I'm designing a light source that will employ a grid of LED emitters powered by constant current drivers. The LEDs will be laid out in a parallel/series combination, in rows of 8 emitters in series, with 3 rows in parallel. There will be 3 of these arrays on the board, for a total of 9 rows of LEDs, driven by 3 constant current drivers. This is the schematic for the board:
In my initial testing, using 8 LEDs in series seems to limit the impact of voltage variation in any individual emitter, and I will be using an aluminum PCB substrate which will double as a heat sink to distribute the heat, but I'm considering also adding a resistor in series because I've heard that helps to ensure that the current stays the same between the parallel strings. From what I understand, I would add a few hundred mV of voltage drop to each string, so if the constant current driver is outputting 120mA, and voltage drop across the 8 LEDs is 18V, I would add a resistor that would result in 18.3V.
What I don't understand, and I'm hoping someone can explain, is how adding this resistor helps to regulate the maximum current in the string.