Abstract:
A plug is diclosed that is adapted to be inserted into a standard electrical outlet and has a built-in integrated circuit for rectifying the alternating current received from the outlet and limiting the direct current produced. The plug is specifically suited for low-current, low-voltage applications such as supplying DC to a string of ornamental lights. The integrated circuit has a diode with a modified cathode that acts as a fuse as it is rectifying the alternating current. The cathode is formed of a plurality of electrically parallel conductors. Excessive current blows out at least some of these conductors in the same way a conventional fuse blows. Various embodiments add other current-limiting elements, elements for producing full wave or near full wave rectification, and for opening the circuit when they detect extraneous currents.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention: 
     The present invention relates to small wattage electrical power needs such as ornamental light strings for decorating trees. In particular, the present invention relates to integrated circuits for rectifying and limiting current from an AC source. 
     2. Discussion of Background: 
     Strings of individual, small wattage lights are widely used to decorate Christmas trees. Although these types of decorations are enjoyed by many people, they are sometimes a fire hazard, resulting in tragedies every Christmas holiday season. Most of these light strings draw a current of at least 200 mA at 120 VAC. Because of the concern over safety, these light strings may be made with heavier gauge wire, 22 gauge rather than 24. 
     In addition to Christmas tree light strings, there are other applications for electrical power for devices that have low power needs and especially those that use direct current (DC) rather than alternating current (AC), such as many devices that use integrated circuits, such as calculators or devices that use light-emitting diodes. AC or large voltage surges can ruin these types of devices. 
     There have been some attempts to make Christmas tree decorations safer. Many of these involve putting fire or smoke detectors in ornaments. An example of this can be found in U.S. Pat. No. 4,075,614 issued to White and in U.S. Pat. No. 5,396,221 issued to Bridges. Fuses have been incorporated into the plugs of light strings, as taught for example by Cheng in U.S. Pat. Nos. 4,227,228 and Lehmann et al in 3,968,398. However, none of these address all of the problems with these types of light strings. 
     Smith et al, in U.S. Pat. No. 4,675,575, describe a light string based on lightemitting diodes (LEDs) which can be driven by pulsed DC power, which in turn is powered by household AC voltage. The pulsed DC is produced by a silicon controlled rectifier from the AC input. 
     DC is not unknown in connection with Christmas tree lighting using more conventional type incandescent bulbs. See Wu&#39;s stages selection type Christmas tree light controller circuit in U.S. Pat. No. 5,345,147, and Kramer&#39;s decorative lighting system in U.S. Pat. No. 3,789,211. However, in both cases the DC power is used only tangentially in achieving a particular lighting effect and not to power the bulbs themselves. 
     Turning briefly to the figures, FIGS. 1A, 1B and 1C illustrate graphs of voltages versus time, with voltage represented by the vertical coordinate and time represented by the horizontal coordinate. FIG. 1A illustrates an alternating current (AC), meaning that the current reverses the direction of flow. So-called household current varies between approximately ±60 volts and does so at approximately 60 times per second. Direct current (DC) does not change direction, although it may change amplitude. FIGS. 1B and 1C both illustrate DC; FIG. 1B shows half-wave DC and Fig. 1C shows full wave DC. Both also illustrate pulsed DC. 
     It is known to generate half-wave and full wave DC from AC by devices called converters or rectifiers. Devices for rectifying AC to DC are sometimes based on diodes, which are solid state circuit elements that conduct current in the forward direction but not in the reverse direction. For half-wave rectification, as illustrated in FIG. 1B, a single diode can exclude negative voltages, leaving only positive voltages. For full-wave rectification, two or four diodes are used. 
     Some of the patents identified above employ diodes in various functions including in relation to power sources, such as disclosed by Smith et al. However, there remains a need for a better way to power low wattage devices and Christmas tree light strings in particular. 
