GB2107110A

GB2107110A - Discharge lamps

Info

Publication number: GB2107110A
Application number: GB08224660A
Authority: GB
Inventors: Robert Brian Page
Original assignee: Thorn EMI PLC
Current assignee: Thorn EMI PLC
Priority date: 1981-09-11
Filing date: 1982-08-27
Publication date: 1983-04-20
Also published as: GB2107110B

Abstract

In high pressure discharge lamps such as Xenon lamps the envelope ends are enclosed in end caps providing electrical connection and support. Existing end caps provide excessively rigid electrical connection with insufficient support allowing the risk of the quartz envelope cracking. Electrical connection of the end cap (23, 24) to the leg (3) of the quartz envelope (2) is provided by a braided wire lead (28) opened out to a wire basket disposed around the end (16) of the leg and electrically connected via a molybdenum wire (26) to the usual foil seals (21). This provides a relatively flexible connection and filling the end cap with saureisen cement (38) then provides relatively rigid support. <IMAGE>

Description

SPECIFICATION Discharge lamps This invention relates to high pressure discharge lamps and in particular discharge lamps containing a fill of a rare gas, typically xenon. Such lamps provide solid tungsten electrodes disposed at spaced positions in gas having a pressure of 10-40 atmospheres of 99% pure xenon and in operation an arc burns between the electrodes, fed by an AC or a DC supply. The lamps may be of compact or linear form, a typical compact lamp having an arc length of a few millimetres located at the centre of an approximately spherical bulb.

The electrodes are relatively massive and are carried on tungsten shanks from which electrical leads are taken by suitable lead through arrangements enclosed in end caps, at the bulb extremes. It has been found that the lead through, seal end cap arrangement commonly used is subject to disadvantages which lead to poor electrical contact and thus reduce the life of the lamp. It is thus an object of this invention to provide an improved arrangement.

According to the invention there is provided in a discharge lamp including a quartz envelope, discharge electrodes spaced within the envelope, and one or more foil seals sealed into the quartz to provide electrical connection from a respective electrode to the outside of the envelope; an end cap arrangement including a metal end cap and a braided wire lead electrically connected to the end cap, the wire lead being opened at one end to form a wire basket which is disposed substantially around the end of the quartz envelope and electrically connected to said foil seals.

According to another aspect of the invention there is provided a high pressure discharge lamp having: a quartz envelope with a bulbous centre section and elongated tubular leg parts; a pair of electrodes spaced within the bulbous centre section and mounted on respective electrode shanks extending into the leg parts; molybdenum foil seals at the end of the leg parts carrying electrical connection from the respective shanks external to the envelope; and end cap arrangements enclosing the ends of the respective leg parts and providing electrical connection to the respective foil seals; the end cap arrangements each comprising a metal end cap, a braided wire lead electrically connected at one end to the end cap and opened at the other end to form a wire basket disposed around the end of the respective leg part and electrically connected to the respective foil seal, and substantially rigid filling material substantially filling the remaining space within the end cap to rigidly attach the end cap to the quartz envelope leg part.

In order that the invention may be clearly understood it will now be described by way of example with reference to the accompanying drawings of which: Figure 1 shows a xenon discharge lamp with which the invention, may be used, Figure 2 shows in a section and prior art end cap arrangement for lamp of Figure 1 and Figure 3 shows in section the end cap arrangement of this invention.

The known lamp shown in Figure 1 comprises a quartz envelope 1 having a generally bulbous centre section 2 and elongated tubular leg parts 3 and 4. Inner quartz seal tubes 5 and 6 are disposed inside and hermetically joined to the leg parts 3 and 4 respectively and are enclosed in end cap members 7 and 8. An anode 9 is carried by a tungsten shank 10 and a cathode 11 is carried by a further tungsten shank 12. The shank members 10 and 12 are supported at 13 and 14 as described and claimed in our co-pending application No. 81 23627 and are further supported within the respective inner seal tubes 6 and 5.

Figure 2 shows in more detail the known construction within an end cap such as 7. The outer and inner quartz tubes 3 and 5 are joined at 1 5 and have a mouth part 16 with a central opening 1 7 extending therefrom.

A brass pin 1 8 is surrounded by a braze fill 19 at the point at which a barrel portion 20 extending into the opening 1 7 through the mouth part 16, commences. Molybdenum foil leads 21 extend from the shank 12 (not shown in Figure 2) being sealed in the quartz. The whole is then wrapped with asbestos sheet material 22 and closed with a brass tube 23 and end shield 24. The molybdenum leads 21 make contact with the tube 23 and final electrical connection is then made by copper braid 25 attached to the brass pin 1 8.

Although this arrangement is commonly used it has been found that, perhaps because of the flexibility of the cap arrangement in comparison with the relative rigidity of the brass pin 1 8 in the quartz, the brass pin tends to work about the mouth part 16 portion 20 causing fracture of the quartz tube which can lead to lamp failure.

In Figure 3 there is shown the improved end cap arrangement of this invention suitable for lamp including that shown in Figure 1. As in Figure 2 the outer quartz tube 3 and inner seal 'tube' 5 are joined at 1 5 and extend to a mouth part 16 with a central opening 17. The proportions are slightly different from those shown in Figure 2 but that is not significant. Also in Figure 3 are the tungsten shank 12 and support 14. Similarly the molybdenum foils 21 are set into the quartz as in Figure 2 but in this case are extended by molybdenum leads 26, joined thereto at 27.

