Hello
I was hoping someone could explain how to do this calculation for me. I've got 3 transistors and 5 RGB LEDs, and I need to use the transistor to drive them, as my microcontroller can't source enough current for all of them on one pin.
Because they're pins on RGB LEDs, and I need individual color control, I can't put them in series, so I was going to use 3 resistors per RGB LED, like so:
Vdd ---- | | | | \\ / \\ / V V --- --- | | / / \\ \\ / / \\ \\ | | ------ | |/ IN/\\/---| |\\>
| | ___ _
(Only two LEDs shown because I'm terrible at ascii art. There are actually 5.)
Since the green and blue have a Vf of 3.2, I'm guessing I'm going to have to use a 5V power supply instead of a 3.3V one, because I can't get a VCE small enough to drive them.
If I need to pass 20mA through each LED, with a voltage drop of 3.2, can someone please explain how I calculate the two different resistors (Base and on each LED) I need?
Thanks in advance!