MXPA99011005A - Ferroresonant transformer ballast for regulating the current of gas discharge lamps - Google Patents

Abstract

The invention relates to a ferroresonant ballast that regulates the current level to a gas discharge lamp (138). The inventative concept primarily resides in the use of an additional winding (136) with the gas discharge lamp being provided in parallel with winding (136). An additional inventative concept resides in the use of an inductor placed in parallel across the output capacitor so as to dim the lamp (138), i.e. control the current draw by the lamp (138).

Description

BALLAST OF FERRORRESONANT TRANSFORMER TO REGULATE THE LIGHT CURRENT OF LIGHT DISCHARGE LAMPS CROSS REFERENCE WITH RELATED PATENT This application incorporates by reference the description in the U. U. Patent No. 3,573,606 granted to Kakalec.

BACKGROUND OF THE INVENTION The present invention relates to lamp ballasts, and more particularly deals with a ferroresonant transformer ballast for regulating the flow of light discharge lamps. The current-voltage characteristics of light discharge lamps, such as mercury vapor lamps, are non-linear where the voltage is constant relatively over a range of lamp current that makes a voltage source an inadequate power source. A current source, on the other hand, has a high output impedance which allows the source voltage to follow the voltage of the lamp. As shown in Figure 1, a commonly used method for energizing a light discharge lamp 10 is by means of a variable or alternating voltage source Vin and a ballast 12 coupled in series with the lamp 10 in order to limit the current and Support the voltage difference between the lamp and the voltage source. However, this method leaves the lamp current, and therefore the output power of the lamp, sensitive to changes in the input voltage and also reduces the input power factor.

Another method for energizing a light discharge lamp is to use a ferroresonant transformer as an alternating voltage source which has additional benefits as the method described with respect to Figure 1. Ferroresonant technology, in general, is known for voltage regulation . For example, the Patent of E. U. No. 3,573,606 to Kakalec, the teachings of which are incorporated herein by reference, teaches a ferroresonant voltage regulator. The ferroresonant transformers maintain a constant output voltage, limit the output current and improve the input power factor. Figure 2 schematically illustrates a ferroresonant transformer 14 of constant voltage. The ferroresonant transformer 14 includes an E-shaped part 16 and an I-shaped part 18 which cooperate to form a core. An input coil 20 is wound around a central leg 22 of the E-shaped part 16, and a capacitor coil 24 is wound around a secondary core portion of the central leg 22. An output capacitor (not shown) ) is coupled in series with the capacitor coil 24. A dispersion inductance branch 26, generally positioned at a longitudinal center point of the central leg 22, cooperates with an opposite surface of the E-shaped part 16 to define a space 28 of air. Figure 3 schematically illustrates an equivalent electrical circuit of the ferroresonant transformer 14 of Figure 2, where the coils 30 represent the input coil, an inductance 32 having a reactance Xs represents the dispersion inductance, an inductance 34 having a reactance XM represents the saturable inductance of a secondary portion of the core where the capacitor coil 24 is coiled, the coils 36 represent the capacitor coil, and the capacitor 38 having voltage c is the output or resonance capacitor. The regulation is achieved as follows: any increase in the capacitor voltage Vc will be further saturated so that the value of XM is decreased. A decrease in the value of XM will also decrease the equivalent capacitance, and in turn decrease the resonant amplification. Conversely, any decrease in Vc will reduce the degree of saturation of the nucleus so that the value of XM increases. An increase in the value of XM will also increase the equivalent capacitance, and in turn will increase the resonant amplification. The effective current (RMS) of the capacitor is virtually constant over a range of input voltage. As shown in Figure 4, if a lamp 40 is inserted in series with the output capacitor 38, the capacitor current will adequately energize the lamp 40 as long as the open circuit voltage (the voltage level just before the lamp 40 is turned on) is sufficiently high to cause the lamp 40 to form an arc. The lamp current can be varied by changing the capacitive value of the resonant capacitor 38 which is usually done by exchanging variable capacitance capacitors. The saturated core of the ferroresonant transformer increases the amplitude factor (Vpßak / Vrms) of the lamp current which shortens the operating life of the lamp and makes it difficult for lamps with metal additive to remain on. The low grade steel reduces the magnitude of the maximum current of the capacitor which makes it the preferred choice for laminations despite the higher core losses and reduced efficiency. High power lamps require high voltage across their terminals. Since the output capacitor 40 is in series with the lamp, as shown in Figure 4, the output capacitor 40 must be rated for the same voltage as the lamp. High voltage capacitors are more expensive, more difficult to obtain, and physically larger than the standard 660 V type. It is therefore an object of the present invention to provide a ferroresonant ballast that overcomes the disadvantages associated with prior ballasts for regulate the flow of light discharge lamps.

