The transformer is a step down transformer from 220Vac to 6Vac.

The book said that suddenly disconnecting the DC input would produce an impulse of about 1000 Volts at the output.

My guess is the because the current through the secondary winding goes abruptly from constang to zero (like a step function), then d(flux)/dt is proportional to a Delta function (because the derivative of a step function is a delta function).

what do you think?

thanks.