Patent Publication Number: US-8970127-B2

Title: Lighting circuit and illumination device

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a divisional of U.S. patent application Ser. No. 12/860,528, filed on Aug. 20, 2010. U.S. patent application Ser. No. 12/860,528 is based upon and claims benefit of priority from Japanese Patent Application No. 2009-192280, filed Aug. 21, 2009 and Japanese Patent Application No. 2010-135705, filed on Jun. 15, 2010. The entirety of all of the above-listed Applications are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a lighting circuit and an illumination device. 
     BACKGROUND 
     Conventionally, an illumination system in which a power source, an illumination load appliance and a controller are connected in series and the controller performs illumination control of the illumination load appliance is sometimes employed. In such illumination system, power is supplied to the illumination load appliance using two-wire wiring. The controller adjusts the power supplied to the illumination load appliance by means of a phase control method to perform dimming control (for example, Japanese Patent Application Laid-Open Publication Nos. 2007-538378 and 2005-011739). 
     In such two-wire wiring illumination system, e.g., a bidirectional triode thyristor (hereinafter, referred to as “TRIAC”) is used as a switching element configured to perform power phase control. By turning on/off the TRIAC, the power supply from the power source to the illumination load is controlled, whereby dimming is performed. In other words, the TRIAC is turned on a period of delay time, which is based on the dimming control, from a zero crossing of the power source voltage, whereby the time of supplying power to the illumination load is controlled to perform dimming. 
     In such power phase control method, since the power is steeply turned on, power supply noise to be generated is large. In order to reduce the effect of such power supply noise, a noise prevention circuit including a capacitor and an inductor is employed. A dimmer including such noise prevention circuit is disclosed in, e.g., Japanese Patent Application Laid-Open No. 11-87072. 
     However, a resonant circuit is formed by the capacitor and the inductor included in the noise prevention circuit, and when a TRIAC, which is a switching element, is turned on, the resonant circuit causes a resonant current to flow in the TRIAC. In other words, at the time of power supply using phase control, a transient oscillation occurs, and a resonant current (transient oscillation current) having a large peak value, which flows at that time, flows also into the TRIAC. It is necessary that a relatively large holding current flow in the TRIAC to maintain conduction. No problem arises during a period in which the resonant current flows in the TRIAC in the same direction as that of the current from the power source. However, during a period in which the resonant current flows in the opposite direction, the current flowing in the TRIAC may be relatively lowered to fall below the holding current. 
     Even in such case, where a bulb, which has a relatively low resistance value, is employed for the illumination load, the bulb, which is the illumination load, acts as a damping resistance, whereby the resonant current is suppressed, enabling a current equal to or higher than the holding current to flow in the TRIAC. 
     However, where a high-resistance element, such as an LED (Light Emitting Diode), is employed for the illumination load, immediately after the TRIAC is turned on, the current flowing in the TRIAC may be reduced by the resonant current to fall below the holding current, which causes the TRIAC to be turned off. Subsequently, the TRIAC may be turned on again. In this manner, the TRIAC may be repeatedly turned on/off in a half cycle of the power source voltage according to the level and polarity of the resonant current of the time when the TRIAC is on. 
     In other words, there has been a problem that depending on the type of the illumination load, the TRIAC may repeatedly be turned on/off even during a period in which the TRIAC is on under normal conditions, which causes flicker in the lighting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating an illumination device including a lighting circuit according to a first embodiment of the present invention; 
         FIG. 2  is a circuit diagram illustrating a specific circuit configuration of a variable impedance circuit  13  in  FIG. 1 ; 
         FIG. 3  is a waveform diagram with the abscissa axis indicating time and the ordinate axis indicating voltage, which illustrates an AC power source voltage of a power source  11  and control of a TRIAC T; 
         FIG. 4  is a waveform diagram with the abscissa axis indicating time and the ordinate axis indicating voltage and current, which illustrates a resonant voltage (dashed line) and a resonant current (solid line); 
         FIG. 5  is a circuit diagram illustrating an effect of a resonant current; 
         FIGS. 6A to 6F  are timing charts illustrating an operation of the first embodiment; 
         FIG. 7  is a circuit diagram of an illumination device according to a second embodiment of the present invention; 
         FIG. 8  is a circuit diagram of a part of the illumination device according to the second embodiment, the part controlling a damping resistor and a converter; 
         FIGS. 9A and 9B  are waveform diagrams illustrating output control of a converter according to a phase angle of an AC voltage half cycle in the illumination device according to the second embodiment; 
         FIG. 10  is a graph illustrating a relationship between a phase angle of an AC voltage half cycle and an output of a filter in the illumination device according to the second embodiment; 
         FIG. 11  is a circuit diagram of an illumination device according to a third embodiment of the present invention; 
         FIG. 12  is a circuit diagram of a part of the illumination device according to the third embodiment, the part controlling a damping resistor and a converter; 
         FIG. 13  is a diagram of an illumination device according to a fourth embodiment of the present invention; and 
         FIG. 14  is a diagram of an illumination device according to a fifth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 
     A lighting circuit according to an embodiment includes: a self-hold element connected in series to an AC power source that generates power for lighting an illumination load, together with the illumination load, the self-hold element being configured to control supply of the power provided by the AC power source by the self-hold element being turned on/off; a noise prevention circuit connected in parallel to the self-hold element; and a damping circuit configured to parallely connect a damping resistance to the noise prevention circuit only for a predetermined period from turning-on of the self-hold element. 
