EP0103425A2

EP0103425A2 - Linear filament assembly for incandescent lamps

Info

Publication number: EP0103425A2
Application number: EP83304798A
Authority: EP
Inventors: Anthony Wade Bollon
Original assignee: Thorn EMI PLC
Current assignee: Thorn EMI PLC
Priority date: 1982-09-02
Filing date: 1983-08-19
Publication date: 1984-03-21
Also published as: AU1780883A; EP0103425A3; JPS5960960A

Abstract

In linear filament incandescent lamps, especially low power tungsten halogen lamps of above 100 volts, support of the filament coil is an important problem. This filament assembly for such a lamp supports the coil (1) by supports (2) extending from a rigid rod (5) parallel thereto. To support the ends of the coil it is formed with gaps (4) therein and support member (16) fixed to the ends of the rods enter the coil at the gaps to run axially along tail portion (15) of the coil and to be welded with the coil to the sealing foils (7) for the pinch seals.

Description

The present invention relates to the support of elongated filaments in incandescent lamps, and more especially in linear, halogen regenerative-cycle lamps.
Elongated incandescent filaments operating above 100V require support along their length. In linear tungsten halogen lamps it has been the practice to support a coiled filament at intervals along the length of the lamp by spaced spirals of refractory metal, usually tungsten. These spirals fit loosely in the lamp tube and terminate in a coil which is secured to or otherwise meshes with the filament coil over two or three turns.
The ends of the lamp envelope are sealed in conventional manner with pinch seals having molybdenum foil strips by which the electrical supply passes through the seals. These are welded to leadwires which are themselves screwed onto the ends of the coiled filament.
In an improved lamp shown in Figure 1 the elongated coiled filament 1 is supported at intervals by support wires 2 substantially rigidly mounted in the lamp envelope 3 independently of the filament. In the preferred embodiment each wire terminates in a coil 4 of two or more turns having a pitch and diameter such that the coil supports the filament without meshing therewith. Such a supporting coil arrangement is disclosed and claimed in our European Patent Application No.80 301645 (publication No.0020075).
The rigid mounting of the support wires is preferably achieved by securing the wires to a rigid insulating rod, shown at 5 in Figure 1, extending alongside or generally parallel to the filament 1. This rod may be made from high silica content glass (for example that identified by the Registered Trade Mark VYCOR), high melting point aluminosilicate or borosilicate glass or possibly an insulating ceramic but quartz is preferred for a tungsten halogen lamp. The supports 2 are preferably introduced into the insulating rod 5 by the method, of heating the quartz or glass and pushing in the wire supports, disclosed and claimed in our European Patent Application No.81 300412 but the method described and claimed in British Patent Application No.2064215A may also be used.
As mentioned the ends of the lamp are sealed by pinch seals 6, incorporating molybdenum foils 7, generally with ceramic caps 8. The rod 5 is supported by further supports 9 attached to the rod in the same manner as supports 2 and welded to foils 7. The ends of the filament 1 are supported by solid plugs 10, also welded to foils 7 and onto the end of which the filament 1 is held by interference fit (at 11).
This form of construction is not entirely satisfactory for some of the lower wattage linear tungsten halogen lamps as for these the filament may be so fine that it is impractical to weld or hot press it to connect it to the plug.
It is an object of the invention to provide an improved filament to foil connection for lower wattage lamps and an improved filament assembly and lamp incorporating that connection.
According to the invention there is provided a linear filament assembly for an incandescent lamp, the assembly including a generally linear coiled or coiled coil filament having a central coil portion and two tail coil portions each separated from the central portion by a respective gap in the coil or secondary coil, a rigid refractory insulating rod extending generally parallel to the filament, a plurality of supports extending between the rod and the filament at spaced positions to support the central portion of the filament coil relative to the rod and two support members, each secured to the rod at respective ends thereof, each support member including a part extending along the axis of the filament coil and engaging the respective tail coil portion through the bore thereof to support the ends of and tension the filament coil.
If the coil portion is a single coil then the gaps are sections of uncoiled filament wire.
If the coil portion is a coiled coil then the gaps are single coil sections.
The invention embraces a linear filament incandescent lamp using such a filament assembly and is particularly applicable to halogen regenerative-cycle lamps.
In order that the invention may be clearly understood and readily carried into effect it will now be described by way of example with reference to the accompanying drawings of which:

Figure 1 shows a prior art linear filament lamp of which the significance has been discussed hereinbefore;
Figure 2 shows a detail of a prior art filament support illustrating two different forms;
Figure 3 shows a filament assembly in accordance with this invention, and
Figure 4 shows an alternative form of the assembly of Figure 3.

