US2849656A - Switch-start discharge lamp circuit - Google Patents

Description

Aug. 26; 1958 w. J. KARASH 2,849,656

SWITCH-START DISCHARGE LAMP CIRCUIT Filed Sept. 15;,1953

Inventor; I Wa|-ber-'J.Kar-ash, a4 y Wfik His Attorney United States Patent Ofice 2,849,656 Patented Aug. 26, 1958 SWITCH-START DISCHARGE LAMP CIRCUIT Walter J. Karash, Euclid, Ohio, assignor to General Electric Company, a corporation of New York Application September 15, 1953, Serial No. 380,313

3 Claims. (Cl. 31597) This invention relates to a circuit arrangement advantageous in prolonging the life of discharge lamps under certain conditions frequently encountered in leading power factor switch-start circuits.

Up to the present time, the efforts of Workers in the field of starting and operating circuits for discharge lamps has been directed in general toward finding ways and means to lower the lamp starting voltage. I have found that there are certain circumstances under which it becomes highly desirable to raise, rather than lower, the lamp starting voltage. These conditions are related to the Well-known phenomenon whereby in lead-lag switch-start circuits the lead lamp has a much shorter life than the lag lamp.

The object of my invention is to provide a new, improved and economical circuit arrangement achieving longer life for the lead lamp in switch-start circuits.

In accordance with my invention, I have discovered that the major reason for the shorter life of lead lamps in switch-start circuits is the higher peak voltages produced in the lead circuit, especialy as a result of the switching transient when the control switch is closed. This often causes instantaneous starting of the lead lamps when the line voltage is high or when the lamps are well aged and have become easy starters, with a resulting decrease in lamp life. The minimum voltage required across the electrodes of a lamp in order to start it, where the lamp has a conductive member in close proximity to it, may be increased by applying to the member a potential which is intermediate the potentials of the electrodes. With the autotr-ansformer type ballast commonly used with 40-watt fluorescent lamps, these requirements are achieved most simply by one or the other of the following wiring arrangements. If an ungrounded alternating supply is used, the fixture is connected to the junction point of the primary and secondary windings of the autotransformer, this point being commonly referred to as the midpoint. If a grounded supply is used, the grounded side of the line is connected to the midpoint rather than to the low end of the primary, and the fixture is grounded in the usual fashion. With either of these arrangements, and with the use of a compensating winding in the lead circuit as is well known, the life of the lead lamp becomes substantially equal to that of the lag lamp.

For further objects and advantages and for a better understanding of the invention, attention is now directed to the following description to be taken 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 schematic diagram of a circuit suitable for investigating the conditions giving rise to the invention.

Fig. 2 is a schematic diagram, partly pictorial in form, illustrating a lead-lag switch-start circuit embodying the invention.

The circuits which have been illustrated are particularly suitable for a standard 40-watt fluorescent lamp, 48 inches long and 1% inches in diameter, and in the description to follow reference, in general, will be made to such a lamp, although it will be appreciated that the invention is not limited thereto.

Referring to Fig. 1, the discharge device or lamp 1 comprises an elongated tubular envelope 2 which has sealed into its opposite ends a pair of filamentary electrodes 3 and 4. These electrodes may consist of coils of tungsten wire activated with oxides of alkaline-earth metals such as barium and strontium oxides. The envelope 2 contains a starting gas, such as argon, krypton, xenon, or mixtures thereof, at a pressure of a few millimeters, and a small quantity of mercury. During normal operation under ambient temperature conditions, the mercury achieves a pressure of the order of 10 microns. The inside of the envelope is coated with a suitable phosphor which transforms the ultraviolet radiation produced by the discharge through the mercury vapor into longer wavelength radiations occurring within the visible range.

