Before I fry something >:( , please
I am using a "Red Wings" on-board power supply to power a nRF24L01 breakout. I would like to use it to supply the UNO as well (since it provides regulated 5v).
Should it be safe to connect Gnd to Gnd and +5V to Vin on the UNO?
Should this power the UNO without using the 2.1 mm power jack or the USG input?
Gnd to Gnd is ok, +5 to Vin not so ok. Vin is the input voltage to the 5 volt regulator and it requires at least 7 volts or more to operate properly. If you are sure the breakout is providing a well regulated +5 then you could apply that to the 5V pin between the double ground and 3.3V pins. Be aware that Arduino.CC does not recommend this. Best not to have the USB cable plugged in and nothing plugged into the barrel jack.
If you're satisfied the 5V is within USB limits, you could hack an old printer cable and put 5V into the USB.
I do that from an ATX PSU, but then of course we know an ATX 5V is USB compliant since that's where a PC USB gets its 5V from in the first place.
Due_unto:
Gnd to Gnd is ok, +5 to Vin not so ok. Vin is the input voltage to the 5 volt regulator and it requires at least 7 volts or more to operate properly. If you are sure the breakout is providing a well regulated +5 then you could apply that to the 5V pin between the double ground and 3.3V pins.
Glad I asked. I thought the 5 V pin was output.
Be aware that Arduino.CC does not recommend this.
Yes, aware of that. It's why I asked in the first place.
Best not to have the USB cable plugged in and nothing plugged into the barrel jack.
My whole object was to have only one input from mains. I am using the nRF24L01 breakout and they are sensitive to fluctuation in the 3.3 V supply they receive. The regulator fixes that but it means a second power infeed. By using the 5 V from the external regulator to feed the UNO I hoped to be able to get away with only the one mains feed.
Is that idea okay, or is there a better way?
Glad I asked. I thought the 5 V pin was output.
It can be used as either input or output, it is just a connection to the 5V rail of the board.
vagulus:
Glad I asked. I thought the 5 V pin was output.
As long as your 5 volt is well regulated it wont hurt the Uno. It WILL however backfeed +5 volts to any device connected through the Uno's USB port. This is why it's not recommended.
Uh Huh! 
I'll just have make sure that I don't have the USB connected when the power supply is.
Thanks
vagulus:
Uh Huh!
I'll just have make sure that I don't have the USB connected when the power supply is.
Thanks
Follow manner_royal's suggestion and use a USB cable to apply the 5V power. That way you must unplug the power supply before you plug into the computer's USB. Idiot-proof.
But you only get 500mA due to the PTC resettable fuse near the USB connector. May or may not be a concern.
CrossRoads:
But you only get 500mA due to the PTC resettable fuse near the USB connector. May or may not be a concern.
You're already splicing two cables together, nothing stops you from making a Y.