PTC thermistor Principle


PTC (Positive Temperature Coefficient) refers to a sharp increase in resistance at a certain temperature, the phenomenon or materials having a positive temperature coefficient thermistor can be specifically used as a constant temperature sensor. The material is based on BaTiO3 or SrTiO3 or PbTiO3 as the main component of the sintered body, wherein the incorporation of a trace amount of Nb, Ta, Bi, Sb, Y, La and other oxides the valence control leaving half of conductivity, normally this The semiconductor of BaTiO3 and other materials referred to as the semiconductive porcelain (body); also added to increase its positive temperature coefficient of resistance, Mn, Fe, Cu, Cr oxide of additives and from other effects, using the general ceramic process forming The high temperature sintering the such as leaving titanate platinum semiconducting its solid solution, to thereby obtain the positive characteristics thermistor material. Its temperature coefficient and Curie point temperature varies with the composition and sintering conditions (especially cooling temperature).

Barium titanate crystals belong to the perovskite-type structure, which is a ferroelectric material, pure barium titanate is an insulating material. Adding small amounts of rare earth elements, in the barium titanate material, after appropriate heat treatment, in the vicinity of the Curie temperature, the resistivity increased sharply by several orders of magnitude to produce the PTC effect, this effect and BaTiO3 crystal ferroelectric properties in the vicinity of the Curie temperature related to the phase change of the material. The barium titanate semiconductive ceramic is a polycrystalline material, the interface between the grains exist between the grains. The semiconductive porcelain when it reaches a certain temperature or voltage, the crystal grain boundary changes, so that the resistance sharply changes.

Barium titanate semiconductor ceramic PTC effect due to the grain boundary (grain boundaries). The interface is equivalent to a potential barrier for the conduction electron, intergranular. The temperature is low, due to the effect of electric field within the barium titanate, resulting in electrons very easily across the barrier, then the resistance value is small. When the temperature is raised to the Curie point temperature (i.e., the critical temperature) near the inner electric field is damaged, it does not help the conduction electrons across the barrier. This corresponds to the barrier rises, the resistance value increases suddenly, to generate the PTC effect. The physical model of the PTC effect of barium titanate semiconductor ceramic the sea looking surface barrier model, Daniels et al the barium absence of models and superimposed barrier model, they were from different aspects of the PTC effect made a reasonable explanation.

PTC thermistor appeared in 1950, and then 1954 to barium titanate as a main material of the PTC thermistor. PTC thermistor in the industry can be used as a measurement and control of temperature, temperature detection and regulation is also used in certain parts of the car, but also a large number for civilian equipment, such as water temperature control instant water boiler, air-conditioner and the temperature of the cold storage itself heating for gas analysis and wind machine.

PTC thermistor In addition to its use as a heating element, but also play the role of the “switch”, both sensitive element, heater and switch three functions, called "thermal switch. Current caused by the components after the temperature rise, i.e. the increase in the temperature of the heating element, when the temperature exceeds the Curie point, the resistance is increased, thus limiting the current increases, then the current causes lower element temperature decreases, the resistance value decreases so that the circuit current increases, the element temperature rises again and again, so it has to keep the temperature in a specific range of function, but also play the switching action. Made of such a resistance temperature characteristic heating source applications as the heating element, air heater, electric iron, drying closet, air conditioning, etc., but also electrical played overheating protective effect.