GB2184293A - Screening enclosures for electronic circuits - Google Patents

Abstract

In an electrical apparatus of the kind in which electronic circuits are housed within a screening enclosure (1) and the leads entering the enclosure for external connection of the apparatus each include a filter element (15), e.g. to provide a barrier to interference picked up by external leads, the filter elements (15) are mounted in an internal wall (9) of the enclosure between the circuits and an external wall (3) carrying a connector (5) adapted to mate with a complementary connector to establish the external connection of the apparatus. By separating the filter elements (15) from the connector (5), the risk of damage to the filter elements when making connection to the apparatus is avoided, and the design of the filter elements is facilitated. <IMAGE>

Description

SPECIFICATION Electrical apparatus This invention relates to electrical apparatus.

More particularly the invention relates to electrical apparatus ofthe kind comprising an enclosure housing electronic circuits.

Where the enclosure is required to provide not merelya housing forthe circuits, butalsoacleanen- vironment for the circuits both in respectofatmos- pheric and electrical conditions the enclosure is re quired, to a greater or lesser degree, to be sealed against the ambient atmosphere, and will also con sist of a suitable material such as to provide electrical screening. It is also, however, sometimes also neces sarytoincludeinatleastsomeoftheleadswhereby the circuits are connected with equipment external oftheenclosure, electric filters to prevent electrical interference signals picked up by external leads from reaching the circuits.Such interference may arise, for example, when the apparatus passes through an area in which a high electric field gradient exists, e.g.

when a vehicle carrying the equipment passes near to a high power radio transmitter or other source of high power oscillatory or switched currents.

In known such arrangement, the filters are incorporated in connectors provided in the wall of the enclosure to allow electrical connections to be made between the circuits within the enclosure and external equipment. Typically a filter is incorporated in each individual pin or socket element of a plug or socket connector mounted on the enclosure wall and adapted to mate with a complementary connector.

Such an arrangement has the disadvantages that the filters are liableto be damaged when the connector of which they form part is coupled or uncoupled from its complementary connector. Furthermore such arrangements either tend to be rather bulky or the performance ofthe filters is severely curtailed due to space limitations.

It is an object of the present invention to provide an electric apparatus wherein these difficulties are overcome.

According to the present invention there is provided an electric apparatus comprising: an enclosure housing electronic circuits; connector means mounted on an external wall ofthe enclosure and adapted to mate with complementary connector means toes- tablish electrical connections between said circuits and equipment external ofthe enclosure; and filter means mounted on an internal wall ofthe enclosure disposed between said circuits and said connector means, said filter means comprising an electrically conductive planar structure having a plurality of apertures extending therethrough, each said aperture housing an individual electricfilter connec- ted in a respective one of a plurality of leads extending between said connector means and said electric circuits.

One electric apparatus in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic perspective view of part ofthe apparatus, partly broken away; Figure 2 is a sectional view of one of a number of filter elements incorporated in the apparatus; Figure3 is a circuitdiagram ofthefilterelementof Figure 2.

Referring to Figure 1,the apparatus comprises a rectangular enclosure 1 made of sheet metal.

In an aperture in one external end wall 3 ofthe enclosure there is mounted a multi-way plug connector 5 whereby connections may be established through the enclosure wall between electronic circuits housedwithin the enclosure and equipmentexternal of the enclosure. To this end eachwaythroughthe connectorterminates at its end outside the enclosure in a pin 7 adapted to mate with a socket of a multiway socket connector (not shown) connected to the ends of leads from the external equipment. At its inner end each way through the connector5ter- minatesin a lead wire (notvisible)adaptedforconnection by soldering to the end of an individual wire ortheconductorofa flexible printed circuit cable.

Inside the enclosure 1 there is an internal wall 9 which forms one end of a sealed compartment 11 within the enclosure which houses the electronic cir- cuits (not shown) of the apparatus. Over an aperture in the internal wall 9 there is mounted a plate 13 through which extend a plurality of circular holes disposed in an arraysimilarto the array of pins 7 in the connector 5.

The plate 13 is required to be electrically conduct- ive and may thus consist of electrically conductive material or comprise an insulating material, e.g. a plastics material, provided with an electrically conductive coating.

Each hole in the plate 13 contains an electricfilter element 15, further described below, each filter element 13 having two leads, one on each side ofthe plate 13. The plate 13forms a common ground connection to each filter element 15, each filter element thus comprising afourterminal network.

The filter leads 17 on the side ofthe plate 13 remote from the electronic components are adapted for connection by soldering to the ends of individual wires, or the ends of conductors of a flexible printed circuit cable, whose other ends are connected to the connector 5. Two such connections in the form ofwires 18 are shown in Figure 1.

The filter leads on the side of the plate 13 nearer the electronic circuits (notvisible) are adapted for connection by soldering to leads (not shown) to and from the electronic circuits. The leads may for example be individual wire or tracks of a flexible printed ci rcuit strip.

