Guest
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!
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!