Abstract:
The purpose of the present invention is to provide an LED lighting circuit, which can normally light an LED even if an alternating current power supply is an electronic transformer, an LED illuminating device, and a socket for an LED illuminating unit. Disclosed is an LED lighting circuit includes a rectifier circuit for rectifying an AC output from an AC power supply, an LED drive unit, which drives the LED by having rectifying output inputted thereto from the rectifying circuit, a reverse current preventing unit, which is provided between the rectifying circuit and the LED drive unit, and a terminal voltage control unit, which reduces the output terminal voltage of the rectifying circuit in the case where the alternating current output from the alternating current power supply is unstable or stopped. Also provided are an LED illuminating device, and a socket for an LED illuminating unit.

Description:
TECHNICAL FIELD 
     The present invention relates to an LED lighting circuit that can properly light an LED even with a power supply obtained from an inverter circuit, and also relates to an LED illumination device and an LED illumination unit socket. 
     BACKGROUND 
     Halogen lamps that operate at a rated voltage or 12 V are in widespread use. The power supply for such halogen lamps is often produced by stepping down a line supply voltage. Since a conventional step-down transformer is bulky, a device (hereinafter referred to as an electronic transformer) is employed that converts the AC line supply voltage to a high-frequency voltage by using an inverter circuit and then converts the voltage to a lower voltage by using a high-frequency transformer. The inverter circuit to be used in the electronic transformer is one of two types, a self-excited oscillation type or an externally excited oscillation type (refer, for example, to  FIG. 6  in patent document 1). 
       FIG. 12  is a diagram showing a lighting device equipped with a self-excited inverter as presented in the description of the prior art in patent document 1. 
     The lighting device comprises an AC power supply  1 , a full-wave rectifier  2 , a halogen lamp  14 , a half-bridge inverter circuit, and other circuit elements, and the entire circuitry, excluding the AC power supply  1  and the halogen lamp  14 , corresponds to the electronic transformer. A circuit comprising a resistor  5 , a capacitor  6 , and a triggering device  7  outputs an oscillation initiating trigger when a predetermined voltage is reached. Self-excited oscillation occurs within a circuit comprising capacitors  3  and  4 , transistors  8  and  9 , and a current feedback transformer  13 . A high-frequency step-down, transformer  12  produces an AC drive voltage for the halogen lamp  14 . Diodes  10  and  11  are protection diodes for providing protection against back electromotive force. The lighting device depicted in  FIG. 12  shows the operating principle of the half-bridge inverter circuit. 
     The invention disclosed in patent document 1 is intended to suppress the rush current that occurs during startup, and for that purpose, a soft start circuit A and a voltage feedback circuit B are provided as shown in  FIG. 1  included in patent document 1. 
     Recently, LED illumination devices have begun to be used widely, replacing not only incandescent lamps but also halogen lamps. Conventional halogen lamps have power ratings of 20 to 40 W but, in the case of LED illumination devices, brightness comparable to halogen lamps can be obtained at 4 W which is about one-fifths of the above power rating. However, if the self-excited electronic transformer is inserted between the AC line power supply and the halogen lamp bayonet, base (MR16 or MR11 in the case of 12-V halogen lamps), as described above, the LED illumination device may not light (or may turn on and off erratically). 
     Electronic transformers widely used for halogen lamps range from small ones for driving single halogen lamps to large ones rated for 100 to 300 W capable of driving several halogen lamps. For these electronic transformers, the minimum operating power necessary for startup is specified in order to produce a stable output of 12 VAC, and generally a circuit load of 10 to 30 W is required. When an LED illumination device is connected to such an electronic transformer (for example, in  FIG. 12 , the LED illumination device is connected to the secondary side n 2  of the high-frequency step-down transformer  12 ), the output of the electronic transformer may become unstable due to an insufficient load, causing the LED illumination device to malfunction by failing to light or turning on and off erratically. 
     In view of the above, there is proposed a circuit that ensures stable operation when the LED illumination device is connected, regardless of whether the inverter circuit used for the 12-V halogen lamp power supply is of the self-excited oscillation type or the externally excited oscillation type (patent document 2). 
       FIG. 13  is a diagram redrawn from  FIG. 2  from patent document 2. 
     In  FIG. 13 , the output of a self-excited electronic transformer (inverter-type voltage conversion circuit) (in the case of  FIG. 12 , the secondary side of the high-frequency step-down transformer  12 ) is connected to a terminal U-IN of a power feed unit  30 . A lighting circuit  40  first supplies current to a startup assisting circuit  42 , inducing self-excited oscillation in the electronic transformer. Next, a constant-current load circuit  43  draws a constant current from the electronic transformer to stabilize the oscillation. After that, the lighting circuit  40  causes the LED  22  to light, while at the same time, stopping the operation of the constant-current load circuit  43 . 
