A relay is an electrically operated switch. Although other operating principles are possible, many commercially available relays use an electromagnet to operate a switching mechanism mechanically. Relays are widely applied to two situations:
When it is necessary to control a circuit by a low-power signal with complete electrical isolation between control and controlled circuits.
When several circuits must be controlled by one signal.
Today, telephone exchange instruments are probably the most extensive users for relays.
Common relay types include
Latching relay
Reed relay
2.2.1 Mercury-wetted relay
Polarized relay
Machine tool relay
Contactor relay
Solid-state relay
Solid state contactor relay
Buchholz relay
Forced-guided contacts relay
Overload protection relay
Note that since relays are essentially special switches, the terminology applied to switches is also applied to relays. A relay will switch one or more poles, each of whose contacts can be thrown by energizing the coil in one of three ways: Normally-open (NO), Normally-closed (NC), and Change-over (CO) (double-throw (DT))
