US3371245A

US3371245A - Discharge lamp circuit having series condenser and shunt switch for discharging condenser through lamp

Info

Publication number: US3371245A
Application number: US442068A
Authority: US
Inventors: Roger E Hume
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1965-03-23
Filing date: 1965-03-23
Publication date: 1968-02-27
Anticipated expiration: 1985-02-27
Also published as: GB1108736A; DE1539413A1

Description

Feb. 27, 1968 R. E. HUME 3,

DISCHARGE LAMP CIRCUIT HAVING SERIES CONDENSER AND SHUNT SWITCH FOR DISCHARGING CONDENSER THROUGH LAMP Filed March 215, 1965 5 Sheets-Sheet l Attorney.

Feb. 27, 1968 R. E. HUME 3,

DISCHARGE LAMP CIRCUIT HAVING SERIES CONDENSER AND SHUNT SWITCH FOR DISCHARGING CONDENSER rrmouen LAMP 3 Sheets-Sheet 2 Filed March 23, 1965 lm/entor. Payer/5 Hume; y zttarzny.

ENHME 4a Feb. 27, 1968 R. E. HUME 3,

DISCHARGE LAMP CIRCUIT HAVING SERIES CONDENSER AND SHUNT SWITCH FOR DISCHARGING CONDENSER THROUGH LAMP 3 Sheets-Sheet Filed March 23, 1965 TIME [In enter.- Fger' El/ame, 57X W Attorvuy.

United States Patent DISQHARGE LAMP CIRCUIT HAVING SERIES CONDENSER AND SHUNT SWITCH FOR DIS- CHARGENG CONDENSER THROUGH LAMP Roger E. Hume, Danviile, 111., assignor to General Electric Company, a corporation of New York Filed Mar. 23, 1965, Ser. No. 442,068 10 Claims. (Ci. 315--240) ABSTRACT OF THE DISCLOSURE Apparatus for starting and operating electric discharge lamps which during operation supplies a current having a more nearly square wave shape. This is achieved by a saturable reactor or switching device in the output circuit to effect a controlled reversal of the voltage across the discharge lamp by discharging a capacitor when a preselected voltage level is reached in each half cycle.

This invention relates to systems and ballast apparatus for starting and operatnig electric discharge illumination means having a negative resistance characteristic. More particularly, it relates to such systems and ballast apparatus capable of operating electric discharge illumination means with an improved light output for a given value of lamp current.

In many lighting applications where electric discharge illumination means, such as fluorescent lamps, are used, it is particularly desirable to obtain maximum light output corresponding to minimum root mean square value of the lamp current. By minimizing the root mean square value of the lamp current, it is possible not only to increase the light output of the electric discharge lamps but also to reduce the volt-ampere requirements of the ballst apparatus. In applications where a high leakage reactance transformer is used in the ballast apparatus, reductions in heat losses can be achieved.

The relative magnitude of the root mean square value of the lamp current is dependent upon the wave shape of the lamp current. Ideally a lamp current having a square Waveform would provide maximum light output for a minimum value of root means square lamp current. It is therefore desirable in many ballast applications to operate electric discharge lamps with a lamp current that is nearly square in waveshape.

Heretofore a number of arrangements have been employed for improving the shape of the lamp current waveform. In conventional apparatus employing high leakage reactance ballast transformers, the waveform of the lamp current is characterized by a pronounced peak. This peaked waveform indicates that a relatively high root mean square value of the lamp current is required to provide a predetermined light output. An arrangement for improving the waveshape of the lamp current by varying the leakage reactance through the use of nonlinear magnetic shunts is described and claimed in United States Patent No. 3,010,050 granted to Hume et al. Another arrangement for improving lamp current Waveshape by controlling leakage reactance by the use of bridged gaps is disclosed in United States Patent No. 2,869,037 granted to Brooks et al.

Since commerically available power sources vary in voltage to some degree, the regulating characteristics of a ballast apparatus are an important consideration particularly in applications where the electric discharge lamps are used in photocopy applications. Generally, a lower supply voltage will tend to reduce the light output and effect the starting performance of the ballast apparatus. On the other hand, a higher starting voltage may cause the light output to increase thereby causing variations from set levels that may impair the operation of the photocopy equipment. In a :ballast apparatus adapted for use in photocopoy equipment it is therefore particularly desirable that variations resulting in light output for a given range of variations in the supply voltage be maintained within predetermined limits.

