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I always see an rgb led ran with 3 resistors, for example a 20ma led on 5v power uses 150ohm's for red due to it's lower voltage but only 100ohms for green and blue. I am running 6 shift registers off of a microcontroller via spi. I have 16 common anode LED's. all of the anode's on the led's share the same source. my question is could I potentially limit the current going to all the anode's and then only have the red cathode's use an additional resistor?

i.e

          g on led no resistance- 1-|--------|-16 -vcc
          b on led no resistance- 2-|        |-15 -50 ohm resistor to R on (rgbled)
50 ohm resistor to R on (rgbled)- 3-|  74    |-14 -serial in from MCU or prev. register
          g on led no resistance- 4-|  HC    |-13 -NA
          b on led no resistance- 5-|  595   |-12 -latch to mcu & next register
50 ohm resistor to R on (rgbled)- 6-|        |-11 -MCU clock and clock on next register
          g on led no resistance- 7-|        |-10 -Master to 5v high for inactive
                             gnd- 8-|________|-9  -to pin 14 on next register
_____________________
|____________________|_R____ 50 ohm resistor to pin 15
|____________________|_+____ shared 5v with 100 ohm's resistance
|______RGB LED_______|_G____ pin 1 no resistance
|____________________|_B____ pin 2 no resistance
|____________________|
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  • \$\begingroup\$ Why not use CC drivers instead? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Apr 30, 2015 at 17:34
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    \$\begingroup\$ I think I see where you are going with this. If I have only a single resistor for the anode then everytime I change the lighting the draw from the leds will change and the current will change as a result. Hmm, I guess I either need to buy more resistors or get a cc driver. \$\endgroup\$
    – Cfoote7
    Apr 30, 2015 at 17:50

1 Answer 1

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If you only have one LED on at a time, that should work, but it's going to be difficult to have Green and Blue visible at the same time. The slightly lower forward voltage will take nearly all the current, and you won't see the other one. Not so sure about Red and something else, because of the extra resistor.

The general rule is that you can't share resistors across multiple LED's that are supposed to be on at the same time and expect anything close to the same brightness.

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