US3226597A

US3226597A - High pressure metal vapor discharge lamp

Info

Publication number: US3226597A
Application number: US306476A
Authority: US
Inventors: Alwin C Green
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1963-09-04
Filing date: 1963-09-04
Publication date: 1965-12-28
Anticipated expiration: 1982-12-28
Also published as: DE1217496B; NL146327B; DE1217496C2; SE303156B; GB1070374A; NL6410100A

Description

Dec. 28, 1965 A. c. GREEN 3,226,597

HIGH PRESSURE METAL VAPOR DISCHARGE LAMP Filed Sept. 4, 1963 lnveen tor: ALwin C. Green 8 His Air-L- w-nel United States Patent 3,226,597 HIGH PRESSURE METAL VAPOR DISCHARGE LAMP Alwin C. Green, Hiram, Ohio, assiguor to General lectric Company, a corporation of New York Filed Sept. 4, 1963, Ser. No. 306,476 Claims. (Ci. 31560) This invention relates to metallic vapor lamps using an arc discharge in mercury and metal halide vapors to produce visible light, and is more particularly concerned with features having to do with the starting thereof.

The mercury arc lamp has achieved commercial acceptance by virtue of its long life and reasonably good efficiency in the range of 50 to 60 lumens per arc watt. However, it suffers the disadvantage of relatively poor color rendition due to the bluish-green quality of its light, and also its efficiency is appreciably below the 70 to 80 lumens per arc watt range of the ordinary fluorescent lamp. A radical improvement in both color rendition and efficiency may be achieved by adding to the mercury one or more vaporizable metal halides under proper control of loading, temperature and pressure, the preferred metal halide additive being sodium iodide, optionally with thallium iodide. Such improved lamps are described and claimed in copending application Serial No. 84,068 of Gilbert H. Reiling, filed Jan. 23, 1961, entitled Gaseous Electric Discharge Lamps and assigned to the same assignee as the present invention. For convenience, such lamps will hence forth be referred to herein as mercury metal halide lamps.

In its general construction and appearance, the mercury metal halide lamp resembles the conventional high pressure mercury vapor lamp. It comprises a quartz arc tube mounted within a glass outer jacket having a screw base at one end. The are tube contains a quantity of mercury and metal halide such as sodium iodide along with an inert gas such as argon for starting purposes. Thermionic main electrodes are provided at the ends of the arc tube, and an auxiliary starting electrode is located adjacent one of the main electrodes in order to facilitate starting, and is connected through a current limiting resistor to the other main electrode. In the case of the mercury metal halide lamp, the space between the arc tube and the outer jacket is evacuated in order to reduce loss of heat from the arc tube to assure the required arc tube wall temperature. The ends of the arc tube are also preferably provided with a heat reflecting coating in order to assure the desired temperature at the ends.

The presence of the starting electrode gives rise to a troublesome condition which appears to be due to electrolysis of the metal halide. In the case of sodium iodide for instance, the salt at the beginning of operation is in liquid form and may electrically connect the starting electrode to the adjacent main electrode. When this happens, the salt decomposes with formation of a dark de posit and this may result in rapid failure of the seals at that end of the lamp. The object of the invention is to remedy this condition.

-Devitrification of the seals between starting and adjacent main electrode in high pressure mercury vapor lamps is well known, and occurs when a unidirectional potential arises between the starting and the adjacent main electrode by reason of current rectification by the starting electrode. The solution is taught in Patent 2,660,692, St. Louis et al., and consists in increasing the electron emission of the starting electrode to the point where its conductivity is substantially symmetrical on both half cycles of the applied alternating current, as by placing the starting electrode so that its tip is heated in the arc. Although this technique is helpful in the mercury metal meters of mercury, within the arc tube to ice halide lamp, it does not eliminate the brown-black deposit around the starting electrode.

In accordance with my invention, I have found that a solution to the problem resides in positively connecting the starting electrode to the adjacent main electrode after starting of the lamp so as to eliminate any possibility of potential difference between them. A practical and inexpensive way of accomplishing this is to provide a bimetal switch between the starting electrode and the adjacent main electrode. The switch is normally open and it is actuated by the heat of the lamp after starting and short-circuits the starting electrode to the adjacent main electrode. For maximum efiiciency, the bimetal switch may be arranged to disconnect the starting electrode from the current limiting resistor at the same time as the short circuit connection to the adjacent main electrode is made.

