Re: 300 mA from a microcontroller pin

In article <47503CD8.4FC75B30@xxxxxxxxx>, cbfalconer@xxxxxxxxx says...
Mark Borgerson wrote:
toe@xxxxxxxxxxx says...

A bi-colour LED that has only two pins (they're in parallel
facing in different directions). If the microcontroller pin is
high, then it'll be red. If low, it'll be green. If high
impedance, it'll be off.

... snip ...

You don't really need a tri-state driver, then. If both LED pins
are at the same level (either high or low), the LED will be off.

If total power dissipation isn't a concern, your circuit could
simplify to:

+5 +5
| |
R1 R2
| |
|---- LED ----|
| |
P1.1---T1 P1.2---T2
| |
| |
gnd gnd

T1 and T2 are logic-level N-channel mosfets controlled by
your MPU pins.

Ignoring total dissipation, a single tri-state driver will do:

+5
|
R1
|
tri state here >---|X|-----+
LED |
R2
|
GND

with R1 approximately equal to R2. The ratio controls brightness.


Given a 2.0V forward drop across the LED, for 300mA, you need
R1 and R2 to be about 10 ohms.

So power dissipation in each resistor is going to be about
0.9Watts---better use fairly large resistors!!! ;-)

My circuit at least had the advantage that there is no power
dissipation when the LED is off.

Mark Borgerson

.

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