Daz1712:
Also recomended that the total power through the board beonly around 200mA.If supplied with less than 7V, however, the 5V pin may supply less than five volts
Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.
A 3.3 volt supply generated by the on-board regulator. Maximum current draw is 50 mA.GND.
Google Arduino Specifications for more.
Arduino pins should only be used to drive transistors, fets or other ICs.
You, or the article you got that from, mixed up 'board' and 'MCU'.
e.g. max current from the DC socket to V-in can be <=1Amp.
May..., but not true for the V-in pin. 6volt on that pin will still produce a constant 5volt.
The 'USB disconnect switch' is set for 6.6volt though.
The DC socket needs 0.7volt more than the above, because of an inline polarity protection diode drop.
The 5volt pin is a rail, and can be input (if you know what you're doing) or output.
The 3.3volt pin is output only, and has nothing to do with the rest of an Uno.
They took the 50mA from an older generation, where the 3.3volt came from the FTDI chip.
The current Uno R3 has a dedicated 150mA regulator.
Right... never mind the many errors.
Not important what a pin drives. As long as you keep pin/port/chip current/voltage below the limits.
johnwasser:
Are you saying that the TJ(max) and RJA in the datasheet are overly optimistic? What values have you measured?
Datasheet is probably correct, but board/heatsink is the problem.
I know for sure that a Mega, with a bigger regulator and board/heatsink, could shut down (depending on ambient temps) after a while with 12volt on the DC socket and a ~200mA ethernet shield.
That's ((12-0.7) - 5) * 0.2 = 1.26watt in the regulator.
Leo..