Electromagnetically shielded cable

Abstract: A shield cable comprising a plurality of metal conductors disposed in parallel to each other, each conductor being surrounded by an insulation layer, an induction prevention member provided integrally around each one of said isolated conductors, a drain wire disposed in said induction prevention member to electrically contact said induction prevention member and a covering insulation member provided to cover said induction prevention member. The induction prevention member is made of an electrically conductive resin including vapor phase-growing carbon fiber and graphitized carbon fiber made of said vapor phase-growing carbon fiber, said induction prevention member having a volume resistivity of 10⁻³ to 10⁵ Ω cm. Preferably, there is provided a shield means to cover periphery of said induction prevention member and said drain wire is disposed at the central portion of said shield cable in parallel to said conductors.

Description: This invention relates to an electromagnetic interference prevention cable. More specifically, a high-frequency interference prevention and/or electromagnetic wave induction prevention wire is used for electrical connection of an electronic device such as an audio device and an office automatic device. In conventional electromagnetic and high-frequency circuits, various kinds of shield cables and shield plates have been used in order to prevent malfunctions due to noise produced from such circuits. In the conventional high-frequency interference prevention, a static coupling and an electromagnetic coupling between the wires is interrupted by a shield cable or a shield plate, thereby removing unnecessary oscillation. However, such method requires a highly-technical layout of shield cables and shield plates, and can not actually be achieved easily. In recent years, computer control for electric devices and electric products has remarkably increased. Electronic circuits of such devices have been highly integrated, and current flowing through elements have been microscopic, and there has arisen a problem that malfunctions of the device may occur due to induction between wires of a wiring bundle. On the other hand, the products have become compact and lightweight, and also the space-saving and lightweight design of the wiring has been strongly desired. In order to achieve the space-saving of the wiring, tape-like cables (flat band cables) have been increasingly used. GB-A-2 047 947 discloses a shield flat cable comprising a plurality of parallel metal conductors, each of which are surrounded by an inner insulation layer. The plurality of insulated conductors is then integrally coated by an electrically conductive polymer layer including carbon black. Also coated by the polymer layer is a single bare wire which is parallel to said metal conductors and serves as a drain wire. The whole assembly is provided with a usual outside insulation jacket. The conductive polymer layer is used for intercepting electrical interferences from the metal conductors and transmitting them to the drain wire. EP-A 2 -0279985 discloses an electrically conductive thermoplastic resin composition which is used for shielding cables from electromagnetic interference. This composition comprises a thermoplastic resin as a major component and carbon fiber as a minor component, the fiber comprising no more than 8% by volume of the composition. The composition is prepared by dry mixing the ingredients to room temperature in a suitable vessel, extruding them through a die so as to form a molten stream of masticated resin having the fiber distributed therein and are then formed to a desired article. The thus generated electrically conductive resin has a resistivity between 1 and 500 Ω cm. DE-B-2 139 848 discloses a cable according to the precharacterizing portion of claim 1. The object of the invention is to provide an inter-conductor induction prevention tape cable which is excellent in induction prevention, lightweight, corrosion-resistant, excellent in production efficiency, inexpensive, and space saving. This object is achieved by the characterizing features of claim 1. According to the invention, there is provided an induction prevention tape cable comprising a plurality of parallel conductors which have a rectangular, flat cross-section, which are electrically insulated from one another and an induction prevention member composed of an electrically-conductive resin having a volume resistivity of 10 -3 to 10 5 Ω cm which is provided between any two adjacent ones of the conductors. The induction prevention member is not only provided between any adjacent conductors electrically insulated from one another, but also covers each conductor over the whole or part of the periphery of each conductor. A drain wire is provided in such a manner that the drain wire is disposed in electrical contact partially or entirely with the induction prevention member so as to provide a shield effect against electromagnetic wave. Further embodiments of the invention are indicated in the appended sub-claims. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 to 6 are perspective views of induction prevention tape cables of the present invention, respectively; Figure 7 is a perspective view of a tape cable for comparative purposes; Figure 8 is a view showing a method of measuring an induction prevention effect; Figure 9 is a graph showing an inter-conductor induction prevention effect of the various tape cables of the present invention; Figure 10 is an illustration showing the principles of the operation of a conventional product; and Figure 11 is an illustration showing the principles of the operation of the product of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION The invention will now be described in detail with reference to the drawings. Fig. 1 shows an induction prevention tape cable (hereinafter referred to merely as "cable") E in which an induction prevention member 203 is provided between any adjacent ones of a plurality of conductors 201, each coated with an insulator 202, to isolate the conductors 201 from one another, and a covering insulation member 206 is provided to cover the induction prevention member 203. The induction prevention member 203 is made of an electrically-conductive resin having a volume resistivity of 10 -3 to 10 5 Ω ·cm, and preferably 10 -3 to 10 0 Ωcm. The electrically-conductive resin is obtained by adding an electrical conductivity-imparting material to a matrix resin. This electrical conductivity-imparting material comprises one or more of metal powder, metal particles, metal flakes, metal fiber, electrically-conductive carbon black, graphite powder, PAN-type carbon fiber, pitch-type carbon fiber, vapor phase-growing carbon fiber, and graphitized one of these carbon fibers. According to a procedure for the production of an ordinary tape cable, as the matrix resin, there can be used a thermoplastic resin such as PVC, EVA, EEA, PE, PP, PET and PBT, a paint thereof, an epoxy-type or phenolic-type thermosetting resin, a paint thereof, rubber such as silicone rubber, EPDM, and fluororubber, or ultraviolet curing resin, and a suitable combination of these materials can also be used. For example, for producing the electrically-conductive resin, 20 to 160 parts by weight of graphitized vapor phase-growing fiber, pulverized into a length of 0.1 to 50 um, is added to 100 parts by weight of ethylene vinyl acetate resin constituting the matrix, and these are kneaded into pellet form by a blender such as a pressure kneader, a Henschel mixer and a double-screw mixer, and according to an ordinary procedure, the mixture is extrusion-molded to produce a highly electrically conductive resin having a volume resistivity of 10 -3 to 10 3 Ω · cm. A cable F shown in Fig. 2 differs from the cable E of Fig. 1 in that a metal foil 205 covers the covering insulation member 206 (not shown). A cable G shown in Fig. 3 differs from the cable E of Fig. 1 in that the induction prevention member 203 is also provided on the lower surfaces of the insulated conductors 201 disposed parallel to one another. A cable H shown in Fig. 4 differs from the cable E in Fig. 1 in that the induction prevention member 203 is provided around the entire outer periphery of each conductor 201. A cable I shown in Fig 5 differs from the cable H of Fig. 4 in that a drain wire 204 is disposed between two conductors 201 and is embedded in the induction prevention member 203. The drain wire 204 is composed of a metal conductor such as a single wire, a plurality of wires, a flattened conductor or a flattened square conductor. It is preferred that the drain wire 204 be disposed parallel to the conductor 201 partially (preferably, entirely) in electrical contact with the induction prevention member 203. To obtain a uniform shield effect with respect to each conductor, it is preferred that the drain wire 204 be disposed at the central portion of the cable I. A cable J shown in Fig. 6 differs from the cable I of Fig. 5 in that a metal foil 205 covers the entire periphery of the induction prevention member 203. To improve wear resistance, the covering insulation member (not shown) is provided as in the cable I. In Figs. 2 and 6, the metal foil 205 is a shield layer, and it may be replaced by a metal mesh or a metal braid. In the induction prevention tape cables of the present invention, the induction prevention member composed of the electrically-conductive resin is provided between the adjacent conductors, and therefore the inter-conductor induction within the tape cable can be prevented. More specifically, Figs. 10 and 11 show the principles of operation of a conventional product and a product of the present invention, respectively. In Fig. 10, the induction prevention member 203 is not provided between two conductors 201 and 201. In Fig. 11, the induction prevention member 203 is provided between two conductors 201 and 201. Reference numeral 204 denotes the drain wire connected to the prevention member 203, and reference numeral C denotes an interconductor capacity. Since the induction prevention member is made of the electrically-conductive resin, it is provided between the adjacent conductor with no gap, and the thickness of the cable can be reduced. If the electrical conductivity-imparting material of the electrically-conductive resin is of the carbon