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 (SW1) to a pulsating power source (AC), second closed circuit for connecting the lamp (DL) to the power source (AC) through a series circuit of second switching means (D1) which operates during OFF period of the first switching means (SW1) and an energy-storing means (C1), and an arrangement 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 discharge lamps in which a first closed-circuit means is connected for connecting a discharge lamp through a first switching means to a pulsating power source for applying a pulsating power source voltage to the lamp, and the first switching element is turned ON and OFF for operating the discharge lamp, characterized in that the apparatus further comprises a second closed-circuit means for connecting the discharge lamp through a series circuit of a second switching element and an energy-storing element to the pulsating power source, the second switching element being turned ON and OFF at least during OFF period of the first switching element for applying the pulsating source voltage to the lamp, a third closed-circuit element including a third switching element for attaining and applying to the discharge lamp a superposed voltage of the pulsating power source voltage and the energy in the energy-storing elements with their polarities made coincident with each other to be in the same direction in a period in which a discharge lamp current supplied to the discharge lamp is insufficient due to a voltage drop at the pulsating power source notwithstanding ON state of the first and second switching elements, and a control element connected to at least one of the first to third switching elements for being capable of controlling their operation. According to the apparatus for operating discharge lamps of the present invention as arranged in the above, the power source, first switching means and discharge lamp are connected in series and 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 in a first embodiment of apparatus for operating discharge lamps according to the present invention; FIG. 2 shows waveforms attained at various parts in the circuit of FIG. 1; FIGS. 3 and 4 are circuitry diagrams in other working aspects of the apparatus according to the present invention than that of FIG. 1; FIG. 5 shows in a circuitry diagram a second embodiment of the apparatus according to the present invention; FIG. 6 shows waveforms obtained at respective parts in the circuit of FIG. 5; FIG. 7 shows in a circuitry diagram another working aspect of the apparatus according to the present invention than that of FIG. 5; FIG. 8 is a circuitry diagram showing a third embodiment of the present invention; FIG. 9 is a circuitry diagram showing a fourth embodiment of the present invention; FIG. 10 shows waveforms at various parts in the circuit of FIG. 9; FIGS. 11 to 16 are circuitry diagrams showing respectively other working aspects of the apparatus of FIG. 9; FIG. 17 is a circuitry diagram showing a fifth embodiment of the present invention; FIG. 18 shows in a circuitry diagram another working aspect of the apparatus shown in FIG. 17; FIG. 19 shows in a circuit diagram a sixth embodiment of the present invention; FIG. 20 shows in a circuit diagram still another embodiment of the apparatus according to the invention; FIG. 21 shows respective waveforms at various parts in the circuit of FIG. 20; FIGS. 22 and 23 show in circuit diagrams further working aspects of the apparatus according to the invention; and FIG. 24 shows fragmentary circuit diagrams respectively applicable to the apparatus according to the invention. 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 to FIG. 1, there is shown a first embodiment of the apparatus for operating discharge lamps according to the present invention, in which such rectifier as a diode bridge DB is connected to such pulsating power source as an alternating current source AC, a first closed-circuit means comprising a first switching means SW1 is connected to output ends of the rectifier DB and eventually to the source AC, and a discharge lamp DL is connected through the first switching means SW1 to the output ends of the rectifier DB and source AC. Furthermore, a second closed-circuit means is connected to the first closed-circuit means so that a voltage of the source AC will be received through the rectifier DB. More specifically, the second closed-circuit means comprises a series circuit of a first diode D1 acting as a second switching element turned ON and OFF at least during OFF period of the first switching element SW1 and a capacitor C1 acting as an impedance and forming an energy-storing element, and this series circuit of the diode D1 and capacitor C1 is connected to both ends of the first switching element SW1. With respect to the first and second closed-circuit elements, further, a third closed-circuit element comprising a second diode D2 acting as a third switching element is connected, and this third switching element functions