Abstract:
A lighting module with improved reliability including a pair of LEDs connected in parallel and with the same polarity and a method for making the said light module are disclosed. The parallel LEDs with the same polarity will increase the reliability of the light module and make it suitable for use in light strings where a relatively large number of such light modules are connected in series and where the failure of one such light module will cause the failure of the entire light string. A light string made of such light modules and a method for making the light string are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to light modules assembled from light emitting diodes (“LEDs”) and to light stings assembled by connecting such light modules together. 
   2. Description of Related Art 
   LEDs are increasingly used as light sources in various applications. Some of the features that make LEDs attractive include: low power consumption, long lifespan, low heat generation, small size and weight, robustness, fast switching time and availability in a variety of colors. In addition, in recent years, the cost of making LEDs has significantly decreased, making their use more economical, even in cost-sensitive applications. 
   One application where LEDs have become particularly popular in recent times has been household and decorative light strings. Such strings are typically formed from between fifty and one hundred LEDs connected together in series. The low power consumption and low heat generation of LEDs make them particularly suitable for such applications, where the cost of power and fire hazard make other types of light such as incandescent lights less attractive. 
   Nevertheless, there are problems with such strings. Despite their robustness, LEDs do sometimes fail. In the event of failure, the whole light string will go dark, which result is both unattractive and challenging to troubleshoot. The user must then locate the one LED out of fifty or one hundred that is faulty. 
   In a competing product, strings of incandescent lamps, a popular solution to this problem has been to connect all of the lamps together in parallel to form a string. When a lamp in such a parallel circuit fails, the rest of the lamps continue to shine and the defective one is easy to identify and replace. 
   However, parallel circuitry has not been embraced with LED strings. In contrast to incandescent bulbs that can be built with a filament resistance suitable for parallel connection to a source of alternating current, LEDs are confined by their semiconductor properties to having a forward voltage drop typically in the neighborhood of 1.1 to 3.0 volts. As a result, unless voltage-reduction circuitry is in a light string, a large number of LEDs must be connected together in series to produce a total voltage drop equal to the voltage at which the alternating current is supplied, being 110 V AC  in North America. For this reason, complete series-strings of LEDs are sometimes connected together in parallel, but the LEDs themselves are connected together in series to form the string. Thus, there is redundancy between the strings and a user can quickly tell if a string is not working, but it is still a challenge to find the LED within a string that is responsible for a malfunction. 
   A second reason that parallel circuitry is not seen in LED strings is that there exists a widely held view in the electronic design community that it is bad practice to connect diodes together in parallel with the same polarity. This view is based on the concern that parallel diodes are not well-suited for carrying more current than a single diode can carry on its own, because unless all parallel diodes have identical forward voltage drops, the one with the lowest forward voltage drop will carry the most current, which will cause its temperature to increase, which will cause its forward voltage drop to decrease further, which will cause it to carry even more current until it perhaps fails. If the failing diode fails open, the other parallel diodes will then be forced to carry more current, until they possibly fail one by one. It is important to note nevertheless, that this view seems to have arisen in the context of power circuits that are tasked with delivering high currents through expensive power diodes. In contrast, LEDs typically have significantly less steep current versus voltage curves than other diodes and, consequently, it is less likely that connecting non-identical LEDs in parallel will give rise to significant current differentials and overheating in one of the LEDs. Furthermore, for typical lower current applications in which LEDs are used, LEDs may be cheap enough to significantly over-specify their rated forward current. 
   Accordingly, what is needed is a way to provide redundancy in an LED light string, such that when an LED fails, the rest of the light string will still function and the failed LED may be identified without undue difficulty. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to this need. 
   According to one aspect of the invention, there is provided a method creating illumination that includes connecting a first light emitting diode (“LED”) and a second LED together in parallel and with the same polarity. 
   According to another aspect of the invention, there is provided an apparatus that includes a light module that has a first LED and a second LED connected to the first LED in parallel and with the same polarity. At least one of the first and second LEDs might have a maximum total current rating sufficient to carry all current conducted through the light module. 
   In one configuration, the first and second LEDs have dissimilar electrical characteristics, such that the first LED carries all of the current conducted through the light module while the second LED remains unused unless and until the first LED fails open. In an alternate configuration, the first and second LEDs have similar electrical characteristics, and in particular the first and second LEDs have substantially the same forward voltage drop over the operating range of the light module. To better achieve similar forward voltage drops, the first and second LEDs are thermally connected to a common heat sink. The apparatus might also have a light-diffuser covering the first and second LEDs. 