     A lighting circuit according to an embodiment further includes: a rectifier circuit to which a voltage from the AC power source is applied via the self-hold element; and a constant current circuit connected in parallel to an output end of the rectifier circuit together with the damping circuit, the constant current circuit being configured to drive the illumination load. 
     In a lighting circuit according to an embodiment, the damping circuit includes: a clipping unit configured to clip an output of the rectifier circuit; a first schmitt trigger circuit configured to shape a waveform of an output of the clipping unit; a differentiating circuit configured to differentiate an output of the first schmitt trigger circuit; and a second schmitt trigger circuit configured to shape a waveform of an output of the differentiating circuit. 
     An illumination device according to an embodiment includes: the lighting circuit; and the illumination load. 
     An illumination device according to an embodiment includes: an input terminal; a rectifier circuit including an AC input end connected to the input terminal; an LED lighting circuit including an input end connected to DC output ends of the rectifier circuit; and a damping resistor configured to be connected to the DC output ends of the rectifier circuit only for a predetermined period at the start of application of each half wave of a power source voltage to the input terminal. 
     The LED lighting circuit is not specifically limited. Preferably, the LED lighting circuit includes a converter configured to perform a high-frequency operation. The converter is preferably a buck converter because an LED has a low operating voltage. However, the converter may be another known converter of various circuit types, such as a boost converter, as desired. 
     The damping resistor connected to the DC output ends of the rectifier circuit only for a short period of time from the start of application of a voltage in each half cycle of a power source voltage functions as means configured to damp a transient oscillation current at the start of application of the power source voltage. In other words, when a sharply-rising voltage in a half-cycle voltage of an AC voltage whose phase has been controlled by a phase-control dimmer, is applied to the illumination device, even if a transient oscillation occurs at a sharp rising part of the voltage whose phase has been controlled, the damping resistor functions as damping means for the transient oscillation. Thus, the transient oscillation is damped and the peak value of the transient oscillation current is thereby lowered. Consequently, the damping resistor is effective for preventing a phase-control dimmer from causing malfunctions at the rising in each half cycle of the power source voltage whose phase has been controlled. 
     It is preferable that the time of the connection of the damping resistor to the DC output ends of the rectifier circuit be within 1 ms from the start of the application of each half cycle of the power source voltage. In such length of time, the damping resistor generates only a small amount of heat, which can be ignored. Although the damping resistor has the effect of preventing the phase-control dimmer from causing malfunctions even though the time of the connection of the damping resistor exceeds 1 ms. But this is not preferable, because, with the connection time longer than the aforementioned length of time, the power loss caused by the damping resistor increases and the amount of heat generation accompanied by the power loss increases considerably. 
     Also, it is preferable that the connection time of the damping resistor at least include a period in which an oscillating voltage is generated, which has a relatively high peak value so that the voltage may cause malfunctions, the oscillating voltage being of a transient oscillation generated as a result of sharp rising of an AC voltage whose phase has been controlled by the phase-control dimmer. Therefore, the connection time of the damping resistor is preferably no less than around 10 μs. With such length of time, the connection of the damping resistor continues for a majority of a ½ cycle of a resonant frequency of a generally-used noise prevention circuit (30 kHz to 100 kHz), enabling provision of substantial damping operation for the transient oscillation current. More preferably, the connection time is no less than 15 μs. In order to more reliably prevent the phase-control dimmer from causing malfunctions, the connection of the damping resistor may be continued for one cycle of the resonant frequency. In other words, the connection time may be 10 μs to no less than 34 μs. 
     The means for connection of the damping resistor for the short period of time is not specifically limited. However, the means can be configured so that the time of the damping resistor connecting to the DC output ends of the rectifier circuit can be controlled using a switch element as desired. In such configuration, the switch element may be included in a control IC for the converter or may also be provided externally. 
     Furthermore, the damping resistor can be a voltage-dependent nonlinear resistor. For such a nonlinear resistor, a surge absorption element, for example, can be used. A surge absorption element is generally used for absorbing external surges such as lightning surges. Accordingly, in such a case, a surge absorption element having a high breakdown voltage that is around four times a rated AC power source voltage is used. Meanwhile, in order to employ a voltage-dependent nonlinear resistor in the embodiments to cause the damping resistor itself to control the connection time, the breakdown voltage is preferably a value close to the peak value of the AC power source voltage, that is, 1.5 to 1.6 times, more preferably 1.5 to 1.55 times a rated AC power source voltage. 
     In the above configuration, when the voltage-dependent nonlinear resistor broke down due to a transient oscillation generated at sharp rising of a voltage in each half cycle of an AC voltage formed by, e.g., the phase-control dimmer, the voltage-dependent nonlinear resistor absorbs the part of the transient oscillation voltage that exceeds the breakdown voltage, and consequently, the peak value of the transient oscillation current is lowered. Accordingly, when a voltage-dependent nonlinear resistor is employed for a damping resistor, the damping resistor is substantially connected to the DC output ends of the rectifier circuit when the voltage-dependent nonlinear resistor broke down. 
