Automatic Compensation for LED/PTX End of Life
Original Publication Date: 1986-Apr-01
Included in the Prior Art Database: 2005-Mar-08
Sensing of paper in impact printers is usually performed by a LED/PTX sensor. This method works well if the LED bias current is chosen high enough to compensate for the end-of-life degradation of the LED. Typically an LED will degrade up to 40% of its initial light output at end-of-life. Therefore, bias networks set the LED current high enough in anticipation of this future degradation. When the LED/PTX is used for sensing paper, this high bias current will cause sufficient energy to be radiated by the LED, such that enough energy will pass through the paper and not be sensed by the PTX. This is especially true when sensing lightweight papers. From a paper point of view, the bias current should be set as low as possible, such that the presence of paper will be detected. This contradiction creates a difficult design problem.