US3519872A

US3519872A - Thermionic electrode with an auxiliary starting coil for a discharge lamp

Info

Publication number: US3519872A
Application number: US639154A
Authority: US
Inventors: Patrick C Ward
Original assignee: Westinghouse Electric Corp
Current assignee: Philips North America LLC
Priority date: 1967-05-17
Filing date: 1967-05-17
Publication date: 1970-07-07
Anticipated expiration: 1987-07-07

Description

July 7, 1970 P. c. WARD 3,519,372

THERMIONIC ELECTRODE WITH AN AUXILIARY STARTING COIL FOR A nzscmmem LAMP Filed May 17. 1967 FIG 4 INVENTOR Patrick C. Ward ATTORNEY United States Patent 3,519,872 THERMIONIC ELECTRODE WITH AN AUXILIARY STARTING COIL FOR A DISCHARGE LAMP Patrick C. Ward, New York, N.Y., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., :1 corporation of Pennsylvania Filed May 17, 1967, Ser. No. 639,154 Int. Cl. H011 61/06, 61/18, 61/54 US. Cl. 313-211 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention generally relates to mercury-metal halide type arc-discharge devices, and particularly to an improved electrode design for such a lamp.

The so-called mercury-additive high-pressure discharge lamp has become familiar in the commercial lighting field. Such a lamp is described in US. Pat. N0. 3,234,421 dated Feb. 8, 1966. In one form, this type of lamp can use a resistive heating element as part of the structure to initiate the discharge. Such an electrode is taught by Walter J. Decker and Hugh D. Fraser in copending application Ser. No. 379,110 filed June 30, 1964, now Pat. No. 3,356,884, entitled Arc-Discharge Device, and owned by the present assignee. The structure disclosed therein uses a fine wire coiled tungsten heating element overwound about the extremities of two solid tungsten support rods. The fine wire coil tightly fitting about the solid support rod acts therewith as the primary discharge sustaining surface. A coiled thermionic emitter associated with the primary portion of the electrode to facilitate initiating the discharge is taught by Hays et al., Pat. No. 2,263,171, dated Nov. 18, 1941, and also owned by the present assignee.

It is desirable when using such starting devices that the discharge shift quickly, after it is initiated, from the heating portion of the electrode to the more substantial primary discharge sustaining surface for improved lamp stability, and longer life of the starting element. A very heavy coiled tungsten member which may or may not retain a low-work function material is known to provide such a substantial discharge sustaining surface. The disposition and composition of the electrode elements determines this capability of quickly shifting the discharge, once initiated, from the starting element to the more substantial portion of the electrode.

SUMMARY It is an object of this invention to provide an improved arc-discharge device which incorporates a very heavy coiled tungsten operating electrode in conjunction with a fine wire heating element to facilitate starting.

It is another object to provide a discharge lamp construction which will quickly stabilize the discharge on the primary discharge sustaining surface after the heating element initiates the discharge.

The aforesaid objects and others which will become apparent as the description proceeds are achieved providing an arc-discharge device wherein one electrode comprises two elongated tungsten support members of unequal 3,519,872 Patented July 7, 1970 length. One end of each support member is embedded in one seal of the arc tube. The longer tungsten support member has a heavy coiled tungsten member tightly overfitting the inwardly extending extremity of the longer member, forming an operating discharge-sustaining surface. A material having a low-work-function may be retained by the heavy coiled tungsten member and the elongated support member. A projecting arm portion is afiixed to the inwardly extending extremity of this heavy coiled tungsten member. One end of a relatively fine wire coiled-coil tungsten heating member is tightly fitted over this projecting arm portion. The other end of the coiled-coil tungsten heating member is tightly fitted over the shorter elongated support member. The body of the coiled-coil tungsten heating member is thereby supported intermediate the shorter support member and the projecting arm portion of the heavy coiled tungsten member which is the primary discharge sustaining surface.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of this invention, reference should be had to the accompanying drawings, wherein:

FIG. 1 is a sectional elevational view of a preferred lamp embodiment which incorporates the improved electrode of the prevent invention;

FIG. 2 is a view showing the electrode-heater element before incorporation into the arc tube;

FIG. 3 is a sectional elevational view of the arc-tube portion of the lamp as shown in FIG. 1, illustrating in detail the electrodes as sealed into and positioned to the are tube;

FIG. 4 illustrates an alternative embodiment arc tube wherein both electrodes are constructed as shown in FIG. 2;

