Capacitive Full-Wave Circuit for LED Light Strings

[0001] This application is based upon and claims the benefit of U.S. provisional application No. 61/111,253, entitled “A Capacitive Full-Wave Circuit for LED Light Strings,” filed Nov. 4, 2008, the entire disclosure of which is herein specifically incorporated by reference for all that it discloses and teaches.

[0002] LEDs have been widely used for decorative lighting purposes because of their low cost, low electrical power consumption, and long life time. For example, LED light strings have taken the place of many conventional incandescent light strings.

[0003] Different from conventional incandescent bulbs that are not sensitive to the electrical current direction, LEDs work only with current in a specific (positive) direction. Therefore, if an LED light string is connected to the household AC power supply directly, the LED bulbs emit light only during the positive half waves and the LEDs produce a 60 Hz glittering. A standard solution to this problem is to insert an AC-DC converter between the AC power supply and the LED string. As implemented by most of existing LED strings, such an AC-DC converter typically takes a four-diode full-wave rectifying circuit.

[0004] An embodiment of the present invention may comprise a capacitive full wave LED light string that is connected to an AC power sources having a first power lead and a second power lead comprising: an LED string module; a first adapter comprising: a first capacitor having a first lead connected to the first power lead of the AC power source and a second lead connected to a first end of the LED string module, the first capacitor having a capacitance that creates an impedance between the first power lead and the LED string module that is sufficient to limit current through the LED string module to prevent damage to the LED string module and cause a positive voltage to be applied to the LED string module whenever a voltage level on the first power lead exceeds a voltage level on the second power lead; a first diode having a cathode connected to the first end of the LED string module and an anode connected to the second power lead; a second adapter comprising: a second capacitor having a first lead connected to the first power lead and a second lead connected to a second end of the LED string module, the second capacitor having a capacitance that creates an impedance between the first power lead and the LED string module that is sufficient to limit current through the LED string module to prevent damage to the LED string module and cause a positive voltage to be applied to the LED string module whenever the voltage level on the second power lead exceeds the voltage level on the first power lead; a second diode having a cathode that is connected to the second power lead and an anode that is connected to the second end of LED string module.

[0005] An embodiment of the present invention may further comprise a method of full wave rectification of an AC power source for application to an LED light string module and minimizing power loss comprising: connecting a first lead of a first capacitor to a first power lead of the AC power source; connecting a second lead of the first capacitor to a first end of the LED string module; connecting an anode of the first diode to a second power lead of the AC power source; connecting a cathode of the first diode to the second lead of the first capacitor and the first end of the LED string module; connecting a first lead of a second capacitor to the first power lead of the AC power source; connecting a second lead of the second capacitor to an anode of a second diode and a second end of the LED light string module; connecting an anode of the second diode to the second power lead of the AC power source; selecting a capacitance for the first capacitor that creates an impedance between the first power lead and the LED light string module that is sufficient to limit current through the LED light string module to prevent damage to the LED light string module and cause a positive voltage to be applied to the LED light string module whenever a voltage level on the first power lead exceeds a voltage level on the second power lead; selecting a capacitance for the second capacitor that creates an impedance between the first power lead and the LED light string module that is sufficient to limit current through the LED light string module to prevent damage to the LED light string module and cause a positive voltage to be applied to the LED light string module whenever the voltage level on the second power lead exceeds the voltage level on the first power lead.

[0010] FIG. 1 is a schematic diagram of an embodiment of a capacitive full-wave LED string 100. Existing LED light strings are typically driven by a voltage power supply. Therefore, resistors are often used to match the LEDs operating voltage. One common problem with such LED strings is that one must use different resistors for strings with a different number of LEDs. The resistors often consume as much electrical power as the LEDs. Moreover, if one LED fails, it will affect the rest of LEDs. Therefore, a constant current power supply is the best driver for a LED string. However, current power supplies are not widely used for light strings because of the high cost associated with these current power supplies.

