rlogiacco:
Looking at your schematic Julia, do I interpret it correctly by saying VCC_1 is the Due 5V pin? If it is and you have 6 of those IR LEDs you are using 660mA out of the maximum 800mA the Arduino due voltage regulator can output. Please don't think the 3.3V and 5V current limit can add up: the 3.3V output is a further regulation out of the 5V regulation (according to the schematics on the Arduino website) which means 800mA is total maximum limit.On top of that, even if you are sending just short energy bursts to those LEDs I bet you are waving in almost 3 times their maximum current rating (unless you are using power LEDs, they are almost always rated at 40mA) which means you are stressing them quite hardly for no benefit: you should just replace the series resistors so to limit the current to 40mA (or whatever is those LEDs current rating if you have the datasheet): that will put the LEDs within their specs and reduce the load on your Due voltage regulator.
When picking the step down buck regulator don't forget the input voltage range: most regulators out there on eBay aren't rated for 40V input.
Now, regarding your GND pin question, you can safely connect the 5V pin to GND through something which limits the current flow within the voltage regulator limits (like a resistor or an LED with a series resistor and so forth) not just because the 3.3V limit does apply to I/O pins only, but because the limit is posed by the voltage regulator and not the MCU. If, by accident, you short out the 5V or 3.3V pin with a GND pin (like by putting a power LED anode and cathode in there without a series resistor) what you will fry is the voltage regulator, the MCU will still be intact.
As stated before, please note I do not own a Due and I'm talking only based on what I can see on paper with my limited electronics knowledge
Hi,
thank you for the useful information. So at 5 V, if i want 40 mA per LED i should switch to 125 Ohm per LED, correct?
Greetings,
Julia