Patent Publication Number: US-8970118-B2

Title: LED lighting tube

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Taiwan patent application No. 102101453, filed on Jan. 15, 2013, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an LED lighting tube and, more particularly, to an LED lighting tube compatible with an electronic ballast having a preheat current. 
     2. Description of Related Art 
     With reference to  FIG. 2 , a conventional fluorescent tube  60  is a gas-discharge lamp, and, accordingly, filaments  62  mounted in a tube  61  of the fluorescent tube  60  has to be heated to ionize gas in the tube  61  before the fluorescent tube  60  begins to glow. 
     In general, the fluorescent tube  60  has a first terminal L and a second terminal N respectively mounted on two ends of the fluorescent tube  60  and respectively connected to AC output terminals of an electronic ballast  70 . The first terminal L has two electrode pins L 1 , L 2  and the second terminal N has two electrode pins N 1 , N 2 . Two filaments  62  are mounted in the tube  61  and respectively connected in series between the electrode pins L 1 , L 2  and between the electrode pins N 1 , N 2 . When the fluorescent tube  60  connected to a rapid start ballast or a program start ballast is turned on, the ballast produces preheat currents to the first terminal L and the second terminal N. After the preheat currents flow through and heat the filaments  62  to ionize the gas in the tube  61 , the fluorescent tube  60  starts to glow. 
     Currently, an LED lighting tube has a structure of the first terminal L and the second terminal N of the fluorescent tube  60  in order to be compatible with a conventional fluorescent tube holder. An LED unit is mounted in the LED lighting tube and is used as a light source of the LED lighting tube. Two ends of the LED unit are respectively connected to the first terminal L and the second terminal N to obtain a power supply. However, the LED lighting tube does not have a structure of the filaments  62 , such that there is no impedance (filaments  62 ) between the electrode pins L 1 , L 2  of the first terminal L, and between the electrode pins N 1 , N 2  of the second terminal N. Therefore, if the LED lighting tube is directly mounted in a fluorescent tube holder having a rapid start ballast or a program start ballast, the electronic ballast  70  produces short-circuit currents respectively to the electrode pins L 1 , L 2  and to the electrode pins N 1 , N 2  when the LED lighting tube is switched on. The electronic ballast  70  and the LED lighting tube may be damaged due to the short-circuit currents. 
     SUMMARY OF THE INVENTION 
     The main objective of the invention is to provide an LED lighting tube compatible with an electronic ballast having a preheat current. 
     The LED lighting tube comprises: 
     two terminals, with each terminal having two electrode pins; 
     two snubber circuits respectively connected to the two terminals, with two input terminals of each snubber circuit respectively connected to two electrode pins of a corresponding terminal, with each snubber circuit having at least one resistor connected in series between the two electrode pins of the corresponding terminal to avoid a short circuit due to no impedance between the two electrode pins; 
     a waveform conversion circuit having multiple rectifier diodes, with input terminals of the waveform conversion circuit respectively connected to the output terminals of the snubber circuits, and with a recovery time of each rectifier diode being under 2.5 us; and 
     at least one LED light string, with two ends of the at least one LED light string respectively connected to output terminals of the waveform conversion circuit, and with each one of the at least one LED light string comprising multiple LED units connected in series. 
     A user mounts the LED lighting tube in a conventional fluorescent tube holder and turns on the power. Due to the snubber circuits, at least one resistor is connected in series between two electrode pins of each terminal. An electronic ballast having a preheat current regards the at least one resistor as a filament of a conventional fluorescent tube. That is, the at least one resistor simulates the filament of the fluorescent tube. The preheat current flows through the at least one resistor. Therefore, the preheat current is restricted by the at least one resistor, and there is an impedance between two electrode pins of each terminal to avoid a short-circuit current damaging the electronic ballast and the LED lighting tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a circuit diagram of a first embodiment of an LED lighting tube in accordance with the present invention; 
         FIG. 1B  is a circuit diagram of a second embodiment of an LED lighting tube in accordance with the present invention; 
         FIG. 1C  is a circuit diagram of a third embodiment of an LED lighting tube in accordance with the present invention; and 
         FIG. 2  is a circuit diagram of a conventional fluorescent tube. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIG. 1A , an input terminal of an electronic ballast  10  is connected to an AC power (60 HZ). The electronic ballast  10  has four AC output terminals respectively connected to electrode pins L 1 , L 2 , N 1 , N 2  of a first terminal L and a second terminal N. 
