Thermistors are temperature-sensitive resistors that exhibit a significant change in resistance in response to changes in temperature. They are commonly used as temperature sensors in various electronic devices and systems due to their high sensitivity and accuracy over a limited temperature range. Thermistors operate based on the principle of the temperature dependence of electrical resistance in semiconductor materials.

Here's how thermistors work:

1. Semiconductor Material: Thermistors are made of semiconductor materials, typically metal oxides such as manganese, nickel, cobalt, or titanium, which exhibit a nonlinear change in resistance with temperature.

2. Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC):

   • NTC Thermistors: NTC thermistors exhibit a decrease in resistance with increasing temperature. As the temperature rises, the number of charge carriers in the semiconductor material increases, resulting in a decrease in resistance.
   • PTC Thermistors: PTC thermistors, on the other hand, exhibit an increase in resistance with increasing temperature. The resistance of PTC thermistors typically increases abruptly at a certain temperature, known as the Curie temperature.

3. Resistance-Temperature Relationship: The resistance of a thermistor varies exponentially with temperature according to the Steinhart-Hart equation or another appropriate model. This relationship is characterized by the thermistor's resistance-temperature curve, which is specific to the type of thermistor and its material composition.

4. Measurement Circuit: Thermistors are connected in electrical circuits as resistive temperature sensors. By measuring the electrical resistance of the thermistor using a voltage divider circuit or Wheatstone bridge configuration, the temperature of the thermistor can be determined.

5. Calibration: To accurately measure temperature using a thermistor, the resistance-temperature curve of the thermistor must be calibrated over the desired temperature range. Calibration involves establishing a relationship between the resistance of the thermistor and the corresponding temperature using calibration equipment and techniques.

Types of Thermistors:

1. NTC Thermistors:

   • Negative Temperature Coefficient (NTC) thermistors are the most common type of thermistor.
   • They exhibit a decrease in resistance with increasing temperature.
   • NTC thermistors are widely used in temperature measurement and control applications, such as in thermostats, automotive sensors, and electronic devices.

2. PTC Thermistors:

   • Positive Temperature Coefficient (PTC) thermistors exhibit an increase in resistance with increasing temperature.
   • PTC thermistors are often used in applications requiring self-regulating heating elements, overcurrent protection, and motor starting circuits.

3. Interchangeability Thermistors:

   • Interchangeability thermistors are a type of NTC thermistor designed to have a standardized resistance-temperature curve.
   • They are used in applications where interchangeability and compatibility between different thermistors are important, such as in medical temperature sensors and consumer electronics.

Thermistors offer advantages such as high sensitivity, small size, and low cost, making them suitable for a wide range of temperature sensing applications in various industries. However, their nonlinear resistance-temperature relationship requires careful calibration for accurate temperature measurement.