DEVICE FOR LIGHTING A DISCHARGE LAMP

Abstract: The inexpensive device having a soft-start function lights a discharge lamp by supplying the AC output of the self-excited half-bridge-type inverter to the lamp. In the inverter, two switching elements are alternately driven by the feedback from the secondary winding of a saturable current transformer. The impedance connected to the control winding of the saturable current transformer is increased for a predetermined period of time from the start of the lighting in order to control the current of the control winding. Further, the saturation time of the saturable current transformer is shortened to increase the output frequency and to decrease the output. Moreover, the impedance is increased if the input voltage is raised, in order to stabilize the output.

Description: Technical Field The present invention relates to an apparatus for operating a discharge lamp, and more particularly, to an apparatus for operating a discharge lamp using an inverter circuit which has a soft start function and can positively light a discharge lamp at a start time by controlling an output as a constant one against a variation in voltage of a power supply. Background Art An apparatus for operating discharge lamp and lighting fixture having such an apparatus for discharge lamp are conventionally known, the lighting device being adapted to light a discharge lamp by rectifying an electric power of a commercial AC power supply and inverting a rectified power by an inverter circuit. In the apparatus and lighting fixture, if a power supply voltage applied to the lighting device varies to an extent of 10%, then an electric current supplied to the discharge lamp varies to an extent of about 15%. Therefore, an output varies greatly against a variation of an input voltage. If such a discharge lamp is to be lighted with an output of the inverter circuit staying low, it is fully not lighted, presenting a problem. Further, an apparatus for operating discharge lamp, having a soft start function, has to employ a large capacity resistor and becomes complex in its circuit arrangement. It has, therefore, been difficult to manufacture a low-cost device of a soft start function type, even considered from manufacturing costs. Prior art document DE-A-32 22 994 discloses an apparatus similar to that of the precharacterizing part of claim 1 or claim 2, wherein however a load is specified as an inductive load or an ohmic load having an inductive component and not as a discharge lamp. However, it is already known from document US-A-4 727 470 that discharge lamps advantageously can be driven by varying the saturation time of a current transformer. Finally, prior art document GB-A-2 106 339 discloses an apparatus wherein an impedance element is connected to a control winding of a base drive transformer so as to control a current in the control winding by varying an impedance. Disclosure of Invention It is an object of the present invention to provide a low-cost apparatus for operating discharge lamp which has a soft start function and can provide a constant output against a power supply voltage variation and a constant light output on a discharge lamp and can positively light the discharge lamp at a lighting time by controlling an output. To solve this object the present invention provides a discharge lamp operating apparatus as specified in claim 1. Brief Description of the Drawings It should be noted that only Fig. 6 shows an embodiment of the present invention, whereas Fig. 1, 4, 5, 9 and 10 show only circuit diagrams which fall not within the scope of the claims but are useful for the understanding of the invention. To be more specific: Fig. 1 is a circuit diagram showing a first apparatus for operating discharge lamp; Fig. 2 is a timing diagram for explaining the operation of a soft start circuit in the apparatus of Fig. 1; Fig. 3 is a characteristic curve showing a frequency-to-output relation of an inverter circuit in the apparatus of Fig. 1; Fig. 4 is a circuit diagram showing a second apparatus for operating discharge lamp; Fig. 5 is a circuit diagram showing a third apparatus for operating discharge lamp; Fig. 6 is a circuit diagram showing a fourth apparatus for operating discharge lamp according to an embodiment of the present invention; Fig. 7 is a view showing a soft start operation in full light and dimmed light on the apparatus of Fig. 6; Fig. 8 is a characteristic curve showing a voltage-current relation of the apparatus of Fig. 6; Fig. 9 is a circuit diagram showing a fifth apparatus for operating discharge lamp; and Fig. 10 is a circuit diagram showing a sixth apparatus for operating discharge lamp. In Fig. 1, reference numeral 12 represents a commercial AC power supply which is connected to an