US2957995A - Instant start discharge lamp - Google Patents

Description

Oct. 25, 1960 R. s. FOX

INSTANT START DISCHARGE LAMP Filed. Dec. 31, 1956 lnven tov: RObT1T 3. Fox, 9 W

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INSTANT START DISCHARGE LAMP Robert S. Fox, University Heights, Ohio, assignor to General Electric Company, a corporation of New York Filed Dec. 31, 1956, Ser. No. 631,591

1 Claim. (Cl. 313-109) This invention relates to elongated low pressure electric discharge lamps such as fluorescent lamps and more particularly to instant start lamps of that kind.

Instant start fluorescent lamps differ from other types of fluorescent lamps in that the electrodes are not preheated in order to start the arc discharge or ignite the lamp. Instead, recourse is had to a relatively high 'voltage applied between the electrodes to start discharge, the electrodes being initially at ambient temperature. Of course, as soon as the lamps are ignited, the electrodes heat up and a hot spot is developed providing the required electron emission at a relatively low cathode drop in the same fashion as with switch start or rapid start fluorescent lamps using preheating of the electrodes. Instant start lamps have, as their name implies, the advantage of starting instantaneously upon the application of line voltage to their operating circuit.

One of the problems Which has arisen with instant start fluorescent lamps is that of safely terminating the operation of the lamps at the end of their useful lives. For practical purposes, an instant start fluorescent lamp has reached the end of its life when one of its electrodes has become deactivated. The electrodes consist of tungsten wire filaments coated with an electron emitting material, generally an alkaline earth oxide such as strontium or barium oxide. The electron emissive or activating material assures a copious flow of electrons with a relatively low cathode drop or expenditure of energy. At starting, some of the activating material is sputtered oil, and during operation more is slowly vaporized. When all of the activating material has become vaporized, the lamp is said to be deactivated. The cathode voltage drop then rises radically from a value less than 20 volts to a value up to 100 volts.

With other types of fluorescent lamps such as switch start and rapid start lamps, when the cathodes become deactivated, the open circuit voltage of the ballast is insuflicient to start or continue the discharge and operation of the lamp simply ceases. However with instant start lamps, the open circuit voltage supplied by the ballast is usually suflicient to maintain the are even after the cathodes have become deactivated. Continued operation of the lamp under these conditions results in rapid vaporization of the tungsten filament due to the heat developed as a result of the excessive cathode voltage drop. Simultaneously the end of the lamp envelope darkens rapidly through deposition of vaporized electrode material. The are then generally attaches itself to a junction of one of the lead-in wires with the stem press and the glass of the stem becomes highly heated. Possibly emission from the junction is facilitated by the presence of sodium in the glass and by alkaline earth metal previously vaporized or sputtered from the cathode during life and which has condensed on the stem. Operation in this fashion may continue for some time until finally the glass of the overheated stem becomes sufficiently softened that external atmosphereic pressure ruptures a hole through its wall and admits the atmosphere.

The lamp is then said to be aired; when this happens, the arc is extinguished and operation ceases.

The operation of an instant start fluorescent lamp after deactivation and until it becomes extinguished by airing is not of itself objectionable. Unfortunately, however, it happens occasionally that the end of the glass envelope of the lamp may fracture violently at the end of life. The lamp may then fall from the fixture and of course this presents a hazard to the occupants of the space below. The problem is especially serious in installations of fluorescent lamps using thick-walled large diameter glass tubing. For instance one lamp designated by applicants assignee 40T17/IS is a 40-Watt lamp, 60 inches in length using a tubular envelope 2% in outer diameter and having a wall thickness of approximately .050 inch. The envelope is relatively heavy and i in falling from a fixture presents a definite hazard.

The object of the invention is to provide a new and improved instant start discharge lamp which substantially eliminates the possibility of fractured lamp ends at the end of life allowing the lamp to fall out of the fixture.