The lamp is shown connected in a leading power factor load circuit 5, 6 across an autotransformer 7 energized from supply lines 8, 9 which may be the usual -120 volt, 60-cycle commercial supply. The autotransformer comprises primary winding 10 and secondary 11 with an adjustable output tap or slider 12 thereon. The windings are coupled together but with the usual magnetic shunt indicated at 13 between them. The shunt provides high leakage reactance in the load circuit which includes both primary and secondary windings connected in series. Primary winding 10 is connected across supply lines 8, 9 in series with manual control switch 14. In series with the secondary circuit, there is connected capacitance 15 having a reactance greater than the inductive leakage reactance of the autotransformer Whereby load circuit 5, 6 operates on a leading power factor.

Lamp 1 is connected across load circuit 5, 6 through one side of filamentary electrodes 3, 4, respectively. The other side of the electrodes is connected in a shunting circuit 16, 17 for preheating the electrodes at starting. This circuit comprises inductive reactance 18, generally known as a compensating winding, and starting switch 19 of the thermal glow type. The switch comprises an envelope 21 containing an ionizable gas such as neon. Within the envelope there are sealed fixed electrode 22 and curved bimetallic strip electrode 23. These electrodes are normally out of engagement with each other, and upon the application of a suitable voltage across them, a discharge takes place through the ionizable gas. This discharge causes heating of the bimetallic strip 23 which thereupon unflexes and engages the fixed electrode 22. The switch is thereby closed, and the discharge is extinguished; after the switch has cooled sufliciently, it reopens and the cycle repeats if sufficient voltage is present across electrodes 22, 23.

Located in close proximity to the envelope of the lamp, there is shown a conductive member 24 extending the length of the lamp. When connected to one of the electrodes, it facilitates starting, that is, it lowers the minimum value of voltage required across the electrodes in order to start the discharge. Such a conductive member having the property of reacting capacitively upon the ionizable medium within the lamp, is often referred to as a starting aid. In the usual case, it consists merely of the metal face plate of the usual commercial fixture in which the lamp is operated. Other well-known forms of starting aids are a ribbon of a conductive paint or other material extending the length of the envelope and generally known as a stripe, and a transparent conductivecoating such as a tin or indium oxide coating on 3 the envelope. These latter forms of starting aids are generally used where it is desired to further reduce the starting voltage of the lamp by applying to the starting aid a voltage above thatof either electrode, that is a voltage great 1' thanthat of the electrode having the higher potentialgj It willbe appreciated that in the ordinary commercial 1amp inst;allation, there is unavoidably present a starting aid in the form of the front metal plate of the fixture. The cond uctiveelement .24 in Fig. 1 may be taken as representing such jametal plate. It has been the practice to' ground the fixture and to connect the low side of the primarylt) of the autotransformer to the grounded side of the supply line, namely to conductor 8 as illustrated. The result is thatconductive member 24 is at the potential of oneof the electrodes, namely that of electrode 3 as .illustrated- In order to illustrate the possibility of varying the potentialapplied to member 24, potentiometer is shown connected across the operating circuit 5, 6. Withthe slider 26 at the lowermost end of the potentiometer,member 24 is effectively at ground potential as in the usual installation.

The. normal, or perhaps it might more properly be said the-intended, mode of operation of the circuit of Fig. 1 is as follows. When control switch 14 is closed, the line voltage, say of 118 volts, applied across primary .10 inducesin secondary 11 approximately 100 volts. The secondary voltage being added in series to the primary, the. open circuit voltage applied across load or operating circuit 5, 6 is approximately 218 volts. The electrodes 3, 4 of the lamp being cold, the open circuit voltage of 218 volts is normally insulficient to start the discharge. However, that voltage is also applied across the electrodes of thermal switch 19, thereby causing it to ionize and conduct current. The heat generated by the discharge within the switch causes bimetallic strip 23 to unflex and engage fixed electrode 22. The thermal switch is now closed and the full preheat current flows through the electrodes 3 and 4 which heat up to thermionic emission. Within a short interval of time, generally fixed at a few'seconds by the design of the switch, the switch has cooled sufliciently to reopen. When the switch reopens, the'open circuit voltage of the autotransformer, assisted by the transient impulse produced when thermal switch 19 opens, is normally sutficient to start the discharge between the heated electrodes within the lamp. When occasionally the discharge fails to start, the thermal switch ionizes again and repeats its closing and opening cycle until'the lamp does start. After the lamp has started, the voltage drop across it is less than that required to cause ionization within the thermal switch, which thereupon remains permanently open.