Each pin 7 of th e co n necto r 5 is th us co n n ected to theelectroniccircuitsinthecompartmentll via a lead including a respective one of the filter elements 15. The filter elements 15 are of course designed so asto preventinterferencesignals picked up by leads external ofthe enclosure from reaching the electronic circuits in compartment 11.

Itwill be appreciated that the filter elements 15 are entirely free of the risk of being damaged when the plug 5 is coupled to and uncoupled from its associated socket. Furthermore the internal wall 9 together with the plate 13 and associated filter elements 15 may be constructed so as to provide a more satisfac tory seal than the end wall 3 and connector5,thus facilitating the provision of a clean atmosphere for the electronic circuits. Moreover, since the filter elements 15 serve solely as filters and the filter elements and plate 13 do not also serve as parts of a connector, the filter elements and plate can be designed to form a compact assembly with the plate providing good electrical screening between the filter elements.A compact arrangement having improved electrical performance compared with the above mentioned prior art arrangements, particularly in respect of attenuation/frequency characteristics ofthe filter elements, is thus provided.

Thefilterelements 15 are required to be of a suit ablycompactformsoastofitwhollywithintheholes in the plate 13, one suitable form forthe filter elements being illustrated in Figures 2 and 3.

As indicated in Figure 3, each element is ofform comprising a series inductor element 11 and two shunt capacitor elements 21 and 23.

Referring now to Figure 2, the inductor element 19 includes a helical metallic coating 25formed on the curved surface of a cylindrical former 27 offerrite material. The former 27 fits within a ferrite sleeve member 29 and two cup-shaped metal ferrules 31 and 33 fit respectively over the two ends ofthe ferrite sleeve 29. Around the ferrules 31 and 33 and in the space between the inner ends of the ferrules 31 and 33 there is a coating 35 of dielectric material which in turn is surrounded by an outertubular metal casing 37, the casing 37 and coating 35 extending at their ends partly overthe flat ends of the ferrules 31 and 33.

Two lead wires 39 and 41 forthefilter element fit respectively into holes extending axially into the ferrite former 27, one from each end, each lead wire 39 or41 being electrically connected by soldering to the adjacent end of the helical metal coating 25 and the adjacent metal ferrule 31 or33. At each end of the filterelementthe remaining space between the lead wire39 or41 and the coating 35 and casing 37 isfilled with a suitable electrically insulating sealant material 43.

Each filter element 15 is suitably mounted in the plate 13 by soldering to a tubular metal mounting plate 45 provided in each hole in the plate.

Itwill be appreciated that the coating 25 with the ferrite former 27 and sleeve 29 forms the inductor element 19 ofthe filter element whilst the two ferrules 31 and 33 respectivelyform one electrode of each ofthe capacitors 21 and 23, the casing 37 forms the grounded electrodes ofthetwo capacitors 21 and 23, and the coating 35 the dielectrics of the capacitors 21 and 23.

If desired, a solder bucket may be provided on one or both ofthe filter element leads 39 and 41 as illustrated at47 in Figure 2.

Typically each filter element 13 has a diameter of 5 millmetres and a length of 7 millimetres.

Itwill beappreciatedthatfilterelementsofdif- fernntformmay be used in otherarrangements in accordance with the invention. For example, the fil terelements may be of rectangular instead of the cir cularcross-section,theholesintheplate 13 then of course being of corresponding cross-section. This can facilitate obtaining a good seal between the filter elements and the plate 13.

Itwill be appreciated that,whilst in the particular apparatus described above byway of example the filter elements 15 are of n form, in other apparatus in accordance with the invention the filters may be of T form or comprise multiple elements of the T or# form.

Claims (5)

1. An electrical apparatus comprising: an enclosure housing electronic circuits; connector means mounted on an external wall ofthe enclosure and adapted to mate with complementary connector means to establish electrical connections between said circuits and equipment external of the enclosure; and filter means mounted on an internal wall of the enclosure disposed between said circuits and said connector means, said filter means com prising an electrically conductive planarstructure having a plurality of apertures extending therethrough, each said aperture housing an individual el ectriofilterconnected in a respective one of a plurality of leads extending between said connector means and said electric circuits.

2. An apparatus according to Claim 1 wherein said internal wall forms one end of a sealed compartmentwithin the enclosure, within which compartmentsaid circuits are housed.

3. An apparatus according to Claim 1 or Claim 2 wherein each said individual electricfiltercomprises a fourterminal network, and said planarstructure constitutes a common connection to all said individual electricfilters.

4. An apparatus according to any one ofthe preceding claims wherein each said individual electric filterfits substantially wholly within the associated said aperture so that said planar structure forms an electrical screen around each said individual electric filter.

5. An electrical apparatus substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.