Hydraulics & Pneumatics

For design and manufacturing engineers involved in buying or specifying fluid power components and systems.

NASA Tech Briefs

Innovations developed by NASA and its industry partners in a wide array of fields.


How OEM's can make their parts better, faster, and more efficient.

Metal 3D Printing Design Guide

Direct Metal Laser Sintering (DMLS) 3D printing for parts with reduced cost and little waste.

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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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