1

Will the 3.3V regulator produce 3.3v when the Arduino is powered from the USB port?

2

Will the 3.3V regulator on an Arduino Leonardo work when powered from USB?

Yes, provided that

  • it is an "official" Leonardo or a competent clone and

  • as long as you do not load the power supply excessively with external loads.

Official circuit diagram here

The USB supply feeds the 3V3 regulator via MOSFET T1 at the bottom of the sheet and U3, an LP2985 LDO regulator. If your board has the same circuitry as this it should provide 3V3 from USB.


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0

The 3.3V regulator is fed by the 5V, whatever that comes from. If you don't use a separate power supply like a wall-wart, the 5V switches automatically to the USB's 5V. That feeds the regulator for the 3.3V.
So yes, it will produce 3.3V.

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