Can the Uno R3 be run on 5v supplied via the 5v V-In pin while it’s also receiving 5v from the USB from the PC ? Reason for the question - it’s a turnkey application running on 5v at V-In pin, but sometimes will need to be reprogrammed via USB.
No
Powering on Vin (not the 5V pin) would be save but the specification for Vin is 7..12V.
Vin must be greater or equal to 7V and less than 12V.
It will not work on 5V connected to Vin
Nope. The Vin pin purpose is to feed the board with 7-12V in order for the regulator to provide +5V to the rest of the board. Thus it is “before” the regulator.
Yes…
It is called the 5V pin.
5V is for 5V. VIN is for 7-12V
Now I understand my error, so I’ll change my question. If I power the Uno R3 with 7v-12v via the barrel jack, can I also plug in the USB with its 5v supply ?
Technically, yes. IF you share grounds VERY carefully. But safe practice is no. Or, build/get yourself a smart power system that only allows one input ant automatically switches…. Depends on your application.
Also the barrel jack is basically connected to the VIN pin, I believe through a diode.
Yes
Now the question is: What is connected to the Uno. You may be overloading the Vin input.
Driving 6x 5v servos from their own 5v supply and 8x 12v solenoid valves from their own 12v supply. The Uno power input is not powering anything apart from the Uno.
Then no problem.
Doesn’t the barrel jack and the USB socket share the common ground?
It’s sleepytime past midnight Sydney Australia - I’ll pick this up in the morning.
Melbourne for me.
Depends on who made the board 
More the shield doesn’t on some….
Yes but why do you ask?
Grounds are common on my Uno and the shield. Does that mean it’s safe to supply voltage via both the barrel jack and USB?
Because @outbackhut raised the question about the common ground.
Forgot - the only drain on the Uno power rails is 4x 10k pots on 2x 2 axis joysticks. All the rest, are controlled via a 16-Channel 12-bit PWM Servo shield I2C interface PCA9685 card, with solenoid valves driven by 2x 4 channel by opto-isolated Mosfet cards.
Thanks for checking on total load.
It's safe but has nothing to do with common grounds.
The Uno is one of the Arduino boards where you do not need to worry about about applying external power and USB power at the same time
Thank you. Marked as the solution.
The danger with injecting 5volt on the 5volt pin is that there is a direct connection to the 5volt USB rail of the PC/laptop. Sleep the PC/laptop, and you could be back-powering the laptop through the USB socket. Your guess is aas good as mine what could happen.
"> 6V6 on V-in or > 7V2" on the DC socket should be safe, because it disconnects the USB lead with a mosfet switch. At least with genuine Arduinos. Not so much with a clone.
Leo..