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))
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