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Thermistor circuit, help wanted

Discussion in 'General Electronics Discussion' started by briancs342, Jun 7, 2010.

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  1. briancs342

    briancs342

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    Jun 7, 2010
    I'm trying to build a circuit with a thermistor, comparator chip, and relay that will cut off power to a heat source when it reaches 180C (or somewhere close). However, I am totally inept at this sort of thing, so I need to know several things, including:
    The exact specifications of what I need to buy in terms of comparators, relays, and thermistors;
    How to hook it all up together; and
    How to hook it up to my power supply so that it works.

    If you could help me out, I would greatly appreciate it.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,501
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    Jan 21, 2010
    What is the heat source and how is it powered? What power source do you have for your circuit?

    The circuit you will want is something similar to this. An op-amp as a comparator will (with suitable components) allow you to do this.
     
  3. briancs342

    briancs342

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    Jun 7, 2010
    Heat source is a ceramic heating element, 20 watts, and power source is 120V outlet.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,501
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    Jan 21, 2010
    OK, you'll need some sort of low voltage supply for the electronics, and probably a relay to turn the heater on and off.

    180C is close enough to the melting point of solder that you're going to have to be very careful that an overtemp doesn't cause the thermistor to detatch. In addition, you may have problems finding a thermistor rated for this temperature.

    Another issue will be hysteresis -- you don't want the ceramic heating element switching on and off rapidly as the temperature varies a small raction of a degree over or under the switching point.
     
  5. briancs342

    briancs342

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    Jun 7, 2010
    Ideally I would have the thermistor leads running down a cable and into the central console which would be far away from the heat source, so I would imagine I could get away with the solder problem. I thought that the rapid switching might be an issue, but unless it would be damaging to the circuits or other instruments I don't think it would be too important-- as long as it keeps the temp relatively constant.

    Perhaps there would be other solutions to a temp reading circuit to control the power? Ideally I'm looking for something relatively cheap (less than $60-80), and hopefully something simple, but I'm open to any idea, really. Thanks for your help thus far, too.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,501
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    It's the wires connected to the thermistor that are the problem. Unless you can get one which already has long leads, you would need to solder leads to it. And if it has leads, and they're soldered, well, that's the same problem.

    As I said, getting a thermistor that operates at this temperature may be an issue.

    Perhaps you can look at a thermal cutout designed to trigger at around that temperature.

    Did you mention the accuracy you require? Would +/- 5 degrees be too much? what about +/- 10?
     
  7. briancs342

    briancs342

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    Jun 7, 2010
    5 degree moe is perfectly fine, but I don't know if I could do 10.
     
  8. briancs342

    briancs342

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    Jun 7, 2010
    So if I used a thermal cutout, what specifically should I be looking for? Also, what else would I need? It seems that the entire circuit I would have could be replaced by this one component.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Pretty much. Depending on the current handling capacity, you may still need it to switch power to a relay. The other issue is how often it is designed to trigger, and the temperature range over which it is rated to operate (the spread of temperatures between triggering and resetting.

    Remember that the old tin/lead solder melts at 183C and is totally inappropriate for soldering devices that will get anywhere near this temperature. You would have to (at least) select a solder with a higher melting point. The easily found options are some of the lead free solders, or "silver solder". But in each case you'll have to ensure that the melting point is high enough.

    Because of the elevated temperature you're looking at, I cannot recommend a sensor. You'll need to locate something and then maybe a circuit can be built around it.
     
  10. briancs342

    briancs342

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    Jun 7, 2010
    OK, the solder I have is rated to 240C, so no problem there.

    As for the current capacity, 20A is standard out of a wall socket, correct?

    If I couldn't find a cutout rated to 180C close on rise, could I use a 180C open on rise and connect it to a relay so that when it opens the relay is activated and the current to the heat source shuts off? I seem to be able to find open on rise cutouts at higher temps than the close on rise.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,501
    2,841
    Jan 21, 2010
    That's an approach you could consider. You need to ensure that if the wires come off it that the power is removed.

    If you get a device that opens when it reaches the correct temperature then it can be used to pull in a relay to turn on the heater. When the temperature is correct, the device goes open circuit, the relay opens and the heater powered via the N.O. contacts shuts down.
     
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