polyfuse

Polyfuse is a resettable device used to protect electrical circuits from overcurrent conditions. Also known as a polymer positive temperature coefficient resettable fuse (PPTC) or Polyswitch, it provides overcurrent protection without replacement after a fault.

Principle and construction

A polyfuse consists of a thin polymer film or composite containing conductive fillers, packaged between electrical terminals in a protective housing. In normal operation, the device exhibits a low resistance. When excess current flows, self-heating raises the temperature of the polymer, triggering a rapid increase in resistance due to the polymer’s PTC behavior. This limits current to a safe level. If the fault is removed and the device cools, the resistance returns to its low, ready state, restoring normal operation.

Ratings and operation

Polyfuses are specified by hold current (I_h), trip current (I_trip), and maximum voltage (V_max). The hold current is the maximum current the device can carry in a stabilized state, while the trip current is the current at which the device switches to high resistance. Formats include radial-leaded disk-style and surface-mount (SMD) variants. They have low on-state resistance to minimize voltage drop during normal operation, but are not designed for continuous high-current or high-energy short circuits.

Applications and limitations

Polyfuses are widely used for protecting consumer electronics, power adapters, USB devices, battery packs, solar electronics, and automotive components. They are favored for their self-resetting nature and silent operation. Limitations include dependence on ambient temperature, aging with repeated trips, slower recovery at high temperatures, and limited effectiveness for high-energy faults. They are not suitable substitutes for all traditional fuses in high-power or high-voltage applications.

See also: PTC thermistor, PPTC, overcurrent protection.