I'm currently using an Arduino Uno (R3), but want to switch to Leonardo for the USB-ability. The Uno can work from the USB's 5V. Looking at the Leonardo's schematic it seems the only input to the voltage regulator is from the power connector. This makes sense as it's a 5V regulator which needs > 5V in. But I don't see the MOSFET which connects the USB's 5V to the board's 5V, like there's one on the UNO.

So, does the Leonardo need the separate power input connected?

1 Answer 1


Power steering is performed by the assembly labelled "+5V AUTO SELECTOR" at the bottom of the page. The voltage at VIN is halved and then compared with the board's 3.3V output in order to determine whether or not to enable USB power via T1.

  • Ow! Didn't see that. It was close enough to the 1117 though. :-( Thanks a lot! Jan 23, 2015 at 14:09

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