to attain a superposed voltage of the pulsating power source and stored energy in the capacitor C1 with their polarities made coinciding with each other in the same direction and to apply the attained superposed voltage to the discharge lamp DL within a period in which lamp current supply to the discharge lamp DL becomes insufficient due to a voltage drop of the pulsating power source notwithstanding that the first and second switching elements are in ON state. Between an end of the rectifier DB and the first switching element SW1, a proper impedance Z may be inserted, if required. For the second and third switching elements, any other element than the diode but achieving the same function may be employed and, further, a control element (not shown) may also be connected to the first to third switching elements for controlling their ON and OFF operation. Now, in the apparatus for operating discharge lamps of FIG. 1, as the first switching element SW1 is turned ON under condition that a rectified voltage Vin of the rectifier DB is higher than the discharge-enabling voltage of the discharge lamp DL, ions are increased, a discharge lamp current IL shows a tendency of increase, and the conductance of the discharge lamp increases. As the first switching means SW1 is turned OFF at a set current value, on the other hand, the discharge lamp current IL is made zero, the ions are caused to decrease with a time constant of the discharge lamp to have the conductance of the discharge lamp also decreased, and the operating of the discharge lamp DL is to be maintained by the turning ON and OFF of the first switching means SW1. This is made to be a basic arrangement of the present invention. In the present embodiment, further, the arrangement is so made that, in an event where, in particular, the first switching means SW1 is in its OFF state and the source voltage VAC is lower than the discharge-enabling voltage VL of the discharge lamp DL, the energy-storing in the capacitor C1 included in the second closed-circuit means as the energy-storing means is utilized for maintaining the operating of the discharge lamp DL. Next, the operation of the apparatus for operating discharge lamps of FIG. 1 shall be described to details also with reference to FIG. 2. Now, the source voltage VAC of such waveform as shown by (a) in FIG. 2 is supplied from the alternating current source AC, the voltage VAC is rectified at the rectifier DB, and a voltage Vin of such waveform as in (b) of FIG. 2 is applied to following stage circuit element. If in this case the voltage Vin is higher than the discharge-enabling voltage VL of the discharge lamp DL, such a current Iin as shown in (c) of FIG. 2 is supplied from the source AC side to the discharge lamp DL, and, in the OFF state of the first switching means SW1, such a charging current Ic as shown in (g) of FIG. 2 is caused to flow to the capacitor C1 through the first diode D1 acting as the second switching means turned ON and OFF in the OFF state of the first switching means SW1, and the capacitor C1 is thereby charged. As the rectifier voltage Vin starts dropping due to a drop in the source voltage VAC, the energy supply from the source AC side to the capacitor C1 stops, the current Iin is made zero, and the energy in the capacitor C1 is supplied to the discharge lamp DL through the second diode D2 acting as the third switching means upon turning ON of the first switching means SW1. When the rectified voltage Vin is higher than the discharge voltage of the capacitor C1, such discharge lamp voltage VL as shown in (d) of FIG. 2 is applied to the discharge lamp DL upon turning ON of the first switching means SW1, and such discharge lamp current IL as in (e) of FIG. 2 is made to flow. When the first switching means SW1 is turned OFF at its predetermined current value, as will be clear from a waveform (f) of FIG. 2, the discharge lamp current IL flowing through the capacitor C1 decreases but the charge voltage to the capacitor C1 increases, on the other hand, and the discharge lamp conductance decreases gradually, upon which the capacitor C1 is so charged that the voltage at the capacitor C1 (corresponds to flat portion of the rectified voltage Vin) will be higher than the discharge-enabling voltage of the discharge lamp DL. Provided that the capacitor C1 has an impedance, it can be regarded that a charging circuit is present from the discharge lamp DL to the capacitor C1 so that the voltage of the capacitor C1 substantially can be raised up to a both end voltage of the series circuit of the discharge lamp DL and the first switching means SW1. The energy discharged from the capacitor C1 is supplied through the diode D2 as the third switching means to the discharge lamp DL, since the diode D1 as the second switching means is disposed to be reverse directional. The thus applied voltage has an envelope substantially equal to such rectified voltage Vin as shown by (b) in FIG. 2, and the discharge lamp voltage VL, discharge lamp current IL and current