PolySwitch PTC Devices-A New Low-Resistance Conductive Polymer-Based PTC Device for Overcurrent Protection

Positive temperature coefficient (PTC) resistors for use as overcurrent protectors are now available with normal conduction mode resistances of about 0.04 . This resistance is significantly lower than devices made from heretofore available barium titanate ceramics. PolySwitchTM overcurrent protectors are made from conductive polymer compositions, which are newly developed materials. Fabrication of a PolySwitch device includes the creation of the base conductive polymer composite, melt forming it to device shape, attachment of leads, and packaging. The final product looks like a disc capacitor but acts like a solid-state circuit breaker or resettable fuse. The source of the PTC effect in these compositions is volume expansion upon heating and the subsequent disruption of connecting carbon black chains. Resistance characteristics of these compositions are reproducible and consistent from device to device; the concepts of a minimum and maximum base resistance and a resistance recovery curve are discussed. Polymer volume compaction processes are responsible for the recovery behavior. The physics of PolySwitch device behavior is explained by Joule heating. The characteristics important to the designer are ultimate trip current as a function of ambient temperature, a property of thermal and electrical equilibrium; and the trip-time characteristic, a property of the nonequilibrium dynamic response. Knowledge of both these charactistics allows the designer to quickly choose a PolySwitch device for his particular job.