Ballast Apparatus For Starting And Operating Arc Lamps

Abstract

Ballast apparatus for starting and operating arc lamps includes a full wave rectifier having its input connected in series with a ballasting reactor across the terminals of an alternating current supply. A filter circuit is connected across the output of the full wave rectifier so that a filtered D.C. voltage equal to the peak supply voltage appears at the output of the rectifier bridge. A lamp supply circuit, connecting the bridge output with input terminals for connection to the arc lamp, includes a series connection of a choke coil and a first autotransformer. A second autotransformer has its primary connected in series with the ballasting reactor across the A.C. supply terminals and its secondary connected to the lamp supply circuit between the choke coil and the first autotransformer. A value low capacitance is connected across the lamp supply circuit at the junction between the choke coil and the first autotransformer and, with the second autotransformer, provides a signal of low energy but at a voltage level substantially higher than the peak supply voltage. A start capacitor is connected in loop with the primary of the first autotransformer and a switch for providing a starting pulse to strike an arc in the arc lamp.

Description

BACKGROUND OF THE INVENTION

This invention relates to imroved ballast apparatus for operating arc lamps. More particularly it relates to such an improved ballast apparatus particularly suitable for operation of electric discharge lamps utilizing a metal halide filling in a quartz tube. Such lamps, which often are used for applications such as light sources in projectors, impose very severe operating and starting requirements on the associated ballast apparatus. Typiallly such lamps have a start or breakdown voltage of at least 8000 volts and in some instances in excess of 10,000 volts. As such lamps begin to operate, until hot spots are established on their cathodes, they have a requirement of an intermediate voltage of about 300 volts. Although this requirement is not completely understood it is known that it is essentially a voltage or potential requirement and that the lamps at this time do not necessarily require a large amount of energy. After the cathode spots are formed, the lamps operate at much lower voltage quite often in the neighborhood of 35 to 45 volts.

U.S. Pat. No. 3,467,886, issued to Robert P. Alley on Sept. 16, 1969 and assigned to General Electric Co., asignee of the present invention, illustrates and describes a ballast apparatus for starting and operating such lamps. The ballast apparatus of the type disclosed in the aforementioned Alley pattent derive the initial voltage, the intermediate voltage and the run voltage from a single alternating current source of electric energy, which may conveniently be a 120 volt 60 hertz domestic supply. Apparatus of the type shown and described by Alley start and run the arc lamp while, at the same time, protecting the lamp against high inrush currents which could cause failures such as breaking of the seals of the lamp when a lamp is being started in a cold condition. With such ballast apparatus it is necessary to use multi-position switches or relays for connecting one portion of the ballast apparatus to the lamp for starting and then disconnecting that portion and connecting another portion for operating the lamp.

Lamps of this general type are operated by direct current electrical energy and any substantial alternating current ripple must be removed from the operating signal as the alternating current ripple may beat with the shutter of a motion picture projector and cause a flicker to appear on the viewing screen.

SUMMARY OF THE INVENTION

Accordingly it is an object of the present invention to provide a new and improved ballast apparatus for starting and operating arc discharge lamps.

It is another object of this invention to provide such a ballasting apparatus for starting and operating arc discharge lamps of the metal halide type.

It is yet another object of this invention to provide such a ballast apparatus in which the need for complicated switching arrangement is eliminated.

It is still another object of the present invention to provide such an improved ballast apparatus in which the filtering of the rectified main power for the lamp is accomplished at a relatively low voltage level and, at the same time, any high voltages required by the lamp are provided.

In accordance with one form of the present invention there is provided a ballast apparatus for starting and operating arc lamps from an alternating current source. The appratus includes a full wave rectifying means having an alternating current input and a direct current output. A reactive ballasting device is connected in a power supply circuit in series with the input of rectifying means across a alternating current source. An alternating current filter circuit is connected across the direct current output of the rectifying means. There is a lamp operating circuit for connecting the rectifying means and filter circuit to terminals for connection to an arc lamp. The lamp operating circuit includes a direct current choke for limiting current flow from the filter circuit to the lamp connection terminals upon energization of the at least one lamp. An intermediate circuit is connected to the lamp operating circuit on the lamp connection terminal side of the direct current choke and provides a high voltage, low energy signal as compared with the output of the rectifying means and filter circuit. A lamp starting circuit is coupled with the lamp operating circuit for introducing an electric energy pulse to strike an arc in the arc lamp.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects and advantages of the invention and the manner of attaining them may better be understood from the following description taken in connection with the accompanying drawing in which:

