I'm using an Uno to communicate with a PC via USB and control several motor controller boards. These boards all operate at 5V, and I would like to power these boards as well as the Arduino from a common 5V source (in this case, the 5V rail from an ATX power supply). Is there a way to bypass the USB power and onboard voltage regulator and just run the Arduino from my own 5V supply?
When I made my bench supply from an ATX, I went and bought a USB A-B cable. Then I cut the "normal" usb plug (the flat one, that goes into PC usually., is that the A or B :~ ) off, and hard-wired that into one of the ATX 5v outputs. So I have a dedicated Arduino 5v USB power plug on my bench, at exactly the same voltage characteristics as a PC USB since it's from a PC's usual style of 5v supply.
JimboZA:
So I have a dedicated Arduino 5v USB power plug on my bench, at exactly the same voltage characteristics as a PC USB since it's from a PC's usual style of 5v supply.
But absolutely none of the usual protection built into the USB ports. 
In regard to the original question - it is dead simple - if you have 5V regulated, you just feed it into the 5V "Vcc" terminal on the Arduino. (Actually "IOREF" is also the same connection in most cases.)
I will have a PC communicating with the Arduino via USB, so I can't use a USB cable to power it.
If I feed 5V directly into the Vcc terminal, will that switch off the USB power from the PC, or will I need to modify the board somehow so it only draws power from my supply and not the USB connection?
As I understand it, the power selection circuitry requires at least 6V through the barrel jack to switch off USB power, and putting 5V on the Vcc terminal won't switch off USB power.
n1kt0:
I will have a PC communicating with the Arduino via USB, so I can't use a USB cable to power it.If I feed 5V directly into the Vcc terminal, will that switch off the USB power from the PC, or will I need to modify the board somehow so it only draws power from my supply and not the USB connection?
As I understand it, the power selection circuitry requires at least 6V through the barrel jack to switch off USB power, and putting 5V on the Vcc terminal won't switch off USB power.
Most of your concerns are true but many suffer no problems powering directly via the 5V pin. What I did some years past when presented with this "working around" the auto-voltage selector circuit was to just take a spare USB cable and hack off the PC end and strip out the 5vdc power and ground wire and wire them to any external regulated +5vdc voltage source/power supply. Then if I needed to download a new sketch I would just unplug the modified USB cable and power the board via a PC using a standard USB cable. Otherwise I would just plug in my external 5vdc voltage source via it's modified USB cable. As long as your external +5vdc is of suffient quality and capacity this method is no different then how millions use and power their arduino via a standard PC via USB cable.
Lefty
retrolefty:
Most of your concerns are true but many suffer no problems powering directly via the 5V pin. What I did some years past when presented with this "working around" the auto-voltage selector circuit was to just take a spare USB cable and hack off the PC end and strip out the 5vdc power and ground wire and wire them to any external regulated +5vdc voltage source/power supply. Then if I needed to download a new sketch I would just unplug the modified USB cable and power the board via a PC using a standard USB cable. Otherwise I would just plug in my external 5vdc voltage source via it's modified USB cable. As long as your external +5vdc is of suffient quality and capacity this method is no different then how millions use and power their arduino via a standard PC via USB cable.Lefty
Clever workaround. Thank you for the advice!
How is this for complicated solution?
Hint - pay attention to "Power"
But if you are unsure about hooking it up with right polarity,don't.
Cheers
Vaclav