BRIEF DESCRIPTION OF THE INVENTION The present invention resides in a ferroresonant transformer ballast for regulating the flow of light discharge lamps. The ballast comprises a magnetic core for supporting the coil windings. A first or incoming coil is wound around the magnetic core and is energizable from a variable source to supply voltage and input current. A second coil or capacitor is wound around the magnetic core and magnetically coupled to the first coil to induce a voltage across the terminals of the second coil in response to a change in current of the first coil. A satellite capacitor is connected through the terminals of the second coil to resonate around a constant average voltage level. A third coil or lamp is wound around the magnetic core and magnetically coupled to the second coil to induce a voltage across the terminals of the third coil in proportion to the average voltage across the output capacitor. At least one light discharge lamp is connected through the terminals of the third coil whereby a current level of the light discharge lamp is regulated in response to the average voltage of the output capacitor. The ferroresonant ballast may also include a control circuit and a control inductor that is plugged in and out of electrical contact with the output capacitor in order to simulate the saturation of the core and to maintain the current of the lamp at a value of generally constant or stable state. An advantage of the present invention is that the ferroresonant ballast provides a low amplitude factor of the lamp current, whereby the ferroresonant ballast is allowed to be used with metal additive lamps without any changes or design modifications. In addition, any type of lamination from low grade strip steel to high grade "El" lamination can be used. Other objects and advantages of the present invention will be apparent in view of the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DIAMETERS A more complete understanding of the invention and many of the advantages thereof will be readily appreciated as the same is better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which: Figure 1 schematically illustrates a conventional ballast used with a discharge lamp. Figure 2 shows schematically a conventional ferroresonant transformer. Figure 3 schematically illustrates an equivalent electrical circuit of the ferroresonant transformer of Figure 2. Figure 4 schematically illustrates an equivalent electrical circuit of the ferroresonant transformer of Figure 2 enhancing a light discharge lamp. Figure 5 schematically illustrates an uncontrolled ferroresonant transformer ballast fabricated from E-shaped and I-shaped laminations according to the present invention. Figure 6 shows schematically a ferroresonant transformer ballast manufactured from strip steel according to another embodiment of the present invention. Figure 7 schematically illustrates an equivalent electrical circuit of the ferroresonance transformer ballasts of Figures 5 and 6.

Figure 8 illustrates schematically a conventional controlled ferroresonant transformer. Figure 9 is a graph illustrating current and voltage waveforms associated with the output capacitor of the ferroresonant transformer of Figure 8. Figure 10 illustrates schematically an equivalent electrical circuit of the ferroresonant controlled transformer of Figure 8. Figure 1 1 schematically shows a ferroresonant transformer ballast controlled manufactured from laminations in the form of E and I-shaped according to a further embodiment of the present invention. Figure 12 schematically illustrates a controlled ferroresonant transformer ballast fabricated from strip steel according to yet another embodiment of the present invention. Figure 13 illustrates schematically an equivalent electrical circuit of the controlled ferroresonant transformer ballast of Figures 11 and 12. Figure 14 is a graph illustrating various waveforms of a capacitor and output lamp associated with a ferroresonant controlled transformer ballast of according to the present invention. Figure 15 is a schematic illustrating one embodiment of a control circuit used in conjunction with a controlled ferroresonant transformer ballast.

