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Sci.Electronics.Basics -> Transistor Current Question
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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
----
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\ / \ /
V V
--- ---
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/ /
\ \
/ /
\ \
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------
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|/
IN/\/---|
|\>
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___
_
(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!
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