Reed relays are electrically-operated switching devices that consist of two thin, magnetic strips (reeds) encapsulated in a glass envelope. When a coil around the glass envelope is energized, the resulting magnetic field brings the reeds together and closes the contacts. Most reed relays have higher coil resistances and provide faster switching speeds than standard relays. With normally open (NO) devices, the relay contact pair is open when the coil is not energized. With normally closed (NC) devices, the relay contact pair is closed when the coil is not energized. Changeover relays typically have one set of Form C contacts.
Reed relays vary in terms of contact ratings, coil ratings, and performance specifications. Contact ratings include maximum switching current, maximum AC switching voltage, maximum DC switching voltage, maximum AC switching power, and maximum DC switching power. Coil ratings include AC coil voltage, DC coil voltage, coil resistance, coil nominal AC power, and coil nominal DC power. Performance specifications for reed relays include make time and break time. Make time (operate time, pick-up time) is the time required from actuation to the full current-carrying state. Break time (release time, drop-out time) is the time from deactivation to the open state.
Selecting reed relays requires an analysis of pole and throw specifications. Single pole (SP) relays have a single, moveable lever with a contact at the end. Double pole (DP) relays consist of two, single pole contacts that are concurrently actuated by the same mechanical system and operation. Typically, the first line is hot and the second is neutral. Triple pole (TP) relays consist of three, single pole contact systems. Relays with more than three poles are indicated by the number of single pole contacts followed by the letter "P". For example, four pole relays are 4P devices. In terms of throw specifications, both single throw and double throw reed relays are available. Single throw (ST) relays have a pair of contacts that are open in one position and closed in the other. Double throw (DT) relays have three contacts. In one position, the first is in contact with the second, but not the third. In the other relay position, the connection is reversed.
Reed relays are available with a variety of features. Devices with convertible contacts can be reconfigured from normally open to normally closed, and vice versa. Push-to-test buttons allow the manual actuation of contacts for circuit testing. Intrinsically safe (IS) reed relays will not produce sparks or thermal effects under normal or abnormal conditions that ignite a specified gas mixture. Sealed relays prevent the ingress of contaminants and water. Current sensitive devices are used to indicate when a preset AC or DC current threshold has been exceeded. Voltage sensitive devices are used to detect abnormal conditions on the line side of the relay. Some reed relays provide visual indicators, time delays, and latching controls. Others include adder or expandable decks that allow the addition of poles to the relay configuration.
Reed relays are used in a variety of applications that require low and stable contact resistance, low capacitance, and high insulation resistance. For example, they are often used in automatic test equipment (ATE) and instrumentation. For high frequency applications, reed relays can be fitted with coaxial shielding. Devices with very low thermal voltages are used in data acquisition equipment and process control applications. By contrast, devices with high isolation voltages are suitable for medical applications. Reed relays are also used in switching solenoids, contactors and starter motors.
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