Abstract:
An electronic ballast is capable of realizing high frequency lighting of a discharge lamp and switching between at least two lighting modes with different light outputs. The ballast includes a preheating circuit having a winding component connected in parallel with a main resonant circuit with a lamp current flowing therein for the discharge lamp. A constant preheating current for the lamp filaments is supplied from a secondary winding of the winding component during lighting of the discharge lamp and a path of a current flowing on a primary winding side of the winding component is switched by a switch according to the lighting mode.

Description:
A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims benefit of the following patent application(s) which is hereby incorporated by reference: Japanese Patent Application No. JP2008-215809, filed Aug. 25, 2008. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO SEQUENCE LISTING OR COMPUTER PROGRAM LISTING APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an electronic ballast for a discharge lamp having at least two lighting modes with different light outputs, and a lighting fixture using such ballast. 
     In a discharge lamp of a thermionic cathode type such as a fluorescent lamp, lamp illumination performance and a lamp life can be secured by maintaining an appropriate filament temperature in the lamp.  FIG. 9  shows an example of typical lamp dimming control data defined according to the IEC. The horizontal axis employs numerical values obtained by dividing a lamp current Id by a reference current Itest. The vertical axis employs numerical values obtained by dividing the sum of squares of a large current side I LH  and a small current side I LL  of a lead wire current by the square of the reference current Itest, wherein a maximum dimming control curve A, a target dimming control curve B and a minimum dimming control curve C are defined. More specifically, a maximum target value, a recommended target value and a minimum target value are shown for a current Id flowing into the lamp filaments for each dimming level. 
     In the present specification, a lead wire current obtained during illumination is divided into a current referred to as a lead wire current which includes a lamp current (i.e. large current side), and a current referred to as a constant preheating current which flows via filaments (i.e. small current side). Indexes employed in the vertical axis in  FIG. 9  are obtained by dividing the sum of squares of the lead wire current and the constant preheating current by the square of the lamp current, and can be said as indicating the conditions of a constant preheating current required for each lamp current. 
     A curve D in  FIG. 9  (without any flow of a constant preheating current) is provided with numerical values plotted on an assumption that a lead wire current is equal to a lamp current and a constant preheating current is 0 [A]. If a lamp current which is substantially equal to a rating of a discharge lamp is made to flow (refer to an area in the vicinity of 1.0 in the lateral axis), the curve D substantially overlaps a target dimming control curve but shows indexes which decrease in accordance with reduction of a lighting output and fall under the lower target value in course of time (refer to an area less than 0.7 in the lateral axis). 
     That is, it can be said that a filament temperature suitable for illumination can be maintained in the vicinity of rated illumination of a discharge lamp by constantly preheating filaments using the lamp current. Dimming control, on the other hand, requires a larger constant preheating current to maintain the appropriate filament temperature in accordance with a lower lamp output. 
     As stated above, it is a well-know fact that a required amount of the constant preheating current is increased when dimming the discharge lamp. 
     An operation in an electronic ballast for a discharge lamp will be explained referring to a first conventional example shown in  FIGS. 10 and 11  with respect to the present invention to achieve an appropriate flow of a constant preheating current during illumination by corresponding to a lighting output in a ballast having at least two lighting modes with different light outputs. 
       FIG. 10  is a circuit configuration of the discharge lamp ballast according to the first conventional example.  FIG. 11  is a graph showing characteristics of a resonance circuit made of an inductor L 2  and a capacitor C 3  in each control state of preheating, starting, rated lighting and dimmed lighting, and a relationship between a constant preheating current flowing into filaments at that time and a driving frequency in an inverter  12 . 
     A low frequency AC power source sent from a commercial power source  10  is rectified by a diode bridge including diodes D 1  to D 4  in a step-up chopper circuit  11 . The voltage is stepped up by a step-up chopper circuit including a choke coil L 1 , a transistor Q 1  and a diode D 5 . Obtained at both ends of an electrolytic capacitor C 2  is a DC voltage of, for example, about 300V. This DC voltage is converted into a high frequency current in inverter  12  and used as a lighting current for a discharge lamp  13 . 
