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Thermistors

THERMal resISTORS

Thermistors are special solid temperature sensors that behave like temperature-sensitive electrical resistors. No surprise then that their name is a contraction of "thermal" and "resistor". There are basically two broad types, NTC-Negative Temperature Coefficient, used mostly in temperature sensing and PTC-Positive Temperature Coefficient, used mostly in electric current control.

There's even more history of the name and development of thermistors and facts about some key NTC parameters at the Kele Electronics website, just be prepared for some strong opinions about one brand of thermistor.

They are mostly very small bits of special material that exhibit more than just temperature sensitivity. They are highly-sensitive and have very reproducible resistance Vs. temperature properties.

During the last 60 years or so, only ceramic materials (a mix of different metal oxides) was employed for production of NTC thermistors. In 2003, AdSem, Inc. (Palo Alto, CA) developed and started manufacturing of Si and Ge high temperature NTC thermistors with better performance than any ceramic NTC thermistors.

Thermistors, since they can be very small, are used inside many other devices as temperature sensing and correction devices as well as in specialty temperature sensing probes for commerce, science and industry.

Some of those new-fangled digital medical thermometers that get stuck in one's mouth by a nurse with an electronic display in her other hand are based on thermistor sensors. They are probably inside your cell phone, automobile, stereo and television, too, but you'd never know it unless you were an engineer or visited here.

Thermistors typically work over a relatively small temperature range, compared to other temperature sensors, and can be very accurate and precise within that range, although not all are.

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.

Thanks for visiting; there's more info at the links highlighted below.

Thermisor Temperature Sensor Resources

  1. Thermistor Types, including applications information and downloadable programs for resistance vs temperature
  2. Standards, Calibration, Traceability
  3. Thermistor-based Temperature Sensor Probes
  4. Thermistor Applications
  5. Thermistor Vendors
  6. Silicon Thermistors

Also note The Temperature Sensors Community Web Site, a companion site set up to enable direct inputs from temperature sensor users and makers. Vendors, do visit there. Sign in and enter your own company data, product and service offerings and news-It's been Improved! Users can post reports & reviews of companies, news, products and services. Note that inputs are moderated for propriety and excess zeal! Best of all, it is freely available and "self service".

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A unique Web Resource that introduces new web page links randomly.

GEC Instruments


AdSem Thermistors





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