   The module might also have a third LED connected to the first and second LEDs in parallel but with opposite polarity and a fourth LED connected to the first and second LEDs in parallel but with opposite polarity. 
   The module could also be connected together in series with other similar modules to provide a string of such light modules. The actual number of light modules that are connected together in series would be selected such that the sum of the minimum operating voltage for each of the light modules is less than or equal to the voltage available to supply the apparatus. For example, the minimum operating voltage of a light module might be the greater of the minimum operating voltage of the first LED and the minimum operating voltage of the second LED. Furthermore, the number of light modules that are connected together in series would be selected such that the sum of the maximum operating voltage for each of the light modules is greater than or equal to the voltage available to supply the apparatus. For example, the maximum operating voltage of a light module might be the lesser of the maximum operating voltage of the first LED and the maximum operating voltage of the second LED. 
   The apparatus might also include a way of limiting the current flowing through the light module, for example a resistor connected in series with the light module. 
   According to another aspect of the invention, there is provided an apparatus that includes a light module having a first polarized photon-emitting semiconductor device (“PPESD”) and a second PPESD connected to the first PPESD in parallel and with the same polarity. At least one of the first PPESD and the second PPESD might have a maximum total current rating sufficient to carry all current conducted through the light module. 
   In one configuration, the first and second PPESDs have dissimilar electrical characteristics, such that the first PPESD carries all of the current conducted through the light module while the second PPESD remains unused unless and until the first PPESD fails open. In an alternate configuration, the first and second PPESDs have substantially the same forward voltage drop over the operating range of the light module. 
   Further aspects and advantages of the present invention will become apparent upon considering the following drawings, description, and claims. 
   DESCRIPTION OF THE INVENTION 
   The invention will be more fully illustrated by way of a detailed description of specific exemplary embodiments in conjunction with the accompanying drawing figures, in which like reference numerals designate like parts throughout the various figures. 

   
     1. BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a light module according to a first embodiment of the invention. 
       FIG. 2  is a schematic diagram of a second light module according to a second embodiment of the invention. 
       FIG. 3  is a schematic diagram of a light string according to a third embodiment of the invention. 
       FIG. 4  is a schematic diagram of a light string according to a fourth embodiment of the invention. 
       FIG. 5  is a schematic diagram of a light string according to a fifth embodiment of the invention. 
       FIG. 6  is a schematic diagram of a light string according to a sixth embodiment of the invention. 
       FIG. 7  is a wiring schematic of a light string according to a seventh embodiment of the invention. 
       FIG. 8  is a pictorial view of the light string illustrated in  FIG. 7 . 
   

   2. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     FIG. 1  shows a light module according to one embodiment of the present invention, generally illustrated at  25 . The light module  25  includes a pair of light emitting diodes (“LEDs”)  22 ,  23  that are connected together in parallel and with the same polarity. 
   It is desirable that the pair of LEDs  22 ,  23  each has a maximum reverse voltage greater than the maximum voltage likely to be encountered in use. It is also desirable that the pair of LEDs  22 ,  23  each has a rated forward current greater than the maximum total current expected to flow through light module  25 , so that in other words either of the pair of LEDs  22 ,  23  is capable of carrying the total current. In general, it is desirable that the pair of LEDs  22 ,  23  have substantially similar electrical characteristics; however, most particularly, it is desirable that the pair of LEDs  22 ,  23  each has the same or a substantially similar forward voltage drop over the typical range of operating conditions so as to reduce the likelihood that a significant difference in forward current will develop between the pair of LEDs  22 ,  23 . In this sense, the term substantially similar means that in operation one of the pair of LEDs  22 ,  23  does not carry all or substantially all of the current flowing through the light module  25 . 
   Besides taking care to select LEDs from a common production batch, there are many well-known design and assembly techniques for encouraging components to have similar electrical operating characteristics and it is understood that those would be used in the present case where applicable. For example, where temperature effects might be expected to influence the behavior of the pair of LEDs  22 , 23 , the pair of LEDs  22 ,  23  could be thermally connected to a common heat sink  48 . 
