Resettable fuse is composed of specially treated polymer resin (Polymer) and conductive particles (Carbon Black) distributed in it.
Under normal operation, the polymer resin tightly binds the conductive particles outside the crystalline structure, forming a chain-like conductive electrical path. The resettable fuse at this time is in the low resistance state (a), and the heat generated by the current flowing through the resettable fuse on the line is small, and the crystal structure will not be changed.
When the circuit is short-circuited or overloaded, the large current flowing through the Resettable fuse will cause the polymer resin to melt, and the volume will increase rapidly, forming a high-resistance state (b). The operating current is rapidly reduced, thereby limiting and protecting the circuit.
When the fault is eliminated, the self-recovery fuse cools and crystallizes again, the volume shrinks, and the conductive particles form a conductive path again. The self-recovery fuse is restored to a low resistance state, thus completing the protection of the circuit without manual replacement.

The operating principle of Resettable fuse

  The operating principle of the Resettable fuse is a dynamic balance of energy. The current flowing through the Resettable fuse generates a certain degree of heat due to the thermal effect of the current (there are resistance values for resettable fuses). All or part of the generated heat is dissipated to the environment, and the heat not dissipated will increase the temperature of the Resettable fuse.
The temperature during normal operation is low, and the heat generated and the heat dissipated reach a balance.
  The resettable fuse element is in a low-impedance state, and the resettable fuse does not work. When the current flowing through the Resettable fuse increases or the ambient temperature rises, but if the balance between the heat generated and the heat dissipated is reached, the Resettable fuse still works normally.
When the current or ambient temperature increases again, the Resettable fuse will reach a higher temperature.

If the current or ambient temperature continues to increase at this time, the heat generated will be greater than the heat dissipated, causing the temperature of the Resettable fuse to increase sharply. At this stage, a small temperature change will cause a substantial increase in resistance, and the Resettable fuse is in a high-resistance protection state. The increase in impedance limits the current, and the current drops sharply in a short period of time, thereby protecting the circuit equipment from damage. As long as the applied current is enough to generate more heat from the Resettable fuse, the Resettable fuse in the protected state can always be in the action blocking state (high resistance).
When the applied voltage disappears, the Resettable fuse can automatically recover.