By reason of capacitance 15 serially included in the load circuit, lamp 1 draws substantially leading current from the autotransformer. In accordance with conventional practice, capacitance 15 is of a value such as to providepa capacitive react-anceovercoming the inductive leakage. reactance of winding 11 and limiting the dischargecurrent through the lamp to its rated value. For instance, with a 40-watt lamp and with a ballast transformer providing the open-circuit voltages in the ranges stated above, capacitance 15 may have a value of approximately 2.5 microfiarads. However, when the maintenance or life of the lamp is compared with that of a similar lamp operated on an equivalent lagging power factor circuit which, in the present instance, could be achieved by short-circuiting capacitor 15, it is found that the life of the lamp in the leading power factor circuit is much less. For many years the lighting industry has been confronted with this problem; for instance, discounting for the moment the improvement which may be effected by means of compensating winding 18, the average life of the lead lamp may be as low as to percent of that of a lag lamp. It has generally been thought that-the differential was due to the lesser short circuit current of the lead winding resulting from the presence of series capacitor 15. Thus when thermal starting switch 19 was closed, the electrode preheating current was less'in the case of the lead than in the case of the lag lamp. Thereafter when the thermal switch reopened, the electrodes in the case of the lead lamp would be insufficiently preheated and sputtering of the electrodes would occur at starting with a resulting decrease in lamp life.

It was sought to remedy this situation by the provision of an additional inductive reactance18, generally known as a compcnsatingwinding, which is connected in series with the thermal switch in the electrode shunting circuit 16, 17. The compensating winding ofisets in part the reactance of capacitor 15 and increases the short circuit current, that is the electrode preheating current. The adoption of the compensating winding did provide some improvement: for instance, where the average life of the lead lamp was about 35 to 40 percent that of the lag lamp, it rose to an average of 50 to 55 percent with a compensating winding added. Whereas these figures show a decided improvement, they also clearly show that a compensating winding is not the complete answer.

I have now discovered that the major part of the difficulty which results in shorter life with lead lamps is due to the higher peak voltages available with lead circuits. In particular, the transient produced when the control or wall switch 14 is closed may result in an excessively high peak voltage in a lead circuit. Such higher peak voltages often produce instantaneous starting of a lamp where the supply voltage is already high or where the lamp is well aged and an easy starter. The resulting differential in life between the lead and lag lamps has entailed increased replacement and lighting costs and has seriously interfered with group replacement programs.

In accordance with my invention, I provide a simple and economical circuit arrangement for increasing the minimum voltage required by the lamp in order to start, and which substantially eliminates instantaneous starting of the lead lamp in a switch-start circuit. This circuit may be demonstrated as follows by reference to Fig. 1. If the electrode shunting circuit 16, 17 is disabled, for instance by removing thermal switch 19, the lamp must startinstantaneously, that is without any electrode preheating. Under these conditions, when the voltage applied to the starting aid or conductive member 24 is the same as that applied either to electrode 3 or 4, which conditionis effected with slider 26 at the lower end of potentiometer 25 as illustrated in solid lines, a lower value of secondary voltage is sufiicient to start the discharge. It is then sufiicient to place slider 12 at some intermediate point on secondary winding 11, as illustrated in solid lines, in order to start the lamp. However, as slider 26 is raised upward on potentiometer 25, it is found that the secondary open circuit voltage must be increased in order to start the lamp. The maximum is reached when slider 26 reaches the midpoint of potentiometer 25, that is, the position shown in dotted lines at 2611; the slider on secondary winding 12 must then be raised to its highest position illustrated in dotted lines at 12a. When slider 26 is raised beyond the midpoint on the potentiometer the secondary open circuit voltage required begins to decrease again. These results may be summarized as follows:v the voltage which is required across the electrodes of the lamp in order to start the discharge will be greatest when the starting aid is midway in potential between the electrodes. Accordingly, in order most effectively to prevent instant starting, the starting aid or conductive member 24 is placed at midpotential with respect to the electrodes, for instance by placing the potentiometer slider at the midpoint, as indicated in dotted lines at 26a.

