Abstract:
A light emitting diode (LED) circuit is provided. A light emitting diode (LED) circuit includes an alternating current (AC) source, a rectifier, a voltage-limiting circuit, and an LED module. The AC source provides an AC voltage, and the rectifier generates a first rectified voltage according to the AC voltage. An upper limit of the first rectified voltage is substantially restricted at a rated voltage by the voltage-limiting circuit, and the voltage-limiting circuit generates a second rectified voltage according to the first rectified voltage, in which the second rectified voltage is lower than the rated voltage. The LED module receives the second rectified voltage. Thereby, when the AC voltage is unstable, fluctuation of current flowing through the LED module can be substantially reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority benefit of Taiwan application serial no. 98116860, filed on May 21, 2009. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to a light emitting diode (LED) circuit, and more particularly to an LED circuit that prevents drastic fluctuations of current flowing through an LED. 
         [0004]    2. Description of Related Art 
         [0005]    Thanks to numerous advantages of long life span, miniature size, high vibration and shock resistance, economical power consumption and so on, LEDs have been widely applied to indicating lights or light sources employed in a variety of household electric appliances and instruments. In recent years, the LED has been developed towards multicolor and high brightness; therefore, its application scope has been expanded to large outdoor display boards, traffic signal lights, and the like. In the future, the LED may even become the main illumination light source with both power-saving and environment-protecting functions. 
         [0006]      FIG. 1  is a schematic circuit diagram of a conventional LED circuit.  FIG. 2  is a voltage waveform diagram of a conventional alternating current (AC) voltage. 
         [0007]      FIG. 3  is a voltage waveform diagram of a conventional rectified voltage. Referring to  FIGS. 1-3 , an AC source Vac can provide an AC voltage AS 1  to a rectifier BD 1 . According to the AC voltage AS 1 , the rectifier BD  1  can provide a rectified voltage DS 1  to a plurality of LEDs such as the LEDs LED 1 -LEDN. An example of a current-limiting resistor R 1  is 896Ω. For example, N=34. 
         [0008]    It should be noted that when the AC voltage AS 1  provided by the AC source Vac is unstable, a current flowing through the LEDs LED 1 -LEDN also undergoes drastic fluctuations, and therefore the LEDs LED 1 -LEDN have substantially large brightness variation and color temperature deviation. For example, ideally the AC source Vac provides an AC voltage AS 1  of 110V, but practically, the AC voltage AS 1  may fluctuate between 100V to 120V. 
         [0009]      FIG. 4  is a voltage waveform diagram of a plurality of different conventional rectified AC voltages. Referring to  FIG. 1  and  FIG. 4 , when the AC voltage AS 1  fluctuates between 100V to 120V, the rectified voltage DS 1  also fluctuates between curve C 1  and C 2  depicted in  FIG. 4 . Drastic fluctuations of the rectified voltage DS 1  result in substantial variation in the current flowing through the LEDs LED 1 -LEDN. A root mean square (RMS) current variation value of the current flowing through the LEDs LED 1 -LEDN can be calculated as (26.53/15.194)≅1.75 times. Consequently, this RMS current variation value causes substantial brightness variation and color temperature deviation for the LEDs LED 1 -LEDN. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 CHART 1 
               
             
             
               
                   
               
               
                 Laboratory Data in accordance with FIG. 1. 
               
             
          
           
               
                   
                 RMS Voltage 
                 RMS Current 
                 RMS Efficiency 
               
               
                 AC Voltage 
                 of LEDs LED1- 
                 of LEDs LED1- 
                 of LEDs LED1- 
               
               
                 AS1(V) 
                 LEDN(V) 
                 LEDN(mA) 
                 LEDN(%) 
               
               
                   
               
             
          
           
               
                 100 
                 88.854 
                 15.194 
                 83% 
               
               
                 110 
                 94.02 
                 20.73 
                 80% 
               
               
                 120 
                 98.72 
                 26.53 
                 78% 
               
               
                   
               
             
          
         
       
     
