What Are Thermistors?

Thermistors are temperature-sensitive resistors whose electrical resistance changes predictably with temperature, fabricated from sintered metal oxide semiconductors. The name is a contraction of "thermal resistor." Unlike metallic resistors, whose resistance rises modestly and linearly with temperature, thermistors exhibit large, nonlinear changes in resistance over a relatively narrow temperature range, which makes them among the most sensitive and lowest-cost temperature transducers available. They are used in applications where the temperature range is moderate, typically from minus 55 to 150 degrees Celsius, and where high measurement sensitivity or rapid response time is required.

Thermistors draw on materials science, semiconductor physics, and instrumentation engineering. Their thermoresistive behavior originates in the activation energy required for charge carriers in a metal oxide lattice to hop between sites, a process governed by Boltzmann statistics.

Negative Temperature Coefficient Thermistors

Negative temperature coefficient (NTC) thermistors are the more widely deployed type. Their resistance decreases as temperature rises because higher temperatures thermally activate more charge carriers, reducing the effective resistance of the material. The resistance-temperature relationship is described by the Steinhart-Hart equation, a three-coefficient empirical model that fits the nonlinear NTC curve to within millikelvins across the operating range, or by the simpler beta-parameter equation when a narrower range and lower accuracy are acceptable.

NTC thermistors are manufactured primarily from oxides of manganese, nickel, cobalt, copper, and iron. Typical base resistances at 25 degrees Celsius range from 100 ohms to 10 megohms, with beta values between 2,000 K and 5,000 K determining the sensitivity slope. Bead thermistors, with diameters below one millimeter, offer thermal time constants under one second in stirred liquid, making them suitable for medical probes, flow sensors, and precision calorimetry. Analog Devices documents thermistor signal conditioning for precision measurement, including linearization circuits and analog-to-digital interfacing techniques that compensate for the inherent nonlinearity of the NTC curve.

Positive Temperature Coefficient Thermistors

Positive temperature coefficient (PTC) thermistors exhibit resistance that increases with temperature. The most widely used PTC type is based on barium titanate ceramic doped with rare earth elements. Below a Curie temperature specific to the formulation, typically between 60 and 120 degrees Celsius for commercial grades, the resistance is low and relatively flat. At the Curie temperature, resistance increases sharply by several orders of magnitude over a few degrees, a transition driven by a ferroelectric phase change in the crystal structure.

This switching characteristic makes PTC thermistors effective as self-resetting overcurrent protection devices. When excess current raises the device to its Curie point, the resistance surge limits current to a safe level; when the fault clears and the device cools, it resets automatically. They are embedded in motor windings, battery packs, and power supply outputs as resettable fuses. The IEC 60738-1 standard for PTC thermistors defines the test methods and resistance-temperature characteristic requirements for overcurrent-protection PTC components, ensuring interoperability across manufacturers.

Temperature Measurement and Calibration

For precision temperature measurement, NTC thermistors provide better sensitivity than platinum RTDs in the minus 10 to 100 degrees Celsius range, where the high beta value produces large resistance changes per degree. Their principal limitation is device-to-device interchangeability: unlike Pt100 RTDs conforming to IEC 60751, thermistors are not produced to a universal resistance-temperature table, so individual calibration or tight tolerance matching is required for interchangeable applications.

NIST's temperature calibration services maintain thermistor calibration references traceable to ITS-90, and NIST Special Publication 250 documents the measurement uncertainty achievable with calibrated NTC thermistors in the range from minus 80 to 260 degrees Celsius.

Applications

Thermistors have applications across a wide range of measurement and protection functions, including:

• Body temperature sensing in medical devices and wearables
• Overcurrent protection in motors, power supplies, and battery packs
• Cold-junction compensation in thermocouple instruments
• Air and liquid flow measurement by anemometry
• Ocean temperature profiling and oceanographic instrument strings