LED and resistor question

Hi peeps.. sorry this might be a stupid question but here goes...

I have a RBG LED.. so have 4 legs on it. From what I understand I need to add a 220 ohm resister but what I was wondering, instead of adding 3 to each of the positive legs, can I not just add a single one to the ground pin or does it have to be before each of the there positive legs ? so 3 resistors are needed ?

thanks

3 resistors are needed. Might even be 3 different values - Blue usually has different voltage drop than Red & Green.

If you only want to light one of the colours at a time, you can get away with a single resistor on the common pin, although you won't get the ideal current for all of the 3 LEDs because, as Crossroads says, they may need different resistors. If you use a single resistor and try to light more than one LED at the same time, then the LED with the lower voltage drop is likely to take all the current, so the other one(s) won't light up.

CrossRoads: 3 resistors are needed. Might even be 3 different values - Blue usually has different voltage drop than Red & Green.

The three forward voltages are always different, principally depending on the photon energy and somewhat on the semiconductor material used for each colour, 3 resistors (or constant current sources) are necessary if you want to operate more than one colour at once.

Here's another way to look at it. You have 1 LED on, with Vf of 2V, with 220 ohm resistor current flow is (5V - 2V)/220 = 13.6mA, and 3V across the resistor. Now you turn on another LED - say the same Vf of 2V. The current flow thru the resistor cannot increase - 3V/220 is still 13.6mA, now shared among 2 LEDs, so brightness will suffer. Add a third LED - now 13.6mA is shared among 3 LEDs. Hence the need for a resistor per LED.

Just about an hour ago, I was testing a board fresh off the assembly line. It had a reversed R-Pack, which creates a similar situation... A shared current path (6 LEDs in this case). I noticed that LEDs got dimmer when more LEDs came-on, and a blinking LED made another LED "blink" bright and dim. I've seen this before, and I know right-away what the problem was...