         [0010]    In light of the foregoing considerations, in the LED industry, overcoming problems such as LED brightness variation and color temperature deviation necessitates substantial improvements in LED current variation and voltage stability while under a plurality of supply voltage conditions. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention provides a LED circuit that prevents drastic fluctuations of current flowing through an LED. 
         [0012]    The present invention provides an LED circuit that includes an AC source, a rectifier, a voltage-limiting circuit, and an LED module. The AC source provides an AC voltage. The rectifier is coupled to the AC source, and the rectifier generates a first rectified voltage according to the AC voltage provided by the AC source. The voltage-limiting circuit is coupled to the rectifier, and the voltage-limiting circuit substantially restricts an upper limit of the first rectified voltage at a rated voltage. According to the first rectified voltage, the voltage-limiting circuit generates a second rectified voltage that is lower than the rated voltage. The LED module is coupled to the voltage-limiting circuit, and the LED module can receive the second rectified voltage. 
         [0013]    In one embodiment of the present invention, the LED circuit further includes a triac dimmer. The triac dimmer is coupled between the AC source and the rectifier. 
         [0014]    In one embodiment of the invention, the rectifier is a full-bridge rectifier. 
         [0015]    In one embodiment of the invention, the voltage-limiting circuit includes a transistor. A collector terminal of the transistor is coupled to an output terminal of the rectifier. An emitter terminal of the transistor is coupled to an input terminal of the LED module. A base terminal of the transistor is coupled to a voltage. 
         [0016]    In one embodiment of the present invention, the voltage-limiting circuit includes a transistor, a current-limiting resistor, and a Zener diode. The collector terminal of the transistor is coupled to the output terminal of the rectifier. The emitter terminal of the transistor is coupled to the input terminal of the LED module. A first terminal of the current-limiting resistor is coupled to the output terminal of the rectifier. A second terminal of the current-limiting resistor is coupled to the base terminal of the transistor. An anode terminal of the Zener diode is coupled to an output terminal of the LED module. A cathode terminal of the Zener diode is coupled to the base terminal of the transistor. 
         [0017]    Furthermore, in another embodiment of the present invention, the voltage-limiting circuit can further include a variable resistor. The variable resistor is coupled between the base terminal of the transistor and the cathode terminal of the Zener diode. In another embodiment of the present invention, the voltage-limiting circuit can further include a thermistor. The thermistor is coupled between the base terminal of the transistor and the cathode terminal of the Zener diode. 
         [0018]    In one embodiment of the present invention, the voltage-limiting circuit includes a plurality of transistors, a current-limiting resistor, and a Zener diode. A collector terminal of each of the transistors is coupled to the output terminal of the rectifier. An emitter terminal of each of the transistors is coupled to an input terminal of a plurality of sets of serially coupled LEDs in the LED module. The first terminal of the current-limiting resistor is coupled to the output terminal of the rectifier. The second terminal of the current-limiting resistor is coupled to a base terminal of each of the transistors. The anode terminal of the Zener diode is coupled to an output terminal of the each set of serially coupled LEDs. The cathode terminal of the Zener diode is coupled to the base terminal of each of the transistors. 
         [0019]    In one embodiment of the present invention, the LED module includes a resistor and a set of serially coupled LEDs. A first terminal of the resistor is coupled to the voltage-limiting circuit. An input terminal of the set of serially coupled LEDs is coupled to a second terminal of the resistor. An output terminal of the set of serially coupled LEDs is coupled to a voltage. 
         [0020]    In one embodiment of the present invention, the LED module includes a Zener diode and a set of serially coupled LEDs. The cathode terminal of the Zener diode is coupled to the voltage-limiting circuit. The input terminal of the set of serially coupled LEDs is coupled to the anode terminal of the Zener diode, and the output terminal of the set of serially coupled LEDs is coupled to a voltage. 
