Powering XIAO nRF52840

I had planned to use a USB power bank to power a XIAO nRF52840 for a project I’m working on, however it turns out a lot of USB power banks have a “feature” to shut off power if the device is below a certain threshold, and because I’m hoping to get my current down to less than 250µA for long periods of time it doesn’t seem like I will be able to do that. The power bank I bought for testing won’t even power stay on when it isn’t even in the deep sleep state.

So how else can I power the XIAO nRF52840? It doesn’t look like there is a VIN pin and I’d like to avoid LiPo batteries. Can I supply 5v from a battery to the 5v pin (and connect ground to the negative obviously) to power the XIAO? Or 3.3v to the 3.3v pin? Are there any considerations I need to factor in doing something like that? What happens when I connect the USB to my PC if I have a 5v supply to the 5v pin?
I’m also going to be supplying 5v to a servo from the same power source so is there anything I need to do to prevent that interfering with the power for the XIAO (I had intended to get a power bank with two USB outputs, using one for the XIAO and the other for the servo to keep them somewhat separate).

I’m pretty new to electrical engineering so I’m very hesitant to just try stuff especially when it comes to directly connecting to a power source.

How about putting a resistor across the output, big enough to raise the current to above 250uA? 18k ohms at 5V would draw enough and whilst you won’t be helping to save the planet 1.4mW isn’t so bad…

I think you misunderstood, I want to keep the project as low power as possible, ideally less than 250µA (though that’s looking like it might be difficult), but to keep the USB power bank from turning off I’d need over 200mA which would far exceed what I’m looking to use.

From playing around it looks like I can power it with the 5v pin, but I think I’d prefer supplying 3.3v, is it possible to power the Xiao by supplying 3.3v to the 3.3v pin?

Ok, my mistake. Yes, I’m pretty sure you can just use a 3.3V supply directly into pin 11 - i.e. the one marked 3.3V (slightly weird that the pins are numbered 0 to 13 instead of the conventional 1 to 14 in my view). I’ve never tried it but I can’t see any reason why not. The internal DC/DC converter drops down to 3.3V but it can cope with a bit more that 5V and the dropout voltage is less than 1V so I expect you’ll find that it would happily work with 4V into the 5V pin and you’d know that the 3.3V is stable. I guess the primary reason for the 5V input is because the DC/DC converter is necessary for it to function directly from the USB 5V. If power is really important, you may find that it will still work with a bit less than 3.3V into pin 11 and it’s important not to put in a higher voltage than 3.3V

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Cool thanks, I’ll try using a 3.3v supply then :slight_smile:. I found a “battery shield” that takes 2x 18650 cells and gives provides USB power, 5v pins, and also some 3.3v pins that seems like it’ll be good for what I need (and a hell of a lot cheaper and easier than putting together my own BMS). My hope is that the built in voltage converters it has for 3.3v and 5v should prevent power draw from occasionally running a 5v servo from interfering with the Xiao and sensors that’ll be connected to the 3.3v.

BTW the pins being numbered 0-13 seemed perfectly normal for me, I’m a programmer by trade so I’m always using 0 to refer to the first item of a list :slight_smile:

Fair enough :grinning:
If you are really trying to be minimalist, try running the XIAO from one 18650 cell with a diode to drop the 3.6V down to about 3V - into pin 11. I expect it’ll still run