     The inverter  12  has a half bridge inverter circuit including a pair of transistors Q 2  and Q 3 , and an inverter control circuit  14  drives the transistors Q 2  and Q 3  to be turned on/off alternately to output high frequency power. The high frequency power is supplied to the discharge lamp  13  via filaments by passing through a DC blocking capacitor C 4  and the inductor L 2 . 
     A control power source circuit  15  which includes a step-down chopper circuit or similar circuit generates a DC low voltage (e.g. 12V) which is supplied to the inverter control circuit  14  and a chopper control circuit  16 . The chopper control circuit  16  may include a control IC (e.g. MC33262 made by Motorola, Inc.) and generates a gate control signal for the transistor Q 1  in the step-up chopper circuit  11 . The inverter control circuit  14  provides each gate of the transistors Q 2  and Q 3  in the inverter  12  with a signal oscillated by using a versatile control IC (e.g. μPC494 made by NEC cooperation) via a driver circuit (e.g. IR2111 made by International Rectifier Corp.). 
     When power is applied, the chopper control circuit  16  and the inverter control circuit  14  starts oscillation to bring an output voltage Vdc in the step-up chopper circuit  11  to about 300V and an oscillation frequency in the inverter  12  to fp=95 kHz. At this time, a voltage obtained at the filaments of the discharge lamp  13  is lower than a lamp starting voltage, which means the discharge lamp  13  does not light. 
     High frequency power outputted from the inverter  12  is also made to flow into a transformer T 2  through a capacitor C 9 . Power induced to a secondary side of the transformer T 2  causes a current to flow into the filaments of the discharge lamp  1  through capacitors C 7  and C 8 . This current obtained before the discharge lamp  13  starts is a required preheating current of, for example, about 700 mA. 
     Preheating is carried out for two to three seconds and is followed by reducing an oscillation frequency in the inverter  12  to fs=80 kHz. As a result, the voltage at the lamp filaments is increased to a required starting voltage, whereby the lamp illuminates. Thereafter, the oscillation frequency in the inverter circuit  12  is reduced to fr=55 kHz to bring the discharge lamp  13  into a rated lighting state. 
     In the case where lamp dimming is desired (luminance which is lower than rated luminance), a dimming control signal is sent to the inverter control circuit  14 . Therefore, the oscillation frequency in the inverter circuit  12  is changed to fd=75 kHz for the discharge lamp  13  to be in a dimmed lighting state. 
       FIG. 11  graphically shows the frequency characteristics of a voltage V 1   a  applied to the discharge lamp  13 , including (a) resonance characteristic at no loading when the discharge lamp  13  is turned off, (b) resonance characteristic in dimmed lighting, and (c) resonance characteristic at rated lighting. Impedance in the discharge lamp  13  is added to the resonance circuit in during lighting, so that the Q of the resonance circuit is reduced with a lower resonance frequency and resonance voltage than those obtained at no loading.  FIG. 11  also shows a frequency characteristic of a filament current. 
     As dimming control is deepened with an increased oscillation frequency in the inverter circuit  12 , the filament current is increased as understood from the filament current curve in  FIG. 11 . This is because of a resonance action in a resonant circuit made of the capacitor C 9  and an inductance in a primary winding of the transformer T 2 . This resonant frequency is higher than a resonant frequency in the inverter in each control state of preheating, starting, rated lighting and dimmed lighting, which suggests that a current flowing into the filaments is larger in accordance with a higher operating frequency in the inverter, or a lower lamp voltage before the start of discharging or a lower lighting output during lighting. 
     It is therefore made possible to appropriately secure a required preheating current before the lamp starts and a constant preheating current during dimming control. 
