I’ve got an Arduino Uno Rev 3 and an Adafruit Motor Shield v2.3 on top of it. My design requires sending input to the Arduino (numbers). Therefore, I need to have the USB cable always connected to the Arduino. I’ve got also a stepper motor (8V @ 0.4A) which is connected to the Adafruit Motor Shield (M1 & M2).
Is this generally okay for the USB port and the Arduino/Shield to run only USB power?
Kickback is a different problem, and I would also trust Adafruit in that respect.
Do yourself a favor, and get a separate power supply for the steppers.
Even if they are rated for 8V, most steppers are driven with higher voltage and current controlled.
You can't run motors off of a 9V battery so don't waste your time/batteries!
Use a big Lead Acid or NiMH battery pack. Its also very much suggested that you set up two power supplies (split supply) one for the Arduino and one for the motors...
The 9V thing is about the little rectangular batteries that were used for smoke alarms. They can't provide enough current to drive a motor for long, if at all, but people keep making that same old mistake because they bought a kit with one of those worthless batteries in it.
That's not to say that you couldn't drive a motor off a more substantial 9V battery.
The Arduino UNO has a 500 mA 'polyfuse' to keep it from drawing more than 500 mA from the USB cable.
8 Volts at .4 Amps is 3.2 Watts.
3.2 Watts at 5 Volts is 0.64 Amps (640 mA).
Even if you had a 100% efficiency DC to DC converter to get the voltage up to 8V you could not draw enough current from the USB cable to feed the stepper.
I would try a 2-cell 18650 Lithium Cell holder. That will give you a nominal voltage of 7.4V which will likely be adequate for your stepper AND your Arduino. Figuring 45 mA for the Arduino and 400 for the stepper you might get about four hours per charge.
Thanks for the detailed answer; I am wondering why I can run my setup (Arduino + Shield + stepper motor + ir beam breaker (soldered to 5V pin) for hours, with just USB power and no external source ... Why is all that working with no external power source?