TECHNICAL TIP >>

Magnetically induced reverse polarity high voltage.

It sounds highly technical and it can cause you and your customer some grief, but it is simply the voltage generated when you turn off the supply (Volts DC) to the magnetic coil of an electric strike or any inductive device. More importantly if you don’t take steps to protect against it, you can damage other devices in the circuit. In the case of an electric strike that will be, the switch, relay contacts or electronic components that control the strike’s operation.

The solution is simple and cheap. A single 1 amp diode.
Connect a diode across the strike with the (+ve) of the diode connected to the supply (+ve) and (-ve) to (-ve). 

When power is applied to the strike, it energises the coil and produces a magnetic field and the strike operates. The diode doesn’t conduct due to its reverse connection.

When the power is removed, the strike de-energises and the magnetic field collapses. The collapsing magnetic field induces a high voltage of reverse polarity in the strike’s coil (many times greater than the supply voltage).

With a diode fitted, the induced voltage (reverse polarity to the supply
voltage) is short circuited by the diode, thus protecting the other components in the circuit.

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