The invention is based upon my discovery that the cause of bulb breakage in instant start lamps operated after deactivation is generally emisstion from the junction of a broken filament and the bulb wall. Continued operation of a lamp after deactivation usually results in rupture of the filament at some point intermediate the supporting lead-in wires or mount legs. If one of the stubs of the filament remaining attached to either leg happens to be long enough, it will sag and touch the bulb Wall. Emission may then take place from the filament-bulb Wall junction which, if sustained, may produce a crack and fracture the bulb. In accordance with the invention, such possibility is practically eliminated or at least substantially reduced by connecting only one of the lead-in wires to the base terminals of the lamp. Actual tests have shown that the probability of occurrence of a filament stub on the connected lead-in Wire long enough to contact the bulb wall is thereby greatly reduced.

. For a fuller understanding of the advantages of the invention and of the manner in which it is to be practiced, attention is now directed to the following description of a preferred embodiment and to the accompanying drawing. The features of the invention believed to be novel will be more particularly pointed out in the appended claim.

In the drawing:

Fig. 1 is a schematic illustration of a prior art instant start fluorescent lamp connected into a typical operating circuit of the lead kind.

Fig. 2 is a pictorial illustration of a lamp end showing a broken filament stub contacting the bulb wall in a manner likely to cause fracture of the bulb under continued operation.

Fig. 3 is a plan view, partly in section, of an end of a lampembodying the invention.

Fig. 4 is an end view of the lamp end shown in Fig. 3.

Referring to the drawing and more particularly to Fig. 1, there is shown a fluorescent low pressure discharge lamp 1 connected into an operating circuit 2. The lamp has an elongated tubular vitreous envelope 3 into the opposite ends of which are sealed filamentary electrodes 4, 4 mounted on the inward projections of lead wires 5, 6 passing through the presses 7 of flared mount stems 8 peripherally sealed to the ends of the envelope. The filamentary electrodes are of the coiled coil type provided with an overwind for retaining securely a relatively large quantity of activating or electron emissive material. The activating material may consist of alkal-ine earth oxides such as barium and strontium oxides.

The electrodes may be constructed according to the teachings of US. Patent 2,306,925-Aicher, to which reference may be made for more complete details including the process of manufacture.

The filling within the envelope comprises an inert startlng gas such as argon at a pressure of a few millimeters and a small quantity of mercury indicated by droplet 9. The mercury is in excess of that vaporized during normal operation of the lamp wherein the partial pressure of mercury vapor may be in the range of 8 to 12 microns. The envelope 3 is coated internally with a luminescent phosphor 11, for instance calcium halophosphate. The phosphor converts the ultraviolet radiation, principally 2537 A., produced by the low pressure discharge through the mercury vapor into visible light.

In the lamp Whose ends including the electrode and base are illustrated in detail in Figs. 2 to 4, and which corresponds to the instant start lamp commercially designa.ed 40T17/IS, the vitreous envelope has nominal dimensions 2% diameter and 60 overall length. The lamp is provided with mogul bipin bases comprising a dished metal shell or cap 12 secured to the lamp end by basing cement at 13. A pair of tubular pins 14, 15 project from the base shell and are staked to insulating washers 16 mounted in apertured recesses on the outer side and to an insulating plate 17 on the inner side of the shell. The two pins 14, 15 are electrically connected together by means of a short oircuiting strap 18 overlying the insulating plate 17 on the inner side.

The operating circuit 2 for the lamp comprises a ballasting transformer including a primary winding 19 which s supplied with 115-120 volts, 60 cycle AC. voltage from the usual commercial supply at input terminals 20, 21. Terminal 20 is connected to the low or ground side of the line whereas terminal 21 is connected to the high side of the line. As a safety measure which is required in all instant start ballasts by reason of the high open circuit voltages which they provide, an interrupting feature is provided in the primary circuit of the ballast. In the usual tombstone type double contact lampholders used with lamps having bipin bases, this is achieved by connecting input terminal 20 to one of the lampholder contacts and the low side of the primary winding 19 to the other terminal of the same lampholder. When a lamp is inserted in the lampholders, the lampholder contacts engage pins 14 and 15 of the base and the primary circuit to the ballast is completed through the shorting strap 18 of the lamp base received in the appropriate lampholder. In the absence of a lamp, however, the primary circuit is interrupted and the ballast is disabled and dangerous open circuit voltages are eliminated.