Accordingly, a general object of the present invention is to provide a system and ballast apparatus for starting and operating electric discharge illumination means wherein the lamp current has an improved waveshape.

Another object of the present invention is to provide an improved system and ballast apparatus for operating electric discharge illumination means having an improved regulating characteristic.

It is still another object of the present invention to provide an improved ballast apparatus wherein maximum light output can be achieved for a given root mean square value of lamp current.

A further object of the present invention is to provide a ballast apparatus incorporating an improved arrangement for improving the lamp current waveshape.

It is a more general object of the present invention to provide an improved system and ballast apparatus for operating a plurality of electric discharge lamps with improved operating characteristics.

In accordance with one form of my invention, I have provided a system including a ballast apparatus for starting one or more electric discharge lamps in a series lead circuit. The ballast apparatus includes an alternating voltage supply means, such as a high leakage reactance transformer, having an input circuit for connection with an alternating supply and also having an output circuit for supplying the output voltage of the alternating supply means to the lamps and including at least one series capacitor for introducing a net capacitive reactance in series with the electric discharge lamps during operation. The output circuit further includes a means for effecting a controlled reversal of the voltage across the electric discharge lamps in response to a preselected operating voltage level. Preferably, as a means for effecting the controlled reversal of the lamp voltage, a saturable reactor is provided in the lamp operating circuit, the reactor being designed to saturate at the predetermined voltage level to switch the voltage on the capacitor and thereby produce a controlled reversal of the lamp voltage in each half cycle.

In another aspect of my invention, I utilize as a means for effecting the lamp voltage reversal early in each half cycle a semiconductor switching device, such as a silicon controlled rectifier or a Shockley diode. The semicon ductor switching device is connected in the lamp operating circuit in series with the capacitor. At a predetermined point in each half cycle the alternating output voltage of the ballast apparatus, the semiconductor device is switched into conduction to switch the voltage on the series capacitor, across the electric discharge lamps and thereby force a reversal of the lamp current. An important advantage of a ballast apparatus incorporating such a means for effecting a controlled reversal of lamp current is that it is possible to achieve a more nearly square lamp current waveform and thereby obtain more light output per ampere of current supplied to the electric discharge lamps.

The subject matter which I regard as my invention is set forth in the appended claims. The invention itself, however, together with further objects and advantages may be better understood by referring to the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a schematic circuit diagram of a ballast apparatus embodying one form of my invention for starting and operating twelve electric discharge lamps;

FIGURE 2 is a schematic circuit diagram of another form of the improved ballast apparatus utilizing a semiconductor switching device for effecting the controlled reversal of the lamp current in each half cycle;

FIGURE 3 illustrates the P-N junction arrangement of the five layer diode and also shows the correspondence between the junction arrangement and the symbol used for the five layer diode in FIGURE 2;

FIGURE 4 is a schematic diagram of a ballast apparatus embodying another form of the invention wherein the saturable reactor not only performs the current reversing operation in accordance with the invention but also carries out the voltage transforming function to provide the stepped-up voltage required to start and operate a pair of hot cathode type of fluorescent lamps;

FIGURE 5 illustrates three curves showing the ideaized waveforms, Curve V representing the idealized waveform of the lamp current, Curve V representing the idealized waveform of the capacitor voltage and Curve V representing the idealized waveform of the saturable reactor voltage, the curves being presented as an aid in the explanation of the operation of the improved ballast apparatus of my invention.

Referring now more specifically to FIGURE 1, I have illustrated therein two interconnected ballast apparatuses and 11 for operating two groups of electric discharge lamps 1, 2 3 4, 5 a and 7 8 9 11, 12 spectively. The six lamps operated by the ballast apparatus 10 are disposed in close proximity to the conductive part or fixture 12 connected to a ground G and providing capacitive coupling between the lamps and conductive part 12 to aid in the starting of the electric discharge lamps. Similarly, a conductive part 13 connected to the ground G is disposed close to the six electric discharge lamps operated by ballast apparatus 11 and provides capacitive coupling between the filaments of the lamps and the conductive part 13 to aid in starting these lamps. It will be noted that each ballast apparatus 10, 11 is also connected to the ground G through resistors R and R by lead 14 joining the low potential side of the ballast circuits. Such a grounding arrangement insures that in applications where the power distribution system is not effectively grounded the output voltage across high leakage reactance transformers T T is initially applied between the lamps and the

conductive parts

12, 13 respectively to cause ionization to be initiated in the vicinity of lamp filaments and to thereby facilitate the starting of the lamps.