         [0021]    In one embodiment of the present invention, the LED module includes a current regulative diode (CRD) and a set of serially coupled LEDs. An anode terminal of the CRD is coupled to the voltage-limiting circuit. The input terminal of the set of serially coupled LEDs is coupled to a cathode terminal of the CRD. The output terminal of the set of serially coupled LEDs is coupled to a voltage. 
         [0022]    In one embodiment of the present invention, the LED module includes a variable resistor and a set of serially coupled LEDs. A first terminal of the variable resistor is coupled to the voltage-limiting circuit. The input terminal of the set of serially coupled LEDs is coupled to a second terminal of the variable resistor. The output terminal of the set of serially coupled LEDs is coupled to a voltage. 
         [0023]    In one embodiment of the present invention, the LED module includes a thermistor and a set of serially coupled LEDs. A first terminal of the thermistor is coupled to the voltage-limiting circuit. The input terminal of the set of serially coupled LEDs is coupled to a second terminal of the thermistor. The output terminal of the set of serially coupled LEDs is coupled to a voltage. 
         [0024]    In one embodiment of the present invention, the LED module includes a resistor, a set of serially coupled LEDs, and a field effect transistor. The first terminal of the resistor is coupled to the voltage-limiting circuit. The input terminal of the set of serially coupled LEDs is coupled to the second terminal of the resistor. A drain terminal of the field effect transistor is coupled to the output terminal of the set of serially coupled LEDs. A source terminal of the field effect transistor is coupled to a voltage. A gate terminal of the field effect transistor receives a pulse-width modulated (PWM) signal. 
         [0025]    In summary, embodiments of the present invention can provide a voltage-limiting circuit that substantially restricts an upper limit of a voltage provided to an LED module at a rated voltage. Consequently, drastic fluctuations of current flowing through the LED module are substantially reduced. Furthermore, LED brightness variation and color temperature deviation are substantially improved. 
         [0026]    To make the above features and advantages of the present invention more comprehensible, several embodiments accompanied with drawings are described in detail as follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
           [0028]      FIG. 1  is a schematic circuit diagram of a conventional LED circuit. 
           [0029]      FIG. 2  is a voltage waveform diagram of a conventional AC voltage. 
           [0030]      FIG. 3  is a voltage waveform diagram of a conventional rectified voltage. 
           [0031]      FIG. 4  is a voltage waveform diagram of a plurality of different conventional rectified AC voltages. 
           [0032]      FIG. 5  is a schematic circuit diagram of an LED circuit in accordance with the first embodiment of the present invention. 
           [0033]      FIG. 6  is a voltage waveform diagram of a plurality of different rectified AC voltages in accordance with the first embodiment of the present invention. 
           [0034]      FIG. 7  is a schematic circuit diagram of an LED circuit in accordance with the second embodiment of the present invention. 
           [0035]      FIG. 8  is a schematic circuit diagram of an LED circuit in accordance with the third embodiment of the present invention. 
           [0036]      FIG. 9  is a schematic circuit diagram of an LED circuit in accordance with the fourth embodiment of the present invention. 
           [0037]      FIG. 10  is a schematic circuit diagram depicting an LED circuit in accordance with the fifth embodiment of the present invention. 
           [0038]      FIG. 11  is a schematic circuit diagram depicting an LED circuit in accordance with the sixth embodiment of the present invention. 
           [0039]      FIG. 12  is a schematic circuit diagram depicting an LED circuit in accordance with the seventh embodiment of the present invention. 
           [0040]      FIG. 13  is a schematic circuit diagram depicting an LED circuit in accordance with the eighth embodiment of the present invention. 
