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3w common anode RGB LED circuit?

Hi all,

Does anyone have a link to a circuit diagram for a common anode led? Here are the specs:
  • Red: 2.5V ~ 3.0V, 350mA
  • Green: 3.2V ~ 3.8V, 350mA
  • Blue: 3.2V ~ 3.8V, 350mA

I picked one up and didn't realize the effects of common anode v. common cathode :/ I can run it with drivers from one of the instructables floating around, but I have to use totally isolated power supplies (i.e. one separate battery for each color). I realize that I could use PWM for them, but I don't have the tools to program and burn a chip right now.

Soooo, either a circuit that can sink current (I believe that is the correct terminology) or some way to isolate power between the 3 colors would be much appreciated.

Here is a link to the led I have: http://www.dealextreme.com/details.dx/sku.4530

homba (author) 5 years ago
Regarding:

That's not a description of a common anode in the case of LEDs. It's not possible to use separate supplies for each component LED if the anodes are connected.

Hmm, well, I built one each of the power supply in steps 10-11 here for each color: http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/

If I connect each driver to each color and then to the same power source, turning one color up turns them all up. If I connect them to separate power sources they work individually. *shrug* I understand why - there's nothing to prevent the current from traveling through the other two leds, but don't know how to change it.

Are you saying that I should be using only one voltage regulator and then different resistors/current limiting methods for each led?

Thanks for the sugestions.
gmoon homba5 years ago
Yeah, the real issue is that you've chosen two circuits; each one requires a path through the anode to regulate properly. I.E., the regulator's output pins are the anode itself. But the anode is shared. So if one regulator is "turned down," the others sense the change and compensate. The methods in steps 6,7,8 and 9 should work. They limit the current on the cathode side. The anode is fixed at V+. Any other connections in those steps are biasing resistors...
homba (author)  gmoon5 years ago
If I could ask you one more question ... If I end up doing PWM for this thing, do I have to mess with voltage and amperage much or can I keep them to acceptable levels through keeping the modulation short? Or would it just be best to drop all of the input power through a voltage regulator so that I know what it voltage and amperage I'm getting and then add small resisters to keep the amperage from running away from me? Thanks again for the advice :)
gmoon homba5 years ago
Most of the solutions in the "High-Power LEDs" 'ible are current limiting--the LED driver is taking care of it. Look at Step 8. It's connected to a microcontroller (or any other PWM device.) The PWM itself is a square wave--the PWM effects are a function of the width and duration of the pulse. No voltage worries...
homba (author)  gmoon5 years ago
That's what I thought - time to break out an atmel programmer - thanks!
homba (author)  gmoon5 years ago
Cool, thanks! That will give me something to pick at. I have a great project to use it for, but don't like the way I have to set things up the way it is now.
homba (author) 5 years ago
If I could ask you one more question ... If I end up doing PWM for this thing, do I have to mess with voltage and amperage much or can I keep them to acceptable levels through keeping the modulation short? Or would it just be best to drop all of the input power through a voltage regulator so that I know what it voltage and amperage I'm getting and then add small resisters to keep the amperage from running away from me? Thanks again for the advice :)
gmoon5 years ago
Any NPN transistor switching setup in common emitter mode should work. I.E., where the loads (LED cathodes) are each connected to a different NPN collector--one transistor per cathode (and one current-limiting resistor, too.)

I'm sure an FET would work, too. But you'll need some type of transistor switching, since the power requirements are high.

Regarding:
but I have to use totally isolated power supplies (i.e. one separate battery for each color).

That's not a description of a common anode in the case of LEDs. It's not possible to use separate supplies for each component LED if the anodes are connected.
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