[0011] Instead of using four diodes in a traditional full-wave rectifying circuit, the embodiment of FIG. 1 uses two capacitors and two diodes to produce full-wave rectification of an AC power signal for driving a LED string. The capacitive circuit 100 not only dramatically reduces electrical power consumption of the light string, but also provides a nearly constant average current, that is not sensitive to loads, that drives the LED string.

[0012] As shown in FIG. 1, the capacitive full-wave LED string 100 comprises three parts: a front adapter 101, a back adapter 102, and a paired LED string 103. The front adapter 101 comprises two input AC ports 104 and 105, a capacitor 106, and a diode 107. The back adapter comprises two output AC ports 108 and 109, a capacitor 110, and a diode 111. The LED string 103 comprises a string of parallel connected LEDs 112.

[0013] In operation, during the positive half wave of the AC inputs (i.e., Ua>Ub), the electrical current follows the path from ports 104/108, to capacitor 106, to LED string 103, to diode 111, to ports 105/109. During the negative half wave of the AC inputs (i.e., Ua<Ub), the electrical current follows the path from ports 105/109, to diode 107, to LED string 103, to capacitor 110, to ports 104/108.

[0014] The charging and discharging capability of capacitors make it possible for AC current to pass through the circuit. Therefore, a positive full wave is always applied to LED string 103. The capacitors 106, 110 also work as a voltage divider to match the LED operating voltages which is usually much lower than the AC household voltage so that resistors are not needed to limit the current to the LED string 103. Because capacitors have only imaginary impedance, the capacitors consume a very small amount of electrical power, unlike resistors. Moreover, because of the large impedance of capacitors at low frequencies, the slowly varying current component through the LEDs is not sensitive to the number of LEDs. Therefore, the same front adapter 101 and back adapter 102 can be used for strings with different numbers of LEDs without any modification.

[0015] FIG. 1A is another embodiment that is similar to the embodiment of FIG. 1. As shown in FIG. 1A, a full-wave capacitive LED string 120 is shown that includes a front adapter 122, an LED string 124 and a back adapter 126. AC input ports 128, 130 are connected to the front adapter 122. Similarly, AC input ports 130, 132 are connected to back adapter 126. Front adapter 122 includes an inductor 134 that is connected to the AC input port 128 and capacitor 136. The cathode of diode 138 is connected to the capacitor 136 and to the LED string 124. The anode of the diode 138 is connected to the AC input port 130. Back adapter 126 includes a series connected inductor 140 and capacitor 142. One side of the inductor 140 is connected to the AC input port 130, while the other side of the inductor 140 is connected to capacitor 142. An anode of diode 144 is connected to the capacitor 142 and to a second end of the LED string 124. The cathode of diode 144 is connected to the AC input port 132.

[0016] The operation of the capacitive full-wave LED string 120, illustrated in FIG. 1A, is similar to the operation of the capacitive full-wave LED string 100, illustrated in FIG. 1. However, inductors 134, 140 have been added to limit current spikes. Current spikes can occur during the charging and discharging of the capacitors 136, 142. Inductors 134, 140 are made sufficiently large to limit the current spikes that may occur as a result of the charging and discharging of the capacitors 136, 142. The advantage of the embodiment illustrated in FIG. 1A is that the inductors 134, 140 and capacitors 136, 142 primarily exhibit imaginary impedance, such that energy is stored in these components. As a result, very little energy is consumed in the front adapter 122 and back adapter 126. As a result, the temperature in the front adapter 122 and back adapter 126 is minimized, which allows the capacitive full-wave LED string 120 to be used in both a safe and efficient manner.

[0017] FIG. 2 is a schematic diagram of another embodiment of a capacitive full-wave LED string. The electrical performance of the embodiment in FIG. 1 can be modified by the addition of resistors 208, 209, 214, 215, as shown in FIG. 2. The capacitive full-wave LED string 200, illustrated in FIG. 2, comprises three parts: a front adapter 201, a back adapter 202, and a LED string 203. The front adapter 201 comprises two input AC ports 204 and 205, a capacitor 206, a diode 207, a first resistor 208, and a second resistor 209. The back adapter comprises two output AC ports 210 and 211, a capacitor 212, a diode 213, a first resistor 214, and a second resistor 215. The LED string 103 comprises a string of paired LEDs 112.

