Abstract:
The illumination intensity of light emitting diode in an appliance display is maintained over time by automatically increasing an amount of electric current supplied to the light emitting diode, preferably incrementally, over the life of the light emitting diode. The automatic increase in current compensates for the gradual decay of the light emitting diodes over time, thereby allowing the light emitting diodes to maintain a constant illumination for the lifetime of the appliance.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention pertains to the art of appliances and, more particularly, to a method for maintaining illumination of an display LED of an appliance over an extended period of time. 
   2. Discussion of the Prior Art 
   In recent years, certain household appliances have been manufactured with display units for allowing users to select different criteria or read messages displayed by the appliances. Such a display unit is often defined by a liquid crystal display with an array of light emitting diode (LED) backlights. 
   A drawback of LED backlights is that the output intensity of the backlights decreases steadily with time. Some factors that affect the rate at which an LED&#39;s intensity diminishes include the amount of electric current used to drive the LED, the temperature of the environment in which the LED is operated, and the humidity of the environment in which the LED is operated. When LEDs are used in an appliance, the minimum amount of electric current required to drive the LEDs at an acceptable brightness is approximately 10 mA. When the LED is operated in an environment of about 25° C., the relative illumination level is reduced by an average of 50% after 50,000 hours. Since the typical lifetime of a household appliance far exceeds 50,000 hours, supplying current to the LEDs at a constant rate of 10 mA undesirably results in a time varying intensity. 
   Several attempts and suggestions have been made to solve the problems associated with the deterioration of LEDs over time. For example, it is possible to double the lifetime of LEDs by doubling the number of LEDs and cutting the current supplied to each LED by 50%. However, there are significant costs associated with doubling the number of LEDs used in appliance control panels, which make this option undesirable. Another method of maintaining a consistent output of LEDs over time is to employ a sensor to detect when the LED output begins to diminish and to increase the voltage supplied to the LED based on the output of the sensor. This option may also be cost prohibitive due to the addition and maintenance of the sensor. Therefore, there is a need for a method and arrangement for maintaining the light intensity of LEDs provided in an appliance display without significantly increasing the costs of manufacturing the appliance. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to maintaining the illumination intensity of a light emitting diode (LED) in an appliance display over an extended period of time. The intensity is maintained by automatically increasing an amount of electric current supplied to the light emitting diode incrementally over the life of the light emitting diode. More particularly, a power supply is connected to an array of light emitting diodes and a controller is employed for automatically increasing the electric current transferred to the array of light emitting diodes. The automatic increase in current compensates for the gradual decay of the light emitting diodes over time, thereby allowing the light emitting diodes to maintain a substantially constant illumination for the lifetime of the appliance. 
   Additional objects, features and advantages of the present invention will become more readily apparent from the following detailed description of a preferred embodiment when taken in conjunction with the drawings, wherein like reference numerals refer to corresponding parts in the several views. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a portion of a side-by-side refrigerator incorporating the LED illumination intensity control arrangement of the present invention; and 
       FIG. 2  is a schematic view of the illumination intensity control arrangement employed in the refrigerator of FIG.  1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With initial reference to  FIG. 1 , a side-by-side refrigerator is generally indicated at  2 . In a manner widely known in the art, side-by-side refrigerator  2  is formed from a cabinet shell  3  to which is pivotably attached a freezer compartment side door  5  and a fresh food compartment side door  7 , each of which includes a respective handle  10 ,  12 . 
   As also shown in  FIG. 1 , freezer compartment side door  5  is provided with a dispensing assembly  50  which can be used to dispense either water or ice. Indicated at  51  is a display, preferably a liquid crystal display (LCD), incorporated in dispensing assembly  50 . Although not shown, dispensing assembly  50  also preferably has associated therewith fountain control buttons used for selecting between crushed and cubed ice, dispensing water, or activating a light button. At this point, it should be recognized that separate fountain control buttons can be employed or display  51  can actually constitute a touch screen display which incorporates the fountain controls. Regardless, display  51  is employed to provide various messages to a user. Display  51  is backlit by light emitting diodes (LEDs), such as schematically indicated at  60  in FIG.  2 . 
