The number of LED emitters selected per
string (y) affects the change in the forward
current of the LED emitters as the input
voltage varies over some range. In general,
as the value of y is increased, the change in
forward current becomes larger due to the
same input voltage variation. In addition, when
the input voltage is less than (yVO) the forward
current through each LED emitter is
through each string of LED emitters as a function
of input voltage. Figure 3.2 shows string lengths
ranging from 2 to 6 emitters. Note that series-
strings with 5 and 6 emitters have the largest
change in forward current and highest “threshold
voltage.” Since the forward voltage of the LED
emitters varies slightly over temperature, then the
forward current through each LED string will
change slightly over temperature. Circuits with
longer string lengths will have a slightly larger
forward current change over temperature.
approximately equal to zero. As the value of y is
increased, this “threshold” voltage increases.
Figure 3.2 shows the change in forward current
Figure 3.1 Circuit Configuration of LED Arrays Used in LED Signal Lamps.
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