Figure 16 is a graph illustrating several waveforms associated with a ferroresonant transformer ballast controlled in accordance with the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Turning now to Figure 5, an uncontrolled ferroresonant transformer ballast is designated generally by the reference number 100. The ferroresonant transformer ballast 100 includes an E-shaped part 102 and an I-shaped part. 104. An output coil 106, a capacitor coil 108 and a lamp coil 1 10 are spaced apart from each other and wound around a central leg 112 of the E-shaped part 102. A magnetic shunt 1 14 of dispersion inductance it is positioned around the central leg 1 12 at a longitudinal location between the input coil 106 and the capacitor coil 108. The dispersion inductance branch 114 cooperates with an opposite surface of the E-shaped part 102 to define a first space 1 16 bypass air. The branch 1 18 of the lamp reactor cooperates with an opposite surface of the E-shaped part 102 to define a second bypass air space 19. An output capacitor (not shown) is to be coupled through the terminals of the capacitor coil 108, and a lamp (not shown) is to be coupled through the terminals of the lamp coil 1 10. Accordingly , the lamp coil 110 (other than the capacitor coil 108) serves to isolate the lamp from the output capacitor. In addition, the lamp reactor shunt 118 serves as a reactor in series with the lamp. Unlike the previous ferroresonant transformer shown by the equivalent electrical circuit in Figure 4, the lamp current as used with the ferroresonant transformer ballast 100 of Figure 5 has a lower amplitude factor due to the dispersion inductance contributed by the lamp reactor bypass 118. The lower amplitude factor allows the use of any type of lamination for the ferroresonant transformer ballast core from a low grade strip steel (see Figure 6) to a high grade "El" laminate as shown in Figure 5. With reference to Figure 6, a ferroresonant transformer ballast 120 has similar reference numbers for similar parts with the ferroresonant transformer ballast of Figure 5. The ferroresonant transformer 120 ballast differs from the ferroresonant transformer 100 ballast of the Figure 5 in which the ballast 120 has a core 122 made from strip steel as opposed to the E-shaped pieces and I 102, 104 used for the ferroresonant transformer 100 ballast of Figure 5. The ferroresonant transformer ballast 120 further includes an input coil 106, capacitor coil 108, lamp coil 110, magnetic inductance of dispersion inductance 121 and d magnetic reactivation of lamp reactor 123. Figure 7 is the equivalent electrical circuit of the integrated ferroresonant transformer ballasts shown in Figures 5 and 6, where the coils 124 represent the input coil, an inductance 126 having reactance Xs represents the dispersion reactance, an inductance 128 having reactance XM represents the reactance of saturable magnetization of the core, the coils 130 represent the capacitor coil, a capacitor 132 having capacitive reactance Xc and voltage Vc is the output or resonant capacitor, the inductance 134 having a reactance X? AmP represents the inductance of the lamp reactor shunt, the coils 136 represent the lamp coil, and the lamp 138 is the discharge lamp charge. The open circuit voltage of the lamp is set by the ratio of turns of the lamp coil and the resonance amplification of the system which must be high enough for the lamp to establish an arc. After the lamp is turned on, its initial voltage will drop to approximately 10% of its steady state value. This low voltage will cause the lamp to pull more current which is limited by the scattering reactance of the lamp shunts. The lamp current l? Amp can be calculated as follows: l lamp = (Vc - V | am p) / X | amp (1) By appropriate selection of X? Amp, the current of the liam lamp will be limited to a predetermined maximum value. This initial increase in current is desirable to heat the lamp faster which in turn prolongs the operating life of the lamp 138. As the temperature and voltage of the lamp reach stable state values, the lamp current will be reduced to its value classified as determined by equation (1). The ferroresonant transformer ballast will regulate the output of the lamp by keeping the output capacitor voltage level Vc constant in the same way as a constant voltage ferroresonant transformer. Since all the terms on the right side of equation (1) are constant, it follows that the current of the lamp l | amp will also be constant. There are several advantages associated with ferroresonant transformer ballasts. First, the high voltage of the lamp is independent of the output capacitor voltage which allows it to use normal 660 volt capacitors for any lamp voltage that can vary from 300 volts rms for low power lamps to over 2000 volts rms for lamps of greater power. The lamp shunt limits the lamp current to a predetermined maximum value and reduces the amplitude factor of the lamp current. Third, a low-voltage insulated sensor winding added to the lamp coil allows a simple and safe method to monitor its voltage. Fourth, any type of lamination can be used from low grade strip steel to high grade "El" laminations. The ferroresonant transformer ballasts of Figures 5-7 can be improved by providing a ferroresonant closed-loop current feedback transformer which provides the user with full control over the output of the lamp. A controlled ferroresonant transformer varies the resonance amplification without saturating the core by changing an external linear inductor in parallel with the output or resonant capacitor in order to simulate the core saturation with respect to output voltage regulation. A control circuit detects both the current and voltage of the lamp, and varies the duty cycle of an AC power switch to generate an appropriate inductance and resonance amplification in order to regulate the output of the lamp. In order to better understand the operation of a controlled ferroresonant transformer ballast, reference will first be made to Figures 8-10 which illustrate the prior controlled ferroresonant transformer technology. Turning first to Figure 8, there is shown a controlled ferroresonant transformer 140 where like elements are marked by similar reference numbers with respect to the ferroresonant transformer ballast of Figure 5. A control inductance coil 142 replaces the lamp coil 1 10 of Figure 5. This type of ferroresonant transformer is discussed more fully in the U.S. Patent. , No. 3,573,606 for Kakalec, and is used as a voltage regulator with a switch control inductor that simulates core saturation. Figure 9 shows a trace of the output voltage Vc and the capacitor current lc. The equivalent electrical circuit of this controlled ferroresonant transformer is shown in Figure 10 where the coils 144 represent the input coil, the inductance 146 having reactance Xs is the dispersion inductance, the resistance R represents the equivalent DC resistance of all the windings , the coils 148 represent the capacitor coil, the coil 152 having reactance XL represents a control inductance, the coil 154 having reactance XM represents the magnetization inductance, and the switch 156 is preferably a solid state switch, operated by a control circuit 158 to change the control inductance on and off the parallel relationship with the output capacitor 150 in order to simulate core saturation.