   In general, the above described structure of a side-by-side refrigerator  2  is known in the art, does not form part of the present invention, and is provided for the sake of completeness. The present invention is particularly directed to a method and apparatus for maintaining the illumination intensity of light emitting diodes  60  in appliance display  51  over an extended period of time. Again, the array of light emitting diodes  60 , hereinafter referred to as LEDs, is preferably used to backlight display  51 . In accordance with the most preferred embodiment, display  51  includes seven white LEDs  60  driven at approximately 10 mA in order to achieve the minimum acceptable brightness level for display  51 . However, the intensities of LEDs  60  will decay over time such that their output intensities decrease. For example, when LEDs  60  are supplied with a constant current of approximately 10 mA at 25° C., the relative luminous intensity will decrease, on average, by 50% in 50,000 hours. As the average life of a household appliance is much greater than 50,000 hours, it is desired in accordance with the present invention to counteract the intensity decay of LEDs  60 . Since the,luminous intensity of LEDs  60  increases linearly as the drive current is increased, it is possible to compensate for the decay of LEDs  60  by gradually increasing the drive current. 
   Therefore, in accordance with the invention, the illumination intensity is maintained at a substantially constant level by automatically increasing the amount of electric current supplied to LEDs  60 . By “automatically increasing” it is meant that the current is increased independent of any outside influence. In other words, the increase in current is not a response to a change detected by a sensor such as a temperature or light intensity sensor. Instead, the current is automatically increased incrementally or periodically. 
   As shown in  FIG. 2 , a controller  80  incorporated into refrigerator  2  functions to distribute power to power consuming devices of refrigerator  2  from a power supply  82 . In particular, controller  80  distributes a drive current from power supply  82  to the array of LEDs  60  through a current regulator  85 . In accordance with the invention, controller  80  automatically signals current regulator  85  to increase the amount of drive current supplied to LEDs  60  over time. For example, with LEDs  60 , having a luminous intensity which decreases exponentially, being initially driven at 9 mA at 25° C., after 10,000 hours, the luminous intensity would be in the order to 0.87, i.e. decreased by 13%, when still driven at 9 mA at 25° C. However, in accordance with the invention, controller  80  will automatically increase the drive current supplied to LEDs  60  by 1 mA, thereby maintaining the desired intensity. For instance, by increasing the drive current by 1 mA, the luminous intensity is increased by approximately 11%. Therefore, the luminous intensity is raised from 0.87 to 0.98, i.e., nearly 100%. 
   Therefore, in accordance with the invention, the luminous intensity will be maintained at a substantially constant level for a period of time much greater than that of an LED  60  driven at a constant current. In accordance with a preferred embodiment, controller  80  is preprogrammed to increase the drive current sent to LEDs  60  by approximately 1 mA per 10,000 hours. Of course, the increase may be varied in other ways, such as by increasing the current by a fraction of 1 mA at set intervals over the 10,000 hours. By automatically increasing the drive current, LEDs  60  will need to be replaced much less frequently. Obviously, costs associated with any output intensity sensors are eliminated. Since the decay of LEDs  60  can be readily predicted in household appliances, the present invention represents a cost efficient and reliable solution to automatically increase the drive current supplied to LEDs  60 . Therefore, with this arrangement, the only limitation would be the maximum drive current that LEDs  60  can handle. 
   Although described with reference to a preferred embodiment of the invention, it should be readily understood that various changes and/or modifications can be made to the invention without departing from the spirit thereof. For instance, although the invention is disclosed in connection with a display on a refrigerator, the invention could be equally employed in cooking appliances, dishwashers, clothes washing machines, drying machines, and the like. In general, the invention is only intended to be limited by the scope of the following claims.