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I'm working on some little projects that'll involve more than one Arduino (right now two of them, an Uno R2 and a Mega R3) along with some other 5V items that may themselves draw a few amps.

Rather than have multiple external power supplies, I'd like to have one 5V supply for the whole project, feeding all the Arduinos (which won't directly draw that much power) and the high-power part as well. (Alternatively, I would be willing to create a 12V->5V regulated supply just for the Arduinos - the base supply has a few buses to work with - since the high-power 5V line may sag a bit under its load (a long LED strip)... or just feed 12V to the wall adapter inputs of the Arduinos instead.)

All that said, if you have a nicely regulated 5VDC supply to work with already, how best to feed that to a few Arduinos? What happens if you then plug one of the powered Arduinos into a computer? Will this direct power supply and the USB power be in conflict?

(Note: My understanding from this related topic is that if you power an Arduino via the external adapter AND plug the USB into a computer, the external adapter still does all the powering. However, if you are feeding 5VDC regulated to the Arduino some other way (e.g. Vin) then I'm not sure how that'll work... assuming that's not Just A Bad Idea.)

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You have a couple of easy options here: feed in an unregulated voltage, like the 12v, to the Arduino's Vin pin (on the Arduino headers), and let the Arduino regulate it, or feed in 5v to the 5v pin. In either case, of course, you should connect the ground pin to your PSU's ground rail. From the Arduino schematic, you can see that Vin is only used to feed the 5v regulator and the comparator for the USB/Vin switch, so if you power the Arduino over the 5v rail, you should not connect anything to the USB port.

Assuming you have a fairly stable and clean 5v source, feeding the 5v is the more efficient option.

You are correct that the Arduino is powered over USB only when no DC supply is present. This is implemented with a comparator, comparing half the voltage on the Vin pin to the 3.3v rail - so the switch will work correctly as long as power is supplied to Vin.

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Is the Vin pin and its trace able to handle the full power draw the Arduino's designed to handle via the normal power plug? Is the 5V pin and trace equally capable for this purpose? I recall reading somewhere that it might be unwise to try to run an Arduino at full power this way. It looks like tying the 5V lines together and running them to external, regulated 5V power is a BAD idea if I ever intend to plug them into a USB port (and if that power is accidentally left attached), whereas using Vin (or probably better, the jacks) should be safe in this mode. –  MartyMacGyver Dec 16 '12 at 16:53
    
@MartyMacGyver That depends what you mean by 'full power'. The pins are rated at about 3 amps, but how thick the trace is depends on the Arduino model in question. They're certainly sufficient for powering the Arduino itself, but if your Arduino has high power external shields that use the same lines you might want to think more carefully. And yes, that is exactly what I meant - you should only power the 5v rail directly if you'll never connect USB in this mode. –  Nick Johnson Dec 17 '12 at 10:22
    
I appreciate the thorough answers! :-) I'll look into powering these either from their power jacks or possibly via customized USB power plugs if I want to take the 5VDC-in route (in that case I would have to unplug the USBs to program them, which is more memorable than unplugging a relatively anonymous wire from each board first). Now to figure out how to tame the 12V supply to feed it to the power jacks (ideally down to around 7.5V to minimize waste heat from the on-board regulators). –  MartyMacGyver Dec 17 '12 at 10:58
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