Bourns® Multifuse® family of Polymer Positive Temperature Coefficient (PPTC) “resettable fuses” are used in a wide variety of circuit protection applications.
Under fault conditions the device resistance will rise exponentially and remain in a “tripped” state, providing continuous circuit protection until the fault is removed. Once the fault is removed and the power cycled, the device will return to its normal low resistance state.
Resettable fuses are manufactured in the form of a conductive plastic, which is comprised of a nonconductive crystalline polymer with highly conductive carbon black particles impregnated throughout the crystal lattice. Because of the close proximity of the carbon black particles within the crystal lattice, under normal conditions current is allowed to flow easily through the conductive plastic. However under a fault condition, when there is an increase in current, the conductive plastic heats at the rate of I2R. As the material continues to heat, it eventually reaches the phase transformation temperature, which changes the crystal structure into an amorphous structure. Once the material has transformed into this amorphous structure, the conductive particles become isolated and are unable to conduct current hence the drastic change in material resistance. It is only when the current is removed that the material is allowed to cool and return to its original crystal structure.