Electromechanical relays are devices that complete or interrupt a circuit by physically moving electrical contacts into contact with each other. A relay involves two circuits: the energizing circuit and the contact circuit. The coil is on the energizing side and the relay contacts are on the contact side. When a relay coil is energized, current flow through the coil creates a magnetic field. Whether in a DC unit, where the polarity is fixed, or in an AC unit where the polarity changes 120 times per second, the basic function remains the same: the magnetic coil attracts a ferrous plate, which is part of the armature. One end of the armature is attached to the metal frame that is formed so that the armature can pivot, while the other end opens and closes the contacts. Relay contacts are designed, built and specified for the type of application for the relay. No single voltage and current rating applies to a given set of contacts under all circumstances. Contact the relay manufacturer for guidance if your requirements are unusual or the specifications seem incomplete.
Relay-type choices available for electromechanical relays include general-purpose relay, machine control or heavy-duty relay, reed relay, and aerospace or MIL-Spec relay. General-purpose relays operate with AC or DC current, at common voltages and they can control currents ranging from 2A to 30A. A heavy-duty relay is used to control starters and other industrial components. Reed relays are capable of switching industrial components such as solenoids, contactors and starter motors. An aerospace or MIL-spec relay meets appropriate military specifications or is intended for aerospace applications. Mounting choices for electromechanical relays include PC board, socket or plug-in style, bracket or flange mount, and DIN rail.
Pole and throw specifications are important to consider when searching for electromechanical relays. Pole choices include single pole (SP), double pole (DP), triple pole (TP), four pole (4P), and greater than four poles. Throw choices are single throw or double throw. Single throw (ST) relays have a pair of contacts open in one position and closed in the other. Double throw (DT) relays have three contacts. The common one is in contact with the second, but not with the third, in one position of the relay, and reverses this connection in the other relay position.
Important contact specifications to consider when searching for electromechanical relays include the number of normally open contacts, the number of normally closed contacts, and the number of changeover contacts. Contact ratings to consider include maximum switching current, maximum AC switching voltage, maximum DC switching voltage, maximum AC switching power, maximum DC switching power. Important coil ratings to consider include AC coil voltage, DC coil voltage, coil resistance, coil nominal AC power, and coil nominal DC power. Important performance specifications to consider for electromechanical relays include make time (operate time) and break time (release time). Common features for electromechanical relays include convertible contacts, poly-phase relay, time delay, intrinsically safe, visual indicators, sealed relay, push-to-test button, current sensitive, voltage sensitive, expandable deck, and latching controls. An important operating parameter to consider is the operating temperature.