The single FIGURE of the drawing is a schematic electric circuit diagram of a ballast apparatus incorporating one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawing there is illustrated, in electric schematic circuit diagram form, a ballasting apparatus 10 incorporating one form of the present invention. Main input terminals 11 and 12 are provided for connection to a suitable source of electric energy which may, in the exemplification, be the normal 120 volt 60 hertz domestic power supply. An on-off switch 13 is connected to one of the main input terminals 11 so to selectively provide power to the ballast apparatus when the switch 13 is in its closed position. The switch 13 is connected to one side of a ballasting reactor 14, the other side of which is connected to one input terminal 15 of a full wave rectifying means in the form of a bridge rectifier 16. The other input terminal 17 of the bridge rectifier is connected to the other main input terminal 12. Thus, when the main input terminals 11 and 12 are connected to a source of a electric energy and the switch 13 is closed, the ballasting reactor 14 and input terminals 15, 17 of the rectifier 16 are connected in series across the power supply.

As is well known in the art the bridge rectifier 16 includes 4 diodes or rectifiers connected in a conventional bridge fashion. If desired a transient supressing capacitor may be connected in parallel with each of the diodes as shown in the aforementioned Alley patent. A direct current signal is derived from the bridge rectifier 16 at its output terminals 18 and 19. Terminal 18 is connected to a conductor 20 and may be considered to be the positive terminal. Terminal 19 is connected to a conductor 21 and may be considered the negative terminal of the bridge.

An A.C. filter circuit including an inductance 22 and capacitors 23 and 24 is connected between the conductors 20 and 21 and thus across the output terminals 18, 19 of the bridge rectifier 16. The inductor 22 is connected in series with the capacitors 23, 24 which are connected in parallel with each other. The parallel connection of the capacitors is shunted by a series connection of resistors 25 and 26. The resistors serve as a discharge path for the capacitors in the event the capacitors are charged when the circuit is de-energized.

The capacitors 23, 24 and inductor 22 are sized so that the filter circuit is near resonance at 120 hertz, which is the frequency of the D.C. output pulses of the bridge rectifier 16. Thus the circuit effectively filters any A.C. ripple so that the D.C. signal available at the output terminals 18, 19 is sufficiently free of alternatng current ripple that it will not cause a flicker in a arc lamp. It will be understood that the inductor 22 could be removed from the filter circuit and the capacitors 23, 24 made somewhat larger and proper filtering still would be provided. However, as will become apparent hereinafter it is desirable that the filtering be accomplished in a manner which stores as little energy as possible. Use of the inductor 22 enables the capacitors 23 and 24 to be smaller and thus the stored energy will be less. The inductor 22 also will assist in limiting discharge current from the capacitors 23, 24 when a lamp arc is struck. It also will be understood that the capacitors 13, 24 could be combined into a single unit. However, at the voltage and energy levels involved in typical applications the use of two separate units is more economical.

The conductor 20 is connected to one side of a D.C. choke coil on inductor 27, the other side which is connected to a diode or rectifier 28. The diode 28 is connected by a conductor 29 to the primary winding 30 of an autotransformer 31. The autotransformer also includes a secondary winding 32 which is connected to a lamp supply terminal 33. The conductor 21 is connected to another lamp supply terminal 34 and thus, when an arc lamp such as a metal halide lamp 35 is connected between the lamp supply terminals 33 and 34, a lamp supply circuit is completed.

The lamp supply loop of the lamp supply circuit extends from the bridge rectifier output terminal 18 through conductor 20, choke 27, diode 28, conductor 29 and autotransformer 31 to the first lamp supply terminal 33. From supply terminal 33 the loop extends through the lamp 35, terminal 34, and conductor 21 to the other output terminal 19 of the bridge rectifier.

A second autotransformer 36 has its primary winding 37 connected across the input terminals 15, 17 of the diode bridge 16. Thus the primary winding 37 of the autotransformer 36 is connected in series with the ballasting reactor 14 across the main input terminals 11, 12. The distal end of the secondary winding 38 of autotransformer 36 is connected through a diode or rectifier 39 and a current limiting resistance 40 to the conductor 29. From the conductor 29 a branch circuit including a serially connected resistance 41, capacitor 42 and capacitor 43 extends to the conductor 21. A pair of serially connected resistances 44 and 45 are connected in parallel with the serially connected capacitors 42, 43 and the junction between the resistances 44, 45 by a conductor 46. The autotransformer 36 steps up the supply voltage from terminals 11 and 12 so that a voltage somewhat higher than that of the supply is impressed on the branch circuit including resistance 41 and capacitors 42, 43. The diode 39 rectifies the output of the autotransformer 36 so that a D.C. voltage is applied while the resistance 40 serves as a current limiting resistance. When a ballasting apparatus is used in conjunction with an arc lamp requiring an intermediate voltage of about 300 volts the output of the autotransformer 36 will be at least 300 volts and may conveniently be as high as about 450 volts. This voltage will build up across the series connection of capacitors 42, 43 to provide the intermediate voltage for operation of the lamp. The resistances 44, 45 are bleed resistances for discharging the capacitors when the circuit is inactivated. It will be understood that capacitors 42, 43 could be combined into a single film type capacitor, however, the circuit arrangement shown with two separate capacitances enables the use of two electrolytic capacitors to provide this intermediate or backup voltage at a lower cost. The resistance 41 limits the current from the capacitors 42, 43 when a lamp arc is struck. The capacitors 42, 43 are of low capacitive values so that while their charge builds to a voltage substantially in excess of the supply voltage the total amount of energy available is relatively low. To this end each of the capacitors 42, 43 may be 10 microfarads while the capacitors 23, 24 in the filter circuit conveninetly may be 200 microfarads.

A start circuit, including a start capacitor 47 and charging resistor 48, is connected from the intermediate tap of the autotransformer 31, that is the tap between the primary and secondary windings, to conductor 21. A switch means, which conveniently may take the form of a spark gap device 49, is connected across the start capacitor 47 and primary winding 30 of the autotransformer 31. When the circuit is completed within the spark gap device 49 a closed loop is formed including the capacitor 47 primary winding 30 and spark gap device 49.

Assuming the lamp 35 to be off and the main terminals 11, 12 to be connected to a 120 volt 60 hertz power supply, operation of the ballast apparatus beings upon closing of the on-off switch 13. Initially very little current flows and the ballasting reactor 14 has little effect. Essentially 170 volts peak appears across the output terminals 18, 19 of the bridge rectifier 16. This charges the capacitors 23, 24 to essentially 170 volts. The 120 volts A.C. input also is applied to the primary winding 37 of autotransformer 36 and the autotransformer steps this voltage up to 450 volts, by way of exemplification. The autotransfromer secondary charges the serially connected capacitors 42, 43 to essentially 450 volts. The diode 28 enables the capacitors 42, 43 to be charged to a higher level than the capacitors 23, 24.

The intermediate circuit, including the autotransformer 36 and the capacitors 42, 43, provides a source for charging start capacitor 47. When the charge on capacitor 47 builds to the breakdown or spark level of spark gap device 49, which may conveniently be about 300 volts, the spark gap device 49 conducts. This, in effect, closes a switch completing the loop including start capacitor 47 and primary winding 30 of autotransformer 31. The start capacitor 47 then discharges through the primary winding 30 causing a much higher voltage about 8000 volts in the exemplification, to appear at the output of the secondary winding 32. This voltage causes the lamp 35 to arc.

Upon arcing of the lamp 35 the capacitors 23, 24 and 42, 43 discharge through the autotransformer 31. The inductor 22, choke coil 27 and transformer 31 limit the current flow as a result of the discharge of capacitors 23, 24, with the choke coil 27 doing the major portion of the current lighting. The current flow as a result of discharge of capacitors 42, 43 is limited only by the resistance 41 and autotransformer 31, however, these capacitors have very low energy and cause relatively small current. Limitation of the discharge current immediately upon lamp arcing is necesary as an excessive inrush current may damage the lamp, particularly the lamp seals.