           [0041]      FIG. 14  is a schematic circuit diagram depicting an LED circuit in accordance with the ninth embodiment of the present invention. 
           [0042]      FIG. 15  is a voltage waveform diagram of an AC voltage AS 2  when a triac dimmer depicted in  FIG. 14  is conductive for a full cycle. 
           [0043]      FIG. 16  is a voltage waveform diagram of a rectified voltage DS 4  when the triac dimmer depicted in  FIG. 14  is conductive for a full cycle. 
           [0044]      FIG. 17  is a voltage waveform diagram of the AC voltage AS 2  when the triac dimmer  70  depicted in  FIG. 14  is conductive for half a cycle. 
           [0045]      FIG. 18  is a voltage waveform diagram of the rectified voltage DS 4  when the triac dimmer depicted in  FIG. 14  is conductive for half a cycle. 
           [0046]      FIG. 19  is a voltage waveform diagram of the AC voltage AS 2  when the triac dimmer depicted in  FIG. 14  is conductive for a quarter of a cycle. 
           [0047]      FIG. 20  is a voltage waveform diagram of the rectified voltage DS 4  when the triac dimmer depicted in  FIG. 14  is conductive for a quarter of a cycle. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0048]      FIG. 5  is a schematic circuit diagram of an LED circuit in accordance with the first embodiment of the present invention. Referring to  FIG. 5 , in the present embodiment of the invention, an LED circuit  10  includes an AC source Vac, a rectifier BD 1 , a voltage-limiting circuit  30 , and an LED module  50 . The AC source Vac is coupled to the rectifier BD 1 , and the AC source Vac can provide an AC voltage AS 1  to the rectifier BD 1 . The rectifier BD 1  is coupled to the voltage-limiting circuit  30 . The rectifier BD 1  can be, for instance, a full-bridge rectifier that rectifies the AC voltage AS 1  in order to provide a rectified voltage to the voltage-limiting circuit  30  that is similar to a rectified voltage DS 1  depicted in  FIG. 3 . 
         [0049]    It should be noted that the voltage-limiting circuit  30  is coupled to the LED module  50 . The voltage-limiting circuit  30  can substantially restrict an upper limit for the rectified voltage DS 1  at a rated voltage, and according to the rectified voltage DS 1 , the voltage-limiting circuit  30  can also provide a rectified voltage DS 2  to the LED module  50  that is lower than the aforementioned rated voltage. 
         [0050]    In the present embodiment of the invention, the voltage-limiting circuit  30  includes a current-limiting resistor R 1 , a transistor Q 1 , and a Zener diode ZD 1 . An example of the current-limiting resistor R 1  is 10KΩ. The transistor Q 1  can be a bipolar junction transistor, for example. A reverse breakdown voltage of the Zener diode ZD 1  is 132V, for instance, but not limited. Therefore, the transistor Q 1  can provide a rectified voltage DS 2  of less than 131.3V to the LED module  50 . In other words, the rated voltage for the present embodiment of the invention is 131.3V. 
         [0051]    In the present embodiment of the invention, the LED module  50  can include a current-limiting resistor R 2  and the LEDs LED 1 -LEDN, in which the LEDs LED 1 -LEDN form a set of serially coupled LEDs. The current-limiting resistor R 2  is for instance  218 Q. For instance, N=34, although it should be noted that other embodiments of the present invention are not limited thereto, and thus the LEDs LED 1 -LEDN can be other quantities not described herein. 
         [0052]      FIG. 6  is a voltage waveform diagram of a plurality of different rectified AC voltages in accordance with the first embodiment of the present invention. Referring to  FIG. 5  and  FIG. 6 , when the AC voltage AS 1  fluctuates between 100V to 120V, the rectified voltage DS 2  vary between curve C 3  and curve C 4  depicted in  FIG. 6 . As shown in Chart 2 below, when the AC voltage AS 1  fluctuates between 100V to 120V, a RMS current variation value of current flowing through the LEDs LED 1 -LEDN is around (27.28/22.1) or 1.26 times, which is substantially smaller than the value found in Chart 1. It should be noted that in the embodiments of the present invention, the lower the reverse breakdown voltage of the Zener diode ZD 1 , the smaller the current varies when the input voltage AS 1  changes. However, the Zener diode ZD 1  having a substantially small reverse breakdown voltage may cause a substantially large energy loss, so the choice between current variation and energy loss can be weighed accordingly. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 CHART 5 
               
