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Straight upfront...electrical engineering is not my forte. However, I am trying to help a client with a particular issue and I'm hoping this community can provide some invaluable insight.

They have a maglock that draws 260mA @ 24VDC. The push bar on the door serves as the switch, which triggers a delay relay that cuts power to the magnet for 5 seconds, allowing for egress.

The problem is that although the circuit is interrupted and power is cut immediately upon pressing against the push bar, the magnet doesn't let go of the door for about 1.5 seconds. This means that people are faceplanting into the door as they expect it to open when they push on it, but it remains locked for an extra moment.

I believe this is caused by back EMF preventing the magnet from completely de-energizing for that extra 1.5 seconds. Therefore, I would like to install some EMF protection, not to protect the circuitry components, but to allow the EMF generated when the field collapses to flow back through the magnet and provide a quicker release.

In doing some research, it seems that there are two common solutions for this situation. Some recommend using a flywheel diode and Zener diode together, while others recommend using a MOV (varistor). I'm not sure which would be more suitable for my particular case.

Furthermore, I don't know which specific diodes/varistor would be best suited to this circuit which normally operates at the stated 260mA/24VDC.

Please forgive my ignorance and help enlighten me if you can.

Thank you very much in advance.

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    \$\begingroup\$ 1.5 seconds sounds too long for the field to decay. Probably there is a charged capacitor involved. Look for a large capacitor in the circuit , probably directly across the coil. \$\endgroup\$
    – Marla
    Commented Apr 14, 2022 at 18:14
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    \$\begingroup\$ If you want to modify the circuit you'll first need to disassemble everything down to the individual components and create a circuit diagram. Kind of a pita I know. \$\endgroup\$
    – Drew
    Commented Apr 14, 2022 at 19:18
  • \$\begingroup\$ Also might help to check the relays for any dirt or sticky stuff too \$\endgroup\$
    – ManRow
    Commented Apr 14, 2022 at 23:39

2 Answers 2

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1.5 seconds sounds too long for the magnetic field to decay.

More likely there is a capacitor (likely an electrolytic capacitor) connected across the solenoid's coil.

It sounds like you are dealing with a commercially purchased system. You may be able to obtain information from the manufacturer. Perhaps even a schematic or advise.

Reducing or eliminating the capacitor likely will solve the problem. However, I do not know if capacitor removal will cause other problems.

This is probably a common problem. And I have seen signs on such doors stating, "Door will open in 4 seconds, keep pushing."

If you advance further into the area that you mentioned trying to eliminate the magnetic field sooner, you can search here on this site for question/answers regarding using flyback diodes, flyback zener, etc.

EDIT : @rdtsc suggested the following good information :

Just a note that "flyback", "free-wheel", and even "anti-parallel" can all be used to describe the diode itself; while the function of the diode (or diode+zener, MOV) can be called a "snubber", "inductive kick-back clamp", "transient voltage suppressor (TVS)", or even a "surge/spike limiter." So many names for essentially the same thing! – rdtsc

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  • \$\begingroup\$ Just a note that "flyback", "free-wheel", and even "anti-parallel" can all be used to describe the diode itself; while the function of the diode (or diode+zener, MOV) can be called a "snubber", "inductive kick-back clamp", "transient voltage suppressor (TVS)", or even a "surge/spike limiter." So many names for essentially the same thing! \$\endgroup\$
    – rdtsc
    Commented Apr 14, 2022 at 19:33
  • \$\begingroup\$ @rdtsc. .. yes, thanks for that. Often I forget to add in all the synonyms to help the OP better. \$\endgroup\$
    – Marla
    Commented Apr 14, 2022 at 19:49
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This is residual magnetism. To test put tape on one corner maglock armature. If that fixes it replace maglock if no change you have a failing relay. Dtk-ess is best to protect from kickback damaging the contacts.

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