Opamp thermistor trigger

Discussion in 'The Projects Forum' started by donryanocero, Apr 16, 2013.

  1. donryanocero

    Thread Starter New Member

    Apr 16, 2013
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    Hi, I constructed a circuit similar to that in Forrest Mims' thermistor opamp trigger on page 37 of science and communications circuits (vol 2) in case someone has it lying around. basically it's non-inverting comparator with a 100k pot between +in and ground and a 10k thermistor between V+ and -in in a voltage divider with a 10k resistor to ground.

    I'm getting quite a bit of output current when i would think it should be off. Anyone have any insight? The output current is like 1/2-2/3 when it's 'off'. I can post a picture if that helps.

    I wanted to use this to switch a MOSFET to turn a fan on in an external hard drive enclosure when the temperature passes a certain point.
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    Can you post a schematic? A hand-drawn copy would not infringe on Mr. Mim's intellectual property rights.
     
  3. edwardholmes91

    Member

    Feb 25, 2013
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    Here is a copy of the circuit. I have tried it in simulation but I can't get the LED to light at all.
     
  4. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Have you looked at the output signal on a scope? You may be getting oscillation near the temperature switching threshold. Adding some hysteresis to the circuit should help considerably.

    Additionally, what you are trying to do is really a job for a comparator, rather than on op-amp. Try replacing the 741 with an LM311. You may (probably will) need to add some hysteresis to prevent oscillation.
     
  5. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Oh! VR1 is connected improperly...

    The way you have it wired, the (-) input will never exceed the (+) input. Sorry, I should have caught that first!
     
  6. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    A few more hints:

    1. Consider grounding your thermistor and swapping the +/- inputs to the amp. This will allow you to use shielded wire to your thermistor if you intend to make a long temperature probe. It will also help to prevent letting the smoke out if your probe accidentally touches the electrical ground.

    2. Rather than VR1 swinging from rail-to-rail (which is what you should have when you hook it up right), consider the addition of two fixed resistors, one on each end of the pot. This will *tremendously* improve the setability of the pot, and limit drift due to temperature and mechanical shock. If you can tell me the *exact* thermistor part number you are using, and the desired range of set points (min temp and max temp), I can tell you what resistors to use (in either 5% or 1% values -- your choice).

    Have fun!
     
  7. #12

    Expert

    Nov 30, 2010
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    I have a very similar circuit made with a 723 chip. Basically an op amp with a zener voltage reference. My circuit demonstrates the grounded shield sensor and the padded potentiometer.
     
  8. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Is that hysteresis you are adding with the 4001 and 680K?
     
  9. #12

    Expert

    Nov 30, 2010
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    Yes.......
     
  10. donryanocero

    Thread Starter New Member

    Apr 16, 2013
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    Edward's circuit is like mine. Why would grounding the thermistor and swapping the inputs allow me to use shielded wire to the thermistor?
    I'm using a TC503 thermistor and would like the fan to trigger at 115-120F.

    Just realized that it's a 100K thermistor. I couldn't find a data sheet and someone posted incorrectly in a forum that's 10K. I'll rebuild the circuit for the 100K and see what happens now.
     
  11. #12

    Expert

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    Because that's the whole point of arranging the circuit to use a grounded shield. If you arrange the circuit to have the thermistor in the non-grounded position, you end up using a twisted, shielded pair to keep noise pickup to a minimum. Two wires and a shield is 3 wires. Cheaper and less noisy to use 1 wire and a shield.
     
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