AC input terminal of a rectifying circuit 14. An electrolytic capacitor 16 for smoothing is connected between the DC output terminals of a rectifying circuit 14. Power MOS type field effect transistors 18 and 20 in series array have their drain-to-source circuits connected in parallel with the electrolytic capacitor 16 as shown in Fig. 1 and serve as switching elements. One end of a primary winding 24 of a saturable current transformer 22 for feedback control is connected to a connection point between the source of the field effect transistor 18 and the drain of the field effect transistor 20. One end of a first secondary winding 26 of the saturable current transformer 22 is connected to a connection point of the primary winding 24. The other end of the first secondary winding 26 is connected to the gate of the field effect transistor 18 via a resistor 28. Similarly, one end of a second secondary winding 30 of the saturable current transformer 22 is connected to the source of the field effect transistor 20. The other end of the secondary winding 30 is connected to the gate of the field effect transistor 20 via a resistor 32 and the saturable current transformer 22 is of such a type that the primary winding 24, first secondary winding 26, second secondary winding 30 and control winding 36 are magnetically coupled with a saturable core 34 as a common magnetic path. These elements, together with two field effect transistors 18 and 20, saturable current transformer 22 and so on, constitute a self-excited half-bridge type inverter circuit 38. The other end of the primary winding 24 in the saturable current transformer 22 is connected via a reactor 40 to one filament 44 of a discharge lamp 42. The other filament 46 of the discharge lamp 42 is connected through a capacitor 48 or 50 to the DC output of the rectifier circuit 14. A starting capacitor 52 is connected between the two filaments 44 and 46. A series circuit of voltage dividing resistors 54 and 56 is connected in parallel with the capacitors 48 and 50. An electrolytic capacitor 58 for charging is connected in parallel with the resistor 56. A connection point of the voltage dividing resistors 54 and 56 is connected via a zener diode 60 to the base of a transistor 64 which provides an impedance circuit 62 1 as a soft start circuit. The transistor 64 has its collector connected to a series circuit of resistors 66 and 68 and its emitter connected to a negative output terminal of the rectifier circuit 14, that is, the collector-to-emitter circuit of the transistor 64 is connected to the resistor 68. A series-circuit of a resistor 70 and capacitor 72 is connected to both the ends of the series-circuit of the resistors 66 and 68, that is, to an output terminal of the impedance circuit 62 1 . Further, the DC output terminal of a series-circuit 74 is connected between both the ends of the aforementioned series-circuit. The control winding 36 of the saturable current transformer 22 is connected to the AC input terminal of the rectifier circuit 74. The operation of this first discharge lamp operating apparatus will be explained below. A commercial alternating current supplied from the commercial AC power supply 12 is rectified and smoothed by the rectifier circuit 14 and field capacitor 16. Then electric current is supplied to the primary winding 24 of the saturable current transformer 22 and, when the saturable core is saturated, is inverted at the current transformer. Upon repeated inversion resulting from the saturation, the field effect transistors 18 and 20 are alternately rendered on and off for each inversion. By so doing, the direct current is converted to alternating current to enable the discharge lamp 42 to be turned on. During a period of time (about 0.5 to 1 second) in which the electrolytic capacitor 58 is charged at a starting time, a potential on the connection point between the resistors 54 and 56 is placed at a lower level than a zener voltage on the zener diode 60. For this reason, the transistor 64 is held in the off state since a base current is supplied to the base of the transistor 64. Thus an electric current flowing through the resistor 68 in the impedance circuit 62 1 is not bypassed, making higher a closed circuit's impedance coupled to the DC output terminal of the rectifier circuit 74. An increase in impedance results in a smaller saturation time of the saturable core 34. An increase in the output frequency shortens a predetermined time period, at the starting time, of about 0.5 to 1 second. Upon the charging of the electrolytic capacitor 58, a potential on the connection point between the resistors 54 and 56 becomes a higher level than a zener voltage across the zener diode 60. Then a base current is supplied to the base of the transistor 64 