For further objects and advantages and for a better understanding of the invention, attention is now directed to the following detailed description of preferred embodiments to be read in conjunction with the accompanying drawing. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

In the drawing:

FIG. 1 is a side view of a mercury metal halide arc lamp embodying the invention.

FIG. 2 is a similar view illustrating a variant of the invention.

Referring to the drawing and more particularly to FIG. 1, there is shown a high pressure mercury vapor lamp 1 comprising an outer vitreous envelope or jacket 2 of generally tubular form modified by a central bulbous portion 3. It is provided at its outer end with a re-entrant stem 4 having a press 5 through which extend relatively stiff inlead wires 6,7 connected at their outer ends to the contacts of the usual screw type base 8, namely the threaded shell 9 and the insulated center contact 10.

The inner arc tube 12 is made of quartz or fused silica and has sealed therein at opposite ends a pair of main arcing electrodes, 13 at the base end and 14 at the outer end, plus an auxiliary starting electrode 15 at the base end adjacent main electrode 13. The. electrodes have inleads 16, 1'7 and 18, respectively, each including an intermediate thin foil section 19 hermetically sealed through full

diameter pinch seals

21, 22 at the ends of the arc tube.

Main electrodes

13, 14 each comprise a tungsten wire helix wrapped around a tungsten core wire. Activation of the main electrodes may be achieved by providing a small elongated piece or sliver of thorium metal (not shown in the drawing) inserted between the core and the helix. Alternatively the electrodes may be activated by a very thin layer of thorium. metal vacuum-deposited thereon. The are tube contains a quantity of mercury which is entirely vaporized during operation of the lamp and which at such time exerts a pressure in the range of l to 15 atmospheres. A quantity of sodium iodide is provided in excess of that vaporized at the operating temperature of the arc tube which should be not less than 500 C. at any place. Since the lowest temperature is encountered at the ends, a heat reflective coating indicated by the speckling is applied to the ends of the arc tube and to the adjacent portions of the pinch seals. Also as a heat conservation measure, the interenvelope space is evacuated; getter material provided in the channeled rings 23 may be flashed after sealing the jacket in order to assure high vacuum. A rare gas such as argon is provided at a low pressure, for instance at approximately 215 millifacilitate starting and warm up.

The are tube is supported within the outer jacket by a frame or harp comprising a single side rod 24 and rod portions'25 on the open side. The frame is welded at its base end to inlead wire 7 and has a pair of

transverse spring members

26, 27 fastened to its upper end WlhiCh bear against the outer tubular portion of the jacket to provide lateral support. The are tube is fastened to the frame by a pair of

metal straps

28, 29 which extend between rod 24 and rod portions 25 and encompass and clamp the pinch seals. The straps are spaced a distance away from the ends of the arc chamber in order to limit the cooling effect and also to prevent dev-itrifi-cation of the quartz about the electrode inleads. A conductor 30 threaded through an insulating glass sleeve 31 connects inlead l6'ofn1ain electrode 13 to inlead wire dot the outer jacket which in turn is connect-ed to base shell 9. A conductor 32 connects inlead 17 of main electrode 14 to rod 24 through which circuit continuity is provided, to center contact of the base.

The practice heretofore in connection with mercury vapor lamps has been to connect auxiliary starting electrode 15 permanently to main electrode 14 at the opposite end of the arc tube by means of a current limiting resistor 33 connected between inlead 18 of the starting electrode and side rod 24 as illustrated in FIG. 1. Only a relatively low starting current is needed to ionize the lamp, and the current limiting resistor 33 may have a value of 40,000 ohms by way of example. When a line voltage of 230 to 240 volts A.C. as indicated is applied across the lamp terminals in series with a ballasting reactor 34, sub stantially the full line voltage appears between main electrode 13 and auxiliary electrode 15, and thisis sufiicie-nt to start the lamp. In normal operation, the voltage drop across the lamp may be approximately 100 vol-ts and resistor 33 then limits the starting electrode current to A milliampere or less.

