing on any controller. Putting 2 sensors in each location is more of a problem when I am trying to read the temperature off of an existing averaging network of multiple sensors.
    Kele has this transducer... takes a voltage/current input, outputs the resistance. You'd need to order the one with a custom range to match the curve of a 20K ohm sensor.
    http://www.kele.com/transducers/res-1e.aspx

  6. #6
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    Quote Originally Posted by rjk_cmh View Post
    Not possible? No one makes something?

    The closest I have found is the Honeywell WRECVR which is the wireless receiver that works with the TR2x series wireless temperature sensors. It connects to a T7351's temp inputs, which usually take a 20k thermistor. So I know that a 20k signal can be provided by an electronic module, but I don't know of anywhere that makes a device like I described in the first post.
    EasyIO FC20

    20k thermistor in ...0-10, 40-20, 0-20, modbus, bacnet out...why not use it as the thermostat.

    or...

    use an FG20 and take advantage if the inbuilt web server
    1 + 1 = 3 ( *** for very large values of 1)

    ...everybody wants a box of chocolates and long stemmed rose

    Be brave. You cannot get eaten by an imaginary tiger.

  7. #7
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    Quote Originally Posted by digo View Post
    Kele has this transducer... takes a voltage/current input, outputs the resistance. You'd need to order the one with a custom range to match the curve of a 20K ohm sensor.
    http://www.kele.com/transducers/res-1e.aspx
    I didn't even think of that... thanks i'll look into it.

  8. #8
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    Quote Originally Posted by MatrixTransform View Post
    EasyIO FC20

    20k thermistor in ...0-10, 40-20, 0-20, modbus, bacnet out...why not use it as the thermostat.

    or...

    use an FG20 and take advantage if the inbuilt web server
    I kind of need to upgrade this thermostat anyways - might be a good idea to try something like that too.

  9. #9
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    Quote Originally Posted by digo View Post
    Kele has this transducer... takes a voltage/current input, outputs the resistance. You'd need to order the one with a custom range to match the curve of a 20K ohm sensor.
    http://www.kele.com/transducers/res-1e.aspx
    I would think twice about this idea. They switch in fixed resistor banks with relays and well....

    1. The resolution your going to get is crap for space temp, covering the full range of the sensor at least.
    2. Lifetime of the relays in this app, assuming you "tune" the resistance range for good resolution...not so good.

    I don't really get what the heck the OP trying to do, but there are better ways I'm sure. Just measuring the voltage across the input of the existing controller with another would likely get the job done without messing with the existing setup, but even that seems like more work than its worth. Fix the problem at hand rather than bandaid bull$hit.
    Last edited by orion242; 04-28-2015 at 11:47 PM.
    Propagating the formula. http://www.noagendashow.com/

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