I have an Arduino Uno R3. At the 5V output port of the board with nothing connected and blinky running while powered through USB, I can measure only 4.5V. Is this normal? Or is there anything wrong with my board?
Now that you've posted the schematic, a straight answer is possible. That schematic is a sloppy mess, but I know that's not your fault. It doesn't say much about the folks that designed your arduino board though. Yucc. It makes you wonder what else they didn't pay much attention to.
The board can be powered from USB or a external supply. When from USB only, there is a diode from the USB power to the "5V" supply. The USB supply voltage is nominal 5V, but can be lower than what you actually measured in some cases. What you measured is about expected with 5V from the USB then with a diode drop in series.
It looks like there is at least a reverse blocking diode (D1) at the external DC power input. That means if you hook up the external supply backwards the board won't run but nothing will be damaged. You therefore didn't break anything, at least due to providing the wrong external supply polarity.
Olin is wrong, there is no diode between USB input and 5V(i suppose he's talking about the internal diode of the T1 mosfet) . when there is voltage from the USB port, the MOSFET will be turned on by the op amp (U5A) and this will bypass the internal diode of the mosfet. depending on the Rds(on) of the mosfet, there will be a series resistance between USBVCC and +5V and that is approxim. 70mOHM according to the mosfet datasheet
I had same issue, dead MOSFET was the reason. I just removed it and solder wire there. Actually it's not good way to fix, but it works, but you should not use external DC in with that fix.
It's just video to show problem (USB 5v in and 4.5 after MOSFET) http://youtu.be/g3qYECB3tc8