ISW flowing through the first switching means SW1 are shown at (d) , (e) and (f) in FIG. 2, respectively. Accordingly, in the case when the source voltage is pulsating type and in particular the alternating current, the discharged energy can be utilized for the energy supply at the time adjacent zero-cross points, so as to be able to prevent the flickering and eventual extinguishing of the discharge lamp from occurring, while stably maintaining the operating of the discharge lamp DL without imparting any large stress to the switching means, in particular, the first switching means. Further, when the capacitor C1 is charged through the discharge lamp DL, the capacitor C1 is capable of realizing an action of smoothing the rectified voltage Vin, prolonging the term for which the charging current to the capacitor C1 flows, and improving effectively the harmonic distortion of the current drawn from the A.C. source. In a working aspect shown in FIG. 3, as will be clear when compared with the apparatus shown in FIG. 1, substantially the same arrangement is adopted, except that the first switching means SW1 is disposed on a higher potential side with respect to the discharge lamp DL and the capacitor C1 is also disposed on a higher potential side with respect to the second diode D2 as the third switching means, and the same effect of preventing the flickering and extinguishing of the discharge lamp DL, reduction of any stress imparted to the switching means, and effective improvement in respect of the harmonic distortion of the current drawn from the A.C. source can be attained. In another working aspect shown in FIG. 4, the apparatus is adapted to the operating of a plurality of discharge lamps DL11, DL12, ... DLn, and a plurality of the first switching means SW1, SW2, ... SWn are provided respectively in correspondence to each of these discharge lamps. Furthermore, a plurality of the first diodes D11, D12, ... Dn acting as the second switching element are connected respectively in correspondence to each of the discharge lamps DL11, DL12, ... DLn and the first switching elements SW1, SW2, ... SWn, while the capacitor C1 as the energy-storing elements and the second diode D2 as the third switching element are utilized in common. Other arrangements are the same as those in the embodiment of FIG. 1, and the same operation and effects as in the case of FIG. 1 can be attained. Referring next to FIG. 5, there is shown a second embodiment of the apparatus for operating discharge lamps according to the present invention, in which four switching elements SW1 to SW4 of the first switching elements are disposed in a full bridge mode, so that alternately turning ON and OFF of two pairs of these switching elements SW1, SW2 and SW3, SW4 will cause an alternating current to be supplied to the discharge lamp DL connected to the middle branch of the full bridge arrangement. Furthermore, two switching elements SW5 and SW6 of the second switching element are connected as inserted respectively between each of both ends of the discharge lamp DL and a junction point between the capacitor C1 as the energy-storing elements and the diode D2 as the third switching element. Other arrangements including the first to third closed-circuit elements and so on are the same as those in the embodiment of FIG. 1, and such function as the charging and discharging of the capacitor C1 and so on are also the same. In the present instance, the first closed circuit element is formed to comprise four switching elements SW1 to SW4 of the first switching element, the second closed circuit element is formed with two switching elements SW5 and SW6 of the second switching element and the capacitor C1, and the third closed circuit element is formed with the diode C2 as the third switching element, as will be readily appreciated. In the apparatus for operating discharge lamps of FIG. 5, when one pair of the switching elements SW3 and SW4 are in OFF state while SW1 in the other pair of the first switching element is ON and SW2 of this pair is ON or OFF, one SW5 of the second switching element is OFF with SW2 is turned ON but is ON with SW2 is turned OFF and the other SW6 of this second switching element is OFF. Furthermore, when the pair SW1 and SW2 of the first switching element are in the OFF state while one SW3 in the other pair is ON and the other SW4 is ON or OFF, one SW5 of the second switching element is OFF and the other SW6 is OFF with SW4 made ON but is ON with SW4 is turned OFF. Referring more specifically in conjunction with FIG. 6, such alternating current source voltage VAC as in (a) of FIG. 6 is rectified to apply such rectified voltage as in (b) of FIG. 6 to other circuit elements at following stages, upon which the pair SW1 and SW2 of the first switching element are made ON as seen in (g) of FIG. 6 and such rectified current Iin as (c) of FIG. 6 is caused to flow but, when the rectified voltage Vin is higher than the discharge-enabling voltage of the discharge lamp DL, such discharge lamp voltage