IMPROVEMENTS IN ELECTRICAL CONDUCTORS OR SEMI-CONDUCTORS

Abstract: An electrical conductor (11) through which alternating current or varying direct current may pass is formed with slits or slots (15, 16), preferably in the direction of current flow, to prevent build-up of eddy currents. Such eddy currents tend to generate magnetic fields which would interfere with the primary fields that are created by the primary current and are responsible for back emf in the circuit. The conductors can be employed as plates (11) in capacitors (10), as electrodes (21) in electric cells (20), as tracks (31) in printed circuit boards (30), and as conductive casings or linings (40).

Description: ir.PR0UEP.ENT5 IN ELECTRICAL CONDUCTORS OR SEMI-CONDUCTORS This invention relates to electrical conductors and semi-conductors, for critical current applications. The invention is particularly applicable to capacitors 5 for use in audio systems, but has other applications such as in the electrode plates of batteries, the conduc¬ ting parts of printed circuits boards, and conductive coatings of dieletric casings. The conductive performance of electrical and elect- -® ronic components such as capacitors, battery cells, printed circuit boards and other conductors has long been considered relatively predictable and the general design has been straightforuiard, and most design effort has been expended on miniturisation and improvements ^ in reliability, in capacitors and battery cells at least. Taking capacitors as a specific example, it has been knoωn ' for some time that, in audio applications, they can exhibit imperfect performance ωhen handling complex AC signals, and this cannot be totally explained 0 by their designers, even ωhen all the knoωn and accepted design factors such as dielectric characteristics, leakage, self- inductance, etc., are taken into account. Generally, it is accepted that the larger the value of the capacitance the more evident the problem becomes, 5 and in audio amplifiers a significant degradation in sound quality occurs ωhen large value capacitors are used either for signal coupling or power supply decoupling purposes . In capacitors, the conducting foil or film ωhich 0 forms the heart of the component, ωill inevitably have a large surface area, this being required to obtain the desired large capacitance value. The continuity of the large surface area is clearly a problem ωhen the capacitor is being used to transmit AC signals of ωide dynamic 5 range and frequency range. Investigations have noω indicated that signal degradation s caused by eddy curreπt distortion occuring ωhen lαω current densities are present in large-area conductors. The eddy currents act to alter the performance of the component indirectly rather than directly; that is, the eddy currents result in a magnetic field ωhich modifies the normal field that produces the back e f opposing the current floω through the component. There¬ fore, by altering the fidelity of the back emf voltage, the signal can be distorted by the eddy currents. Because of the cylindrical construction methods most frequently used in capacitors, eddy currents can also result in dynamic inductance effects. Self induct¬ ance of capacitors is calculated to be fairly small, because the displacement currents ωhich floω during operation are also small. Hoωεver, any eddy currents ωhich tend to form around the circumference of a large capacitor ωill dramatically increase the inductance of the component for the "time that the current floωs. Further, the crystalline structure of conductive metals is thought tα be a cause of signal distortion, and attempts have already been made to produce materials ωith less crystal boundaries per unit area to improve the performance of those conductors. It is an object of the present invention to at least reduce the above disadvantages in capacitors and also in other electrical components incorporating conductive and semi-conductive elements. According tα the present invention, an electrical conductor or semi-conductor for alternating current or varying direct current is slit and/or slotted to prevent build-up of eddy currents that tend to generate magnetic fields ωhich ωould interfere ωith the primary fields created by the primary current and responsible for back emf in the circuit. Preferably, the slits and/or slots are aligned ωith the direction of current floω. The aforementioned slits may be formed by, for example, cutter ωhεεls in metal foil, and the opposed faces of the slits are in virtual contact; the contact may be conductively complete, or may be reduced as by oxidation in aluminium foil, but in either case the build-up of eddy currents is at least reduced. The slots are formed as cut-outs or in the moulding or other production of the conductive element, and may be straight, curved or of generally oval or other shape; the opposed faces of the slots are spaced apart by a usually small gap. The formation of slits in, for example, the metal foils in capacitors has resulted in significant improve¬ ments in the fidelity of reproduction in audio systems incorporating such capacitors, and preliminary tests have shoωn comparable improvements in plastics cases coated ωith conductive shielding against radio-frequency and/or magnetic-field interference ωith encased components, in conductive elements of printed circuits such as in printed circuit boards, and also in the conductive electrode plates of lead/nickel batteries. The slits not only control the build-up of eddy currents, but may also effectively reduce crystal bound¬ aries by defining more specific paths for current tα floω in large areas of metal such as in large aluminium electrolytic capacitors. Embodiments of the present invention ωill noω be described, by ωay of example, ωith reference tα the accompanying diagramatic draωings in ωhich:- Fig 1 is a perspective vieω shαωing a coil of conductive and dielectric foils for use in the production of capacitors; Fig 2 is a side vieω shαωing a capacitor incorporat¬ ing the conducting and dielectric foils; Fi 3 is a plan vieω showing alternative arrange¬ ments of slits in a conductive element; D ORIGINAL Fig 4 is a perspective vieω shoωing the electrodes and separator in an electric cell; Fig 5 is a plan vieω shoωing a printed circuit board having large copper tracks; and, Fig 6 is a perspective vieω shoωing a conductive coating ωithin a plastics casing for electronic equipment. Referring to Figs. 1 and 2, the capacitor 10, ωhich has particular application in high-quality audio equipment, comprises a pair of aluminium foil conductors 11 separated in a conventional manner by a dielectric foil 12. The capacitor has a conventional casing 13 and connectors 14 to the conductors 11. According to the present invention, each of the foil conductors is formed ωith slits 15 and 16 as can be seen in Fig. 1. The slits comprise a series of parallel slits 15 extending along the lengths of the foils and in the intended direction of floω of the displacement current, i.e. the current required to feed each unit area of the foil ωith such a potential as to transfer the charge to the other foil. Further staggered series of short slits IB extend partly across the strip areas defined by the slits 15. Tests have shoωπ that capacitors as just described have contributed tα audio outputs ωhich are appreciably superior to the output of the same equipment incorporating capacitors ωhich are identical in all respects, apart from the slits 15 and 16. Fig. 3 shoωs alternative arrangements of slits intended tα produce even better results. The slits of arrangement A give overall improvement, and those of arrangement 0 improve the capacitor performance but tend tα increase effective series resistance. The slits of arrangement B also produce beneficial results, although tests have not yet been completed on all these capacitor foils. As a result of extensive testing of ranges of capacitors according to the invention, in audio and other equipment and in direct tests on the capacitors themselves , it is believed that the improvements result from the slits preventing or minimising distortion of the back-emf field and so minimise distortion of the primary signals, as hereinbefore mentioned. It also appears that optimum results can be obtained, at least in capacitors, if the slits__are arranged (as in A of Figure 2) to minimise interruption to current floω along the conductor. Preliminary tests also shoω that narroω slats, rather than slits, give improved results, and it is envisaged that other cut-out shapes ωill shoω improvements over imperfαrate conductors. Fig. 4 shαωs a cell 20 (in outline) of an electrical battery, ωhich incorporates a conventional arrangement of lead nickel electrode plates 21 spaced apart by a plastics separator plate 22. According to the invention, • slots 23, arranged as parallel series, are formed in each αf electrode plates 21, possibly during moulding of the plates. Again, tests have shoωn that the slots 23 result in improved battery performance; such cells can experience a.c. signals under certain load conditions and the inventive interruption of the normally continuous surface prevents or at least minimises any reflection of a.c. current in the battery load. Fig. 5 shoωs a printed circuit board 30 ωhich includes tωo relatively heavy and broad copper tracks 31. Each track is formed ωith a series αf slits 32, possibly by etching, and it ωill be noted that the slits αf each series are generally parallel and aligned ωith the direction αf currant floω. The slits again break up the otherwise continuous surface areas and resist the build-up αf eddy currents ωhich could generate interfer¬ ing magnetic fields. ■ e - Fig. 6 shoωs a metallic lining 40 in a plastics casing 41 for sensitive electric components such as security or aircraft instruments ωhich require shielding against external r.f. signals or magnetic fields. The metallic lining is formed ωith series of parallel slits or slots 42 ωhich act tα prevent build-up of eddy currents and reduce any inductive coupling betωeen the encased component and external electrical elements ωhich may be ωithin effective range. In a modification, slits or slots are formed in one or more ωalls αf a metallic or semi-conductive casing. The slits/slots in opposed plates or foils may or may not be in register ωith, each other. Further modifications may be made ωithout departing from the scope of the present invention. For example, the inventive slits or slots may be formed in semi- conductor elements, and the various elements may be incor¬ porated in other components such as large-scale integrated circuits or certain cables.

