First my english is no so well,but i'll try my best to make you understand my problem. :blush: Dear all, I was learning the arduino uno rev3 schemtic, and i read about the overview about Uno 3 from the mainpage ,it says that "The Arduino Uno can be powered via the USB connection or with an external…

The Arduino Uno R3 is powered from the USB port as the 'default' power supply. when a DC power supply of more than 6 or 7 volts is applied to the DC input connector a comparator composed of 1/2 of the LM358 switches off and the USB power is disconnected via the Mosfet labelled T1 and thus is the boa…

Thank you for the quick help .And LarryD FDN340P is exactly what confused me. In the schematic FDN340P's symbol is a enhancement-mode other than a depletion-mode MOSFET . If FDN340P is a enh mode p type MOSFET ,then it should be supplied a negative volt in its gate to get it on, this is what i learn…

Here goes. When there is a Vin voltage to the Arduino the O/P from the regulator is 5V. The 5 volts is connected to the source of the FET. If USB is connected to the Arduino at the same time, the PC places 5V on the Drain of the FET. Hence there is no current flow through the FET (5-5=0 results …

Note: If you then plug Vin back in the jack, the regulator will supply the current and the FET will again have no current flow (5-5=0V).

how the Arduino Uno power automatically slected between USB and external power?

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LarryD January 9, 2013, 5:32pm 2

fdn340p is a p-Channel mosfet.
0v at the gate will turn it on

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how the Arduino Uno power automatically slected between USB and external power?

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