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I have a very simple project that is working off of an external "wall wart" power supply and I would like to add a back up battery in case of power failure. I have breadboarded a backup power supply with two DC barrel jacks and 2 diodes and it works, supplying the higher of the two voltages.

My question is: do I have to use this circuitry to connect to Vin, ignoring the already in place barrel jack on the Arduino board; or can I eliminate the second of my barrel jacks and diodes and rely on the on-board jack for the wall power and simply take the batteries voltage through my extra jack and diode and then into Vin? I.e., can the Arduino handle power supplies from the on-board jack and Vin simultaneously -- switching back and forth as needed -- or will it just use Vin if it detects an input there?

The Arduino docs don't say this specifically and the question Arduino backup power supply seems to indicate that this is the case, but I just want to ask the question simply and decisively.

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up vote 5 down vote accepted

If you look at the Arduino Uno v3 schematic, you'll see that the onboard jack has a series diode between it and VIN. If you've got a series diode between your battery backup and VIN, then you are replicating the dual-diode configuration that you're already using...you'd just be leveraging one of the diodes on the Arduino PCA.

enter image description here

Note that this doesn't address charging of your battery, but it doesn't sound like that's addressed in your current arrangement, either.

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