The brass pin is dispensed with, however, and connection is by a length of braided wire 28 in which the multiple threads, typically six in number, are opened out to form a basket which is disposed around the end of the quartz tube mouth 1 6. Also disposed around the quartz tube are a brass washer 29 and a fibre washer 30 which serves to prevent the metal washer making contact with the quartz end at 31. The molybdenum leads 26 are threaded through the fibre washer 30 and brass washer 29 and positioned adjacent to the threads of the braided wire basket 28 the assembly being initially held and supported by nickel tape spaced 32 which axially locates the braided wire basket 28 around the quartz tube mouth 1 6. The assembly of the brass washer 29, the copper wire basket ends and the molybdenum leads 26 are then brazed together as indicated at 33.The assembly is again enclosed in an end cap comprising a brass tube 23 and end shield 24 brazed thereto, there being about a 1 mm gap between the tube 23 and the quartz envelope. The remaining spaces therein are then filled through the gap with a ceramic cement, shown at 38, which serves further to support and hold the assembly. The ceramic cement is preferably Saureisen cement but other suitable cements include Fortafix and Autostik. Compared with the prior end cap arrangement the cap is thus supported on the quartz leg in a more rigid manner.

The basket of braided wire 28 provides an axial location for the end cap and a flexible connection which avoids the problems of the previous pin design. The combination thus better preserves the integrity of the electrical connections and reduces the possibility of quartz fracture around the tube mouth 1 6.

The end shield is provided with an internally threaded aperture into which is screwed an externally threaded steel insert 34 having an axial opening 35, inside which the braided wire 28 is brazed. For better electrical connection the braided wire 28 is cut at a slant before brazing therein to expose more surfaces of the individual wires. A suitable electrical connector 36, internally threaded, is screwed onto the insert 34. It has been found useful for the thread on the steel insert 34 to be broken at 37 so that, when it is screwed fully into the shield 24 and the connector 36 is screwed thereon, they are jammed into firm connection.

The end cap arrangement illustrated in Figure 3 is for a 1 kw xenon lamp. However it will be appreciated that the invention is applicable to lamps of other ratings and with other fills for which the problems are similar.

Other embodiments of the invention will be apparent to those skilled in the art.

Claims

1. In a discharge lamp including a quartz envelope, discharge electrodes spaced within the envelope, and one or more molybdenum foil seals sealed into the quartz to provide electrical connection from a respective electrode to the outside of the envelope; and an end cap arrangement including a metal end cap and a braided wire lead electrically connected to the end cap, the wire lead being opened at one end to form a wire basket which is disposed substantially around the end of the quartz envelope and electrically connected to said foil seals.

2. An end cap arrangement according claim 1 in which the end cap is filled with a substantially rigid filling material supporting the end of the quartz envelope therein.

3. An end cap arrangement according to claim 2 in which the filling material is a ceramic cement.

4. An end cap arrangement according to claim 3 in which the filling material is Saureisen cement.

5. An end cap arrangement according to any preceding claim including wire leads of the same material as the foil seals connecting said seals to the braided wire lead.

6. An end cap arrangement according to either claim 4 or claim 5 in which the wire leads are connected to a metal washer and brazed to the ends of the braided wire lead.

7. An end cap arrangement according to any preceding claims in which the end cap comprises a metal tube part and an end shield part.

8. An end cap arrangement according to claim 7 in which the end cap includes a central member having an axial opening within which the braided wire lead is electrically connected.

9. An end cap arrangement according to claim 8 in which the braided wire is cut at an angle to expose individual wires to different lengths and brazed into said axial opening.

10. An end cap arrangement according to claim 8 or claim 9 in which the shield part of the end cap has an internally threaded aperture into which the central member is screwed.

11. An end cap arrangement according to claim 10 including an electrical connector, having an internally threaded aperture, screwed into said central member.

12. An end cap arrangement according to claim 11 in which the central member has an external thread, broken at one point so that when it is fully screwed into the shield part of the end cap and the electrically connector is fully screwed thereon, a firm connection is achieved.

1 3. In a discharge lamp an end cap arrangement substantially as herein described with reference to Figure 3 of the accompanying drawings.

14. A high pressure discharge lamp containing a fill of rare gas and having two spaced tungsten electrodes, electrical connections to the electrodes being by means of respective end cap arrangements each being according to any of the preceding claims.

1 5. A high pressure discharge lamp having: a quartz envelope with a bulbous centre section and elongated tubular leg parts; a pair of electrodes spaced within the bulbous centre section and mounted on respective electrode shanks extending into the leg parts; moiybdenum foil seals at the ends of the leg parts carrying electrical connection from the respective shanks external to the envelope; and end cap arrangements enclosing the ends of the respective leg parts and providing electrical connection to the respective foil seals; the end cap arrangements each comprising a metal end cap, a braided wire lead electrically connected at one end to the end cap and opened at the other end to form a wire basket disposed around the end of the respective leg part and electrically connected to the respective foil seal, and substantially rigid filling material substantially filling the remaining space within the end cap to rigidly attach the end cap to the quartz envelope leg part.

1 6. A lamp according to claim 1 5 in which the wire basket is electrically connected to the molybdenum foil by the intermediary of onebr more molybdenum wires.

17. A lamp according to either claim 1 5 or claim 1 6 in which the filling material is Saureisen cement.

1 8. A high pressure lamp substantially as herein described with reference to Figures 1 and 3 of the accompanying drawings.

19. A high pressure Xenon discharge lamp according to any one of claims 15 to 18.