FIG. 5 is a view of an alternative embodiment of the electrode heater element before incorporation into the arc tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT With specific reference to the form of the invention illustrated in the drawings, the lamp 10 in FIG. 1 generally comprises a radiation-transmitting, sealed inner quartz envelope or are tube 12 having seals at either end thereof, with

electrodes

14 and 16 disposed proximate the arc tube seals and operable to sustain a vapor discharge therebetween. For operation with the power input of 400 watts, the arc tube 12 encloses a volume of 28 cc. A predetermined charge of mercury 18 is included in the arc tube 12 in an amount of 117 milligrams and a predetermined amount of selected additive material such as 5 milligrams of

thallous iodide

20, 26 milligrams of sodium iodide 22, and 15 milligrams of thallium metal 23 are also included within the arc tube 12. Many other combinations of so-called additive metallic halides can also be used, as is known in the art. A small charge of inert ionizable starting gas, such as argon at a pressure of 20-25 millimeters, completes the discharge-sustaining fillmg.

A radiation-transmitting, sealed, outer envelope 24 is spaced from and surrounds the arc tube 12, and electrical lead-in conductors are sealed through both the inner envelope 12 and the outer envelope 24 and serve to electrically connect the

operating electrodes

14 and 16 to a power source. The volume between the outer envelope 24 and the arc tube 12 preferably encloses a hard vacuum, although a nitrogen fill can be used.

The ends of the are tube 12 have provided thereabout metallic end caps or other suitable heat shields 26-. Aluminum silicate fiber or similar material is desirably packed intermediate the end caps 26 and the are tube 12, in order to provide a good heat insulation effect and prevent condensation of the additive material at the ends of the arc tube. The arc tube 12 is supported within the outer envelope 24 by a conventional supporting frame 28. Conventional ribbon seals 30 facilitate making electrical connection through the arc tube.

The electrode 16 is fabricated as an electrode-heating element and the heater element end thereof electrically connects through a switching or bimetal means 32, in order to facilitate starting of the device, as described in detail in the aforementioned Pat. 3,307,069. One end of the bimetal 32 connects through the arc tube frame 28 and the base member 34 to a conventional power source and the other end of the bimetal 32 is permanently connected through an insulated flexible leadin conductor 36 to the electrode 14.

The specified lamp is subject to considerable modification utilizing materials and techniques well known in the art. Lamps which are to operate at greater or less input power will have design parameters effecting such use, and the foregoing detailed specific example is not intended to limit these parameters.

In FIG. 2 is shown the electrode 16 prior to incorporation into the arc tube. The elongated

tungsten support members

40, 42 are also shown, The longer support member 40 has a relatively heavy coiled tungsten member 44 tightly fitting about its extending extremity. Afixed to the extending extremity of the heavy coiled tungsten member 44 is a projecting arm portion 46 generally directed toward but spaced from the shorter elongated member 42. One end 48 of the fine wire tungsten coiled-coil heating element 50 is tightly overfitted on this projecting arm portion 46. The other end 49 of the coiled-coil heating element 50 is tightly overfitted on the shorter elongated member 42. The fine wire coiled-coil heating element 50 is supported between these

members

46, 42. The coiled-coil barrel portion 50 is positioned intermediate the projecting arm 46 and the shorter elongated member 42. In the specific embodiment 12 as shown in FIG. 3, a substantial portion of the coil end 49 is embedded in the seal 38a. If desired, only the solid tungsten member 42 need be embeded in the seal 38a. While the fine wire heating element has been specified as a coiledcoil member another embodiment is for it to be comprised of a simple fine wire coil 60, as illustrated in embodiment 17 in FIG. 5.

The other electrode 14 consists essentially of tungsten and is operatively positioned within the arc tube as shown in embodiment 12 illustrated in FIG. 3, proximate the seal 38. This electrode comprises an elongated tungsten support member 52 and a relatively heavy coiled tungsten member 54. This heavy coiled tungsten member 54 may be wound with space between the turns for retaining a low-work-function material 58 such as thorium oxide. A variety of other emissive materials that can be used are well known in the art.

Lead-in conductors connect to the

electrodes

14 and 16 through the conventional ribbon seals 30. As a specific example of the electrode assembly, the solid tungsten member 40 has a diameter of 30 mils and the solid tungsten member 42 has a diameter of mils. The overwound fine wire tungsten coil 50 is formed of wire having a diameter of 12 mils. The primary winding comprising the coil ends 48- and 49 has an interior coil diameter of 33.0 mils wound with 70 turns per inch. The coiled-coil barrel portion 50 is formed of three secondary winding turns with an interior barrel diameter of 60 mils, wound with 13 turns per inch. After this electrode assembly is sealed into the arc tube, the projecting portions 56 of the hairpin-shaped assembly are cut away.

