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Square wave magnetic field
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ricko_uk:
Hi,
if you drive a coil with a square wave you clearly don't get a true square wave current (hence magnetic field intensity) across it.

What is the best way to "force" a square wave current through it "as square as possible"?

For example increasing the voltage when ramping it up would increase the steepness of the front rising edge but then it would overshoot the desired amplitude. So maybe there are some clever circuits?

Also, by using a ferrite core (maybe of some ferrrites with special characteristics) would it help or worsen the sharpness of the square wave current through it?

Many thanks :)
Buriedcode:
Increasing voltage across the coil (inductor) or reducing the inductance of the coil.   Thats the only way to "increase the steepness" as you put it.

I'm sure you're aware of di/dt = V/L which makes that pretty clear.  Inductors, by their nature resist changes in current.
duak:
BuriedCode summed it up, I think.   The problem with trying to change the current rapidly is that the winding develops a counter-EMF that opposes the change in current.  There is a way to reduce the inductance and that is to add something called a shorted turn that turns the inductor into a transformer.   By adding a shorted turn, the counter-EMF is reduced (ie. shorted out) and the winding current can increase much more rapidly.  Once the current reaches its desired value the driver reduces the applied voltage to where the requisite winding current can be maintained.   If the winding current isn't changing then no current is induced in the shorted turn and it has no effect on the magnetic flux.  Since the magnetic flux is determined by the winding current, the number of turns and the magnetic reluctance then when the current reaches the desired value, the flux also reaches the desired level.

The shorted turn is usually a heavy wire or foil.  It will take some experimentation to determine the optimum configuration.

This technique is used in hard drive head actuators to improve their spreed.  Look for "shorted turn actuators"
ricko_uk:
Thank you Duak and  Buried code.

Duak, that's very interesting!!
duak:
I should have added that with a shorted turn, the self-inductance also drops.  If the repetition rate and current is high, the windings could get hot.
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