and a current across the resistor 68 is bypassed to the transistor 64. Thus an impedance in the closed path connected to the DC output terminal of the rectifier circuit 74 is made smaller. A smaller impedance leads to a shorter saturation time for the saturable core 34. In this way, as shown in Fig. 2, the output frequency is lowered and the output is raised, enabling the discharge lamp 42 to be fully lighted. The load circuit constitutes an LC resonant circuit constructed of the primary winding of the saturable current transformer 22, reactor 40 and capacitors 48 and 50. The frequency-to-output relation is represented as shown in Fig. 3. Let it be assumed that a greater impedance is set for the maximal output value (frequency f0) of the LC resonant circuit and, in this case, the frequency is made at, for example, f2. Then an output involved is lowered to a dimmed state. On the other hand, let it be assumed that the frequency is made at, for example fl against a smaller frequency. Then an output is made high compared with the frequency f2. It will be seen that a fully lighted state is obtained. A second apparatus for operating discharge lamp (not within the scope of the invention) will be explained below with reference to Fig. 4. The apparatus for operating a discharge lamp as shown in Fig. 4 is of such a type that a regulation improving circuit 76 1 acting as an output variable circuit and constant output circuit is added to the apparatus as shown in Fig. 4. The same reference numerals are employed to designate parts or elements corresponding to those shown in Fig. 4 and any further explanation is, therefore, omitted. In the arrangement shown in Fig. 4, a series-circuit of a resistor 78 and variable resistor 80 is connected between a positive output terminal of a rectifier circuit 14 and a connection point between the collector of a transistor 64 and a resistor 68. A positive output terminal, that is a DC output terminal, of a rectifier circuit 74, is connected via a resistor 82 to a connection point between the resistor 78 and the variable resistor 80. The emitter of a transistor 84 and collector of a transistor 86 are connected between the resistor 82 and the DC output terminal of the rectifier circuit 74. The transistor 84 has its base connected to a tap of the variable resistor 80 and its collector connected to the base of the transistor 86. Further, the emitter of the transistor 86 is connected via a resistor 88 to a negative output terminal, that is a DC output terminal, of the rectifier circuit 74. A diode 90 and zener diode 92 are connected, in a indicated polarity relation, between the positive output terminal, that is the DC output terminal, of the rectifier circuit 74 and a common point between the resistor 68 and the emitter of a transistor 64. Here, resistors 54, 56 and 68, electrolytic capacitor 58, transistor 64 and zener diode 60 provide an impedance circuit 62, serving as a soft start circuit. The transistors 78, 82 and 88, variable resistor 80 and transistors 84 and 86 constitute a regulation improving circuit 76 1 serving as both the aforementioned output variable circuit and the aforementioned constant output circuit. The operation of the second discharge lamp lighting device will be explained below. A self-excited half bridge type inverter circuit 38 converts, as in the first apparatus, direct current to alternating current. If, at this time, the resistive value of the variable resistor 80 varies, the base currents of the transistors 84 and 86 vary, varying a saturation time for a saturation core 34 and a frequency. Therefore, an output of the inverter circuit 38 varies in accordance with a variation in a resistive value of the variable resistor 80. At a time of starting, a predetermined period of time the electrolytic capacitor 58 takes to charge the electrolytic capacitor 58 allows a higher impedance to be achieved in a closed path on the output side of the rectifier circuit 74. By so doing, a higher frequency is involved at the inverter circuit 38, heating a discharge lamp 42 at a low output level. When the transistor 64 is turned ON owing to the charging of the electrolytic capacitor 58, a lower impedance is involved in the closed path on the DC output side of the rectifier circuit 74, causing the frequency to be lowered. At a time of a normal output level, the discharge lamp 42 is started. With an input voltage rise on the inverter circuit 38, an input base current is lowered in the transistor 86. Therefore, an impedance goes high in the closed path across the DC output terminals of the rectifier circuit 74, shortening a saturation time for the saturable core 34. By so doing, a higher frequency is involved on the inverter circuit 38 in which case even a rise of an input voltage causes no