GB2084799A - Electrical capacitors - Google Patents

Abstract

A method of forming a plurality of contact areas (of sprayed metal) for independent connections to a plate (metallised films 2,3-Fig. 1) of a capacitor, the plate comprising an array of edges (2B) of plate material lying adjacent to a surface (4) of the capacitor body (1-Fig. 1), in which contact material (9) is deposited over the whole area of that surface (4) to bond with the plate material (2B) over that area, and is subsequently slotted or grooved (at 10, e.g. by a saw blade 11) to divide it into the plurality of contact areas (5A,5B). <IMAGE>

Description

SPECIFICATION Improvements in or relating to electrical capacitors This invention relates to electrical capacitors.

It has arisen in relation to the type of capacitor known as a film capacitor (or metallized film capacitor) but could also be applied to other types of similar construction whose plates each comprise an array of edges of plate material adjacent to a respective surface of the capacitor body and in which, for the connection of terminals to the plates of the capacitor, areas of conductive material are deposited over the respective surfaces of the capacitor body so as to bond with the material of the respective plates at their edges.

For certain applications (such as those hereinafter described with reference to the drawings) two independent connections have to be made to each of the plates of the capacitor in such a way that no resudual contact material interconnects them electrically. Previous methods of achieving this have always involved the use of a mechanical mask to shield a portion of the area to which the contact material is applied. This is a nuisance in the production of capacitors, and the masking technique has other drawbacks, such as squashing of the adjacent portions of the capacitor and possible penetration of contact material under the mask.

According to the present invention, contact material is deposited over the whole of the relevant area of the surface in question, to bond with the material of the respective plate of the capacitor over that area, and is subsequently slotted or grooved to divide it into a plurality of contact areas for the independent connections to that plate of the capacitor.

The slotting is conveniently done by means of a saw, most conveniently a circular saw.

There are also other possibilities, such as the use of abrasive wheels or diamond wheels for example, or the use of a technique involving heat (laser cutting for example) though such a technique would probably not be satisfactory in the case of metallized film capacitors because of the consequent fusion of the plastics material of the metallized films or tapes.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows the construction of a wound metallized film capacitor of cylindrical form, which may or may not be subsequently flattened; Figure 2 shows the wound capacitor with a contact area of conductive material at one end divided into two; Figure 3 is a diagrammatic cross-sectional illustration of one end of the capacitor to a larger scale, showing the use of the known masking technique to divide the contact area into two independent areas; Figure 4 is generally similar to Fig. 3 but shows a method of dividing the contact area into two in accordance with the present invention; Figures 5 to 9 are electrical circuit diagrams illustrating certain applications of such capacitors; and Figure 10 shows the end of a capacitor, such as that in Fig. 2, in connection with the explanation given with regard to Figs. 5 to 9.

Referring now to the drawings and first to Fig. 1, there is shown a film capacitor 1 which is formed by winding two metallized plastics material tapes 2, 3 on a core or a removable mandrel (not shown, for simplicity of illustration). The tapes 2, 3 have margins 2A, 3A which are clear of metallizing 2B, 3B.

Thus when the winding is completed, the two plates of the capacitor constituted by the metallized films 2B, 3B are staggered so that, at each end of the capacitor, one of the metallized films extends to the end surface of the winding while the other is recessed below the surface to the extent of the clear margin 2A or 3A. Thus, considering the upper end 4 of the capacitor as shown in Fig. 1, the metallizing 2B on the outer tape 2 comes flush with the end 4 while the metallizing 3B on the inner tape 3 is recessed from the surface of the end 4 by the clear margin 3A.

If the capacitor has been wound on a mandrel, it may or may not, as desired, be flattened after removal of the mandrel.

Referring now to Fig. 2, metal in molten particular form (usually tin-zinc amalgam) is sprayed normal to the winding end such as 4, whether circular (as shown) or flattened (not shown), penetrating the interleaving of the tapes slightly to form a mechanically robust end connection to the capacitor plate constituted by the metallizing 2B or 3B. For certain applications of the capacitor, two independent connections such as 5A, 5B have to be made to each end of the winding in such a way that no sprayed metal interconnects them electrically across a central diametrical gap 6.

Previous methods of making the two independent connections have always involved a mechanical mask of some form which is pressed into the winding end to shield a portion from the metal spray. Fig. 3 shows such a mask 7 pressed into the winding end beyond the spray penetration depth at 8, so that the metal spray 9 is limited to the areas of the two contacts 5A, 5B. Fig. 3 also shows the edge of the metallizing 2B of the first film 2, which extends to the surface of the winding end 4, and the clear margin 3A of the second film 3 which extends to a point beyond the lower edge of this Figure. It will of course be appreciated that the illustrations are extremely diagrammatic and exaggerated in regard to the relative dimensions.

Referring now to Fig. 4, this illustrates a method in accordance with the invention, in which the metal spray 9 is first applied over the whole of each winding end such as 4.

