GB2294815A - Non-rewirable electrical plug with noise suppression - Google Patents

Abstract

A mains plug 14 has connected thereto an electrical cable with at least two insulated cores 11 and 12. A radio frequency interference suppressing ferrite annulus 30 is located in the plug 14 and encircles the cable 10 at or adjacent the point of entry of the cable. The first cable core 11 is connected in a first chamber of the plug to one end 32 of a first conductor 27 which is wound several times round the ferrite annulus 30, the other end of that conductor 27 being connected to a first terminal 18 of the plug. The second cable core 12 is connected in a second chamber to one end 31 of a second conductor 26, which also is wound several times around the annulus 30 and has its other end connected to a second terminal 25, 17 of the plug. The plug top is sealed to the base of the plug. <IMAGE>

Description

NON-REWIRABLE ELECTRICAL PLUG This invention concerns a non-rewirable electrical plug, and in particular relates to the combination of a non-rewirable mains plug and an electrical cable connected thereto, which cable has at least two, but possibly three, separately insulated cores.

It is well known that a common cause of electrical breakdown in portable electrical equipment powered from the mains supply is a failure in the connections between a mains plug for connecting into a mains outlet socket, and an electrical cable connected to that plug.

Not infrequently, people may connect the wires incorrectly to the wrong pins of the plug, but more commonly, the standard of workmanship in making the connections is not particularly good, which may lead to short circuits or disconnections within the plug. The clamping of the cable within the plug is also of importance if the connections within the plug are to be reliable, but often the clamping is only inadequately performed.

In an attempt to alleviate the problems associated with a consumer fitting a plug to a mains cable provided as a part of a piece of equipment, many manufacturers are now supplying portable equipment with a plug already connected to the free end of the mains cable. Further to enhance reliability, many such plugs are of the so-called non-rewirable type, where the plug is permanently attached to the end of the cable and can be removed only by cutting the cable adjacent the plug.

Many such non-rewirable plugs have the cable conductors welded or otherwise permanently secured to the electrodes of the plug, and there may be a grommet sleeve moulded around the cable, to relieve strain at the point where the cable leaves the plug body.

Many kinds of electrical equipment generate electrical noise (that is, radio frequency inter ference). Electricai regulations are increasingly insisting that any such noise generated by equipment must not be fed back into the electrical mains supply and manufacturers are having to take steps to ensure that the equipment includes adequate suppress ion of electrical noise. At the same time, such suppression may assist with the isolation of a piece of equipment from any electrical noise which may be present on the mains supply - perhaps originating from some other inadequately-suppressed equipment.

It is known to provide an electrical mains plug including noise-suppression components. Typicaliy, such a plug includes a plug top defining a chamber within which those components are mounted, the mains cable being electrically connected to screw-type terminals provided within the plug top, for the purpose of electrically-connecting to the suppression components. Such an arrangement gives rise to a relatively bulky plug which cannot be arranged as a permanently affixed, non-rewirable plug on the end of the mains cable of an item of portable equipment.

It is an aim of the present invention to address the problem of having a non-rewirable plug permanently attached to the end of a mains cable, which plug is also able to assist with the suppress ion of electrical noise.

According to one aspect of the present invention, there is provided the combination of a non-rewirable mains plug and an electrical cable connected thereto which cable has at least two insulated cores, in which combination there is a ferrite annulus located within the plug and encircling the cable at or adjacent the point of entry of the cable into the plug, a first insulated conductor having one end permanently connected to a first electrode of the plug, the first conductor being wound several times around the ferrite annulus and having its other end located in a first chamber of the plug, a second insulated conductor having one end permanently connected to a second electrode of the plug, the first conductor being wound several times around the ferrite annulus and having its other end located in a second chamber of the plug, the respective other ends of the first and second conductors being permanently attached to the two cores of the cable, within the respective plug chambers.

The present invention stems from the recognition that, in the case of a non-rewirable mains plug, a ferrite annulus having current-carrying windings may be disposed within the plug body with the mains cable itself passing through that annulus, so enabling the achievement of electrical noise suppression within a most compact voiume. Such a configuration would not be acceptable in the case of a consumer rewirable plug, for the majority of consumers would not be able successfully to connect a cable in the proper manner to such a plug.