Turning now to Figures 11-16, a ferroresonant transformer controlled ballast according to the present invention will be explained in detail where similar elements with respect to the ferroresonant transformer of Figure 8 are marked with similar reference numerals. With reference to Figure 11, a controlled ferroresonant transformer ballast is designated generally by the reference number 200. The controlled ferroresonant transformer ballast 200 is different, in part, from the ferroresonant transformer of Figure 8 with respect to type and placement of windings around the central leg 112. The windings around the central leg 1 12 are an input coil 106, a capacitor coil 108, a power supply coil 202, a lamp coil 100 and a voltage sense coil 204. As can be seen in Figure 11, the capacitor coil 108 and the power supply coil 202 generally occupy the same longitudinal position in the central leg 1 12 between a branch 1 18 of the lamp reactor and a branch 1 14 of dispersion inductance. The lamp coil 110 and voltage sensing coil 204 generally occupy the same longitudinal position in the central leg 1 12 between the lamp reactor shunt 1 18 and the I-shaped piece 104. As can be seen from Figure 1 1 , the controlled ferroresonant transformer ballast is manufactured from "El" laminations. However, a controlled ferroresonant transformer ballast can also be made of strip steel due to a low amplification factor associated with the ferroresonant transformer ballast 200. As shown in FIG. 12, a ferroresonant controlled transformer ballast 206 employs steel in strips for the core 208. Figure 13 schematically shows an equivalent electrical circuit 210 of the controlled ferroresonant transformer ballasts of Figures 11 and 12. The coils 212 represent the input coil, an inductance 214 having a reactance Xs represents the inductance of dispersion, the coils 216 represent the capacitor coil, the capacitor 218 having reactance Xc and voltage Vc is the output capacitor, the coil 220 having reactance X? amp is the inductance of the lamp branch, the coils 222 represent the lamp coil, and the coil or inductor 224 having XL reactance represents an inductance tor with external switch. A control circuit 226 receives inputs from a lamp voltage sensor 228 and a lamp current sensor 230 and has a control output 232 to open and close a switch 234 to turn on and off the inductor 224 of parallel relationship with the capacitor output 218 in response to sensors 228 and 230 in order to simulate core saturation.

