Filter assemblies

Abstract: A filtered connector 1 has an outer metal shell 13 within which extend several electrical contacts 26 to 28. Primary capacitors 30 to 32 of tubular shape have their inner plates electrically connected to the contacts. The primary capacitors extend through holes in a metal board 33 to which their outer plates are electrically connected. The board 33 is insulated from the outer shell 13 of the connector apart from a single secondary capacitor 40 connected between the board and the shell. The secondary capacitor 40 is larger than the primary capacitors 30 to 32, so that it is capable of withstanding higher voltages, but it has a lower value of capacitance.

Description: This invention relates to filter assemblies of the kind for filtering high frequency signals, the assembly including a plurality of electrical signal lines and a plurality of primary capacitors one associated with each line. The invention is more particularly concerned with filtered connectors, couplings, feedthroughs or the like. Filtered connectors are used to reduce high frequency interference signals on electrical paths through the connector. The connectors are often used to make connection to electrical apparatus within a housing, so as to minimize the amount of external electrical interference that passes along the electrical path into the housing and to minimize the amount of interference generated within the housing that is transmitted externally of the housing. The construction of filtered connectors usually comprises an external metal shell secured to the housing and adapted to mate with a shell of a cooperating connector. A filter element is connected between the shell and each contact or wire within the filtered connector. The filter element acts as a shunt path for high frequency signals on the contact or wire so that these pass to earth via the shell. The filter element may be a single component or a combination of components such as capacitors and ferrite inductors. The filter element may be a simple capacitive filter, a PI filter, a T filter or a combination of these. It may also be necessary for the connector to include a transient voltage suppressor to reduce the effect of lightning strikes. An example of a filtered connector is described in GB 2233511A. In some applications, the connector may have a large number of contacts each of which has to be filtered. Because space within the connectors is limited, the size of the filter assembly that can be used is also limited. This creates a particular problem with contacts or lines in the connector required to carry a high dc voltage, because capacitors with a high capacitance and capable of handling a high voltage have a large size. In order to reduce the size sufficiently for the capacitors to be incorporated in a connector, it is usual to use capacitors with a lower capacitance. This has the disadvantage that it reduces the effectiveness of the filtering. It is an object of the present invention to provide an improved filter assembly. A filter assembly according to the preamble of claim 1 is know from GB-A-22 33 511. According to one aspect of the present invention there is provided a filter assembly as defined in claim 1. In this way, the assembly need have only one relatively large, secondary capacitor however many lines are filtered, but can still provide effective filtering. The primary capacitors preferably each have a larger capacitance than the secondary capacitor and may be 30 times that of the secondary capacitor. The primary capacitors may be tubular capacitors embracing respective signal lines. The primary capacitors are preferably connected between the signal lines and a conductive board extending transversely of the signal lines and the tubular capacitors may be mounted in holes in the conductive board with the secondary capacitor connected between the conductive board and ground. According to another aspect of the present invention as defined in claim 8 there is provided an electrical connector including a filter assembly according to the above one aspect of the invention. The connector preferably includes an outer metal shell, the primary capacitors being connected between electrical contacts in the connector and a conductive member, the secondary capacitor being connected between the conductive member and the outer metal shell. According to a further aspect of the present invention as defined in claim 10 there is provided a system including an apparatus and a filter assembly according to the above one aspect of the invention the apparatus being connected to a common point between the primary capacitors and the secondary capacitor. A system including a connector and filter assembly according to the present invention, will now be described, by way of example, with reference to the accompanying drawing, which is a sectional side elevation of the system. The filter assembly is in the form of a female filtered electrical connector 1 mounted on the housing 2 of electrical apparatus 3, the housing being at ground potential. The connector 1 is adapted to mate with a cooperating male connector 4 mounted at one end of a screened electrical cable 5. For simplicity, the cable 5 is shown as having only three wires 6 to 8 although, in general, the cable would have more than three wires. Each wire 6 to 8 is connected to a respective pin 16 to 18 in the connector 4, the pins being supported in an insulating insert 9 within an outer metal shell 10. The shell 10 supports a rotatable coupling nut 11. The nut 11 is internally threaded at its right-hand end for engagement with an external thread 12 around the left-hand end of an outer metal shell 13 of the female filtered connector 1. As its right-hand end, the shell 13 of the female filtered connector 1 has an external flange 14 making electrical connection with the inside of the housing 2 via a conductive gasket 15. The flange 14 is urged against the gasket 15 by means of a threaded ring 16 on the connector, which is tightened against the outside of the housing 2. The female connector 1 contains three electrical signal lines in the form of sockets 26, 27 and 28 in alignment with the pins 16, 17 and 18 respectively in the male connector 4 so that, when the two connectors are mated, the pins extend within the sockets and make electrical connection. Three wires 26' to 28' connect the apparatus 3 to respective ones of the sockets 26 to 28. The sockets 26 to 28 are supported in an insulative insert 29 and each socket supports a primary tubular ceramic capacitor 30 to 32 respectively. The primary capacitors 30 to 32 are of small size, typically about 15mm long and 6mm in diameter with a capacitance of about 1.5nF and a maximum operating voltage of 50 volts. The capacitors 30 to 32 have one plate on their inner surface soldered to respective ones of the sockets 26 to 28 and a second plate on their outer surface soldered to the periphery of respective holes in a conductive, metal board 33 extending transversely of the sockets 26 to 28, parallel and to the right of the insert 29. The board 33 is electrically isolated from direct contact with the shell 13 of the connector by a gap 34 around its outer edge. The board 33, however, is electrically connected to the shell 13 via a secondary capacitor 40. The secondary capacitor 40 is larger in size than the other capacitors 30 to 32 and has a lower capacitance. Typically the capacitance of the secondary capacitor 40 is above about 15pF, such as 50pF, making the capacitance of the primary capacitors 30 to 32 larger by a factor of about 30. The size of the secondary capacitor 40 also enables it to survive much higher voltages, typically up to about 3.2kV. The board 33 is a common point joining the low voltage capacitors 30 to 32 and this provides a reference point for the apparatus 3, being connected to the apparatus via line 41. The unwanted high frequency common mode and differential mode voltages on lines 6 to 8 are attenuated by the filter assembly 30 to 32, 40 to this reference point. By measuring attenuation of the unwanted high frequency signals between the lines and this reference point, the attenuation of the unwanted signals remains the same, irrespective of the value of the high voltage capacitor 40. This enables the value of the high voltage secondary capacitor 40 to be less than the low voltage primary capacitors 30 to 32, thereby keeping the size of the capacitor to a minimum. The filters described above are simple capacitor filters but alternative filters including a capacitor could be used. The filters could be connected to the electrical lines in other ways, instead of by tubular capacitors embracing a socket. The filter assembly need not be a connector but could, for example, be a feedthrough assembly for the housing of apparatus.

