mitsakos:
Hi Paul, there is a note just right below the cable of the input power jack, its 5V via a voltage regulator. i read that UNO can do 6-20V, and safe margins are 7-12V as input. i already burned 2 boards for whatever reason and the dc jack input no longer works. so i am using a 5V power jack input on the new board.
The Uno can run on 7-12 volts thru the power jack because it has an on-board 5 volt regulator to power the processor. If the DC power jack no longer operates, it's because the regulator has died.
If you connect your 5 volt power to the 5 volt pin of the Uno board, that will power the Uno directly, No regulator.
There is NO 5 volt power jack on any of my Arduino boards. There are 5 volt pins. Is that what you are using?
Paul