SMT Low Profile PTC Resettable Fuses | First Look

Bel Fuse’s polymer formulation positive temperature coefficient (PTC) devices offer circuit protection and an added resettable feature in low-profile, SMT packages. The Bel PTC consists of a block of polymeric material containing conductive carbon granules that is sandwiched between two conductive metal plates. When this polymer block reaches approximately 125oC, either due to current passing through it via conductive chains of carbon particles or due to an external heat source; it swells volumetrically. This expansion breaks apart a majority of the chains of carbon granules that run between the two conductive plates. This behavior results in a sharp increase in resistance across the two plates, which all but eliminates current flow through the device, allowing just enough residual current flow to maintain the block’s internal temperature at 125oC. Once this “tripped” state current is cut off, the polymer brick cools and shrinks to its original size, thereby allowing the broken carbon chains to reestablish themselves and permit the part to return to its low resistance state. Once cooled to room ambient, the PTC will reset. This reset-ability protects electronic equipment, which remains fully functional, and eliminates the need for service personnel to physically replace a fuse. This behavior allows the fuses to protect against harmful overcurrent, short circuit conditions, and over temperature conditions not necessarily caused by excessive current. The surface mount series of resettable PTC devices are all AEC-Q200 compliant, have an operating temperature of -40°C to +85°C, offer voltage ratings from 6 to 60 Volts DC, and an operating hold current range from 10 milliamps all the way up to 5 amps.