If k {\displaystyle k} is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient ( PTC) thermistor, or posistor, there are two types of PTCr, switching thermistor and silistor. If k {\displaystyle k} is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient ( NTC) thermistor. Resistors that are not thermistors are designed to have a k {\displaystyle k} as close to 0 as possible, so that their resistance remains nearly constant over a wide temperature range. Thermistors are divided based on their conduction model. Negative Temperature Coefficient (NTC) thermistors have less resistance at higher temperatures, while Positive Temperature Coefficient (PTC) thermistors have more resistance at higher temperatures. [1]

With PTC thermistors, resistance increases as temperature rises; usually due to increased thermal lattice agitations, particularly those of impurities and imperfections. PTC thermistors are commonly installed in series with a circuit, and used to protect against overcurrent conditions, as resettable fuses. Instead of the temperature coefficient k, sometimes the temperature coefficient of resistance α T {\displaystyle \alpha _{T}} ("alpha sub T") is used. It is defined as [6] α T = 1 R ( T ) d R d T . {\displaystyle \alpha _{T}={\frac {1}{R(T)}}{\frac {dR}{dT}}.} With NTC thermistors, resistance decreases as temperature rises; usually due to an increase in conduction electrons bumped up by thermal agitation from the valence band. An NTC is commonly used as a temperature sensor, or in series with a circuit as an inrush current limiter. This article's lead section may be too short to adequately summarize the key points. Please consider expanding the lead to provide an accessible overview of all important aspects of the article. ( February 2022) Thermistors differ from resistance temperature detectors (RTDs) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a greater precision within a limited temperature range, typically −90°C to 130°C. [5] Basic operation [ edit ]A thermistor is a semiconductor type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of thermal and resistor. This α T {\displaystyle \alpha _{T}} coefficient should not be confused with the a {\displaystyle a} parameter below. where a, b and c are called the Steinhart–Hart parameters and must be specified for each device. T is the absolute temperature, and R is the resistance. The equation is not dimensionally correct, since a change in the units of R results in an equation with a different form, containing a ( ln ⁡ R ) 2 {\displaystyle (\ln R)

Depending on type of the thermistor in question the k {\displaystyle k} may be either positive or negative. NTC thermistor are widely used as inrush current limiters, temperature sensors, while PTC thermistors are used as self-resetting overcurrent protectors, and self-regulating heating elements. An operational temperature range of a thermistor is dependent on the probe type and is typically between −100 °C and 300 °C (−148 °F and 572 °F). Thermistors are generally produced using powdered metal oxides. [3] With vastly improved formulas and techniques over the past 20 years [ when?], NTC thermistors can now achieve accuracies over wide temperature ranges such as ±0.1°C or ±0.2°C from 0°C to 70°C with excellent long-term stability. NTC thermistor elements come in many styles [4] such as axial-leaded glass-encapsulated (DO-35, DO-34 and DO-41 diodes), glass-coated chips, epoxy-coated with bare or insulated lead wire and surface-mount, as well as thin film versions. The typical operating temperature range of a thermistor is −55°C to +150°C, though some glass-body thermistors have a maximal operating temperature of +300°C.