     Patent document 1: Japanese unexamined Patent Publication No. H02-66890 (FIGS. 1 and 6) 
     Patent document 2: Japanese Unexamined Parent Publication No. 2010-135139 (FIG. 2) 
     SUMMARY 
     In the circuit shown in  FIG. 13 , the constant-current load circuit  43  supplements the power consumed by the LED  22  and thereby provides the power necessary for the electronic transformer to continue to operate. That is, since the power for driving the constant-current load circuit  43  becomes necessary in addition to the power necessary for driving the LED  22  itself, a large amount of power is required as a whole, thus partially defeating the purpose of the LED illumination device that features low-power operation. Furthermore, when a halogen lamp having a small external size is replaced by an LED illumination device, there arises a need to take account of not only the limitation on the heat dissipation capability of the LED illumination device itself but also the heat dissipation of the constant-current load circuit. In this case, heat generation has to be reduced by reducing the power of the LED. If the power of the LED is reduced, the brightness of the LED illumination device drops, resulting in a performance degradation. Moreover, the circuit disclosed in patent document 2 requires a larger number of components and wiring lines because of the provision of the constant-current load circuit  43 , operation stopping circuit  45 , and current stopping circuit  44 . 
     It is an object of the present invention to provide an LED lighting circuit that aims to solve the above deficiencies, and an LED illumination device and an LED illumination unit socket that incorporate such an LED lighting circuit. 
     It is another object of the present invention to provide an LED lighting circuit that can properly light an LED even when an electronic transformer is employed for supply of AC power, and an LED illumination device and an LED illumination unit socket that incorporate such an LED lighting circuit. 
     It is a further object of the present invention to provide an LED lighting circuit that can properly light an LED even when an electronic transformer is employed for supply of AC power, and that can reduce the number of circuit components and minimize line power not relevant to lighting, and an LED illumination device and an LED illumination unit socket that incorporate such an LED lighting circuit. 
     An LED lighting circuit according to the invention includes a rectifier circuit for rectifying an AC output from an AC power supply, an LED driving unit which takes as input a rectified output from the rectifier circuit and drives an LED; a reverse current blocking unit provided between the rectifier circuit and the LED driving unit, and a terminal voltage control unit which operates to reduce an output terminal voltage of the rectifier circuit when the AC output from the AC power supply becomes unstable or stops. 
     Preferably, in the LED lighting circuit, the terminal voltage control unit includes at least one capacitor. 
     Preferably, in the LED lighting circuit, the terminal voltage control unit is a parallel circuit which includes a capacitor and a resistor. 
     Preferably, in the LED lighting circuit, the capacitor in the terminal voltage control unit is a tantalum capacitor or an electrolytic capacitor. 
     Preferably, in the LED lighting circuit, the terminal voltage control unit includes a second resistor which is connected in series with the capacitor. 
     Preferably, in the LED lighting circuit, the terminal voltage control unit includes a Zener diode which is connected in parallel with the capacitor. 
     Preferably, in the LED lighting circuit, the terminal voltage control unit includes a second capacitor which is connected in parallel with the capacitor and the second resistor. 
     Preferably, an the LED lighting circuit, when the output of the rectifier circuit steps, the capacitor is discharged, with a time constant that is set longer than two cycles of the AC output of the AC power supply. 
     Preferably, in the LED lighting circuit, the reverse current blocking unit comprises at least one diode. 
     An LED illumination device according to the invention is equipped with an LED lighting circuit which includes a rectifier circuit for rectifying an AC output from an AC power supply, an LED driving unit which takes as input a rectified output from the rectifier circuit and drives an LED; a reverse current blocking unit provided between the rectifier circuit and the LED driving unit, and a terminal voltage control unit which operates to reduce an output terminal voltage of the rectifier circuit when the AC output from the AC power supply becomes unstable or stops. 
     An LED illumination unit socket with which is connected an LED illumination unit having an LED driving unit for driving an LED includes a rectifier circuit which takes as input an AC output from an AC power supply, and which supplies a rectified output to the LED driving unit, a reverse current blocking unit provided between the rectifier circuit and the LED driving unit; and a terminal voltage control unit which operates to reduce an output terminal voltage of the rectifier circuit when the AC output from the AC power supply becomes unstable or stops. 
     According to the LED lighting circuit, LED illumination device, and LED illumination unit socket described above, the problem that the LED does not light or turns on and off erratically can be avoided, regardless of whether the electronic transformer for the AC power supply is of a self-excited oscillation type or an externally excited oscillation type. 
