An operational temperature range of a thermistor is dependent on the probe type and is typically between −100 and 300 °C (−148 and 572 °F).

], 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.

[5] Assuming, as a first-order approximation, that the relationship between resistance and temperature is linear, then where Depending on type of the thermistor in question the

is negative, the resistance decreases with increasing temperature, and the device is called a negative-temperature-coefficient (NTC) thermistor.

[7] They're typically pressed into a bead, disk, or cylindrical shape and then encapsulated with an impermeable material such as epoxy or glass.

[9][10] these oxides form a ceramic body with terminals composed of conductive metals such as silver, nickel, and tin.

[9][10] In practical devices, the linear approximation model (above) is accurate only over a limited temperature range.

Over wider temperature ranges, a more complex resistance–temperature transfer function provides a more faithful characterization of the performance.

In practice, the equation gives good numerical results for resistances expressed in ohms or kΩ, but the coefficients a, b, and c must be stated with reference to the unit.

can be solved, the real root of which is given by where The error in the Steinhart–Hart equation is generally less than 0.02 °C in the measurement of temperature over a 200 °C range.

Many NTC thermistors are made from a pressed disc, rod, plate, bead or cast chip of semiconducting material such as sintered metal oxides.

They work because raising the temperature of a semiconductor increases the number of active charge carriers by promoting them into the conduction band.

In certain materials like ferric oxide (Fe2O3) with titanium (Ti) doping an n-type semiconductor is formed and the charge carriers are electrons.

In materials such as nickel oxide (NiO) with lithium (Li) doping a p-type semiconductor is created, where holes are the charge carriers.

[16] 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.

Below the Curie point temperature, the high dielectric constant prevents the formation of potential barriers between the crystal grains, leading to a low resistance.

They are stable devices which are hermetically sealed in an axial leaded glass encapsulated package.

The dynamics of PTC thermistors being powered lends to a wide range of applications.

When the plastic is cool, the carbon grains are all in contact with each other, forming a conductive path through the device.

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.

If the thermistor is being used to measure the temperature of the environment, this electrical heating may introduce a significant error (an observer effect) if a correction is not made.

It can, for example, make a sensitive air-flow device employed in a sailplane rate-of-climb instrument, the electronic variometer, or serve as a timer for a relay as was formerly done in telephone exchanges.

The rate of transfer is well described by Newton's law of cooling: where T(R) is the temperature of the thermistor as a function of its resistance R,

is the temperature of the surroundings, and K is the dissipation constant, usually expressed in units of milliwatts per degree Celsius.

At equilibrium, the two rates must be equal: The current and voltage across the thermistor depend on the particular circuit configuration.

The power dissipated in a thermistor is typically maintained at a very low level to ensure insignificant temperature measurement error due to self-heating.

[6] 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.