Interchangeable LED bulbs and light string assembly therewith

This invention relates to a type of interchangeable LED bulb, and to a light module assembled by connecting two interchangeable LED bulbs in parallel and with the same polarity, and further to a light string assembled by connecting such light modules together in series.



In another configuration of forming a light module, two LEDs are connected in parallel and with the same polarity, but with no other electrical elements; similar light modules are then connected in series to form a light string. Reliability of the light string is improved, as with the parallel configuration, when one light bulb burns open, the other bulb in parallel will still keep current flowing through the string, provided that it has the capacity of conducting all current flowing through a light module. Only the faulty bulb will become dark and all other bulbs in the string will stay lit, so it is easy to identify the failing bulb and replace it.



The disadvantage of such series configuration is that, the whole light string Will turn dark if one of the LEDs in the string burns open or is somehow disconnected from the string; and it is a time consuming task in identifying the faulty one among many good but unlighted LEDs. That is why LED strings currently available in the market have LED bulbs molded together with the lampholders and thus making it impossible to replace any bulbs without cutting off the electrical wires. As a result, the whole string will usually be disposed if one failing LED causes the string to become dark.



LEDs have increasingly been used as luminance sources in various applications. In addition to other desirable features, the availability in a variety of colors makes LEDs very attractive and suitable for decoration purposes. As a result, one application where LEDs become particularly popular in recent years is decorative light strings. Such light strings are usually formed from a plurality of LEDs connected in series, where the LEDs may have different colors.



A disadvantage of this configuration is the inability to provide a unrestricted choice of color in replacing LED bulbs within a light string. We know LEDs are made of different semiconductor materials which have different ranges of emitted light wavelength and different electrical characteristics. When two LEDs of different colors are connected in parallel and with the same polarity, but with no other electrical elements, the one with a lower threshold forward voltage will conduct current first and usually take more current than the other one that has a higher threshold forward voltage. In an extreme case, one LED conducts all current while the other one has no current flowing through it. It is unacceptable that some light bulbs are illuminating while others are dark in a light string. Therefore, it is a common practice that two substantially similar LEDs, which have the same electrical characteristics; and usually have the same color, must be selected for the parallel connection to form a light module. Otherwise, normal illumination can not be ensured for every bulb. As a result, an LED bulb of one color can not be used for replacing an LED bulb of another color, even though the lampholder shall allow and accommodate such replacement activities.



Accordingly, interchangeable LED bulbs and a light module therewith are provided in the present invention to ensure the light string formed from such interchangeable bulbs and light modules are not only reliable in terms of functioning, but also flexible in terms of free choice of color change for a light module



According to another aspect of the present invention, a resistor should be selected such that the current flow through the LED in an interchangeable LED bulb is less than the maximum current of the LED. Usually a resistor must be selected based on both LED electrical characteristics and the number of light modules connected in a light string, so that nominated current will flow through the LED bulb. The actual power consumption of a resistor must be less than its maximum power consumption.



According to yet another aspect of the present invention, a plurality of such light modules is connected together in series to provide a string of such light modules. The 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 module is less than the voltage available to supply the string. For example, the minimum operating voltage of a light module may be the greater of the minimum operating voltage of the two LEDs.



According to another aspect of the present invention, a resistor should be selected such that the current flow through the LED in an interchangeable LED bulbs are less than the maximum current of the LED. Usually a resistor must be selected based on both LED electrical characteristics and the number of light modules connected in a light string, so that nominated current will flow through the LED bulb. The actual power consumption of a resistor must be less than its maximum power consumption.