The operation of the controlled ferroresonant transformer ballast incorporated in Figures 11-13 consists of three stages: ignition, heating and steady state. With respect to the ignition stage: at start-up, the control circuit 226 forces the open circuit voltage of the lamp to rise to a maximum value in order to turn on the lamp. During heating, the control circuit 226 will detect the low voltage of the lamp and increase its current by keeping the switch 234 open while V? Amp is below its steady state value. As the lamp heats up, its V? Amp will increase and control circuit 226 will gradually increase the duty cycle of switch 234 by bringing the lamp current to its set value by reducing the equivalent capacitive reactance Xeq = XL in parallel with XC- After the lamp reaches its steady state value, the control circuit 226 will detect the lamp current via the lamp current sensor 230 and will keep the lamp current at a constant value regardless of the input voltage V . Figure 14 is a trace of the various V? Amp waveforms, l? amp, Vc and le of the ferroresonant controlled ballast transformer represented by the equivalent electrical circuit of Figure 13. Important advantages when using a ferroresonant controlled ballast transformer is a low amplification factor of the lamp current which is critical for the use of light discharge lamps with metal additive, and a high input power factor which is a characteristic of all ferroresonant transformers. Figure 15 illustrates schematically one embodiment of the control circuit 226 of Figure 13 used in conjunction with a ferroresonant transformer to form a controlled ferroresonant ballast 235 which is an embodiment of the present invention. The control circuit includes a lamp voltage sensor 236 preferably wound around a magnetic core of the ferroresonance transformer ballast 235 to detect the lamp voltage, and further includes a lamp current sensor 238 preferably located adjacent to the line supply to the lamp in order to detect the current of the lamp. The lamp voltage sensor 236 is coupled to an input of a DC reference module 240, and the lamp current sensor 238 is coupled to an input of a first rectifier 242. A power supply coil 244 is coupled to an input of a second rectifier 246. An output of the first rectifier 242 is coupled via a potentiometer 248 to a first input of an error amplifier 250. An output of the DC reference module 240 is coupled to a second input of the error amplifier. 250. An output of the error amplifier 250 is coupled to a first input of a comparator 252. A ramp generator 254 has an input coupled to an output of the second rectifier 246, and an output coupled to a second input of the comparator 252. A The output of the comparator 252 is coupled to an input of a driving or compensating circuit 256. An output of the driving circuit 256 is coupled to a control input. and a switch 258, such as the gate of a silicon-controlled rectifier switch, which is coupled in series with a switch control inductor 260. The control inductor 260 is electrically coupled in parallel with an output capacitor 262 of a ferroresonant transformer ballast circuit when the switch 258 is closed. The operation of the control circuit of Figure 15 will now be explained with respect to the three stages of operation of the lamp: ignition, heating and steady state. During the ignition stage the average lamp voltage rises with that of the output capacitor, and the lamp current is zero before the lamp is turned on. The operation of the control circuit of Figure 15 will now be explained with respect to the three stages of operation of a ferroresonant ballast: ignition, heating and steady state. During the ignition stage, the lamp voltage sensor 236 and the lamp current sensor 238 respectively generate voltage signals proportional to the voltage level through the lamp 40 and the current level flowing through the lamp. Because the lamp 40 has not yet been turned on, the current flowing through the lamp 40 is approximately zero amps, and therefore the voltage level generated by the current sensor is approximately zero volts. Accordingly, the difference between the voltage signals generated by the voltage sensor 236 and the current sensor 238 is a relatively high value which is amplified by the error amplifier to produce an error signal Ve. An alternating voltage is induced in the power supply coil 244 which in turn is rectified by the second rectifier 246. The rectified voltage signal is then input to the ramp generator 254 to produce a sawtooth signal having a period equal to one half of the alternating input signal supplied to the ferroresonant transformer in the input coil. The relatively high signal Vß and the ramp signal are then input to the comparator 252. The comparator generates a digital output of "1" (i.e., the output goes high) during the portion of the ramp signal cycle when the ramp signal rises above the Ve level. Because Ve is a relatively high signal before ignition, the ramp signal is generally not rises above the Ve level. Consequently, the comparator output remains at a digital output of "0" (ie, the output remains low), and switch 258 remains open such that no current can be diverted from the capacitor output 262 to control inductor 260 with switch. Therefore, the entire current can be directed to charge the output capacitor 262 such that the voltage across the output capacitor 262 can be raised. Because the lamp coil 1 10 is magnetically coupled to the capacitor coil 108, as the voltage rises through the output capacitor 262, the voltage across the lamp 40 also rises until the voltage level of the capacitor the lamp is high enough to light the lamp (that is, turn on the lamp).