rise in an output level of the inverter circuit 38. On the other hand, a drop in the input voltage of the inverter circuit 38 causes an increase in an input base current of the transistor 86. Then a lower impedance is involved in the closed path between the DC output terminals of the saturable core 34. By doing so, a lower frequency is involved for the inverter circuit 38, not lowering an output level of the inverter circuit 38 even when an input voltage is lowered. It follows that even a variation in an input voltage level of the inverter circuit 38 causes no variation in the output voltage level. Thus an electric current supplied to a discharge lamp 42 can be held at a predetermined brightness level without being varied. A third apparatus for operating discharge lamp (not within the scope of the present invention) will be explained below with reference to Fig. 5. In Fig. 5, the same reference numerals are employed to designate parts or elements corresponding to those shown in Figs. 1 and 4 and any further explanation is, therefore, omitted for brevity's sake. A varistor 96 is connected between both ends of a commercial AC power supply 12 via a fuse 94. First and second windings 100 and 102 of a transformer 98 for noise prevention are connected, at one end, across the varistor 96. A capacitor 104 is connected to the other end of the first and second windings 100 and 102 of the transformer. The first winding 100 in the transformer is connected at the other end to the drain of an electrolytic effect transistor 108 via a diode 106 of polarities indicated and to the source of an electrolytic effect transistor 20 via the source of an electrolytic effect transistor 20. That is, the diodes 106 and 108 are connected cathode-to-anode and constitute a rectifier circuit 14 as set out above. A series-circuit of a resistor 110 and capacitor 20 is connected between the drain of the field effect transistor 18 and the source of the field effect transistor 20. A capacitor 114 is connected between the drain and the source of the field effect transistor 18. A series-circuit of a resistor 118 and diode 116 of polarities indicated in Fig. 5 is connected between the source and the field effect transistor 18 and a connection point between the resistor 110 and the capacitor 112. A bidirectional two-terminal thyristor (SSS) 120 is connected between the gate of the field effect transistor 20 and a connection point between the resistor 110 and the capacitor 112. A heat-sensitive switch 122 which is thermally coupled to a reactor 40 is connected between the other terminal of the second winding 102 of the transformer 98 and a filament 46 of a discharge lamp 52. The heat-sensitive switch 122 serves to enable the commercial AC power supply 12 to be cut off from the inverter circuit 38 when an abnormal excess current flows through the circuit involved. A parallel-circuit of an electrolytic capacitor 124 and resistor 126 is connected between the cathode of the diode 106 and the filament 46. Similarly, an electrolytic capacitor 128 is connected between the anode of a diode 108 and the filament 46. A resistor 130 is connected between a connection point between the electrolytic capacitors 124 and 128 and a connection point between a resistor 56 and a zener diode 60. A series circuit of a resistor 78 and capacitor 132 is connected between the cathode of the diode 106 and the anode of the diode 108 and has its connection point connected via a resistor 134 to one end of a variable resistor 80. The resistor 80 is connected, at the other end, to a connection point between the collector of a transistor 64 and a resistor 68 via a resistor 136. Here, resistors 56, 68 and 136, electrolytic capacitor 58, transistor 64 and zener diode 60 provide an impedance circuit 62 3 functioning as a soft start circuit. Resistors 82, 88 and 134, variable resistor 80 and transistors 84 and 86 provide a regulation improving circuit 76 2 serving as an output variable circuit and constant output circuit. Further, a series-circuit of diodes 138 and 140 and that of diodes 142 and 144 are connected in parallel with the series-circuit of a resistor 70 and capacitor 72, these diodes in these series circuits having their polarities indicated in Fig. 5 and serving as a rectifier circuit 74. A control winding 36 as set out above has one end connected between diodes 138 and 140 and the other end connected between diodes 142 and 144. The operation of the third apparatus will be explained below. A commercial alternating current is supplied from the commercial AC power supply 12 to the rectifying circuit 14 where it is rectified. A voltage is alternately applied to the field effect transistors 18 and 20 through first And second secondary windings 26 and 30 in a saturable current transformer 