VL as in (d) of FIG. 6 is applied to the discharge lamp DL, and such discharge lamp current IL as in (e) of FIG. 6 flows through the pair SW1 and SW2 of the first switching element. When one SW1 of the pair of the first switching element is turned to be in ON state whereas the other SW2 is turned OFF, then the one SW5 of the second switching element is made ON as in (i) of FIG. 6 and such charging current as in (h) of FIG. 6 is made to flow to the capacitor C1. When, on the other hand, the other pair SW3 and SW4 of the first switching element are both turned ON as in (f) of FIG. 6, the discharge lamp current IL is caused to flow through these switching elements SW3 and SW4, whereby the switching element SW4 is turned OFF but the switching elements SW3 and SW6 are turned ON as in (f) and (j) of FIG. 6, and the capacitor C1 is charged. Furthermore, when the rectified voltage Vin drops to be lower than the discharge-enabling voltage of the discharge lamp DL, the discharged energy from the capacitor C1 is supplied through the diode D2 as the third switching element to the first switching element comprising the elements SW1 to SW4, and eventually the alternating current is to be supplied to the discharge lamp DL. In the embodiment of FIG. 5, other function and effect are the same as those in the embodiment of FIG. 1. In a working aspect shown in FIG. 7, the energy-storing capacitor C1 is disposed on the higher potential side with respect to the diode D2 as the third switching element. When, in this aspect, the pair SW3 and SW4 of the first switching element are in OFF state while one SW2 of the other pair of the first switching element is ON and the other SW1 of this pair is ON or OFF state, one SW5 of the second switching element is OFF and the other SW6 is OFF with SW3 made ON but is ON with SW3 made OFF. Furthermore, when the pair SW1 and SW2 of the first switching element are in OFF state while one SW4 of the other pair is ON and the other SW3 of this pair is ON or OFF, one SW5 of the second switching element is OFF with SW3 made ON but is ON with SW3 made OFF, and the other SW6 is OFF. Other arrangements are the same as those in the embodiment of FIG. 1 or 5, and the same functions as in such embodiment can be obtained. Referring to FIG. 8, there is shown a third embodiment of the apparatus for operating discharge lamps according to the present invention, in which a control circuit CON is connected across the both output terminals of the rectifier DB which is connected on the other hand to the alternating current source AC, and the discharge lamp DL is further connected across the terminals of the rectifier DB through a series circuit of the diode D1 as the second switching element, capacitor C1 as the energy storage element and diode D2 as the third switching element. Furthermore, each of a pair of the switching elements SW1 and SW2 forming the first switching elements is connected between a junction point of the diode D1 and capacitor C1 and a junction point of the diode D2 and discharge lamp DL and between junction point of a higher potential side terminal of the rectifier DB and diode D1 and a junction point of the capacitor C1 and diode D2. With respect to these switching elements SW1 and SW2, the control circuit CON is providing ON and OFF control signals. In the present embodiment, the switching element SW1 is driven with a constant pulse, and the switching element SW2 achieves the ON and OFF operation at the discharge-enabling voltage VL1. Furthermore, it is made possible to stabilize the discharge lamp current by rendering it to be sinusoidal with the peak values of input current restrained when input voltage is high, in such manner that ON period of the switching element SW1 in the first switching element is controlled in accordance with momentary voltage value of the voltage Vin rectified by the rectifier DB so that, when Vin is high, the ON period is shortened but, when Vin is low, the ON period is prolonged. According to the present embodiment, the arrangement of the switching elements can be made simpler than in the case of, in particular, the embodiment of FIG. 8. Further, other arrangements, function and effect are the same as those in the embodiment of FIG. 1, and it will be readily appreciated that the first to third closed circuit elements respectively comprising the first to third switching elements may be formed for achieving the same function as in the foregoing embodiments. Referring to FIG. 9, there is shown a fourth embodiment of the apparatus for operating discharge lamps according to the present invention, in which, in particular, an example of the control circuit CON not shown in FIG. 1 but connected across the output terminals of the rectifier DB is shown, and a further diode D3 is inserted between the higher potential side terminal of the rectifier DB and the discharge lamp DL. At this diode D3, the rectified voltage is divided into V1 and Vin, high or low level of the source voltage is detected by means of the value of