Quick Understanding of Thermistors in Electronic Component Knowledge Collection

Thermistors are sensitive elements, which are divided into positive temperature coefficient (PTC) thermistors according to the temperature coefficient. Thermistors have Negative temperature coefficient (NTC). The common characteristic of thermistors is their sensitivity to temperature, exhibiting different resistance values at different temperatures. Positive temperature coefficient (PTC) of thermistors When the temperature is high, the resistance value is larger; when the temperature is high, the resistance value of Negative temperature coefficient thermistors (NTC) is smaller. They belong to semiconductor devices.

Thermistor characteristics: ① Highly sensitive, its resistance temperature coefficient will be about 10-100 times larger than that of metals; ② The working temperature range is wide, with room temperature devices suitable for -55 ℃ to high temperature devices suitable for 315 ℃ (currently up to 2000 ℃), and low temperature devices suitable for -273 ℃ to 55 ℃; ③ Small in size, it can measure the temperature of gaps, cavities, and blood vessels in organisms that cannot be detected by other temperature gauges; ④ Convenient to use, the resistance value can be freely selected from 0.1 to 100k Ω; ⑤ Easy to make into complex forms, capable of mass production; ⑥ Stable performance and strong overload capacity.

Thermistor type: positive temperature coefficient (PTC) and Negative temperature coefficient (NTC). As the temperature increases, the resistance of PTC thermistors increases, while the resistance of NTC thermistors decreases. NTC thermistors are currently the most widely used.

PTC thermistor

PTC (PosiTIve Temperature Coeff1Cient) refers to a material in a thermistor state or with a positive temperature coefficient, which rapidly increases in resistance at a certain temperature and can be specifically used as a stable temperature sensor. Calcinated materials mainly composed of BaTIO3, SrTIO3, or PbTIO3, with trace amounts of Nb, Ta, Bi, Sb, Y, La and other oxides added for atomic valence manipulation to make them semi conductive. This semiconductor based BatiO3 and other materials are commonly referred to as semiconductor (bulk) ceramics; In addition, Mn is added to increase its positive resistance temperature coefficient, oxides of Fe, Cu, Cr and other effective additives. Thermistor materials are formed by general ceramic process, sintered at high temperature, semiconducting Solid solution such as platinum titanate, and obtain positive characteristics. Its temperature coefficient and Curie temperature point temperature change with composition and calcination standard (especially refrigeration temperature).

Barium titanate crystal belongs to perovskite structure and is a ferroelectric material. Pure Barium titanate is an insulating material. Adding a small amount of Trace element into Barium titanate materials, after proper heat treatment, the resistance will increase by several orders of magnitude around the indoor temperature, resulting in PTC effect, which is related to the Ferroelectricity of BatiO3 crystals and the changes in materials around the indoor temperature. Barium titanate semiconducting ceramics is a kind of polycrystalline material. When the semiconducting porcelain reaches a specific temperature or voltage, the Grain boundary changes and the resistance changes sharply.

Due to the grain boundary (crystal boundary), the PTC effect of Barium titanate semiconducting ceramics. The pages between crystals act as potential barriers for conducting electrons. When the temperature is low, due to the effect of the electric field in Barium titanate, electrons are easy to cross the potential barrier, and the resistance is small. When the temperature rises to the temperature near the Curie temperature (i.e. the critical pressure), the internal electric field is damaged and cannot help the conductive electrons cross the barrier. This is equivalent to an increase in potential barrier and a sudden increase in resistance, resulting in PTC effect. The PTC effect conceptual models of Barium titanate semiconducting ceramics include the Haiwang surface barrier model, Daniels barium deficiency model and cumulative barrier model, which mainly explain the PTC effect reasonably from different aspects.

PTC thermistors occurred in 1950, and then in 1954, PTC thermistors with Barium titanate as the common material appeared. PTC thermistors can be used for industrial temperature measurement and control, as well as for temperature measurement and adjustment at a certain position of the vehicle, and are also widely used for civil equipment, such as water temperature, air conditioning and cold storage temperature. They use their own heating for gas analysis and Wind turbine.

PTC thermistors can not only be used as heating elements, but also serve as "switches". It has three functions: sensitive components, heaters, and switches, known as "thermal switches". After the current passes through the element, the temperature will rise, that is, the heater temperature will rise. When the temperature exceeds the Curie temperature, the resistance will rise, which will limit the current rise. Therefore, the decrease of the current will lead to the decrease of the element temperature, and the decrease of the resistance value will increase the circuit current, the increase of the element temperature, and the cycle. Therefore, it has the function of maintaining temperature within a specific range and serves as a switch. The heating source adopts this temperature resistance characteristic as a heating element, using heaters, electrochromic iron, drying wardrobes, air conditioning, etc., which can also play a role in protecting electrical appliances from overheating.

NTC thermistor

NTC (Negative Temperature Coeff1Cient) refers to the state and material of thermistor, which decreases exponentially with increasing temperature and has a Negative temperature coefficient. The material is a semiconductor ceramic, which is made of two or more kinds of hydroxides, such as manganese, copper, silicon, cobalt, iron, nickel, zinc, etc., by fully mixing, molding, and calcining, and can be made into Negative temperature coefficient (NTC) thermistors. According to the proportion of material composition, calcination atmosphere, sintering temperature, and structural conditions, the resistance and material constant change. Non oxides represented by silicon carbide, tin selenide, tantalum nitride, etc. are NTC thermistor materials.

The development of NTC thermistors has gone through a long period. In 1834, scientists found for the first time that Silver sulfide had the characteristic of Negative temperature coefficient. In 1930, scientists found that Copper(I) oxide Copper(II) oxide also had the performance of Negative temperature coefficient, and it was successfully applied in the temperature compensation circuit of aviation equipment. Subsequently, due to the continuous progress of transistor technology, significant breakthroughs were made in the exploration of thermistors. NTC thermistors were developed in 1960 and are widely used in temperature measurement, temperature control, temperature compensation, and other fields.

The detection range is generally -10~300 ℃ or -2000 ℃~10 ℃.

The accuracy of the thermal Resistance thermometer can reach 0.1 ℃, and the temperature sensing time can be less than 10s. It is not only suitable for grain depot thermometer, but also suitable for temperature detection in food storage, health care, scientific farming, ocean, deep wells, high places, glaciers, etc.

Due to the unique characteristics of semiconductor thermistors, they can be used not only as measuring elements (such as temperature, flow, liquid level, etc.), but also as control elements (such as thermal switches, current limiters) and circuit compensation elements. Thermistors are widely used in household appliances, power industry, communication, military science, aerospace and other fields, and have broad development prospects.

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