problem with arduino uno + L293D shield

Using Arduino Motors, Mechanics, Power and CNC
#1

hi everyone, i'm Michele from Italy.

i've got a strange problem:

i'm using Adafruit L293D shield motor with arduino uno to drive a double coil stepper motor.

the motor run correctly and it seems all right with my code but, if i disconnect my usb cable from pc after transferring code, the motor stop running and start to vibrate, if i reconnect usb cable to arduino and pc, all return to run correctly.

can some one explain me the reason?

thankyou all

#2

When you disconnect the USB cable, what is supplying power to the Arduino and the motor shield?

#3