     The present inventor has discovered that when the electronic transformer stops due to an insufficient load, if the output terminal voltage of the rectifier circuit is reduced, the electronic transformer resumes oscillation. The present invention has been devised by utilizing this phenomenon. Upon detecting that the output of the AC power supply has stopped (or has become unstable), the terminal voltage control unit operates to reduce the output terminal voltage of the rectifier circuit, at this time, the reverse current blocking unit acts to block the current flowing from the LED driving unit back to the output terminal side of the rectifier circuit. That is, when the voltage at the input side becomes a lower than the voltage at the output side of the reverse current blocking unit, the reverse current blocking unit electrically isolates the LED driving unit from the terminal voltage control unit; as a result, the voltage at the output terminal side of the rectifier circuit quickly drops, and thus the electronic transformer quickly resumes oscillation. In this way, the LED lighting circuit, the LED illumination device, and the apparatus constructed by connecting the LED illumination unit with the LED illumination unit socket are allowed to continue to operate in a stable manner. Furthermore, according to the LED lighting circuit, LED illumination device, and LED illumination unit socket of the invention, the provision of the terminal voltage control unit in conjunction with the revere current blocking unit serves to reduce the number of circuit components and minimize the power not relevant to lighting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram showing the configuration of an LED lighting circuit  100 . 
         FIG. 2  is a detailed circuit diagram showing a circuit incorporating the LED lighting circuit  100  of  FIG. 1 . 
         FIGS. 3(   a ) to  3 ( e ) are diagrams showing voltage waveforms taken at various parts in the circuit of  FIG. 2 . 
         FIG. 4  is a cross-sectional view schematically illustrating a cross section of an LED illumination device  140 . 
         FIG. 5(   a ) is a cross-sectional view schematically illustrating an LED illumination unit  150 , and  FIG. 5(   b ) is a cross-sectional view schematically illustrating an LED illumination unit socket  160 . 
         FIG. 6  is a detailed circuit diagram showing a circuit incorporating an alternative LED sighting circuit  170 . 
         FIG. 7  is a detailed circuit diagram, showing a circuit incorporating another alternative LED lighting circuit  180 . 
         FIG. 8  is a detailed circuit diagram showing a circuit incorporating still another alternative LED lighting circuit  190 . 
         FIG. 9  is a detailed circuit diagram showing a circuit incorporating yet another alternative LED lighting circuit  200 . 
         FIG. 10  is a detained circuit diagram showing a circuit incorporating a further alternative LED lighting circuit  210 . 
         FIG. 11  is a detailed circuit diagram showing a circuit incorporating a still further alternative LED lighting circuit  220 . 
         FIG. 12  is a diagram showing a lighting device equipped with a self-excited inverter as presented in the description of the prior art in patent document 1. 
         FIG. 13  is a diagram redrawn from  FIG. 2  from patent document 2. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An LED Lighting circuit, an LED illumination device, and an LED illumination unit socket will be described below with reference to the drawings. It will, however, be noted that the technical scope of the present invention is not limited to any specific embodiment described herein, but extends to the inventions described in the appended claims and their equivalents. Further, throughout the drawings, the same or corresponding component elements are designated by the same reference numerals, and the description of such component elements, once given, will not be repeated thereafter. 
       FIG. 1  is a schematic block diagram showing the configuration of an LED lighting circuit  100 . 
     The LED lighting circuit  100  comprises a rectifier circuit  101 , a terminal voltage control unit  102 , a reverse current blocking unit  103 , and an LED driving unit  104 . In  FIG. 1 , three LEDs  105  are shown as being connected to the LED lighting circuit  100 . The rectifier circuit  101  has terminals  101   a  and  101   b  at which power is supplied from an AC power supply, and thus the output of the AC power supply is supplied as input to the rectifier circuit  101 . The output of the rectifier circuit  101  is coupled via a wiring line  106  to both the terminal voltage control unit  102  end the reverse current blocking unit  103 . The output of the reverse current blocking unit  103  is coupled via a wiring line  107  to the LED driving unit  104 . The LED driving unit  104  drives the three LEDs  105  connected an series. Current from the terminal voltage control unit  102  and the LED driving unit  104  is returned to the rectifier circuit  101  via a wiring line  108 . 
     When the output of the AC power supply becomes unstable or stops, the terminal voltage control unit  102  operates to reduce the output terminal voltage of the rectifier circuit  101  (the voltage on the wiring line  106 ). When the output terminal voltage of the rectifier circuit  101  becomes lower than the output terminal voltage of the reverse current blocking unit  103 , the reverse current blocking unit  103  acts to block the current flowing from the LED driving unit  104  back to the output terminal side of the rectifier circuit  101 . 
       FIG. 2  is a detailed circuit diagram showing a circuit incorporating the LED lighting circuit  100  of  FIG. 1 . 