     An LED lighting tube in accordance with the present invention has a first snubber circuit  20 , a second snubber circuit  30 , a waveform conversion circuit  40 , and an LED light string  50 . 
     Two input terminals of the first snubber circuit  20  are respectively connected to the electrode pins L 1 , L 2 , and an output terminal of the first snubber circuit  20  is connected to an input terminal of the waveform conversion circuit  40 . In a preferred embodiment, the first snubber circuit  20  has a resistor  21 , and a resistance value of the resistor  21  equals a resistance value of a filament before being heated. Two ends of the resistor  21  are respectively connected to the electrode pins L 1 , L 2  of the first terminal L. That is, the resistor  21  is connected in series between the electrode pins L 1 , L 2  to avoid a short-circuit current due to no impedance between the electrode pins L 1 , L 2 . Furthermore, each input terminal of the first snubber circuit  20  is also the output terminal of the first snubber circuit  20 . In addition, in another preferred embodiment as shown in  FIG. 1B , the first snubber circuit  20 ′ has two resistors  21 ′ connected in series. A sum of resistance values of the two resistors  21 ′ equals a resistance value of a filament before being heated. Two ends of the connected resistors  21 ′ are the input terminals of the first snubber circuit  20 ′, and a connected node of the resistors  21 ′ is the output terminal of the first snubber circuit  20 ′. The resistors  21 ′ can also avoid the short-circuit current due to no impedance between the electrode pins L 1 , L 2 . In addition, in another preferred embodiment as shown in  FIG. 1C , the first snubber circuit  20 ″ has multiple resistor circuits  201 ″, and each resistor circuit  201 ″ has two resistors  21 ″ connected in series between the electrode pins L 1 , L 2 . That is, the resistor circuits  201 ″ are connected in parallel, and the resistors  21 ″ are connected in series and parallel. A connected node between the electrode pin L 1  and the resistor circuits  201 ″ is an input end of the first snubber circuit  20 ″, and a connected node between the electrode pin L 2  and the resistor circuits  201 ″ is another input end of the first snubber circuit  20 ″. Furthermore, a connected node between the two resistors  21 ″ of each resistor circuit  201 ″ is the output terminal of the first snubber circuit  20 ″. A sum of resistance values of the resistors  21 ″ equals a resistance value of a filament before being heated. The first snubber circuit  20 ″ has the resistors  21 ″ connected in series and parallel to increase a heat dissipation area of the first snubber circuit  20 ″. 
     Input terminals of the second snubber circuit  30  are respectively connected to the electrode pins N 1 , N 2  of the second terminal N, and an output terminal of the second snubber circuit  30  is connected to an input terminal of the waveform conversion circuit  40 . In a preferred embodiment, the second snubber circuit  30  has a resistor  31 , and a resistance value of the resistor  31  equals a resistance value of a filament before being heated. Two ends of the resistor  31  are respectively connected to the electrode pins N 1 , N 2  of the second terminal N. That is, the resistor  31  is connected in series between the electrode pins N 1 , N 2  to avoid a short-circuit current due to no impedance between the electrode pins N 1 , N 2 . Furthermore, each end of the input terminals of the second snubber circuit  30  is also the output terminal of the second snubber circuit  30 . In addition, in another preferred embodiment as shown in  FIG. 1B , the second snubber circuit  30 ′ has two resistors  31 ′ connected in series. A sum of resistance values of the two resistors  31 ′ equals a resistance value of a filament before being heated. Two ends of the connected resistors  31 ′ are the input terminals of the second snubber circuit  30 ′, and a connected node of the resistors  31 ′ is the output terminal of the second snubber circuit  30 ′. The resistors  31 ′ can also avoid the short-circuit current due to no impedance between the electrode pins N 1 , N 2 . In addition, in another preferred embodiment as shown in  FIG. 1C , the second snubber circuit  30 ″ has multiple resistor circuits  301 ″, and each resistor circuit  301 ″ has two resistors  31 ″ connected in series between the electrode pins N 1 , N 2 . That is, the resistor circuits  301 ″ are connected in parallel, and the at least two resistors  31 ″ are connected in series and parallel. A connected node between the electrode pin N 1  and the resistor circuits  301 ″ is an input end of the second snubber circuit  30 ″, and a connected node between the electrode pin N 2  and the resistor circuits  301 ″ is another input end of the second snubber circuit  30 ″. Furthermore, each input terminal of the second snubber circuit  30 ″ is also the output terminal of the second snubber circuit  30 ″. A sum of resistance values of the at least two resistors  31 ″ equals a resistance value of a filament before being heated. The second snubber circuit  30 ″ has the at least two resistors  31 ″ connected in series and parallel to increase a heat dissipation area of the second snubber circuit  30 ″. 
