thermistor

A thermistor is a solid-state resistor made from a semiconductor material, typically a metal oxide ceramic, whose electrical resistance changes sharply with temperature. Unlike conventional resistors, which resist change across a wide temperature range, thermistors are engineered to respond dramatically to small temperature shifts, making them useful as temperature sensors and thermal compensators in circuits. The resistance change is not linear, but follows a predictable curve that can be characterized mathematically.

There are two main classes: NTC thermistors (negative temperature coefficient) decrease in resistance as temperature rises, while PTC thermistors (positive temperature coefficient) increase in resistance with heat. NTC thermistors dominate industrial applications because their sensitivity is greater. An NTC bead thermistor might drop from 100 kilohms at 25 degrees Celsius to 1 kilohm at 100 degrees Celsius. Common base materials are manganese oxide, nickel oxide, and cobalt oxide sintered together to form a ceramic compound with the desired electrical properties.

Thermistors come in several physical forms: bead thermistors are tiny spheres of sintered material with wire leads, disk thermistors are flat elements useful for surface mounting, and rod or probe thermistors house the element in a metal or glass sheath for immersion in liquids or gases. Thin film thermistors deposited on substrates offer precision and reproducibility for high-volume applications. Response time varies from milliseconds for exposed beads in a gas stream to several seconds for protected probes in thermal masses.

Measurement and compensation uses

In temperature measurement, a thermistor is placed in a circuit where the changing resistance produces a changing voltage or current that can be read by an analog-to-digital converter or a dedicated meter. The accuracy depends on knowing the thermistor's response curve, often described by the Steinhart-Hart equation, which relates resistance to absolute temperature. Thermistors are cheaper and faster-responding than resistance temperature detectors (RTDs) but typically cover smaller temperature ranges and are less stable over decades of use.

Thermistors also function as automatic thermal compensators in frequency oscillators, power supplies, and audio amplifiers, where they adjust bias currents or component values to counteract temperature drift in other parts of the circuit. A small PTC thermistor placed in series with a heating element can act as a self-regulating heater, raising its resistance as it warms and thus limiting its own current draw. Failure modes include drift due to aging of the ceramic material, cracking from thermal shock or mechanical stress, and open circuits from lead corrosion or solder joint fatigue.