22. In this way, the field effect transistors 18 and 20 are alternately operated. In a predetermined period of time up to the charging of the field capacitor 58 at a starting time, a greater impedance is involved in a closed circuit on the output side of the rectifier circuit 74 because the transistor 64 is not turned ON. Thus the output frequency of the impedance circuit 62 3 is lowered, enabling the discharge lamp 42 to be operated in a soft start fashion. In a normal time following the charging of the electrolytic capacitor 58, the transistor 64 is turned ON, allowing an electric current in the resistor 68 to be bypassed to the transistor 64. By doing so, an impedance is lowered in the closed circuit on the DC output of the rectified circuit 74 and an output level of the inverter circuit 38 is increased, enabling the discharge lamp 42 to be lighted in a normal fashion. At this time, the saturation time of the saturable core 34 varies by varying a resistive value for the transistor 84 connected to the variable resistor 80. It is thus possible to vary the output of the inverter circuit 38. With a power supply voltage raised, an equivalent resistive value is enhanced in the collector-to-emitter circuit of the transistor 86 and an impedance is raised in the closed path on the DC output terminal of the rectifier circuit 74. Then the saturation time is shortened in the saturable core 34 and a high frequency is involved at the inverter circuit 38, producing a low output. With the power supply voltage lowered, on the other hand, an equivalent resistive value is lowered in the collector-to-emitter circuit of the transistor 86 and an impedance is lowered in the closed path on the DC output end of the rectifier circuit 74. The time in which the saturable core is saturated is lengthened and the frequency is lowered in the inverter circuit 38, producing a high output level. Even when a power supply voltage varies, an input current of the discharge lamp 42 is held at a predetermined level by making an output of the inverter circuit 38 constant. It is thus possible to obtain constant brightness on the discharge lamp. An embodiment of the present invention will be explained below with reference to Fig. 6. In Fig. 6, the same reference numerals are employed to designate parts or elements corresponding to those shown in Figs. 1, 4 and 5 and any further explanation is omitted for brevity's sake. The embodiment comprises the third apparatus (Fig. 5) and a dimmend light switching circuit 146 and full-light start circuit 148. Stated in another way, the dimmer circuit 146 includes a transistor 150 having its collector and emitter connected to the base and emitter of a transistor 64 in a soft start circuit 62 3 . A series-circuit of a zener diode 152 and switch 154 and an electrolytic capacitor 156 in a full-light start circuit 148 are connected, as shown in Fig. 6, between the base and the emitter of the transistor 150 as shown in Fig. 6. In the full-light start circuit 148, a resistor 158 is connected between an electrolytic capacitor 156 and a connection point of electrolytic capacitors 124 and 128 and a series-circuit of resistors 160 and 162 is connected across the electrolytic capacitor 128. The base of the transistor 166 is connected to a connection point between the resistors 160 and 162 via a zener diode 164. The collector of the transistor 166 is connected via resistor 168 to another connection point between resistors 160 and 162 and to the base of the transistor 170. The emitter of the transistor 166 is connected to that of the transistor 170 and to a connection point of the electrolytic capacitors 124 and 128. The collector of the transistor 170 is connected to the switch 154 and a resistor 172 is connected between the collector and the emitter of the transistor 170. Even in the apparatus for operating a discharge lamp thus arranged, a self-excited half bridge circuit 38 converts direct current to alternating current as in the case of the apparatus shown in Fig. 5. With the switch 154 OFF, no current is supplied to the base of the transistor 150 and turned OFF. By so doing, a base current is supplied to the transistor 64, causing it to be turned ON. A current in the resistor 68 is bypassed to the transistor 64. Then the impedance of the impedance circuit 76 3 is lowered and the frequency involved is lowered, lighting a discharge lamp 42 in a full-light fashion. With the switch 154 ON, on the other hand, a current is supplied to the base of the transistor 150, enabling the transistor 150 to be turned ON. The base current of the transistor 64 is bypassed. The transistor 64 is turned OFF and a higher impedance is involved in the impedance circuit 76 3 . A high impedance leads to a decrease in output level of the discharge