     An operation of an electronic ballast will also be explained by referring to a second conventional example in  FIGS. 12 and 13 , with respect to an invention to lower power consumption by carrying out an operation to secure a preheating current in a preheating period and to prevent a constant preheating current from flowing after stable lighting. 
       FIG. 12  shows a configuration of an electronic ballast according to a second conventional example, including an AC power source  10 , a rectifier DB for rectifying an output of the AC power source  10 , a DC power source circuit  11   a  for smoothing an output of the rectifier DB. An inverter  12  converts a DC voltage outputted from the DC power source circuit  11   a  into high frequency power. A preheating circuit  5  includes a series circuit having a capacitor C 9  connected across the output of inverter  12 , a primary winding N 1  of a preheating transformer T 2  and a preheating switching element SW 1 . A control circuit  4  controls the preheating switching element SW 1  to be turned on/off and the inverter  12 . A load circuit  6  is made of a series circuit including a DC-blocking capacitor C 4  connected between output ends of the inverter  12 , a resonant inductor L 2 , and a discharge lamp  13  of a thermionic cathode type, and a resonant capacitor C 3  connected in parallel with the discharge lamp  13 . The two preheating windings N 21  and N 22  arranged in the preheating transformer T 2  are connected to filaments F 1  and F 2  in the discharge lamp  13  via capacitors C 7  and C 8  respectively. 
     The control circuit  4  which controls the inverter circuit  12  and the preheating circuit  5  carries out each control of filament preheating, starting and lighting for the discharge lamp  13  after the AC power source  10  is supplied to start the inverter  12 . The control circuit  4  includes: a timer circuit  41  for setting each switching time to switch operations in the inverter  12  from a preheating state to a starting state, and from the starting state to a lighting state, and switching time to switch operations in the preheating circuit  5  from a preheating current supplying state to a preheating current stopping state respectively, and outputting a control signal corresponding to each switching time. A frequency setting circuit  42  sets each operating frequency in the inverter  12  in the preheating state, the starting state and the lighting state in accordance with each control signal outputted from the timer circuit  41 . A driving circuit  43  outputs a driving signal to determine switching of the switching elements in the inverter  12  on the basis of a frequency set by the frequency setting circuit  42 . An inverter  44  outputs a control signal δ obtained by inverting a control signal Y which is outputted from the timer circuit  41  to control preheating switching element SW 1 . 
     An operation of the control circuit  4  will be explained below by using a timing chart shown in  FIG. 13 . First, after time point t 0  to start driving the control circuit  4 , the discharge lamp  13  is brought into the preheating state (preheating mode). An amount of time t 1  to maintain the preheating state is set by a control signal α outputted by the timer circuit  41 , during which the inverter  12  is subjected to a switching operation at a frequency fp set for the preheating state. 
     Next, after passing the time t 1 , the control signal α is switched from “L” to “H” for switching to a starting state (i.e. starting mode) to apply a voltage required for starting to both ends of the discharge lamp  13 . An amount of time t 2  to maintain the starting state is set by a control signal β outputted by the timer circuit  41 , during which the inverter  12  is subjected to a switching operation at a frequency fs (fs&lt;fp) set for the starting state. 
     Next, after passing the time t 2 , the control signal β is switched from “L” to “H” for switching to a lighting state (i.e. lighting mode) to supply power required for rated lighting of the discharge lamp  13 . At the time t 2  and thereafter, the control signal α is set to “H” and the control signal β is set to “H”, wherein the inverter circuit  12  is subjected to a switching operation at a frequency fr (fr&lt;fs&lt;fp) set for the lighting state at this time to realize lighting of the discharge lamp  13  by a predetermined output. 
     In the present conventional example, the control signal δ obtained by inverting the control signal Y which is switched from “L” to “H” at time t 3  set as t 1 &lt;t 3 &lt;t 2  is used to turn on the preheating switch element SW 1  up to the time t 3  so as to supply a preheating current, and the preheating switch element SW 1  is turned off at the time t 3  and thereafter to stop supplying a preheating current If. 
