Different from the best answer of this one: Why does a resistor need to be on the anode of an LED? But well, I'm also an electronics noob ^.^
Recently I bought the official Arduino starter kit and played around with the common cathode RGB LED that came with this kit. In the project book, the instructions for the colour mixing lamp is to use separate 220ohm resistor for each of the RGB legs. But I thought, why can't I put the resistor on just the cathode of the LED?
I tried that by plugging the LED into the breadboard and connecting the cathode through a 220ohm resistor to the ground of Arduino UNO. When I touched any one of the R, G or B anode legs with a jumper wire connected to +5V, the respective colour lights up alright. However, when I bridge the G and B pins together, only the green LED lights up. When I bridge all three anodes together, only the red LED lights up. But if I connect the anodes to +5V through their separate 220ohm resistors(according to the instruction), the colours will combine.
Why is it so?
Answer
The red led will hog all the current because it might only need 2 volts across it to begin conducting. The green and blue LEDs need a higher voltage but because they are all in parallel the red led dominates. Try measuring their respective volt drops when each operates.
It's like putting a 5 volt zener in parallel with a 10 volt zener. The 10 volt zener will never conduct.
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