   If one of the pair of LEDs  22 ,  23  fails open, the other will still function and therefore the light module  25  will still illuminate. The remaining one of the pair of LEDs  23 ,  22  will carry the full current on its own. Furthermore, so long as it has been over-specified with a suitably high rated forward current it will continue to function properly. This LED-redundancy is inexpensive at these low current levels and because the pair of LEDs  22 ,  23  are connected in parallel to achieve redundancy rather than higher current carrying capacity, there is good reason for going against the conventional view that diodes should not be connected in parallel with the same polarity. 
   It will be appreciated that, even though it might be desirable for the pair of LEDs  22 ,  23  to be well-matched such that they both carry a share of the current conducted through the light module  25 , this arrangement is not required for the invention to yield benefit. So long as each of the pair of LEDs  22 ,  23  is over-specified with a high enough rated forward current to carry the full current conducted through the light module  25 , then improved redundancy is achieved even if one of the pair of LEDs  22 ,  23  conducted the full current before it failed open and thereafter the other one of the pair of LEDs  23 ,  22  conducted the full current. In fact, there may be a benefit to mismatching the pair of LEDs  22 ,  23 , such that the first of the pair of LEDs  22 ,  23  is held in reserve while the second carries all the current conducted through the light module  25  until it fails open, at which point the fresh first of the pair of LEDs  22 ,  23  takes over carrying all of the current conducted through the light module  25 . Those skilled in the art will appreciate that increased redundancy, and therefore reliability for the light module  25 , may be obtained by connecting more than two LEDs in parallel and with the same polarity. 
     FIG. 2  shows a light module according to a second embodiment of the present invention, generally illustrated at  46 . The light module  46  includes a first pair of LEDs  42 ,  43  that are connected in parallel and with the same polarity and a second pair of LEDs  44 ,  45  that are connected in parallel and with the same polarity, which is opposite to the polarity of the first pair of LEDs  42 ,  43 . 
   Unlike the first embodiment light module  25 , which is configured to provide illumination only when a source of electromotive force is applied to forward bias the pair of LEDs  22 ,  23 , the second embodiment light module  46  is configured to provide illumination both when a source of electromotive force is applied to forward bias the first pair of LEDs  42 ,  43  and when a source of oppositely polarized electromotive force is applied to forward bias the second pair of LEDs  44 ,  45 . Thus, when connected to a source of alternating current, the first embodiment light module  25  is configured to provide illumination during approximately half of the cycle of the source current whereas the second embodiment light module  46  is configured to provide illumination during substantially the full cycle of the source current, thus providing a brighter appearance. In addition to illumination during substantially the full cycle of the source current, which will result in the light module  46  appearing brighter, light module  46  has further the advantage of generating less electromagnetic interference. Because the current flowing through light module  46  is substantially a full sinusoid, it will contain lower levels of higher order harmonics, which can cause coupled wires to act as an antenna propagating electromagnetic waves with frequencies corresponding to these higher harmonics. 
   It is further desirable that the first embodiment light module  25  and the second embodiment light module  46  each also includes a light-diffuser  27  covering its respective LEDs  22 ,  23 ,  42 ,  43 ,  44 ,  45 . Each respective light-diffuser  27 ,  47  is configured to diffuse the light emitted by the respective LEDs  22 ,  23 ,  42 ,  43 ,  44 ,  45  such that an observer of the respective light module  25 ,  46  will be unable to readily distinguish which of the LEDs is the source of the light from the light module  25 ,  46  or in what relative proportions. 
     FIG. 3  shows a light string according to a third embodiment of the invention, generally illustrated at  108 . The light string  108  includes a block  120  of light modules  25  connected in series, all with the same polarity. The light string  108  may include more than one block  120  of light modules  25 , as is the case with this third embodiment, which includes a second parallel block  120 ′ of light modules  25 . 
   If an LED, for example one of the pair of LEDs  22 ,  23  in a particular light module  25 , fails open, then the remaining one of the pair of LEDs  23 ,  22  will carry all of the current flowing through that light module  25 , and therefore that light module  25  as a whole will continue to provide light and conduct current and therefore the whole block  120  will continue to provide light and conduct current. 
     FIG. 4  shows a light string according to a fourth embodiment of the invention, generally illustrated at  200 . The light string  200  includes a block  220  of light modules  46  connected in series. The light string  200  may be more than one block  220  of light modules  46 , as is the case with this fourth embodiment, which includes a second parallel block  220 ′ of light modules  46 . 
