Thermistors are types of semiconductors whose resistance changes according to any changes in temperature in the system to which the thermistor is connected. They are typically composed of ceramic or polymer materials which tend to be more sensitive to changes in temperature, with the amount of resistance in a thermistor being increased or decreased according to an increase in temperature.
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Because of their dependence on resistance and temperature, thermistors can be used as temperature sensors or heat sensors. Out of all of the types of temperature sensors, thermistors are the most sensitive and so are used in precision applications which rely on system adjustments based on even slight temperature changes. Industries such as automotive, residential heating and electronics, medical, pharmaceutical and food and beverage processing use thermistors because of their ability to read and adjust to even slight changes in temperature. While they have a higher accuracy than other types of thermocouples such as resistance temperature detectors and high temperature thermocouples, thermistors do have a limited working temperature range due to the sensitive nature of the materials typically used to construct them. Manufacturers are able to design thermistors according to the specific needs of a temperature control system and can adjust the required resistance of the thermistor itself.
Modern thermostats and incubation systems, automotive applications such as monitoring the temperature of engine coolant or oil, as well as basic residential processes like electrical circuit protection all employ a type of thermistor to monitor and control the system. There are two main types of thermistors which are chosen depending on their intended applications. The first type thermistors are called a positive temperature coefficient thermistor (PTC). In PTC thermistors, if the first-order temperature coefficient of resistance is positive, the resistance will increase when the temperature increases. This is seen in current limiting devices such as those used for circuit protection in place of fuses. The increase in resistance of the thermistor due to a rise in temperature results in lowering the voltage traveling to a device. In the second type of thermistors, called negative temperature coefficient (NTC) thermistors, resistance decreases as temperature increases. This occurs when the first-order temperature coefficient of resistance is negative. These types of thermistors can be used in a wide range of applications. PTC thermistors can take the place of fuses in current-limiting devices for circuit protection, while NTC thermistors can be used as resistance thermometers. The temperature of battery packs and digital thermostats can be observed with either type of thermistor.
