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Topic: ProMini 3.3V DigitalPin Output for OSRAM LRTB (RGB-LED) (Read 2888 times) previous topic - next topic


Hello, and a happy new Year,

I am usin a ProMini(3.3V) and I want to use it with two RGB LEDs through the digital Pins.

Id like to use those:

What voltage is coming out of the digital pins reliable?

Do I need resistors (at all)? Which?



Jan 04, 2013, 11:49 pm Last Edit: Jan 04, 2013, 11:51 pm by Grumpy_Mike Reason: 1
What voltage is coming out of the digital pins reliable?

Well if you are running the processor at 3V3 then that is the voltage of the output.

Do I need resistors (at all)?



Well looking at the data sheet it looks like you can drive the LEDs at 60mA which is way above what you can get from a pin, so do you want to use transistors to drive them?
This is because the forward voltage drop is greater than the output voltage you have, so use a transistor to get a 5V output. Then you can calculate the resistor value for each depending on how much current you want to drive it at.

For direct connection I would go for a 90R on the blue and 150R on the other two, but it is less than optimal.


What voltage is coming out of the digital pins reliable?
You don't run LEDs  from a constant-voltage source...  You run them from a constant-current source.   (Or, from an approximately-constant current source, such as a known-voltage across a resistor.) 

If you try to run from a voltage-source, a very small change in voltage will make a large change in current (and a large change in brightness).   A small increase in voltage could even result in a big-enough current jump to fry the part.    The actual voltage drop (at a given current) will vary from part-to-part, and with temperature.

And, the different colors in an RGB LED will have different voltage drops (at the same current).

I think you are going to need more than 3.3V (and a transistor or MOSFET), especially for the blue... You need enough "extra" voltage, so that you can have a (relatively) constant voltage drop across the resistor (which is the "remaining" voltage after you subtract the LED voltage-drop).   


What voltage is coming out of the digital pins reliable?


Do I need resistors (at all)?

Depending on how you intend to drive them.

If you look at the datasheet, the green / blue leds have forward voltage greater than 3.3 (max), with 3.2v typical.

So you may not be able to drive them to full brightness in a 3.3v environment. Alternatively, you can put a larger resistor on the red led.

Or pwm.



thanks for your answers.

I think I will use Low Current LEDs instead, because I need to run the device from a battery. Using 250mAh battery won't lead to ongoing fun if I am using that hungry LEDs ;)

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