Battery options and USB-PWR

A shrunk down Leonardo-compatible board, thumb drive sized with native USB support. [Product info]
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Joined:Mon Jul 08, 2013 1:25 pm
Battery options and USB-PWR

Post by noisymime » Mon Aug 26, 2013 2:48 am

I'm trying to find the best solution for providing power from a battery and am currently thinking that one of the relatively cheap USB phone charger packs would work well. They're a nice regulated 5V supply and have decently large capacities, even when you take a certain amount off for marketing BS.

Firstly, is there any reason why this wouldn't work? It seems like as a standard USB power supply it should be fine, but thought I'd ask.

Secondly though, I also need some way to physically turn power on/off as this project can potentially go quite some time between uses and charges. Am I correct in thinking that if I cut the USB-PWR trace and wire up a switch to it, this will completely kill power to the Leostick? I know it will kill power to the CPU, but is there anything else that will remain powered up on the 5V rail even with USB-PWR cut?


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Re: Battery options and USB-PWR

Post by cef » Mon Aug 26, 2013 4:00 am

Hi noisymime,

The USB-PWR jumper only removes power from one side of the AVR, however some other pins (the ones for the USB port) are still connected to the USB. This is so that the Leostick uses the USB voltages for USB communication, even if the AVR itself is powered by other means. I think that the 32u4 might draw a small amount of current from the USB in this case (with USB-PWR cut), but I'm not 100% sure without testing it.

Note: This is only due to the fact that you're aiming to power the Leostick from the USB connector. If you were powering it from the 5V header pin on the board itself, and wanted to only connect USB intermittently (without removing the other power supply) then cutting the USB-PWR jumper would be the right thing to do.

What you could do is desolder the 500mA fuse that's on the Leostick, and replace the fuse with an external switch and an inline fuse (to offer the same protection as the existing fuse). As this is directly connected to the 5V input from the USB connector, you'd definitely have zero current draw when the switch is off.

FWIW: The Schematics are at: ... f?raw=true

Hope this helps.

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