5V.This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.
I'm a bit concerned that it's not advisable to supply power to the 5V pin. Is it just because you could get the polarity and voltage wrong and wreck your arduino
or is there something else to be concerned about?
Do you know what happens if I supply voltage to the 5V pin and also have the USB connected to my computer?
The power pins are as follows:VIN. The input voltage to the Arduino board when it's using an external power source (as opposed to 5 volts from the USB connection or other regulated power source). You can supply voltage through this pin, or, if supplying voltage via the power jack, access it through this pin.5V. The regulated power supply used to power the microcontroller and other components on the board. This can come either from VIN via an on-board regulator, or be supplied by USB or another regulated 5V supply.3V3. A 3.3 volt supply generated by the on-board FTDI chip. Maximum current draw is 50 mA.GND. Ground pins.
The reason was discussed a while ago. The 1117 5V regulator is susceptibe to damage when reverse driven. The earlier posts discussed bricked boards after putting 5V into the board via the header pin. A member of the team discussed it with us. http://forum.arduino.cc/index.php?topic=141639.0
I read that a suitable voltage to power a board thorough the Vin pin is 9V but I don't know how much current is enough. Will a 9V 1A supply be enough? I have a 5V supply but I wouldn't occupy the USB port.
If USB is not connected, you can power the Mega from the 5V header. Be sure to connect a diode from 5V (anode) to Vin (cathode) to avoid totally reverse biasing the 5V regulator. See page 10 here:http://www.onsemi.com/pub_link/Collateral/NCP1117-D.PDFEarlier boards like the Duemilanove used a different 5V regulator that wasn't susceptible to being damaged by reverse driving.