Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have to look up the inductance... big open air coil. So a user had a problem and it looks like the pass transistor blew. (2N6059... I did this design ~15-20 years ago.) The circuit is a simple opamp voltage to current converter. And has never had problems before (that I know of.)* So the tech at the school who is replacing the transistor was speculating that maybe the students unplugged the coil while it was under power and the back emf blew out the pass transistor. Playing around on the bench I see very little back emf.. a few volts at most. I do see some sparks when hot plugging the coil in and out (with 36 V supply voltage.) But I think this is just arcs from the power supply... it looks to be the same size spark when plugging in or out.
Anyway I'm not sure what happened, and is there something obvious I might not be thinking about?
George H.*One can always worry that it failed on someone and then they never asked for help, but just stopped using it.