Been trying to wrap my head around these common cathode RGB LEDs...
Am I correct in saying that you just can NOT connect these in series, and therefore they're only actually usable in parallel with 3 resistors each?
They wouldn’t be any use in series — how would you individually control each primary colour?
Common cathode RGBs are relatively rare - where did you get them?
eBay! A whole bag of 40 of them for a lighting project... but I've spent a while now trying to fathom just how I'm going to use them only to discover that they're pretty useless. No wonder they were cheap. :I
Precisely what is it that makes them useless?
Am I correct in saying that you just can NOT connect these in series,
You can't connect common anode or common cathode LEDs in series. Common cathode LEDs are not so common because most drivers sink current and so you need common anode. However if you want to drive them with current sourcing then they are fine.
Mm. I'm wanting to stick them all in a 20 x RGB LED array and control each colour with PWM. Cannot fathom a method to do that with these though... oh well, back to eBay then! lol
Not sure what a 20 x RGB LED array is. Is it a 20 X 20 matrix?
If so you will have problems multiplexing that and controlling the brightness with PWM as the multiplexing has to be synchronised to the PWM.
How would you use common anode RGB LEDs for this?
Basically, 5 LEDs in series with a resister at the end, connected in parallel with 4 other strings. Times that by 3 (one for each colour of the RGB LED).
Having common cathodes means using 1 resistor per colour, which is 3 resistors per RGB LED! That's 60 resistors total! A common anode RGB LED would only use one resistor per RGB LED. A lot more manageable.
... I think. :S I'm still trying to work it all out. 
A common anode RGB LED would only use one resistor per RGB LED
No.
With common anode you would need a resistor in each cathode. That is the same number of resistors.