lamp 42. In this way, light is dimmed at two steps by turning the switch 154 ON and OFF. When the discharge lamp 42 is started, a soft start is carried out. Since the electrolytic capacitor 156 is not charged, even if, for example, the switch 154 is turned ON, the transistor 150 is not turned ON until a voltage on the electrolytic capacitor 156 exceeds a zener voltage on the zener diode 152. For this reason, the discharge lamp is operated necessarily in a full-light start fashion for about 2 seconds after the discharge lamp has been started as shown in Fig. 7. According to the embodiment, even if the discharge lamp 42 is to be lighted in a dimmed fashion with, for example, the switch 154 ON, it is started necessarily in a full-light fashion and there is no drop in a secondary voltage, improving the lamp's startability. A fifth discharge lamp operating apparatus (not within the scope of the invention) will be explained below with reference to Fig. 9. In Fig. 9, the same reference numerals are employed to designate parts or elements corresponding to those shown in Figs. 1 and 4 to 6 and any further explanation is omitted for brevity's sake. In Fig. 9, a capacitor 174 is connected between the drain and the source of a field effect transistor 18. Similarly, a capacitor 176 is connected across the drain and the source of a field effect transistor 20. A series-circuit of a diode 178 and resistor 180 is connected in parallel with a resistor 28 which is connected between the gate of the field effect transistor 18 and a first secondary winding 26 of a saturable current transformer 22. A series-circuit of a diode 182 and resistor 184 is connected in parallel with a resistor 32 which is connected between a second secondary winding 28 of the saturable current transformer 22 and the gate of the field effect transistor 20. The operation of the fifth apparatus will be explained below. A commercial alternating current is supplied from a commercial AC power supply 12 to a rectifier circuit 14 where it is rectified. A corresponding current is converted to alternating current by the operation of two field effect transistors 18 and 20. In a soft start circuit 62 4 , since a transistor 64 is turned OFF until an electrolytic capacitor 58 is charged at a start time, an impedance is enhanced in a closed path on the output side of a rectifier circuit 74. Thus the saturation time of a saturable core 34 is hastened and the output frequency of an impedance circuit 62 3 becomes higher, suppressing the output of an inverter circuit 38 at a lower level. As a result, a discharge lamp 42 is started in a soft start fashion. Thereafter, upon the charging of the electrolytic capacitor 58, the transistor 64 is turned ON and a lower impedance is produced in a closed path on the DC output of the rectifier circuit 74. The saturation of the core 34 takes a longer period of time. In this way, the frequency involved is lowered and the inverter circuit 38 produces a normal output level. The discharge lamp 42 is lighted at a normal, predetermined brightness level. A sixth discharge lamp operating apparatus (not within the scope of the invention) will be explained below with reference to Fig. 10. In Fig. 10, the same reference numeral is employed to designate parts or elements corresponding to those of the preceding embodiments and any further explanation is omitted for brevity's sake. An apparatus for operating discharge lamp as shown in Fig. 10 includes a plurality (2 in this embodiment) of discharge lamps in place of the discharge lamp 42 in the apparatus shown in Fig. 9 and a regulation improving circuit 76 2 connected between the DC output terminals of a rectifier circuit 14 via resistors 186 and 78 and resistors 136 and 68. That is, a discharge lamp 42 1 is connected to a starting capacitor 52 1 provided between two filaments 44 1 and 46 1 and a discharge lamp 42 2 is connected to a starting capacitor 52 2 provided between two filaments 44 2 and 46 2 . The discharge lamps 42 1 and 42 2 are connected, in parallel fashion, between a connection point, on one hand, of capacitors 48 and 50 and reactors 40 1 and 40 2 , on the other hand, which are connected to the other end of a primary winding 24 of a saturable current tra sformer 22. The operation of the sixth apparatus of Fig. 10 is the same as that of the fifth apparatus of Fig. 9. It is thus possible to perform a soft start. The saturation time of a saturable core 34 varies by varying a base current in the transistor 84 through a variable resistor 80. The output of an inverter circuit 38 can vary by varying the frequency for the inverter circuit 38. Further, a rise in input voltage of the inverter circuit 38 leads to a decrease in the base current of a transistor 86 and a rise of impedance in the