Archive for December, 2013|Monthly archive page

Mag Lock Slow to Release?

Thursday, December 19th, 2013

If you are finding a mag (electromagnetic) lock to be “sticky” or slow to release it is likely that it has a diode connected across the power terminals. Remove it, problem solved.

This is a common mistake. The reverse biased diode is supposed to prevent a current spike (often call back EMF) caused by the decay of the magnetic field around a coil. This is true of basic coils such as those found in relays and unprotected strikes. However the combination of the diode and the internal electronics of modern mag locks combine to cause a slow release of the armature plate.

This is often very noticeable to the door user. In one particular case I came across recently an entire buildings worth of doors had this problem. It was exacerbated by the fact that the push to exit buttons used the wrong contact. The problem was so bad that the security manager had put up signs that read “press button… wait… open door.” The same company that incorrectly installed the diodes and exit buttons had also glossed over the problem for five years of annual maintenance.

Protection diodes should not be installed in modern electromagnetic locks. The delay can cause excessive force to be placed on the lock by users and I have even seen people bump into the door expecting it to open sooner. It is also a major problem when a mag lock is combined with a power swing door operator, the operator fights against the mag for a second shortening the life of both parts.