     More specifically, a preheating current is made to flow in filaments in the precedent preheating period and a constant preheating current supplied to the filaments is stopped after stable lighting. Therefore, power consumption by a constant preheating current which is unnecessary in normal lighting and adverse effects to a lamp life are prevented. 
     BRIEF SUMMARY OF THE INVENTION 
     In the electronic ballast described as the first conventional example, a lighting output and a constant preheating current are appropriately supplied as stated above by a combination of the two independent resonant circuits including a main resonant circuit for supplying lighting power and a preheating resonant circuit for supplying filament preheating power, and the interrelationship therebetween is largely affected by variations in the characteristics of components which constitute the resonant circuits, thereby making it difficult to design the circuits. 
     In designing the preheating resonant circuit so as to have relatively fewer effects from component variations, the design needs to be realized such that preheating power applied to filaments has fewer variations resulting from a frequency characteristic in an inverter operating range during lighting, and preheating power exhibits an output curve which is substantially flat relative to variations of lighting power. In this case, a current which makes little difference to a current in a dimming control state is made to flow into filaments even in a full-lighting mode in which a constant preheating current is unnecessary, causing concern about an increased power loss without contributing to a light output and adverse effects to a lamp life. 
     Meanwhile, in the electronic ballast described as the second conventional example, the constant preheating current is stopped after achieving stable lighting, thereby eliminating the concern considered as a problem in the first conventional example about power loss without contributing to a light output and adverse effects to the lamp life. However, this configuration will result in having an insufficient preheating current because no constant preheating current is supplied during dimming control, and there is another concern about adverse effects such as premature filament failure. 
     The present invention was achieved by taking the above problems into consideration, having an object to improve efficiency of an electronic ballast by cutting off an unnecessary constant preheating current in a full-lighting mode and thereby reducing power loss which does not contribute to a light output. The invention also prevent problems related to a short life of a discharge lamp such as premature filament failure by maintaining an appropriate filament temperature during lighting resulting from securing a constant preheating current in a dimming control mode with a reduced light output. 
     A first aspect of the present invention is characterized by an electronic ballast as shown in  FIG. 1  which is capable of realizing high frequency lighting of a discharge lamp  13  and switching at least two lighting modes with different light outputs. The ballast includes a preheating circuit (including transformer T 2  and capacitors C 7 , C 8  and C 9 ) having a winding component (i.e. T 2 ) connected in parallel with a main resonant circuit (including inductor L 2  and capacitor C 3 ) with a lamp current flowing therein for the discharge lamp  13 . A constant preheating current for filaments is supplied from a secondary winding of the winding component T 2  during lighting of the discharge lamp  13  and a path of a current flowing on a primary winding side of the winding component T 2  is switched by a switch Q 4  according to a lighting mode. 
     A second aspect of the present invention is based on the electronic ballast according to a first aspect of the present invention, wherein the preheating circuit constitutes an LC resonant circuit including the primary winding of the winding component T 2  and a serially connected capacitance C 9 . An oscillation frequency in the electronic ballast during lighting is operated to be higher than a resonant frequency in the first resonant circuit (including L 2  and C 3 ) with a lamp current flowing therein, and lower than a resonant frequency in a preheating resonant circuit (including T 2  and C 9 ). 
     A third aspect of the present invention is based on the electronic ballast according to the first or second aspect of the present embodiment, having a first lighting mode (i.e. full-lighting mode) and a second lighting mode (i.e. dimming control mode) allowing an operation over a plurality of stages with a light output less than that of the first lighting mode as shown in  FIGS. 5 and 6 . The first lighting mode (i.e. full-lighting mode) turns off the switch Q 4  arranged in the current path so as to stop or reduce a constant filament preheating current during lighting. The second lighting mode (i.e. dimming control mode) turns on the switch Q 4  arranged in the current path so as to supply a constant preheating current to the filaments during lighting. 
