An NTC thermistor is a temperature-sensitive resistor

An NTC thermistor is a temperature-sensitive resistor whose resistance decreases as temperature increases. In this circuit, the thermistor R₁ and the fixed resistor R₂ form a voltage divider powered by a 12 V supply. The voltage developed at the junction of the two resistors is labeled VTEMP. An operational amplifier is connected as a voltage follower, so the output voltage VOUT is equal to VTEMP while providing a low-impedance output that can drive other circuits without affecting the measurement.

At 25°C, the thermistor resistance is 10 kΩ and R₂ is 1 kΩ. Using the voltage divider equation,

VOUT = (R₂ / (R₁ + R₂)) × V

the output voltage becomes

VOUT = (1000 / (10000 + 1000)) × 12 = 1.09 V

At 100°C, the thermistor resistance decreases significantly to about 100 Ω. Applying the same equation,

VOUT = (1000 / (100 + 1000)) × 12 = 10.9 V

These calculations show that the output voltage increases as temperature rises because the resistance of the NTC thermistor decreases. By measuring VOUT, the temperature can be estimated accurately after calibration. Thermistor-based temperature sensing circuits are widely used in thermostats, battery management systems, medical instruments, household appliances, and industrial control systems because they are inexpensive, sensitive, and easy to interface with analog or digital electronics.