Patent Application
20080231204
1. Field of the Invention (Technical Field)
The present invention relates to backlights for transmissive displays and more particularly to a method and apparatus and method for replacing a fluorescent lamp with a light emitting diode (LED) module.
2. Background Art
The invention resulted from the desire to replace fluorescent lamps with LED assemblies without changing the fluorescent lamp inverter or ballast. LEDs are favorable over fluorescent lamps because LEDs have a lower failure rate, they stay bright longer than fluorescent (they last longer) and LEDs do not contain mercury, a compound considered toxic when disposed. Further, LEDs are as efficient as fluorescent lamps and continue to become more efficient because fluorescent technology is mature and improvements continue to be made with LEDs. Eventually LEDs will provide the same amount of light with less power as fluorescent lamps. The present invention provides an apparatus and method to replace the prior art fluorescent lamps with a LED assembly without any modification to the existing hardware, the assembly merely replaces the fluorescent lamp.
The majority of fluorescent lamps can be categorized as either hot cathode or cold cathode. Hot cathode fluorescent lamps have filaments while cold cathode fluorescent lamps have electrodes without filaments. Hot cathode fluorescent lamps are primarily used for general lighting and avionic displays. Cold cathode fluorescent lamps are generally used to backlight LCD displays used in computer monitors, TVs, laptops and avionic displays.
The present invention provides a means to replace a fluorescent lighting system with an LED system. By connecting the LEDs in accordance with the preferred embodiment, the fluorescent lamp driving circuit can be reused without modification. The current rating of the LEDs must be equal to or less than the current output of the fluorescent lamp driving circuit. The LED replacement lamp may consist of as few as two LEDs but multiple LEDs may be used as long as the power consumed by the LEDs does not exceed the power rating of the fluorescent lamp driving circuit.
A primary object of the present invention is to provide a means to replace a fluorescent tube lamp with an LED lamp. This system can be used for backlighting transmissive displays or as a general lighting system.
A primary advantage of the invention it that it allows a fluorescent lamp to be replaced with an LED lamp without modifying the fluorescent lamp inverter driving circuit or ballast. The fluorescent lamp driving circuit is a current source and a current source is the preferred method used to operate LEDs.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating a preferred embodiment of the invention and are not to be construed as limiting the invention. In the drawings:
First LED string 22 and second LED string 20 are connected in parallel but opposite in polarity. The anode 28 of the first LED in first string 22 is connected to cathode 30 of the last LED in second string 20. Anode 32 of the first LED in second string 20 is connected to cathode 34 of the last LED in first string 22.
When AC voltage source 12 is energized, one of the LED strings begins to conduct current. During the positive half of the AC current output of the inverter current 36 conducts through first string of LEDs 22 and the first string produces light. During the negative half of the AC current output of the inverter current 38 conducts through second LED string 20 and the second string produces light. Ballast impedance 14 serves the same purpose in the LED replacement lamp assembly 18 by limiting the current through the LED strings. Unlike the fluorescent lamp circuit 10 a high voltage does not appear across LED strings 20 and 22 because the LED impedance is low. The LED string voltage is determined by the current voltage (IV) characteristics of the LEDs.
Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above, are hereby incorporated by reference.