     A fourth aspect of the present invention is based on the electronic ballast according to any one of the first to third aspects and makes it possible to realize a light output control over a plurality of stages with a visually continuous dimming operation. The amount of a preheating current is controlled in accordance with a lighting mode by changing the switch Q 4  to operate corresponding to a lighting control signal or a signal secondarily generated from the lighting control signal as shown in  FIG. 7 . 
     A fifth aspect of the present invention is a lighting fixture including the electronic ballast according to any one of the first to fourth aspects of the present invention. 
     According to the first and second aspects of the present invention, a power loss without contributing to a light output due to a constant preheating current flowing into filaments serving as a current path can be reduced in a lighting mode with a large light output in which an appropriate filament temperature can be maintained by a lamp current. An appropriate filament temperature can be maintained by securing the constant preheating current flowing into the filaments serving as a current path in a lighting mode with a small light output in which an appropriate filament temperature cannot be maintained only by a lamp current, so that problems related to premature filament failure (i.e. short life of lamp) can be prevented. 
     According to the third aspect of the present invention, power consumption can be efficiently converted into a light output and filament overheating can also be prevented in a first lighting mode. This prevents blackening of the lamp bulb, premature filament failure, and premature emitter exhaustion. Meanwhile, in a second lighting mode to obtain power saving and lighting effects by suppressing power consumed in a lamp, these effects can be obtained while preventing premature blackening of the lamp bulb, filament failure, and emitter exhaustion. 
     According to the fourth aspect of the present invention, it is possible to establish any amount of a filament current in accordance with the degree of a dimming control for a lamp without requiring an operation performed for a constant preheating current by a resonance effect, thereby making it easier to design a circuit for supplying preheating power. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing a first embodiment of the present invention. 
         FIG. 2  is a graphical diagram showing transition of lighting power and preheating power according to the first embodiment of the present invention. 
         FIG. 3  is a dimming data sheet showing plotted data for a lighting state realized by a electronic ballast according to the first embodiment of the present invention. 
         FIG. 4  is a front view showing an appearance of a remote control transmitter for use in a second embodiment of the present invention. 
         FIG. 5  is a graphical diagram showing transition of lighting power and preheating power according to the second embodiment of the present invention. 
         FIG. 6  is a dimming data sheet showing plotted data for a lighting state realized by a electronic ballast according to the second embodiment of the present invention. 
         FIG. 7  is a circuit diagram according to a third embodiment of the present invention. 
         FIG. 8  is an exploded perspective view showing a schematic configuration of a lighting fixture according to a fourth aspect of the present invention. 
         FIG. 9  is a characteristic diagram showing one example of a general dimming control data sheet. 
         FIG. 10  is a circuit diagram showing a electronic ballast according to a first conventional example. 
         FIG. 11  is a characteristic diagram showing transition of a lighting output/preheating output in the electronic ballast according to the first conventional example. 
         FIG. 12  is a circuit diagram showing a electronic ballast according to a second conventional example. 
         FIG. 13  is a timing chart showing how to control the electronic ballast according to the second conventional example at the time of starting. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Shown in  FIG. 1  is a circuit configuration of an electronic ballast according to a first embodiment of the invention to explain the configuration and operation thereof. 
     In this embodiment, an AC voltage of 100V and 50/60 Hz supplied from a commercial power source  10  is rectified to a DC voltage with a peak value of about 141V by a diode bridge including diodes D 1  to D 4 . The DC voltage is stepped up by a step-up chopper circuit including a choke coil L 1 , a transistor Q 1  and a diode D 5 . Obtained at both ends of an electrolytic capacitor C 2  connected to an output end of the step-up chopper circuit is a DC voltage of, for example, about 300V. This DC voltage is converted into high frequency power in a subsequent inverter  12  and used as lighting power for a discharge lamp  13 . 
