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    A relay is an electrically operated switch. Relays are used where it is necessary to control a circuit by a low-power signal, or where several circuits must be controlled by one signal. Here we are using two types of relays (i) solid state relay (ii) electromechanical relay.

    Solid state relay (SSR)

    It is a solid state electronic component that provides a similar function to an electromechanical relay but does not have any moving components, increasing longterm reliability.

    Solid state and electromagnetic relay

    (a) Solid state relay (b) Electromagnetic relay

    Electromagnetic relay

    A simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core, an iron yoke which provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets of contacts. One of the two sets of contacts in the relay pictured is closed, and the other set is open. When an electric current is passed through the coil it generates a magnetic field that activates the armature, and the consequent movement of the movable contact(s) either makes or breaks (depending upon construction) a connection with a fixed contact.

    If the set of contacts was closed when the relay was deenergized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position.

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