According to a second aspect of the present invention, there is provided a method of assembling to a non-rewirable mains plug an electrical cable having two insulated cores, which method comprises the steps of: - winding with several turns first and second insulated conductors around a ferrite annulus; - threading an end portion of the cable through the annulus; and then performing the following steps in any order: - permanently connecting one end of the first conductor to a first electrode of the plug; - permanently connecting one end of the second conductor to a second electrode of the plug; - permanently connecting the other end of the first conductor to one of the cores of the cable; - permanently connecting the other end of the second conductor to the other of the cores of the cable; and - locating the said conductor other ends in respective chambers of the plug.

In the arrangement of the present invention, the respective one ends of the first and second conductors may be welded to the respective first and second electrodes of the plug, such as by a spot-welding process. Other essentially permanent connection techniques could be employed, such as crimping, brazing or soldering. In the case of said other ends of the first and second conductors, these may be connected to the respective cores of the cable by means of respective sleeves which surround a conductor and a core and are crimped to connect together the conductor and core, both electrically and mechanically. Again, other essentially permanent connection techniques could be employed. It will be appreciated that in both cases, the object is to achieve an essentially permanent connection which will therefore display high reliability.

It will be appreciated that the term "permanent" as used herein refers to a connection which cannot be removed and remade by a consumer, using conventional domestic tools.

Most preferably, the plug includes a replaceable fuse of an appropriate rating, disposed electrically between a pin of the plug and an internal electrode to which one conductor is permanently connected. In the case of the other conductor, the internal plug electrode to which that conductor is connected may be defined by an internal (of the plug body) part of the pin itself.

The use of a ferrite annulus (or torus) for the purpose of electrical noise suppression is well known and understood. In the present invention, it is convenient for the first conductor to be wound around a first circumferential portion of the annulus surface, and a second conductor to be wound around a second circumferential portion of the annulus surface, spaced from the first portion thereof. In this way, enamelinsulated wire may be employed for the two conductors whilst still maintaining a high degree of isolation between the two conductors.

The first and second chambers of the plug, in which the ends of the two cable cores are respectively located, need not be wholly separate with no access from one to the other. It is sufficient for the walls defining the chambers to be such that when the two core ends have been located in their respective chambers, there is no possibility whatsoever, by virtue of the wall configuration, of the two ends coming into contact. Conveniently, the chambers are on opposite sides of the plug body and once the cable core ends have been located thereon and the plug fully assembled, each end is constrained to stay in its respective chamber.

The mains cable may have a third core, in which case the third core would be permanently connected to a third electrode of the plug. Such a third core is normally employed for earthing purposes.

The plug itself may comprise a moulded plastics material shell, together with a moulded plastics cover, which cover is permanently attached to the shell once the internal wiring has been completed. This permanent attachment may be performed in a variety of ways, such as by gluing, high-frequency welding, thermal welding or the like.

By way of example only, one specific embodiment of non-rewirable plug together with a two-core mains cable arranged in accordance with the present invention will now be described in detail, reference being made to the accompanying drawings, in which: Figure 1 is a side view of the plug and cable combination; Figure 2 is an under plan view of the plug and cable combination; and Figure 3 is a plan view on an enlarged scale of the plug with the cable connected thereto, prior to the permanent attachment of the cover.

The embodiment of plug and cable combination shown in the drawings comprises a cable 10 having two separate insulated cores 11 and 12 within an outer sheath, a cable grommet 13 tightly fitted on to the end portion of the cable 10, and a plug 14 having a plug body 15 and a cover 16. The plug 14 is configured as a conventional UK three-pin 13 amp plug, having a live pin 17, a neutral pin 18 and an earth pin 19. Of course, the principles of the present invention may be applied to other plug configurations.

Both the plug body 15 and cover 16 are separately moulded from a plastics material. The cover 16 has four downwardly-projecting legs 20 which pass through corresponding holes 21 in the plug body when the cover is fitted to the body. The free ends of those legs 20 are then thermally deformed and bonded to the plug body 15, so as thereby to connect in a permanent manner the cover to the body. At the same time, this serves to clamp the grommet 13 in place and frictionally to lock the cable within that grommet.

The plug body 15 defines a rectangular chamber 22 within which is located a fuse carrier 23, accessible from the under-face of the plug. Within the plug body, a spring-metal blade 24 is welded to the upper end of pin 17 and projects into the fuse chamber 22, for connection to a carried fuse. At the other end of the fuse chamber, a further spring-metal blade 25 is adapted for connection to a carried fuse, and also defines a crimp-connector for an enamelled wire 26. A similar crimp-connector is welded to the upper end of pin 18, for a second enamelled wire 27.

