GB2032205A

GB2032205A - Low-current Fuse and Method of Production

Info

Publication number: GB2032205A
Application number: GB7931098A
Authority: GB
Original assignee: Wickmann Werke AG
Current assignee: Wickmann Werke AG
Priority date: 1978-09-09
Filing date: 1979-09-07
Publication date: 1980-04-30
Also published as: JPS5556328A; FR2435800B1; GB2032205B; DE7826855U1; FR2435800A1; NL7906716A

Abstract

A fuse for low normal current applications has an elongate fusible conductors of very small dimensions formed by applying a layer of electrically conductive material to an electrically non-conductive heat- resistant base 1 and thereafter removing material from the layer to form a fusible conductor of predetermined shape and dimensions. The base may be of ceramics, the conductor of e.g. Cu or Ag, applied by e.g. evaporation, coating, electrodeposition or gluing and shaped by laser or sand blasting. Enlarged ends 3 are left to facilitate mounting in and electrical contact with a housing, Fig. 3 not shown. <IMAGE>

Description

SPECIFICATION Low-current Fuse and Method of Production The invention relates to fuses, especially for low normal currents, having an elongate fusible conductor of very small dimensions, and to a method of producing such fuses.

Fuses are used for overload and short-circuit protection of individual components in electrical apparatus or else of electrical apparatus as a whole, the power consumption of which, depending upon the kind of apparatus, may be very small. The development of semiconductor components has led to a still further reduction in the current consumption of certain electrical apparatuses so that electrical apparatuses are now known, which exhibit a current consumption of only a few milliamps.

Fuses for use in apparatus of that kind, especially if they are intended for low normal currents, have very thin wires as the fusible conductors, the diameters of which frequently amount to only a few thousandths of a millimetre.

These wires are clamped freely inside the fuse with a length between clamps of about 20 mm.

It will be appreciated that very thin wires can be handled only with difficulty. Consequently, the rejection rate in the assembly of the wires in fuse housings is very high because of wire breakage. If the freely clamped wire in the finished fuse is subjected to impact and vibratory loadings, it tends to tear, so that frequent interruptions caused by this reduce the operating reliability of such fuses. In clamping the thin wire in the fuse housing a danger exists of the wire getting stretched undesirably or being caused to tear. The straightest possible run of the wire is striven for because the fusible conductor, in the case of an electrical loading, for example, in the range of normal current, expands by heating and touches the inner wall of the fuse housing with the stretched part.This produces an undesirable cooling of the fusible conductor, with the result of an alteration of the fusing times.

The problem therefore exists of creating a method of producing fuses especially for low normal currents, which makes possible accurate production of a fusible conductor which, both in assembly and also later in service, is resistant to mechanical loadings and stable when thermally loaded.

According to the present invention a fuse is formed by applying a layer of electrical conductive material to an electrically non-conductive heatresistant base and thereafter removing material from the layer to form a fusible conductor of predetermined shape and dimensions.

A ceramic carrier, for example, may be employed as the heat-resistant base upon which a layer of electrically conductive material, for example, copper or silver or conductive paste of different compositions is applied. The layer may be applied, for example, by evaporation, coating, electrolytic depositing or gluing. From this layer, by means of a progressive method of removal acting upon the surface of the layer, material is removed in predetermined regions until the part of the layer remaining forms a fusible conductor of predetermined shape and dimensions.

The working out of the desired shape of the fusible conductor from the layer, in particular taperings or narrowings of the cross-section of the fusible conductor by partial removal of the thickness of layer, is effected by narrowing the width of layer or the thickness of the layer or the width of fusible track or by a combination of the removal of the material in several directions or dimensions of the layer. For this purpose the thickness of layer may be relatively large as compared with the cross-sectional dimensions necessary for the fusible conductor.

A fuse formed in accordance with this method is insensitive and stable in relation to mechanical and thermal loadings even if the electrically conductive layer in the region intended for melting-through exhibits, depending upon the normal current, only extremely small crosssectional dimensions. The difficulties hitherto in the assembly of the fuse are eliminated, the more so since the clamping process is omitted.

Furthermore the fusible conductor can be produced with very exact maintenance of the dimensions necessary for the required or desired normal current, so that the desired electrical resistances and fusing times may be realised with great accuracy.

