So basically, the Arduino board only monitors the USB connector and the Vin input voltages. Also, you must refrain from powering the 5V pin together with the USB connector. If you look closely at the simplified schematic above, you will see that you should not simultaneously power the DC jack and the Vin pin. If Arduino finds an external voltage coming from either the Vin or the DC jack, it disconnects the USB connector VBUS (+5V) voltage. To prevent this from happening, Arduino monitors the Vin for the presence of any voltage. As a result, two or more input voltages may compete in supplying power to the board. As a matter of fact, in four different (4) ways. How the Auto Switching of USB Power WorksĪnyway, why, in the first place, is there a need for a switch on the USB connector? The schematics above show that the Arduino board accepts power in many ways. Its only use within the Arduino board is serving as a voltage reference for controlling the USB connector voltage.įinally, the simplified schematic shows that the USB connector connects to the 5V (and hence, VCC) thru a simplified switch. Its main purpose is for supplying external circuits with 3.3 volts. Observe that this 3.3V does not supply anything on the Arduino board. It comes out as a 3.3 volts that also terminates on the female header with a 3.3V label. The 5-volt output of the regulator is also fed to the input of the 3.3V voltage regulator. Anyway, both AMS1117 and NCP1117 are the same 1117 device from two different manufacturers. However, the maximum input voltage is 18 volts, a two (2) volts difference from the NCP1117. It has a similar dropout voltage of 1.2 volts. A quick check on the datasheet indicates that it is also a 1 ampere voltage regulator. In the Chinese Arduino clone that I have, the 5V voltage regulator is an AMS1117. In addition to that, it boasts of input operation up to 20 volts. Also, it has a maximum dropout voltage of 1.2 volts. According to the datasheet, it can output a current in excess of one (1) ampere. It is a three-terminal voltage regulator. In the original Arduino Uno R3e, this is a NCP1117ST50T3G chip. The main parts of the power supply are the two (2) voltage regulators, the op amp comparator, the MOSFET transistor, and the protection diode.

In the picture above, the important parts of the Arduino power supply are all annotated. Arduino Uno R3 with the Power Supply Annotated

Schematic diagram of arduino uno