I have a problem I can imagine probably a lot of people might have. In a project that requires 'high power' output say a lot of leds or motors etc, I often power the Arduino from the same power source as the load supply, 5V, 12V... This becomes a problem when the Arduino is connected to USB for programming. If the load power supply is ever disconnected when the USB is connected, the USB backfeeds the load, which in some circumstances can be like a short circuit. Also, to do a power cycle requires cycling both powers in a particular order, no mistakes! I was wondering how people address this common issue, or am I missing something?
I guess you could add a diode and a switch between the Arduino 5V pin and the external power supply.
The diode would prevent the USB from accidentally supplying the external circuit. The switch could be used to cycle the power when USB not connected, although pressing the Arduino's reset button should be enough.
I am using this kind of USB-Hub on which I can switch on/off the powersupply from my PC. So when I have my Arduino circuit powered externally I switch off the power from the PC. The Data lines are always connected. This works great and my cat likes it too
Thanks. No offense, but I don't understand why this feature isn't a part of all Arduino circuitry? Sometimes I have motors and leds that will pull up to 10A at 12V. I've also seen cases where devices that are no longer powered are somehow still working by drawing power through the data communication lines connected to the Arduino. Very scary when they are high power devices.
Good question. May be it's got something to do with the attitude of the Arduino developers that Arduino boards are meant for the hobbyists, for people that start out with anything microcontroller. As these people learn more about microcontrollers, programming and electronics, they might want to do more with their boards, but at the same time they know more.
Having said that, I agree with you that Arduino could do better, both in making their boards more universally applicable and in documentation.
Hi,
You can leave your project fully powered up and use a USB cable with the 5V wire cut so there is no power conflict between programming computer and Arduino.
This is my isolator;
It is a USB extension cable, the white wire bridges the 5V supply for normal operation.
Remove it to isolate the power supplies.
Tom.... 
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