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Counteracting electromagnetic remanence

Counteracting electromagnetic remanence

Counteracting electromagnetic remanence

Hi guys, just discovered this great site, hopefully you can help me!

I am tweaking a lock mechanism. It has a steel disk connected to a manually operated lever. The lever moves the disk into position on top of a 12vdc electromagnet while mechanically locking the system. The electromagnet prevents the user unlocking the system while it is active. There is some natural 'spring' in the system which i had hoped would release the disk from the magnet when it is powered down.

The problem I'm having is that when the electromagnet is deactivated, there is some remanent magnetism between the magnet and the disk which keeps the disk firmly mated with the electromagnet. This magnetism dissipates as soon as the 'seal' is broken.

After some experimentation, I have found the putting a small negative voltage across the electromagnet (~1V) releases the disk, but unfortunately I do not have a -ve voltage on th epower supply (only 0v, 5v, 12v, and running the EM off 7v is not strong enough). I have tried spacing the disk from the magnet slightly with a thin layer of tape, this also works, but also heavily reduces the strength of the system and makes it far too easy for the user to force the lock open. I also experimented with placing small permanent magnets on the reverse side of the disk, in an attempt to counteract this remanence, but so far with no success.

Does anyone have any bright ideas? Are there any clever ways of overcoming this remanence? Could I be using a different material for the lock disk?

My final option would be to add a more spring to the system, but this would by my 'all out of good ideas' solution.

Thanks in advance,


RE: Counteracting electromagnetic remanence

If playing with the spring force and the gap is not enough
you may try to connect a non-polarized capacitor parallel
with the coil. measure the coil inductance L and select
a C value which resonates with L at no more than 100Hz
so when you turn off the current it generates an exponentially decreasing oscillation.

It may be cheaper to redesign the mechanism.

Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>

RE: Counteracting electromagnetic remanence

It's a shame you can't get negative voltage.  A sinusoidally decaying pulse (alternating between positive & negative) would eliminate a lot of the residual magnetism in the core of the electromagnet.

I admit this is not a great suggestion, but I'll throw it out there:  How about a secondary coil around the electromagnet, with the windings reversed?  It would need it's own power supply, but you could cycle between the two coils to produce an alternating field that would decay to zero.  I realize that this really complicates things.

RE: Counteracting electromagnetic remanence

around zero i.e. with alternating polarity and decreasing amplitude -- it can perfectly degauss the coil.

Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>

RE: Counteracting electromagnetic remanence


A star for you.  That is one of the best things about this site, I learn something new almost every day.  Thanks for the tip.  I hope it helps the original poster.

RE: Counteracting electromagnetic remanence

Thank you for all your ideas.

I had considered wrapping a coil around the outside of the electromagnet, but had not got quite desperate enough to try it yet!

The old simple harmonic LC oscillator - should have remembered that from my student years. Thanks nbuckska, I shall definitely be giving this a try. Next problem, how to measure inductance!? now where's my multimeter...

RE: Counteracting electromagnetic remanence

Joe, you might also want to look at the properties of your parts.  A better anneal may reduce the residual field enough to minimize the 'ringing' needed to zero out the field.

Measure inductance?  We always just tried caps and keep going bigger until it worked.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

RE: Counteracting electromagnetic remanence

You might want to try lamination steel.  It has a lower retained magnatism that low carbon steel.

RE: Counteracting electromagnetic remanence

Problem completely solved with a 47uF capacitor! A nice simple mod to do as well.

Thanks for all your help guys, nbucska especially.


RE: Counteracting electromagnetic remanence


It MUST be non-polarized cap -- a polarized electrolytic cap will be destroyed in the circuit !!!!!

Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>

RE: Counteracting electromagnetic remanence

Yup sure, got some non-polarised electrolytic caps - Didn't know they existed before today!

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