During the heating step immediately after ignition of the light discharge lamp 40, V? Amp falls into voltage, l? Am is high, and in turn Ve is relatively high such that switch 258 remains open to increase l? Amp while V | amp is below its steady state value. As the lamp heats up, its voltage V? Amp will increase, which in turn will decrease Ve generated by the error amplifier 250. As Ve decreases, the portion of each cycle of the ramp signal that is at a level greater than the of Ve will increase resulting in the comparator becoming high for a larger portion of each cycle of the ramp signal. As a consequence, the driver circuit 256 closes the switch 258 for an incrementally larger portion of each cycle of the ramp signal (i.e., the duty cycle of the switch 262 is increased). Increasing the duty cycle of the switch 258 brings the current of the lamp 40 to its set value by reducing the equivalent capacitive reactance Xeq = XL in parallel with Xc. After the lamp 40 reaches its steady state, the control circuit will detect the current of the lamp and maintain it at a constant level regardless of the input voltage received from the input coil. Figure 16 is a graph of an error amplifier voltage signal 264, a ramp generator voltage signal 266, a gate voltage signal 268 or switch control, and a control inductor current signal 270. As can be seen in Figure 16, when the voltage of the ramp signal 266 rises above that of the error signal 264, the gate signal 268 used to control a silicon controlled switch is activated in response to the comparator. 252 that rises in order to allow the current (as shown by the inductor signal 270) to flow through the control inductor 260. The lamp current can be adjusted by components (not shown) to vary the voltage of the lamp. error amplifier reference. Such components can be, for example, resistors and opto-isolators with logic control switches which interface with PLCs. Although the present invention has been described in several preferred embodiments, it will be understood that numerous modifications and substitutions may be made without departing from the spirit or scope of the invention. Accordingly, the present invention has been described in several preferred embodiments by way of illustration, rather than limitation, and the scope of this patent disclosure will not be determined primarily from the scope of the appended claims.

Claims (14)

REIVI NDICATIONS

1 . A ferroresonant transformer ballast for regulating the flow of light discharge lamps, the ballast comprising: a magnetic core; a first coil wound around the magnetic core and energizable from a variable source to supply a changing input voltage and current; a second coil wound around the magnetic core and magnetically coupled to the first coil to induce a voltage across the terminals of the second coil in response to a change in current from the first coil; an output capacitor connected through the terminals of the second coil to resonate around a constant average voltage level; a third coil wound around the magnetic core and magnetically coupled to the second coil to induce a voltage across the terminals of the third coil in proportion to the average voltage across the output capacitor; and at least one light discharge lamp connected through the terminals of the third coil whereby the voltage across the light discharge lamp is regulated in response to the average voltage of the output capacitor.

2. A ferroresonant transformer ballast as defined in claim 1, further including: a first magnetic shunt extending from the magnetic core to a longitudinal position between the first and second coils, the first shunt serving as a shunt of dispersion inductance; and a second magnetic shunt extending from the magnetic core to a longitudinal position between the second and third coils, the second shunt serving as a lamp reactor shunt.

3. A ferroresonant transformer ballast as defined in claim 1, wherein the magnetic core comprises cooperating I-shaped and I-shaped parts, the I-shaped part positioned through the free ends of the part in E-shape to form a three-legged magnetic core, the first, second and third coils being wound around a central leg of the three-legged magnetic core.

4. A ferroresonant transformer ballast as defined in claim 3, wherein the first and second magnetic shunts each extend outwardly from the central leg toward opposite portions of the outer legs of the magnetic core, the magnetic shunts and the opposite portions of the external legs of the magnetic core cooperating to form air spaces between them.

5. A ferroresonant transformer ballast as defined in claim 1, wherein the magnetic core is fabricated from strip steel.

6. A ferroresonant transformer ballast as defined in claim 2, wherein the interrupting means includes a voltage sensor coil wound around the core and a current sensor coil positioned adjacent to the lamp coil, the switch being open and closed in response to current and voltage levels detected by voltage and current sensors.

