# transformer response to a step input

#### AlexK

Joined May 23, 2007
34
I saw a circuit in a book, in which 6V DC was connected to it's secondary winding and the output voltage is the voltage on the primary winding.
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.

#### thingmaker3

Joined May 16, 2005
5,083
I think you have accurately described a part of "inductive kick."

The current in the primary goes from constant current to zero current in short time. Rapid collapse of the magnetic field induces voltage across the secondary. We can use Mr Faraday's equation: E= -N(dFlux/dt) where N=number of turns.

#### AlexK

Joined May 23, 2007
34

So how can I calculate the exact voltage that will appear at the secondary winding? How fast does the current go from constant to zero? What number of turns should there be to produce 1000 volts?

Edit: I've read about the inductive kick and I have some more questions.
Suppose I have the circuit given here http://www.coilgun.info/theory/inductivekickback.htm.
Normally the current flows from A to B, when the switch is opened the inductor should oppose the change and will want to impose the current in the same dirrection as before, right? so why does it say that terminal B will become positive relative to terminal A, doesn't it mean that current will flow in the opposite direction?

And could you elaborate more about the protection methods given there? I didn't understand how the diode and the zener diode protection work.

Thnaks for the help.