When supply over coil is switched off

Thread Starter


Joined Jun 27, 2008
I have an assignment question that reads like following:
A220V, 125Hz supply is suddenly switched across a coil. the current in the coil reaches 692mA after 3.2 ms and the circuit is then switched off.

At this point I must calculate a bunch of stuff, but the first one baffles me.
It says I must calculate the final steady value of the current.

What I know:
Current will reach a max. twice at 4ms interval each. This may not mean much for the following question though.

My reasoning, there is no parallel resistor for any further current flow, so no circuit will exist, you'll be stuck with a coil with no supply to maintain the charge in the coil. What happened to the charge stored in the coil I can only guess, but it could have arced out when the switch was opened.
I do not know. But I'm sure that no circuit for current flow exists.

Any help would be appreciated.


Joined Mar 27, 2008
arrie, can you upload what your looking at?

You said there is no resistance in parallel [with the coil] for current to flow. You are correct--that resistance would be used to keep arcing (kickback) from happening.

Either way, your final steady state current would be 0 A; It arced when the switch was opened, it discharged through a parallel resistance, or the circuit is open and there is no current flow.

Thread Starter


Joined Jun 27, 2008
That is it, the question was asked like I stated it in words.
Thanks silvrstring.
You have confirmed what the logic part of my brain said: Because the circuit consists only of a supply and a coil, when the supply is taken away, you have no circuit and any current will drop to zero almost instantaneously.
I'm just baffled why I got this question in the first place.
It's not done yet, I have to still calculate the resistance of the coil, inductance of the coil, energy stored in the coil, and funny enough, the current in the coil 3.6ms after the instant of switching off.
Thanks again anyhow.