In the mercury metal halide lamps having only the construction so far described, a brown-black deposit has been observed to form around the starting electrode which may cause violent failure of the lamp within a short interval of time. The effect is most pronounced when the lamp is operated with the starter electrode lower-most. The metal halide in the form of liquid sodium iodide then collects at the lowest point of the arc tube which is also the coolest point. The liquid salt may electrically connect thev starter and the adjacent main electrode. Under these conditions, even a fraction of a milliampere of direct current is enough to cause electrolytic decomposition of the sodium iodide. Seal failure then results within a short time, probably due to the attack of the sodium metal on the quartz. However, even when the lamp is operated such that the starter electrode is uppermost, seal failure occurs. Under these conditions, the failure appears to be more similar to electrolytic devitrification failure in mercury vapor lamps, only aggravated by the presence of the sodium iodide. The damage is most pronounced in the quartz about the starting electrode and the adjacent main electrode. In accordance with my invention, I have found, a solu tion to the foregoing problem which consists in providing a thermal switch which short-circuits the starter electrode to the adjacent rnain electrode after the lamp has started. As illustrated in FIG. 1, the thermal switch may consist of a strip of dissimilar metals'bent toa U-shape and having one leg welded or otherwise fastened to inlead 16 of main electrode 13. At room temperature, the switch extremities lie close together. As the lamp warms up, the U-shaped piece opens out and the piece of molybdenum wire 36 welded across the free end of the bimetal strip engages molybdenum inlead 18 of the starting electrode. 1 have found it suitable to use a strip of bimetal having a thickness of approximately .010 inch, 43 x 3 millimeters in dimensions, and formed to a U about /4 inch in overall length. By way of example, in a 400-watt lamp the bimetal switch closes 2 to 4 minutes after starting of the lamp and substantially eliminates the seal failure problem.

In the embodiment of FIG. 1, the bimetal switch by short-circuiting the starter electrode to the adjacent main electrode, in effect leaves the starting resistor 33 connected in parallel with the arc tube during operation of the lamp. This means that the arc voltage is applied at all times across the starting resistor and entails a wattage loss therein of A to /2 watt duringoperation. This may be avoided according to embodiment in in FIG. 2 by connecting the starting resistor 33 between the harp and a blind pin terminal 37 provided in the pinch seal of the arc tube. The bimetal is fastened to inlead 18 of the starting electrode and its free end is disposed to move between blind pin 37 and inlead 16 of main electrode 13. In the rest position of the bimetal at ambient temperature, it engages blind pin 37 so that the starting electrode is connected through the starting resistor torod 24. After the lamp has started, the free end of the bimetal disengages itself from blind pin 37 and engages inlead 16 of main electrode 13. Thus in operation after the lamp has warmed up, the starting resistor is disconnected from the supply and wattage loss in it is avoided. At the same time, the starting electrode is short-circnited to the adjacent main electrode as required in order to prevent electrolysis of the sodium iodide, or electrolytic damage to the seals. I

While the invention has been described by reference to specific preferred embodiments of same, the details of construction described are intended as exemplary and not in order to limit the invention there-to except insofar as included in the accompanying claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric discharge lamp comprising a vitreous outer jacket enclosing a quartz arc tube, said are tube having a pair of main thermionic electrodes sealed therein at opposite ends and a starting electrode located adjacent one of said main electrodes sealed therein at one end, an ionizable filling within said are tube comprising mercury, a metal halide, and an inert starting gas, means supporting said are tube within said outer jacket and providing circuit connections to said main electrodes, means including a current limiting resistor within said, outer jacket connecting said starting electrode to the main electrode at the opposite end of the arc tube, and a thermal switch actuated by the heat of said are tube during normal operation and arranged to short-circuit the starting electrode to the adjacent main electrode when actuated.

2. An electric discharge lamp comprising a vitreous outer jacket enclosing a quartz arc tube, said are tube having a pair of main thermionic electrodes sealed therein at opposite ends and a starting electrode located 'adjacent one of said main electrodes sealed therein at one end, an ionizable filling within said are tube comprising mercury, sodium iodide, and an inert starting gas, means supporting said are tube within said outer jacket and providing circuit connections to said main electrodes, means including a current limiting resistor within said outer jacket connecting said starting electrode to the main electrode at the opposite end of the arc tube, and a thermal switch unactuated at room temperature andactuated by the heat of said are tube during operation of said lamp, said thermal switch being arranged to short-circuit'the starting electrode to the adjacent mainelectrode when actuated.

