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       Polymer PTC thermistor for overcurrent protection 　 　

       1. PTC effect 　　

       A material has a PTC (Positive Temperature Coefficient) effect, which means 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. Nonlinear PTC effect　 　

       The material undergoing phase change will exhibit a phenomenon of a sharp increase in resistance of several to more than ten orders of magnitude along a narrow temperature range, known as the nonlinear PTC effect, as shown in Figure 1. Various types of conductive polymers exhibit this effect, such as polymer PTC thermistors. These conductive polymers are very useful for manufacturing overcurrent protection devices. 　 　

       3. Polymer PTC thermistors for overcurrent protection　 　

       Polymer PTC thermistors, also known as self-healing fuses (hereinafter referred to as thermistors), are highly suitable as overcurrent protection devices due to their unique positive temperature coefficient resistance characteristics (i.e. PTC characteristics, as shown in Figure 1). 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 switching temperature (Ts, see Figure 1), 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), thus providing 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 ambient temperature on polymer PTC thermistors 　　

       Polymer PTC thermistor is a direct heating, step type thermistor, and its resistance change process is related to its own heating and heat dissipation situation. Therefore, its holding current (Ihold), operating current (Itrip), and operating time are affected by environmental temperature. Figure 4 is a schematic diagram of the relationship between the typical holding current, operating current, and ambient temperature of a thermistor. 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 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. Figure 5 is a schematic diagram of the relationship between the operating time of the thermistor and the current and ambient temperature. 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.　　

       Recovery characteristics of polymer PTC thermistors after action 　 　

       Polymer PTC thermistors can be reused multiple times due to their 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. Generally speaking, thermistors with smaller area and thickness recover relatively quickly; However, the recovery of thermistors with larger area and thickness is relatively slow.