Thermistor Triggered Relay

Discussion in 'The Projects Forum' started by sallc5, Jun 12, 2012.

  1. sallc5

    Thread Starter New Member

    Feb 20, 2010
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    I would like to use this circuit or one similar to output a low to switch a relay when a certain temperature is reached and shut off once drops below that set temperature.

    [​IMG]



    The relay is always hot 12v and takes a ground to activate. The thermistor I am using correlates as such:

    [​IMG]

    I would like the circuit to to turn on at about 212*F. I am having troulbe picking out the values of the components from there. Or are the components listed suitable for my purpose? Still new to electronics and appreciate your help greatly.:D
     
  2. #12

    Expert

    Nov 30, 2010
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    Your thermistor and adjuster are reversed. You need about 177 ohms for the trigger point, not 100,000

    Change to a 500 ohm adjuster pot.

    This will allow quite a bit of curerent...34 ma.
    You should switch to a higher resistance thermistor to avoid it being heated by the sense current.
    Perhaps one that is rated at 10,000 ohms or 20,000 ohms at 25 centigrade.
     
    Last edited: Jun 12, 2012
  3. sallc5

    Thread Starter New Member

    Feb 20, 2010
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    Thanks for the quick response! Any other suitable circuit to use that will not heat up the thermistor? I do not have much of a choice in the thermistor being used.
     
  4. #12

    Expert

    Nov 30, 2010
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    The circuit is good except for the amplifier chip. It's antique. It's miserable about using low voltages. Isolate the 4 parts of the voltage divider circuit and add a 390 ohm resistor at the top and another at the bottom. That reduces the voltage range on the sensor section to 4 volts instead of 12 volts and reduces the waste heat by a factor of 9.
     
  5. sallc5

    Thread Starter New Member

    Feb 20, 2010
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    Would you have another circuit or amplifier chip in mind that would be more suitable or just go with the added resistors to reduce voltage?
     
  6. #12

    Expert

    Nov 30, 2010
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    I'm lazy. I'm not going to start the whole project from scratch when you can fix what you have for 30 cents.
     
    Last edited: Jun 12, 2012
  7. wayneh

    Expert

    Sep 9, 2010
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    FWIW, the LM358 dual op-amp is widely available, cheap, and can sense down to the ground rail. It's too slow for audio and and higher frequencies, but fine for this DC stuff.
     
  8. sallc5

    Thread Starter New Member

    Feb 20, 2010
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    Sounds good. Thanks for your assistance!
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Depending on how much current your relay needs, you might find an op-amp that can drive it directly, so that you don't need the transistor. Or I should say you could replace the function of the transistor with the second op-amp of a dual.
     
  10. #12

    Expert

    Nov 30, 2010
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    That won't stop the thermistor from dissipating .2 watts
     
  11. wayneh

    Expert

    Sep 9, 2010
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    Quite right. Your simple fix is the way to go for that aspect. Improving the op-amp only...improves the op-amp.
     
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  12. #12

    Expert

    Nov 30, 2010
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    The simple fix also keeps the antique amplifier chip inside its proper working range. I've done these before. Hard to catch me making a mistake on this kind of circuit. I just didn't want to go into an LM35 circuit with a beginner.
     
  13. sallc5

    Thread Starter New Member

    Feb 20, 2010
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    Which pins on opamp to add the 390R resistor to?
     
  14. #12

    Expert

    Nov 30, 2010
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    None of them. The opamp isn't a voltage divider circuit, it's an amplifier. Using the drawing you provided, you put the 2 resistors just to the right of the connections for the 10k resistors.
     
  15. wayneh

    Expert

    Sep 9, 2010
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    Ha ha, I suppose you're right. To me, the LM35 is much easier to understand and deal with than all this non-linear thermistor stuff, but yeah, it's easier to improve an existing circuit than start all over.
     
  16. #12

    Expert

    Nov 30, 2010
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    When you only have one set point, this circuit reduces to almost digital simplicity. The OP provided the temperature/resistance chart and the hysteresis was already on the correct side of the input. That made it a snooze for me.

    Probably have to do the hysteresis tomorrow.
     
    Last edited: Jun 13, 2012
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