     SUMMARY OF THE INVENTION 
     According to its preferred embodiments and briefly described, the present invention is a plug, attachable to an electrical outlet and containing an integrated circuit designed to yield a DC output to a low-current, low-voltage load such as, for example, a light string that might be used for decorating a Christmas tree. Within the plug, the novel and non-obvious elements and combinations of elements provide significant and, in some embodiments, redundant protection from current surges and current leakage while reducing energy consumption. Moreover, the resulting light is whiter than typical incandescent bulbs illuminated by AC. 
     A major advantage of the present invention is that it operates on a much lower current, typically one fifth the amperage of conventional light strings. This advantage results in reduced operating temperature for which translates into significant cost savings when the vast number of these types of light strings in use is considered. Notwithstanding the lower current requirements, the quality of light is improved and appears to be whiter light. 
     Another important advantage of the present invention is that the simplicity of the circuits translates into low manufacturing costs, in fact, significantly lower costs than conventional strings with heavier gauge wire. 
     Still other features of the present invention are certain circuit components that are included for safety reasons, for example, a special diode, to be described more fully herein, but which, simply put, operates like a fuse. Also, the present circuit includes special current sensing devices that open the circuit to shut down the lights if an over-current condition should arise from some internal and/or extraneous load condition, and diodes that prevent AC from getting to the lights. 
    
    
     Other features and their advantages will be apparent to those skilled in low wattage power supplies from a careful reading of the Detailed Description of Preferred Embodiments accompanied by the following drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the figures, 
     FIGS. 1A, 1B and 1C are graphs illustrating, respectively, alternating current, half-wave pulsed direct current and full-wave pulsed direct current versus time; 
     FIG. 2 is a circuit diagram according to a first preferred embodiment of the present invention; 
     FIG. 3 is a circuit diagram according to a second, alternative preferred embodiment of the present invention; 
     FIGS. 4A and 4B are circuit diagrams according to third and fourth alternative preferred embodiments of the present invention, which are variations of the first and second embodiments, respectively; 
     FIG. 5 is a circuit diagram illustrating yet a fifth alternative embodiment of the present invention, which is a variation of the third embodiment of FIG. 4A; 
     FIG. 6 is a circuit diagram illustrating a circuit for use with any of the first through fifth embodiments of FIGS. 2-5; 
     FIG. 7 is an exterior front perspective view of a plug for a wall socket containing a circuit, according to a preferred embodiment of the present invention; and 
     FIG. 8 is a rear perspective view of the plug of FIG. 7 with a portion of the housing of the plug cut away to show an integrated circuit inside, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention can be viewed from several standpoints. First, the present invention is circuitry for use in a plug housing adapted to be attached to an electrical outlet by inserting its contacts into the holes of an outlet and to provide power for a low wattage, low voltage device such as a series of Christmas lights. It will be clear to those familiar with these types of devices that other applications, especially those that use integrated circuits, can employ the present circuitry in such a plug. 
     The present invention is also a &#34;string of lights&#34;, which means a plurality of incandescent light bulbs, each in one of a series of sockets adapted to apply an electric potential across the filament in the bulb so as to cause it to glow from resistance heating, each light being connected by electrically conducting wires so that it is electrically in series and/or in parallel with the other, and having the improved plug at one end of the string for insertion into a wall socket that delivers household current, which plug is in electrical connection with the electrically conducting wires and thus each of the light bulbs. 
     One of several preferred, alternative embodiments is illustrated in FIG. 2. Inputs are generally indicated by reference numerals 10 and 12; outputs of an integrated circuit 18 are indicated by reference numbers 20 and 22. Fuses 14 and 16 may be installed in-line between inputs 10 and 12, respectively, and integrated circuit 18, and, if so, are preferably rated at no more than approximately 180 milliamperes each. The output of integrated circuit 18 is half-wave, 54 volts rectified DC for 120 volts AC input and is fed to a load such as a light string 28. The components shown inside integrated circuit 18 are preferably manufactured as a unit. 
