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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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