Hi, I'm interested in powering a nano and about 30 WS2812b LEDS from a USB battery pack.
For the sake of cleanliness, I would like to keep all my power connections consolidated to the mini USB plug on the nano. I know you're not supposed to run more than 500ma through the nano, so I was wondering if there is a pin directly connected to the USB port's 5v connection so the LEDs can draw power directly from the source without passing current through any other components on the board.
I did some continuity testing with a multimeter, but didn't see anything on vcc or 5v, so just wondering if I just happened to miss a diode or something. Or alternatively, are there any other locations on the board that would be an easy solder?
No, the USB voltage (USBV) connects directly to the anode of a Shottky diode which prevents backfeeding a higher voltage to your computer and damaging it's power supply.
Which diode is limited to half an Amp and does drop a few hundred millivolts.
You can remove the diode and bridge the contacts if you like. It might be wise to disconnect the LED strip when plugged into the computer USB port though if you have the data input to the WS2812s held down to ground with a 22k resistor on the Arduino pin during boot-up, the LED strip of 30 LEDs will draw only its quiescent current of 30 mA until it is programmed.
The problem with feeding 5 V back into a laptop USB port is not damage to the power supply as such, but to the actual USB controller which is designed to limit the current delivered out of the port.
On my Lenovo laptop if an external 5V is connected to the USB 5V, the PC complains of a current surge and shuts down the port on me, requiring a restart of the PC for the port to work again. Having a diode to prevent voltage/current from an external source is a good idea.
You need power, Gnd, data from the Nano to the WS2812B strip.
If you use the ICSP header, and one of D11-12-13 for the WSB2812B signal, you would be all set. Just unplug from there when you have the PC connected to avoid accidentally trying to pull 1.5A from the PC's USB supply and thru the diode.
If you don't want to crimp together a 2x3 connector & ribbon cable (which I think would be too thin for 1.5A), then a Pololu housing
female terminated wires https://www.pololu.com/product/1800 (different lengths are available)
could be used to make up a little cable to connect to your strip.
If you are connecting up an external 2A supply to the Nano to supply 5V,
then I would also suggest adding a wire from the 5V IO pin to the 5V pin on the ICSP connector. The onboard trace is only 16mil wide and a trace width calculator suggests that is only wide enough for 1.25A
CrossRoads:
On my Lenovo laptop if an external 5V is connected to the USB 5V, the PC complains of a current surge and shuts down the port on me, requiring a restart of the PC for the port to work again.
Which means you cannot use a "powered" USB hub on it!