Subsequently a groove 10 is cut across the winding end 4 through the sprayed metal 9 to a point beyond the spray penetration depth at 8 but not beyond the clear margin 3A of the second film 3. This is conveniently done by means of a saw blade 11 of a circular saw, but other grooving or slotting means could be used as previously mentioned herein. The bottom 1 OA of the groove 10 being below the spray penetration depth at 8, the desired insulation band 6 (Fig. 2) is produced.

By monitoring the electrical resistance between the two halves 5A, 5B of the end spray, completion of the cut can be detected by a sharp increase in resistance. This is an important advantage of the method in addition to the advantage of avoiding the drawbacks of the masking technique as previously mentioned herein. It is of course very important to stop the cutting as soon as completion of the cut has been detected, in view of the small dimensions concerned example, in a small capacitor, the slot may need to be cut to a depth of about 1.3 millimetres, this being just beyond the spray penetration depth of about 1 millimetre.

A capacitor formed as described above will of course be completed by having terminals (usually wires) bonded to the respective contact areas and then usually being encapsulated in insulating material. Methods of thus completing a capacitor are known and will not be described herein as they do not form part of the present invention.

In addition to being applicable to the cylindrical form of film capacitor illustrated in Figs.

1 and 2, which may or may not be flattened, the invention is also applicable to the flat form which may be obtained by winding a mother capacitor and then dividing it up into individual capacitor units. In this latter form of capacitor of course the metallized films are not continuous but they will again be in staggered disposition at opposite ends of the unit and will be individually interconnected by the sprayed metal contact material to constitute the two plates of the capacitor.

In regard to the inventive nature of the method of dividing the contact material into a plurality of contact areas by slotting or grooving rather than masking, it is pointed out that there has hitherto been a general reluctance to cause physical damage to capacitor windings. Moreover, the slotting or grooving is quite a critical operation, and, as mentioned above, it has been found to offer distinct advantages over the masking technique.

An important advantage not previously mentioned herein is that the monitoring of the completion of the cut can be logged and this will serve to reassure the user of the capacitor as to the absence of residual contact rnaterial between the independent connections to each of the plates of the capacitor. Such quality assurance is highly desirable in regard to the applications of such capacitors which will now be described with reference to the remaining Figures of the drawings.

Where a shunt capacitor is required in a circuit in which failure would result in a malfunction of the circuit, it is desirable to use a capacitor of special manufacture (such as those which have been described herein) which may be termed a "fail-safe" device.

Circuits in the form of Figs. 5 and 6 are examples of circuits in which the "fail-safe" capacitor may be used. If either of the connections Ito the capacitor C in Fig. 5 was to fail (e.g. if a lead was to come adrift) the circuit would operate but possibly in an abnormal fashion. If the capacitors C and C in Fig.

6 were to fail in the same way the circuit would not function as intended.

To overcome such abnormal modes of operation it is essential that the capacitors, when exhibiting a failure mode, should disable the circuit. To achieve this condition it is necessary for current flow to be interrupted. A method of obtaining this condition is to have, as shown in Fig. 7, two connections a, b or c, dto each plate of the capacitor, so that current must flow via the plates of the capacitor, as indicated by the arrows. If such a fourterminal capacitor C' is now substituted for each of the capacitors in Figs. 5 and 6, as shown in Figs. 8 and 9, it will become apparent that any failure mode (open circuit) of the capacitor will cause the circuit to stop operating.

In the manufacture of a capacitor of this nature where metal spray is used for the end termination, it is essential that the end connection is so split that direct connection between the two D-shaped contact areas, 5A, 5B as shown in Fig. 10, occurs via the plates of the capacitor. This may be done (as previously mentioned herein) either by masking the required metal-free area 6 so that no metal spray impinges onto it or by spraying normally and cutting a slot in the end whose depth is such that it creates a metal-free area without causing electrical deterioration of the capacitor. The latter method which is in accordance with the invention, is more economical and better for several reasons, previously mentioned herein.

Regarding the most important advantage to users of such capacitors, namely the facility to make "before and after" resistance measurements, this may readily be done by means of a direct-current measurement. While resistance values will vary in dependence upon the size of the capacitor and materials being used, a considerable change in resistance occurs after slot cutting, owing to the higher resistance path through the metallized film plates.

When monitoring the resistance during slot cutting, the resistance change is very rapid and noticeable; it can be tabulated and charted to prove that the slot has been cut effectively.

Capacitors made in accordance with the invention will be particularly valuable in failsafe electronic equipment for railway signalling systems.

Claims (5)

1. A method of forming a plurality of contact areas for independent connections to a plate of a capacitor, the plate comprising an array of edges of plate material lying adjacent to a surface of the capacitor body, in which contact material is deposited over the whole area of that surface, to bond with the plate material over that area, and is subsequently slotted or grooves to divide it into the plurality of contact areas.

2. A capacitor having a plurality of contact areas, for independent connections to a plate of the capacitor, formed by a method according to claim 1.

3. A method of forming a plurality of contact areas for independent connections to a plate of a capacitor, substantially as herein described with reference to Fig. 4 of the accompanying drawings.

4. A capacitor substantially as herein described with reference to Fig. 4 of the drawings.

5. A capacitor substantially as herein described with reference to Figs. 1, 2 and 4 of the drawings.