As shown in Figure 3, a ferrite annulus 30 is disposed over the end portion of the cable 10 where that cable projects beyond the grommet 13, with the cores 11 and 12 of that cable passing through the annulus 30. The annulus has two separate windings formed from the wires 26 and 27, on opposed sides of the annulus 30. The other ends of the wires 26 and 27 are connected to the cores 12 and 11 respectively, by means of crimped sleeves 31 and 32 respectively, whereby the two windings are in series with the two cores 11 and 12, between the main part of the cable 10 and the respective pins of the plug.

The crimped connections 32 are disposed in respective chambers of the plug, between the outer peripheral wall 33 of the plug and internal mouldings associated with the fuse chamber 22. The cover 16, when fitted to the body 15, closes off the top of those chambers, the bottom thereof being defined by the base wall of the body 15. Moreover, the pins 20 of the cover serve to guide the cores into those chambers, so that when the plug has fully been assembled, there is no risk whatsoever of the uninsulated crimped connections 32 coming into contact with any other conducting part of the plug.

Though it is not critical, the inductance provided by each winding wound on the annulus 30 and in series with the cable cores 11 and 12 should be about 300pH.

The gauge of the wires 26 and 27 should be adequate for the intended duty of the plug, which should be defined by the fuse supported by the carrier 23.

It will be appreciated that the above described configuration of plug and cable permanently attached to the plug allows a most compact arrangement whilst still giving adequate electrical noise isolation. The finished plug need not be significantly bigger in size than a conventional, non-suppressed plug.

Claims (12)

1. The combination of a non-rewirable mains plug and an electrical cable connected thereto which cable has at least two insulated cores, in which combination there is a ferrite annulus located within the plug and encircling the cable at or adjacent the point of entry of the cable into the plug, a first insulated conductor having one end permanently connected to a first electrode of the plug, the first conductor being wound several times around the ferrite annulus and having its other end located in a first chamber of the plug, a second insulated conductor having one end permanently connected to a second electrode of the plug, the first conductor being wound several times around the ferrite annulus and having its other end located in a second chamber of the plug, the respective other ends of the first and second conductors being permanently attached to the two cores of the cable, within the respective plug chambers.

2. A combination as claimed in claim 1, wherein the respective one ends of the first and second conductors are welded to the respective first and second electrodes of the plug.

3. A combination as claimed in claim 1 or claim 2, wherein said other end of each of the first and second conductors is connected to the respective core of the cable by means of a respective sleeve which surrounds the conductor and core and is crimped to connect together both electrically and mechanically the conductor and core.

4. A combination as claimed in any of the preceding claims, wherein the plug includes a replaceable fuse disposed between a pin of the plug and an internal electrode to which one conductor is connected.

5. A combination as claimed in any of the preceding claims, wherein the first conductor is wound around a first circumferential portion of the annulus surface, and the second conductor is wound around a second circumferential portion of the annulus surface spaced from said first portion thereof.

6. A combination as claimed in any of the preceding claims and in which the cable has a third core, wherein the third core is permanently connected to a third electrode of the plug.

7. A combination as claimed in any of the preceding claims, wherein the plug comprises a moulded plastics material shell together with a moulded plastics cover which cover is permanently attached to the shell.

8. A combination as claimed in claim 1 and substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.

9. A method of assembling to a non-rewirable mains plug an electrical cable having two insulated cores, which method comprises the steps of: - winding with several turns first and second insulated conductors around a ferrite annulus; - threading an end portion of the cable through the annulus; and then performing the following steps in any order: - permanently connecting one end of the first conductor to a first electrode of the plug; - permanently connecting one end of the second conductor to a second electrode of the plug; - permanently connecting the other end of the first conductor to one of the cores of the cable; - permanently connecting the other end of the second conductor to the other of the cores of the cable; and - locating the said conductor other ends in respective chambers of the plug.

10. A method as claimed in claim 9, wherein the permanent connecting steps are performed by one of crimping, welding, brazing or soldering.

11. A method as claimed in claim 9 or claim 10 and wherein the plug comprises a plug body and a plug cover, in which method the plug cover is permanently attached to the plug body following completion of the connecting steps.

12. A method as claimed in claim 9, and substantially as hereinbefore described, with reference to the accompanying drawings.