What is PTC?

2020-11-03 19:36:05 hongling

PTC has been widely used in various fields of the electronics industry, so what is PTC? How does it work?

PTC is a kind of semiconductor heating ceramics. When the outside temperature decreases, the resistance value of PTC decreases, and the heat generation increases accordingly.

How PTC works

PTC thermistor (positive temperature coefficient thermistor) is a temperature-sensitive semiconductor resistor. Once it exceeds a certain temperature (Curie temperature), its resistance value is almost a step as the temperature rises. The increase in the formula. The temperature change of the PTC thermistor body can be obtained by the current flowing through the PTC thermistor, or by the external heat input or the superposition of the two. Ceramic materials are usually used as high-resistance excellent insulators , And the ceramic PTC thermistor is based on barium titanate, doped with other polycrystalline ceramic materials, and has lower resistance and semiconducting characteristics. Through purposeful doping of a material with higher chemical valence It is achieved as a lattice element of the crystal: a part of barium ions or titanate ions in the crystal lattice is replaced by higher valence ions, thus obtaining a certain amount of free electrons that generate conductivity.  

For the PTC thermistor effect, that is, the reason for the step increase in the resistance value, the material structure is composed of many small crystallites, and the potential is formed on the grain boundary, the so-called grain boundary (grain boundary). The barrier prevents electrons from crossing the boundary into the adjacent area, thus generating high resistance. This effect is cancelled at low temperatures: the high dielectric constant and spontaneous polarization at the grain boundary hinder it at low temperatures The formation of a potential barrier allows electrons to flow freely. This effect greatly reduces the dielectric constant and polarization at high temperatures, resulting in a substantial increase in the potential barrier and resistance, showing a strong PTC effect.

PTC thermistors are sensitive components that have been developed early, with many types and more mature development. PTC thermistors are composed of semiconductor ceramic materials, and the principle used is that temperature causes resistance to change. If the concentration of electrons and holes are n and p, and the mobility is μn and μp, the conductivity of the semiconductor is: σ=q (nμn+pμp) because n, p, μn, and μp are all temperature-dependent functions , So the conductance is a function of temperature, so the temperature can be calculated by measuring the conductance, and the resistance-temperature characteristic curve can be made. This is the working principle of semiconductor thermistor.

Thermistors include positive temperature coefficient (PTC) and negative temperature coefficient (NTC) thermistors, and critical temperature thermistors (CTR). Their resistance-temperature characteristics are shown in Figure 1. The main features of PTC thermistors are: ①High sensitivity, its resistance temperature coefficient is more than 10-100 times larger than that of metal, and can detect temperature changes of 10-6℃; ②Wide operating temperature range, normal temperature devices are suitable for- 55℃~315℃, the applicable temperature of high temperature device is higher than 315℃ (currently the highest can reach 2000℃), low temperature device is suitable for -273℃~55℃; ③Small size, can measure the gap, cavity and other thermometers can not measure The temperature of blood vessels in living organisms; ④Easy to use, resistance value can be arbitrarily selected between 0.1~100kΩ; ⑤Easy to process into complex shapes, can be mass produced; ⑥Good stability and strong overload capacity.

PTC thermistor

PTC (Positive Temperature Coeff1Cient) refers to the thermistor phenomenon or material that has a positive temperature coefficient and a sharp increase in resistance at a certain temperature. It can be used 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 Nb, Ta, Bi, Sb, Y, La and other oxides for atomic valence control to make it semiconducting. Semiconductorized BaTiO3 and other materials are referred to as semiconducting (bulk) porcelain for short; at the same time, oxides of Mn, Fe, Cu, Cr and other additives that increase the positive temperature coefficient of resistance are added, and they are formed by general ceramic technology. High temperature sintering makes platinum titanate and its solid solution semiconducting, thereby obtaining positive characteristic PTC thermistor materials. The temperature coefficient and Curie point temperature vary with the composition and sintering conditions (especially the cooling temperature).

The PTC thermistor appeared in 1950, and then in 1954, a PTC thermistor with barium titanate as the main material appeared. PTC thermistors can be used for temperature measurement and control in industry, as well as for temperature detection and adjustment of certain parts of automobiles. They are also widely used in civil equipment, such as controlling the water temperature of instantaneous boilers, air conditioners and cold storage, Use its own heating for gas analysis and wind speed machine. The following is a brief introduction to the application of heating and overheating protection to heaters, motors, transformers, high-power transistors and other electrical appliances.

In addition to being used as a heating element, PTC thermistor can also function as a "switch". It has three functions: sensitive element, heater and switch, called "thermal switch", as shown in Figures 2 and 3. Show. After the current passes through the element, the temperature rises, that is, the temperature of the heating element rises. When the Curie point temperature is exceeded, the resistance increases, thereby limiting the current increase, so the decrease of the current causes the temperature of the element to decrease, and the decrease of the resistance value causes the circuit current Increase, the temperature of the element rises and repeats itself, so it has the function of keeping the temperature in a specific range, and it also plays the role of a switch. Use this resistance to temperature characteristics to make a heating source, as heating elements, such as heaters, electric irons, drying wardrobes, air conditioners, etc., can also play a role in overheating protection for electrical appliances.