the voltage V1, such detection is provided to the control circuit CON, and a signal is provided from the control circuit CON to the first switching element SW1 for controlling the same. Referring to the operation of this embodiment of FIG. 9 in conjunction with FIG. 10, the source voltage VAC such as (a) in FIG. 10 is applied to the apparatus, and, through the rectifier DB, such voltage V1 as (c) in FIG. 10 is applied from cathode side of the diode D3 to the control circuit CON. When this voltage V1 is discriminated at a discriminating circuit in the control circuit CON to be high, an output pulse Vd of the control circuit CON is made narrower, while this pulse width is made wider when the voltage V1 is discriminated to be low, as seen in (d) of FIG. 10. Thus, the output Vd of the control circuit CON is provided to the switching means SW1, and the voltage applied to the discharge lamp DL is made somewhat wavy as in (e) of FIG. 10 in accordance with the operation of the switching means SW1 in response to this output Vd. At this time, the current IL made to flow through the discharge lamp DL including a charge current made to flow to the capacitor C1 through the diode D1 when the switching means SW1 is in OFF state will be as in (f) of FIG. 10. Here, such stable current as in (g) of FIG. 10 is made to flow to the switching means SW1, and such charge and discharge current Ic as in (h) of FIG. 10 is caused to flow to the capacitor C1. In the present embodiment, therefore, the controlling signal is varied in the pulse width in accordance with the level of the input voltage. That is, the signal pulse width is made narrower to restrict the discharge lamp current value when the source voltage VAC is high but is made wider to enlarge the current value, whereby the discharge lamp current IL can be made to be of the sinusoidal mode and the discharge lamp operation can be stabilized. In the present embodiment, other arrangement, function and effect are the same as those in the embodiment of FIG. 1, and the first through third closed circuit means respectively including the first to third switching means can be provided for attaining the same function as in the foregoing embodiments, as will be readily appreciated. Furthermore, the control circuit CON shown in the present embodiment may be also employed in the embodiment of FIG. 1 and in the foregoing embodiments in the identical arrangement or with the arrangement properly modified. In a working aspect shown in FIG. 11, in contrast to the embodiment of FIG. 9, the arrangement is so made that the input to the control circuit CON is obtained from the rectified voltage Vin without use of the diode D3, and is so controlled that, when the rectified voltage Vin is high in such period tx as shown in FIG. 12, in particular, the signal pulse width is made narrower but is made wider when the voltage is low, whereby the control can be carried out with the pulse width preliminarily set in accordance with the value of the rectified voltage Vin, and the discharge lamp operation can be stabilized with the discharge lamp current made to be of the sinusoidal mode. Other arrangements, function and effect of the present aspect are the same as those in the embodiment of FIG. 9. In another working aspect of FIG. 12, the arrangement is so made that the diode D3 is employed similarly to the embodiment of FIG. 11 but as connected at both ends to the control circuit CON, the current flowing to the discharge lamp DL and first switching element SW1 is detected at the control circuit CON, and the control signal is provided from the control circuit CON to the switching means SW1 in accordance with the detected current value, whereby the same operation as that in FIG. 9 and FIG. 11 as well can be attained and the discharge lamp operation can be stabilized. In this arrangement, it is possible to employ, in place of the diode D3, such element as a detecting resistor or the like which does not constitute any current limiting element. In the present aspect, other arrangement, function and effect are the same as those in the embodiment of FIG. 9. In another working aspect of FIG. 13, the arrangement is so made that the source voltage VAC is detected at the input terminal of the rectifier DB, the detected voltage value is provided as an input to the control circuit CON, and the control signal is provided from the control circuit CON to the switching means SW1, whereby the same operation as in the case of FIG. 9, 11 or 12 is carried out and the discharge lamp operation can be stabilized. Other arrangements, function and effect of this aspect are the same as those in the embodiment of FIG. 9. In a further working aspect shown in FIG. 14, the arrangement is so made that an impedance Z2 which does not constitute any current limiting element is inserted in series with the switching means SW1, the impedance Z2 is connected at both ends to the control circuit CON, the current flowing to the discharge lamp DL and switching means SW1 is provided to the control