Apparatus for operating discharge lamps

Abstract: An apparatus for operating discharge lamps includes first closed circuit for connecting a discharge lamp (DL) through first switching means to a pulsating power source (AC), second closed circuit for connecting the lamp to the power source through a series circuit of second switching means which operates during OFF period of the first switching means and an energy-storing means (C1), and an arrangement (CON) for controlling the both switching means. A pulsating source voltage is thereby applied to the discharge lamp (DL). The apparatus is made thus capable at least of minimizing current-limiting impedance and realizing a continuously stabilized discharge of the lamp (DL).

Description: SPECIFICATION BACKGROUND OF THE INVENTION This invention relates to an apparatus for operating discharge lamps which comprises an impedance element allowing an input current to a discharge lamp to be continuously fed to the lamp, and which is capable of minimizing in size the required current-limiting element. DESCRIPTION OF RELATED ART In the apparatus for operating discharge lamps of the kind referred to, conventionally, there has been taken a measure for operating the discharge lamp by connecting a full-wave rectifier to an A.C. source for full-wave rectification, and an inverter across output ends of the full-wave rectifier with a smoothing capacitor interposed, so as to light and operate the discharge lamp, while carrying out a desired current limitation through a choke coil connected to the inverter. With this measure, it is made possible by the use of the inverter circuit to minimize in size the apparatus by an extent for which the frequency is higher than in a case where the choke coil as the current limiting element for the discharge lamp is directly relied on the frequency of the A.C. source, so long as the same level of impedance is to be obtained. Further, since the alternating current source is rectified and smoothed to be a direct current source, it is possible to provide a stable operating current to the discharge lamp. In this apparatus for operating discharge lamps, however, only a current of a limited phase close to the largest value of the A.C. source is contributive to a charging of the smoothing capacitor, so that an input current will have a high harmonic distortion which causes the smoothing capacitor required to be of a larger value and the choke coil as the current limiting element also required to be of a larger value, whereby there has arisen a problem that the apparatus has been unable to be minimized in size in these respects. DE-A-35 37 517 discloses a switch arrangement for operating discharge lamps. In view of the above problem, there has been a suggestion in which a power source, switching element and discharge lamp are connected in series and the switching element is so set as to be of a short conduction time, to thereby omit the choke coil as the current-limiting element. In U.S. Patent No. 3,619,716 to Joel S. Spira et al, there has been suggested an apparatus for operating discharge lamps in which ON pulse time width in the switching element is made short while making effective impedance of the discharge lamp higher and charging an RC circuit connected in parallel with the discharge lamp, so that charged energy can be supplied to the discharge lamp when the switching element is in OFF state. According to this arrangement, it has been possible to attain the size minimization of the apparatus in respect that series-connected large size choke coils can be omitted, but there has arisen another problem that a larger size switching element is necessitated due to that all energy supply and interruption including the energy supplied to the discharge lamp are relying on the operation of the switching element. In another U.S. Patent No. 3,906,302 to Dan B. Wijsboom, there has been suggested an operating arrangement in which a control mechanism is connected to a discharge lamp through a switching transistor to which an RC circuit is connected across collector and emitter of the transistor, and an inductance is connected in parallel with the discharge lamp. With this arrangement, it is made possible to apply a higher voltage to the lamp during the OFF period of the switching element, but there still has been a problem in sufficiently minimizing the size, similarly to the foregoing known art. Further, there has been suggested in Japanese Patent Laid-Open Publication No. 50-153471 of Hiroshi Nishimura, an assignor to the same assignee as in the present case, another lamp lighting apparatus in which a switching element is connected through a control section to a source, and an RC circuit is connected in parallel with the switching element. A discharge lamp may require a relatively high starting voltage, but, after the arc is initiated, will have a considerably lower discharge-enabling voltage. With this arrangement, however, there should be a risk that the operating of the discharge lamp cannot be maintained so as to cause the lamp to flicker and finally to extinguish if the source voltage becomes lower than the discharge-enabling voltage of the discharge lamp in the event where the source voltage happens to pulsate. That is, it