Hi everyone.
I am designing my first custom Arduino for a hobby project. I am using the ATmega 328p and using the Arduino Nano schematic as reference.
I see there is a diode that blocks the Arduino from attempting to power the PC. But when the battery is disconnected and Arduino is plugged in to the PC, +5V from VCC of Arduino will connect to the output pin of the regulator. There is no protection for backfeeding. How does this work?
The output of the regulator is 5v when it has power input.
The cathode of the diode is connected to the regulator, the anode is connected PC 5v, hence the diode does not conduct.
When the is no power input to the regulator and the PC is connected, the diode forward biases giving 5v - .3v = 4.7v for the controller.
This 4.7v goes to the regulator output which is capable of withstanding power on the output when not powered.
Ah, I see. I did not now this. I thought the regulator will break if there is power on its output but no power on its input, especially when there are capacitors connected to the regulator. What would happen if there was another component, like for instance an LED connected to the input pin of the regulator. Current would flow from the output pin of the regulator to the input pin, would that damage the regulator?
Cannot comment on that but I think this should help.
A diode, anode to output and cathode to input is sometimes added in some circuits.
Hi,
Have you thought of keeping the Nano assembly and mounting it to your PCB with header sockets.
Then you will be assured of a good working controller when you trouble shoot your PCB.
Here is a pic of a prototype I used, with a Nano plugged in, makes life so much easier.
Tom.. 
Thank you very much 
Edit: The schematic shows that the regulator has 4 pins, but I have only seen these regulators with 4 pins (in, gnd, out), do you know why it shows 4 in the schematic?
Yes. I have done this before with Arduino and modules. But now I want to make my own PCBs with just the microcontroller instead of using the Arduino and make my own modules
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