After the lamp hot spots form on the lamp cathodes the voltage drop across the lamp, that is the lamp run voltage, will drop to about 35 to 45 volts. However, the current through the lamp will rise. At this time the lamp has a relatively flat characteristic and it is necessary to limit the lamp current. This function is performed by the ballasting reactor 14. The lamp current flows through the ballasting reactor 14 which causes a voltage drop across the reactor. This limits the voltage available to the A.C. input terminals of the bridge rectifier 16 and to the primary winding 37 of the autotransformer 36 so that the voltage available to the lamp supply circuit is essentially the designed lamp run voltage.

Once the lamp has ignited the start circuit and the intermediate voltage circuit could be completely removed. The supply voltage and current for the lamp basically is supplied through the bridge rectifier 16 with the filter circuit, including inductor 22 and capacitors 23, 24 filtering out the A.C. ripple so that there will be no flicker of the lamp.

It will be seen that a ballast apparatus incorporating the present invention provides for A.C. filtering at a relatively low voltage level so that the energy stored within the filtering circuit is much lower. Thus supression of the current flow from the filtering circuit upon the lamp arc first being struck is simplified. Additionally the higher level intermediate voltage requirement of such arc lamps is provided by a low energy circuit so that the starting current associated with it is much lower and requires less supression. This greatly simplifies the switching functions within the circuit so that, in effect, all that is required is a single pole single throw switch for the start circuit. This may conveniently be performed by spark gap device. It will be understood that this switching function could be performed by any number of other forms of switches such as a relay or silicon controlled rectifier and associated firing circuit for example. However, the spark gap device is very economical and is quite satisfactory in performance.

Since both the bridge rectifier 16 and the primary of auto-transformer 36 are supplied through the ballasting reactor 14, the single ballasting reactor in the A.C. supply circuit will provide a complete ballasting function for the lamp. Both the supply and ballasting functions can be performed by other arrangements. When isolation is desired, as in medical applications for instance, the isolated supply and ballasting functions both can be provided by a high reactance isolation transformer. In such an arrangement the bridge rectifier can be connected to an intermediate secondary terminal and the intermediate voltage circuit can be connected to the high voltage terminal of the isolation transformer secondary. Alternatively the bridge rectifier can be connected across the isolation transformer secondary, with the intermediate circuit connected to an autotransformer having its primary connected across the isolation transformer secondary.

A ballasting apparatus generally as shown in the figure has been successfully built and operated with components having the following value or characteristics:

Ballasting reactor 14 14 millihenries Capacitor 23 200 microfarads, 200 volts Capacitor 24 200 microfarads, 200 volts Inductor 22 4.4 millihenries Choke coil 27 4.8 millihenries Resistors 25, 26 2.5 kilohms, 5 watts Resistor 41 33 ohms, 2 watts Capacitors 42, 43 10 microfarads, 350 volts Resistances 44, 45 470 kilohms, 2 watts Capacitor 47 2 microfarads, 400 volts Resistance 48 100 kilohoms, 1/2 watt Resistor 40 10 kilohms, 2 watts Autotransformer 31 Copper foil wound on E-E laminated core Primary winding 6 turns Secondary winding 123.5 turns Autotransformer 36 Magnetic wire wound, .0113" diameter wire Primary winding 847 turns Secondary winding 1898 turns

While, in accordance with the patent statutes, the preferred embodiment of the invention has been described changes may be made therein without actually departing from the true spirit and scope of the invention. We intend by the appended claims to cover all such modifications as fall within the true spirit and scope of the invention.