will be appreciated that, according to the conventional apparatus for operating discharge lamps, a large stress is apt to be applied to the switching element in the case when the apparatus is attempted to be minimized in size by omitting the current limiting element while operating desired function. In addition, the foregoing conventional apparatus for operating discharge lamps involve a problem that, in particular, the harmonic distortion of the current drawn from the A.C. source becomes remarkable in the case where a rectifying and smoothing circuit is connected to the A.C. source providing a source voltage involving the pulsation for obtaining a relatively stable direct current voltage. SUMMARY OF THE INVENTION A primary object of the present invention is, therefore, to provide an apparatus for operating discharge lamps which is capable of omitting the large size current limiting element while still maintaining a stable operation of the discharge lamp even when such pulsating power source as the A.C. source is employed, reducing any stress to the switching element for restraining the flickering and eventual extinguishing while eliminating any harmonic distortion of the current drawn from the A.C. source, and, if desired, allowing the current supply to the discharge lamp to be made continuous so as to render the apparatus to be of a low noise. The foregoing object of the invention can be attained by an apparatus for operating a discharge lamp comprising: a pulsating power source (AC), a bridge circuit comprising a parallel connection of a first series circuit of first and second switching elements (SW1, SW2) connected in series, a second series circuit of third and fourth switching elements (SW3, SW4) connected in series, and a capacitive energy-storing element (C1) connected between a junction point of the first and third switching elements (SW1, SW3) and a junction point of the second and fourth switching elements (SW2, SW4), a discharge lamp (DL) connected between a junction point of the first and second switching elements (SW1, SW2) in the first series circuit and a junction point of the third and fourth switching elements (SW3, SW4) in the second series circuit through said pulsating power source (AC) for receiving a power supplied from the power source (AC), and a control circuit (CON) for controlling ON/OFF operation of said first to fourth switching elements (SW1-SW4), characterized in that in one of half-cycles of the power from the pulsating power source (AC) said control circuit (CON) is so arranged that, in a period when a pulsating source voltage (VAC) of the pulsating power source (AC) is higher than a discharge lamp voltage (VL) of the discharge lamp (DL), said control circuit maintains the first switching element (SW1) in ON state and the second switching element (SW2) in OFF state, while said third and fourth switching elements (SW3, SW4) are alternately turned ON and OFF, so that, when the third (SW3) is ON, the pulsating power source (AC) and the discharge lamp (DL) are directly connected to cause the source power directly supplied to the discharge lamp (DL) and, when the fourth switching element (SW4) is ON, the source power is supplied through the capacitive energy-storing element (C1) to the discharge lamp (DL) while charging this element (C1); in a period when the pulsating source voltage (VAC) of the pulsating power source (AC) is lower than the discharge lamp voltage (VL) of the discharge lamp (DL), said control circuit maintains the first switching element (SW1) in OFF state and second switching element (SW2) in ON state while the third and fourth switching elements (SW3, SW4) are alternately turned ON and OFF, so that, when the fourth switching element (SW4) is ON, the pulsating power source (AC) and discharge lamp (DL) are directly connected to cause low power supplied to the discharge lamp (DL) from the pulsating power source (AC) but, as the third switching element (SW3) turn ON, the capacitive energy-storing element (C1) and pulsating power source (AC) are connected in series to cause the total voltage of the pulsating power source (AC) and the voltage component of the capacitive energy-storing element (C1) to be supplied to the discharge lamp (DL) while discharging a stored charge in the capacitive energy-storing element (C1); in the other half-cycle of the power from the pulsating power source (AC), said control circuit (CON) is so arranged that, in a period when the source voltage (VAC) of the pulsating power source (AC) is higher than the discharge lamp voltage (VL) of the discharge lamp (DL), said control circuit maintains the fourth switching element (SW4) in OFF state and the third switching element (SW3) in ON state while the first and second switching elements (SW1, SW2) are alternately turned ON and OFF; and in a period when the source voltage (VAC) of the pulsating power source (AC) is lower than the discharge lamp voltage (VL) of the discharge lamp (DL), said control circuit maintains the fourth switching element (SW4) in ON state and the third switching element (SW3) in OFF state while the first and second switching elements (SW1, SW2) are alternately turned ON and OFF. According to the apparatus for operating discharge lamps of the