closed path on the output side of the rectifier circuit 74. Therefore, the saturation of the saturable core 34 is hastened and the impedance circuit 62 3 produces a higher output frequency. The output level of the inverter circuit 38 is suppressed. Upon a decline in an input voltage of the inverter circuit 38, on the other hand, the input base current of the transistor 86 is increased and a lower impedance is produced in the closed path on the DC output of a rectifier circuit 74, lengthening the saturation time of the saturable core 34. Therefore, the frequency involved is lowered and an output level of the inverter circuit 38 is not decreased. Even if the input voltage largely varies, the variation of an output involved can be lowered. In the aforementioned apparatus for operating discharge lamp, when the variation of an input voltage is 10%, a variation in a lamp current is about 5% and the variation of the lamp current can be suppressed to one-third that of a conventional counterpart. As set forth above, according to the present invention an apparatus for operating discharge lamp of less cost can be provided which can be driven in a soft start fashion, provide a constant output and constant light output against the variation of a power supply voltage and positively light a discharge lamp by controlling an output level. Industrial Applicability An apparatus for operating discharge lamp of the present invention can be built in a lighting fixture, such as a fluorescent lamp, gas discharge lamp, which includes a discharge lamp.

Claims: An apparatus for operating a discharge lamp comprising: self-excited inverter circuit means (38) including a DC power supply, a feedback-controlled, saturable current transformer having primary (24) and secondary windings (26, 30) and a control winding (36) and two switching elements (18, 20), and adapted to alternately operate the two switching elements (18, 20) by an output of the secondary windings (26, 30) of the saturable current transformer; discharge lamp means (42) for receiving an AC output from the inverter circuit means (38) so that it is lighted; and output variable circuit means (62 3 ) for varying a saturation time of the saturable current transformer in the inverter circuit means (38) and an output of the inverter circuit means (38), characterized by dimmed light switching circuit means (146) for varying an output of said output variable circuit means (62 3 ) and full-light start circuit means (148) which, when said discharge lamp means (42) is started, provides an output, in spite of switching said dimmed light switching circuit means (146), with an output of said inverter circuit means (38) not placed in a lowered state (Fig. 6). An apparatus according to claim 1, characterized in that said output variable circuit means (62 3 ) includes impedance circuit means, having a soft start function, for varying a saturation time of said saturable current transformer for a predetermined time period after said discharge lamp means (42) started and for lowering an output of said inverter circuit means (38), and said impedance circuit means comprises a voltage dividing resistor (78, 80, 68) connected to a DC output terminal of a rectifier circuit (90, 92) to be coupled to said control winding (36), and a transistor (64) connected between the voltage dividing resistor (78, 80, 68) and an output of said inverter circuit means (38). The apparatus according to claim 1 or 2, wherein said output variable circuit means (62 3 ) varies a current in said control winding so that a saturation time of said saturable current transformer is varied. The apparatus according to claim 3, wherein said impedance circuit means controls a current in said control winding (36) to vary an impedance of said saturable current transformer and said inverter circuit means (38) varies a current in said control winding (36) in accordance with an impedance varied by said impedance circuit means to provide an output variation. The apparatus according to claim 2, wherein said output variable circuit means (62 3 ) includes constant output circuit means for varying a saturation time of said saturable current transformer in accordance with a voltage supplied from said power supply to make an output of said inverter circuit means (38) constant. The apparatus according to claim 1, wherein said discharge lamp means (42) comprises a plurality of discharge lamps connected in a parallel array.

Inventors: INUI KENICHI, KAKITANI TSUTOMU, INUI, KENICHI, KAKITANI, TSUTOMU, INUI, Kenichi Toshiba Lighting &amp; Techn. Corp., KAKITANI, Tsutomu Toshiba Lighting &amp; Techn. Corp.
Applicants: TOSHIBA LIGHTING & TECHNOLOGY, TOSHIBA LIGHTING & TECHNOLOGY CORPORATION
Application year: 1990
Main classification: H05B41
IPC codes: H05B41