Output relay Back-EMF Protection


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Description

When an electric lock is de-energised, the collapsing magnetic field produces energy to be
generated within the windings of the coil. This energy produces a voltage (or EMF) within the
coil, of the opposite polarity to the normally applied voltage used to energise the magnet.
This ‘back-EMF’ can result in hundreds of volts and can cause damage to the switching relay,
as it sparks across the contacts.
It is normal practice to install a diode, or bridge rectifier in the circuit, close to the lock, to
dissipate this voltage and protect the switching relay.
Mag locks sometimes have this Back-EMF protection built into the monitoring PCB supplied with
the lock. If you chose not to use this, then the same diode bridge could be used to protect the
installation. (The built in monitoring PCB uses a hall effect device to detect that the lock is
energised and the door is closed. unfortunately this cannot be used to replace the door
monitoring reed relays, as our controllers expect a open/short circuit, and this hall effect
generates some excessive voltages, which upset the controller.)