Hello,
Thank you very much for your comments. A silly question (sorry but not an electronics expert). My understanding is that the SMD voltage regulator within Arduino Mega ( NCP1117ST50T3G) drives up to 1A. The stepper motor I am connecting needs much lower current - why this may makes the voltage regulator fail?
In any case for sure next time I will use external voltage regulation circuitry to avoid this happening again.
Best regards,
Jaime.