In the embodiment of the invention shown in FIGURE 1, which was used to provide the illumination required in photocopy equipment, separate filament transformers T and T were used since continuous cathode heat was required in order to obtain fast starting and better lamp life. Filament transformers T has seven cathode heating windings H H H H H H and H inductively coupled with a primary winding F on a suitable magnetic core 15. It will be seen that the upper ends of the primary windings P and P of the filament transformers T and T are connected by lead 16 to a common connection 17, which electrically joins these ends to the high potential side of the alternating current supply. In the embodiment of the invention illustrated in FIGURE 1 the filament transformer T of ballast apparatus 11 is essentially identical to the one used in ballast apparatus 10. The filament transformer T also includes a primary winding P a magnetic core 20 and cathode heating windings H H H H 12 13 and 14 To provide the necessary starting and operating voltages high leakage reactance type of transformers T is employed in the ballast apparatus 10 with leads 18 and 19 adapted for connection to a suitable alternating current source (not shown). A similar high leakage reactance transformer T is used in ballast apparatus 11 and includes a pair of input leads 21 and 22 for connecting the primary winding P across an alternating current source. Step-up transformers are required since the starting and lamp operating voltages required to start six fluorescent lamps are greater than the applied 60 cycle, volt line supply. The high leakage reactance ballast transformers T T include the primary windings P P secondary windings S S

magnetic cores

23, 24 and magnetic shunts 25, 26.

A type of magnetic core which may be used in the high leakage reactance transformers T and T is described in Hume et al. Patent 3,010,050. The magnetic shunts 25, 26 supply a path for the leakage flux between the primary and the secondary windings. The effect of leakage fiux is that during operation the voltages induced in the windings cause the transformers T T to have a relatively high leakage reactance, and the load current in the output circuit supplied to the lamps is thereby limited. It will be understood that the flux leakage path, if desired, may be formed by nonmagnetic material such as air of by shunts formed integrally on the magnetic core or by inserted shunts of magnetic material or other suitable means.

A pair of capacitors C and C connected in series circuit with the secondary winding S introduce a net capacitive reactance into the output circuit and serve as an energy storage element. Generally the net capacitive reactance in the output circuit helps to minimize the load that the ballast apparatus contributes to the distribution system. Capacitor discharge resistors R and R are connected in shunt with the capacitors C and C and provide a path for the discharge of these capacitors when the ballast apparatus 10 is disconnected from the alternating current supply. If desired, a radio interference capacitor may also be connected across the input terminal leads or across the electric lamps where it is necessary to suppress radio interference from the lamps which may feed back to the power lines through the ballast.

In balast apparatus 11 a pair of series capacitors C and C are also employed to provide a net capacitive reactance in the output circuit of transformer T and a pair of capacitor discharge resistors R and R are connected in shunt with the series capacitors C and C Although a pair of series capacitors were used in each of the ballast circuits shown in FIGURE 1 it will be appreciated that a single capacitor may be substituted for each pair of series capacitors.

The starting of the electric discharge lamps L L L L L and L is effected by providing a starting capacitor C connected in shunt across lamps L L L and L so that the open circuit starting voltage is initially applied across the pair of lamps L and L A second starting capacitor C is connected across lamps L and L so that the open circuit starting voltage is applied across lamps L and 1...; after lamps L and L are ignited. In ballast apparatus 11 starting capacitors C and C serve the same function as starting capacitors C and C of ballast apparatus 10.

In accordance with one important aspect of my invention I have placed a saturable reactor X consisting of a reactor winding W and the saturable core 30 across the parallel-connected series capacitors C C and the electric discharge lamps L L L L L and L A saturable reactor X comprised of a reactor winding W and saturable core 31 is similarly connected in the circuit of ballast apparatus 11. The saturable cores used in the illustrated exemplification of the invention are formed of laminations and are designed to saturate at a predetermined voltage level in each half cycle of the alternating output voltage of the high leakage reactance ballast transformers T and T When the saturable reactors X and X saturate, they present a low impedance to the current in the output circuit and thereby cause the voltage across the series capacitors C C and C C to be switched across the group of lamps in its respective output circuit. The capacitor discharge current flows in a closed loop which includes a pair of series capacitors, a saturable reactor and a group of lamps. By effecting such a reversal of the lamp voltage early in each half cycle, it was found a more square-like lamp current waveform is obtained, and an improved light output is achieved for a given value of root mean square lamp current.

