THERMISTOR

In real this is how an Thermistors looks like..

Thermistors are thermally sensitive resistors and have, according to type, a negative (NTC), or positive (PTC) resistance/temperature coefficient. Thermometrics product portfolio comprises a wide range of both types.

Thermistor symbols
temperature dependent resistor

Thermistors

What is a thermistor? A thermistor is a temperature-sensing element composed of sintered semiconductor material which exhibits a large change in resistance proportional to a small change in temperature. Thermistors usually have negative temperature coefficients which means the resistance of the thermistor decreases as the temperature increases.


Types of thermisters:

Thermistor Elements The thermistor element is the simplest form of thermistor. Because of their compact size, thermistor elements are commonly used when space is very limited. OMEGA offers a wide variety of thermistor elements which vary not only in form factor but also in their resistance versus temperature characteristics. Since thermistors are non-linear, the instrument used to read the temperature must linearize the reading.
Linear Response Thermistor Elements For applications requiring thermistors with linear response to temperature change, OMEGA offers linear components. These unique devices consist of a thermistor composite for temperature sensing and an external resistor composite for linearizing.
Thermistor Probes The standalone thermistor element is relatively fragile and can not be placed in a rugged environment. OMEGA offers thermistor probes which are thermistor elements embedded in metal tubes. Thermistor probes are much more suitable for industrial environments than thermistor elements.

Benifits of thermisters

Accuracy
Thermistors are one of the most accurate types of temperature sensors. OMEGA thermistors have an accuracy of ±0.1°C or ±0.2°C depending on the particular thermistor model. However thermistors are fairly limited in their temperature range, working only over a nominal range of 0°C to 100°C .

Stability
Finished thermistors are chemically stable and not significantly affected by aging.

Thermistor Terminology

A glossary slightly modified from that given in a US government publication: MIL-PRF-23648D. Note that the term being described is in bold typeface.

A thermistor is a thermally sensitive resistor that exhibits a change in electrical resistance with a change in its temperature. The resistance is measured by passing a small, measured direct current (dc) through it and measuring the voltage drop produced.

The standard reference temperature is the thermistor body temperature at which nominal zero-power resistance is specified, usually 25°C.

The zero-power resistance is the dc resistance value of a thermistor measured at a specified temperature with a power dissipation by the thermistor low enough that any further decrease in power will result in not more than 0.1 percent (or 1/10 of the specified measurement tolerance, whichever is smaller) change in resistance.

The resistance ratio characteristic identifies the ratio of the zero-power resistance of a thermistor measured at 25°C to that resistance measured at 125°C.

The zero-power temperature coefficient of resistance is the ratio at a specified temperature (T), of the rate of change of zero-power resistance with temperature to the zero-power resistance of the thermistor.

A NTC thermistor is one in which the zero-power resistance decreases with an increase in temperature.

A PTC thermistor is one in which the zero-power resistance increases with an increase in temperature.

The maximum operating temperature is the maximum body temperature at which the thermistor will operate for an extended period of time with acceptable stability of its characteristics. This temperature is the result of internal or external heating, or both, and should not exceed the maximum value specified.
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The maximum power rating of a thermistor is the maximum power which a thermistor will dissipate for an extended period of time with acceptable stability of its characteristics.

The dissipation constant is the ratio, (in milliwatts per degree C) at a specified ambient temperature, of a change in power dissipation in a thermistor to the resultant body temperature change.

The thermal time constant of a thermistor is the time required for a thermistor to change 63.2 percent of the total difference between its initial and final body temperature when subjected to a step function

The resistance-temperature characteristic of a thermistor is the relationship between the zero-power resistance of a thermistor and its body temperature.

The temperature-wattage characteristic of a thermistor is the relationship at a specified ambient temperature between the thermistor temperature and the applied steady state wattage.

The current-time characteristic of a thermistor is the relationship at a specified ambient temperature between the current through a thermistor and time, upon application or interruption of voltage to it.

The stability of a thermistor is the ability of a thermistor to retain specified characteristics after being subjected to designated environmental or electrical test conditions.

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