     A person skilled in the art could easily understand from the nature of the present invention that since the illumination device is an illumination device using an LED as a light source, the illumination device may have any shape. When the illumination device is used in combination with a household phase-control dimmer, a bulb-shaped LED lamp is often employed. 
     The illumination device according to the embodiments is effective for an LED lighting system that connects with an AC power source via a phase-control dimmer. However, the above system is not necessarily employed because the LED can be lighted without difficulty even if the illumination device according to the embodiments is used by connecting the illumination device directly to the AC power source. 
     An illumination device according to an embodiment further includes: a switch connected in series between a positive output end and a negative output end of the rectifier circuit, the positive output end and the negative output end being included in the DC output ends of the rectifier circuit, together with the damping resistor; and a control unit configured to detect a voltage of the DC output ends of the rectifier circuit to control on/off of the switch, thereby connecting the damping resistor to the DC output ends of the rectifier circuit. 
     Furthermore, in an illumination device according to an embodiment, the control unit turns on the switch using an output of a monostable circuit, the monostable circuit being configured to generate an output only for a predetermined short period of time at the start of application of each half cycle of the power source voltage. 
     Furthermore, in an illumination device according to an embodiment, the damping resistor includes a voltage-dependent nonlinear resistor. 
     In an illumination device to an embodiment, the control unit turns off the switch within 1 ms after application of each half cycle of the power source voltage. 
     An illumination device according to an embodiment further includes a phase-control dimmer including an input end connected to an AC power source, and an output end connected to the input terminal. 
     A bulb-shaped LED lamp according to an embodiment includes the aforementioned illumination device. 
     &lt;First Embodiment&gt; 
       FIG. 1  is a circuit diagram illustrating an illumination device including a lighting circuit according to a first embodiment of the present invention.  FIG. 2  is a circuit diagram illustrating a specific circuit configuration of a variable impedance circuit  13  in  FIG. 1 . 
     The illumination device illustrated in  FIG. 1  supplies power from a power source  11  to an illumination load appliance connected between terminals I 1  and I 2  via two-wire wiring. An illumination load appliance in the present embodiment employs an LED as an illumination load  15 . 
     Between the power source  11  and the illumination load appliance connected to the terminals I 1  and I 2 , a TRIAC T, which performs phase control, is provided, and the power source  11 , the TRIAC T and the illumination load appliance are connected in series. The power source  11  generates an AC power source voltage of, for example, 100 V. The present embodiment is described in terms of an example in which a TRIAC is used for an element for performing phase control, a thyristor, which is also a self-hold element as with a TRIAC, or another switching device may be employed. 
       FIG. 3  is a waveform diagram with the abscissa axis indicating time and the ordinate axis indicating voltage, which illustrates the AC power source voltage of the power source  11  and control of the TRIAC T. 
     The TRIAC T is connected between the AC power source  11  and the terminal I 1 , and a series circuit of a variable resistance VR and a capacitor C 2  is connected in parallel to the TRIAC T. The point of connection between the variable resistance VR and the capacitor C 2  is connected to a control end of the TRIAC T via a bidirectional diode (hereinafter, referred to as “DIAC”) D. 
     The variable resistance VR is configured so as to be set to have a resistance value according to the dimming control. When the TRIAC T is off, the capacitor C 2  is charged by the AC power source  11  via the variable resistance VR. After a predetermined period of delay time based on the time constant of the variable resistance VR and capacitor C 2  from the start of the charge of the capacitor C 2 , the terminal voltage of the capacitor C 2  reaches a voltage allowing the DIAC D to be turned on. Consequently, pulses are generated in the DIAC D and supplied to the control end of the TRIAC T. Consequently, the TRIAC T is brought into conduction. 
     The TRIAC T maintains conduction as a result of being supplied with a current from the power source  11 . During the period in which the TRIAC T is on, the capacitor C 2  is discharged, and the TRIAC T is turned off when its holding current is not maintained. When the polarity of the power source voltage applied to the TRIAC T is inversed, the capacitor C 2  is charged again, the DIAC D is turned on after the elapse of the delay time. Consequently, the TRIAC T is turned on after a predetermined period of delay time from a zero crossing of the AC power source voltage. Subsequently, the operation is repeated in a similar manner, during a period of a power supply cycle with the delay time excluded (hereinafter, referred to as “power supply period”), the power from the power source  11  is supplied to the illumination load appliance via the TRIAC T. 
     The AC waveform illustrated in  FIG. 3  indicates a voltage generated by the power source  11 . The shaded areas each indicate a power supply period during which the TRIAC T is brought into conduction. The delay time can be adjusted by changing the resistance value of the variable resistance VR. 
     A noise prevention circuit including a capacitor C 1  and a coil L is connected to opposite ends of TRIAC T. The noise prevention circuit prevents noise from leaking into the power source  11  side. 
     A rectifier circuit  12  is provided between the terminals I 1  and I 2 . The rectifier circuit  12  may be, for example, a diode bridge. The rectifier circuit  12  rectifies a voltage supplied to the terminals I 1  and I 2  and outputs the voltage. 
