|Saturday, 16 December 2006|
Fuses are very important in protecting circuitry from overload conditions. Fuses are devices which, by melting of one or more of their parts intended and designed for this purpose, open the circuit by interrupting the current if the current exceeds a predetermined value for a sufficiently long period of time. They are designed to blow open at predetermined current levels and are selected based upon safety specifications designated for a particular circuit. The fusible element or fuse link is intended to melt away under the influence of a current which exceeds a particular value for a particular length of time. There are thermal fuses, mechanical fuses, spark gap surge arrestors, varistors, and other similar devices, each designed specifically as a solution to one or more extreme electrical events. Each device provides benefit in particular situations that may be greater than other types of devices. In general, an electrical fuse combines both a sensing and interrupting element in one self-contained device and is direct acting in that it responds only to a combination of magnitude and duration of current flowing through it. The fuse normally does not include any provision for making or breaking the connection to an energized circuit but requires separate devices to perform this function. A fuse is a single-phase device, such that only the fuse in the phase or phases subjected to overcurrent will respond to de-energize the affected phase or phases of the circuit that is faulty. After having interrupted an overcurrent, it is replaced to restore service. Currently, two basic types of fuses are employed, the expulsion fuse and the current limiting fuse. Each type employs a fusible element designed to melt when a current of a predetermined magnitude and duration passes through the element. The expulsion type fuse interrupts overcurrents through the deionizing action of gases that are liberated when the fusible element melts. An expulsion fuse typically employs a relatively short length of a fusible element contained within a tubular enclosure that is part of a larger assembly known as a fuseholder. The enclosure used in the expulsion type fuse is lined with an organic material. Interruption of an overcurrent takes place within the fuse by the deionizing and explosive action of the gases which are liberated when the liner is exposed to the heat of the arc that is created when the fusible element melts in response to the overcurrent. The operation of the expulsion-type fuse is characterized by loud noise and violent emission of gases, flame and burning debris, all of which pose a danger to personnel who may be in close proximity to the fuse when it operates. Because of its violent mode of operation, this type of fuse has generally been restricted to outdoor usage only. The current-limiting type interrupts overcurrents when the arc that is established by the melting of the fusible element is subjected to the mechanical restriction and cooling action of a sand filler that surrounds the fusible element. A current-limiting fuse typically consists of one or more silver wire or ribbon elements of a required length which are electrically connected at their ends to a pair of electrical terminations. The assembly is placed in a tubular housing that is made of a highly temperature-resistant material, and the housing is then typically filled with high-purity silica sand and sealed.
Electrical fuses have taken many forms and generally comprise fuses having a fusible link extending between a pair of terminal portions. The fusible link may be provided either with notches cut in one or more sides of the fusible portion or with holes formed therethrough to create narrower and therefore weaker portions within the fusible portion. One of the more common types of fuses is the thermal fuse (electrothermal fuse). In the thermal fuse, electrical current flowing through the fuse causes the fuse to heat. The current path within a typical fuse is through the end caps or ferrules to a metallic fusible element. The resistance of the fusible element develops heat that causes a portion of the metal to melt or disintegrate upon reaching the melting temperature of the metal. This property is exploited to achieve accurate thermal activation of a fuse in response to a particular level of overload current. In normal operation, the temperature of the device remains relatively low and the resistance of the device also remains low. When an overload current flows through the device, the internal temperature of the fuse rises sufficiently to cause the fuse to electrically open. An alloy type thermal fuse is widely used as a thermo-protector for an electrical appliance or a circuit element, for example, a semiconductor device, a capacitor, or a resistor. Such an alloy type thermal fuse has a configuration in which an alloy of a predetermined melting point is used as a fuse element, the fuse element is bonded between a pair of lead conductors, a flux is applied to the fuse element, and the flux-applied fuse element is sealed by an insulator. A time delay fuse is a type of fuse that is designed to allow temporary and harmless currents to pass there through without triggering the fuse. Time delay fuses are used in connection with equipment having temporary current surges, such as motors and transformers. Time delay fuses often employ a fusible element and a spring-loaded heat mass. A deposit of solder retains the heat mass from movement by the spring. The dimensions of the fusible element are selected such that it melts quickly under short-circuit conditions. Time delay fuses are typically used in circuits subject to temporary transients such as motor starting currents. A typical high-voltage, current-limiting fuse comprises a tubular insulating housing, an elongated core within the housing, and one of more fusible elements wound about the core and connected between terminals at opposite ends of the housing. A core is needed in fuses of this type rated at 5 KV and above in order to enable the fuse to accommodate the required length of fusible element within a housing of practical length. The fuse housing materials may consist of glass, ceramic, porcelain, and glass-filament-wound epoxy tubing. Copper ferrules or sand cast caps are typically glued to the ends of the fuse body with an epoxy or pressed onto the fuse housing with an interference fit to form end enclosures. A card type fuse is suitable for use in various devices having a low electrical power of less than 1A. For example, such a fuse is suitable for fuse-matching in a wire harness composed of wires having a small diameter, and which connects a series of electronic elements in a car. In such fields, utilization of card type fuses has been increasing. Solid state fuses are also known in which transistors and thyristors are placed in series with the load and turn off in response to a load fault condition.
