Editing Thermistor (section)

===PTC (positive temperature coefficient)===
Most PTC thermistors are made from doped polycrystalline [[ceramic]] (containing [[barium titanate]] (BaTiO<sub>3</sub>) 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.<ref>[http://www.resistorguide.com/ptc-thermistor/ "PTC Thermistors and Silistors"] The Resistor Guide</ref>  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.<ref>{{Cite web|url=https://www.ei-sensor.com/what-is-a-thermistor/|title = What is a Thermistor? How do thermistors work?}}</ref>

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 [[Cathode-ray_tube|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 [[electrical conductor|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 BaTiO<sub>3</sub> 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 [[Flip-flop (electronics)|latch/memory circuit]], the effect being enhanced by using two PTC thermistors in series, with one thermistor cool, and the other thermistor hot.<ref>Downie, Neil A., ''The Ultimate Book of Saturday Science'' (Princeton 2012) {{ISBN|0-691-14966-6}}</ref>

The [[International Electrotechnical Commission|IEC]] standard symbol for a PTC thermistor includes a "+t°" under the rectangle.<ref>
{{cite web
 | title= PTC thermistor – Positive Temperature Coefficient
 | website= Resistor Guide
 | url= http://www.resistorguide.com/ptc-thermistor/
}}</ref>