dual color LEDS, 2 signals to drive, need help

[John]

I have a dual color led that I need to control from a programmable
device. I have 2 signals (LEDYAGC and LEDGAYC) to control if the led is
yellow or green.
I'm OK with all that.

I need to figure out the voltage and drive strentgh of the programmable
device (or the buffer if it needs that) that drives the LED signals. I
also need to figure out the series resistor I need to use. I've never
used this type of LED, so I just don't know where to start. Here's the
schematic of the LED and it's requirements. The programmable
device/buffer IOs can be 3.3v or 2.5v.



LEDYAGC ------
|
|
-----------
| |
-- --
yellow \/ /\ green
-- --
| |
-----------
|
|
LEDGAYC ------





LED Requirements

Description Absolute Maximum Rating
Green Yellow Unit
Wavelength At Peak Emission, If=20ma 565 590 Nm
Dominant Wavelength, If=20ma 566 588 Nm
Lens Clear Clear ---
Luminance Intensity, Iv@20mA 14 5 mcd
Capacitance, If=20ma 15 20 C
Forward Voltage, Vf=20mA 2.5 2.5 V
Reverse Voltage 5 5 V
Peak Forward Current 140 140 mA
DC Forward Current 25 30 mA
Reverse Current, Vr=5V 5 5 V
Power Dissipation 105 105 mW


Thanks, John

 

[Bob]

You need to add one series R from one of the IO pins (it doesn't matter
which one).

Set the IO strength such that its outputs go all the way to the supplies
when they're delivering the desired current (into the LED). The series R
will then determine the LED current. Read the specs for the IO driver to
convince yourself that the chosen IO strength will provide enough drive to
insure that the outputs get close to the supplies under this load (i.e.,
within at least 200mV, or so).

Assuming that you want to operate the LED at 20mA (this will be VERY
bright), and the LED has 2.5V across it (at 20mA), then this implies that
you'll want to use 3.3V IO. The series R will, then, be the controlling
factor for the current.

The LED current will be (3.3V-2.5V)/R. You can do the math.

Bob



"John" <josepugmima@comcast.net> wrote in message
news:1109245641.142028.175290@z14g2000cwz.googlegroups.com...

I have a dual color led that I need to control from a programmable
device. I have 2 signals (LEDYAGC and LEDGAYC) to control if the led is
yellow or green.
I'm OK with all that.

I need to figure out the voltage and drive strentgh of the programmable
device (or the buffer if it needs that) that drives the LED signals. I
also need to figure out the series resistor I need to use. I've never
used this type of LED, so I just don't know where to start. Here's the
schematic of the LED and it's requirements. The programmable
device/buffer IOs can be 3.3v or 2.5v.



LEDYAGC ------
|
|
-----------
| |
-- --
yellow \/ /\ green
-- --
| |
-----------
|
|
LEDGAYC ------





LED Requirements

Description Absolute Maximum Rating
Green Yellow Unit
Wavelength At Peak Emission, If=20ma 565 590 Nm
Dominant Wavelength, If=20ma 566 588 Nm
Lens Clear Clear ---
Luminance Intensity, Iv@20mA 14 5 mcd
Capacitance, If=20ma 15 20 C
Forward Voltage, Vf=20mA 2.5 2.5 V
Reverse Voltage 5 5 V
Peak Forward Current 140 140 mA
DC Forward Current 25 30 mA
Reverse Current, Vr=5V 5 5 V
Power Dissipation 105 105 mW


Thanks, John

 

[Rich Grise]

On Thu, 24 Feb 2005 15:48:22 +0000, Bob wrote:

You need to add one series R from one of the IO pins (it doesn't matter
which one).
....
Assuming that you want to operate the LED at 20mA (this will be VERY
bright),

I've found that an LED is visibly just as bright at 10 mA as at 20, at
least I couldn't tell the difference. From there down, the apparent
brightness is pretty much linear with current.

Cheers!
Rich