     Outputs appearing at one output end and another output end of the rectifier circuit  12  are supplied to a constant current circuit  14 . The constant current circuit  14  generates a constant current from the outputs of the rectifier circuit  12 , and supplies the constant current to the illumination load  15  via terminals O 1  and O 2 . For the illumination load  15 , for example, an LED may be employed. As a result of the time of voltage supply to the rectifier circuit  12  being controlled by the TRIAC T, the value of the constant current from the constant current circuit  14  varies according to the on time of the TRIAC T. Consequently, the brightness of the illumination load  15  is controlled by dimming. 
     The noise prevention circuit inserted to prevent leakage of power supply noise forms a resonant circuit, which makes a resonant current flow in the TRIAC T during the TRIAC T being on. 
       FIG. 4  is a waveform diagram with the abscissa axis indicating time and the ordinate axis indicating voltage and current, which illustrates a resonant voltage (dashed line) and a resonant current (solid line).  FIG. 5  is a circuit diagram illustrating an effect of a resonant current.  FIG. 5  is a simplified diagram of  FIG. 1 , and indicates an example in which an illumination load appliance  16  is connected between terminals I 1  and I 2 . 
     The resonance frequency of the noise prevention circuit is around 30 kHz to 100 kHz, and the resonance cycle is sufficiently short compared to the AC cycle of the power source  11 . As illustrated in  FIG. 5 , when the TRIAC T is on, during a period in which a current a flows into the TRIAC T from the power source  11 , a resonant current b having a same direction as that of the current a and a resonant current c having a direction opposite to that of the current a flow. Even in the power supply periods illustrated in the shaded area in  FIG. 3 , the TRIAC T is turned off when a current that is the sum of the current a and the resonant current c falls below the holding current of the TRIAC T. 
     As illustrated in  FIG. 4 , the level of the resonant current immediately after the TRIAC T being turned on after the elapse of the delay time is relatively large, and also, when an LED is used for the illumination load appliance, the resistance value of the illumination load appliance is a relatively large. Thus, immediately after the TRIAC T is turned on, the TRIAC T is turned off by the resonant current. The TRIAC T is turned on again by the capacitor C 2  being charged, and thus, even during a power supply period, the TRIAC T is repeatedly turned on/off for a period of time according to the level of the resonant current. The resonant current and resonant voltage waveforms in  FIG. 4  represent only the resonant condition of the noise prevention circuit, and a current component flowing into the illumination load  15  (current component a in  FIG. 5 ) from the power source  11  via the TRIAC T is excluded. Accordingly, the waveform of a current actually flowing in the TRIAC T is the resonant current waveform in  FIG. 4  plus the component a from the power source  11 . 
     Also, a holding current of a TRIAC is several tens of milliamperes (30 mA to 50 mA). In a period close to a zero-crossing of the AC voltage, the current flowing in the TRIAC T becomes relatively small. However, when a bulb is used for the illumination load, the resistance of the bulb during dimming also become small, and thus, even during dimming, a sufficient current flows in the TRIAC T, thereby the holding current being maintained. 
     On the other hand, when an LED, which is a high-resistance element, is employed for the illumination load, during dimming, the current flowing in the TRIAC T becomes relatively small, and thus, the effect of the resonant current flowing in the TRIAC T becomes large. 
     Therefore, in the present embodiment, a variable impedance circuit  13  is provided as a damping circuit that suppresses the effect of the resonant current. In the present embodiment, the variable impedance circuit  13  is provided between the output end and the other output end of the rectifier circuit  12 , that is, in parallel to the resonant circuit formed by the noise prevention circuit. 
     The variable impedance circuit  13  includes, for example, a switch element and a resistive element, and the resistive element is connected between the output end and the other output end of the rectifier circuit  12  only for a period in which the switch element is on. For example, only for one resonance cycle from the start of a power supply period, the switch element is turned on to make the resonant current flow in the resistive element, whereby the resonance is damped to reduce the peak value of the resonant current, enabling a sufficient current exceeding the holding current to flow in the TRIAC T even when the resonant current (current c) flows in a direction opposite to that of the current a. 
       FIG. 2  indicates an example in which an FET Q 1  is employed for the switch element and a resistance R 4  is employed for the resistive element. A 100 W bulb for a 100 V AC power source has a resistance value of 100Ω under a dimming control to 100%, and a cold resistance is around 1/10 to 1/20 of the resistance value. In other words, during dimming, the resistance value of the bulb is several tens of ohms, and the bulb acts as a damping resistance. In the present embodiment, the resistance value of the resistance R 4  is similar to the resistance value of the bulb during dimming. Consequently, the resistance R 4  acts as a damping resistance, and sufficiently suppresses the effect of the resonant current. 
     In  FIG. 2 , the resistance R 4  and a drain-source path of the FET Q 1  are connected between the output end and the other output end of the rectifier circuit  12 . A series circuit of a diode D 1 , a resistance R 1  and a zener diode ZD is also connected between the output end and the other output end of the rectifier circuit  12 . A resistance R 2  and a capacitor C 3  are connected in parallel to the zener diode ZD. 
