Uno... Automatically Switch to 6V Battery's when 12VDC Supply Main Disconnects?

I've seen lots of posts regarding battery backups with charging batteries etc...

Hopefully a simple question...

Will the Arduino Uno automatically start grabbing power from the 6V (4 x AA's) connected once the main port 12VDC drops out? I realize that the 4AA's won't last very long, but will last long enough during a standard power outage.

Thanks, DJM

No, you need a diode or other type of battery isolator. If you have 4XAA's, one or two (safer) 1N4001 diodes between battery + and Vcc (not RAW) will work, as a diode will drop about 0.7 volts.

If the 6V supply is sent in through the USB cable the UNO will switch to the USB cable when the Vin goes below 6.6V. Your current will be limited to 500 mA by the USB polyfuse.

Is it OK to put 6V on the USB V+ line for an UNO?

The ATmega328P and ATmega16u2 can tolerate a Vcc of 6.0V (Absolute Maximum Rating). I don’t know what else you have connected and if it is 6V tolerant. To be safer you could add a series diode or two and use the forward voltage drop of the diodes to bring the voltage down a bit.

jremington: No, you need a diode or other type of battery isolator. If you have 4XAA's, one or two (safer) 1N4001 diodes between battery + and Vcc (not RAW) will work, as a diode will drop about 0.7 volts.

Than you all for the responses. Great info. I believe that this best suits my needs. Going to have a 12V 2A 2.1mm Wall Wart powering the unit primarily, and have the 6V (4xAA) diode isolated going to the VIN.

Thanks for the great answers. And actually made me think about possibly powering the unit via USB too.