The electrode element 44 is constructed as an inner coil of 30 mil tungsten, and an outer coil of 30 mil tungsten tightly wound at approximately 28 turns per inch and substantially covering the inner coil. The electrode element 44 may be wound with space between the turns for retaining a low-work-function material 58. While this electrode element 44 comprises two concentrically wound coils of relatively heavy tungsten wire more than just the two can be utilized. The coil is approximately 5 mm. in length. The projecting arm portion 46 is also 30 mil diameter and is affixed to or is part of this outer coil and projects from the turn of the coil which extends furthest toward the other electrode. This type of relatively heavy coiled electrode is described in detail by Rokosz, Pat. No. 3,170,081, dated Feb. 15, 1965. The electrode embodiment 14 has a similar coiled electrode element 54 associated with it. This electrode element 54 is constructed the same as electrode element 44 except there is no projecting arm portion from the outer coil of electrode element 54.

An alternative arc tube embodiment is shown in FIG. 4 wherein both ends of the arm tube 12a incorporate electrode-heating elements 16, as specified hereinbefore. With such a construction, the lamp can be started at still lower voltage by heating at both ends of the arc tube, preliminary to applying the arc-initiating potential between the electrodes. In other respects, this arc tube embodiment is similar to the embodiment as shown in FIG. 3.

When the potential is initially applied to the lamp, the bimetal 32 electrically connects directly to the embedded leg 49 of the heating element and the initial heating potential is applied directly across the electrode. The coiled-coil heating element 50 rapidly heats to an incandescent status to heat the lamp. As soon as the bimetal is heated, it is electrically disconnected from the heating element 50 and as a consequence, tl-ie potential is applied between the

electrodes

14 and 16. Since the projecting arm portion 46 projects farther into the arc tube body portion than the heating element 50, the are quickly switches from the heater element 50 to the more substantial extensive portion of the electrode. In electrode 16 this projecting arm portion 46 is an extension of the farthest projecting turn of the coiled electrode element 44. It is this electrode element 44 which acts as the prime discharge sustaining element after the heating element 50 initiates the discharge.

When an additive lamp as shown in FIG. 1 is operated in conjunction with a conventional ballast and a starting circuit and incorporates the improved electrode structures of the present invention as shown in detail in FIG. 3, the starting voltage averages less than volts. If heater elements are provided at both ends of the arc tube, as in the embodiment shown in FIG. 4, the average starting voltage will be even lower. In either case, the starting voltage is substantially constant throughout the life of the device.

It will be recognized that the objects of the invention have been achieved by providing an improved arc-discharge device in which a fine wire heating element is operatively positioned with respect to the predetermined primary discharge sustaining surface. This determines that the discharge will quickly switch to the more stable electrode portion after the discharge is initiated.

While the best embodiments of the invention have been illustrated and described in detail, it is to be particularly understood that the invention is not limited thereto or thereby.

I claim as my invention:

1. In combination with an arc-discharge device comprising an elongated radiation-transmitting envelope having a seal at each end and containing a predetermined amount of discharge-sustaining filling, spaced electrodes positioned within said envelope proximate each envelope seal, two spaced elongated tungsten support members of unequal length projecting into said envelope from at least one of said seals, and at least one of said electrodes comprising a fine wire coiled tungsten member comprising end portions and a heating element portion supported between said spaced tungsten support members, the improvement which comprises:

(a) a heavy coiled tungsten member tightly fitting over the inwardly extending extremity of said longer 5 tungsten support member and forming an operating discharge-sustaining surface;

(b) a projecting arm portion affixed to the inwardly extending extremity of said heavy coiled tungsten member and generally directed toward but spaced from said shorter tungsten support member; and

(c) one end portion of said fine wire coiled member tightly overfitted on said projecting arm portion, and the other end portion of said fine wire coiled member overfitted on said shorter tungsten support memher to support said fine wire coiled member therebetween in a position where said projecting arm portion extends farther into the arc tube body portion than said heating element portion of said fine wire coiled member.

2. The arc-discharge device as specified in claim 1, wherein a low-work-function substance is retained 'by said heavy coiled tungsten member and said elongated support member.

3, The arc-discharge device as specified in claim 1, wherein said electrodes proximate each seal of said envelope include said fine wire coiled member as a part thereof.

References Cited UNITED STATES PATENTS 2,315,286 3/1943 Hays et a1. 313-33 X 2,524,455 10/ 1950 Macksoud 315-49 2,899,583 8/1959 Macksoud 313-109 3,170,081 2/1965 Rokosz 313-217 X 3,356,884 12/1967 Decker et a1 313-212 3,363,132 1/1968 Olson et a1. 313-198 JAMES W. LAWRENCE, Primary Examiner P. C. DEMEO, Assistant Examiner US. Cl. X.R.