I'm having a problem with an LED matrix that I built from Radio Shack LEDs. The matrix has 3 rows by 3 columns, where each column shares a common power and each row shares a common ground.
G G G P P P | | | | | | | | | | | | | | | | | | | | |--------G------LED(B)--LED(Y)--LED(R) | | | | | | |--------------G------LED(B)--LED(Y)--LED(R) | | | | |--------------------G------LED(B)--LED(Y)--LED(R)
In the first column are identical blue LEDs that are 20 mA and 3.2-3.8 volts (350 mcd). In the second column are identical yellow LEDs pulling 20 mA, 1.9-2.4 volts (80 mcd). In column three are identical red LEDs that have the same specs as those in column two. When I built this, I had to use what I had lying around. ;) I knew that there would be a difference in brightness between the blue ones and the rest of them.
When three identical LEDs (the columns) come on together, there is no difference in brightness. But when I light them by row, something strange happens. When I light a blue one by itself, it's okay. When I light a yellow one next to a blue one that is already lit, the blue one dims slightly. And when I add a red LED in column three next to a blue and yellow that are already lit, the blue one dims to practically nothing.
As I said earlier, this does not happen when I light the columns. Nor does it happen when I light all nine of them.
I thought that there wouldn't be a problem in running them in parallel because they all have the same current specs. But apparently the voltage difference affects the current? Can somebody explain to me what is happening, and tell me if there is a way to fix it (without replacing all of the LEDs with identical ones)? Thanks.