     Inside integrated circuit 18 are two diodes 24 and 26. Diode 24 acts as a half-wave rectifier and a fuse, and is referred to herein as a solid state rectifying fuse (SSRF) diode, or &#34;Gibboney&#34; diode, after the inventor. The N junction of this diode is modified from that of conventional diodes to limit current. If the design current for diode 24 is exceeded, say, going past 100 milliamperes during the positive current flow portion of the cycle, diode 24 will open in a manner similar to a fuse. The modification to diode 24 comprises forming a plurality of small conducting strips at the cathode that are sufficient in combination to carry the rated current. These strips are preferably on the order of one to ten microns in width. Diode 24 can be manufactured with a nominal rating of one or more amperes, but by burning out or severing some of the strips, its rating can be adjusted downward from the nominal rating for specific applications. In this sense, it is a programmable diode. For Christmas tree light strings, an amperage rating of 150 milliamps may be suitable. 
     Diode 26, a conventional diode rated at 100 times the peak half cycle surge current of fuses 14 and 16, shunts the current from input 12 to output 20 to prevent AC from getting to the lights or other loads if integrated circuit 18 shorts out. Diodes 24 and 26 obviate the need for fuses 14 and 16, especially fuse 14, but fuses 14 and 16 are shown in FIGS. 2-6 because they are required under current industry standards. Alternatively, if diode 24 is replaced by a conventional diode, fuse 14 and possibly also fuse 16 may still be needed. 
     FIG. 3 illustrates an alternative embodiment of the present invention. As with the embodiment shown in FIG. 2, there are inputs 10 and 12, outputs 30 and 32, fuses 14 and 16 and integrated circuit 40. There is also SSRF diode 24 and resistors 34, 36 and 38. The output of circuit 40 is full-wave rectification; however, the amplitude of the second half-wave is reduced compared to that of the first half-wave in order to limit current. Circuit 40 uses the negative half of the cycle to fill in between positive halves with a lower voltage pulse to keep the lamp filaments from cooling down. To achieve this result, the voltage of the positive half of the cycle passes through SSRF diode 24 and then to output 30. During the negative half of the cycle, current flows across resistor 36 and then across resistor 34, but after the first of these two voltage drops, part of the current is tapped for output 32. The level of the output voltage from the negative half of the input cycle will depend on the ratio of resistors 34 to 36, and the magnitude of these resistors determines the amount of current that can be sourced by the load. This embodiment, especially when connected to a household current system having a 50 cycle frequency instead of 60, would avoid the shimmering effect that the embodiment of FIG. 2, or any other half-wave embodiment, would inevitably produce. 
     FIGS. 4A and 4B illustrate integrated circuits 18 and 40 modified as will be described below. However, components common to FIGS. 2 and 3 will have the same reference numbers to simplify the understanding of the drawings. In FIG. 4A, a circuit 50 is in line between inputs 10 and 12 and outputs 20 and 22. Diodes 24 and 26 have the same functions in circuit 50 as they do in circuit 18. In addition to diodes 24 and 26, however, is a conventional thermistor 52, or other current-limiting devices, to vary the resistance with the temperature or voltage, generally increasing in resistance as its temperature rises. Here, until the rated current of thermistor 52 is exceeded, its resistance is very low. Once that current is exceeded, however, the temperature of thermistor 52 rises significantly as does its resistance, to the order of megaohms, essentially opening the circuit. Once current is interrupted, thermistor 52 cools down in a second or two. 
     FIG. 4B illustrates an integrated circuit 60 that parallels circuit 40 of FIG. 3, including inputs 10 and 12, outputs 30 and 32, diode 24, resistors 34, 36, and 38. Circuit 60 includes thermistor 52 for the same purposes as described with respect to circuit 50. 