     A point of connection between the resistance R 1  and the zener diode ZD (hereinafter referred to as “point A”) is connected to a negative logic schmitt trigger circuit S 1  via a resistance R 3 . An output of the rectifier circuit  12  appears at the point A via the diode D 1  and the resistance R 1 . The voltage at the point A is clipped to a predetermined level by the zener diode D 1  and the capacitor C 3 . 
     The schmitt trigger circuit S 1 , which shapes the waveform of an input voltage, outputs a rectangular wave that falls when the output of the rectifier circuit  12  rises, and rises from a zero crossing. An output end of the schmitt trigger circuit S 1  is connected to a power source terminal via a capacitor C 4  and a variable resistance VR 2 . A diode D 2  is connected in parallel to the variable resistance VR 2 . A differentiating circuit is formed by the capacitor C 4 , the variable resistance VR 2  and the diode D 2 , and at a point of connection between the capacitor C 4  and the variable resistance VR 2  (hereinafter referred to as “point B”), a waveform obtained as a result of differentiating an output of the schmitt trigger circuit S 1  appears. 
     The waveform at the point B is supplied to an input end of a negative logic schmitt trigger circuit S 2 . The schmitt trigger circuit S 2 , which shapes the waveform of an input voltage, outputs pulses rising when an output of the differentiating circuit falls. The pulse width of the output pulses of the schmitt trigger circuit S 2  can be adjusted by changing the resistance value of the variable resistance VR 2 . 
     The output of the schmitt trigger circuit S 2  is supplied to a gate of the FET Q 1 . The FET Q 1  is turned on by the high-level pulses supplied to the gate to connect the resistance R 4  between the output end and the other output end of the rectifier circuit  12 . In other words, the resistance R 4  is connected between the output end and the other output end of the rectifier circuit  12  only for a period determined by a constant of the differentiating circuit from rising of the output of the rectifier circuit  12 . 
     Next, an operation of the embodiment configured as described above will be described with reference to the timing charts illustrated in  FIGS. 6A to 6F .  FIG. 6A  illustrates an input to the rectifier circuit  12 ,  FIG. 6B  illustrates an output of the rectifier circuit  12 ,  FIG. 6C  illustrates a waveform at the point A,  FIG. 6D  illustrates an output of the schmitt trigger circuit S 1 ,  FIG. 6E  illustrates an output of the differentiating circuit (waveform at point B), and  FIG. 6F  illustrates an output of the schmitt trigger circuit S 2 . 
     An AC voltage from the power source  11  is supplied to the illumination load appliance between the terminals I 1  and I 2  through the TRIAC T via the two-wire wiring. The TRIAC T is brought into conduction after the elapse of the delay time, which is based on the time constant of the variable resistance VR and the capacitor C 2  from a zero crossing of the power source voltage, and provides power to the illumination load appliance during a power supply period. 
     Now, it is assumed that power is supplied from the TRIAC T between the terminals I 1  and I 2  during the shaded power supply periods in  FIG. 6A . The rectifier circuit  12 , as illustrated in  FIG. 6B , outputs a positive voltage. The output of the rectifier circuit  12  is provided to the variable impedance circuit  13 . 
     At the point A in the variable impedance circuit  13 , a waveform obtained as a result of the output of the rectifier circuit  12  being clipped to a predetermined level based on the zener diode ZD and the capacitor C 3  ( FIG. 6C ) appears. The waveform is supplied to the schmitt trigger circuit S 1  via the resistance R 3 . The schmitt trigger circuit S 1  shapes the input waveform, and outputs a waveform that falls as the input waveform rises and rises from a zero crossing. 
     The output of the schmitt trigger circuit S 1  is supplied to the differentiating circuit formed by the capacitor C 4 , the variable resistance VR 2  and the diode D 2 . The differentiating circuit outputs a waveform that falls and rises at the inclination based on the time constant of the capacitor C 4  and the variable resistance VR 2  as the output of the schmitt trigger circuit S 1  falls ( FIG. 6E ). Because of the presence of the diode D 2 , the output of the differentiating circuit does not change as the output of the schmitt trigger circuit S 1  rises. 
     The timing of the output of the rectifier circuit  12  rising, that is, the timing of TRIAC T being turned on is detected by the differentiating circuit. The output of the differentiating circuit is supplied to the schmitt trigger circuit S 2 , and the schmitt trigger circuit S 2  outputs a pulse-formed waveform that rises and falls as the output of the differentiating circuit falls and rises ( FIG. 6F ). The pulse width of the output pulse of the schmitt trigger circuit S 2  can be adjusted by the inclination of the output of the differentiating circuit, that is, the resistance value of the variable resistance VR 2 . 
     The output of the schmitt trigger circuit S 2  is supplied to the FET Q 1 , and the FET Q 1  is turned on during a positive pulse period of the schmitt trigger circuit S 2  to connect the resistance R 4  between the output end and the other output end of the rectifier circuit  12 . 
