electromechanical relay (EMR)
The electromechanical relay(EMR) is a switch whose switching contacts are triggered by the induction in a coil. The switching contacts are mechanically connected to a lever, which is attracted by the iron core of the coil when current flows through it and moves the spring contacts with this movement. In addition to the electromechanical relay, there are also solid-state relays( SSR), which are smaller and have no switch bounce.
Functionally, relays have either one stable state, which are called monostable relays, or two stable states, which are the bistable relays. In a monostable relay, the relay switches back as soon as the induction current for the relay coil is interrupted. Such relays are found in building, control and telecommunication applications. The bistable relays switch to the second stable state when triggered again and remain there until triggered again.
Important characteristics of relays are the switching load, the transient response caused by the bouncing of the switching contacts, and the contact resistance of the relay contacts, which changes over the number of switching cycles. To achieve low contact resistance, good corrosion resistance and high burn-off resistance of the relay contacts, the relay contacts are made of tungsten, or special gold and silver alloys, such as hard gold (AuNi), silver nickel (AgNi), silver cadmium oxide (AgCdO) and silver tin oxide (AgSnO).
Relays are used for switching circuits and have the advantage that there is no potential between the triggering coil and the spring contacts. They are mainly found in telecommunications, in time-dependent switches, on printed circuits in encapsulated form as reed relays, and in power engineering, where they are called contactors.
The relay contacts can be open in the rest position, in which case they are called normally open contacts; if they are closed in the rest position, they are called normally closed contacts.