suppling power to +5v pin AND saving USB communication

Hi! I would like to power my Arduino Nano through pin +5v.
I'll use a 12volt battery, connected to pin VIN of a Pololu5v voltage regulator

Then I will connect the Pololu VOUT pin to arduino +5v pin.

I read on many posts that this connection is ok, but that in this case, it is dangerous to connect the USB cable too, because of the power from USB is no more switched off .
I'm wondering if there is some way to save both the powering to Arduino +5v pin and the USB communication.
Thanks a lot.

If you check the schematic, there is a diode between the +Vusb and +V5, so nothing shouldn't happen. I have some nano clones and, at least on mine, the diode is present. Check your just to be sure.

Ciao, Ale.

There is a lot of mystical concern about the possibility of "back-feeding" power into the PC's USB system however in reality, almost all common "powered USB hubs" have their 5 V adapter directly connected to both USB input and all outputs, while the Arduino provides a polyfuse in that path for protection.

The concern is moot.

Ok, thanks. I tested it right now, and I confirm it seems to work! Bye, Valerio

Paul__B:
There is a lot of mystical concern about the possibility of "back-feeding" power into the PC's USB system however in reality, almost all common "powered USB hubs" have their 5 V adapter directly connected to both USB input and all outputs, while the Arduino provides a polyfuse in that path for protection.

The concern is moot.

I've had one or two of those direct-connect hubs before. Not the best quality and certainly the wrong design. I used to use the upstream cable to power a raspberry pi and downstream with USB flash drive and wifi dongle :smiley:

I attached here the circuit that I succesfully tested.

https://plus.google.com/u/0/105563585590680769581/posts/fXBMecgLULN?pid=6302279960445282434&oid=105563585590680769581

One more question:
I inserted a diode 1N4007 between arduino +5V and Pololu VOUT. I did this to prevent current flow in the wrong way (from +5V to VOUT), if I switch off the battery and power the arduino through USB only. I thought it could damage the Pololu. Is it a real problem? Is that diode useful? Thanks.

In that way you have to consider that you are feeding your arduino with less than 5V, should be around 4.3V. This is not a big problem for the mcu itself, but take care of it if you are using the supply voltage as reference for analog reading (as is for default).

Ciao, Ale.

ilguargua:
In that way you have to consider that you are feeding your arduino with less than 5V, should be around 4.3V. This is not a big problem for the mcu itself, but take care of it if you are using the supply voltage as reference for analog reading (as is for default).

Ciao, Ale.

Atch! I didn' think about it! I guess this is because there is a 0.7 voltage drop due to the 1N4007, is it right? In your opinion, is that diode useful? Maybe it is not necessary to protect the Pololu from reverse current...I'm not an expert in electronics :slight_smile:
Thanks a lot, Valerio

Yes, is the diode Vf voltage drop. I don't know which polulu adapter are you using, try to go on their site and see if schematic is available (usually it is), so you can see the kind of regulator is used and if need backfeed protection or not. Say that, it's not so common that you need 2 feed for your arduino, so if USB is used for example just for load now sketch you can put a small plug on polulu output and disconnect when reprogramming, or if you need instead a serial monitor always active you can set up a USB cable without the +5V.

Ciao, ALe.