I have a musical instrument that needs 5Volt to work. It draws about 200MaH. It normally gets that from a USB input.
I am now adding a 5V power pack, in order to go wireless. Can I leave both 5V sources connected at the same time or will this cause overheating when USB power is plugged in?
Thanks in advance!
Wrenze
It is drawing 200mA, not 200mAh.
Give us a circuit. Drawn on a sheet and photographed will do.
Weedpharma
Thanks for asking for a diagram - in doing that I have just answered my own question: it seems that I was unwittingly considering connecting the powerbank directly to the USB port on the laptop - maybe not such a good idea 
Power supplies are not designed to be operated that way... It would probably be OK, but if the USB power is coming from your computer I wouldn't take risk because even though the odds of frying your computer are slim, one fried computer would ruin your whole day. 
It won't hurt your Arduino.
You can put a diode in series with each supply and that will prevent current from flowing backwards into one supply or the other, but you'll get a ~0.7V drop with a regular silicon diode and a ~0.2V drop with a Schottky diode.
Thank you Doug, very helpful. I think I'll fit a switch to choose one or the other...
Worth watching.
Don't ever let two regulated supplies fight it out by connecting them to the same load,
the worst case if everything gets fried (seen it happen) - voltage regulators are
at heart high current high gain amplifiers and are designed to be stable only into
a dissipative load, not another regulator's output.
You can power an Arduino via the DC jack or Vin pin (7V or above) while USB power
is applied as there is a switching circuit on the board especially to enabled this.
You can share supplies via diodes at a pinch, which protects against instability but
loses about 0.7V and generally only takes power from one of them depending on
relative voltages.
