NTC and comparator to detect temperature trip point

In summary, in the circuit described in "The Art of Electronics," a comparator is used to switch its output based on the relative voltages at its two input terminals. The voltage at point X (thermistor) is lower than at point Y (temperature-insensitive resistor) when the temperature is above 30°C because the voltage across the NTC (thermistor) decreases as its resistance decreases. This is due to the mathematical relationship \frac{dV_{NTC}}{dR_{NTC}}=\frac{R_{fixed}}{(R_{fixed}+R_{NTC})^{2}}, which shows that the voltage across the NTC changes in the same direction as the change in NTC resistance.
  • #1
Dina1999
2
0
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:)
 
Engineering news on Phys.org
  • #2
Dina1999 said:
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:)
Welcome to PF!

Is it possible to post the circuit diagram?
 
  • Like
Likes berkeman
  • #3
cnh1995 said:
Welcome to PF!

Is it possible to post the circuit diagram?
It's not in the 1st Edition, must be in the 2nd Edition...
 
  • #4
Dina1999 said:
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:)
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:
  • #5
Svein said:
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}}=\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.
Thank you so much! It makes sense:)
 

FAQ: NTC and comparator to detect temperature trip point

What is an NTC?

An NTC (Negative Temperature Coefficient) is a type of thermistor that has a resistance that decreases as the temperature increases. It is commonly used in temperature sensing applications.

What is a comparator?

A comparator is an electronic component that compares two input voltages and outputs a signal based on which voltage is higher. It is often used in conjunction with an NTC to detect temperature trip points.

How does an NTC and comparator work together to detect temperature trip points?

The NTC is connected to one of the input voltages of the comparator, while the other input is connected to a reference voltage. As the temperature changes and the resistance of the NTC changes, the voltage at its input also changes. When this voltage reaches a certain level (the reference voltage), the comparator outputs a signal, indicating that the temperature trip point has been reached.

What is a temperature trip point?

A temperature trip point is a specific temperature at which a device or system is designed to trigger a response. In the case of an NTC and comparator, the trip point is the temperature at which the comparator outputs a signal, indicating that a certain temperature has been reached.

What are some common applications of NTCs and comparators for temperature sensing?

NTCs and comparators are commonly used in temperature control systems, such as in ovens, refrigerators, and air conditioners. They can also be used for temperature monitoring in industrial processes and in electronic devices to prevent overheating.

Back
Top