Referring to Fig. 2, there is shown a two-lamp leadlag switch-start circuit embodying the invention inpractical form. A pair of lamps 31, 32, which may be similar to lamp 1 previously described, are shown disposed immediately in front of a metal plate 33. It will be appreciated that this represents the usual commercial fixture arrangement wherein the conventional two-terminal type lamp holders locate the lamp with a spacing of approximately inch between it and the front panel of the fixture.

Lamp 31 is connected across a leading power factor load circuit 34, 35, whereas lamp 32 is connected across a lagging power factor load circuit 34, 36. The load circuits are energized by a high reactance autotransformer 37 as follows: lead circuit 34, 35 includes primary winding 38, secondary winding 39, and capacitor 41 connected in series; lag circuit 34, 36 includes primary winding 38 and secondary winding 42 connected in series. In both load circuits, the primary and the respective secondary are connected in aiding voltage relationship. For this reason, the ends of secondaries 39 and 42 which may be referred to as their low sides, are connected to common point 43 which is the high side of the primary winding 38. Point 44, which is the low side of primary 38 is connected to electrodes 31a, 32a of the lamps, whereas the high sides of the load circuits, namely conductors 35, 36, are connected to electrodes 31b, 32b respectively. The'electrode shunting circuit of lead lamp 31 includes thermal glow switch 45 and compensating winding 46 connected in series, whereas that of lag lamp 32 includes only thermal glow switch 47.

As thus far described, the circuit of Fig 2 is in all respects similar to ordinary commercial lead-lag switchstart transformer ballast combinations for standard 40- watt fluorescent lamps. In most installations, the transformer ballast is energized from a 115 to 120 volt, 60- cycle alternating current supply, one side of which is grounded. The voltages generated in the secondary winding 39,, 42 are approximately 100 volts; these voltages add to the primary or line voltage to provide open-circuit voltages across the lamps prior to starting of approximately 200-220 volts. The voltage across the lead lamp is usually made slightly higher than that across the lag lamp in order to improve regulation after starting. The conductors 8, 9 in the drawing represent the supply lines, conductor 8 being the grounded side of the line. It has been the practice until my present invention to connect the low side of the autotransformer. primary to the grounded side of the supply, and to place the control or Wall switch between the ungrounded side of the supply and the high side of the primary. In accordance with my invention, I reverse the mode of connections; as illustrated in Fig. 2, the high side ofthe primary, namely point 43, is connected to the grounded line 8, whereas control switch 48 is inserted between the low side of the primary, namely point 44, and ungrounded line 9. With this arrangement, I ground the fixture in the same manner as in prior installations, the grounding being indicated by the solid line jumper 49. The secondary windings generate open circuit voltages which are of approximately the same magnitude as the supply voltage. For instance, where the supply voltage is 118 volts, secondary windings 39 and 42 may each generate approximately 100 volts on open circuit. Thus, by the very simple expedient of connecting the high side of the autotransformer primary to the grounded side of the line, when the metal fixture 33 is grounded, is is placed substantially midway in the potential between the electrodes at starting. The arrangement thus fulfills the condition for achieving the highest starting voltage requirements for the lamp.

With the arrangement of Fig. 2, I find that the voltage required to cause instantaneous starting of a lamp is as much as 50 percent higher than that required with the prior art arrangements where the low side of the autotransformer primary is grounded. This differential is sufficient to eliminate substantially all instantaneous starting of the lead lamps, even including instantaneous starting of well-aged lamps under excessively high line supply conditions. Furthermore, I find that this arrangement produces no measurable deleterious effect on the normal starting of the lamps, that is on the desired mode of starting which occurs when the thermal switches 45 and 47 reopen after the lamp electrodes have become preheated.