present invention as arranged in the above, the discharge lamp current is directly controlled for controlling the ionization, whereby the effective impedance of the discharge lamp can be elevated to be high, the current limiting impedance can be made the minimum required or even omitted, and the continuous power supply to the discharge lamp can be realized by providing the impedance element for storing the energy through the discharge lamp and supplying the discharge lamp current while charging the impedance element with the second switching means is turned ON and the first switching means is turned OFF. Furthermore, the flickering or eventual extinguishing of the discharge lamp can be restrained even upon a low input voltage of the pulsating power source, by utilizing the energy-storing in the impedance element, and the harmonic distortion of the current drawn from the A.C. source can be more effectively improved than in the case of the conventional rectified and smoothed source power, by rendering ON period of the first switching means shorter when momentary input source voltage is high but longer when the momentary input source voltage is low to have the current to the discharge lamp controlled, consequent to which the effective impedance of the discharge lamp is made high when the input source voltage is high but is made low when the input source voltage is low. Other objects and advantages of the present invention shall be made clear in following description of the invention detailed with reference to accompanying drawings. BRIEF EXPLANATION OF THE DRAWINGS FIG. 1 is a circuitry diagram showing a first embodiment of the present invention; FIG. 2 shows waveforms at respective parts in the circuit of FIG. 1; FIG. 3 is a circuitry diagram showing a second embodiment of the present invention; FIG. 4 shows waveforms at various parts in the circuit of FIG. 3; FIG. 5 is a circuitry diagram showing another working aspect of the apparatus of FIG. 3; FIG. 6 shows in a circuitry diagram a third embodiment of the present invention; FIG. 7 shows waveforms at various parts in the circuit of FIG. 6; FIG. 8 shows in a circuit diagram another working aspect of the apparatus shown in FIG. 6. While the present invention shall now be described in detail with reference to the respective embodiments shown in the accompanying drawings, it should be appreciated that the intention is not to limit the present invention only to these embodiments shown but rather to include all alterations, modifications and equivalent arrangements possible within the scope of appended claims. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a first embodiment of the apparatus for operating discharge lamps according to the present invention, in which the switching elements SW1 to SW4 are connected in a bridge circuit to the alternating current source AC, so that the switching elements SW1 and SW4 will act respectively as the first to third switching elements as later described, the capacitor C1 is connected in the middle branch of the bridge circuit, and the discharge lamp DL is connected to the source AC with the bridge circuit interposed. The operation of this embodiment shall be detailed with reference also to FIG. 2. Upon application of such source voltage VAC as in (a) of FIG. 2 to the apparatus for operating discharge lamps of FIG. 1, the voltage VAC that has exceeded discharge-enabling voltages VL1 and VL2 of the discharge lamp DL causes such discharge lamp current IL as in (g) of FIG. 2 to be increased whereas, as the source voltage VAC does not reach the discharge-enabling voltage, the discharge lamp current IL decreases. Now, provided that the source voltage VAC exceeds the discharge-enabling voltage VL1 at time t1, the switching element SW1 acting as the first switching element is turned ON while another element SW2 is made OFF and SW3 is made ON, and in the present instance the switching elements SW3 and SW4 operate complementarily (see (b) to (e) of FIG. 2). As the switching element SW3 turns ON, the source voltage VAC is applied to the discharge lamp DL, and a current flows through a path of AC → SW1 → SW3 → DL → AC. Here, the source voltage VAC is higher than the discharge-enabling voltage VL1 and the discharge lamp current IL elevates. And the switching element SW3 turns OFF, the current flows through a path AC → SW1 → C1 → SW4 → DL → AC, the discharge lamp current IL decreases as shown in (g) of FIG. 2, and the capacitor C1 is caused to be charged as in (h) of FIG. 2. This capacitor C1 carries out the charging in response to the turning ON and OFF of two switching elements SW3 and SW4 which operate complementarily. Furthermore, as the source voltage VAC becomes lower than the discharge-enabling voltage VL1 at time t2, the switching element SW1 turns OFF while another element SW2 turns ON, and the total voltage of the source voltage VAC and voltage of the capacitor C1 is applied to the discharge lamp DL as in (f) of FIG. 2 in response to the turning ON and OFF of the switching element SW3. As time t3 is reached, the source voltage VAC makes a zero-cross, and the polarity of the source voltage VAC is reversed after this point, upon which the switching element SW3 turns OFF while the switching element SW4 