Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Homework Helper
    Gold Member

    Welcome to PF!

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


    User Avatar

    Staff: Mentor

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


    User Avatar
    Science Advisor

    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:)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted