BACKGROUND OF THE INVENTION An arc discharge lamp is a device for producing light output when electrical current is applied to the connecting electrodes. Due to the very low resistive nature of an arc lamp, a means has to normally be provided to limit the current in the lamp. For lamps that operate in domestic and industrial power supplies, which are alternating voltage sources, the simplest and cheapest method of limiting current is by means of a ballast or reactor placed in series with the lamp and the power supply. The ballast, which is sometimes known as a reactor, is an inductive element which provides an impedance to the alternating current flowing through the lamp. In equipment used for driving arc lamps, it is well known that iron and copper reactors or electronic ballasts are used for regulating current to the lamp. Such iron and copper ballasts produce an approximately sinusoidal waveform of current versus time, and hence a modulated light output is obtained.

Electronic ballasts produce either square wave or quasi sinusoidal current waveforms of varying frequencies, depending upon the application, and often do not exhibit light modulation. An avantage of electronic ballasts over conventional ballasts is that they are often of lighter weight and of higher efficiency. However, the higher cost and lower reliability of electronic ballasts in comparison to magnetic ballasts has limited their application and usage. In addition, such ballasts

are often electronically and acoustically noisy in comparison to conventional magnetic ballasts.

The design and manufacture of ballasts is known in the art, and the use of these devices is found on street lighting, fluorescent lighting, in film studios, and in many other applications. Ballasts used for supplying arc lamps are also known, in which iron and copper ballasts are typically used. However, conventional prior arts ballasts used in arc lamps often provide undesirable characteristics, such as visually observable flickering, acoustic noise, and high levels of heat output, which make these devices undesirable when used in film and video applications.

U. S. Patent 5, 283, 502 to Piaskowski et al. issued on February 1, 1994 discloses an improved arc discharge ballast which comprises harmonic resonators, simple switching means and a drive circuit. The harmonic resonators, switching means and drive circuit are coupled to the lamp to provide squaring of the current waveform and even light output. This device produces an even light output and substantial power savings, although the object of the present invention is to provide even greater power savings and a more even light output than what was achieved in this prior art device.

SUMMARY OF THE INVENTION It is a feature of the present invention to provide a circuit for supplying current to an arc lamp, which reduces flickering and acoustic noise in the arc lamp.

It is another feature of the present invention to provide a circuit for supplying current to an arc lamp which reduces heat generation and energy consumption during operation of the arc lamp.

It is another feature of the present invention to provide current inputs into the circuit to accurately control the shape of the current waveform in the lamp and produce a current waveform in the lamp that is almost perfectly square.

It is a further feature of the present invention to sense the current running through a portion of the circuit and control the current inputs used for controlling the shape of the current waveform.

According to the above features, from a first broad aspect, the invention provides a circuit for supplying a substantially square waveform of current to an arc discharge lamp, which is comprised of : a source of alternating voltage; primary current controlling means connected between the source of voltage and the lamp to produce an approximately sinusoidal waveform of current; current sensing and control means connected between the current controlling means and the lamp; and supplementary current controlling means connected between the current sensing means and the lamp which responds to control signals from the sensing and control means to provide supplementary current controlling to produce a nearly square waveform of current passing through the lamp.

According to the above features, from a second broad aspect, the invention provides a method for supplying a substantially square waveform of current to an arc discharge lamp in a circuit, including the steps of : (1) passing the current through a current controlling means so as to produce an approximately sinusoidal waveform of current; (2) passing the approximately sinusoidal waveform of current through a wave sensing and control means to sense the shape of the current waveform, said sensing and control means generating a control signal l in response to said circuit; (3) passing the approximately sinusoidal waveform of current through supplementary current controlling means which are responsive to said control signals to inject supplementary current waveforms into said approximately sinusoidal waveform of current to produce a final square waveform of current ; and (4) passing said final waveform of current through said lamp.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment to the invention will now be described with reference to the accompanying drawings, in which: FIG. 1 is a known prior art arc discharge lamp circuit including a lamp ballast.

FIG. 2 is a graph of the current waveform showing the plot of current versus time produced by the ballast in the circuit of Fig. 1.

FIG. 3 is a graph of the relative light intensity versus time produced by the lamp circuit of Fig. 1.

FIG. 4 is a graph of current versus time illustrating the ideal square current waveform which would produce optimal operation of an arc lamp.

FIG. 5 is a graph of light intensity versus time which would be produced by the optimal current waveform of Fig. 4.

FIG. 6 is a graph of two plots of current versus time. One of the plots shows the ideal square waveform, while the second shows the reduced sinusoidal waveform produced by inductive elements utilized in a circuit according to a preferred embodiment of the invention.

FIG. 7 is a plot of current versus time, illustrating a required current which must be injected to change the sinusoidal waveform of Fig. 6 into the perfect square waveform of Fig. 6.

FIG. 8 is a plot of current versus time, illustrating the current injected by two separate semiconductors in a circuit according to a preferred embodiment of the present invention.

