GB2213322A

GB2213322A - Terminating a flat wound capacitor used in a discharge circuit

Info

Publication number: GB2213322A
Application number: GB8719855A
Authority: GB
Inventors: Harold Hammond
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1987-08-21
Filing date: 1987-08-21
Publication date: 1989-08-09
Also published as: GB8719855D0

Abstract

A capacitor (40) comprises a flat wound capacitor made on a flat winding mandrel from successive windings of a pair of electrically conductive strips interleaved with a pair of electrically insulating strips and a pair of terminals each secured between a respective one of said conductive strips and an insulating strip and projecting laterally from the strips. In a preferred embodiment one terminal (20) is one foil strip conductor of a foil strip transmission line (29) which comprises two conductors separated by a layer of insulation (24), the other terminal being soldered to the second foil strip conductor (26). The terminal (20) contacts one conductive layer (54) of a further transmission line (50), the other terminal being connected to line layer (56) via a trigger switch (e.g. spark gap) (70) mounted in holes of the transmission lines. On capacitor discharge, a bridgewire or foil initiator (42) explodes to trigger a chemical explosion. <IMAGE>

Description

"IMPROVEMENTS IN AND RELATING TO CAPACITORS" This invention relates to capacitors and in particular to low inductance capacitors.

It is known to initiate an explosion by discharging a capacitor through an exploding bridgewire or an exploding foil initiator. In both cases the effect is similar - the bridgewire or foil initiator is vaporized and the resulting plasma is directed onto a secondary explosive to trigger a conventional chemical explosion.

Such systems require however a very fast rising current pulse which in turn calls for the minimum of inductance in the capacitor and its discharge circuit through the bridgewire or foil initiator.

A typical capacitor previously proposed for this purpose is disclosed is US PS 4,502,096 and comprises a flattened, spiral wound capacitor provided at each end with a number of projections from a respective one of the foil strips which constitute the plates of the capacitor. The projections at each end are clamped to form the capacitor terminals. Such a capacitor however involves a large number of manufacturing steps so that it is relatively expensive and is moreover awkward to connect to the foil strip transmission line used in the discharge circuit.

It is an object of the present invention to provide a method of manufacturing a capacitor that is simple and inexpensive and which produces a capacitor which has a low impedance and is easily connected to a foil strip transmission line.

The present invention is a method of making a capacitor comprising providing to a flat former a pair of elongated electrically conductive strips interleaved with a pair of elongated electrically insulating strips, winding a first length of said interleaved strips on said former, placing each of a pair of strip electrodes between an insulating strip and a respective one of the conductive strips, the electrodes extending laterally of the conductive strips, winding a second length of said interleaved strips on said former thereby securing the strip electrodes in position, and securing the free ends of the strips of the second length to the preceding winding.

The present invention is also a capacitor comprising a flat wound capacitor made from successive windings of a pair of electrically conductive strips interleaved with a pair of electrically insulating strips and a pair of terminals, each secured between a respective one of said conductive strips and an insulating strip and projecting laterally from the strips.

The present invention is further a detonator circuit comprising a capacitor as described in the last preceding paragraph in series with a trigger switch and a load capable of triggering a chemical explosion upon discharge of the capacitor.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 diagrammatically illustrates a stage in the manufacture of a capacitor according to the present invention; Fig. 2 is a perspective view of a capacitor according to the present invention; Fig. 3 illustrates the insertion of the capacitor terminals during manufacture; Fig. 4 is a diagram of a detonator circuit incorporating a capacitor according to the present invention; and Fig. 5 illustrates a discharge circuit for a capacitor according to the present invention.

Referring now to Fig. 1. a capacitor is wound on a flat former 10 from a pair of electrically conductive strips 12 and 14, in this embodiment aluminium strips, interleaved with a pair of electrically insulating strips 16 and 18, typically made of a polyester.

The former 10 is rectangular in cross section and has a width greater than its thickness.

In the present embodiment the width of the former is 50 mm while the thickness is only 2 or 3 mm.

The former is also split in layers to facilitate its removal from the wound capacitance.

After approximately half the intended length of the interleaved strips has been wound on the former the winding process is halted to permit the capacitor terminals to be inserted.

In principle the terminals are simply electrically conductive foil strips which are inserted between a respective one of the strips 12 and 14 and an adjacent insulating strip 16 or 18 and which project laterally of the interleaved strips but to ensure adequate insulation between the terminals it is preferred that at least one of the terminals is provided with a layer of insulation on that surface facing the other terminal.

A possible arrangement of the terminals is illustrated in Fig. 3 which shows an enlarged cross section of the particular winding of the interleaved strips 12 to 18 in which the terminals 20 and 22 are inserted. The terminal 20 is placed between the conductive strip 12 and the insulating strip 16 while the terminal 22 is located between the conductive strip 14 and the insulating strip 16. Other locations for the electrodes are of course possible, e.g. the terminal 20 could be placed between the conductive strip 12 and the preceding turn of the strip 18 (i.e. below the strip 12 in Fig. 3) while the terminal 22 could be located between the strips 14 and 18.

In Fig. 3 the terminal 20 is provided with a layer of insulation 24 on its upper surface i.e. the insulation comes between the two electrodes 20 and 22, and it is preferred that the insulation 24 extend at least some way beneath the layer 16 as shown in Fig. 3 to ensure proper insulation between the terminals. In practice it is convenient for the layer 24 to extend across the full width of the layer 16.

In the preferred terminal arrangement illustrated in Fig. 3, the insulating layer 24 is a polyimide strip provided on each side with bonded copper strips 20 and 26, the strip 26 having been shortened by chemical milling to overlap the terminal 22 laterally of the strips 12 to 18 thus to enable electrical connection with the terminal 22 at a later stage in the winding process.

