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Topic: driving a single 3w RGB led (Read 3751 times) previous topic - next topic

Sparkyfunfun

Why hello great Arduino community!  I come before all of you with a problem.  I am trying to drive a single common anode LED (http://www.dealextreme.com/p/3w-led-emitter-on-star-multicolored-rgb-4530) with a Li-battery (http://www.dealextreme.com/p/ultrafire-3-7v-18650-2400mah-battery-1213).

The problems I have are:
Providing 350ma (or slightly less) in constant current to the LED's three cathodes from a varying voltage source
Finding a small component to do this
And (this one is not as important because I could use a PNP transistor) a piece that can deliver constant current and dim LEDs based off of PWM (from arduino uno)

When I found this chip (http://www.onsemi.com/pub_link/Collateral/NCP380-D.PDF) I was so excited because I thought I had found what I needed, but then I figured out it can only regulate down to 500ma (150ma more than the max of the LEDs).  I know constant voltage + resistors are not the way to go because of thermal runaway, but I'm am new to the world of high power LEDs.  I need your help community!

I should also mention that I am tight on space for this project, so the smaller the piece the better!

johnwasser

For each of the three cathodes you can use a resistor for current limiting and an NPN transistor (and base resistor) for PWM.  You can use a single ~1.5 A 5 V regulated supply for the LED and Arduino.
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Sparkyfunfun

I see what you're saying, but a NPN tranistor would just dim the whole LED correct?  I do want color switching so I would need a transistor on the power side of the load (so a PNP yes?).  I looked into driving each cathode with a resistor, but in high power LEDs,  apparently you can run into some thermal run away (as led heats up, Vf goes down, which increases amperes and heat, which lowers Vf, etc.) so I would need a current controller chip for each cathode...at least that is what I am lead to assume from my reading.  My problem is finding a current regulator chip that will regulate ~350 ma (or a little lower) that will accept a voltage input of ~3.7V (from li-battery) or just finding an alternative to current regulator chips at all.  Thank you for your contribution though; if I was driving a smaller RGB LED I'm sure it would work!

Any thought from anyone else?

johnwasser

The NPN transistor acts as a "low side" switch, between the cathodes and ground.  Using three you can individually control the three color channels.

A cheap LM317 adjustable voltage regulator will act as a current regulator:
http://led.linear1.org/a-cheap-current-regulated-luxeon-star-driver-design/

Unfortunately it needs about 3V of headroom to regulate properly so 3.7V power source won't do it.  I guess you need a current regulated LED driver with PWM input.
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