I have a number of 8x8 RGB LED matrices.
Each has 32 pins, and you could (barely) characterise them as common anode - all the anodes in a row are common, but so are all the Red, Green and Blue cathodes in each column.
In short, I don't know where the resistors should go: but I figure that 24 resistors on the cathodes is better than 8 on the anodes. The Red voltage drop is 2.2V, while both the Green and Blue are 3.3V - and all are reportedly 50 mA (max 70 mA) LEDs.
To avoid brightness "droop", do I (5V supply, 30 mA drive each):
- Provide no resistors at all (I don't think so!);
- Provide the maximum for 1 LED (100Ω for Red, 68Ω for Green and Blue);
- Provide the minimum for 8 LEDs (12Ω for Red, 8.2Ω for Green and Blue);
- Average the results (stuff it: 47Ω for the lot of 'em!)
Before you start, I don't want to use an LED driver like the MAX7219 or AS1107 - there are too many matrices, and I'm multiplexing the multiplexors already...