Circuit protection devices

Abstract

A generally rectangular, planar electrical overcurrent sensing device having a top major surface and a bottom major surface includes a patterned metal foil conductor defined along the top major surface. The metal foil conductor has a first electrode region at one end region, a second electrode region at an opposite end region, and a current-concentrating region extending between the first electrode portion and the second electrode portion. The device further includes a planar sheet of a composition which exhibits PTC behavior and which comprises an organic polymer having a particulate conductive filler dispersed therewithin. The planar sheet has a first major surface in thermal contact with the bridging portion and has an opposite second major surface. A third patterned metal foil electrode secured to the second major surface of the planar PTC sheet is sized and aligned with the current-concentrating region such that heat generated in the current-concentrating region from electrical overcurrent flowing through the metal foil conductor is transferred to the planar sheet exhibiting PTC behavior and reduces a control current flowing to said third patterned metal foil electrode. An insulation layer may be imposed between the patterned metal foil conductor and the PTC sheet layer, and in such case the third patterned metal foil electrode is divided into two conductive areas separated by a gap aligned with the current-concentrating region, thereby providing a four terminal device. Tin pellets may be included in the current-concentrating region to reduce a melting/fracture temperature thereof below a flaming temperature of the organic polymer sheet forming the PTC layer.