Referred to the spec of my stepper motor, the stepper motor operated at 12V. I tested it using 5V. It worked fine. When I used 12V to make it rotate, actually, the free wheel diode didn't burn out but the TIP31C's collector pin burnt with black colour (component side). I checked the solder side, the solder point of the common ground for 1st, 2nd, 3rd, 4th phases had been totally destroyed

from:

XXXXXXXX (linked)

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X X X X (discontinue)

I didn't know why. I desolder the free wheel diode and the TIP31C and check them using multi-meter by comparison between good one and damaged one. They had no difference! What can I do? Actually, I used this circuit for three days testing. It worked fine. When I connected the heavy load with stepper motor. The above problems had been arose. How about if I can replace 12V to 5V to reduce the current passed through the stepper motor so that the TIP31C won't burn?