1. Field of the Invention
The present invention relates to a fuse element used in protecting an electric circuit provided in an automobile or the like, and more particularly, to the structure of a fusible portion formed integrally with an electrical connection portion, from a thin metal plate.
The present application is based on Japanese Patent Application No. 2001-149914, which is incorporated herein by reference.
2. Related Art
A related-art automobile fuse of this type will be described by reference to that described in, e.g., Japanese Patent Application Laid-Open No. JP-A-07-65690, and shown in FIG. 5. As shown in FIG. 5, a fuse element P is formed integrally from a conductive metal plate, such as copper. A pair of connection terminals 53 are formed at the respective ends of a narrow portion 51. A thin metal layer is formed from tin on the surface of the fusible portion 51 by plating. A low-fusing-point metal chip 54 is provided in the center and at a position where the greatest amount of heat will develop when an electric current flows through the fusible portion 51. The low-fusing-point metal chip 54 is held by wrapping and caulking crimp pieces formed on the respective side edges of the fusible portion 51, thus forming a fusing portion 52. The fusible portion 51 is bent into an inverted U-shape with the fusing portion 52 taken as a center. The connection terminal portions 53 provided at the respective ends of the fusible portion 51 are connected to an electric circuit.
In order to cause the fusible portion 51 to satisfy a fusing characteristic with as low a resistance value as possible, there is utilized a phenomenon of the low-fusing-point metal chip 54 spreading into the fusible portion 51. A predetermined time lag is ensured by the thin metal layer formed on the surface of the fusible portion 51. However, if a comparatively low overcurrent which does not induce fusing of the fusible portion 51 within the period of the time lag flows through the fuse element repeatedly or if there has frequently arisen a phenomenon of the fusing portion 52 being instantaneously heated as a result of rushed flow of an overcurrent through the fusible portion 51, the low-fusing-point metal chip 54 induces gradual progress in diffusion alloying of the fusing portion 52. As a result, deterioration arises in the characteristic of the fusing portion 52; that is, a characteristic of shortening a time required for fusing the fusing portion 52. Hence, there arises a problem of the fuse element failing to attain predetermined durability.
In order to ensure a fusing characteristic and durability of a fuse, the low-fusing-point metal chip 54 requires complicated manufacturing processes and special management in relation to the purity of material, dimensional accuracy, and maintenance of fixed adhesion of the low-fusing-point metal chip 54 to the fusible portion 51, thereby resulting in low productivity and adding to manufacturing costs.