Fuses are commonly used in automotive electrical systems to protect circuits against damage caused by overload conditions. Fuses for various circuits are often grouped together at clustered locations where circuit junctions exist in a fuse box, power distribution block, or junction block. Many automotive vehicles are equipped with a fuse junction box which serves to hold a plurality of fuses associated with the various electrically powered devices of the vehicle. A typical automotive fuse has a generally rectangular plastic body with a pair of parallel, blade-like fuse terminals extending therefrom. The outer surface of the fuse box is provided with fuse sockets to allow the fuse terminals to be inserted into electrical engagement with the circuit terminals, thereby completing and fuse-protecting the associated circuits. Typical fuse boxes are connected to the positive pole of the motor vehicle battery via one or more cables leading to the fuse box whereat power is supplied to a plurality of fuses contained within the box. The ends of the fuses opposite the end connected to the positive terminal of the battery generally are connected to outgoing cables or cable strands to supply power to electrical consumers such as, for example, motor vehicle lighting systems, sensors and switches, and power accessories. Generally, the type of fusion of fuses used for protecting an electric circuit in an automobile or the like is classified into the fusion in a high current region and the fusion in a low current region. Fuses of the relatively flat, plug in type which have a fuse link encapsulated in a plastic fuse body with a pair of terminal legs extending from the body have become very popular, especially in automotive applications. One commonly used type of automotive fuse takes the form of a pair of parallel blade type contacts with the fusable portion constituting a bridge between the two blades. Blade type fuses are increasingly used in automobile equipment, for the purposes of space requirements, protective qualities and ease of plugging in. Blade type fuses generally comprise an insulating case or body in which is partially mounted a conductive unit constituted by two connection terminal blades joined together by a fuse link element or gauging part. The fusable link is encased in a transparent insulative plastic and may be color coded according to the ampere rating of the fuse. A fuse junction box comprises a plurality of fuse receptacles containing spring clip contacts which engage the blades when the buses are pushed blade-end first into the receptacles. When properly inserted, each fuse forms part of an electrical circuit and protects the circuit from over-current conditions.
Fuses are commonly used in integrated circuits to provide redundancy and programming capabilities. Such fuses can provide for redundancy for the purpose of preventing reduction of yield of the semiconductor devices, which may be caused by random defects generated in the manufacturing process. Redundancy in integrated circuit memories is part of current wafer and chip manufacturing strategy to improve yield. By providing redundant circuits on chips, integrated circuit memory yields are increased by eliminating from circuit operation those circuits or modules which are defective or are not needed. The practice is to blow fuses which allow redundant memory cells to be used in place of cells that are nonfunctional. The redundant circuit portion is provided as a spare circuit portion having the same function as a specific circuit portion so that the specific circuit portion, which has a defect caused during manufacturing may be replaced with the redundant circuit in order to maintain the function of the entire semiconductor. Fuses are typically used in the design of monolithic integrated circuits (IC), and in particular in memory devices as elements for altering the configuration of the circuitry contained therein. To increase yield in integrated circuits such as memory chips, it is common to include redundant memory cells on the memory chips. If a memory circuit is found to be defective or is not needed, the fuse may be blown thereby activating or deactivating the redundant memory cells. Fuses incorporated in semiconductor constructions are typically provided as fuse links, which are narrow lines in a patterned conductive layer. The links are arranged in vectors or arrays, with the separation between the fuses being determined by, among other things, the spot size achievable by an energy input device utilized to heat up and blow the fuses, as well as by the positional accuracy of the energy input device. Electrical fuses, particularly silicided and non-silicided polysilicon fuses, have been widely adopted in integrated circuit fabrication over conventional metal fuses because of their package level reparability, field programming abilities, and built in self-test abilities. Fuses for integrated circuits are typically classified as two types: laser blown and electrical fuses. Both laser blown and electrical fuses are typically located near the surface of the semiconductor device so that fuses can be blown without damage to underlying integrated circuits.