Fuse And Method Of Making Same

Abstract

A fuse comprising a fusible element mounted in a housing and carrying a plurality of relatively closely spaced activator cores. A method of making the fuse comprises the steps of punching discs from a strip of activator material, applying these discs to the fusible element in the desired spaced apart relationship, heating the fusible element carrying the discs so as to melt these discs to form the activator cores, and mounting this fusible element in the housing. An alternative method of making the fuse comprises the steps of applying a photoresist layer to the fusible element, locally exposing and etching the photoresist layer, selectively applying activator material to the fusible element by electroplating, and mounting this fusible element in the housing.

Description

The present invention relates to a fuse comprising a fusible element mounted in a housing between two end terminals, the fusible element having the shape of a thin wire or strip and including activator means.

Such a fuse is known, for example, from an article in Bulletin des Schweiz. Elektrot. Ver. 53 (1962), pp. 585-592, and is generally considered particularly suitable for use as a retarded fuse. In the known fuse the activator means may consist of a single activator core provided in the middle of the fusible element.

Activator cores on the basis of metals having low melting points, such as tin, lead and indium, are frequently used in retarded miniature fuses. In these fuses the activator core has the shape of a relatively thick blob of, for example, a tin-lead alloy secured by melting in the middle of the fusible wire, or the activator core consists of an eye filled with activator material in the middle of the fusible wire, which wire has been bent previously for this purpose. Particularly in the case of very thin fusible wires it is difficult to reproducibly provide such alloy cores, which renders it difficult to reproducibly manufacture retarded fuses for low current intensities. Moreover, the fusible element with the single activator core must be mounted highly accurately in the housing of the fuse, as in incorrect position has a detrimental effect on the limiting current of the fuse. Due to these difficulties and due to the poor control of the pre-diffusion of the activator material into the fusible wire or strip, the lower limit of the nominal current intensity for such fuses will be about 1.5 amp., while for lower current values one must resort to more complex structures employing extension springs, as disclosed on page 589 of the above article.

British Pat. No. 473,335 discloses fuses wherein an activator core containing magnesium powder is provided on the fusible wire. At a given current through the fusible element the magnesium in the core ignites and the interruption is established.

The magnesium-containing core may be provided, for example, by applying a paste including the material to the fusible wire. However, the application of paste to very thin wires is a difficult operation, while also the dosage and accurate postioning entails many problems. Moreover, an amount of oxygen is required for the burning of the magnesium, which causes difficulties in the case of fuses having very small dimensions. Furthermore, the high ignition temperature of about 500.degree. C causes a high degree of sagging of the fusible wire due to expansion, which entails the danger that the activator core located in the middle of the wire is inactivated as it contacts the wall of the housing. This danger is also present in the earlier mentioned fuses having an activator core on the basis of low melting point metals. In magnesium powder core fuses it was tried to solve this problem by providing two cores, each adjacent the respective end of the fusible wire (see e.g. Wireless World, Feb. 1947, pp. 51-53). An incorrect position of the fusible element due to assembling inaccuracies of the fuse detrimentally affects the limiting current both of the single and the double activator core fuse, while the attainable retardation is relatively poor due to the high reaction temperature and is insufficient for many uses.

It is an object of the present invention to provide a fuse wherein the above drawbacks are eliminated fully or to a great extent. This object is achieved as according to the present invention the activator means consist of a plurality of small activator cores closely spaced apart on the fusible wire or strip such that the axial position of the fusible element does not or hardly affect the limiting current of the fuse.

Due to the small dimensions of each one of the plurality of activator cores, the drawback of possible cooling upon contact with the wall of the housing is limited, while due to the cumulative effect of the diffusion zones the average temperature of the fusible wire and, consequently, its expansion remain limited. Furthermore, in the case of sufficiently small interspaces between the cores the axial position of the retarded fusible element does no longer affect the limiting current. An advantage of the fuse according to the present application is further that the assemblage of the retarded fusible elements is considerably facilitated as the accurate positioning of a single activator core is no longer necessary. This advantage applies both to manual and automatic assemblage.

According to the present invention retarded fuses comprising fusible elements including activator cores for hitherto unattainably low nominal current values can be achieved with high accuracy.

As an example of the fuses according to the present invention may serve a retarded fuse wherein very small activator cores having a weight of some tens of micrograms or more are uniformly and closely spaced apart on a thin fusible wire having a thickness of 20 .mu.um.

The present invention may also be applied to advantage to fuses wherein the fusible wire is wound about a core of insulating fibres. According to the present invention, in such a structure the fusible wire carries uniformly spaced small alloy cores.

The present invention also relates to methods of making the fuses according to the present invention. In a suitable method small discs are punched from a strip of thinly rolled activator material whether or not applied to a thin flim substrate, the discs are disposed on the fusible wire of strip in the desired spaced apart relationship with the activator side facing the wire or strip, the wire being previously provided with an adhesive coating, the wire or strip with the discs is passed along a heating means so as to melt each disc to form an activator core on the fusible wire of strip, whereafter a desired length of the fusible wire or strip carrying the activator cores is mounted in an appropriate housing.

