Diode and transistor failure

[Weatherman]

Hi to u all,
My mini drill stopped working and ì open it to assess the cause of the breakdown.inside on a small pcb was the control circuìt built around an LM358 ic and a transistor.The transistor casing was cracked but fortunately,i could still read its type which was an npn darlington tip120. The diode across the motor was also destroyed but the dc motor and the rest of the circuit protected by a zener diode were spared.my question is why has the transistor failed.?was the diode not supposed to protect it from back emf?could the harm be a thermal issue?i occasionally use the drill at moderate speeds for some woodwork .awaiting your answer.thanks for helping.

 

[(*steve*)]

it is possible that the diode failed short circuit first and then the transistor failed because it was switching the entire current available from the power source.

 

[dave9]

Sometimes things just die. Bad design, manufacturing fault, whatever. What you should do now is repair it and make measurements such as current and temperature, then go from there deciding if the repair is done or something was too close to the limits and needs beefed up.

 

[duke37]

Check the brushes, commutator and armature for condition.