Plug-in resettable fuse (PPTC: polymer resettable fuse) is a polymer thermistor with positive temperature coefficient, which is used for overcurrent protection and can replace the current fuse. When the circuit is working normally, its resistance value is very small (the voltage drop is very small). When the circuit is over-current and its temperature rises, the resistance value increases sharply by several orders of magnitude, so that the current in the circuit is reduced below the safe value. In this way, the following circuits are protected, and the low resistance value is automatically restored after the overcurrent disappears. Its effect is similar to that of a switching element, but with a slower response.