When the terminals have been located in position, the winding of the interleaved strips 14 to 18 is continued until the intended or designed length of strips is reached. This second winding step ensures that the terminals are trapped in position and are in intimate contact with their respective foil strip.

When the winding is completed, the interleaved strips are cut, the ends of the wound strips are secured in position by, for example, wrapping woven glass tape over the windings, and the former 10 is removed. The terminal 22 is soldered to the conductive strip 26. The capacitor 40 at this stage is as illustrated in Fig. 2 and it can be seen that the terminals are presented as a short length of strip transmission line 29 readily adapted to be secured by means of the hole 30 to the other elements in the discharge circuit. The capacitor is of course subject to the conventional post-winding treatments of, for example, clamping and potting, prior to use.

In Fig. 4 is illustrated a discharge or detonater circuit incorporating a capacitor according to the present invention. The capacitor 40 is connected in a series circuit with a load 42, which may be an exploding bridgewire or an exploding foil initiator and a trigger switch 44, typically a spark gap. Both the load and the switch are mounted on a strip transmission line easily connectible to the terminal transmission line 29.

In Fig. 5 is shown an example of a discharge circuit. A strip transmission line 50, comprising an insulating polyimide strip 52 with upper and lower copper strips or conductors 54 and 56 is provided at one end with a load 42 having aluminium strip terminals 58 and 60 the terminal 58 being soldered to the upper conductor 54 which does not extend to the end of the line 50, and the terminal 60 being wrapped round the end of the transmission line 50 and soldered to the conductor 56.

The other end of the transmission line 50 is provided with a hole and is located beneath the transmission line 29 from the capacitor 40, the holes 30 and 62 being in register, and the conductors 54 and 20 being in contact. A trigger switch 66 extends through the holes and clamps- the transmission lines together between a fixed clamp 68 at the bottom of the ceramic coated body 70 of the switch 66 and a movable upper clamp 72 at the top, the clamp 72 being secured by a screw member 74 and engaging the strip 26. Within the body 70 of the switch 66 are provided two spaced main electrodes connected respectively to the clamps 68 and 72 and a trigger electrode connected to the terminal 76.Thus, the discharge path for the capacitor 40 is from the strip 26 to the upper clamp 72, across the gap between the main electrodes of the switch 66 to the clamp 68, and thence to the conductor 56, load 42, conductor 54 and strip 20.

A capacitor according to the present invention measured 60mm by 56mm by 6mm, and had a capacitance of 63nF. It was tested in a ringdown test employing the discharge circuit shown in Figure 5 with load 42 comprising a very low resistance load (approximately 10 m5L) across which the discharge waveform of the circuit was viewed with an oscilloscope. When charged to 3KV, the capacitor yielded 5.94KA. The circuit impedance was calculated at 11.7nH.A conventional 60nF capacitance capacitor, supplied by Reynolds Industries Inc. and manufactured in accordance with US Patent No. 4,502,096, measured 70mm by 70mm by 14.5mm, i.e. 3.5 times the volume, had a capacitance of 60nF and yielded 5.65KVA at 3KV with a total circuit inductance of 1OnH in the same ringdown test as that described above with the conventional capacitor replacing capacitor 40. Thus the capacitor of the present invention was technically satisfactory, was much simpler and cheaper to manufacture and was markedly less bulky than the conventional capacitor.

Modifications may be made to the embodiments described. For example two or more pairs of conductive strips, each with a respective pair of insulating strips, could be wound together, each pair of conductive strips having its own terminals. Moreover each pair of conductive strips could be provided with more than one pair of terminals spaced along the length of the strips.

Claims

CLAIMS:

1. A method of making a capacitor comprising providing to a flat former a pair of elongated electrically conductive strips interleaved with a pair of elongated electrically insulating strips, winding a first length of said interleaved strips on said former, placing each of a pair of strip electrodes between an insulating strip and a respective one of the conductive strips, the electrodes extending laterally of the conductive strips, winding a second length of said interleaved strips on said former thereby securing the strip electrodes in position, and securing the free ends of the strips of the second length to the preceding -winding.

2. A method as claimed in claim 1, in which the first and second lengths are approximately equal.

3. A method as claimed in claim 1 or claim 2, in which the lateral extension of one electrode is secured to one conductor of a foil strip transmission line which comprises a pair of foil strip conductors separated by a layer of insulation, the other foil strip conductor forming the other electrode.

4. A capacitor comprising a flat wound capacitor made from successive windings of a pair of electrically conductive strips interleaved with a pair of electrically insulating strips and a pair of terminals each secured between a respective one of said conductive strips and an insulating strip and projecting laterally from the strips.

5. A capacitor as claimed in claim 4, in which the terminals engage the conductive strips at approximately their mid points.

6. A capacitor as claimed in claim 5 or claim 6, in which the lateral extension of one electrode is secured to one conductor of a foil strip. transmission line which comprises a pair of foil strip conductors separated by a layer of insulation, the other foil strip conductor forming the other electrode.

7. A capacitor discharge circuit comprising a capacitor as claimed in any of claims 4 to 6, a trigger switch, a transmission line comprising an insulating strip and two foil conductor strips located on opposite sides of the insulating strip, the transmission line providing two load terminals and connecting to the capacitor and switch to provide a series circuit from one load terminal through the capacitor and the switch to the other load terminal.

8. A circuit as claimed in claim 7, including a load connected across the load terminals, the load being capable of triggering a chemical explosion upon discharge of the capacitor.

9. A circuit as claimed in claim 8, in which the load is an exploding bridgewire or an exploding foil.