Thermistors and thermistor sensors

　　Similar to RTDs, thermistors are also temperature-sensitive semiconductors whose impedance varies with temperature. Thermistors are fabricated from metal oxide semiconductor materials encapsulated in glass or epoxy beads. Also, thermistors typically have much higher nominal impedance values than RTDs, ranging from 2000Ω to 10,000Ω, so they can be used for lower current measurements.

　　Each sensor has a set nominal impedance that varies proportionally with temperature according to some linearized approximation. Thermistors have either a negative temperature correlation coefficient (NTC) or a positive temperature correlation coefficient (PTC), the former (also more common) having an impedance that decreases with increasing temperature, and the latter having an impedance that increases with increasing temperature.

　　PTC thermistors or PTC thermistors can be used as current limiting devices (instead of fuses), or as heating components in small temperature-controlled furnaces. The NTC thermistor is mainly used for temperature measurement and is widely used in digital thermostats or automobiles to monitor the temperature of the engine.

　　Typically, a thermistor has a high sensitivity (about 200Ω/°C), which makes it very sensitive to changes in temperature. Although the thermistor has an extremely high response rate, its use is limited to a temperature range of up to 300°C. This feature, along with its high nominal impedance, helps provide accurate measurements in lower temperature applications.

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