A thermistor is a semiconductor type of resistor in which the resistance is strongly dependent on temperature. The word thermistor is a portmanteau of thermal and resistor. The varying resistance with temperature allows these devices to be used as temperature sensors, or to control current as a function of temperature. Some thermistors have decreasing resistance with temperature, while other types have increasing resistance with temperature. This allows them to be used for limiting current to cold circuits, e.g. for inrush current protection, or for limiting current to hot circuits, e.g. to prevent thermal runaway. Thermistors are categorized based on their conduction models. Negative-temperature-coefficient (NTC) thermistors have less resistance at higher temperatures, while positive-temperature-coefficient (PTC) thermistors have more resistance at higher temperatures. NTC thermistors are widely used as inrush current limiters and temperature sensors, while PTC thermistors are used as self-resetting overcurrent protectors and self-regulating heating elements. The operational temperature range of a thermistor is dependent on the material and is typically between −100 and 300 °C (−148 and 572 °F).
Types Depending on materials used, thermistors are classified into two types:
With NTC (negative temperature coefficient) thermistors, resistance decreases as temperature rises; usually because electrons are bumped up by thermal agitation from the valence band to the conduction band. An NTC is commonly used as a temperature sensor, or in series with a circuit as an inrush current limiter. With PTC (positive temperature coefficient) thermistors, resistance increases as temperature rises; usually because of 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. Thermistors are generally produced using powdered metal oxides. With formulas and techniques vastly improving over the past 20 years as of 2020, 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, 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. 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.
Basic operation Assuming, as a first-order approximation, that the relationship between resistance and temperature is linear, then
Δ R = k Δ T , {\displaystyle \Delta R=k\,\Delta T,}
where
Δ R {\displaystyle \Delta R} , change in resistance,
Δ T {\displaystyle \Delta T} , change in temperature,
k {\displaystyle k} , first-order temperature coefficient of resistance. Depending on type of the thermistor in question the k {\displaystyle k} may be either positive or negative. 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 PTC resistor – 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. 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
α T = 1 R ( T ) d R d T . {\displaystyle \alpha _{T}={\frac {1}{R(T)}}{\frac {dR}{dT}}.}
This α T {\displaystyle \alpha _{T}} coefficient should not be confused with the a {\displaystyle a} parameter below.
Construction and materials
Thermistors are typically built by using metal oxides. They're typically pressed into a bead, disk, or cylindrical shape and then encapsulated with an impermeable material such as epoxy or glass. NTC thermistors are manufactured from oxides of the iron group of metals: e.g. chromium (CrO, Cr2O3), manganese (e.g. MnO), cobalt (CoO), iron (iron oxides), and nickel (NiO, Ni2O3). these oxides form a ceramic body with terminals composed of conductive metals such as silver, nickel, and tin. PTC thermistors are usually prepared from barium (Ba), strontium, or lead titanates (e.g. PbTiO3). Thermistors can also be produced by resonant acoustic mixing of the previously mentioned oxides, followed by a sintering process. This effort reduces production time and can eliminate the calcination step entirely.
Steinhart–Hart equation
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