By way of a more specific exemplification of the invention, the ballast apparatuses and 11 shown in FIGURE 1 were constructed with the following circuit components, which are given by way of illustration:

Transformers T T Primary winding P P 396 turns of .0508 of an inch copper wire. Secondary winding S S 1330 turns of .0320 of an inch copper wire.

' Filament Transformers T T Primary windings P P 703 turns of .0142 of an inch copper wire. Cathode heating windings H17 H29 H31 i H5 H6 H7 H8, H9 H10 H11:

In order to demonstrate the improved regulating characteristics and the light output achieved by the improved series capacitor switching arrangement a ballast apparatus utilizing the components as described above Was operated and tested. The lamp current and percent lumen maintenance were determined for line voltages of 105, 115 and 125 volts and with series capacitors having an average capacitance of 8.022 microfarads and 8.14 microfarads. The light output of the lamps was measured by a photocell connected to a DC. meter calibrated to indicate the lumen output with light output of the lamps assumed to be 100 percent when operated on a reference ballast adjusted for since wave current of .0800 amperes. In Table I the results of these determinations are summarized.

TABLE I Series Lamp Current Lumen Capacitance Input (volts) (amperes) Maintenance (microfarads) (percent) circuit voltage, which is the sum of the voltages across the primary and secondary windings P and S of transformer T is applied initially across lamps L and L due to the connection of the starting capacitor C Since both ballast apparatuses 10 and 11 start and operate in essentially the same manner, I will describe herein only the manner in which ballast apparatus 10 is started and 0perated.

Due to the connection of the starting capacitor C across the four lamps L L L and L substantially all of the open circuit voltage of transformer T appears in the first instance across the lamps L and L Also, at the same instant the filaments of the lamps are being heated to electron emitting temperatures by the filament transformer T After lamps L and L are ignited, current flows through the starting capacitor C and produces a relatively high voltage drop which is now impressed across lamps L and L Once lamps L and L have ignited the voltage drop across starting capacitor C is impressed across lamps L and L these last two lamps are fired. With all of the lamps operating the impedance across the lamps is sufiiciently greater than the impedance of the starting capacitors C and C so that the current flow through the starting capacitors is negligibly small during operation.

The saturable reactor X is designed so that at the peak value of the voltage applied across its terminals, the saturable reactor X is at or near saturation, and its impedance falls down to zero to switch the voltage on capacitors C and C across the lamps and effect a quick reversal of the lamp current in each half cycle. The controlled reversal of the lamp current produces a lamp current having a more nearly square waveshape.

In contrast with the embodiment of the invention shown in FIGURE 1, a ballast apparatus 35 shown in FIGURE 2 does not utilize a saturable reactor to effect a quick reversal of the lamp current in each half cycle but employs a five layer diode D The voltage transforming and current limiting functions of the ballast apparatus 35 are carried out by a high reactance transformer T having a primary winding P a secondary winding S and cathode heating winding H H and H The secondary winding 5 is loosely coupled with the primary P on a magnetic core 36. A magnetic shunt 37 is interposed between the primary winding P and the secondary winding S to provide a path for leakage flux.

As is shown schematically in dashed outline in FIGURE 2, the ballast apparatus 35 is housed in a case or canister 38. The ballast apparatus 35 is adapted for operating a pair of hot cathode fluorescent lamps L and L The lamps L and L are connected so that they can be started and operated in series and are shown disposed in proximity to a conductive part 39, usually the lamp fixture, connectedto a ground G The conductive part 39 provides for capacitive coupling between the lamps L and L and the conductive part 39 in order to aid in starting the lamps. The low potential side of the primary winding P is connected through a resistor R to the ground G through the case 38. With this grounding arrangement the voltage across the primary winding P and secondary winding S applied during starting condition between a lamp electrode and the conductive part 39 to initiate ionization in the vicinity of the lamp electrode thereby to facilitate the starting of the lamps L and L In order to energize the ballast apparatus 35, a pair of input terminals or leads 40, 41 are brought out of the ballast case 38 for connection to a suitable alternating power supply such as a 60 cycle, volt commercial supply. The output of the ballast apparatus 35 is supplied to the lamps L and L by three pairs of

leads

42, 43, 44, 45 and 46, 47.

