Relays are electronic control devices that pair a control system (or input loop) with a controlled system (or output loop). They are typically used in automatic control circuits, with a smaller current to control a larger “automatic switch” of current. Relays are used in the role of automatic adjustment, safety protection, and conversion circuits.
Electromagnetic relays are composed of a core, a coil, an armature, and one or more contacts. When an electric current passes through the coil, it generates a magnetic field that activates the armature, and the consequent movement of the movable contact or contacts makes or breaks a connection with a fixed contact. Relays can be built for either direct current or alternating current, with modifications for both. Direct current relays have diodes placed along the coil to dissipate energy from the collapsing magnetic field that occurs at deactivation, while alternating current relays uses methods to split the flux into two out-of-phase components which then add together.
Relays come in numerous different types and configurations for their various roles. Here are some of the most common and significant, listed below:
Coaxial relays are used when a radio transmitter and receiver share an antenna and are used to allow the antenna to switch back and forth between the two. This protects the receiver from the high power of the transmitter.
Forced-guided contacts relay have relay contacts that are mechanically linked together, so that when the relay coil is energized or de-energized, all of the linked contacts move together. This guarantees that contacts are never in opposite states.
Latching relays maintain contact position indefinitely without power applied to the coil via mechanical linkage. The advantage is that one coil consumes power only for an instant while the relay is being switched, and the relay contacts maintain this setting during a power outage. A latching relay allows for remote control of a building’s lighting for instance, without the hum that is generated by a continuously energized coil. Early computers stored bits in magnetically latching relays.
Solid state relays are solid-state electronic components that do not have any moving components, similar to solid-state drives in PCs. This lack of moving parts increases long-term reliability.
Vacuum relays have their contacts mounted in an evacuated glass housing, to permit radio-frequency voltages as high as 20,000 volts without flashover between contacts.