circuit CON, and thereby the control signal is caused to be provided from the control circuit CON to the switching element SW1, whereby the discharge lamp operation can be stabilized in the similar manner to that in the case of FIG. 9 or any of FIGS. 11 to 13. Other arrangements, function and effect of the present aspect are the same as those in the embodiment of FIG. 9. In a further working aspect of FIG. 15, the arrangement is so made that the switching element SW1 is connected on higher potential side of the discharge lamp DL in contrast to the aspect of FIG. 13, whereby the discharge lamp operation can be stabilized in the similar manner as in the case of FIG. 9 or any of FIGS. 11 to 14. Other arrangements, function and effect of the present aspects are the same as those in the embodiment of FIG. 9. In a still further working aspect shown in FIG. 16, the arrangement is so made that, in the apparatus for the plurality of discharge lamps DL11, DL12, ... DLn as in the foregoing aspect of FIG. 4, the input to the control circuit CON is obtained from the rectified voltage Vin in the similar manner to the aspect of FIG. 11, and the control signal is provided from the control circuit CON to the respective switching elements SW1, SW2, ... SWn, forming the first switching element so that the discharge lamp operation can be stabilized in the similar manner to that in the foregoing case of FIG. 9 or any of FIGS. 11 to 15 and to that in the aspect of FIG. 4. Other arrangements, function and effect of the present aspect are the same as these in the embodiment of FIG. 9. In providing the input to the control circuit with the detection of the level of the source voltage VAC, it will be possible, in contrast to the foregoing arrangement of FIG. 9 or any of FIGS. 11 to 16, to detect the voltage across the diode D1 or D2 or the voltage or current of the capacitor C1, so as to realize the control of the first switching element in the similar manner to that in the foregoing aspects. Referring now to FIG. 17, there is shown a fifth embodiment of the apparatus for operating discharge lamps according to the present invention, in which the switching elements SW1 to SW4 forming the first switching element are connected in the full bridge mode as has been described with reference to FIG. 5, and the control signal is provided from the control circuit CON to each one SW2 or SW4 of each pair of the switching elements SW1 and SW2 or SW3 and SW4. When, in this case, the switching elements SW3 and SW4 are in OFF state while the switching element SW1 is in ON state and the switching element SW2 is in ON or OFF state, the second switching element SW5 turns OFF with SW2 made ON and turns ON with SW2 made OFF to cause the other second switching element SW6 is turned OFF. When, on the other hand, the switching elements SW1 and SW2 are OFF while the switching element SW3 is ON and the switching element SW4 is ON or OFF, the second switching element SW5 turns OFF and the other second switching element SW6 turns OFF with SW4 is made ON but turns ON with SW4 made OFF. With this arrangement, too, the same stabilization of the discharge lamp operation as in the embodiment of FIG. 9 can be attained while supplying an alternating current voltage to the discharge lamp DL through the capacitor C1 and diode D2. In the present embodiment, other arrangements, function and effects are the same as those in the embodiment of FIG. 1 or 5. In a working aspect of FIG. 18, the arrangement is so made that the capacitor C1 is disposed on the higher potential side with respect to the diode D2, in contrast to the embodiment of FIG. 17 but similarly to the foregoing aspect of FIG. 7, and the control signal is provided from the control circuit CON to each one SW1 or SW3 of each pair of the switching elements SW1 and SW2 or SW3 and SW4 forming the first switching element. When, in this case, the switching elements SW3 and SW4 are in OFF state while the switching element SW2 is ON and the switching element SW1 is ON or OFF, the second switching element SW5 is turned OFF and the other second switching element SW6 is turned OFF with SW1 made ON but is turned ON with SW1 is made OFF. When, on the other hand, the switching elements SW1 and SW2 are OFF while the switching element SW4 is ON and the switching element SW3 is ON or OFF, the second switching element SW5 is turned OFF with SW3 made ON but is turned ON with SW3 made OFF and the other second switching element SW6 is turned OFF. With this arrangement, too, the stabilization of the discharge lamp operation can be attained in the similar manner to that in the embodiment of FIG. 9 or 17, while supplying the alternating current voltage to the discharge lamp DL. In the present aspect, other arrangement, function and effect are the same as those in the case of FIG. 1 or 7. Referring to FIG. 19, there is shown as sixth embodiment of the apparatus for operating discharge lamps according to the present invention, in which the arrangement is so made that such diodes D1 