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Type of resistor whose resistance varies with temperature

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. 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. 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).

Most PTC thermistors are made from doped polycrystalline ceramic (containing barium titanate (BaTiO3) and other compounds) which have the property that their resistance rises suddenly at a certain critical temperature. Barium titanate is ferroelectric and its dielectric constant varies with temperature. Below the Curie point temperature, the high dielectric constant prevents the formation of potential barriers between the crystal grains, leading to a low resistance. In this region the device has a small negative temperature coefficient. At the Curie point temperature, the dielectric constant drops sufficiently to allow the formation of potential barriers at the grain boundaries, and the resistance increases sharply with temperature. At even higher temperatures, the material reverts to NTC behaviour. Another type of thermistor is a silistor (a thermally sensitive silicon resistor). Silistors employ silicon as the semiconductive component material. Unlike ceramic PTC thermistors, silistors have an almost linear resistance temperature characteristic. Silicon PTC thermistors have a much smaller drift than an NTC thermistor. They are stable devices which are hermetically sealed in an axial leaded glass encapsulated package. Barium titanate thermistors can be used as self controlled heaters; for a given voltage, the ceramic will heat to a certain temperature, but the power used will depend on the heat loss from the ceramic. The dynamics of PTC thermistors being powered lends to a wide range of applications. When first connected to a voltage source, a large current corresponding to the low, cold, resistance flows, but as the thermistor self heats, the current is reduced until a limiting current (and corresponding peak device temperature) is reached. The current limiting effect can replace fuses. In the degaussing circuits of many CRT monitors and televisions an appropriately chosen thermistor is connected in series with the degaussing coil. This results in a smooth current decrease for an improved degaussing effect. Some of these degaussing circuits have auxiliary heating elements to heat the thermistor (and reduce the resulting current) further. Another type of PTC thermistor is the polymer PTC, which is sold under brand names such as "Polyswitch" "Semifuse", and "Multifuse". This consists of plastic with carbon grains embedded in it. When the plastic is cool, the carbon grains are all in contact with each other, forming a conductive path through the device. When the plastic heats up, it expands, forcing the carbon grains apart, and causing the resistance of the device to rise, which then causes increased heating and rapid resistance increase. Like the BaTiO3 thermistor, this device has a highly nonlinear resistance/temperature response useful for thermal or circuit control, not for temperature measurement. Besides circuit elements used to limit current, self limiting heaters can be made in the form of wires or strips, useful for heat tracing. PTC thermistors "latch" into a hot / high resistance state: once hot, they stay in that high resistance state, until cooled. The effect can be used as a primitive latch/memory circuit, the effect being enhanced by using two PTC thermistors in series, with one thermistor cool, and the other thermistor hot. The IEC standard symbol for a PTC thermistor includes a "+t°" under the rectangle.

As a thermometer for low temperature measurements of the order of 10 K.
As an inrush current limiter device in power supply circuits, they present a higher resistance initially, which prevents large currents from flowing at turn on, and then heat up and become much lower resistance to allow higher current flow during normal operation. These thermistors are usually much larger than measuring type thermistors, and are purposely designed for this application. As sensors in automotive applications to monitor fluid temperatures like the engine coolant, cabin air, external air or engine oil temperature, and feed the relative readings to control units like the ECU and to the dashboard. To monitor the temperature of an incubator. Thermistors are also commonly used in modern digital thermostats and to monitor the temperature of battery packs while charging. Thermistors are often used in the hot ends of 3D printers; they monitor the heat produced and allow the printer's control circuitry to keep a constant temperature for melting the plastic filament. In the food handling and processing industry, especially for food storage systems and food preparation. Maintaining the correct temperature is critical to prevent foodborne illness. Throughout the consumer appliance industry for measuring temperature. Toasters, coffee makers, refrigerators, freezers, hair dryers, etc. all rely on thermistors for proper temperature control. NTC thermistors come in bare and lugged forms, the former is for point sensing to achieve high accuracy for specific points, such as laser diode die, etc. For measurement of temperature profile inside the sealed cavity of a convective (thermal) inertial sensor. Thermistor Probe Assemblies offer protection of the sensor in harsh environments. The thermistor sensing element can be packaged into a variety of enclosures for use in industries such as HVAC/R, Building Automation, Pool/Spa, Energy and Industrial Electronics. Enclosures can be made out of stainless steel, aluminum, copper brass or plastic and configurations include threaded (NPT, etc. ), flanged (with mounting holes for ease of installation) and straight (flat tip, pointed tip, radius tip, etc.). Thermistor probe assemblies are very rugged and are highly customizable to fit the application needs. Probe assemblies have gained in popularity over the years as improvements in research, engineering and manufacturing techniques have been made. UL Recognized NTC thermistors in the XGPU2 category helps save equipment manufacturers time and money when applying for safety approvals for their end product. DO 35 hermetically sealed glass encapsulated thermistors can operate up to 250 °C which gives them an advantage in many applications when UL is requested for a sensing element.

The first NTC thermistor was discovered in 1833 by Michael Faraday, who reported on the semiconducting behavior of silver sulfide. Faraday noticed that the resistance of silver sulfide decreased dramatically as temperature increased. (This was also the first documented observation of a semiconducting material.) Because early thermistors were difficult to produce and applications for the technology were limited, commercial production of thermistors did not begin until the 1930s. A commercially viable thermistor was invented by Samuel Ruben in 1930.