Electrical Engineering

Why does a switch spark when disconnecting a coil carrying high dc current?

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2011-03-25 14:21:22

An inductor resists a change in current. If you have a steady

state current going through an inductor and you attempt to suddenly

increase the current, the inductor will nearly instantaneously

present a higher resistance so that the current does not

immediately change. Its resistance, then, will start to decrease as

the current ramps up to the new value.

Similarly, if you have a steady state current going through an

inductor and you attempt to suddenly decrease the current, the

inductor will nearly instantaneously present a lower resistance so

that the current does not immediately change. Its resistance, then,

will start to increase as the current ramps down to the new

value.

That's all just background information so you can understand

what an inductor is.

If you have a steady state current going through an inductor and

you attempt to suddenly decrease the current to zero by opening the

circuit, the inductor will respond by attempting to maintain the

current, but that current has nowhere to go. This creates a large

negative voltage spike across the inductor. Think about it. Ohm's

law says that voltage is current times resistance. You have some

current; you have infinite resistance; therefore you must have

infinite voltage. In fact, a theoretical pure inductor will do

exactly that - generate an infinitely large negative voltage spike.

That does not happen in practice, but it is very common to see

transients of several hundred or thousand volts. This is why you

need to have some kind of suppression circuit in place - otherwise

that transient will go back and blow out whatever circuit is

driving it.


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