From Electronics Technology Handbook
Electromechanical components for electronics are those that are compatible with electronic circuitry in size, weight and ratings for mounting on circuit boards nearby in cases, cabinets or enclosures. Examples include relays, solenoids, switches, and fractional horsepower DC motors.
A relay is the simplest form of remote controller. The input circuits of all relays are electrically isolated from their output circuits, and their contacts can be closed remotely by passing current sufficient to actuate the relay coils through wires controlled by a manual switch or another relay. Relays are classed as electromechanical (EM) and solid-state relays (SSRs). Solid-state relays are discussed in Sec. 5, "Fundamental Electronic Circuits."
All electromechanical relays are identified by:
Output current rating
Number of poles
Composition of output contacts
Packaging style and form factor
The general classifications for electromechanical relays are:
General-Purpose Electromechanical Relays
A general-purpose electromechanical relay used in electronics applications is a modern version of the traditional relays invented more than a century ago and manufactured primarily for use in electrical power and lighting, telegraph, and telephone systems. A conventional coil-and-contact relay, as shown in Fig. 3-1, consists of an electromagnetic coil, movable spring-loaded armature, and electrically isolated input and output circuits. When the coil is energized, the electromagnet attracts the hinged and spring-loaded armature against the tension of the return spring, causing the upper normally closed (NC) contacts to break and the lower normally open (NO) contacts...
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