     The secondary side n 2  of a high-frequency step-down transformer  111   a  in the AC power supply  111  is connected to the terminals  101   a  and  101   b . The primary side n 1  is inserted in a path through which flows a high-frequency alternating current output from an inverter circuit contained in the AC power supply  111 . 
     Next, a description will be given of how  FIG. 2  corresponds to  FIG. 1 . 
     The LED lighting circuit  100  in  FIG. 1  corresponds to the portion  112  shown in  FIG. 2 . The rectifier circuit  101  in  FIG. 1  is constructed from a diode bridge circuit comprising four diodes  23 . The terminal voltage control unit  102  in  FIG. 1  is constructed from a parallel circuit  115  comprising a resistor  24  and a capacitor  25 . The reverse current blocking unit  103  in  FIG. 1  is constructed from a diode  26 . The LED driving unit  104  in  FIG. 1  is constructed from a combination of a capacitor  27  and a resistor  28 . 
     The LED lighting circuit  100  includes the terminals  101   a  and  101   b , and the secondary side n 2  of the high-frequency step-down transformer  11   a  is connected to the diode bridge circuit via the terminals  101   a  and  101   b . The current output terminal of the diode bridge circuit is connected to the positive terminal of the resistor  24 , the positive terminal of the capacitor  25 , and the anode of the diode  26 . The cathode of the diode  26  and the positive terminal of the capacitor  27  are together connected to the positive terminal of the LED array  113  constructed from the three LEDs  105  connected in series. The negative terminal of the LED array  113  is connected to the current limiting resistor  28 . The negative terminals of the resistor  24 , capacitors  25  and  27 , and resistor  28 , respectively, are connected to a wiring line through which current is returned via the diode bridge circuit to the AC power supply  111  (the secondary side n 2  of the high-frequency step-down transformer  111   a .) 
     When the output of the AC power supply becomes unstable or stops, the parallel circuit  115  of the resistor  24  and capacitor  25 , which, corresponds to the terminal voltage control unit  102 , releases the charge stored on the capacitor  25  and thereby reduces the output terminal voltage of the rectifier circuit  101  (the voltage on the wiring line  106 ). At this time, the diode  26 , which corresponds to the reverse current blocking unit  103 , acts to block the current flowing from the LED driving unit  104 , i.e., the positive terminal of the capacitor  27 , back to the output terminal side of the rectifier circuit  101 . The capacitor  27  in the LED driving unit  104  is a smoothing capacitor for smoothing the current to be supplied to the LED array  113 , while the resistor  28  is a current limiting resistor for limiting the current flowing to the LED array  113 . That the output of the AC power supply becomes unstable means that the oscillation of the inverter in the AC power supply becomes weaker and the amplitude of the AC power supply output decreases (for example, to 50% or less of the normal amplitude level) or that such a situation is beginning to occur. 
     The rectifier circuit  101  is net limited to the diode bridge configuration, font may be constructed from a single diode. 
     The terminal voltage control unit  102  may be constructed from a circuit comprising a switching device such as an FET or a bipolar transistor, since it is only required to reduce the output terminal voltage of the rectifier circuit  101 . In that case, the oscillating state of the AC power supply  111  is monitored, and the switching device is turned on based on the monitoring information. For example, when the output of the AC power supply is unstable or stops, the terminal voltage control unit  102  operates to reduce the output terminal voltage of the rectifier circuit  101  to about 5 to 7 V. This helps to restore the oscillation of the inverter in the AC power supply  111 . 
     The reverse current blocking unit  103  is not limited to a diode, but instead, use may be made of a switching device. In that case, the oscillating state of the AC power supply  111  or the result of a comparison made between the magnitude of the drive voltage of the LED driving unit  104  and the magnitude of the output terminal voltage of the rectifier circuit  101  is monitored to control the on/off operation of the switching device. 
     The LED driving unit  104  may be constructed using a known voltage step-up circuit, voltage step-down circuit, or constant-current circuit. The resistor  28  may be replaced by a known constant-current circuit. If stray capacitance can be used, the capacitor  25  may be omitted. 
     In the circuit depicted in  FIG. 2 , the value of the resistor  24  is 160Ω, the value of the capacitor  25  is 10 μF, and the value of the capacitor  27  is 220 μF, though these values may vary depending on the rating of the electronic transformer and the current consumption of the LED array. The time constant of the parallel circuit of the resistor  24  and capacitor  25  is 1.6 ms; since the oscillation frequency of the inverter is about 30 kHz to 80 kHz, it follows that the time constant is several to several tens of times as long as the oscillation period. If the time constant is at least about twice the oscillation period, erroneous oscillation of the inverter contained in the electronic transformer can be prevented. Further, since the power supply is a 12-V power supply, three LEDs  105 , each with a forward voltage of about 3 V, are connected in series. 