The secondary winding 22 of the ballast is arranged to have high leakage reactance relative to the primary winding; to this end, primary l9 and secondary 22 are wound respectively on magnetic cores indicated schematically at =23, 24 with a magnetic shunt path indicated at 25. To assure reliable starting of the particular lamp which has been described, the open circuit voltage applied across the lamp by primary winding 17 and secondary winding 21 connected in series as shown and including ballasting capacitor 26, will be in the range from 450 to 550 volts. Capacitor 26 has a reactance approximately twice the leakage reactance of secondary winding 24. In normal operation, lamp current is maintained at approximately 425 milliamperes, the operating voltage drop across the lamp being approximately 105 volts.

The relatively high open circuit voltage, which is necessary to assure reliable starting, is usually sufficient to maintain the arc after a lamp has become deactivated. As previously mentioned, the majority of lamps continued in operation after deactivation of the electrodes fail harmlessly by airing through an overheated mount stem. However, the occasional lamp fails violently with fracture of the envelope, and when the lamp is supported in its fixture solely by the engagement of the bases in the lampholders, the lamp may fall. The cause of these violent failures has been very elusive inasmuch as the lamps upon falling generally become shattered. However, as a result of extensive observation and experimentation, I have concluded that a condition for the occurrence of a violent failure is that emission takes place from a cathode-bulb wall junction.

Continued operation of the lamp after deactivation of the filamentary cathode generally results in rupture of the filament. The commercial practice in the past has been to connect both lead-in 'wires supporting the filament to the base pins and to short-circuit the base pins together, as indicated in dotted lines in Fig. 2. Under these circumstances, the filament may break at almost any point intermediate its ends. If one of the remaining filament stubs should be long enough to touch the bulb wall upon sagging, emission may take place from the cathode-bulb wall junction. Such a condition is illustrated in Fig. 2 wherein the filamentary cathode 4 has become broken off near its point of attachment to upper lead wire 6 and has sagged from its original position indicated in dotted lines to the position indicated in solid lines at 4a and has effected a junction with the bulb wall at 27. The heat resulting from sustained emission at the junction has produced the cracks indicated at 28. Complete fracture of the envelope may occur when the junction has become sufficiently heated to allow atmospheric pressure to puncture the glass wall.

In accordance with the invention and in reliance upon these findings, in order to provide an instant start fluorescent lamp substantially free of the possibility of violent failure, I eliminate or substantially reduce the possibility of sustained emission from a filament-bulb wall junction. This result may be achieved through the use of a filament which is short enough that even though it should become detached from one of the lead wires and swing freely from the other, it could not contact the bulb wall irrespective of the angular orientation of the lead wires and filament, that is whether the filament is supported in horizontal or vertical planes, or in some intermediate plane as in Fig. 2.

For the case where it is desired to maintain the same length or spread of filament, and thereby avoid substantial redesign of the lamp making machinery, I have discovered that a simpler solution to the problem resides in connecting only one of the lead-in wires to the pins or terminals of the base. Such an arrangement is shown in Fig. 3 wherein lead wire 5 is connected to base pin 14 but lead wire 6 is cut otf short at 6a. In the instant lamp, the spread of the filament, that is its linear length between the points of engagement of the lead-in wire clamps, is approximately 1.1. Where the lead wires are oriented to support the filament in a horizontal plane, as shown in Fig. 4, a filament stub having a length of approximately 0.75" is long enough to contact the glass wall of the bulb. However, emission from a given filament-bulb wall junction can occur only when the filament stub is attached to a lead-in wire which is connected to the base terminals or pins. Connecting only one lead to the terminals of the base substantially reduces the possibility of a filament stub of sufiicient length remaining attached to that lead.