There is shown in Figure 2 a filament 1 for a linear incandescent lamp supported by a quartz backbone rod 5 by supports 2 and electrically connected to foils 7 in the prior art manner. The left hand end of the filament 1 as illustrated is in fact supported and connected to the foil 7 in essentially the manner shown in Figure 1 except that end support 9 is introduced axially into rod 5 in an acceptable alternative form. The right hand end has plug 10 replaced by a wire 12, which like the support wires 2 and 9 may be of material such as tungsten, molybdenum or tantalum. This wire is formed with a coil part 13 which is screwed into and enmeshed with the coil of the filament 1 and subsequently welded or hot pressed to foil 7.
It will be apparent that these two constructions will be difficult to achieve satisfactorily with very fine filament wires. Furthermore the lit length,Q , of the filament, is defined by the extent to which the plug 10 or coil 13 are screwed into the filament and it is difficult accurately and reproducibly to define this length.
To improve this construction it is proposed to use a construction of which one form is shown in Figure 3. In this construction the filament 1, is a conventional single coil or coiled coil filament supported in the manner of the said two European patent applications on a quartz backbone rod 5. At each extreme of length ( the filament coil is, however, formed with discontinuity which may be called a a gap or "lazy" turn 14. This ensures that the desired lit length is i and as such is defined with the accuracy to which the position of the lazy turn can be determined. It should be clear that the filament wire (or the primary coil of a coiled coil) is continuous and the gap is in the coil (or secondary coil).
Outward of the lazy turns 14 the coiled coil or single coil construction resumes with filament tail coils 15. The quartz rod 5 is supported at its ends by end supports 16, in this example introduced axially into the rod. These supports are cranked to provide a part 17 which in each case is on the axis of the filament support loops 4 and therefore with the filament. The filament tail coils 15 outside lazy turns 14 are then disposed around and co-axial with parts 17 and the two are welded to foils 7. In this manner the filament and rod (5) are supported by supports 16 and the join to the foil 7 is by a single weld at each end which is advantageous. The tails 15 do not light because of the substantial conductive core provided by part 17 of support 16 so that the lit length is solely the well defined length ℓ .
In construction the filament 1 is first threaded through the support loops 4 and one filament tail 15 is located on the part 17 of one support 16 through the gap of lazy turn 14 so that the support engages through the bore of the tail. The filament is then tensioned appropriately to engage the other support 16 with the other tail 15 in the same manner and the sub-assembly welded to the molybdenum sealing foils 7. It will be apparent that the form of construction thus provides a rigidly mounted filament sub-assembly prior to welding to the foils which firmly holds the thin low wattage filament for medium and high voltage lamps (100-250 volts). The construction also allows the filament in such an assembly to be pre-set to the required tension to prevent filament sag throughout lamp life as determined by the crank length.
Figure 4 shows essentially the same form of construction as that of Figure 3 except that the supports 16 are simple 'L' shaped members which are introduced into the side of the backbone rod 5 in the same manner as the support 2.
The constructions shown in Figures 3 and 4 do not illustrate the lamp envelopes or seals but it will be appreciated that these will typically be as shown in Figure 1.
In a typical practical lamp the filament is of 0.0427mm diameter wire coiled on itself to form a primary coil of 0.177mm diameter and coiled on itself again to form a coiled coil of which the secondary coil is of 0.742mm diameter.
It will be appreciated that the use of this invention is not limited to the low wattage lamps for which it is particularly advantageous. It may be used with other linear filament incandescent lamps of both double and single ended construction and may use materials and constructions of other parts which are known for or which may transpire to be suitable for such lamps.

Claims

1. A linear filament assembly for an incandescent lamp, the assembly including a generally linear coiled or coiled coil filament having a central coil portion and two tail coil portions each separated from the central portion by a respective gap in the coil or secondary coil, a rigid refractory insulating rod extending generally parallel to the filament, a plurality of supports extending between the rod and the filament at spaced positions to support the central portion of the filament coil relative to the rod and two support members, each secured to the rod at respective ends thereof, each support member including a part extending along the axis of the filament coil and engaging the respective tail coil portion through the bore thereof to support the ends of and tension the filament coil.

2. A filament assembly according to Claim 1 in which the filament is a single coil and the gaps comprise sections of uncoiled filament wire.

3. A filament assembly according to Claim 1 in which the filament is a coiled coil and the gaps comprise sections of single coil.

4. A filament assembly according to any of the preceding Claims in which the support members are cranked members fixed axially into respective ends of the rod.

5. A filament assembly according to any of Claims 1-3 in which the support members are "L"-shaped members fixed substantially radially into the rod substantially at the ends thereof.

6. A filament assembly according to any preceding claim including two foil sealing members, each welded to one of said tail portions and the respective support member engaged therein.

7. A linear filament incandescent lamp including: a linear filament assembly according to any of the preceding claims; a tubular light transmitting envelope closed at each end; and at least one closure being a pinch seal through which electrical connection is made to the filament assembly wherein the pinch seal includes a foil sealing member welded to one of said tail portions and to the respective support member engaged therein.

8. A lamp according to Claim 7 having a pinch seal at each end of the envelope each including a foil sealing member welded to a respective tail portion and to the respective support member engaged therein.

9. A halogen regenerative cycle incandescent lamp in accordance with either Claim 7 or Claim 8 in which the envelope is of high melting point glass.

10. A method of assembling a linear filament incandescent lamp, the method including:

forming a generally linear coiled or coiled coil filament having a central coil portion and two tail coil positions each separated from the central portion by a respective gap in the coil or secondary coil;

securing to a rigid refractory insulating rod at a plurality of spaced positons therealong, a plurality of supports each terminating in a support loop with the loops lying axially substantially on a line parallel with the rod and securing to each end of the rod a respective support member, each support member having a linear part which is disposed on the axis of said loops;

threading the filament through said loops,

locating one filament tail portion on one said linear part through the respective gap;

tensioning the filament; and

engaging the other said linear part in the other tail portion through the respective gap.

11. A method according to Claim 11 including the further step of welding each linear part and the tail portion through which it is engaged to a respective sealing foil member.