       FIG. 3  is a diagram showing voltage waveforms taken at various parts in the circuit of  FIG. 2 . 
       FIG. 3(   a ) shows one cycle of the AC waveform of the AC line power supply in order to provide a general idea of the time relationship. 
       FIG. 3(   b ) shows the voltage waveform taken at the cathode of the diode  26 . That is,  FIG. 3(   b ) shows the drive voltage of the LED driving unit  104 , which is also the voltage applied to the positive side of the LED array  3 . This voltage waveform rises with a short delay relative to the rising of the AC line power supply, has a peak near the peak of the AC line power supply, and thereafter gradually falls. When the voltage (in absolute value) of the AC line power supply rises above a certain level, the oscillation of the inverter circuit contained in the electronic transformer continues to persist, as will be described later, and extra power is available to charge the capacitor  27 , as a result of which the voltage waveform shown in  FIG. 3(   b ) rises. 
     During the period that the inverter circuit is oscillating, current flows to the LED array  113  via the diode  26 . As the AC line supply voltage of  FIG. 3(   a ) rises, and the oscillation period increases, extra charge is stored on the capacitor  27 . As a result, the voltage waveform of  FIG. 3(   b ) rises with a delay relative to the AC line power supply. 
       FIG. 3(   c ) shows a voltage waveform applied to the terminal  101   a . Each pentagonal portion corresponds to the period during which the oscillation output is produced. In each pentagonal portion, the period defined by the parallel top and bottom sides represents the full oscillation period, and the period defined by the sloping sides represents the decay period. That is, in the lighting circuit shown in  FIG. 2 , the oscillation of the electronic transformer rises rapidly, and decays after a certain period of oscillation.  FIG. 3(   c ) also shows that the oscillation period is short when the AC line supply voltage is low, and long when the AC line supply voltage is high.  FIG. 3(   c ) further shows that when the AC line supply voltage exceeds a given value (in absolute value), the inverter circuit begins to oscillate, and also that the oscillation is intermittent. 
       FIG. 3(   d ) shows, in enlarged form, the periods corresponding to two pentagonal portions in  FIG. 3(   c ). One oscillation period corresponding to one pentagonal portion in  FIG. 3(   c ) is shown as containing AC pulses for four cycles, but actually, it contains ten to several tens of AC pulses. The AC pulses are distorted and generally appear as a square wave. The frequency is about 30 to 80 kHz, as earlier noted. In the figure, the oscillation is shown as decaying in two cycles, but it decays relatively quickly in a few cycles. That is, the oscillation of the inverter circuit continues for a certain period, and then decays and stops. When the terminal voltage control unit  102  reduces the output voltage of the diode bridge circuit by detecting the above situation, the inverter circuit resumes oscillation. This operation is repeated. 
       FIG. 3(   e ) shows the voltage waveform taken at the output terminal of the diode bridge circuit. The voltage rises to its maximum value almost simultaneously with the start of the oscillation, and the maximum voltage is maintained during the oscillation period; then, when the oscillation ends, the voltage decays. Actually, some ripple occurs daring the oscillation period, but such ripple is not shown here. When the output terminal voltage drops to a few volts, the inverter circuit resumes oscillation, and the above operation is repeated. When the waveform is closely observed, it is seen that the oscillation resumes when the output terminal voltage drops to a given fraction of the maximum voltage maintained during the oscillation period. 
       FIG. 4  is a cross-sectional view schematically illustrating a cross section of an LED illumination device  140 . 
     Two pins  144  protruding from the bottom of the outer casing  141  of the LED illumination device  140  correspond to the terminals  101   a  and  101   b  in  FIG. 2  and are connected to a lighting circuit block  143 . The LED lighting circuit  112  shown in  FIG. 2  is contained in the lighting circuit block  143 . Three LEDs  105  are mounted on a substrate  142  disposed above the lighting circuit block  143 . The lighting circuit  143  is connected to the substrate  142  via interconnecting lines  145 . The sloping face in the upper part of the outer casing  141  is formed in the shape of a reflecting mirror. The LED illumination device  140  shown here is compatible with a pin socket. (MR16 bayonet base), but the first and second terminals  101   a  and  101   b  may be designed so as to be compatible with a screw-in socket. 
       FIG. 5  is a diagram showing a construction in which an LED illumination unit  150  and an LED illumination unit socket  160  are separated from each other.  FIG. 5(   a ) is a cross-sectional view schematically illustrating the LED illumination unit  150 , and  FIG. 5(   b ) is a cross-sectional view schematically illustrating the LED illumination unit socket  160 . 