     Accordingly, the resistance R 4  is connected between the output end and the other output end of the rectifier circuit  12 , that is, in parallel to the resonant circuit during the pulse periods in  FIG. 6F  in which the output is at a high level during a period of time determined by the time constant of the differentiating circuit from the turning-on of the TRIAC T. The resistance value of the resistance R 4  is set to, for example, a resistance value equivalent to a resistance value during dimming when a bulb is used for the illumination load, and the resistance R 4  acts as a damping resistance configured to make the resonant current of the resonant circuit formed by the capacitor C 1  and the coil L flow therein. Consequently, the resonant current that flows in the TRIAC T is suppressed, enabling the on state of the TRIAC T to be maintained. 
     Since a resonant current attenuates with time, the resistance R 4 , which is a damping resistance, may be connected in parallel to the resonant current only for a predetermined period from the turning-on of the TRIAC T. More specifically, the resistance R 4  is connected in parallel to the resonant circuit only for one cycle from occurrence of the resonant current illustrated in  FIG. 4 , enabling the effect of the resonant current to be effectively suppressed. 
     As illustrated in  FIG. 4 , when the resonant current is positive, the resonant current flows in a same direction as that of the current flowing from the power source  11  into the TRIAC T, and thus, it is not necessary to connect the resonant circuit to the resistance R 4  simultaneously with the turning-on of the TRIAC T. The resistance R 4  only needs to be connected in parallel to the resonant circuit by the elapse of a half cycle of the resonant current from the turning-on of the TRIAC T. 
     The resistance R 4  is connected between the output end and the other output end of the rectifier circuit  12  only for the positive pulse periods illustrated in  FIG. 6F , enabling power wastefully consumed by the resistance R 4  to be suppressed to the minimum. 
     As described above, in the present embodiment, when the TRIAC is turned on, a resistance for damping is inserted in parallel to the resonant circuit for a predetermined period of, e.g., around one cycle of a resonant current to suppress the resonant current flowing in the TRIAC, enabling prevention of the TRIAC from being turned off by the effect of the resonant current. Consequently, the TRIAC is on continuously during a power supply period according to dimming control, enabling provision of lighting with no flicker. 
     Although the above-described embodiment has been described in terms of an example in which a variable impedance circuit is provided between output ends of a rectifier circuit, the variable impedance circuit only needs to be provided in parallel to a resonant circuit, and thus, it is clear that the variable impedance circuit may be provided, for example, on the input side of the rectifier circuit, that is, between the terminals I 1  and I 2 . 
     Also, the terminals I 1  and I 2  may include terminal fittings or may also be mere conductive wires. Where the illumination device is a bulb-shaped LED lamp including a base, the base functions as an input terminal. 
     &lt;Second Embodiment&gt; 
     A second embodiment of the present invention will be described. In the second embodiment, as illustrated in  FIG. 7 , an illumination device includes input terminals t 1  and t 2 , a rectifier circuit Rec, an LED lighting circuit LOC, and an LED LS, which is a load, and a damping resistor Rd. 
     The input terminals t 1  and t 2  are means configured to connect the illumination device to an AC power source AC, for example, a commercially-available 100V AC power source. The AC power source AC may be connected to the illumination device via or not via a known phase-control dimmer, which is not illustrated, as described above. 
     Furthermore, the input terminals t 1  and t 2  may include terminal fittings, or may also be mere conductive wires. Where the illumination device is a bulb-shaped LED lamp including a base, the base functions as an input terminal. 
     A rectifier circuit Rec is means configured to convert an AC to a DC, and includes AC input ends and DC output ends. The AC input ends are connected to the input terminals t 1  and t 2 . A person skilled in the art should know that the AC input ends are connected to the input terminals t 1  and t 2  via noise filters (not illustrated), which should therefore be allowed. 
     Also, the rectifier circuit Rec is not limited to a full-wave bridge rectifier circuit as illustrated, and it is allowed to arbitrarily select and use a known rectifier of various circuit types as desired. Furthermore, the rectifier circuit Rec can include smoothing means. For example, a smoothing capacitor C 11  including, e.g., an electrolytic capacitor as illustrated in the Figure, can be connected to the DC output end for the LED lighting circuit LOC directly or in series via a diode D 11  as illustrated in the Figure. 
     The LED lighting circuit LOC only needs to be circuit means configured to light LED LS, which will be described later, and no specific configuration of the LED lighting circuit LOC is particularly limited. However, for, e.g., circuit efficiency enhancement and easy control, it is preferable to employ a configuration including a converter CONV as its main component. The illustrated converter CONV indicates an example using a buck chopper. 
     The converter CONV, which includes a buck chopper, includes first and second circuits AA and BB, and a control unit CC. The first and second circuits AA and BB include a switching element Q 11 , an inductor L 11 , a diode D 12 , an output capacitor C 12  and a current detection element CD as their elements. 
     In the first circuit AA, a series circuit of the switching element Q 11 , the inductor L 11 , the current detection element CD and the output capacitor C 12  is connected to the DC output end of the rectifier circuit Rec whose output voltage has been smoothed. When the switching element Q 11  is turned on, an increasing current, which linearly increases, flows from the DC output end of the rectifier circuit Rec, and electromagnetic energy is accumulated in the inductor L 11 . The current detection element CD is connected to the position illustrated in  FIG. 7  so as to detect the increasing current. 