     The inverter  12  has a half bridge inverter circuit including serially connected transistors Q 2  and Q 3 , and provides a high frequency rectangular wave voltage at a connection point between the switching transistors Q 2  and Q 3 , by a control circuit  17  which carries out a high frequency switching operation to turn on the transistors Q 2  and Q 3  alternately. The high frequency voltage is converted into lighting power of a substantially sinusoidal wave by a resonance action of an inductor L 2  and a capacitor C 3 . This lighting power is supplied to the discharge lamp  13  via a step-up transformer T and a DC blocking capacitor C 4 . The discharge lamp  13  which is a thermionic cathode fluorescent lamp is connected to the lighting device via a lamp socket. 
     The control circuit  17  may include an integrated circuit (or other components) to control a lighting output of the discharge lamp  13  to a predetermined output by driving the transistor Q 1  in step-up chopper circuit  11  and the transistors Q 2  and Q 3  in the inverter  12  in response to signals for turning on/off and dimming control or other signals sent from a dimming control output circuit  18  such as a remote control signal receiving device. 
     High frequency power outputted from the inverter circuit  12  is also made to flow into a transformer T 2  through a capacitor C 9 . Power induced to a secondary side of the transformer T 2  causes a current to flow into filaments of the discharge lamp  13  through capacitors C 7  and C 8 . Also connected in series to the capacitor C 9  and the transformer T 2  is a transistor Q 4  whose switching operation is used to switch an amount of current supplied to the filaments. The transistors Q 1  to Q 4  may be MOSFETs or other semiconductor switching elements. 
     A driving signal for the transistor Q 4  is supplied from an output terminal of a comparator  19  which has a positive input terminal for inputting a fixed voltage Vref and a negative input terminal for inputting a dimming control level signal Vdim from the dimming control output circuit  18  to the control circuit circuit  17 . 
     The dimming control level signal Vdim corresponds to a smoothed DC voltage which is increased as a light output is controlled to be higher and is decreased as a light output is reduced by a dimming control. If it is assumed that a dimming control level signal obtained in a maximum light output is Vdim1 and a dimming control level signal obtained in a minimum light output is Vdim2, a relationship therebetween relative to the fixed voltage Vref will be Vdim2&lt;Vref&lt;Vdim1. 
     More specifically, the transistor Q 4  is turned off to stop supplying a constant preheating current in a lighting mode with a dimming control level equal to or greater than nominal or rated brightness. Here, if an impedance element such as a capacitor is connected in series with the transistor Q 4 , supplying a constant preheating current can be suppressed by turning off the transistor Q 4 . On the contrary, in a lighting mode with a dimming control level equal to or less than rated or nominal brightness, the transistor Q 4  is turned on to supply a constant preheating current. 
     Note that, in shifting the discharge lamp  13  from a turned-off state to a turned-on state by a lighting signal inputted upon power supply or from a remote control transmitter, the dimming control level signal Vdim is fixed to an “L” level during a preheating period prior to lamp lighting and the transistor Q 4  is turned on to supply a preheating current. 
       FIG. 2  shows transition of lighting power in the discharge lamp and preheating power in the filaments in a dimming control operation during lighting.  FIG. 3  also shows changes observed in a lamp current Id, a lead wire current I LH , and a constant preheating current I LL  as plotted data on the aforementioned dimming control data sheet. ▴ indicates a characteristic observed when the transistor Q 4  is turned on and ♦ indicates a characteristic observed when the transistor Q 4  is turned off. 
     In  FIG. 3 , a constant preheating current is substantially 0 [A] in a lighting state a with a maximum light output and operated at a point close to a target dimming control curve B. 
     In accordance with reduction of a light output, a preheating current undergoes a transition to a direction in which the preheating current becomes insufficient as shown in a lighting state b. The transistor Q 4  is switched from a turned-off state to a turned-on state when a light output reaches a fixed point or less (i.e. less than 0.7 on the horizontal axis) and begins supplying a constant preheating current. This is followed by increasing the constant preheating current as a light output decreases thereafter and carrying out an operation at a point along a target dimming control curve B on the dimming control data sheet. That is, an appropriate filament temperature is maintained during lighting. 
