Ring = Cathode...

Frank,
What current are you trying to drive. When V is applied, I ramps up at a rate proportional to L and V. Are you letting I get too big, and the stepper is saturating ? If that happens I >>>

That will punch out a TIP 31 C
Ic average = 3A
Ic Peak = 5A, you don't have any leeway there, a mosfet might make a better choice.


Steve