Claims: A filter assembly for filtering high frequency signals, the assembly including a plurality of electrical signal lines (26 to 28) and a plurality of primary capacitors (30 to 32) one associated with each line such that each primary capacitor has a plate connected with a respective one of said lines, characterised in that the assembly includes a single secondary capacitor (40), said secondary capacitor being compatible with higher voltages than the primary capacitors, one plate of said secondary capacitor (40) being connected to ground, and a second plate of said secondary capacitor (40) being connected to another plate of each of said primary capacitors (30 to 32). A filter assembly according to Claim 1, characterised in that the primary capacitors (30 to 32) each have a larger capacitance than the secondary capacitor (40). A filter assembly according to Claim 2, characterised in that the capacitance of the primary capacitors (30 to 32) is about 30 times that of the secondary capacitor (40). A filter assembly according to any one of the preceding claims, characterised in that the primary capacitors are tubular capacitors (30 to 32) embracing respective signal lines (26 to 28). A filter assembly according to any one of the preceding claims, characterised in that the primary capacitors (30 to 32) are connected between the signal lines (26 to 28) and a conductive board (33) extending transversely of the signal lines. A filter assembly according to Claims 4 and 5, characterised in that the tubular capacitors (30 to 32) are mounted in holes in the conductive board (33). A filter assembly according to Claim 5 or 6, characterised in that the secondary capacitor (40) is connected between the conductive board (33) and ground. An electrical connector (1) including a filter assembly (30 to 32, 40) according to any one of the preceding claims. A connector according to Claim 8, characterised in that the connector (1) includes an outer metal shell (13), that the primary capacitors (30 to 32) are connected between electrical contacts (26 to 28) in the connector and a conductive member (33), and that the secondary capacitor (40) is connected between the conductive member (33) and the outer metal shell (13). A system including an apparatus and a filter assembly according to any one of Claims 1 to 7, characterised in that the apparatus (3) is connected to a common point between the primary capacitors (30 to 32) and the secondary capacitor (40).

Inventors: BRYANT ANEURIN WAYNE, STEPHENS JOHN, BRYANT, ANEURIN WAYNE, STEPHENS, JOHN
Applicants: SMITHS INDUSTRIES PLC, SMITHS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1995
Main classification: H03H7
IPC codes: H01R13