     An electronic ballast according to a second embodiment of the present invention will be explained. It has basically a same circuit configuration as the first embodiment (i.e. circuit shown in  FIG. 1 ) and there is a difference only in that an LED not shown is added as a night-light to the aforementioned control circuit circuit  17 . The discharge lamp  13  here is also a thermionic cathode fluorescent lamp. 
     The fluorescent lamp has lighting modes including a first lighting mode (i.e. full-lighting mode) with a large light output, and a second lighting mode (i.e. dimming control mode), allowing an operation over a plurality of stages with a light output smaller than that of the first lighting mode. 
       FIG. 4  shows an appearance of an infrared remote control signal transmitter for use in operating the electronic ballast according to the present embodiment. Arranged in a transmitter  20  are a full-lighting button  21  for controlling to the first lighting mode (full-lighting mode), a preference button  22  for switching to the second lighting mode (i.e. dimming control mode), an LED button  23  for turning on the LED night light by turning off a fluorescent lamp, a brightness button  24  and a darkness button  25  for operating a lighting output over a plurality of dimming stages in the second lighting mode (i.e. dimming control mode) or the LED lighting mode, and a turn-off button  26  for turning off both a fluorescent lamp and the LED to bring the lighting fixture into a standby mode. 
     If the full-lighting button  21  is pressed in the transmitter  20  to realize the first lighting mode (i.e. full-lighting mode), the transistor Q 4  arranged in the preheating resonant circuit is turned off to bring a constant preheating current into substantially 0 [A]. 
     If the preference button  22  is pressed in the transmitter  20  to realize the second lighting mode (i.e. dimming control mode), the transistor Q 4  arranged in the preheating resonance circuit is turned on to supply the constant preheating current. 
       FIG. 5  shows transition of lighting power in a discharge lamp and preheating power in filaments in a dimming control operation during lighting.  FIG. 6  also shows changes observed at this time in the lamp current Id, the lead wire current I LH  and the constant preheating current I LL  as plotted data on the aforementioned dimming control data sheet. 
     In  FIG. 6 , a constant preheating current is substantially 0 [A] and disposed adjacent to the target dimming control curve B in a lighting state of the first lighting mode (i.e. full-lighting mode). 
     In the second lighting mode (i.e. dimming control mode), the constant preheating current is supplied to increase the constant preheating current as a light output is decreased thereafter, followed by an operation at a point along the target dimming control curve B on the dimming control data sheet. That is, an appropriate filament temperature can be maintained during lighting. 
     In the case of carrying out a feedback control for a lamp light output by power consumed in the inverter, power required for constant preheating is also combined for feedback, wherein there is a danger that a discontinuous change may happen in switching to a constant preheating current in the middle of continuously reducing a light output as shown in the first embodiment (refer to  FIG. 3 ). Such visual discontinuity can be prevented by creating a sufficient difference in the light output using the first lighting mode (i.e. full-lighting mode) and the second lighting mode (i.e. dimming control mode) as shown in  FIG. 6 . 
     An unexpected operation may also happen in response to a power saving operation by a user in the first embodiment such that a light output is slightly reduced due to decreased discharge lamp lighting power before and after switching the transistor Q 4  from a turned-off state to a turned-on state. As understood from  FIG. 2  while power consumption in the ballast as a whole is almost free from any changes resulting from increased filament preheating power, the present embodiment creates a sufficient difference in the light output by using the first lighting mode (i.e. full-lighting mode) and the second lighting mode (i.e. dimming control mode) so as to realize a remarkable reduction in the discharge lamp lighting power in comparison with an increase in the filament preheating power and obtains certain power saving effects by selecting the second lighting mode (i.e. dimming control mode), so that an unexpected operation in response to a power saving operation by a user can be prevented. 
