You cannot power the board with the barrel jack and VIN GPIO at the same time, because there is a polarity protection diode, connecting between the positive of the barrel jack to the VIN pin, rated at 1A. Therefore you are able to power the Uno with an external 9 Volt battery. The voltage has to be between 7V and 12V. VIN Pin: If you use an external power supply like a battery, you can use the VIN pin.If you buy a DC power jack, make sure the power adapter of the plug supplies a voltage between 7V and 12V. DC Power Jack: It is possible to use the DC power Jack as power supply.The standard USB connection delivers 5V and allows you to draw 500mA in total. USB cable: The most popular and also the easiest way to power the microcontroller is via USB cable.You can power your Arduino Uno in 3 save ways because a voltage regulator provides a regulated and stable voltage for the microprocessor: The 5V pin of the Arduino Uno is directly connected to the 5V voltage regulator and supports a maximum current that is defined by the difference of the current provided by the voltage regulator and the current from the ATmega328p. I never needed more than 50mA on the 3.3V pin, but in my opinion, a current draw of around 100mA should be possible. Regarding the data sheet of the LP2985, the maximum current is 150mA but on the official Arduino website, the maximum current should be 50mA.

The second voltage regulator, 3.3V LP2985, has an input voltage of 5V and reduces the voltage to 3.3V for the 3.3V pin of the Uno. The maximum current draw from the USB connection is 500mA. There is no need for a voltage regulator because the USB connection is already regulated by the USB output from your PC or laptop. The ATmega328p can also be powered via the USB connection, that I use a lot in my projects. The NCP1117 provides a stable output voltage of 5V and a maximum current of 800mA for the ATmega328p.

Technically the maximum input voltage is 20V but because at 20V the voltage regulator is producing a lot of heat and would break after a short time period, it is recommended to supply an input voltage between 7V and 12V. The NCP1117ST50T3G is connected to the VIN pin and the DC power jack.

Arduino uno r3 board datasheet