In FIGURE 2 the capacitor discharge resistors R and R are shown connected in shunt with a series 7 capacitor C and a starting capacitor C respectively. The resistors R R provide a path for the discharge of the capacitors C C when the power supply is disconnected from the ballast apparatus 35.

The voltage required to start and operate the lamps L and L is applied across the lamps essentially by the output leads 43 and 47. The voltages induced in cathode heating windings H H and H are applied across the filaments of lamps L and L by leads 42, 43, 44, 45, 46 and 47 to maintain a continuous supply of current to these filaments when the ballast apparatus 35 is operating the lamps.

In the embodiment illustrated in FIGURE 2, the five layer diode D effects a quick reversal of the currents supplied to the lamp at or near the peak value of the voltage applied across its terminals 48, 49. The resistor R connected in series with the five layer diode D is provided to limit the current flowing through the five layer diode D; when it is in a conducting state.

As is shown in FIGURE 3, the five layer diode, which may be employed in the practice of the invention, is comprised of five P-type and N-type zones of semiconducting material. The five layer diode is switched from its high impedance state to its low impedance state when the voltage across its terminals 48, 49 exceeds the switching voltage of the diode. The value of this switching voltage for the five layer diode used in this exemplification of the invention was such that when the output voltage of the transformer T reached its peak value, the five layer diode D switched to its low impedance state in each half cycle to cause the voltage across capacitor C to be applied across lamps L and L It will be appreciated that the five layer diode D is a bidirectional switching device and can be fired during each alternation of the voltage at approximately the point at which its switching voltage is exceeded.

Although I have illustrated a five layer diode in the ballast apparatus 35 of FIGURE 2, it will be apparent that other bidirectional semiconductor switching arrangements, such as a silicon controlled rectifier or a four layer diode in a full wave bridge, can be employed to perform the switching action to effect a controlled reversal of lamp current at about the peak point of the transformer output voltage in each half cycle.

The starting of lamps L and L by ballast apparatus 35 is accomplished in a conventional manner. The open circuit voltage of the ballast apparatus 35 is initially applied across lamp L since the starting capacitor C effectively shunts the lamp L Also, starting of lamp L facilitated by a small auxiliary current flow between one of its lamp filaments and the conductive part 39 disposed in capacitive relationship therewith. After lamp L is ignited, the voltage across the starting capacitor C is sufficient to start lamp L Thus in each half cycle when the voltage across the ballast transformer T is at or near the peak point value, the five layer diode D fires and effects a controlled reversal of the lamp current to render the waveform more square-like.

Referring now to the schematic circuit diagram shown in FIGURE 4, I have illustrated therein a ballast apparatus 50 employing a saturable reactor T which not only performs the current reversing function but also supplies the step-up in voltage required to start and operate a pair of fluorescent lamps L and L The saturable reactor T includes a primary winding P and a secondary winding S inductively coupled with the primary winding P on a saturable magnetic core 51. The magnetic core 51 is designed to saturate at or near the peak value of the alternating voltage across primary winding P Also inductively coupled with the primary winding P are three cathode heating windings H H and H The series capacitor C the starting capacitor C and the capacitor discharge resistors R R perform the same functions as the corresponding parts do in ballast apparatus shown in FIG- URE 2. A conductive part 54 connected to a ground G is positioned in proximity to the lamps L and L and a resistor R is connected to ground G through ballast case 55.

The current limiting function in the ballast apparatus 50 is effected by a reactor X The reactor X also serves the purpose of isolating the lamp operating loop from the alternating current supply when saturable reactor T saturates to effect a reversal of the lamp current in accordance with one form of my invention.

The starting of the lamps L and L is accomplished in the same manner as was previously described in connection with the ballast apparatus 35 shown in FIGURE 2. During the starting condition the open circuit voltage of saturable reactor T is initially applied to lamp L which is the first lamp to be ignited. After lamp L is ignited, the voltage drop across the starting capacitor 0,, is sufficient to fire lamp L and the lamps L L are operated in series. As in the other embodiments of my invention, a controlled reversal of the lamp current is effected in each half cycle. When the saturable reactor T saturates in each half cycle, it presents a very low impedance as compared with its unsaturated impedance and thereby causes the voltage across the series capacitor C to be applied across the lamps L L and brings about a controlled reversal of the lamp current.

