RGB leds and resistors values for DOUTx4

Parts Questions
1

hi experts,

before to finish soldering my DOUTx4 modules w/ ULN to drive my rgb matrixes, I have to choose resistors values.

The rgb leds I got are those :

http://www.sparkfun.com/datasheets/Components/YSL-R596CR3G4B5C-C10.pdf

Vred = 2V

Vgreen= 3.2V

Vblue= 3.2V

all for I=20mA

so

I guess:

Rred = 100 ohms

Rgreen = 160 ohms

Rblue = 160 ohms

am I right ??

the fact I use ULN, the fact I use DOUT modules would alter these values ???

I guess, no.

But I’d prefer to ask before, if anyone has experiences with that

all the best,

protofuse

2

Hmm, I think 150, 100, 100 looks closer.

Try this website:

http://led.linear1.org/1led.wiz

For a quick first run, Source Voltage is 5, Diode forward voltages and currents you already provided.

This will provide safe resistor values that can be left on forever without damaging the LEDs.

So what can change the numbers? The DOUTs that supplies power may have a slight voltage drop. You could figure that out and deduct it from 5V. The ULN outputs may not QUITE get all the way to ground, you could deduct that from the 5V also. In truth, neither of those will be enough to make a difference.

LEDs that are being scanned in a matrix CAN be overdriven for bit more light, but it will shorten the life of the LEDs, and if the scanning ever stops unexpectedly, the LEDs will be unhappy.

Have Fun,

LyleHaze

3

thanks a lot lylehaze.

it will be a common cathode led matrix.

I did the calculation: 150, 100, 100 seems to be fine indeed!

I’ll buy my resistors :slight_smile: thanks a lot!