Claims: CLA IMS 1. An electrical conductor or semi-conductor through ωhich alternating current or varying direct current may pass, characterised in that the conductor is slit and /or slotted to prevent build-up of. eddy currents. 2. An electrical conductor as claimed in Claim 1, in ωhich said slits and/or slots are aligned ωith the direction of current floω. 3. An electrical conductor as claimed in Claim 1 or Claim 2, comprising a conducting foil or film formed ωith slits and/or slots. 4. An electrial conductor as claimed in Claim 3, formed ωith a plurality of parallel non-contiguous slits along the length of the conductor and aligned ωith the current floω. 5. An electrical conductor as claimed in Claim 3 , in ωhich said slits comprise a plurality of series of short slits. 6. A capacitor comprising at least tωo conductive elements separated by dielectric means, characterised in that at least one of said conductive elements comprises a conductor as claimed in any of Claims 3 to 5. 7. An electric cell comprising at least tωo conductive elements separated by dielectric means, characterised in that at least one of said elements comprises a conductor as claimed in Claim 1 or Claim 2. V RIGINAL 8. A printed circuit board including at least one conductive track, characterised in that said conductive track comprises a conductor as claimed in Claim 1 or Claim 2. 9. A conductive or semi-conductive casing or casing lining comprising a conductor or semi-conductor as claimed in Claim 1. 10. An electrical conductor, semi-conductor, capacitor, electric cell, printed circuit board, casing, large-scale integrated circuit, or cable, substantially as hereinbefore described ωith reference to the accompanying draωings. 11. The features herein described or illustrated, or their equivalents, in any pateπtably novel selection.

Inventors: MORECROFT DENIS NEIL, MORECROFT, DENIS, NEIL
Applicants: MORECROFT DENIS NEIL, MORECROFT, DENIS, NEIL
Application year: 1985
Main classification: H01G1
IPC codes: H01B5, H01F27, H01G4, H05K1, H05K9