In order to more fully explain what I believe is the mode of operation of the improved capacitor switching arrangements, I have illustrated in FIGURE 5 idealized voltage waveforms, V representing the waveform of the voltage across the saturable reactor, V representing the waveform of the voltage across the series apacitor and, V representing the waveform of the lamp voltage. It will be undrestood that these waveforms are not actual voltage waveforms but represent idealized shapes of these voltages.

It will be noted that the saturable reactor voltage waveform V in the first half cycle is increasing at a constant rate and the voltage waveform V is essentially triangular in shape, the apex of the triangle representing the point of saturation of the saturable reactor. During the same half cycle the series capacitor voltage waveform is decreasing at essentially the same rate as the saturable reactor voltage V It will be understood that the sum of the instantaneous values of the saturable reactor voltage waveform V and the series capacitor voltage waveform are equal to the instantaneous value of the lamp voltage. Because of the relative phase relationship and position of the waveforms V and V with respect to the zero axis, it will be seen that ideally the sum of the instantaneous values of voltage waveforms V and V are constant in magnitude in any half cycle or in other words, the voltage waveform V ideally would be a square waveshape. According to my understanding of the operation of the improved capacitor switching arrangement, by quickly switching the series capacitor voltage across the lamps in each half cycle when the voltage of the supply means is at or near its peak, more nearly linear type of reactor voltage and capacitor voltage relationships are approached to produce a more square-like lamp voltage waveform.

An important advantage derived from the use of the various means for effecting the improved controlled reversal of the lamp current is that it is possible to obtain a maximum lumen output per ampere of lamp current. Further, a ballast apparatus characterized by good regulating characteristics is provided wherein the light output is only minimally affected by variations in supply voltages.

From the foregoing description of the various embodin ents of my invention, it will be apparent that many modifications may be made. It will be understood, however, that these embodiments of the invention are intended as exemplifications of the invention and that the invention is not limited thereto. It is to be understood, therefore, that I intend by the appended claims to cover all such modifications that fall 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:

1. A ballast apparatus for starting and operating an electric discharge illumination means having a negative resistance characteristic, said ballast apparatus comprising a high leakage reactance transformer having at least a primary winding and a secondary winding inductively coupled on a magnetic core, said high leakage reactance transformer including an input circuit for connection to an alternating current supply and having an output circuit for applying an operating voltage to the electric discharge illumination means, a saturable reactor having a saturable core and a winding wound thereon and connected in shunt across said primary and secondary winding in the output circuit, said output circuit including aseries capacitor connected in series circuit relationship with said secondary winding, said saturable reactors having a saturating characteristic wherein the saturable core saturates in each half cycle when the operating voltage reaches a predetermined level thereby to effect a discharge of the capacitor in each half cycle and produce a current in the operating circuit having a more nearly square wave shape.

2. A ballast apparatus for starting and operating an electric discharge illumination means having a negative resistance characteristic, said ballast apparatus comprising a high leakage reactance transformer having at least a primary and a secondary winding inductively coupled on a magnetic core, said high leakage reactance transformer including an input circuit for connection to'an alternating current supply and having an output circuit for applying an operating voltage to an electric discharge illumination means, said output circuit including a series capacitor connected in series circuit relationship with said secondary winding, semiconductor switching means connected in said output circuit across the primary and secondary winding of said high leakage reactance transformer for switching the voltage across the series capacitor to produce a controlled reversal of the current supplied to the electric discharge illumination means in each half cycle when the operating voltage reaches a predetermined level thereby to supply to said electric discharge illumination means a current having a more nearly square waveshape.

3. The ballast apparatus set forth in claim 2 wherein an impedance element is connected in series circuit relation with said semiconductor switching means for limiting the current discharge from said series capacitor.