[0018] During the positive half wave of the AC input (i.e., Ua>Ub), the electrical current follows the path from ports 204/210, to first resistor 208, to capacitor 206/resistor 209, to LEDs 203, to diode 213, to ports 205/211. During the negative half wave of the AC input (i.e., Ua<Ub), the electrical current follows the path from ports 205/211, to diode 207, to LED string 203, to capacitor 212/resistor 215, to resistor 214, to ports 204/210. Hence, a positive full wave signal is always applied to LED string 203.

[0019] The principle of operation of the circuit of FIG. 2 is similar to FIG. 1. In the front adapter 201, the resistor 209 is connected in parallel to the capacitor 206. The parallel connection is then connected in series to the resistor 208. In the back adapter 202, the resistor 215 is connected in parallel to the capacitor 212. The parallel connection is then connected in series to the resistor 214. The resistors 208 and 214 are used to limit the charging current of the capacitors during charging periods of the capacitors 206, 212, and the resistors 209 and 215 are used to provide discharging loops for the capacitors 206, 212 during discharging periods. Use of these additional resistors can reduce the charging and discharging current spikes that can occur at the LEDs with, however, some additional power consumption by the added resistors. A series connected inductor can also be used to limit current spikes.

[0020] FIG. 3 shows an embodiment of the invented capacitive full-wave LED string module implemented with circuits such as illustrated in FIGS. 1 and 2. A capacitive full-wave LED string 300 comprises a front power plug 301, a back power socket 302, a LED string module 303, two power wires 306 and 307 connecting the front power plug 301 and the back power socket 302, an intermediate wire 308 connecting the front power plug 301 and the front part of the LED string module 303, and an intermediate wire 309 connecting the back part of the LED string module 303 to the back power socket 302. The front power plug 301 comprises two AC power line connectors 304 and 305, a capacitive front adapter circuit (101 or 201) (not shown in FIG. 3) that is embedded in plug 301, and three output wires 306, 307 and 308. The back power socket 302 comprises two AC power line connectors 310 and 311, a capacitive back adapter circuit (102 or 202) (not shown in FIG. 3) that is embedded in socket 302, and three input wires 306, 307 and 309.

[0021] FIG. 4 shows a second embodiment of a capacitive full-wave string. In this embodiment, the capacitive front and back adapter circuits are not integrated and packaged into the front power plug 401 and back power socket 402, such as shown in FIG. 3. Standard power plug 401 is connected to a front adapter 403. Socket 402 is connected to adapter 404. The front adapter 403 and back adapter 404 are two independent units from power plug 401 and power socket 402. The front adapter 403 is connected between the front power plug 401 and the front part of the LED string module 406. The back adapter 404 is connected between the back power socket 402 and the back part of the LED string module 406.

[0022] Hence, a capacitive full-wave electrical circuit for LED light strings is disclosed that makes use of the charging and discharging capability of capacitors. Two capacitors and two diodes are used to realize full-wave rectification to drive a LED string load. A capacitive full-wave LED light string comprises a capacitor and a diode that may or may not be embedded in a front power plug (FIG. 3), a capacitor and a diode that may or may not be embedded in a back power socket (FIGS. 3), and a LED string between the front plug and back socket. The capacitor/diode combination can be included in a module that is separate from the plug/socket. Full-wave rectification is produced by the capacitors and diodes. Besides the full-wave rectification function, the two capacitors have two more advantages. First, compared to a conventional resistor voltage divider circuit, the capacitors consume an extremely small amount of electrical power because of their imaginary impendence. Second, because of the large imaginary impendence of the capacitors, the current is not sensitive to the number of LEDs in the string and therefore the disclosed embodiments can be used with strings having different numbers of LEDs. The electrical current-voltage performance can be further modified by introducing four resistors with a cost of some additional power consumption.

[0023] The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.