             
             
               
                   
               
               
                 Laboratory Data in accordance with FIG. 5. 
               
             
          
           
               
                   
                 RMS Voltage 
                 RMS Current 
                 RMS Efficiency 
               
               
                 AC Voltage 
                 of LEDs LED1- 
                 of LEDs LED1- 
                 of LEDs LED1- 
               
               
                 AS1(V) 
                 LEDN(V) 
                 LEDN(mA) 
                 LEDN(%) 
               
               
                   
               
             
          
           
               
                 100 
                 93.15 
                 22.1 
                 89% 
               
               
                 110 
                 96.96 
                 25.35 
                 83% 
               
               
                 120 
                 99.86 
                 27.78 
                 76% 
               
               
                   
               
             
          
         
       
     
         [0053]    Persons skilled in the art should know that LED brightness and current flowing through the LED are directly related. In the present embodiment of the invention, when the AC voltage AS 1  fluctuates between 100V to 120V, the RMS current variation value of a current flowing through the LEDs LED 1 -LEDN is around 1.26 times. Compared with conventional techniques, the present embodiment of the invention substantially improves upon problems such as LED brightness variation and color temperature deviation. 
         [0054]    Although the aforementioned embodiment has disclosed a possible type of an LED circuit, persons of ordinary skill in the art realize that different manufacturers may develop different designs of LED circuits, and the application of the present invention should not be limited to this type only. In other words, as long as the voltage-limiting circuit limits a voltage provided to an LED module at the rated voltage, these implementations do not depart from the spirit of the present invention. Some other embodiments are further discussed hereinafter to allow persons of ordinary skill in the art to comprehend and embody the present invention. 
         [0055]    In the aforementioned embodiment of the present invention, although the current-limiting resistors R 1  and R 2  and the Zener diode ZD 1  can each be implemented with a single component, the present invention is not limited thereto. In other embodiments of the present invention, the current-limiting resistors R 1  and R 2  and the Zener diode ZD 1  can be implemented with a plurality of components coupled in series or in parallel. Although the rectifier BD 1  is exemplified as a full-bridge rectifier, the present invention is not limited thereto. In other embodiments of the present invention, the rectifier BD 1  can be implemented in alternate means. For example, the rectifier BD 1  can be implemented with a half-bridge rectifier in combination with a capacitor. Even though the current-limiting resistor R 1  is exemplified as 10KΩ, and the current-limiting resistor R 2  is exemplified as 218Ω, but nevertheless, the present invention should not be construed as limited to the embodiments set forth herein. In other embodiments of the present invention, current-limiting resistors of different resistance values can be implemented to replace the current-limiting resistors R 1  and R 2 . 
         [0056]    In addition, although the rated voltage is set at 131.3V as an illustrative example, the present invention is not limited thereto. In other embodiments of the present invention, persons skilled in the art can alter the rated voltage as they see fit. For example, the reverse breakdown voltage of the Zener diode ZD 1  can be changed in order to vary the rated voltage. Another example can be changing a reference voltage provided to a base terminal of the transistor Q 1  in order to vary the rated voltage. 
         [0057]    More specifically, in the first embodiment of the present invention, the voltage-limiting circuit  30  and the LED module  50  depicted in  FIG. 5  represent only an exemplary embodiment, and the present invention is not limited thereto. In other embodiments of the present invention, persons skilled in the art can alter the implementations of the voltage-limiting circuit  30  and the LED module  50  as they see fit. 