7. A ferroresonant transformer ballast as defined in claim 2, wherein the interrupting means further includes: a coil of energy supply wound around the magnetic core generally in a longitudinal position occupied by the capacitor coil so that the The power supply coil is magnetically coupled to the capacitor coil to induce a voltage across the terminals of the power supply coil in proportion to the average voltage level across the output capacitor; a voltage sensor positioned adjacent to the lamp coil to generate a voltage level in proportion to the voltage level through the light discharge lamp; a current sensor positioned adjacent a current path of the lamp to generate a voltage level in proportion to a current level flowing through the lamp; a first rectifier having an input coupled to the current sensor; a DC reference module having an input coupled to the voltage sensor; a second rectifier having an input coupled to the power supply coil; a differential or error amplifier having a first input coupled to the output of the first rectifier, a second input coupled to an output of the DC reference module, and an output to generate an error voltage signal; a ramp generator having an input coupled to an output of the second rectifier, and an output to generate a ramp signal; and a voltage comparator having a first input coupled to the output of the error amplifier, a second input coupled to the output of the ramp generator, and an output coupled to a switch control terminal.

8. A ferroresonant transformer ballast as defined in claim 7, wherein the interrupting means further includes a drive circuit interposed between the voltage comparator and the switch control terminal.

9. A ferroresonant transformer ballast as defined in claim 7, further including a variable resistor interposed between the output of the first rectifier and the input of the error amplifier.

10. A ferroresonant transformer ballast as defined in claim 2, wherein the switch is a silicon controlled switch.

1 1. A ferroresonant transformer ballast for regulating the voltage through light discharge lamps, the ballast comprising: a three-legged magnetic core having a central leg and external legs; a first coil wound around the central leg of the magnetic core to supply a changing input voltage; a second coil wound around the central leg of the magnetic core whereby the second coil is magnetically coupled to the first coil to induce a voltage across the terminals of the second coil in response to a change in current from the first coil; an output capacitor connected through the terminals of the second coil to resonate at approximately a constant average voltage level; a third coil wound around the central leg of the magnetic core whereby the third coil is magnetically coupled to the second coil to induce a voltage across the terminals of the third coil in proportion to the average voltage across the output capacitor; and a light discharge lamp connected through the terminals of the third coil whereby a current of the light discharge lamp is regulated in response to the average voltage of the output capacitor.

12. A ferroresonant transformer ballast as defined in claim 1, further including: a first magnetic shunt extending outwardly from the central leg of the magnetic core in a longitudinal position between the first and second coils, the first shunt serving as a derivative of dispersion inductance; and a second magnetic shunt extending outwardly from the central leg of the magnetic core in a longitudinal position between the second and third coils, the second shunt serving as a lamp reactor shunt.

13. A ferroresonant transformer ballast as defined in claim 11, wherein the magnetic core comprises E-shaped and I-shaped parts, the I-shaped part placed through the free ends of the shaped part. of E to form a three-legged magnetic core.

14. A ferroresonant transformer ballast for regulating the current level of light discharge lamps, the ballast comprising: a magnetic core that includes cooperating I-shaped and I-shaped parts, the I-shaped part placed through of the free ends of the E-shaped piece to form a three-legged magnetic core having a central leg and external legs; a first coil wound around the central leg of the magnetic core to supply a changing input voltage; a second coil wound around the central leg of the magnetic core whereby the second coil is magnetically coupled to the first coil to induce a voltage across the terminals of the second coil in response to a change in current from the first coil; an output capacitor connected through the terminals of the second coil to resonate at approximately a constant average voltage level; a third coil wound around the central leg of the magnetic core whereby the third coil is magnetically coupled to the second coil to induce a voltage across the terminals of the third coil in proportion to the average voltage across the output capacitor; a light discharge lamp connected through the terminals of the third coil whereby a current level of the light discharge lamp is regulated in response to the average voltage of the output capacitor; a first magnetic shunt extending outwardly from the central leg of the magnetic core to a longitudinal position between the first and second coils, the first shunt serving as a shunt inductance shunt; and a second magnetic shunt extending outwardly from the central leg of the magnetic core to a longitudinal position between the second and third coils, the second shunt serving as a lamp reactor shunt.