A quick understanding of thermistors

　　Thermistor is a type of sensitive element, which is divided into positive temperature coefficient thermistor (PTC) and negative temperature coefficient thermistor (NTC) according to different temperature coefficients. A typical characteristic of thermistors is that they are sensitive to temperature and exhibit different resistance values at different temperatures. The positive temperature coefficient thermistor (PTC) has a larger resistance value when the temperature is higher, and the negative temperature coefficient thermistor (NTC) has a lower resistance value when the temperature is higher. They are both semiconductor devices.

　　Thermistor features: ①High sensitivity, its temperature coefficient of resistance is more than 10-100 times larger than that of metal; ②Wide operating temperature range, room temperature devices are suitable for -55℃～315℃, and high temperature devices are suitable for temperatures higher than 315℃ ( At present, the highest temperature can reach 2000 ℃) The low temperature device is suitable for -273 ℃ ~ 55 ℃; ③ Small size, can measure the temperature of voids, cavities and blood vessels in the body that cannot be measured by other thermometers; ④ Easy to use, the resistance value can be in the range of 0.1～ 100kΩ can be arbitrarily selected; ⑤It is easy to process into complex shapes and can be mass-produced; ⑥Good stability and strong overload capacity.

　　Types of thermistors: Positive Temperature Coefficient (PTC) and Negative Temperature Coefficient (NTC). The resistance of the PTC thermistor increases as the temperature increases, and the resistance of the NTC thermistor decreases as the temperature increases. Currently the most widely used is the NTC thermistor.

　　PTC (PosiTIve Temperature Coeff1Cient) refers to a thermistor phenomenon or material with a positive temperature coefficient that increases sharply in resistance at a certain temperature, and can be used exclusively as a constant temperature sensor. The material is a sintered body with BaTIO3 or SrTIO3 or PbTIO3 as the main component, which is doped with a small amount of oxides such as Nb, Ta, Bi, Sb, Y, La and other oxides to control the atomic valence to make it semiconducting. The semiconducting BaTiO3 and other materials are referred to as semiconducting (bulk) porcelain; at the same time, oxides of Mn, Fe, Cu, Cr and other additives that increase the positive temperature coefficient of resistance are also added. High temperature sintering makes platinum titanate and its solid solution semiconducting, so as to obtain a thermistor material with positive characteristics. Its temperature coefficient and Curie point temperature vary with composition and sintering conditions (especially cooling temperature).

　　Barium titanate crystal belongs to perovskite type structure, which is a ferroelectric material, and pure barium titanate is an insulating material. After adding trace rare earth elements to the barium titanate material, after proper heat treatment, the resistivity increases sharply by several orders of magnitude near the Curie temperature, resulting in the PTC effect, which is related to the ferroelectricity of BaTiO3 crystal and its proximity to the Curie temperature. phase transition of the material. Barium titanate semiconducting ceramic is a polycrystalline material, and there is an inter-grain interface between the grains. When the semiconducting ceramic reaches a certain temperature or voltage, the crystal grain boundary changes, and the resistance changes sharply.

　　The PTC effect of barium titanate semiconducting ceramics originates from grain boundaries (grain boundaries). For conducting electrons, the inter-grain interface acts as a potential barrier. When the temperature is low, due to the action of the electric field in the barium titanate, the electrons are very easy to cross the potential barrier, and the resistance value is small. When the temperature rises near the Curie point temperature (i.e. critical temperature), the internal electric field is destroyed and it cannot help the conducting electrons to cross the potential barrier. This is equivalent to raising the potential barrier and suddenly increasing the resistance value, resulting in the PTC effect. The physical models of the PTC effect of barium titanate semiconducting ceramics include the Haiwang surface barrier model, the barium vacancy model and the superposition barrier model of Daniels et al., which provide reasonable explanations for the PTC effect from different aspects.

　　PTC thermistors appeared in 1950, and then PTC thermistors with barium titanate as the main material appeared in 1954. PTC thermistors can be used for temperature measurement and control in industry, and also used for temperature detection and adjustment of certain parts of automobiles. The use of its own heating for gas analysis and fan speed machine and so on.

　　In addition to being used as a heating element, the PTC thermistor can also play the role of a "switch", which has three functions of a sensitive element, a heater and a switch, which is called a "thermal switch". After the current passes through the element, the temperature rises, that is, the temperature of the heating element rises. When the temperature exceeds the Curie point, the resistance increases, thereby limiting the increase of the current. Therefore, the decrease of the current causes the temperature of the element to decrease, and the decrease of the resistance value makes the circuit current. Increase, the temperature of the component rises, and it repeats itself, so it has the function of keeping the temperature in a specific range, and it also acts as a switch. Using this temperature resistance characteristic to make a heating source, as a heating element, there are air heaters, electric soldering irons, drying cabinets, air conditioners, etc., and it can also protect electrical appliances from overheating.

Read recommendations:

NTC temperature sensor for electric heating floor

NTC thermistor in smart bracelet temperature module

2359 PIR LENS

PIR Lens 2134-1

TP-L80