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Relay (23)

A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically,
 but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal
(with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal.

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.

Basic operation of the relay:  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 de-energized, 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. Usually this force is provided by a spring, but gravity is also used commonly
in industrial motor starters. Most relays are manufactured to operate quickly. In a low-voltage application this reduces noise; in a high
voltage or current application it reduces arcing.

Relays are used to and for:
1, Amplify a digital signal, switching a large amount of power with a small operating power.
2, Detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers.
3,Isolate the controlling circuit from the controlled circuit when the two are at different potentials.
4,Time delay functions. Relays can be modified to delay opening or delay closing a set of contacts.
5,Vehicle battery isolation.
6, Switching to a standby power supply.