   The light string  108 ,  200  may be configured to connect directly to a source of household alternating current (“AC”). In this configuration, the number of light modules  25 ,  46  in each block  120 ,  220  must be selected such that the sum of the minimum operating voltage for each of the light modules  25 ,  46  is less than or equal to the voltage of the available supply and that the sum of the maximum operating voltage for each of the light modules  25 ,  46  is greater than or equal to the voltage of the available supply. The minimum and maximum operating voltages of the light modules  25 ,  46  is essentially the minimum and maximum operating voltages of the respective pairs of LEDs  22 ,  23 ,  42 ,  43 ,  44 ,  45 . 
   For example, assume that all the light modules  25 ,  46  in the block  120 ,  220  are identical and that all the pairs of LEDs  22 ,  23 ,  42 ,  43 ,  44 ,  45 , have a forward AC voltage operating range of 1.5 V AC  to 2.5 V AC  and a corresponding current range of 10 mA to 50 mA. If the AC supply voltage is 110 V AC , then with 50 light modules  25 ,  46  in the block  120 , 220  the voltage drop across each light module  25 ,  46  will be approximately 2.2 V AC , which is well within the operating range of each light module  25 ,  46  and the respective pairs of LEDs  22 ,  23 ,  42 ,  43 ,  44 ,  45 . 
     FIG. 5  shows a light string according to a fifth embodiment of the invention, generally illustrated at  408 . The light string  408  includes at least one block  420  of light modules  25  connected in series. The light string  408  further includes a resistor  54  connected in series with the block  420 . The value of resistor  54  may be selected to provide current-limiting in the event of a short circuit in block  420  and to produce during regular operation of the block  420  a voltage drop sufficient to replace one or more light modules  25  if less modules are desired in block  420  than would be required as discussed above with respect to the third and fourth embodiment blocks  120 ,  200 . Those skilled in the art will appreciate that, besides a resistor, other means may be used to limit the current in block  420  or produce a voltage drop equivalent to one or more light modules  25  in series. 
     FIG. 6  shows a light string according to a sixth embodiment of the invention, generally illustrated at  508 . The light string  508  includes at least one block  520  of light modules  25  connected in series. The light string  508  further includes a full-wave rectifier  64  coupled to the block  520 . This embodiment of the light string  508  is configured to provide current to each light module  25  over the entire AC cycle, such that each light module  25  will appear brighter and steadier. 
   While not shown in  FIG. 6 , those skilled in the art will appreciate that means for smoothing the ripple in the output of the rectifier  64  may also be coupled to rectifier  64 . For example, an inductor may be placed in series between the rectifier  64  and the light modules  25  in the block  520 , or a capacitor may be placed in series with the rectifier  64  and in parallel with light modules  25  to smooth the ripple. 
     FIGS. 7 and 8  show a light string according to a seventh embodiment of the invention, generally illustrated at  608 . The seventh embodiment light string  608  is similar to the third embodiment light string  108 , except that it includes only a single block  620  of light modules  25  connected in series. 
   The light string  608  further includes a plug  71  attached in series to one end of the light string  608 , adapted to connect the light string  608  to a source of AC. The light string  608  also includes a receptacle  76  attached in series to the other end of light string  608 , adapted for connecting the light string  608  to another appliance (not shown) that requires AC, for example another light string  608 . The plug  71  and receptacle  76  are connected together in parallel to the light string  608 , so that an open circuit in the light string  608  will not interrupt the AC being provided to the other appliance (not shown). 
   While specific embodiments of the invention have been described and illustrated, such embodiments should be considered illustrative of the invention only and not as limiting the invention as construed in accordance with the accompanying claims. 
   It will be understood by those skilled in the art that various changes, modifications and substitutions can be made to the foregoing embodiments without departing from the principle and scope of the invention expressed in the claims made herein. For example, although the invention has been discussed in terms of light emitting diodes, those skilled in the art may recognize that similar benefits could be achieved by substituting other similar polarized photon-emitting semiconductor devices, such as light emitting transistors. 
   While the invention has been described as having particular application for decorative lighting, and in particular Christmas lighting, those skilled in the art will recognize it has wider application, for example in optical communications.