     The waveform conversion circuit  40  has multiple rectifier diodes  41  to form a half-bridge circuit or a full-bridge rectifier circuit. Two input terminals of the waveform conversion circuit  40  are respectively connected to the output terminal of the first snubber circuit  20  and the output terminal of the second snubber circuit  30 . The recovery time of each rectifier diode  41  is under 2.5 us, which is obtained by taking a frequency of the electronic ballast  10 , which is 40 KHZ, as a basis and taking 1/10 of a cycle of the electronic ballast  10  to avoid overheating. A preferred rectifier diode  41  has a recovery time under 1 us, or the recovery time is 0.2 us. 
     Two ends of the LED light string  50  are respectively connected to two output terminals of the waveform conversion circuit  40 . The LED light string  50  has multiple LED units  51 . A capacitor  52  is connected in parallel with the LED light string  50 . The capacitor  52  is charged when the AC power goes up from low voltage to high voltage, and then releases the charge when the AC power goes down from high voltage to low voltage to solve a problem of stroboscopic effect when the LED light string  50  starts to glow. In general, the frequency of the stroboscopic effect is high (20 KHZ to 40 KHZ), and the stroboscopic effect can be eliminated by the capacitor  52 . A preferred capacitance of the capacitor  52  is between 1 uF and 20 uF due to the high frequency of the electronic ballast  10 . Only a low capacitance of the capacitor  52  is needed. The capacitor is not necessary if a high frequency flicker can be tolerated. 
     A user mounts the LED lighting tube in a conventional fluorescent tube holder and turns on the power. With the first snubber circuit  20 ,  20 ′,  20 ″, at least one resistor  21 ,  21 ′ is connected in series between the electrode pins L 1 , L 2  of the first terminal L. With the second snubber circuit  30 ,  30 ′,  30 ″, at least one resistor  31 ,  31 ′,  31 ″ is connected in series between the electrode pins N 1 , N 2  of the second terminal N. The electronic ballast  10  having a preheat current regards the resistors  21 ,  21 ′,  21 ″,  31 ,  31 ′,  31 ″ as filaments of a conventional fluorescent tube. That is, the resistors  21 ,  21 ′,  21 ″,  31 ,  31 ′,  31 ″ simulate the filaments of the fluorescent tube. The preheat current of the electronic ballast  10  flows through the resistors  21 ,  21 ′,  21 ″,  31 ,  31 ′,  31 ″ connected to the first terminal L and the second terminal N. The preheat current is restricted by the resistors  21 ,  21 ′  21 ″,  31 ,  31 ′,  31 ″ to avoid a short-circuit current damaging the electronic ballast  10  and the LED lighting tube due to no impedance in the first terminal L and the second terminal N. 
     Even though the resistors  21 ,  21 ′,  31 ,  31 ′ are drawn as a lumped resistor, in practice each individual resistor can be made of a string of resistors in parallel or in series, as long as the sum of the resistor string equal the desired value. To distribute the resistor as a string of parallel or series resistors can dissipate the heat more evenly in a large area. 
     In conclusion, principles of the first embodiment, the second embodiment and the third embodiment are almost the same. The three embodiments of the LED lighting tube in accordance with the present invention both avoid a generation of a short-circuit current between the electrode pins L 1 , L 2 , N 1 , N 2 . In addition, the embodiments also have a rectifying effect. 
     Even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and features of the invention, the disclosure is illustrative only. Changes may be made in the details, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.