3. An electric discharge lamp comprising a vitreous outer. jacket enclosing a quartz arc tube, said are tube having a pair of main thermionic electrodes sealed therein atopposite ends and a starting electrode located adjacent one of said main electrodes sealed therein at one end, an ionizable filling within said are tube comprising mercury, a metalhalide, and an inert starting gas, means supporting said are tube within said outer jacket and providing circuit connections to said main electrodes, 21 current limiting resistor connected to the main electrode at the opposite end of the arc tube, and a thermal switch within said outer jacket, said thermal switch when unactuated at room temperature providing circuit continuity between said starting electrode and said resistor, and when actuated by the heat of said arc tube during normal operation providing circuit continuity between said starting electrode and the adjacent main electrode.

4. An electric discharge lamp comprising a vitreous outer jacket enclosing a quartz arc tube having main thermionic electrodes supported by inleads sealed therein at opposite ends, a starting electrode adjacent one of the main electrodes supported by an inlead sealed through said are tube proximately to that of the adjacent main electrode, an ionizable filling within said arc tube comprising mercury, sodium iodide and an inert gas at a low pressure, a frame including conductors supporting said arc tube within said outer jacket and providing circuit connections to said main electrodes, a current limiting resistor connected between said starting electrode and said frame, and a U-shaped bimetal strip fastened at one end to the inlead of one of said starting electrode and the adjacent main electrode and having its free end disposed to engage the inlead of the other when actuated by the heat of said are tube, whereby to shortcircuit the starting electrode to the adjacent main electrode during normal operation of said lamp,

5. An electric discharge lamp comprising a vitreous outer jacket enclosing a quartz arc tube having main thermionic electrodes supported by inleads sealed therein at opposite ends, a starting electrode adjacent one of the main electrodes supported by an inlead sealed through said arc tube proximately to that of the adjacent main electrode, an ionizable filling within said arc tube comprising mercury, sodium iodide and an inert gas at a low pressure, a frame including conductors supporting said are tube within said outer jacket and providing circuit connections to said main electrodes, a blind pin mounted on the starting electrode end of said are tube, a starting resistor connected between said blind pin and said frame, and a U-shaped bimetal strip fastened at one end to the inlead of said starting electrode and having its free end movable between said blind pin and the inlead of the adjacent main electrode, said bimetal strip operating to engage said blind pin at room temperature whereby to connect the starting electrode through said resistor to the opposite main electrode, and engaging the inlead of the adjacent main electrode at higher temperatures corresponding to the operating condition of said lamp whereby to short-circuit the starting electrode to the adjacent main electrode and leave said starting resistor disconnected.

No references cited.

JOHN W. HUCKERT, Primary Examiner. DAVID J. GALVIN, Examiner.

Claims

1. AN ELECTRIC DISCHARGE LAMP COMPRISING A VITREOUS OUTER JACKET ENCLOSING A QUARTZ ARC TUBE, SAID ARC TUBE HAVING A PAIR OF MAIN THERMIONIC ELECTRODES SEALED THEREIN AT OPPOSITE ENDS AND A STARTING ELECTRODE LOCATED ADJACENT ONE OF SAID MAIN ELECTRODES SEALED THEREIN AT ONE END, AN IONIZABLE FILLING WITHIN SAIDS ARC TUBE COMPRISING MERCURY, A METAL HALIDE, AND AN INERT STARTING GAS, MEANS SUPPORTING SAID ARC TUBE WITHIN SAID OUTER JACKET AND PROVIDING CIRCUIT CONNECTIONS TO SAID MAIN ELECTRODES, MEANS INCLUDING A CURRENT LIMITING RESISTOR WITHIN SAID OUTER JACKET CONNECTING SAID STARTING ELECTRODE TO THE MAIN ELECTRODE AT THE OPPOSITE END OF THE ARC TUBE, AND A THERMAL SWITCH ACTUATED BY THE HEAT OF SAID ARC TUBE DURING NORMAL OPERATION AND ARRANGED TO SHORT-CIRCUIT THE STARTING ELECTRODE TO THE ADJACENT MAIN ELECTRODE WHEN ACTUATED.