The discs are preferably punched with a substantially larger diameter than the diameter of the fusible wire or strip and the discs are melted by absorption of infrared radiation, the fusible wire or strip remaining at relatively low temperature.

Thus, for example, a very thinly rolled strip of tin as activator material may be applied to a thin film substrate of a low melting point synthetic material, such as polythene. It is also possible to use a film of synthetic material whereon an extremely thin layer of tin or lead is deposited by evaporation. Discs having a diameter of about 1 mm are punched from such a strip. The discs can be prefolded and disposed on the fusible wire by means of a vacuum forceps. By passing the fusible wire carrying the discs along an infrared irradiator, the discs will absorb heat due to their relatively large surface areas that is just sufficient to melt the activator mass, the surface tension of this melted mass ensuring that a blob is formed around the relatively cool fusible wire. As a result of this method and pre-diffusion remains very slight. By using a simple mechanism the interspaces between the activator cores formed can be kept to close tolerances. The vacuum forceps can be used to advantage for punching and folding the discs of activator material as well as for transporting and disposing the discs on the fusible wire.

By the above method in appeared possible to provide copper wires having a thickness of 15 .mu.m with tin blobs of about 60 .mu.g spaced apart distances of about 5 mm.

According to a further suitable method of making fuses according to the present invention, very small activator cores are provided on a fusible element by applying a photoresist layer to the fusible element, locally exposing and etching the photoresist layer, selectively applying activator material to the fusible element by electroplating, and finally mounting the fusible element carrying the activator cores in an appropriate housing. This method may be considered an analogy of etching processes for manufacturing printed circuitry panels. The local exposure is performed by selective intermittent use of a very fine light beam.

The present invention will be elucidated hereinafter with reference to the accompanying drawing, wherein:

FIG. 1 represents a fuse according to the present invention;

FIG. 2 represents a similar fuse showing the most unfavourable location of the activator cores that may occur when the fuse is assembled; and

FIG. 3 represents a different embodiment of the fuse according to the present invention.

As shown in FIG. 1, the fuse according to the present invention comprises a housing consisting of a tubular section 1, which may be of glass or quartz, having end caps 4 of, for example, a conductive metal. The fusible wire 2 carrying a plurality of uniformly spaced apart activator cores 3 of, for example, an alloy of tin and lead is stretched in the housing. The sealing material 5 provides a sealing of the interior of the housing relative to the environment. The length of the tube 1 may be e.g. 20 mm and the diameter may be e.g. 4.5 mm.

In the mechanical manufacture of the fuse according to the present invention, for example, first a long fusible wire is provided with uniformly spaced apart activator cores. This wire is subsequently cut into sections having the desired length, which sections are mounted in housings. In the most unfavourable case, as shown in FIG. 2, the activator cores at the ends of the wire are in contact with the wall of the housing 1. As observed above, in prior fuses such a contact between the activator core and the tube wall is disastrous for the operation of the fuse. In the fuse according to the present invention, however, owing to an appropriate choice between the interspaces of the cores and the weight of the cores in relation to the diameter of the fusible wire, the contact between maximally two activator cores and the tube wall does not appear to affect the height of the limiting current of the fuse. In an embodiment as shown in FIG. 2, wherein a fusible wire of s silver alloy having a thickness of about 60 .mu.m carries tin-lead cores of about 75 .mu.g spaced apart distances of 5 mm, a retarded miniature fuse is obtained having a nominal current of only 315 mamp.

FIG. 3 shows an embodiment of the fuse according to the present invention wherein the fusible element consists of a core 6 of insulating fibre material wound with an extremely thin silver wire 2. Such a structure is disclosed in Dutch Pat. application No. 70 13620. According to the present invention the silver wire 2 carries closely spaced activator cores 3. A fuse of this type comprising a wound copper wire having a thickness of 15 .mu.m carrying alloy cores of each about 60 .mu.g povides a nominal current of 80 mamp.

Just as the embodiment shown in FIG. 2, the fuse shown in FIG. 3 can be conveniently mechanically manufactured.

Claims

We claim:

1. A method of making a fuse wherein small discs are punched from a strip of thinly rolled activator material, the discs then being disposed on a fusible wire in spaced apart relationship with the activator side facing the wire, the wire being previously provided with an adhesive coating, the wire with the discs being passed along a heating means so as to melt each disc to form an activator core on the fusible wire, whereafter a desired length of the fusible wire carrying the activator cores is mounted in an appropriate housing.

2. A method according to claim 1, wherein the discs are punched with a substantially larger diameter than the diameter of the fusible wire and the discs are melted by absorption of infrared radiation, the fusible wire remaining at a relatively low temperature.

3. A method of making a fuse according to claim 1 wherein very small activator cores are provided on a fusible element by applying a photoresist layer to the fusible element, locally exposing and etching the photoresist layer and selectively applying activator material to the fusible element by electroplating.

4. A fuse comprising a fusible element mounted in a housing between two end terminals, the fusible element having the shape of a thin wire and including activator means, the activator means consisting of a plurality of small activator cores spaced apart on the fusible wire such that the axial position of the fusible element does not appreciably affect the limiting current of the fuse, wherein the fusible wire of the fusible element is wound about a core of insulating fibres.