turns ON and the switching elements SW1 and SW2 are complementarily turned ON and OFF. As the switching element SW1 turns ON, a superposed voltage of the source voltage VAC and voltage of the capacitor C1 is applied to the discharge lamp DL, the discharge lamp voltage and current VL and IL are caused to rise and, as the switching element SW1 turns ON at time t4, the switching element SW3 turns ON while the other element SW4 turns OFF. Then, the source voltage VAC is applied to the discharge lamp DL, and the discharge lamp current IL is caused to increase in negative polarity of the source voltage. At time t5, the source voltage VAC is lower than the other discharge-enabling voltage VL2, the switching element SW4 turns ON while the other element SW3 turns OFF, and the sum of the source voltage VAC and capacitor C1 voltage is applied to the discharge lamp DL in response to turning ON and OFF of the further switching elements SW1 and SW2. When time t6 is reached, the source voltage VAC makes the zero-cross, the switching element SW1 turns OFF while the switching means SW2 turns ON, and the sum of the source voltage VAC and capacitor C1 voltage is applied to the discharge lamp DL in response to the turning ON and OFF of the switching elements SW3 and SW4. At time t7, the same state as that at time t1 is reached, and the above operation is repeated. It should be appreciated that the switching elements SW1 to SW4 in the foregoing embodiment may be subjected to an ON and OFF control by means of an optimum control circuit (not shown). In response to wavy variation of the pulsating source voltage, the first switching element is constituted, in certain half cycles, by the first pair SW1 and SW2 in the four bridge-connection switching elements or by the second pair SW3 and SW4, the second switching element cooperating with the capacitor C1 is constituted by one SW3 or SW4 of the second pair or by one SW1 or SW2 of the first pair of the first switching element, and the third switching element is constituted by the other SW4 or SW3 of the second pair or by the other SW2 or SW1 of the first pair of the first switching element. Accordingly, it will be also appreciated that the closed-circuit elements of the same action as that of the first to third closed-circuit elements in the foregoing embodiment can be formed with these switching elements. Referring next to FIG. 3, there is shown a second embodiment of the apparatus for operating discharge lamps according to the present invention, in which the control circuit CON to which the discharge lamp voltage VL is provided as a detection input is inserted in a circuit in which the switching elements SW1 to SW4 are connected in the bridge mode to the alternating current source AC so as to act as the first to third switching elements, and the capacitor C1 is connected to the middle branch of the bridge as has been described with reference to FIG. 1, and the control signal is provided from this control circuit CON to the respective switching elements SW1 through SW4. Referring to the operation of this embodiment of FIG. 3 in conjunction with FIG. 4, the application of the source voltage VAC to the switching elements SW1 to SW4 acting as the first to third switching elements, causes them to operate as has been described with reference to FIGS. 1 and 2, as shown in (a) to (e) of FIG. 4, the signal pulse width is made narrower when the discharge lamp voltage VL as shown in (f) of FIG. 4 is high but is made wider when the voltage VL is low so that, as in (g) of FIG. 4, the envelope of the discharge lamp current IL will be made closer to the sinusoidal wave upon turning ON of the switching element SW1, and the harmonic distortion of the current drawn from the A.C. source can be effectively improved. At this time, the current IC1 of the capacitor C1 will be as (h) in FIG. 4. It will be appreciated that, in the present embodiment, too, the discharge lamp current IL can be stabilized by varying the signal pulse width in accordance with the level of the discharge lamp voltage VL. While in the embodiment of FIG. 3 the detection of the discharge lamp voltage VL is carried out at a point x as shown in the drawing, it will be possible to arrange the device for carrying out the detection at one of connecting points Y1 and Y2 of the capacitor C1, so that the discharge lamp voltage VL can be detected for optimum setting of the pulse width, prior to that the switching element which actuates at a relatively slow cycle is actuated to have the capacitor C1 voltage added to the source voltage VAC and the switching element which actuates at a relatively quick cycle is actuated. Furthermore, in the foregoing embodiment of FIG. 3, as well as in the embodiment of FIG. 1, it will be possible to employ as the switching elements SW1 to SW4 in the bridge mode to form the first to third switching elements, such reverse parallel circuits of transistors and diodes as shown in FIG. 5 or such element as a power MOSFET and the like. Referring to FIGS. 6 and 7, there is shown a third embodiment of the apparatus for operating discharge lamps according to the present invention, in which the circuit arrangement itself as shown schematically in FIG. 6 may be substantially the same as that of