     The second circuit BB includes a closed circuit of the inductor L 11 , the diode D 12  and the output capacitor C 12 . When the switching element Q 11  of the first circuit AA is off, the electromagnetic energy accumulated in the inductor L 11  is released and a decreasing current flows in the closed circuit. 
     The LED LS is connected in parallel to the output capacitor C 12  of the converter CONV. 
       FIG. 8  is a circuit diagram illustrating a part of a circuit in a control C 21  in  FIG. 7 . 
     The damping resistor Rd is connected between the non-smooth DC output ends of the rectifier circuit Rec via a switch element Q 12  illustrated in  FIG. 8 . Where the illumination device is for a commercially-available 100V AC power source, the resistance value of the damping resistor Rd can be set to around several hundreds of ohms. The switch element Q 12  may be included in the control IC  21  as illustrated in  FIG. 8  or may also be an external component for the control IC  21  as described later. 
     In the present embodiment, the control unit CC is means configured to control the LED lighting circuit LOC and the damping resistor Rd. The control unit CC includes a control IC  21  and a control power source  22 . 
     The control IC  21  includes a plurality of pin terminals, a pin VDC is connected to a positive electrode of the smoothing capacitor C 11  for the rectifier circuit Rec, a pin Vin is connected to the positive side of the damping resistor Rd, a pin Vcc is connected to a positive terminal of the control power source  22 , a pin G is connected to the switch element Q 11  of the converter CONV, a pin CS is connected to a detection output end of the current detection element CD, a pin Inr is connected to the negative side of the damping resistor Rd, and a pin GND is connected to a negative terminal of the control power source  22 . 
     Furthermore, in the second embodiment, the control IC  21 , which controls the time of connection of the damping resistor Rd to the output ends of the rectifier circuit Rec, includes a switch element Q 12 , and also includes a control circuit for the switch element Q 12 , which will be described below. 
     The control circuit for the switch element Q 12 , as illustrated in  FIG. 8 , is configured to detect a non-smooth DC output voltage of the rectifier circuit Rec, which is input from the pin Vin, using a comparator COM 1 , and turn the switch element Q 12  on via a timer TIM and a driver GSD 1  only for a predetermined short period of time as each half cycle of a power source voltage rises. For example, the control circuit in  FIG. 8  turns the switch element Q 12  off within 1 ms after application of each half cycle of the power source voltage. 
     Also, the comparator COM 1 , as illustrated in  FIG. 8 , controls the switching element Q 11  of the converter CONV via a filter F, a comparator COM 2  and a driver GSD 2  to control an output of the converter CONV so as to adjust a conduction angle for each half cycle of the power source voltage. An output (voltage) of the filter F, as illustrated in  FIG. 10 , varies according to the conduction phase angle, and the output voltage of the filter F is a reference voltage for the comparator COM 2 . When a detection value from the current detection element CD reaches the reference voltage, the comparator COM 1  turns off the switching element Q 11  of the converter CONV. 
     The control power source  22 , which includes a secondary winding w 2  to be magnetically coupled to the inductor L 11  of the converter CONV, rectifies an induced voltage in the secondary winding w 2 , which is generated when an increasing current flows in the inductor L 11 , by means of a diode D 13  and smoothes the rectified induced voltage by means of a capacitor C 13  to output a control voltage between the pin Vcc and the pin GND of the control IC  21 . 
     Next, a circuit operation will be described. 
     The control IC  21  in the control unit CC is provided with a function that, when AC power for the illumination device is applied, acts so as to first receive a control power supply from the pin VDC to start the converter CONV, and thus, the converter CONV is promptly started. Once the converter CONV is started, a gate signal is supplied to a gate of the switching element Q 11  from the pin G of the control IC  21  for the converter CONV to start a buck chopper operation. Then, as a result of an increasing current flowing in the inductor L 11 , a voltage is induced in the secondary winding w 2  magnetically coupled to the inductor L 11 , and thereafter, the operation is continuously performed with control power supply provided from the control power source  22 . 
     Consequently, the LED LS connected in parallel to the output capacitor C 12  of the converter CONV is driven to light up. When the detection output from the current detection element CD is input to the pin CS of the control IC  21  as a control input, the converter CONV performs a negative feedback control operation for the increasing current within the control IC  21 . Then, an output current of the converter CONV is proportional to the increasing current, and the LED LS lights up under a constant current control. 
     Meanwhile, when an AC power source voltage is applied, the timer TIM in the control IC  21  generates a gate signal from the driver GSD 1  to turn on the switch element Q 12  simultaneously with the comparator COM 1 &#39;s detection of a non-smooth DC output voltage, and thus, immediately after the power application, the damping resistor Rd is connected between the DC output ends of the rectifier circuit Rec. 
     Consequently, as a result of interposing a phase-control dimmer between the AC power source AC and the illumination device according to the present embodiment, when each half cycle of the power source voltage sharply rises, even though a transient oscillation occurs for the reason described above, the damping resistor Rd damps the transient oscillation. Consequently, the peak value of the transient oscillation is lowered, and thus, a phase-control dimmer causes no malfunctions, enabling provision of desired dimmed illumination. 