     The difference between the LED illumination device  140  shown in  FIG. 4  and the LED illumination unit  150  to be fitted into the LED illumination unit socket  160  shown in  FIG. 5  is that the lighting circuit block  143  of the LED illumination device  140  is replaced by an LED driving unit  153  in the LED illumination unit  150 . The LED driving unit  153  corresponds to the LED driving unit  104  in  FIG. 1 , and may be constructed from a smoothing circuit or a constant-current circuit or the like. A substrate  152  on which three LEDs  105  are mounted is connected via interconnecting lines  155  to the LED driving unit  153  to which pins  154  are also connected. Since the LED illumination unit socket  160  outputs DC, as will be described later, the pins  154  connected to the LED driving unit  153  have polarities. 
     Recesses  161  to accommodate the pins  154  for electrical connections are formed in the upper part of a mold  162  of the LED illumination unit socket  160  shown in  FIG. 5(   b ). The electrodes of the recesses  161  are connected to a lighting circuit block  163  via interconnecting lines  165 . The rectifier circuit  101 , terminal voltage control circuit  102 , and reverse current blocking circuit  103  shown in  FIG. 1  are contained in the lighting circuit block  163 . The lighting circuit block  163  corresponds to a portion shown as an LED illumination device socket circuit  114  in  FIG. 2 . Accordingly, when the LED illumination unit  150  is connected to the LED illumination unit socket  160 , the configuration is the same as that shown in  FIG. 1  and thus the entire circuit operates properly when connected to an electronic transformer. 
       FIG. 6  is a detailed circuit diagram showing a circuit incorporating at alternative LED lighting circuit  170 . 
     In  FIG. 6 , the same electronic components as those in  FIG. 2  are designated by the same reference numerals, and the description of such components will not be repeated here. The difference between the circuit of  FIG. 2  and the circuit of  FIG. 6  is that the parallel circuit  175  that functions as the terminal voltage control unit in the LED lighting circuit  170  in  FIG. 6  further includes a resistor (second resistor)  67  in series with a capacitor  68 . Otherwise, the circuit of  FIG. 6  is identical to the circuit of  FIG. 2 . 
     In the circuit shown in  FIG. 2 , if a ceramic capacitor is used as the capacitor  25 , the capacitor  25  may oscillate and generate noise through the mounting substrate. It is believed that the noise is associated with the intermittent operation of the electronic transformer. 
     By connecting the resistor  67  in series of the capacitor  68  as shown in the LED lighting circuit  170 , it becomes possible to reduce the noise that may be generated when the capacitor  68  is formed from a ceramic capacitor. It is believed that the resistor  67  serves to smooth changes in the voltage developed across the terminals of the capacitor  68  and thus contributes to reducing the noise. However, the resistance value of the resistor  67  must be held to a minimum necessary value in order to not interfere with the induced oscillation of the electronic transformer. 
     If a tantalum capacitor or electrolytic capacitor, not a ceramic capacitor, is used as the capacitor  68 , the noise from the capacitor  68  can be further reduced. This is presumably because the electrode structure of a tantalum capacitor or electrolytic capacitor is not a rigid structure such as that of a ceramic capacitor and the noise (chatter) is suppressed due to the damping of an electrolyte contained in the tantalum capacitor or electrolytic capacitor. That is, by not only inserting the resistor  67  but also using a tantalum capacitor or electrolytic capacitor as the capacitor  68 , the effect of suppressing the noise can be further enhanced. 
     The circuit shown in  FIG. 6  can be incorporated in the LED illumination device shown in  FIG. 4 . The circuit shown in  FIG. 6  can also be incorporated in the illumination device/sectors combination shown in  FIG. 5 . 
       FIG. 7  is a detailed circuit diagram showing a circuit incorporating another alternative LED lighting circuit  180 . 
     In  FIG. 7 , the same electronic components as those in  FIG. 2  are designated by the same reference numerals, and the description of such components will not be repeated here. The difference between the circuit of  FIG. 2  and the circuit or  FIG. 7  is that the circuit  185  that functions as the terminal voltage control unit in the circuit of  FIG. 7  further includes a resistor (second resistor)  72  which is inserted at the output terminal of the rectifier circuit. Otherwise, the circuit of  FIG. 7  is identical to the circuit of  FIG. 2 . 
     The resistor  72  is connected in series with the resistor  24 , the capacitor  25 , and the diode  26  (series-parallel connection). By inserting the resistor  72 , the noise from the capacitor  25  can be reduced when a ceramic capacitor is used as the capacitor  25 . 