         [0058]    For instance,  FIG. 7  is a schematic circuit diagram of an LED circuit in accordance with the second embodiment of the present invention. Referring now to  FIG. 5  and  FIG. 7 , an LED circuit  11  is similar to the LED circuit  10 . The differences between the two circuits reside in that LED circuit  11  has a structure having a plurality of parallel components, including a voltage-limiting circuit  31  and an LED module  51 . In the present embodiment of the invention, the voltage-limiting circuit  31  can include the current-limiting resistor R 1 , the Zener diode ZD 1 , and a plurality of transistors such as transistors Q 1  to Q 3  in the present embodiment. The LED module  51  can include a plurality of sets of serially coupled LEDs, such as three sets of serially coupled LEDs in the present embodiment of the invention. The first set of serially coupled LEDs can be formed by a current-limiting resistor R 2 - 1  and the LEDs LED 11 -LED 1 N. The second set of serially coupled LEDs can be formed by a current-limiting resistor R 2 - 2  and the LEDs LED 21 -LED 2 N. The third set of serially coupled LEDs can be formed by a current-limiting resistor R 2 - 3  and the LEDs LED 31 -LED 3 N. Therefore, a similar effect of the first embodiment is substantially achieved. 
         [0059]      FIG. 8  is a schematic circuit diagram of an LED circuit in accordance with the third embodiment of the present invention. Referring now to  FIG. 5  and  FIG. 8 , an LED circuit  12  is similar to the LED circuit  10 . The differences between the two circuits reside in a voltage-limiting circuit  32  of the LED circuit  12 . In the present embodiment of the invention, the voltage-limiting circuit  32  can include a transistor Q 1  that is coupled between the rectifier BD 1  and the LED module  50 . The base terminal of the transistor Q 1  is coupled to a reference voltage VCC. In addition, the brightness of the LEDs LED 1 -LEDN can be adjusted according to variations of the reference voltage VCC. 
         [0060]      FIG. 9  is a schematic circuit diagram of an LED circuit in accordance with the fourth embodiment of the present invention. Referring now to  FIG. 5  and  FIG. 9 , an LED circuit  13  is similar to the LED circuit  10 . The differences in the two circuits reside in an LED module  52 . In the present embodiment of the invention, the LED module  52  includes a current regulative diode (CRD) CRD 1  and the LEDs LED 1 -LEDN. CRD CRD 1  can replace current-limiting resistors. In other embodiments of the present invention, a plurality of CRDs can be implemented in a series or parallel configuration in order to maintain the current of the LED module  52  under a predetermined current level. The Zener diode ZD 1  can limit the voltage at a predetermined suitable range so that the voltage swing across the CRD is not too large so as to cause component burn out. 
         [0061]      FIG. 10  is a schematic circuit diagram of an LED circuit in accordance with the fifth embodiment of the present invention. Referring now to  FIG. 5  and  FIG. 10 , an LED circuit  14  is similar to the LED circuit  10 . The differences in the two circuits reside in an LED module  53 . In the present embodiment of the invention, the LED module  53  includes a Zener diode ZD 2  and the LEDs LED 1 -LEDN. The Zener diode ZD 2  can replace current-limiting resistors. If resistor R 2  is replaced by the Zener diode ZD 2 , the Zener diode ZD 2  can absorb leftover voltage swings on points A and B to prevent the LED module  53  from suffering component burn out. 
         [0062]      FIG. 11  is a schematic circuit diagram of an LED circuit in accordance with the sixth embodiment of the present invention. Referring now to  FIG. 5  and  FIG. 11 , an LED circuit  15  is similar to the LED circuit  10 . The differences in the two circuits reside in an LED module  54 . In the present embodiment of the invention, the LED module  54  includes a variable resistor VR 1  and the LEDs LED 1 -LEDN. Variable resistor VR 1  can be used as a current-limiting resistor. In addition, by adjusting the variable resistor VR 1 , the brightness of the LEDs LED 1 -LEDN can be changed. Therefore, the present embodiment not only achieves a similar effect of the first embodiment of the present invention, but the present embodiment also adds a capability of adjusting the brightness of the LEDs LED 1 -LEDN. 
         [0063]    It should be noted that in another embodiment of the present invention, the variable resistor VR 1  depicted in  FIG. 11  can also be replaced by a thermistor. Therefore, the present embodiment not only achieves a similar effect of the first embodiment of the present invention, but the present embodiment also adds a capability of adjusting the brightness of the LEDs LED 1 -LEDN according to an environmental temperature. 
         [0064]      FIG. 12  is a schematic circuit diagram of an LED circuit in accordance with the seventh embodiment of the present invention. Referring now to  FIG. 5  and  FIG. 12 , an LED circuit  16  is similar to the LED circuit  10 . The differences between the two circuits reside in a voltage-limiting circuit  33 . In the present embodiment of the invention, the voltage-limiting circuit  33  includes the variable resistor VR 1 , the current-limiting resistor R 1 , the transistor Q 1 , and the Zener diode ZD 1 . By adjusting the variable resistor VR 1 , the voltage at the base terminal of the transistor Q 1  can be changed accordingly, and therefore the brightness of the LEDs LED 1 -LEDN can be adjusted. Therefore, the present embodiment not only achieves a similar effect of the first embodiment of the present invention, but the present embodiment also adds a capability of adjusting the brightness of the LEDs LED 1 -LEDN. 
