Any time you have a wire coil, such as an electric strike, you need an MOV at the device. When the power is removed from an electric strike, the magnetic field around the coil collapses back into the coil producing a high voltage surge. That surge can travel down the wires and damage other components in the system. Damage doesn’t happen instantly, instead it accrues over time. We prove this theory in classes I teach by connecting a small neon bulb to the wires of the strike, applying power, and then releasing it. When the power is released, the bulb will flash. Neon bulbs wont flash unless high voltage is applied. (The braver students have been known to just hold the wires in their fingers while we drop the power.) The MOV absorbs the high voltage surge.
The ANSI/BHMA standard 156.25, which pertains to electrified locking devices, specifies that the “Inductive Kickback Test” has a maximum allowable momentary voltage of 92 volts. Exceeding this voltage leads to a failure of the test. Therefore, 92 volts serves as an agreed reference point, indicating that surpassing this limit may cause damage to other components, such as relay contacts and solid-state devices.
The Digi-Key and Panasonic Part numbers for 12V and 24V MOVs.
* 12V MOV/P7284-ND(Digi-Key) ERZ-V05D270(Panasonic)
max continuous voltage of 22 VDC or 17 VAC, Clamps at 24 to 30 Volts.
* 24V MOV/P7286-ND(Digi-Key) ERZ-V05D390(Panasonic)
max continuous of 31 VDC or 25 VAC, clamps at 35 to 43 Volts.
About 22 cents in quantities of 100. Cheap insurance.
Typical image.
A simple wiring diagram showing the MOV across the lock coil.
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