I have an arduino mega that works perfectly when it is connected via USB, but it does not when it is supplied by a 12V adapter instead of USB. The green LED is pale with the latter.
I have measured the voltages and there is only 3,05V between 5V and GND, and the same between 3,3V and ground. (There is 11,35V between Vin and GND).
I have measured the 5V regulator on the board and the input voltage is only 8,29V and of course the output is 3,06V.
Do you have any idea what is wrong with the board?
I use an LED adapter (5A 12V -> 60W). I have tested with another arduino and it works just fine with that. And I also tried to use a different 12V adapter for the board with the issue but result is the same.
There is nothing connected to the Arduino. I have used 12V relays (14pieces) bound to the Arduino pins but they weren't supplied through the arduino (although they received the power from the same adapter but from different branch of wires than what goes to arduino input).
If you mean the Mega2560 R3, then the voltage regulator circuit is pretty simple (see below). But your measurements don't make much sense.
There is 11,35V between Vin and GND.
I have measured the 5V regulator on the board and the input voltage is only 8,29V and of course the output is 3,06V.
With 8.29V on the input, the regulator should produce 5V on the output. So it is evidently overloaded by something.
Only D1 is between PWRin and the regulator input, and it makes no sense for it to drop 3V.
