I am new to this, so be gentle with me.
I want to use a Uno to operate 6 Servos, and 6 small 5VDC relays to control electrofrog points on a railway layout. As I need a 5VDC feed for the servos, can I supply the Uno with 5VDC to the 5V pins on the board.
If I do this, and then plug my PC to the board, will the two voltage supplies fight with each other.
Hope that is clear.
Thanks for your help
BC
If you have enough regulated 5V to not drop when the relays and servos run then that 5V can power the Uno but IIRC on older Uno boards USB and external 5V might conflict.
PS - on checking, if your 5V is 5V then no problem. See reply #6 here:
https://forum.arduino.cc/index.php?topic=194598.0
OK, Many thanks.
BC
Further to my explanation quoted above, the concept of "fighting" relates to the possibility that the additional power supply may feed current back into the USB power of the PC and this has in some cases been reported to damage the PC or laptop. Clearly you would not want the external power supply to be powered if the PC is not.
How likely this is, is dubious as it turns out that the majority of powered USB hubs do precisely this, and without a polyfuse as protection, as all 5 V lines in these hubs connect directly together. There seems to be little report of this causing damage.
With this caution, it is certainly the case that for any project involving significant current requirements, powering the Arduino by a properly regulated 5 V supply via the "5V" pin is the only practical and sensible approach. (The "barrel jack" and "Vin" pin are essentially useless for powering other than the on-board components alone.)
We can still run small power uses on USB/VIN external, it means beginners not having to learn what more than 6 leds begins to need when getting started -- but the next steps take learning all these new caveats to keep the smoke away.
Err, yes! 
It's like learning to code with bad habits to make getting started easier and then finding that if you want to do much more then what it will take.
I learned a lot from OPC (other people's code) and still do. I gave karma for help I got from a 2015 post just yesterday. TY to Hazard's Mind!
Thanks for the replies regarding the 5 volt supply issue. I think I might try cutting the 5 volt wire in the USB lead to stop and conflict between supplies.
BC
I tried that, but it does not work.
just unplug the Uno from the servos to re-program if required.
Thanks for your help, subject closed.
BC