     After the elapse of a predetermined short period of time from the start of application of the voltage of each half cycle of the power source voltage, the timer TIM stops the driver GSD 1 &#39;s gate signal generation, and thus, the damping resistor Rd is released from between the DC output ends of the rectifier circuit Rec. Therefore, the heat generation caused by the power consumed by the damping resistor Rd is extremely small. 
     Next, an operation in which the LED lighting circuit LOC controls its output so as to adjust to the conduction angle control by the phase-control dimmer to dim and light the LED LS will be described with reference to  FIGS. 8 to 10 . 
     In other words, in  FIG. 8 , when each half cycle of the power source voltage is applied between the input terminals and a non-smooth DC output voltage of the rectifier circuit Rec is input from the pin Vin of the control IC, a gate signal is supplied to the switching element Q 11  via the comparator COM 1 , the filter F, the comparator COM 2  and the driver GSD 2 , to drive the switching element Q 11  to be turned on. When the switching element Q 11  is turned on, an increasing current flows in the first circuit AA in the converter CONV, and the current detection element CD detects the increasing current, and thus, the detection output is input from the pin CS of the control IC. 
     Meanwhile, the filter F integrates the half cycle of the power source voltage to perform effective value conversion, and outputs a voltage with the relationship illustrated in  FIG. 10  as described above. Then, at the point of time when the detection output from the pin CS corresponds to the output voltage of the filter F, the comparator COM 2  stops sending a gate signal from the driver GSD 2 . As a result, the switching element Q 11  of the converter CONV is turned off. Consequently, a decreasing current from the inductor L 11  flows in the second circuit BB. In the present embodiment, off time Toff of switching element Q 11  illustrated in  FIGS. 9A and 9B  is fixed, and when the off time has elapsed, the driver GSD 2  starts operating, and the switching element Q 11  is turned on again. Subsequently, the above-described operation is repeated, and thus, the converter CONV continues the operation to generate an output corresponding to the conduction angle of the power source voltage. 
       FIG. 9A  illustrates an example of a waveform appearing at the pin CS of the control IC where the conduction angle of the power source voltage is 180°, that is, the phase angle is 0°. 
       FIG. 9B  illustrates an example of a waveform appearing at the pin CS of the control IC where the conduction angle of the power source voltage is 90°, that is, the phase angle is 90°. 
     In both of the above examples, when the detection output of the current detection element CD (input to the pin CS) reaches the output voltage level of the filter F, which is indicated by dotted lines in the Figures, the comparator COM 2  stops sending a gate signal from the driver GSD 2 , and thus, it can be understood that an output of the converter CONV varies according to the conduction angle of the power source voltage. 
       FIG. 10  is a graph illustrating a relationship between a phase angle of the power source voltage and an output of the filter, which are set to be proportional to each other in the present embodiment. 
     &lt;Third Embodiment&gt; 
     A third embodiment of the present invention will be described. In the third embodiment, as illustrated in  FIGS. 11 and 12 , a switch element Q 12  configured to control the connection time of a damping resistor Rd is provided external to a control IC  21 . Accordingly, only a control circuit for the damping resistor Rd is included in the control IC  21 . In the Figures, the same components as those in  FIGS. 7 and 8  are provided with the same symbols, and a description of those components will be omitted. 
     &lt;Fourth Embodiment&gt; 
     A fourth embodiment of the present invention will be described. The fourth embodiment, as illustrated in  FIG. 13 , is different from the second and third embodiments in terms of a control circuit for a damping resistor Rd, and a converter CONV. In the Figure, the same components as those in  FIG. 7  are provided with the same symbols, and a description of those components will be omitted. 
     The control circuit for the damping resistor Rd is configured to turn on a switch element Q 12  by means of an output of a monostable circuit ASM configured to generate an output only for a predetermined short period of time at the start of application of each half cycle of a power source voltage. 
     The converter CONV is of a flyback transformer-type. In other words, a buck-flyback converter CONV includes a switching element (not illustrated) included in a control IC  21 , a flyback transformer FT, a diode D 14 , a current detection element CD and the control IC  21  as its main components. The switching element turns on/off the connection of a primary winding in the flyback transformer FT to a DC output end of a rectifier circuit Rec. The diode D 14  rectifies a voltage induced in a secondary winding in the flyback transformer FT to obtain a DC output. The current detection element CD feeds an output current obtained from the secondary side of the flyback transformer FT back to the control C 21  via a photocoupler PC. The control IC  21  performs constant current control of the converter CONV to light an LED LS. 
     &lt;Fifth Embodiment&gt; 
     A fifth embodiment of the present invention will be described. As illustrated in  FIG. 14 , the fifth embodiment is different from the second to fourth embodiments in that a damping resistor Rd includes a voltage-dependent nonlinear resistor. In the Figure, the same components as those in  FIG. 13  are provided with the same symbols, and a description of those components will be omitted. 
     In the present embodiment, the voltage-dependent nonlinear resistor is a surge absorption element having a breakdown voltage set so as to absorb a voltage higher than a peak value of a power source voltage from a transient oscillation voltage generated in a sharp rise in each half cycle of a voltage. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.