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Relays were used for control of automated systems for machine tools and production lines. The Ladder programming language is often used for designing relay logic networks. Early electro-mechanical computers such as the ARRA, Harvard Mark II, Zuse Z2, and Zuse Z3 used relays for logic and working registers. However, electronic devices proved ...
A solid state relay (SSR) is an electronic switching device that switches on or off when an external voltage (AC or DC) is applied across its control terminals. They serve the same function as an electromechanical relay , but solid-state electronics contain no moving parts and have a longer operational lifetime.
A few million reed relays were used from the 1930s to the 1960s for memory functions in Bell System electromechanical telephone exchanges. [2] Often a multiple-reed relay was used, with one of the reeds latching the relay, and the other or others performing logic or memory functions.
Elevators are another common application - large relay logic circuits were employed from the 1930s onward to replace the human elevator operator, but have been progressively superseded with modern solid-state controls in recent years. Relay logic is also used for controlling and automation purposes in electro-hydraulics and electro-pneumatics.
The relays can also be classified on the type of power source that they use to work. A dual powered protection relay powered by the current obtained from the line by a CT. The striker is also shown. Self-powered relays operate on energy derived from the protected circuit, through the current transformers used to measure line current, for example.
One or more reed switches inside an electromagnetic coil constitute a reed relay. Reed relays are used when operating currents are relatively low, and offer high operating speed, good performance with very small currents that are not reliably switched by conventional contacts, high reliability and long life.
Normally closed mercury relay, with coil around top of tube and adjustable angle. A mercury relay (mercury displacement relay, mercury contactor) is a relay that uses mercury as the switching element. They are used as high-current switches or contactors, where contact erosion from constant cycling would be a problem for conventional relay contacts.
Digital/numerical relays provide a front panel display, or display on a terminal through a communication interface. This is used to display relay settings and real-time current/voltage values, etc. More complex digital relays will have metering and communication protocol ports, allowing the relay to become an element in a SCADA system.
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