         [0065]    It should be noted that in another embodiment of the present invention, the variable resistor VR 1  depicted in  FIG. 12  can also be replaced by a thermistor. Therefore, the present embodiment not only achieves a similar effect of the first embodiment of the present invention, but the present embodiment also adds a capability of adjusting the brightness of the LEDs LED 1 -LEDN according to an environmental temperature. 
         [0066]      FIG. 13  is a schematic circuit diagram of an LED circuit in accordance with the eighth embodiment of the present invention. Referring now to  FIG. 5  and  FIG. 13 , an LED circuit  17  is similar to the LED circuit  10 . The differences in the two circuits reside in an LED module  55 . In the present embodiment of the invention, the LED module  55  includes a transistor Q 4 , a current-limiting resistor R 2 , and the LEDs LED 1 -LEDN. A gate terminal of the transistor Q 4  can receive a pulse-width modulated (PWM) signal. The brightness of the LEDs LED 1 -LEDN can be adjusted according to changes in the cycle of the PWM signal. Therefore, the present embodiment not only achieves a similar effect of the first embodiment of the present invention, but the present embodiment also adds a capability of adjusting the brightness of the LEDs LED 1 -LEDN. 
         [0067]      FIG. 14  is a schematic circuit diagram of an LED circuit in accordance with the ninth embodiment of the present invention. Referring now to  FIG. 5  and  FIG. 14 , an LED circuit  18  is similar to the LED circuit  10 . The differences between the two circuits reside in that the LED circuit  18  further includes a triac dimmer  70 . The triac dimmer  70  is coupled between the AC source Vac and the rectifier BD 1 . Therefore, the present embodiment of the invention can provide adjustments to the brightness of the LEDs LED 1 -LEDN.  FIG. 15  is a voltage waveform diagram of the AC voltage AS 2  when the triac dimmer  70  depicted in  FIG. 14  is conductive for a full cycle.  FIG. 16  is a voltage waveform diagram of a rectified voltage DS 4  when the triac dimmer  70  depicted in  FIG. 14  is conductive for a full cycle.  FIG. 17  is a voltage waveform diagram of the AC voltage AS 2  when the triac dimmer  70  depicted in  FIG. 14  is conductive for half a cycle.  FIG. 18  is a voltage waveform diagram of the rectified voltage DS 4  when the triac dimmer  70  depicted in  FIG. 14  is conductive for half a cycle.  FIG. 19  is a voltage waveform diagram of the AC voltage AS 2  when the triac dimmer  70  depicted in  FIG. 14  is conductive for a quarter of a cycle.  FIG. 20  is a voltage waveform diagram of the rectified voltage DS 4  when the triac dimmer  70  depicted in  FIG. 14  is conductive for a quarter of a cycle. 
         [0068]    Comparing the voltage waveform diagrams from  FIG. 15  to  FIG. 20 , the figures show that the AC voltage AS 2  is adjusted by varying the triac dimmer  70  from conducting in a full cycle to conducting in a quarter cycle. Therefore, the present embodiment of the invention uses the triac dimmer  70  to make adjustments to the AC voltage AS 2  in order to vary the brightness of the LEDs LED 1 -LEDN. 
         [0069]    According to the above descriptions, embodiments of the present invention can implement a voltage-limiting circuit to substantially restrict an upper limit of a voltage provided to an LED module at a rated voltage, and thereby preventing drastic fluctuations of current flowing through the LED modules. Embodiments of the present invention can also include the following features: 
         [0070]    1. A variable resistor can be added to the voltage-limiting circuit or the LED module in order to adjust the LED brightness. 
         [0071]    2. A thermistor can be added to the voltage-limiting circuit or the LED module in order to adjust the LED brightness. 
         [0072]    3. A transistor can be added to the LED module in order to adjust the LED brightness according to a PWM signal. 
         [0073]    4. A triac dimmer can be added in an LED circuit in order to adjust the LED brightness. 
         [0074]    5. A transistor can be added to the LED module, and the base terminal of the transistor can be coupled to a reference voltage in order to adjust the LED brightness according to the changes in the reference voltage. 
         [0075]    Although the present invention has been described with reference to the above embodiments, it will be apparent to one of ordinary skill in the art that modifications to the described embodiments may be made without departing from the spirit of the invention. Accordingly, the scope of the invention will be defined by the attached claims and not by the above detailed descriptions.