     It is believed that the resistor  72  serves to smooth changes in the voltage developed across the terminals of the capacitor  25  and thus contributes to reducing the noise. Further, the resistor  72  can be made smaller in value than the resistor  24 , and if a fusible resistor is used as the resistor  72 , the circuit safety can be further increased. By not only inserting the resistor  72  but also using a tantalum capacitor or electrolytic capacitor as the capacitor  25 , the effect of suppressing the noise can be further enhanced. 
     The circuit shows in  FIG. 7  can be incorporated in the LED illumination device shown in  FIG. 4 . The circuit shown in  FIG. 7  can also be incorporated in the illumination device/socket combination shown in  FIG. 5 . 
       FIG. 8  is a detailed circuit diagram showing a circuit incorporating still another alternative LED lighting circuit  190 . 
     In  FIG. 8 , the same electronic components as those in  FIG. 2  are designated by the same reference numerals, and the description of such components will not be repeated here. The difference between the circuit of  FIG. 2  and the circuit of  FIG. 8  is that the LED driving unit  104   a  (the suffix “a” is appended to distinguish it from the LED driving unit  104  shown in  FIG. 1 ) includes a back converter (DC converter)  192 . That is, the LED driving unit  104   a  is constructed from a combination of the smoothing capacitor  27  and the back converter  192 . The back converter  192  is a step-down voltage converter, and converts the voltage developed across the terminals of the capacitor  27  into a lower voltage which is applied to an LED  191 . The back converter  192  comprises a resistor  194 , a coil  193 , an IC  195 , and a diode  196 . 
     The resistor  194  is provided to detect current flowing to the LED  191 , and has a resistance value not larger than 1Ω. The coil  193 , working in conjunction with a switch circuit incorporated in the IC  195 , maintains the current flowing to the LED  191  at a constant value. The diode  196  acts to return to the capacitor  27  the current that the coil  193  tries to continue to flow when the switch circuit is turned off. The IC  195 , which takes power from the voltage developed across the terminals of the capacitor  27 , includes an oscillation circuit and performs control so that desired current flows to the LED  191 . 
     In the LED lighting circuit  112  shown in  FIG. 2 , the LED driving unit (see  FIG. 1 ) is a simple one constructed from a combination of the capacitor  25  and the resistor  26 . In this case, the voltage across the terminals of the capacitor  27  has to be maintained at about 9 V or higher in order for the LED array  113  to continue to operate. That is, as far as the voltage across the terminals of the capacitor  27  is concerned, the lighting circuit  112  including the LED array  113  cannot be said to have a wide operating range. Further, when the voltage across the terminals of the capacitor  27  is high (for example, 12 V), power loss due to the resistor  28  increases. Furthermore, when the voltage across the terminals of the capacitor  27  changes, the brightness of the LEDs  105  also changes. 
     By contrast, in the case of the LED driving unit  104   a  equipped with the back converter  192  as shown in  FIG. 8 , the circuit operates properly as long as the voltage across the terminals of the capacitor  27  is maintained at 6V or higher, for example, though it may depend on the specification of the IC  195 . In this way, by using the back converter  192 , the operating range associated with the voltage across the terminals of the capacitor  27  can be increased. 
     Further, the IC  195  performs switching to control the current flowing from the LED  191  to the coil  193  and, when the current flowing to the LED  191  increases, shuts off the current flowing from the coil  193  within the IC  195 . In this case, the energy stored across the coil  193  as returned via the diode  196  to the capacitor  27 . During this time, the LED  191  operates at a constant current. The LED driving unit  104   a  of  FIG. 8  is thus free from the energy loss that occurs due to the heating of the resistor  28  in  FIG. 2 . Furthermore, the amount of light emission from the LED  191  is maintained constant even when the voltage across the terminals of the capacitor  27  changes to a certain extent. 
     Further, since the IC  195  is used to control the lighting of the LED  191 , it is easy to add a dimming function (brightness control) and a protection function against temperature and overvoltage. Examples of such ICs include ZD850 by Zywyn Corporation, AL8805 by Diodes Inc., LM3405 by National Semiconductor Corporation, and RT8453 by Richtek Technology Corporation. Back converters constructed using these ICs are provided with a current measuring resistor, a coil, and a diode, though the connections of the peripheral circuitry may differ for each IC ( FIG. 8  depicts the circuitry for ZD850 in simplified form). 
     Further, instead of the back converter, a step-up voltage converter may be used as the DC convertor. In this case, the supply voltage range for the step-up voltage converter is wide, and the high energy efficiency for light emission and the constant-current driving of LEDs are secured. However, in the case of the step-up voltage converter, the number of LED series connection stages has to be increased. 
     The circuit shown in  FIG. 5  can be incorporated in the LED illumination device shown in  FIG. 4 . The circuit shown in  FIG. 8  can also be incorporated in the illumination device/socket combination shown in  FIG. 5 . 