A further advantage of the invention consists in the fact that the end portions of the fusible conductor, intended for connection to the contact caps of the fuse by means of soft solder may readily be made so thick that during the soldering process onto the end portions only a fraction of the thickness of layer becomes partially alloyed, the remaining thick residual layer in this region guaranteeing satisfactory operation of the fusible conductor. Hitherto, in the case of soldering in thin wires as fusible conductors, an undesirable pre-alloying frequently results and the alloying continues at both points of soft solder connection into the contact caps with slight heating of the fuse, so that alloying may very soon cover the whole cross-section of wire.The formation of alloy alters the interruption time of very thin fusible wires. Inthe case of the construction of the fusible conductor in accordance with this invention alloying cannot cause any impairment of the operation of the fusible conductor.

The removal of material is preferably carried out in such a way that in the region lying between the end portions of the fusible conductor, intended for the connection to caps, a tapering shape is produced. This tapered region forms an actual zone of fusion whilst the two soldering ends or end portions of the fusible conductor are not subjected to material removal.

The removal of the electrically conductive material may be effected by means of sand blasting the layer. Preferably, however removal of the electrically conductive material is achieved by melting off the material by means of a laser beam.

Depending upon the kind of profile desired for the fusible conductor the regions of the electrically conductive layer which are to be processed are treated perpendicular and/or parallel and/or obliquely to the base either with a fine sand blast or with laser beams dosed in a predetermined manner until the desired dimensions as well as the necessary shape of the fusible conductor are achieved. The method can be completely automated. In particular the removal of material by means of laser beam allows a simultaneous equalization of the spatial tolerances and the electrical tolerances dependent upon them, so that the electrical resistance desired in each case may be achieved with great accuracy.

The assembly of the fuse can, because of the attachment of the thin fusible conductor to a rigid base, be easily performed without damage to the fusible conductor.

An example of a fuse according to the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a front elevation of a fusible conductor on a heat-resistant base; Figure 2 is a sectional side elevation of the fusible conductor and the base along the line A A in Figure 1; and, Figure 3 is a sectional elevation of a fuse having a fusible conductor and base in accordance with Figures 1 and 2.

In the embodiment illustrated in Figures 1 and 2 there is applied, first of all to a base 1 of ceramic material, a silver or copper coating or a coating of a conductive paste of different composition from which a fusible conductor 2 of the shape illustrated is then constructed by removal of material by means of a laser beam or by means of at least one finely controlled sandblasting operation, in particular, in the centre between the end portions 3. A sand-blast treatment or a laser beam application allows accurate maintenance of the desired shape and dimensions. The drawing reveals clearly in front and side elevation the tapering of the fusible conductor 2 in the region between the end portions 3. The end portions 3 themselves are excluded from the laser beam or sand blast treatment and are fastened in use by their metal layer (of unaltered thickness) in contact caps 4 in a fuse housing 5 by soft solder 6 as is shown diagrammatically in Figure 3.

In contrast to the example illustrated and described -- depending upon the desired shape of fusible conductor-the layer may also be subjected, only in the direction of the thickness or in the direction of the width or in both directions or dimensions respectively, and where necessary simultaneously, to the removal of material at points at which the desired shape of fusible conductor requires a certain tapering of the cross section.

Claims

-Claims

1. A fuse having an elongate fusible conductor of very small dimensions, characterized in that the fusible conductor comprises a layer of electrically conductive material, on an electrically non conductive heat-resistant base, shaped by removal ot material from the layer.

2. A fuse according to claim 1, in which the fusible conductor has end portions for the connection to caps, a region of the fusible conductor therebetween being tapered by the removal of electrically conductive material.

3. A fuse according to claim 1, substantially as described with reference to the accompanying drawings.

4. A method of producing a fuse for carrying low normal currents, the fuse having an elongate fusible conductor of very small dimensions, in which the fusible conductor is formed as a layer of electrically conductive material on an electrically non-conductive heat-resistant base, material being removed from the layer to form the fusible conductor in a predetermined shape and dimensions.

5. A method according to claim 4, in which the removal of material is carried out in such a way that a tapering shape is imparted to the fusible conductor between end portions for connection to caps.

6. A method according to claim 4 or claim 5, in which the electrically conductive material is removed by means of sandblasting.

7. A method according to claim 4 or claim 5, in which the electrically conductive material is removed by melting off by means of a laser beam.

8. A fuse produced by a method according to any of claims 4 to 7.