Nano and double alimentatiom

Hello i m working on a small project where i have a nano supplied by a typical circuit with a lm7805, but sometimes i will put the miniusb to get serial data from the nano.
Is a problem for the nano to be supplied by two different source? If i m not wrong it has a selector but i would like that it uses only the lm7805 and never the usb

The Nano doesn't have a fancy power switching circuit like on the Uno/Mega - the 5v rail is connected to the 5v on the USB connector. It is generally inadvisable to connect two power supplies together like that. You'd need to modify the board to cut that line. Or you could connect a MicroUSB cable with the power wire cut.

The Arduino Pro Mini would be a much better board for this purpose (they're super cheap on ebay too), since the serial adapter is not on the board at all. Then, disconnect the Vcc line between the Pro Mini and the external serial adapter (this is of course easiest on the serial adapters that you use dupont line to connect to the target, instead of the ones that plug in directly, since you can just not connect that wire, or if using a 6-pin housing, just don't put the Vcc line into the connector). I have a bunch of serial adapters with the Vcc line cut for this sort of thing.

DrAzzy:
The Nano doesn't have a fancy power switching circuit like on the Uno/Mega - the 5v rail is connected to the 5v on the USB connector. It is generally inadvisable to connect two power supplies together like that. You'd need to modify the board to cut that line. Or you could connect a MicroUSB cable with the power wire cut.

The Arduino Pro Mini would be a much better board for this purpose (they're super cheap on ebay too), since the serial adapter is not on the board at all. Then, disconnect the Vcc line between the Pro Mini and the external serial adapter (this is of course easiest on the serial adapters that you use dupont line to connect to the target, instead of the ones that plug in directly, since you can just not connect that wire, or if using a 6-pin housing, just don't put the Vcc line into the connector). I have a bunch of serial adapters with the Vcc line cut for this sort of thing.

I already have 5 nano or i would have used the pro mini as you said. And you answered to my next question before i asked it:

So i can use something like it with the power but not ground, cut, correct?
http://s.aliexpress.com/aUrmuqUB

Hm - I rechecked the schematic, and was wrong - there's a schottky diode between VUSB and VCC - so even if you have it powered both ways, the power supplies won't fight eachother (though if you don't want it to be powered from the USB ever, you would still have to do something)

Cutting the power wire in cables like that should work. Also, thanks for that link, that cable price is significantly better than I found looking for mini USB cables recently.

But if it is powered both ways which one does it use?

Anyway i found this one also if you prefer

http://s.aliexpress.com/fYRrEVZJ

I think that cutting the wire directly on the board is better for my project, do you have any idea how to do that?

It’s got a schottky from VUSB to Vcc, so if Vcc is more than 0.3v below VUSB, it will come from VUSB, otherwise, from the other supply. If VUSB-0.3v is very close to Vcc, some power will be drawn from each of them (per the I-V curve of the schottky diode). Note that it’s not unusual to see up to 5.2v on USB +5v rail - they do this to help things charge better, to counteract loss in the cable while drawing highish current to charge a phone or something.

Probably removing that diode is the best way to modify the board to disconnect the USB in, actually… There’s only one diode on the board, so it’s easy to find, and removing a diode is more reversible than cutting a trace, normally.

Very interesting! Ok i will try to remove the diode as you said, i m not so bad with soldering so i think i will be able to restore it after, tomorrow i will do it and then i write here if it worked
Thank drAzzy karma++++

here on #7 i have also found another, but more traumatic, way of doing it

done, chottky diode (B2) removed and everything work as you expected, thank you