     FIG. 5 illustrates yet another embodiment of the present invention. Between inputs 10 and 12, fuses 14 and 16, and outputs 20 and 22, is an integrated circuit 70 containing SSRF diodes 24 and diode 26, as in the embodiments of FIGS. 2 and 4. In addition, integrated circuit 70 has thermistor 52 of embodiments 4A and 4B. These elements have the same functions as previously described. Integrated circuit 70 also has an optional full wave diode bridge 72 composed of four standard diodes 74 to feed full wave DC through SSRF diode 24. Diode 26 is not needed if bridge 72 is used. 
     Further, integrated circuit 70 has a programmable voltage array 76, a standard circuit element, in line so that the output voltage can be determined by the user, preferably by specifying that voltage to the manufacturer. Array 76 is a voltage divider or a chopper circuit, depending on the current demands of the application and is well known in the art. For a Christmas tree light string, a chopper circuit is preferred, and its frequency would be less than 10 kilohertz. 
     All of the foregoing integrated circuits, 18, 40, 50, 60 and 70 may be used in combination with an extraneous current load interruptor 80 which can sense a current leakage out of the loop by looking at the current supplied by the source and demanded by the load. Interruptor 80&#39;s inputs 82 and 84 couple directly to outputs 20 and 22 of each of the integrated circuits. The voltage drop of the current flowing into each input 82, 84 of interruptor 80 is sampled using small, well-matched resistors 86 and 88, preferably on the order of one milliohm each, by operational amplifiers 90 and 92. The outputs of operational amplifiers 90 and 92 are input to a summing amplifier 94. As long as the currents in both inputs are the same, then the voltage of the outputs of operational amplifiers 90 and 92 at summing amplifier 94 are the same and its output is zero. If there is a difference, on the order of one or two milliamps or less, then the output of summing amplifier 94 goes to saturation and causes a control logic element 96 to produce an&#34;on&#34;output that opens a solid state relay or switching device such, as CMOS 98, to interrupt power in the circuit. Because the control logic element 96 samples the output of summing amplifier 94 at at least 1 megahertz frequency, shutting down after detecting extranous currents takes about six closck cycles or less than one millisecond. To reset, the plug incorporating interrupter 80 must be pulled from the wall socket and then reinserted. Interruptor 80 can be used with DC or AC. 
     These integrated circuits, with or without extraneous current load interrupter, can be used with various small lights commonly found in conventional AC light strings, such as &#34;minis&#34;, C7s and C9s, as well as other small lights used now in only DC applications, including the so-called &#34;grain of rice&#34; lights that run from 1/8th inches to 3/16th inches long and 1/16th inches in diameter. 
     FIGS. 7 and 8 illustrate a front and back perspective view of a plug according to the present invention that is adapted for insertion into a conventional two- or three-hole electrical outlet (not shown). Plug 100 includes a non-conducting housing 110 from which two electrical contacts 112 and 114 extend. Contacts 112 and 114 fit into two of the holes of the outlet. As is known in the art, contact 114 is slightly wider than contact 112 so that plug 100, when inserted in the electrical outlet or socket, is always inserted with the same orientation to assure that the polarity is correct. 
     From housing 110 also extends a pair of electrically conducting, insulated wires 116, 118, for conducting electricity to a load 124. Load 124 may be a string of lights or other low voltage, low current device, such as a calculator or LED array. 
     Inside housing 110 are fuses 120 and 122 which correspond to fuses 14 and 16, respectively, of integrated circuit 18 of FIG. 2 described above. Wire 118 runs to fuse 122; wire 116 runs to integrated circuit 130 and then to fuse 116. Integrated circuit 130 can comprise integrated circuit 18, 40, 50, 60 or 70, either alone or in combination with interrupter 80. For simplicity, support structure for the internal components of housing 110, and preferably molded integrally with it, has been eliminated in FIG. 8. 
     It will be clear to those skilled in the art of integrated circuits and low voltage power supplies that many modifications and substitutions can be made to the foregoing preferred embodiments without departing from the spirit and scope of the invention, which is defined by the following claims.