Hi all! First time poster 
I'm running a bunch of LEDs (~120 RGBW) from a 12V battery, and I need a way to power the Arduino (Nano) from the same supply. I already have a buck converter in the circuit, as the LEDs require 5V, and the only thing that the board itself is powering via 5V pin is a single IC ship (CD4011, ~10mA current) plus itself (~20mA I think?).
I'm a bit unsure how to power the board (up until now, in all of my tests I've been using USB power). As I see it, I have a few options:
- Connect the 12V battery directly to V_In pin.
- Connect the post-buck converter 5V line to the 5V pin.
- Find an old USB mini cable to behead, connect the post-buck converter 5V line to the USB head, and power via the USB slot.
Each has its advantages and disadvantages
- 12V->V_In takes advantage of the Nano's onboard regulator. However, the board makes a scary buzzing sound when I do this, and heats up a bit
According to MATH, the regulator should be able to easily dissipate the generated power/heat from converting 12V->5V at ~30mA, but I'm unsure about continuing considering the noise (perhaps you can reassure me). - 5V->5V pin would be incredibly convenient and would excuse the regulator of any duties, but...well...then the regulator is excused of its duties. And we like the regulator! Other folks online have said that a "regulated 5V supply" connected to 5V should be fine (despite the official stance being not to do it), but I'm unsure what qualifies as "regulated", and whether or not a buck converter with 1uF and 10uF caps across 5V->Gnd counts, even if I throw in a diode to prevent reverse biasing. The thing will be in a box, so I won't have to worry about accidentally connecting both the battery and the USB cable, though (I know that's bad news).
- 5V->USB slot allows my 5V to be regulated, and the Arduino would probably be happiest, but honestly it just feels the messiest. I've never touched a dissected USB cable, so I would have to check that the polarity is right, it would use up the USB slot in case I have to update the script so it'll be a hassle, and having the entire USB head in there would probably significantly increase my footprint (and weight/size is pretty vital in my application).
These are the options I've considered, and the arguments for and against that I've thought about. I'm currently leaning towards (1) and just crossing my fingers that the buzzing is probably nothing (MATH says so, right?). Although, (2) would probably be ideal if I could get it to work. I could theoretically just get another buck converter down to a more reasonable 7V, but I'd rather do with what I have on hand, if possible.
Before committing, I figured I would ask you fine folk. Any thoughts?