     An electronic ballast according to a third embodiment of the present invention will be explained with reference to  FIG. 7 . Since the present embodiment has a basic circuit operation in common with the first embodiment, explanation thereof will be omitted here. 
     The preheating resonant circuit does not have a resonance effect to increase a constant preheating current as light output decreases as shown in the first and second embodiments. Also, the capacitor C 9  has a sufficiently large capacitance so that a resonance frequency calculated for the preheating resonant circuit is much smaller than an operating frequency in the inverter. A constant preheating current is obtained when the transistor Q 4  is turned on and is characterized as being substantially flat relative to a change in a light output, that is an oscillation frequency. 
     Meanwhile, inputted from the dimming control output circuit  18  (such as a remote control signal receiving circuit) to the control circuit  17  is a DIM signal which determines a dimming control level. The DIM signal is a duty cycle signal with a frequency of 1 kHz, having an ON time which is made larger in accordance with a smaller light output. In order to realize a dimming control level corresponding to this signal, the light output is controlled by the control circuit  17 . The DIM signal is further used to drive the transistor Q 4  to be turned on/off. 
     Therefore, an appropriate filament temperature can be maintained in accordance with each lamp output mode by carrying out an operation to increase/decrease an amount of time to supply a constant preheating current, which is characterized as being flat by nature as stated above, in response to a lighting control signal. 
     Moreover, in the case where light outputted from a lamp is subjected to a feedback control by power consumed in the inverter, power required for constant preheating is also combined for feedback, so that there is a danger that a discontinuous change may happen in switching to a constant preheating current in the middle of reducing a light output continuously. Such a visual sense of incompatibility can be eliminated by applying stepwise changes to an effective value of the constant preheating current in the same manner with the light output. 
     Note that the transistor Q 4  is controlled to be turned on/off in the present embodiment by using a duty cycle signal (of 1 kHz) for dimming control without making any changes, but it may be replaced with a control to turn on/off the transistor Q 4  by an output of a PWM control circuit which is arranged to convert the dimming control level signal Vdim made of a smoothed DC voltage as shown in  FIG. 1  into a duty cycle signal with variable ON time. For example, if the fixed voltage Vref inputted to a positive input terminal of the comparator  19  is replaced with a triangular wave oscillator for carrying out oscillation with an amplitude including the aforementioned Vdim2 to Vdim1 and/or an oscillator with any waveforms in the circuit configuration shown in  FIG. 1 , it is possible to realize a control so that the transistor Q 4  has short ON time in a large light output and long ON time in a small light output. A switching period in the transistor Q 4  may be set by taking a thermal time constant in the filaments or other factors into consideration. 
       FIG. 8  shows an appearance of a lighting fixture using the electronic ballast according to any one of the aforementioned first to third embodiments. The lighting fixture shown in  FIG. 8  is a ceiling light directly attached to a ceiling, including a lighting fixture main body  31  incorporating the electronic ballast, a ring fluorescent lamp  13  serving as a light source, a reflector plate  32  for reflecting light of the fluorescent lamp  13 , a lamp socket  33  for fixing the fluorescent lamp  13  to the lighting fixture main body  31  and supplying lighting power from the electronic ballast to the fluorescent lamp  13 , a lamp supporting spring  34  for fixing the fluorescent lamp  13  to the lighting fixture main body  31 , a power supply mechanism  35  for fixing the lighting fixture main body  31  to a ceiling surface and supplying a commercial power source to the electronic ballast, a light transmitting cover  36  attached to the lighting fixture main body  31  so as to disperse light from the fluorescent lamp, a receiving device  37  for controlling the electronic ballast by receiving an infrared signal from the remote control transmitter from the outside of the fixture, and an LED  38  arranged on the receiving device  37  and used for the purpose of a night-light. 
     Thus, although there have been described particular embodiments of the present invention of a new and useful electronic ballast with preheat current control, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.