Hey everyone, I have a question about powering LEDs alongside a Pro Micro.
My setup:
I'm using a Pro Micro that must stay connected via USB, since it's sending data to the host computer. That USB connection also powers the board.
The USB-C port on my computer should be able to supply up to 900 mA (USB 3.x spec).
I'm driving a bunch of LEDs that draw roughly 600 mA in total.
The problem:
The Pro Micro's VCC pin is fed through its onboard voltage regulator (or the USB trace), so I can't just pull 600 mA through the board — I'd risk damaging the regulator or the traces. The Pro Micro is only rated for maybe 200–300 mA total on VCC.
The obvious fix would be an external 5 V supply connected directly to the LEDs, sharing a common ground with the Pro Micro.
But: since I already have ~900 mA available from the USB-C cable, couldn't I tap the 5 V / GND lines from the USB cable before they reach the Pro Micro? That way the LED current never flows through the board itself, but I still only use one cable.
Has anyone done this, or is there a reason this wouldn't work? Any gotchas I should be aware of?
Ha! Okay, gotcha. You're right … it seems the regulator is only relevant when powering via the RAW pin with a higher voltage, or on the 3.3V version.
That said, my concern still stands, just for a different reason: even without the regulator in the path, I'd still be pulling 600+ mA through the Pro Micro's PCB traces and the USB connector pads. Are they designed for that?
So the question is — is that concern valid, or can the 5V Pro Micro actually handle ~650 mA through VCC without issues in practice?
And a side question: can I actually draw 650 mA from a USB port? As far as I understand, USB 2.0 only allows 100 mA by default, and the device has to negotiate with the host to get the full 500 mA. Since the Pro Micro is a USB 2.0 device, I wouldn't even get the 900 mA that USB 3.x allows. Does the ATmega32U4 handle that negotiation automatically, or is there something I'd need to configure?
Some USB C to Micro USB cables have a 56k pull-up in the USB C plug that allows them to supply up to 900 mA at 5V. It's connected between A5 and VBUS I think.
If you decide to use the suggested "splitter" be sure to find one that is not just a charger cable and that passes the data through as well.
Good point about the fuse — I wasn't fully aware of that. So even if the USB port could deliver more, the onboard fuse would cut me off at around 500 mA. That pretty much settles it: you can't pull more than ~500 mA through the Pro Micro itself, whether via VCC or RAW. Once and for all
Which brings me back to the splitter idea: if I split the 5V line from the USB cable before it reaches the board, the fuse wouldn't be in the path. I'm on USB 3.x (USB-C) — but since the Pro Micro is a USB 2.0 device, would the host still only allocate 500 mA to that port? Or does the port's own capability matter here?
Probably should have asked what brand Pro Micro you have. If you have the Sparkfun QWIIC Pro Micro then it has USB C and can negotiate 5V3A. It also looks like it has pads on the PCB to pick off 5V before the fuse.
Considering you need a splitter cable and a USB-C breakout board so you can connect to the USB power, a 5V @1A wall wart may be much easier and cheaper.