type, the cable is lightweight, and excellent corrosion resistance is achieved. By the use of the drain wire 204 electrically connected to the electrically-conductive resin, the connection to the earth can be easily made. Examples With respect to the cables of Figs. 1, 4 and 6, a tinned hard copper material of a flattened square shape (thickness: 0.15 mm; width: 1.5 mm; plating thickness: not less than 1 um) was used as the conductor 201. An enamel paint was coated on each conductor to form thereon the inner insulator 202 having a thickness of 0.05 mm. An electrically-conductive resin, which was composed of EVA and graphitized vapor phase-growing carbon fiber and was adjusted to a volume resistivity of 2 x 10 -1 Ω · cm, was used as the induction prevention member 203. In this manner, the various cables E, H, I and J were prepared. In the cables E and H, a polyester film having a thickness of 0.1 mm was used as the covering insulation member 206, and a Cu foil having a thickness of 0.05 mm was used as the metal foil 205. As Comparative Example shown in Fig. 7, there was prepared a tape cable E' similar in construction to the cable E but having no induction prevention member 203 between adjacent conductors 201. The induction prevention effects of these cables E, H, I, J and E' at frequency f were measured according to a method shown in Fig. 8. In Fig. 8, reference numeral 207 denotes a FET probe, and reference numeral 208 denotes a spectrum analyzer. The induction prevention effect was calculated by the following formula: S = 20 log Vo Vm S: induction prevention effect (dB) Vo: induced voltage (V) of a tape cable without the electrically-conductive resin or without the metal foil. Vm: induced voltage (V) of the tape cables of the Examples and the Comparative Example. These measurement results are shown in Fig. 9. As is clear from Fig. 9, the cables E and H exhibited the inter-conductor induction prevention effect, as compared with the cable E'. With respect to the cable I having the drain wire and the cable J having the drain wire and the metal foil, the effect was markedly improved. As described above, the tape cable of the present invention, having the induction prevention member between the adjacent conductors, has an excellent inter-conductor induction prevention effect, and by the use of the electrically-conductive resin having a volume resistivity of 10 -3 to 10 5 Ω · cm, the thin and compact design can be achieved. If the electrical conductivity-imparting material of the electrically-conductivity resin is of the carbon type, the lightweight design and the corrosion resistance can be enhanced. By the addition of the drain wire and the shield layer, an easy earth connection can be made in addition to the electromagnetic wave shield effect.

Claims: A shield cable comprising: a plurality of conductors (201) having a flat rectangularly shaped cross-section disposed in parallel to each other by a predetermined gap; an insulation layer (202) is provided around an outer periphery of each conductor (201); an induction prevention member (203) made of an electrically conductive resin layer having a volume resistivity of 10 -3 to 10 5 Ωcm provided to cover at least one side wall of said conductors (201) ; and a covering insulation member (206) provided to cover said induction prevention member (203), characterized in that the insulation layer (202) electrically isolates the conductors from each other; and said induction prevention member (203) is disposed at least between the adjacent side walls of each pair of adjacent isolated flat conductors. A shield cable as claimed in claim 1, characterized in that said electrically conductive resin layer includes vapor phase-growing carbon fiber and graphitized carbon fiber made of said vapor phase-growing carbon fiber. A shield cable as claimed in claim 1 or 2, characterized by shield means (205) for shielding said shield cable from electromagnetic interference to cover periphery of said induction prevention member (203). A shield cable as claimed inn any of claims 1 to 3, characterized in that a drain wire (204) is disposed to electrically contact with said induction prevention member (203) at the central portion of said shield cable in parallel to said conductors (201). A shield cable as claimed in any of claims 1 to 4, characterized in that said drain wire (204) is rectangularly shaped in a cross-section. A shield cable as claimed in claim 5, characterized in that the ratio of the width to the thickness of said drain wire (204) is not less than 1. A shield cable as claimed in any of claims 1 to 6, characterized in that said conductors (201) have a rectangularly shaped cross-section with a ratio of width to thickness being not less than 1.

Inventors: IKEGAYA AKIRA, KATSUMATA MAKOTO, USHIJIMA HIROSHI, YAMANASHI HIDENORI, IKEGAYA, AKIRA, KATSUMATA, MAKOTO, USHIJIMA, HIROSHI, YAMANASHI, HIDENORI, IKEGAYA, AKIRA, C/O YAZAKI PARTS CO., LTD., KATSUMATA, MAKOTO, C/O YAZAKI PARTS CO., LTD., USHIJIMA, HIROSHI, C/O YAZAKI PARTS CO., LTD., YAMANASHI, HIDENORI, C/O YAZAKI PARTS CO., LTD.
Applicants: YAZAKI CORP, YAZAKI CORPORATION
Application year: 1991
Main classification: H01B7
IPC codes: H01B7, H01B11