While the arrangement of Fig. 2, wherein the conductive member 33 representing the metal fixture is directly grounded through the jumper 49, is that which is preferred and which meets the Underwriters requirements, it will be appreciated that the purpose of the invention may also be effected by connecting conductive member 33 directly or through a current limiting resistance to the high side of primary 38, namely to point 43. This may be done by placing a jumper at 51, indicated in dotted outline, whereupon member 33 is connected to point 43 in series with resistance 52. This mode of connection may be used whether or not one side of the supply is grounded, but will normally be used where neither side of the supply is grounded, that is where a floating supply is used. In such case resistance 52 serves to reduce the personnel shock hazard from touching the fixture. In accordance with conventional practice, resistance 52 may have a value of approximately 50,000 ohms whereby to limit the current through the body of any person coming in contact with the fixture to a few milliamperes. It will be appreciated, however, that, for an arrangement of this type, it is preferable to use some other type of starting aid than the fixture itself, for instance to use a conductive stripe or coating on the lamp and to apply the desired potential directly to the stripe or coating as the case may be, rather than to the conductive member constituted by the fixture.

While a certain specific embodiment of the invention has been shown and described, it will, of course, be appreciated that various modifications may be made which will readily occur to those skilled in the art. Obviously, recourse may always be had to a voltage divider or potentiometer connected in parallel with the lamp, as illustrated in the circuit of Fig. 1, or to additional windings to achieve the same result. The appended claims are therefore to cover any such modifications coming within the true spirit and scope of the invention.

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

l. A switch-start leading power factor circuit combination comprising an elongated electric discharge device having a pair of filamentary thermionic electrodes sealed into opposite ends thereof, a leading power factor load circuit including said device and comprising a voltage source and a ballasting capacitance connected in series therewith, an electrode preheating circuit comprising a thermal starting switch connected in shunt across said device and in series with its filamentary electrodes, a conductive member located in proximity to said device and extending the length thereof, and means for applying to said member a potential intermediate those applied to said electrodes by said load circuit at starting.

2. A switch-start leading power factor circuit combination comprising an electric discharge device including an elongated envelope having a pair of filamentary thermionic electrodes sealed into opposite ends thereof, a leading power factor load circuit including said device and comprising an autotransformer having primary and secondary windings connected in series with a ballasting capacitance across said device, an electrode preheating circuit comprising a thermal glow starting switch shunted across said device in series with its electrodes, a conductive member located in proximity to said envelope and extending the length thereof, and a connection between said member and the junction of said primary and secondary windings.

3. A lead-lag ballast and fixture arrangement for switch-start lamps comprising a pair of discharge lamps each comprising an elongatedtzenvelope having a pair of filamentary thermionicelectrodes sealed ,into opposite ends thereof, an autotransformer comprising ,a primary winding anda pair of secondary windings coupled to said primary with high leakage reactance, a ballasting capacitance, a leading power factor load circuit comprising said primary winding, one of said secondary windings connected to the high ,side of said primary winding and in aiding voltage relationship therewith, and said ballasting capacitance connected in series across one of said lamps, a lagging power factor load circuit comprising said primary winding, and the other of said secondary windings connected to the high side of said primary winding and in aiding voltage relationship thereto and connected across the other of said lamps, each of said lamps having an electrode heating circuit comprising a thermal glow starting switch connected in shunt with the lamp and including its electrodes in series, a conduc- 8 tive member located in close proximity to the envelopes of said lamps and extending the length thereof, an alternating current power-supply having one side grounded,

a connection between the high side of said primary winding and thegrounded side of said supply, means for connecting the other side of said supplyto .the low side of said primary winding, and means grounding said conductive member.

References Cited in the file of this patent UNITED STATES PATENTS 2,305,487 Naster Dec. 15, 1942 2,462,336 Ruff Feb. 22, 1949 2,491,854 Force Dec.,20, 1949" 2,504,549 Lemmers Apr. 18, 1950: 2,509,188 Feinberg May 23, 1950 2,597,604 Waguet May 20, 1952

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