The main characteristics of thermistors are:

① High sensitivity, its resistance temperature coefficient is more than 10-100 times larger than that of metals, and can detect temperature changes of 10-6 ℃;

② The working temperature range is wide, suitable for normal temperature devices ranging from -55 ℃ to 315 ℃, high temperature devices suitable for temperatures higher than 315 ℃ (currently up to 2000 ℃), and low temperature devices suitable for temperatures ranging from -273 ℃ to -55 ℃;

③ Small in size, able to measure the temperature of gaps, cavities, and blood vessels in organisms that cannot be measured by other thermometers;

④ Convenient to use, the resistance value can be freely selected between 0.1 and 100k Ω;

⑤ Easy to process into complex shapes, capable of mass production;

⑥ Good stability and strong overload capacity

The thermistor will remain inactive for a long time; When the ambient temperature and current are in the c-zone, the heat dissipation power of the thermistor is close to the heating power, so it may or may not act. When the ambient temperature is the same, the operating time of the thermistor sharply shortens with the increase of current; Thermistors have shorter operating time, smaller maintenance current, and operating current when the ambient temperature is relatively high.

1. The PTC effect is a material with a positive temperature coefficient effect, which refers to the fact that the resistance of the material increases with increasing temperature. Most metal materials exhibit the PTC effect. In these materials, the PTC effect is manifested as a linear increase in resistance with increasing temperature, which is commonly known as the linear PTC effect.

2. The nonlinear PTC effect is a phenomenon in which the resistance of materials undergoing phase change increases sharply along a narrow temperature range by several to more than ten orders of magnitude, known as the nonlinear PTC effect. Various types of conductive polymers, such as polymer PTC thermistors, exhibit this effect. These conductive polymers are very useful for manufacturing overcurrent protection devices.

3. Polymer PTC thermistors are often referred to as self-healing fuses (hereinafter referred to as thermistors) for overcurrent protection. Due to their unique positive temperature coefficient resistance characteristics, they are extremely suitable as overcurrent protection devices. The use of thermistors is similar to that of ordinary fuses, which are connected in series in a circuit.

When the circuit is working normally, the temperature of the thermistor is close to room temperature and the resistance is very small. Series connection in the circuit will not hinder the passage of current; When overcurrent occurs in the circuit due to a fault, the thermistor increases in temperature due to an increase in heating power. When the temperature exceeds the switch temperature, the resistance will instantly increase, and the current in the circuit will quickly decrease to a safe value. This is a schematic diagram of the change in current during the protection process of the AC circuit by the thermistor. After the action of the thermistor, the current in the circuit has significantly decreased, and t in the figure is the action time of the thermistor. Due to the good designability of polymer PTC thermistors, their sensitivity to temperature can be adjusted by changing their switching temperature (ts). Therefore, they can provide both over temperature protection and over current protection. For example, the kt16-1700dl specification thermistor is suitable for over current and over temperature protection in lithium-ion batteries and nickel hydrogen batteries due to its low operating temperature. The influence of environmental temperature on polymer PTC thermistors. Polymer PTC thermistors are a type of direct heating, step type thermistor, and their resistance change process is related to their own heating and heat dissipation. Therefore, their holding current (ihold), operating current (itrip), and operating time are affected by environmental temperature. When the ambient temperature and current are in zone a, the thermistor will act when the heating power is greater than the heat dissipation power; When the ambient temperature and current are in zone b, the heating power is less than the heat dissipation power, and the polymer PTC thermistor can be reused multiple times due to its recoverable resistance. Figure 6 shows a schematic diagram of the resistance changing over time during the recovery process after the thermistor is activated. The resistance can generally recover to a level of about 1.6 times the initial value within a few seconds to several tens of seconds. At this point, the maintenance current of the thermistor has been restored to its rated value and can be used again. Thermistors with smaller area and thickness recover relatively quickly; However, the recovery of thermistors with larger area and thickness is relatively slow