       FIG. 9  is a detailed circuit diagram showing a circuit incorporating yet another alternative LED lighting circuit  200 . 
     In  FIG. 9 , the same electronic components as those in  FIG. 6  are designated by the same reference numerals, and the description of such components will not be repeated here. The only difference between the circuit of  FIG. 6  and the circuit of  FIG. 9  is that the circuit  205  that functions as the terminal voltage control unit in the circuit of  FIG. 9  further includes a Zener diode  201  in parallel with the capacitor  68 . Otherwise, the circuit of  FIG. 9  is identical to the circuit of  FIG. 6 . 
     As earlier described, when the capacitor  68  is formed from a tantalum capacitor, the effect of suppressing the noise can be further enhanced. However, since the rated voltage of a tantalum capacitor is low, there arises the possibility that the rated voltage may momentarily be exceeded. To address this, in the LED lighting circuit  200  shown in  FIG. 9 , the Zener diode  201  is inserted in parallel with the capacitor  68 , a tantalum capacitor, to provide protection so that a voltage greater than the rated voltage will not be applied to the capacitor  68  momentarily. 
     Likewise, when the capacitor  68  is formed from an electrolytic capacitor, the Zener diode  201  may be inserted if it is required to reduce the overall circuit size, requiring the use of a capacitor with a low breakdown strength; in this case also, the provision of the Zener diode  201  offers the advantage of being able to achieve the effect of enhancing the breakdown strength and noise suppression. 
     The circuit shown in  FIG. 9  can be incorporated in the LED illumination device shown in  FIG. 4 . The circuit shown in  FIG. 9  can also be incorporated in the illumination device/socket combination shown in  FIG. 5 . 
       FIG. 10  is a detailed circuit diagram showing a circuit incorporating a further alternative LED lighting circuit  210 . 
     In  FIG. 10 , the same electronic components as those in  FIG. 6  are designated by the same reference numerals, and the description of such components will not be repeated here. The only difference between the circuit of  FIG. 6  and the circuit of  FIG. 10  is that the circuit  215  that functions as the terminal voltage control unit in the circuit of  FIG. 10  further includes a ceramic capacitor (second capacitor)  211  which is inserted in parallel with the series connection of the capacitor  68  and the resistor  67  provided for noise suppression. Otherwise, the circuit of  FIG. 10  is identical, to the circuit of  FIG. 6 . 
     When the noise suppressing resistor  67  is inserted, the series resistance of the capacitor  68  and resistor  67  increases. This gives rise to the possibility that the normal operation of the parallel circuit (comprising the resistor  24  and capacitor  25  in the circuit shown in  FIG. 2 ) may become unstable. 
     In view of this, the ceramic capacitor  211  is inserted in parallel with the capacitor  68  so that the parallel circuit can operate stably as intended. Here, if the capacitance of the ceramic capacitor  211  is large, the ceramic capacitor  211  itself may cause noise. It is therefore preferable that the capacitance of the ceramic capacitor  211  is held to several tens of nanofarads. 
     The circuit shown in  FIG. 10  can be incorporated in the LED illumination device shown in  FIG. 4 . The circuit shown in  FIG. 10  can also be incorporated in the illumination device/socket combination shown in  FIG. 5 . 
       FIG. 11  is a detailed circuit diagram showing a circuit incorporating a still further alternative LED lighting circuit  220 . 
     In  FIG. 11 , the same electronic components as those in  FIG. 6  are designated by the same reference numerals, and the description of such components will not be repeated here. The only difference between the circuit of  FIG. 6  and the circuit of  FIG. 11  is that the circuit  225  that functions as the terminal voltage control unit in the circuit of  FIG. 11  further includes a Zener diode  221  in parallel with the capacitor  68  and a ceramic capacitor (second capacitor)  222  in parallel with the series connection of the resistor  67  and capacitor  68 . Otherwise, the circuit of  FIG. 11  is identical to the circuit of  FIG. 6 . 
     The reason that the Zener diode  221  is inserted in parallel with the capacitor  68 , a tantalum capacitor, is to provide protection so that a voltage greater than the rated voltage will not be applied to the capacitor  68  momentarily, as earlier described with reference to the circuit of  FIG. 9 . 
     The reason that the ceramic capacitor  222  is inserted in parallel with the series connection of the resistor  67  end capacitor  68  is to ensure that the parallel circuit will operate stably as intended, as earlier described with reference to the circuit of  FIG. 10 . It is preferable that the capacitance of the ceramic capacitor  222  is held to several tens of nanofarads. 
     The circuit shown in  FIG. 11  can be incorporated in the LED illumination device a shown in  FIG. 4 . The circuit sheen in  FIG. 11  can also be incorporated in the illumination device/socket combination shown in  FIG. 5 .