and D2 forming the second and third switching elements as those used in the embodiment of FIG. 1 are replaced by any other switching elements, optimumly, switching elements SW7 and SW8 which are equal or approximate to the switching elements SW1 acting as the first switching means. According to this arrangement, the charging of the capacitor C1 can be carried out independent of the ON and OFF operation of the switching element SW1, and the capacitor C1 voltage can be applied to a series circuit of the switching element SW1 and discharge lamp DL for a desired period by means of the ON and OFF operation of the switching element SW8. According to the present embodiment, therefore, the discharge lamp current IL is made more precisely controllable than in the case of the embodiment of FIG. 1. Other arrangement, function and effect of the present invention are the same as those in the embodiment of FIG. 1. In another working aspect of FIG. 20, the arrangement is so made that the current detecting element CDE is connected in series with the discharge lamp DL in the embodiment of FIG. 8. Referring to the operation of this aspect in conjunction with waveforms of FIG. 21, the switching element SW2 is turned ON as in (c) of FIG. 21 when the rectified voltage Vin is lower than the threshold voltage VT1 as in (a) of FIG. 21 to cause the capacitor C1 voltage superposed on the rectified voltage Vin is higher than the threshold voltage VT1 to cause a current to flow through a path DB → D1 → SW1 → DL when the switching element SW1 is in ON state in which the diode D2 is reverse biased through the capacitor C1. When the switching element SW1 is in OFF state, on the other hand, the current is caused to flow through a path DB → D1 → C1 → D2 → DL, upon which an energy is stored in the capacitor C1. When the switching element SW1 is turned ON as in (b) of FIG. 21 when the switching means SW2 is ON, the current is caused to flow through a path DB → SW2 → C1 → SW1 → DL. At this time, so long as the switching element SW2 is ON, the diode D1 is reverse biased through the capacitor C1. When the threshold voltage is set from VT1 to VT1H as in (a) of FIG. 21, the discharge lamp voltage will shift from VL of (d) to VLH of (e) in FIG. 21, and, when the threshold voltage is set from VT1 to VT1L, the discharge lamp voltage will be VLL as in (f) of FIG. 21 and the voltage value decreases, whereby the discharge lamp current is applied as rectified, but the switching elements as well as the control circuit CON can be attempted to be simplified. In the working aspect of FIG. 20 as well as in the embodiment of FIG. 8, the discharge lamp current is being rectified so that, so long as the charge in the capacitor C1 is stored with the switching element SW2 is turned ON, the diode D1 is reverse biased and, when the switching element SW1 is turned ON, the diode D2 is reverse biased, so that the switching means can be attempted to be simplified. In a working aspect of FIG. 22, the switching elements SW1 and SW2 are mutually replaced in contrast to the working aspect of FIG. 20, and the same function and effect as those in the embodiment of FIG. 20 are attained. In a further working aspect of FIG. 23, the switching circuit of the switching elements SW1 and SW2, capacitor C1 and diodes D1 and D2 is disposed on the lower potential side, and the same function and effect as those in the foregoing embodiment of FIG. 20 are attained. For the switching elements employed in the respective embodiments and working aspects as has been described, transistors alone, thyristors, FET's, such reverse parallel connection of the transistors and diode as described with reference to FIG. 23, coupled elements of FET with diode incorporated, or any other equivalents can be properly employed. Further, while in the respective embodiments and working aspects described the capacitor alone has been disclosed to be used as the energy-storing elements, it will be possible to employ such series connection of the capacitor C1 and a reactor L1 of a small value as shown in (a) of FIG. 32, such series connection of the capacitor C1 and a resistor R1 as in (b) of FIG. 24, and such series connection of reactor L1 and resistor R1 to the capacitor Cl as in (c) of FIG. 24.

Claims: An apparatus for operating a discharge lamp, said apparatus comprising a first closed-circuit part connecting at least a discharge lamp (DL:DL11-DLn) through a first switching element (SW1, SWn, SW2, SW3, SW4) to a pulsating power source (AC), characterized in that the apparatus further comprises: a second closed-circuit part connecting the discharge lamp through a series circuit of a second switching element (D1;D11-Dn;SW5,SW6;D1-D1n;D1,SW7) and a capacitive energy-storing element (C1) to the pulsating power source (AC), a third closed-circuit part including a third switching element (D2) as well as said pulsating power source (AC) and said capacitive energy-storing element (C1), and control circuit (CON) connected to said first to third switching elements (SW1;...), (D1;...), (D2), said control circuit (CON) comprising first means for turning said first switching element (SW1;...) ON and OFF for operating the discharge lamp (DL;...), second means for turning said second switching element (D1;...) ON and OFF at least during OFF period of said first switching element (SW1;...), and third means for turning said third switching element (D2) ON and OFF for applying to the discharge lamp (DL;...) the pulsating power source voltage (VAC) as superposed on the voltage of said energy-storing element (C1) with their voltage polarities made coincident with each other in the same direction in a period in which a discharge lamp current (IL) supplied to the discharge lamp is insufficient due to a voltage drop at the pulsating power source (AC). The apparatus according to claim 1, characterized in that said first means of said control circuit (CON) is connected to said first switching element (SW1) to turn it from ON state to OFF state in response to a predetermined value reached by the discharge lamp current (IL). The apparatus according to claim 1, characterized in that said first means of said control circuit (CON) is connected to said first switching element (SW1) to turn it from OFF state to ON state prior to interruption of the discharge lamp current (IL) to the discharge lamp (DL). The apparatus according to claim 1, characterized in that said first means of said control circuit (CON) is connected to said first switching element (SW1) to form said second closed-circuit part, causing a charge current (Ic) to flow through said second switching element (D1) to said energy-storing element (C1) to charge it up to a level of energy capable of being supplied to the discharge lamp (DL). The apparatus according to claim 1, characterized in that said first and second means of said control circuit (CON) are connected to said first switching elements which comprise either one of two pairs of the switching elements (SW1,SW2 and SW3,SW4), one pair of which being maintained in ON state while the other pair is in OFF state. The apparatus according to claim 1, characterized in that said first to third means of said control circuit (CON) are connected to said first, second and third switching elements (SW1;...), (D2) for a continuous flow of the discharge lamp current (IL) to the discharge lamp (DL) through one or more of the first to third switching elements, in response to the pulsating power source voltage (VAC). The apparatus according to claim 1, characterized in that said discharge lamp (DL) is connected in series to said first switching element (SW1) of said first closed-circuit part, said series connection being connected in parallel with said pulsating power source (AC), and said energy-storing element (Cl) in said second closed-circuit part is connected to the lamp (DL) to supply thereto a continuous flow of the discharge lamp current (IL) when the first switching element (SW1) is in OFF state. The apparatus according to claim 7, characterized in that said control circuit (CON) is connected to said pulsating power source (AC) and said first switching element (SW1), an output control signal of varying pulse width controlled in response to variation in the pulsating source voltage (VAC) being thereby provided to the first switching element (SW1) when said first switching element (SW1) is in its ON state, for reducing variation in the discharge lamp current (IL). The apparatus according to claim 7, characterized in that said second and third switching elements respectively comprise diodes (D1,D11-Dn,D2), and said control circuit (CON) is connected to the first switching means (SW1). The apparatus according to claim 1, characterized in that said first switching element comprises four switching members (SW1-SW4) connected in a bridge circuit which is connected in parallel with said pulsating power source (AC), said discharge lamp (DL) being connected intermediately across said bridge circuit, said energy-storing element (C1) comprising a capacitor, and said third switching element (D2) comprising a diode, said capacitor (Cl) and diode (D2) forming a series circuit connected in parallel with said bridge circuit; and said second switching element comprises a pair of switching members (SW5,SW6) respectively inserted between each of both ends of the discharge lamp (DL) and a junction point of the seriesconnected capacitor (C1) and the diode (D2). The apparatus according to claim 10, characterized in that said control circuit (CON) is connected to said pulsating power source (AC) and said first switching element (SW1-SW4) in said bridge circuit, the first switching element being thereby provided with an output control signal of varying pulse width controlled in response to variation in the pulsating power source voltage (VAC) upon predetermined actuation of said first switching element (SW1-SW4) in said bridge circuit, to reduce fluctuation in the discharge lamp current (IL).

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