4. A ballast apparatus for starting and operating an electric discharge illumination means having a negative resistance characteristic, said ballast apparatus comprising a high leakage reactance transformer having at least a primary and a secondary winding inductively coupled therewith on a magnetic core, said high leakage reactance transformer including an input circuit for connection to an alternating current supply and also having an output circuit for applying an operating voltage to the electric discharge illumination means, said output circuit including a series capacitor connected in series circuit relationship with said secondary winding for introducing a net capacitive reactance in series therewith, said trans-former providing a path for charging current to said series capacitor, and means in the output circuit for periodically providing a low impedance discharge path for said series capacitor and a low impedance shunt path in the output circuit of said transformer, said last mentioned means producing a controlled reversal of the voltage supplied to the electric discharge illumination means by effecting a discharge of the capacitor when the operating voltage reaches a predetermined level in each half cycle thereby to provide a current to the electric discharge illumination means having a more nearly square waveshape.

5. A system for starting and operating electric dis charge illumination means comprising: electric discharge i illumination means, a high leakage reactance ballast trailsformer having a magnetic core with a primary and a secondary winding inductively coupled thereon for supply ing operating voltage and current to the electric discharge illumination means, said high leakage reactance transformer having an input circuit for connecting to an alternating current supply and having an output circuit, said input circuit including the primary winding and the output circuit including at least the secondary winding of said high leakage reactance ballast transformer and said electric discharge illumination means, at least one capacitor connected in series with said electric discharge illumination means in the output circuit and means in the output circuit for periodically providing a low impedance path across at least said secondary winding and switching the voltage on said at least one capacitor at a predetermined point in each half cycle of the operating voltage thereby to produce a controlled reversal of the current supplied to the electrical discharge means and supply a voltage to said electric discharge means having a more nearly square waveshape.

6. The system set forth in claim 5 wherein said means in the output circuit for switching the voltage includes a semiconductor switching device and a serially connected impedance element, said semiconductor switching device being switched on in each half cycle when the operating voltage reaches a predetermined level.

7. The system as set forth in claim 5 wherein the electric discharge illumination means is comprised of a plurality of hot cathode lamps and a filament transformer haVing a plurality of cathode heating windings is connected in the output circuit to supply the cathode heating current Ito the plurality of hot cathode lamps.

8. A system for starting and operating electric discharge illumination means having a negative resistance characteristic, said system comprising: an electric discharge illumination means, a high leakage reactance ballast transformer having an output circuit connected to the electric discharge illumination means and supplying the operating voltage thereto, said high leakage reactance transformer having at least a primary and a secondary winding and also having an input circuit including said primary winding for connection to an alternating current source, said output circuit including at least the secondary winding, a capacitor in series with the secondary winding, and a filament transformer having a plurality of cathode heating windings coupled with said electric discharge illumination means and supplying cathode heating current thereto, a saturable reactor having a reactor winding connected in a closed loop with said capacitor and said electric discharge illumination means, said reactor winding wound on a saturable core having a saturating characteristic such that the saturable core saturates in each half cycle when the operating voltage reaches a predetermined value to discharge the capacitor and produce an operating current for the electric discharge illumination means having a more nearly square waveshape.

9. In a system for operating a first and second group of electric discharge lamps, a first high leakage reactance ballast transformer having a primary and a secondary winding for supplying the operating voltage and current for the first group of electric discharge lamps, a second high leakage reactance transformer having a primary and a secondary winding for supplying the operating voltage and current to said second groups of electric discharge lamps, each of said high leakage reactance ballast transformers having an input circuit for connection to an alternating current supply and also having an output circuit for supplying the operating current to its respective group of electric discharge lamps, each output circuit including a capacitor connected in series circuit with the associated secondary winding and a filament transformer for supplying cathode heating current to the electric discharge lamps of its respective groups, and current reversing means seperate from said transformers in each of said 1 1 first and second output circuits for producing a controlled reversal of the current at a predetermined point in each half cycle of the operating voltage of its respective high leakage reactance transformer thereby to supply to said first and second groups of electric discharge lamps a cur- 5 rent having a more nearly square waveshape.

10. In the combination set forth in claim 9 wherein each of said current reversing means in the output circuits is comprised of a saturable reactor having a Winding and a core that saturates in each half cycle in response to a 10 12 preselected value of voltage applied across the winding of said saturable reactor.

References Cited UNITED STATES PATENTS 3,125,705 3/1964 Feinberg et a1 315276 3,135,894 6/1964 Oglesbee et a1. 315-284 X 3,250,952 6/1966 Barriball 315-100 DAVID J. GALVIN, Primary Examiner.