Having a hard time Controlling a 10W RGB LED with my Arduino.

I have a 10W RGB LED.
Red forward voltage is 6-7V (I assume I have to take an avarage?)
Blue-green is 9-12v
it also says each color needs 150ma.

I want to control it with my Arduino nano. there are two problems:

  1. The Mosfet with feedback method needs precise sense resistor values that I can't find.
  2. my 3.3v output with such a small max current has a problem turning the whole thing on and off (PWM). I think I can use a 3904/3906 at the 12v to mosfet's gate. Not sure.
  3. I'm really feeling guilty about the whole 5W power waste. this is going to be plugged in as my nightstand all day.
  4. Kind of the main problem: I have hard time finding the R(sense) value that I need.

I was thinking of just using something like lt3003 but I can't find it in my country and can't order from ebay. Any ideas?

  1. Then you're out of luck. Setting precise currents with a small voltage does require specific resistor values...

But, using a linear constant current driver is very ineffecient for 10W LEDs. Just get yourself a switch mode constant current driver. For example one based on the PT4115 which has a PWM pin you can use. This is the most common chip on drivers but most sellers don't mention the chip :confused:

  1. Why are you limited to 3,3V?

  2. Then part 1.b again, use a switch mode driver :slight_smile: Just get three 150mA drivers :slight_smile: But 150mA is a bit low / harder to get. You can get a different driver and change the resistor value on that one. I think this is a PT4115 driver but it has a 300Ohm resistor for the current resulting is a bit over 300mA. Swap it for a 680Ohm resistor for 150mA.

  3. You do know you can put resistors in series / parallel do you?

And no, don't take the average for the voltage. Measure it if you want to know it. But you just want to control it with current so the exact value doesn't matter. All you need to know is you need more voltage then the max led voltage (+ drop of the current source).

septillion:

  1. Then you're out of luck. Setting precise currents with a small voltage does require specific resistor values...

But, using a linear constant current driver is very ineffecient for 10W LEDs. Just get yourself a switch mode constant current driver. For example one based on the PT4115 which has a PWM pin you can use. This is the most common chip on drivers but most sellers don't mention the chip :confused:

  1. Why are you limited to 3,3V?

  2. Then part 1.b again, use a switch mode driver :slight_smile: Just get three 150mA drivers :slight_smile: But 150mA is a bit low / harder to get. You can get a different driver and change the resistor value on that one. I think this is a PT4115 driver but it has a 300Ohm resistor for the current resulting is a bit over 300mA. Swap it for a 680Ohm resistor for 150mA.

  3. You do know you can put resistors in series / parallel do you?

And no, don't take the average for the voltage. Measure it if you want to know it. But you just want to control it with current so the exact value doesn't matter. All you need to know is you need more voltage then the max led voltage (+ drop of the current source).

multiple resistors will be hard big and messy

and I"m limited to 3.3v because My MCU Only gives 3.3v on the output pins

I couldn't find any PT4115 in my country. maybe a simple buck converter IC with sense resistor will work? that way the sense resistor can be anything. and no more 5w losses!

and the led drivers I found are kinda expensive but I guess I have no choice. they're big too c:

Please don't quote without reason.

kamhagh:
multiple resistors will be hard big and messy

Then you'll need to be satisfied with the closest E24 value I guess.

kamhagh:
and I"m limited to 3.3v because My MCU Only gives 3.3v on the output pins

Fair enough. But in the future, please tell us which Arduino you use from the start.

kamhagh:
I couldn't find any PT4115 in my country.

I can't "smell" where you live and what you can get. But to be honestly, I can't get driver modules in the Netherlands as well. At least not the common PT4115 or other common maker drivers and not for a reasonable price.

kamhagh:
maybe a simple buck converter IC with sense resistor will work?

That can work, I've done it in the past. But couple of disadvantages

  • Most buck converters have a sense voltage of 1,25V (the PT4115 just 100mV) which means more losses in the resistor. For 150mA that's 188mW compared to 15mW with the PT4115.
  • All buck converters I know sense from GND. So in order to make a constant current driver you need to add the sense resistor between GND and the led aka on the cathode of the led. So this only works with common anode leds or leds with individual anode and cathode.
  • They are not that great at dimming. A driver like the PT411 is designed to have a fast signal on it's dim-pin and will act fast on it. Most buck converters have some sort of slow start in them which makes dimming harder / more ugly.

kamhagh:
and the led drivers I found are kinda expensive and are only available with 300ma current :c

Driving 10W led's isn't a light task :wink:

kamhagh:
it also says each color needs 150ma.

Then it's not a 10watt LED.
Common 10watt RGB cob LEDs need 300-350mA per channel.

Switches (transistors) and CL resistors are possible, but waste about 2watt (on a 12volt supply).
Not too bad if you're only using one RGB LED.
Leo..

You're right, that's a 5W RGB.

And with a forward voltage of around 12V a 12V supply is to low...

Vf of three blue/green/white LEDs in series is usually listed as 9-12volt.
Practically that range is a lot smaller.
More like ~10.5volt when cold, and ~9.5volt when hot.
Red is different, with a Vf of ~7.2volt.
CL Resistors are just fine, untill you can afford/get a proper switching CC driver.

Leo..