FIG. 9 is a plot of current versus time, for current deriving from inductors in a circuit according to a preferred embodiment of the invention.

FIG. 10 is a plot of current versus time, illustrating a current injected by semiconductors in a circuit according to a preferred embodiment of the present invention.

FIG. 11 is a schematic diagram of an arc discharge lamp circuit according to a preferred embodiment of present invention.

FIG. 12 is an illustration of a permanent magnetic core inductor utilized in a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Before describing the preferred embodiments of the present invention, a reference is made to prior art, illustrated in Figures 1-3.

Figure 1 illustrates a known arc discharge lamp having an alternating voltage source 16 connected in series to an arc discharge lamp 21. In order to start the lamp, the circuit utilizes an igniter 19 connected to an inductor or transformer 20. The igniter may also be replaced by a transformer-capacitor arrangement known as a semi-resonant start (SRS) which is known in the art. Connected in series with the lamp and alternating voltage source are inductors 17 and 18, and which alternatively may be formed as a single device. The inductors 17 and 18 are alternatively referred to as"ballast"or"reactors", and can be manufactured using ferromagnetic cores, and copper or aluminum electrical conductors. The current waveform produced by the inductors 17, 18 is illustrated in Figure 2, and this figure shows the current produced by the inductors as having regular, approximately sinusoidal variations. These variations gradually increase to the maximum positive current and gradually decrease to the maximm negative current.

The actual variation in light intensity produced by the circuit of Figure 1 is illustrated in Figure 3. As can be seen from this figure, the fluctuations in current produce fluctuations in light intensity over the time intervals shown.

Although the entire time line of Figure 3 represents only 4/100 of a second, the

light fluctuation shown in Figure 3 would be detectable as flickering at certain camera and recording speeds, and is capable of producing other problems, such as acoustic resonance, where standing waves are created in the lamp which cause instability in the arc. This, in turn, can lead to even greater problems with flickering and can even lead to cases where the arc is extinguished altogether.

Theorv Supportions Preferred Embodiment Figure 4 represents the ideal current waveform which would be ideally generated by a ballast system in the arc lamp circuit. Figure 5 shows the resultant ideal lamp output from the ideal current waveform. The square waveform shown in Figure 4 results in an almost perfectly steady light output as shown in Figure 5. Although there may be dips in the light output, these dips occur over very narrow spaces of time, less than several microseconds in length, or may not even be existent, therefore not detectable by standard photographic equipment, such as fixed image cameras or video cameras. Thus, the square current waveform as shown in Figure 5 is the ideal current waveform which should be produced by an optimized circuit and the production of this waveform represents one of the primary objects of the present invention.

Figure 6 illustrates a comparison of the reduced sinusoidal current waveform when superimposed with the ideal square current waveform. As can be seen from this chart, a difference of current exists between sinusoidal waveform and the square waveform for most of the time period during which the sinusoidal current is generated. For any given point in time, this difference of current is measured by the vertical gap between the reduced sinusoidal curve and the square waveform shown in Figure 6. Thus, in order to generate the ideal square waveform, one must inject additional current into the arc lamp circuit having a waveform that will result in a final current waveform that has the ideal square

shape. The waveform of the additional current which must be injected is illustrated in Figure 7, and represents harmonics of the sinusoidal waveform.

Our previous U. S. Patent 5,283,502 to Piaskowski et al., issued February lm 1994, discloses the prior art of supplementing current by means of harmonics to the roughly sinusoidal waveform in order to effect the square waveform of current. However, the method of injection in the present invention is an improvement over our previous patent, since it provides an infinite Fourier series instead of a limited Fourier series, and hence produces an improved current waveform over our prior patent. The present invention also includes additional structural distinctions over the prior patent, as will be seen from the description which follows.

The preferred embodiment of the invention which is capable of injecting difference currents into sinusoidal waveform output of the ballast is shown in Figure 11. The circuit includes an AC power source 1 connected in series to inductors 4,5 which serve as a ballast for the circuit. The inductors 4,5 are connected in series with the lamp bulb 13. An igniter circuit having an igniter 11 and a starter transformer or inductor 12 for passing the current from the igniter are also connected to the circuit. The AC voltage source 1, lamp 13 and igniter circuits 11, 12 are the same as those shown in Figure 1, and their use and operation is discussed with respect to Figure 1 above. However, unlike the circuit of Figure 1, the circuit of Figure 11 includes voltage regulating circuits 2 and 3. These voltage regulating circuits are in turn connected to capacitors 14 and 15. The voltage regulating circuits are conventional electronic circuits that are well known in the art. Such circuits may be constructed out of combinations of electronic elements, such as resistors, Zener diodes, or semiconductors, and other combinations of elements as would be known and appreciated by the person of ordinary skill in the art. The purpose of these voltage regulating circuits is to control the voltage on the power rails 2A and 3A, such that the voltage of the rail 2A is maintained positive

and slightly greater in magnitude than the positive voltage excursion of the lamp 13, while the voltage of the rail 3A is maintained negative and slightly greater in magnitude than the negative voltage excursion of the lamp 13.