FIG. 3 but, as will be clear when operating waveforms of FIG. 7 for the present embodiment are compared with those in FIG. 4, its operation is so made that the timing of charge and discharge of the capacitor C1 is varied to have the supplied energy from the capacitor to the discharge lamp controlled. Referring more specifically, such raising of the threshold voltages VT1 and VT2 to be VT1H and VT2H for varying the timing of charging and discharging of capacitor C1 as shown in (a) of FIG. 7 will cause the ON period of longer cycle portions of the respective switching elements SW1 to SW4 in the bridge mode of FIG. 1 to be shorter as shown in (b) to (e) of FIG. 7, due to which the voltage VL applied to the discharge lamp DL is made to vary from the one shown in (f) of FIG. 7 to the voltage VLH as in (g) of FIG. 7, and a required time for superposing the capacitor C1 voltage on the source voltage VAC and applying the supercharged voltage to the discharge lamp is prolonged. When on the other hand the threshold voltages VT1 and VT2 are lowered to VT1L and VT2L as shown in (a) of FIG. 7, the ON period of the longer cycle portions of the respective switching elements SW1 to SW4 will then be made longer as in (b) to (e) of FIG. 7, due to which the discharge lamp voltage VL is made to be the voltage VLL as in (h) of FIG. 7, and the required time for superposing the capacitor C1 voltage on the source voltage VAC is shortened. Accordingly, it is made possible to vary the voltage to be applied to the discharge lamp DL by varying the threshold voltages VT1 and VT2. That is, with the threshold voltages VT1 and VT2 made higher to be VT1H and VT2H by sufficiently charging the capacitor C1 within the charging time, the discharge lamp DL can have a higher output and, with the threshold voltages made lower to be VT1L and VT2L, the discharge lamp output can be lowered. In this way, it is made possible to control the output of the discharge lamp DL by controlling the ON period of the switching elements SW1 and SW2 in positive half cycle of the alternating current source voltage VAC, in accordance with which, for example, the output of the discharge lamp DL can be varied without requiring any control for further shortening the ON period of shorter cycle portions of the switching elements SW3 and SW4 in the positive half cycles of the source voltage VAC, and an apparatus for operating discharge lamps capable of restraining noise can be thereby provided. Other arrangement, function and effect of the present embodiment are the same as those in the case of FIG. 1 or of FIG. 3 and so on. In a working aspect of FIG. 8, the arrangement is so made that, in the embodiment of FIG. 6, a current detecting element CDE for detecting the current to the discharge lamp DL is connected in series with the discharge lamp DL, and a voltage across this current detecting element CDE is subjected to a feedback to the control circuit CON. In this case, the threshold voltages VT1 and VT2 can be shifted to lower levels by increasing output value of the current detecting means CDE and to higher levels by reducing the output value, whereby the output of the discharge lamp DL can be made constant, while rendering the threshold voltages VT1 and VT2 to be at lower levels with respect to the shift to higher side of the source voltage VAC and to be at higher levels with respect to the shift to lower side. Other arrangement, function and effect of the present aspect are the same as those in the embodiment of FIG. 6.

Claims: An apparatus for operating a discharge lamp comprising: a pulsating power source (AC), a bridge circuit comprising a parallel connection of a first series circuit of first and second switching elements (SW1, SW2) connected in series, a second series circuit of third and fourth switching elements (SW3, SW4) connected in series, and a capacitive energy-storing element (C1) connected between a junction point of the first and third switching elements (SW1, SW3) and a junction point of the second and fourth switching elements (SW2, SW4), a discharge lamp (DL) connected between a junction point of the first and second switching elements (SW1, SW2) in the first series circuit and a junction point of the third and fourth switching elements (SW3, SW4) in the second series circuit through said pulsating power source (AC) for receiving a power supplied from the power source (AC), and a control circuit (CON) for controlling ON/OFF operation of said first to fourth switching elements (SW1-SW4), characterized in that in one of half-cycles of the power from the pulsating power source (AC) said control circuit (CON) is so arranged that, in a period when a pulsating source voltage (VAC) of the pulsating power source (AC) is higher than a discharge lamp voltage (VL) of the discharge lamp (DL), said control circuit maintains the first switching element (SW1) in ON state and the second switching element (SW2) in OFF state, while said third and fourth switching elements (SW3, SW4) are alternately turned ON and OFF, so that, when the third (SW3) is ON, the pulsating power source (AC) and the discharge lamp (DL) are directly connected to cause the source power directly supplied to the discharge lamp (DL) and, when the fourth switching element (SW4) is ON, the source power is supplied through the capacitive energy-storing element (C1) to the discharge lamp (DL) while charging this element (C1); in a period when the pulsating source voltage (VAC) of the pulsating power source (AC) is lower than the discharge lamp voltage (VL) of the discharge lamp (DL), said control circuit maintains the first switching element (SW1) in OFF state and second switching element (SW2) in ON state while the third and fourth switching elements (SW3, SW4) are alternately turned ON and OFF, so that, when the fourth switching element (SW4) is ON, the pulsating power source (AC) and discharge lamp (DL) are directly connected to cause low power supplied to the discharge lamp (DL) from the pulsating power source (AC) but, as the third switching element (SW3) turn ON, the capacitive energy-storing element (C1) and pulsating power source (AC) are connected in series to cause the total voltage of the pulsating power source (AC) and the voltage component of the capacitive energy-storing element (C1) to be supplied to the discharge lamp (DL) while discharging a stored charge in the capacitive energy-storing element (C1); in the other half-cycle of the power from the pulsating power source (AC), said control circuit (CON) is so arranged that, in a period when the source voltage (VAC) of the pulsating power source (AC) is higher than the discharge lamp voltage (VL) of the discharge lamp (DL), said control circuit maintains the fourth switching element (SW4) in OFF state and the third switching element (SW3) in ON state while the first and second switching elements (SW1, SW2) are alternately turned ON and OFF; and in a period when the source voltage (VAC) of the pulsating power source (AC) is lower than the discharge lamp voltage (VL) of the discharge lamp (DL), said control circuit maintains the fourth switching element (SW4) in ON state and the third switching element (SW3) in OFF state while the first and second switching elements (SW1, SW2) are alternately turned ON and OFF. The apparatus according to claim 1, characterized in that said control circuit (CON) is connected to said first and second series circuits of said switching elements (SW1-SW4), said discharge lamp (DL) and pulsating power source (AC) being thereby connected alternately directly to each other and through said capacitive energy-storing element (C1). The apparatus according to claim 2, characterized in that said control circuit (CON) is connected to said pulsating power source (AC) and said first or second series circuit of said first and second or third and fourth switching elements (SW1,SW2 or SW3,SW4), said first or second series circuit of the switching elements receiving an output control signal of varying pulse width upon a predetermined actuation of said first or second series circuit of the switching elements, said varying pulse width being controlled in response to variation in the pulsating source voltage (VAC) of the power source (AC) to reduce fluctuation in a discharge lamp current (IL). The apparatus according to claim 2, characterized in that said control circuit (CON) is connected to said pulsating power source (AC) and said first and second series circuits of the switching elements (SW1-SW4), superposition of stored energy in said capacitive energy-storing element (C1) on the pulsating source voltage (VAC) being thereby varied to vary light output of the discharge lamp (DL). The apparatus according to claim 1, characterized in that said first and fourth switching elements (SW1,SW4) or said second and third switching elements (SW2,SW3) comprise first and second diode (D1,D2). The apparatus according to claim 5, characterized in that said control circuit (CON) connects said pulsating power source (AC) in series with said capacitive energy-storing element (C1) through said first and second series circuits of said switching elements (SW1,SW2 and SW3,SW4) controlled. The apparatus according to claim 6, characterized in that said control circuit (CON) is connected to said pulsating power source (AC) and said first series circuit of said first and second switching elements (SW1,SW2), said first series circuit of the switching elements SW1, SW2), receiving an output control signal of varying pulse width upon a predetermined actuation of the first series circuit of the switching elements, the pulse width being controlled in response to variation in the pulsating source voltage (VAC) of the power source (AC) to reduce fluctuation in the discharge lamp current (IL). The apparatus according to claim 6, characterized in that said control circuit (CON) is connected to said pulsating power source (AC) and said capacitive energy-storing element (C1) which is connected to said pulsating power source (AC) and said first and second series circuits of the switching elements (SW1-SW4), superposition of the energy stored in said capacitive energy-storing element (C1) on the pulsating source voltage (VAC) being thereby varied to vary light output of the discharge lamp (DL).

Inventors: KANDA TAKASHI, OHNISHI MASAHITO, KANDA,TAKASHI, OHNISHI,MASAHITO
Applicants: MATSUSHITA ELECTRIC WORKS LTD, MATSUSHITA ELECTRIC WORKS, LTD.
Application year: 1991
Main classification: H05B41
IPC codes: H05B41