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I am using an electro magnetic door lock which I am powering with 12 volts. It is switched on and off using a transistor array (uln2003.) I have also included a flyback diode. My problem is this: after I have switched off the magnet, there appears to be a lingering field that holds the door shut until it is pushed open. It is not a strong field, and it goes away after the door is pressed open. I need to build a circuit that can neutralize this field. How?

My inexperienced mind is telling me that I need to send a short burst of current through the magnet in the opposite direction.

Hoytman
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  • So is the magnet retaining some magnetism, or is it magnetizing the door catch which then sticks to the magnet? – Ryan Griggs Sep 23 '16 at 01:10
  • I'm assuming a little of both. – Hoytman Sep 23 '16 at 02:05
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    A small airgap when at closest approach will (should) make a substantial difference. This could be achieved with a non magnetic material if desired. – Russell McMahon Sep 23 '16 at 02:31
  • Is it a metal door? it may need a layer of polyamide tape or thicker carpet/thermal insulation tape , like Russ said for an nonmagnetic gap to reduce remenance. – Tony Stewart EE75 Sep 23 '16 at 04:20
  • wooden cabinet doors. I am using magnetic security locks, so the sticking is really problematic with the lighter doors. – Hoytman Sep 23 '16 at 04:57
  • Do you have a link to the lock ? Does it have any sort of guide such as a pin or is it purely a flat armature plate ? – D-on Sep 23 '16 at 06:19

3 Answers3

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I am using an electro magnetic door lock which I am powering with 12 volts. It is switched on and off using a transistor array (uln2003.) I have also included a flyback diode. My problem is this: after I have switched off the magnet, there appears to be a lingering field that holds the door shut until it is pushed open. It is not a strong field, and it goes away after the door is pressed open. I need to build a circuit that can neutralize this field. How?

(1) A small airgap when at closest approach will (should) make a substantial difference. This can easily be achieved with a small layer of non magnetic material if desired. Try tape, paper etc initially. A more permaent solution can follow. This applies even with a non-magnetic material for the door as the plate the magnet attracts may suffice.

(2) A low force spring should be able to be made to work. Magnet-on attracts plate and compresses spring. Magnet off - spring is designed to exceed remaining attraction.

Again, start informally - you can eg use bubble wrap air bubble with suitable mechanical placement and sellotape to hold it.

Russell McMahon
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There is some level of reverse pulse you could put through the lock coil to fully demagnetise it. However, what that level is would be dependent on a number of factors, some you could not control, and would vary between samples of the same lock.

However, even if not perfectly demagnetised, some demagnetisation may be beneficial to you. Go do an experiment on some.

A more robust way to demagnetise things is to use an initially large, then decreasing steadily down to zero, AC current. This takes the sample round the hysteresis loop in ever decreasing amplitudes. Unfortunately the means to provide it could be fairly complicated. In CRT TVs, it used to be provided by the mains, fed through a PTC resistor, that steadily heated up and reduced the current.

Neil_UK
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Remnant magnetism is a concern to relay designers.

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Figure 1. The addition of a brass "pip" on the armature prevents magnetic latching. Source: David McAudrey.

When a small electric current passes through the energising coil, the armature is magnetically attracted to the end of the rod against the bias force of the spring and, if the Ampere-turns are sufficient, it will rock to close the gap in the magnetic circuit. In practice a small brass pip on the underside of the armature prevents the gap from becoming fully closed; otherwise it would tend to stick in the energised position when the energising current is switched off.

A similar approach is what is required in your application but, as others have said, a thin layer of non-ferrous material may suffice.

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