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NTC and comparator to detect temperature trip point

  1. Feb 10, 2017 #1
    In "The Art of Electronics" the following is said: "A comparator in a circuit is a handy device that switches its output according to the relative voltages at its two input terminals. There is a temperature sensing device, a thermistor, which decreases in resistance by about 4%/°C. So we’ve made it the lower leg of a voltage divider. The comparator then compares the voltage from the thermistor and from a temperature-insensitive resistor. When it’s hotter than 30°C, point “X” (thermistor voltage) is at a lower voltage than point “Y" (temperature-insensitive resistor), so the comparator pulls its output to ground." My question is why the voltage at point X (thermistor) is lower than the other one. I thought it would be more logical that if the resistance of the thermistor decreases, the current would increase so that the voltage would also increase. The comparator wouldn't then measure a lower voltage than at Y. Thanks in advance:)
     
  2. jcsd
  3. Feb 10, 2017 #2

    cnh1995

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    Homework Helper

    Welcome to PF!

    Is it possible to post the circuit diagram?
     
  4. Feb 10, 2017 #3

    berkeman

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    Staff: Mentor

    It's not in the 1st Edition, must be in the 2nd Edition...
     
  5. Feb 11, 2017 #4

    Svein

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    OK. The voltage across the NTC is given by [itex] V_{NTC}=V_{ref} \frac{R_{NTC}}{R_{fixed}+R_{NTC}}[/itex]. If you want, you can input some values in the formula and verify that the voltage across the NTC decreases as the NTC resistance decreases. If you want a mathematical reason, observe that [itex]\frac{dV_{NTC}}{dR_{NTC}}=V_{ref} \frac{R_{fixed}}{(R_{fixed}+R_{NTC})^{2}} [/itex] which is positive. This again means that the voltage across the NTC changes the same direction as the change in NTC resistance.
     
    Last edited: Feb 11, 2017
  6. Feb 11, 2017 #5
    Thank you so much! It makes sense:)
     
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