The positive and negative voltage rails 2A and 3A are regulated by the low power loss voltage regulators to also maintain direct current on both of the respective rails. The rail 2A maintains direct positive voltage while the rail 3A maintains direct negative voltage.

The rail 2A connects to a semiconductor 7 which serves as a positive current regulator, while the rail 3A is connected to a semiconductor 8 which serves as the negative current regulator. The combined effect of the positive voltage rail 2A and the semiconductor 7 and the negative voltage rail 3A and semiconductor 8 is to create a system which injects positive and negative currents which closely resemble the ideal difference currents shown by Figure 7. These difference currents are injected into the sinusoidal current waveform produced by the inductors 4,5 and result in a waveform going to the lamp that is almost perfectly square.

In order to control the currents injected into the circuit by the semiconductors 7,8, sensing and control circuit 6 is connected in series between the AC power source 1 and lamp 13. The sensing and control circuit senses incoming current simals (via element 6 of Figure 11) and then sends control signals to semiconductors 7, 8 which in turn pass pulses of current from the power rails 2A and 3A into the lamp current waveform. The injection of current is made at the junction 16. The semiconductors 7,8 serve as current regulators and regulate the injection of current from the power rails 2A and 3A respectively. The semiconductor 7 regulates the injection of positive current form the positive current power rail 2A, while the semiconductor 8 regulates the injection of negative current from the negative power rail 3A. The sensing and control means may be a simple analog circuit which processes incoming current signals and sends out the

appropriate control signal in response to the incoming signal. Other electronic circuits which perform these functions may serve as the sensing and control means, and other types of electronic circuits which are capable of sensing waveforms and sending control signals in response to these waveforms are considered within the scope of the present invention. The semiconductors 7,8 are preferably transistors, but may be other types of electronic devices capable of controlling a flow of current.

The waveform of the injected currents is illustrated in Figure 8, which shows the positive current injections on the solid line and the negative current injections on the dotted line. In comparing Figures 7 and 8, the resulting current injection closely resembles the ideal current injection waveform necessary for the production of a consistent square wave. The demonstrated magnitude of the current injections ranges from about 12 amperes to about 2 amperes in waveforms lasting about 8/1000 of a second, as an example for one type of lamp. The resulting current waveform should also be perfectly square and produce the steady light output shown in Figure 5.

In an alternative embodiment of the invention, it has been found that an alternate structure can be used to replace the inductors 4,5 to produce a more nearly square wave prior to the current injection. Because the waveform which exists this structure is more nearly square than the waveform exiting the set of conventional inductors 4,5, less current injection is required, and thus less power is consumed.

The alternate structure which produces this power savings is a pair of permanent magnetic core devices of the type shown in Figure 12. Two of the devices shown in Figure 12 are arranged in series with a circuit, such that the polarities of these devices oppose each other. A pair of devices of the type shown in Figure 12 can replace the conventional inductors 4,5 and afford a significant

reduction in power due to the fact that these devices produce a more nearly square wave prior to the injection of current.

The device in Figure 12 includes a permanent rare earth magnet 30 sandwiched between a set of permanent magnetic pole pieces 10 to form an assembly. Two such assemblies are constructed, and then placed between two layers of ferromagnetic material 20. The assemblies are placed along the ends of the sheets of magnetically conductive material 20, such that a ferromagnetic material is defined between the assemblies, with the assemblies extending between the sheets 20. Coils 40,50 are then wrapped around each of the layers of ferromagnetic material and extended within the ferromagnetic core between the assemblies.

Each of the devices shown in Figure 12 will produce a highly efficient inductor of fixed polarity. Two of these inductors are then arranged in series within the circuit with their respective polarities opposing one another. Such a pair of efficient inductors replace the conventional inductors 4,5 within the circuit, and have been found to reduce the power usage in the circuit by about 6- 10%.

Figure 9 illustrates the current waveform output produced by the set of permanent magnetic core devices, compared with the ideal square waveform. In comparing this figure with Figure 6, it can be readily observe that the current waveform produced by the permanent magnetic core devices is more nearly square than the current waveform produced by conventional inductors. The ultimate result of this improvement is that less current needs to be injected by the semiconductors 7,8 to produce the ideal square waveform, and thus less power gets consumed during the operation of this improved device.

Figure 10 illustrates the current injection required to produce the square waveform when the permanent magnetic core devices are utilized instead of conventional inductors. This compared favourably with the current injections that

are illustrated in Figure 8, where the circuit employs conventional inductors, and the injection of lower amounts of current means that the alternate embodiment of the invention will produce less power loss.

The arc lamp circuit described herein is not limited to the exact configurations shown or described, but may be varied in any manner consistent with the scope of the appended claims.