# uln2803apg and cree mc-e rgbw

Hi,

I’m quite a noob and I’m trying to connect 3 cree mc-e rgbw leds to my arduino and I am going to use the uln2803apg to connect them to arduino, an aduino ethernet powered with 12v.

Here the schematic:

Can you confirm that is correct?

Here you can find the specs for the leds http://www.ledsupply.com/leds/cree-mce-rgbw-high-power-led
According to the specs the Forwars voltage for 3 leds is:
W: 11,7 V → resistor needed: 1 ohm
R: 7.5 V → resistor needed: 9.1 ohm
G: 11,7 V → resistor needed: 1 ohm
B: 11,7 V → resistor needed: 1 ohm

Do I need the 1ohm resistors or can I not use them?

The point of a resistor is to prevent small variations in the LED’s forward voltage from producing runaway currents.

To do this, the LED needs to use up a decent percentage of the total voltage. A 1 ohm resistor doesn’t do that so it isn’t going to protect you from anything, ie. your LEDs could die.

You need to raise your voltage to 18 or 24 volts so that the resistor is a decent size and can do its job.

Or run the LEDs in parallel, each with its own resistor.

Or get a proper LED controller.

Hi fungus,

thanks for the reply; so If I use them in parallel for each led would be:

R 2.5 V → 20ohm resistor
G 3.9V → 20ohm resistor
B 3.9V → 20ohm resistor
W 3.9V → 20ohm resistor

But what about the schematic, is it correct?

Thanks

heckmac:
But what about the schematic, is it correct?

No. (Even if.)
The LEDs are backward.

Oh sorry, I messed up with the drawing, yes the plus and the minus are backward.

Anything else? The resitors are ok now?

heckmac:
The resitors are ok now?

3.9V * 3 = 11.7V
12V - 11.7V = 0.3V
0.3V / 20? = 15mA
Use 22?
Still, there will be a voltage drop across the 2803 (output), minimal, but since you’re working with remnants <1V, that could be noticeable. Try it, it may work out (it won’t absolutely “not work”, nothing will get killed). Should be “OK”.

2.75V * 3 = 8.25V
12V - 8.25V = 3.75V
3.75V / 20? = 188mA !!
Use 220?