Suitably rated capacitors and the associated resistor are sold as a single packaged component for this commonplace use. In this case a snubber circuit (a capacitor and resistor in series) across the contacts may absorb the surge. If the relay is driving a large, or especially a reactive load, there may be a similar problem of surge currents around the relay output contacts. The "cradle" term refers to the shape of the relay's armature In a low-voltage application this reduces noise in a high voltage or current application it reduces arcing.Ī small cradle relay often used in electronics. Most relays are manufactured to operate quickly. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. If the set of contacts was closed when the relay was de-energized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When an electric current is passed through the coil it generates a magnetic field that activates the armature, and the consequent movement of the movable contact(s) either makes or breaks (depending upon construction) a connection with a fixed contact. This ensures continuity of the circuit between the moving contacts on the armature, and the circuit track on the printed circuit board (PCB) via the yoke, which is soldered to the PCB. The relay in the picture also has a wire connecting the armature to the yoke. Other relays may have more or fewer sets of contacts depending on their function. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. The armature is held in place by a spring so that when the relay is de-energized there is an air gap in the magnetic circuit. The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. Operation with flyback diode, arcing in the control circuit is avoidedĪ simple electromagnetic relay consists of a coil of wire wrapped around a soft iron core (a solenoid), an iron yoke which provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets of contacts (there are two contacts in the relay pictured). Magnetic latching relays are useful in applications when interrupted power should not affect the circuits that the relay is controlling. Another pulse applied to a second set of control terminals, or a pulse with opposite polarity, resets the switch, while repeated pulses of the same kind have no effects. Latching relays require only a single pulse of control power to operate the switch persistently. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults in modern electric power systems these functions are performed by digital instruments still called protective relays. The traditional form of a relay uses an electromagnet to close or open the contacts, but relays using other operating principles have also been invented, such as in solid-state relays which use semiconductor properties for control without relying on moving parts. Relays were used extensively in telephone exchanges and early computers to perform logical operations. Relays were first used in long-distance telegraph circuits as signal repeaters: they refresh the signal coming in from one circuit by transmitting it on another circuit. Relays are used where it is necessary to control a circuit by an independent low-power signal, or where several circuits must be controlled by one signal. The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof. It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. An automotive-style miniature relay with the dust cover taken offĪ relay is an electrically operated switch.
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