In tutorials I've seen, people connect every LED of a RGB LED to a resistance. I understand the purpose, but could we simply put the same resitance once, if we connect it to the common cathode or annode?
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The problem with one resistor in common anode/cathode is that when you activate more than one colour of the RGB LED, you will have additional current flowing through the one resistor which will cause increased voltage drop across the one resistor due to ohms law which will cause less forward voltage across the LED and likely extinguish it or at least make it much less bright.