PTC thermistor: PTC generally refers to semiconductor materials or components with high temperature coefficient. PTC thermistor (positive temperature coefficient thermistor, namely self recovery fuse) is a kind of semiconductor resistance with temperature sensitivity. Once it exceeds a certain temperature (Curie temperature), its resistance value almost increases step by step with the increase of temperature.

When it is used, it is connected in series in the circuit. Under normal circumstances, its resistance value is very small, and the loss is also very small, which does not affect the normal operation of the circuit. However, if there is over-current (such as short circuit), its temperature rises, and its resistance value rises sharply, which limits the passage of large current, so as to play the role of over-current protection and avoid damaging the components in the circuit. When the fault is removed, the temperature will drop automatically and return to the low resistance state, so it is also called self recovery fuse.

Thermal switch: thermal switch is a device to realize current on-off by using the different thermal expansion coefficient of each component layer of bimetallic sheet. When the temperature changes, the deformation of the active layer is greater than that of the passive layer, so that the whole bimetallic sheet will bend to the side of the passive layer, and the curvature of the composite material will change, resulting in deformation.

Under the influence of temperature, the two pieces of metal will have different deformation. The result of this deformation is that the whole will bend towards one side. The influence of this curvature and deformation controls the closing or breaking of the circuit, so it has the effect of temperature control switch.