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During the selection of relays for my project I got to know about polarized relay which I had never heard of before. Could somebody please explain how exactly it works and its areas of applications.

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These relays contain a permanent magnet that supplements the magnetic flux of the coil. The permanent magnet supplies flux to either of two permeable paths than can be completed by an armature. To transfer the armature and its associated contacts from one position to the other requires energizing current through the electromagnetic coil using the correct polarity.

This principle is illustrated by the two magnetic circuit configurations below.

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Energization of the coil by a current pulse of proper polarity reverses the flux in the iron structure of the relay; in the other, flux in the armature is reversed. The number of ampere turns provided by the coil must be sufficient to overcome the reluctance of the open pole gap (see dotted line flux path). One advantage of this type of relay is the capability of latching operation where the contacts remain in the last position without continued application of electromagnetic power. It also augments the amount of armature force available with a given relay size and input power.

Polarized relays are usually small, have faster operate and release times, and are more resistant to shock and vibration forces.

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Unlike adjusting spring tension to manipulate relay sensitivity, adding bias by a permanent magnet increases positive feedback, achiving faster on and off times and higher sensitivity. Magnetic biasing is also usefull to create bistable relays.

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