This chapter covers representative general-purpose relays used in the control circuits of most industrial, commercial, and consumer products. Relay names and terminology are in accordance with preferred usage as practiced by the National Association of Relay Manufacturers (NARM).

Relays open and close electrical contacts to operate other devices. They are often used because they cost less than corresponding electronic switches. But some relay qualities are superior to solid-state devices. For example, input and output circuits in relays are electrically isolated unlike those of most solid-state devices. And relays can have numerous contacts electrically isolated one from another. In addition, electromechanical relays are becoming smaller, now available in PCB-mount and surface-mount packages that are suitable for automated soldering.

One of the advantages of electromechanical relays over solid-state switches is that relays have much lower contact resistance. Contact capacitance is also less, which may benefit high-frequency circuits. Relays are less likely to be turned on by transients than solid-state switches. And relays are less easily damaged by brief shorts or overloads.

Electromechanical relays differ in three important ways from solid-state switches. First, relay coils are highly inductive, and the inductance value is not constant. Inductance is low immediately after energization and increases as current approaches a steady-state level and the relay armature closes. In contrast, solid-state switches have mainly resistive inputs and a constant input current.

Second, relays have a much longer switching time than solid-state switches. Coil inductance is the primary cause, but the mass of armature and contact structures are also factors.

Third, relay coil inductance can produce unacceptably high-voltage transients when the device is deenergized. Protective circuits can reduce the transients to an acceptable level, but they delay relay drop out as well.

Relays also can be a source of EMI. Arcs at the contacts, for example, are produced when a contact bounces on energization and when contacts open on deenergization. Transients produced by deenergizing the coil are another source. EMI can be severe when switching inductive loads at high current and voltage levels.

Reed relays: A reed relay consists of reed switches within an operating coil. The reeds can be any type of configuration, but the quantity is limited by the coil size. Most manufacturers limit coil size to handle 12 standard switches maximum. To obtain additional contacts, relay coils are connected in parallel. Reed relays are available with contact forms from 1A to 12A, 1B to 8B, 1C to 4C, and combinations of these up to the maximum coil size. Coils may be wound with each magnet-wire size to create a large selection of operating parameters.

Reed-relay contacts typically produce 1 to 3 Vpp at 20 to 30 kHz. The voltage, which is produced by magnetostriction, generally decays about 3 msec after contact closure. Miniature reed relays in six-lead DIP and surface-mount packages are used for PC-board applications or wherever space is a constraint. Sensitive relays with coil pickup as low as 1.6 Vdc at 40 mW are available.

Mercury-wetted contact relays: Basically, a mercury-wetted contact relay consists of one or more glass switch capsules surrounded by a coil. These relays maintain their original resistance to within 1 mOmega Signthroughout their life.

When two contacts wetted with mercury are joined, the area of contact between the surfaces is somewhat large because a fillet of mercury surrounds the mated surfaces. When the two surfaces are separated, the mercury stretches into a thin filament and then breaks at two points that isolate a thin rod of mercury in the middle. The thin rod then snaps into a ball and drops to the bottom of the switch.

Mercury loss from the contacts disturbs the equilibrium of the capillary system, and more mercury is fed up the armature from the pool. Thus, in effect, the mercury-wetted contact relay provides a new contact surface for each closure.

Armature relays: Armature relays have pivoting armatures that actuate electrical contacts in response to small control signals.

Ac relays: Alternating current is widely available but is the least flexible power source for relay operation. However, most ac relays designed for 120-Vac line operation tolerate line fluctuations from 102 to 132 Vac.

Most ac applications are for 60-Hz current. Telephone relays operate on 20-Hz current but are similar in construction. For 400-Hz current, as found in aircraft, a radical departure from the 60-Hz relay construction is necessary. Reliable performance is attained by rectifying the 400-Hz ac to dc and using a dc relay motor. Packaged relays containing rectifiers are available.

Dc relays: Relays operated on direct current have inherently greater mechanical life expectancy than ac relays. The most frequent source of dc is rectified ac. Often ac ripple influences relay operation. Some dc relays can tolerate ripple, others need filtering.

When the power source is a rechargeable battery, voltage variations of 25% are possible. Relays are usually designed to operate at 75% of nominal voltage. Coils are designed not to overheat at 125% of pickup voltage.

PCB-mounted relays are generally armature devices. Typical devices are either spdt or dpdt and contain contacts rated at 0.5 A to 2 A. Typical operating voltages are 5 to 24 Vdc and 120 Vac. Power dissipation is in the range of 75 to 400 mW. These units are often available in sealed versions that can be immersion-cleaned during assembly.

Relays standards: NEMA Class A and B relays are specified in the publication, Industrial Control, ICS-1970. These relays control and interlock starters, contactors, and other devices. Relay contacts are also used to open and close circuits to other relays and pilot devices. Relays do not control power-consuming devices, except motors and solenoids drawing under 2 A.

Many manufacturers use MIL-R-5757 as a standard and as a guide for producing Government-acceptable relays. This specification covers relays with contacts capable of switching loads up to 10 A. In general, MIL-R-6106 covers and exceeds the requirements of MIL-R-5757. It also covers relays capable of switching currents in excess of 10 A.

Contacts: Common contact materials are fine silver, coin silver, silver cadmium oxide, and the noble metals. In addition to these materials, special contact finishes may be required for certain applications. For example, if a relay is to be used in a mildly corrosive atmosphere, contacts should have a chromate-conversion coating.

Contactors: Contactors are devices for repeatedly establishing and interrupting electric power circuits. Two types of contactors are defined by NEMA -- electronic and magnetic. Electromagnetic contactors are actuated by electromechanical means. They make and break power circuits to such loads as electric furnaces, lights, transformers, capacitors, heaters, and -- when overload relays or inherent protectors are used -- motors.

The magnet design of an ac contactor consists of a stationary core and a movable armature as in NEMA-A and B control relays. Some contactors are of the horizontal design; others have a hinged or pivoted clapper magnet. Coils are available in voltages up to 600 V, commonly in 110, 220, 240, 380, 440, 480, and 500 V for 25, 50, and 60 Hz.

A dc contactor operates like an ac contactor. However, while an ac magnet is laminated steel, a dc magnet is made of solid steel.

Because copper contacts are used on some contactors, the current rating for each size is an 8-hr open rating -- the contactor must be operated at least once every 8 hr to prevent copper oxide from forming on the tips and causing excessive contact heating. For contactors with silver to silver-alloy contacts, the 8-hr rating is equivalent to a continuous rating. This rating also applies to contactors mounted in the open without an enclosure. Contactors installed in an enclosure have a rating equal to 90% of the open rating because of reduced contactor cooling.

Stepping switches: Stepping switches connect one or more input circuits to one of many output circuits. The switch responds to current pulses supplied by an external source, or operates by interruption of its circuits through interrupter springs on the switch.

Stepping switches count, sequence, program, select, and control and are often applied in machine-tool controls, conveyor systems, test equipment, and communication switching. Rotary stepping switches are available in many sizes and shapes, primarily dependent on the number of contact points in the bank assembly.

Meter relays: Meter relays provide an analog or digital panel indication of a measured variable together with a switching function at a preset level. There are four types of analog meter relays: magnetic contact, locking coil, optical, and solid state. These meters can be used with the user's control circuitry, with a control module option, or with control circuitry contained in the meter.