Will a power supply of 5v work for the DC barrel jack?

I am starting a project that involves a power bank in order to supply the arduino. Will a power supply of 5v work ( turn the green light on and supply 5v through the 5v pin) for the DC barrel jack?

vinny3777

The regulator will drop a volt.
Can be avoided by feeding to 5V pin instead, or make a bypass of the regulator

No. The 5V regulator requires about 2.5 volts above the 5V to regulate properly. Plus there is a diode between the power jack and the regulator to prevent reverse connection from damaging the regulator. So the power jack needs 7.5 to 8V to feed the regulator. You can connect the 5V (if it is a solid regulated 5V) to the 5V input.