Claims

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

1. A ballast apparatus for starting and operating at least one arc lamp from an alternating current source, said ballast apparatus including:

a. a full wave rectifying means having an alternating current input and a direct current output;

b. a reactive ballasting device; a power supply circuit for connecting said alternating current input of said rectifying means and said reactive ballasting device in series circuit relation across an alternating current source;

c. an alternating current filter circuit connected across said direct current output of said rectifying means;

d. a lamp supply circuit for connecting said rectifying means and filter circuit to lamp connection terminals for at least one arc lamp, said lamp supply circuit including direct current choke means for limiting current flow from said filter circuit to said lamp connection terminals upon the striking of an arc in the at least one arc lamp;

e. an intermediate circuit connected to said lamp supply circuit, said intermediate circuit providing a high voltage, low electric energy level as compared with the output of said rectifying means and filter circuit; and

f. a lamp starting circuit coupled with the lamp supply circuit for introducing an electric energy pulse to strike an arc in the at least one arc lamp.

2. A ballast apparatus as set forth in claim 1, wherein said intermediate circuit includes an autotransformer; the primary of said autotransformer being connected in series with said reactive ballasting device; the secondary of said autotransformer being connected to a low capacitive value capacitance.

3. A ballast apparatus as set forth in claim 1 wherein said lamp starting circuit includes an autotransformer having a primary and a secondary, connected in said lamp operating circuit, and a start capacitance and switch means connected in a loop with said primary of said autotransformer for providing a pulse of electric energy to said primary of said autotransformer.

4. A ballast apparatus as set forth in claim 3 wherein said switch means completes the said loop through said start capacitance and said autotransformer primary upon the charge on said start capacitance reaching a predetermined level and interrupts said loop upon substantial dissipation of energy from said start capacitance.

5. A ballast apparatus as set forth in claim 4 wherein said switch means is a spark gap device.

6. A ballast apparatus for starting and operating at least one direct current arc lamp from a source of alternating current electric energy, comprising:

a. main input terminals for connection to a source of alternating current electric energy;

b. full wave rectifying means having a pair of rectifier input terminals and a pair of rectifier output terminals;

c. a ballasting reactor, said rectifier input terminals and said ballasting reactor being connected in series across said main input terminals;

d. an alternating current filter circuit connected across said rectifier output terminals so that a filtered, direct current signal will appear at said rectifier output terminals;

e. a pair of lamp supply terminals for connection to at least one direct current arc lamp;

f. a lamp supply circuit including a choke coil and a first autotransformer connected in series relationship between one of said rectifier output terminals and one of said lamp supply terminals, the other of said rectifier output terminals and the other of said lamp terminals being electrically connected;

g. a second autotransformer; the primary of said second autotransformer being connected in series relationship with said ballasting reactor across said main input terminals; the secondary of said second autotransformer being connected to said lamp supply circuit between said choke coil and said first autotransformer; a low capacitive value capacitance connected in said lamp supply circuit on one side between said choke coil and said first autotransformer and on the other side to said other lamp supply terminal to provide a low energy signal having a voltage substantially in excess of that of the signal appearing at said rectifier output terminals; and

h. a start capacitance and switch means connected in a loop with said primary of said first autotransformer for providing a pulse of electric energy to said primary of said first autotransformer to strike an arc in the at least one direct current arc lamp.

7. A ballast apparatus as set foth in claim 6 wherein said switch means completes said loop through said start capacitance and said first autotransformer primary upon the charge on said start capacitance reaching a predetermined level and interrupts said loop substantial dissipation of energy from said start capacitance.

8. A ballast apparatus as set forth in claim 7 wherein said switch means is a spark gap device.

9. A ballast apparatus for starting and operating at least one arc lamp from an alternating current source, said ballast apparatus including:

a. main input terminals for connection to a source of alternating current electric energy;

b. full wave rectifying means having a pair of rectifier input terminals and a pair of rectifier output terminals;

c. first inductive means coupling said rectifier input terminals to said main supply terminals;

d. an alternating current filter means connected across said rectifier output terminals;

e. a pair of lamp supply terminals for connection to at least one arc lamp;

f. a lamp supply connecting said rectifier output terminals to said lamp supply terminals;

g. a low capacitive value capacitance connected in said lamp supply circuit; circuit means, including second inductive means, coupling said capacitance to said main supply terminals for charging said capacitance to a voltage level in excess of that appearing at said rectifier output terminals; and

h. a lamp starting circuit coupled with the lamp supply circuit for introducing an electric energy pulse to strike an arc in the at least one arc lamp.