The explanation for this phenomenon appears to be that so long as there is some activating material on the filamentary cathode, the hot spot or point of attachment of the arc will always occur at an activated point on the cathode closest to the connected lead. As the activating material is gradually depleted, the hot spot works its way across the cathode to the other lead. Finally the condi tion is reached where the cathode is completely deactivated and since the are or hot spot can no longer find an activated area on which to settle, it will move to the bare tungsten filament nearest the connected lead. This shift is brought about through the voltage drop caused by the resistance of the tungsten filament between the hot spot and the connected lead, the connected lead itself having of course substantially zero resistance by comparison. The are then causes the filamentary cathode to become overheated at a point close to the attachment of the filament to the connected lead and the heat developed at that point finally causes the filament to break. The break point thus occurs close to the connected lead and the short filament stub remaining on this lead cannot contact the bulb wall. Even though the longer stub remaining on the unconnected lead can contact the bulb wall, since that lead has no connection to the operating circuit, emission will not take place from the cathodebulb wall junction. If operation of the lamp is continued, the arc will attach itself to the junction of the connected lead and the stem press; finally the stem press becomes overheated to the point where it is punctured by atmospheric pressure. The lamp then becomes aired and further operation ceases without violent fracture.

In a large group of lamps made in accordance with the invention with only one lead connected as illustrated in Figs. 3 and 4, and wherein the mount stems were oriented to maintain the cathodes horizontal as illustrated in Fig. 4, actual tests have shown that the probability of occurrence of a long filament stub on the connected lead is greatly reduced. For the lamp which has been specifically described, actual tests show the probability to have been reduced by a factor of approximately 100 over the case where both leads were connected as in the past.

In general the probability of a filament stub of a given length contacting the bulb wall is least when the cathodes are oriented in a horizontal plane. This requires that, in the manufacture of the lamp, both mount stems be sealed to the envelope with the lead wires and filaments oriented in the same plane, and that the bases be attached with the pins likewise in the same plane. Thereafter when the lamp is mounted in a fixture having standard tombstone type lampholders which conventionally socket the lamp with the pins in a horizontal plane, the desired purpose is effected. Unfortunately, however, there are many fixtures in commercial use wherein the sockets or lampholders do not follow this practice with the result that the lamp filaments are oriented in other planes than horizontal. Even under these circumstances however, the single lead connection of the filament to the base terminals substantially reduces the probability of a filament stub remaining attached to the connected lead long enough to contact the bulb wall and allow emission from the junction.

Whereas a certain specific embodiment of the invention has been shown and described in detail, it will be understood that this has been done for illustrative purposes only. In general the invention is applicable to any instant start lamp wherein the problem of continued operation after deactivation of the cathodes with resultant possibility of violet failure is present. The hazard presented with a T17 lamp having a relatively thick-walled glass envelope is, of course, greater than with a T12 lamp of 1% diameter having a relatively thin-walled glass envelope. However with lighter-walled glass envelopes the invention offers in the same fashion a solution to the problem of possibility of violent failure.

The essential feature of the invention which consists in connecting only one of the filament supporting lead-in wires to the base terminal or terminals may be used irrespectively of the type of base used with the lamp. For instance with the slimline type of lamp using a single relatively large diameter rounded pin at the center of the base, the invention is practiced by connecting a single one of the lead-in wires to that pin. The appended claim is intended to cover any such modifications coming within the true spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

An elongated tubular electric discharge lamp of the instant start type comprising a sealed vitreous envelope containing an ionizable medium and having a pair of filamentary activated cathodes at opposite ends, a pair of lead-in wires sealed through said envelope at each end and supporting the filamentary cathode from its ends, the length of said filamentary cathode being long enough to permit contact with the envelope wall upon becoming detached from one lead-in wire and hanging freely from the other, a base comprising a pair of pin terminals on each end of said lamp for elfecting connection to an operating circuit, the pair of terminals in each base being short circuited together, and a single one of the lead-in wires at each end of the lamp being connected to the pair of pin terminals.

References Cited in the file of this patent UNITED STATES PATENTS 2,030,805 Wiegand Feb. 11, 1936 2,240,353 Schnetzler Apr. 29, 1941 2,441,863 Zabel May 18, 1948 2,748,306 Bjorkman May 29, 1956

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