Types of Thermistors

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Types of Thermistors
Thermistors are important components in devices that measure and regulate temperature. (Jupiterimages/Creatas/Getty Images)

A thermistor is a type of resistor that is specially manufactured to decrease resistance whenever there is a rise in temperature. The term is derived from the combination of the words "thermal" and "resistor." Thermistors are small pieces of resistors, previously made from ceramic mixed with different metal oxides. Today, semiconductor materials like silicon and germanium, which are more abundant and exhibit better performance, are used. These materials are highly sensitive to temperature changes and are used in most tools and appliances for temperature sensing and correction, giving accurate and precise temperature readings within a small temperature range.

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Michael Faraday made several discoveries in the field of electromagnetic induction and electrochemistry. He is credited with the first recorded discovery of a negative temperature coefficient (NTC) thermistor in 1833, when he reported the semiconducting behaviour of silver sulphide. Because thermistors were difficult to reproduce during Faraday's time, it was 100 years before they were produced commercially when Bell Telephone Laboratories developed a technique to improve the manufacturing process during the early 1940s. The expanding aerospace industry was one of the many industries that relied on thermistors for its aircraft and maintenance equipment.

Positive Temperature Coefficient (PTC)

Positive Temperature Coefficient (PTC) thermistors exhibit electrical resistance that increases as the temperature increases. A thermistor with higher coefficient will have a greater electrical resistance as the temperature rises higher. PTCs are used in devices that limit current flow for circuit protection, acting like fuses or as timers for the degaussing coil circuit in CRT displays such as a regular computer monitor.

Negative Temperature Coefficient (NTC)

The Negative Temperature Coefficient (NTC) is the second type of thermistor. It exhibits decreasing resistance as temperatures increase. NTCs provide higher resistance to temperature when appliances are turned on and gradually decrease their resistance as the appliances continue to heat. NTCs are widely used in battery packs, like those used in cell phones, to protect the batteries while they are charging.

Uses of Thermistors

Thermistors are used to regulate temperature in various household appliances. Most electronic appliances found in the home use both PTC and NTC thermistors. Induction motors like those installed on refrigerators, washing machines and air conditioners have PTCs. Some appliances, such as washing machines, clothes dryers, refrigerators and freezers, as well as small appliances like hair dryers, curling irons, ovens, toasters, thermostats, air conditioners and fire alarms also have NTCs for temperature control. PTC thermistors are also used in automotive temperature gauges.

Thermistors are installed on these appliances to control their temperatures. This prevents them from going beyond a set temperature range. Modern appliances, communication tools and accessories like mobile phones, computers, LCD displays, CPUs, rechargeable batteries, and medical and patient monitoring equipment are all equipped with thermistors so they can be used continuously without fear of overheating and appliance damage. The aerospace, automotive and telecommunications industries (where precise temperature monitoring is crucial) are also heavy users of PTCs and NTCs.

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