Telecom pushes relay design

November 2, 1998

9 Min Read
Telecom pushes relay design

The global explosion in telecommunications services is a major driver of electronic component design--and relays are no exception. In communications, as with many other applications such as PCs and office equipment, the emphasis is on smaller, lower power, multi-functional components. Hand-held products such as cell phones and pagers are space-tight envelopes begging for diminutive devices. But not so obvious are dictates from other phone-line based applications, such as central offices, that require component designers to "get small."

Omron’s recent G6K relay magnetic circuit (left) improves upon the previous moving loop design, resulting in a smaller but more powerful relay. When turned on, an entire loop of magnetic flux, rather than only in one half coil, is generated, making use of the complete coil. Also the new circuit does not have a permanent magnet in the middle of the coil, permitting more windings, and thus a proportional increase in current.

Terry Harmon, product specialist for Omron Electronics (Schaumburg, IL), whose products include telecom relays, emphasizes the trend to smaller devices has accelerated in recent years. "While hand-held was the initial driver, today it's any equipment. For example, in a central office, users need two or three channel banks where there was only one before. More functions have to be added to circuit boards."

To this end relay sizes have been coming down. Solid-state relays (SSRs), not having any moving parts, have an inherent size advantage over less costly electromechanical relays (EMRs). But materials improvements and electrical efficiencies have shrunk the envelope of EMRs by 40-60% over the last five years, according to Harmon. Omron, which makes both types, has introduced G6K low-signal electromechanical relay that is 10.036.535.3 mm in size thanks to its coil configuration and automated manufacture (see diagram). Applications for the relay include modems and data acquisition and transmission equipment.

Design density. More functionality in less space is also a characteristic of the recently released Fujitsu Takamisawa America (FTA, Sunnyvale, CA) FTR-B2 relay for line card switching applications. This unique 4-pole signal relay for switching between phone lines takes the place of two 2-pole relays (or a more general 4-pole design) previously used, all within a package that is about 25% smaller and costs 40% less.

FTA’s unique 4-pole FTR-B2 relay is 60% the size of the previous telecom 4-pole design shown (photo). When it is substituted for a similar functioning, but smaller, twin 2-pole relay configuration (diagram), elimination of two normally closed (transfer), nonfunctioning poles cuts size about a quarter, with the same functionality. The test-relay feature shown is computer controlled by the phone system to automatically check line integrity periodically.

Seiki Sato, FTA product manager for telecom relays, notes the size reduction comes from eliminating two normally closed (transfer) poles in the twin relay configuration that were not used (see diagram). The relay design is also "non-polarized" in that there is no permanent magnet used to help open the switch. Again, without the magnet, the volume shrinks substantially, which is traded off for the somewhat higher power thus needed to be pushed through the coil, to operate both poles simultaneously.

Sato also cites automated production as a key to reducing relay size. This ability to make small components, within tighter piece-part tolerances, means no provision is needed in the design for later calibration or adjustment--a physical space and process savings. He notes higher dielectric materials, such as liquid-crystal polymers, permit higher operating temperatures as well as the more stringent safety standards that will be imposed worldwide in the near future (see sidebar). Sato adds that such standards for voltage and spacing requirements will be adjustable based on material characteristics.

FTA seals all its relays with epoxy not so much for operational considerations but for protection against contamination during circuit board fabrication soldering processes. "It is easier to offer a single part number rather than several," with sealed and unsealed relays, Sato notes. Sealing standards now under consideration will also have to be accounted for by designers in the future, he adds.

The Solid State Optronics TR115 solid state relay and ac detector replaces a reed relay and loop current sensor (which determines if a phone line is already in use). Incorporation of such multi-function circuitry along with relay functions is one advantage of such circuit-based devices. This device is geared to PCMCIA fax modem use and multiplexing.

Finally Sato mentions that relay contacts are bifurcated for redundancy and to ensure contact if "bounce" of one branch occurs when closing. Overlaying of a gold foil layer fused to the contact retains low lifetime contact resistance, as opposed to gold plating thin layer that may have voids.

It's a small world. Afterall, for microprocessor controlled telecom applications, such as PCMCIA modems or multiplexing, it's hard to compete with solid state relays. These are typically MOSFET (metal-oxide semiconductor field-effect transistor)-based with an input applied to a gallium arsenide (GaAs) infrared LED. The light produced is reflected into a series of photodiodes, whose voltage, via driver circuitry, controls the gates of the MOSFETs. Operation is at low signal levels, for fast turn-on, and, without any moving parts, size is essentially that of the semiconductor while producing lifetimes of over 1010 cycles. The semiconductor material also allows incorporation of operational features of many other non-relay devices. And the material properties produce high input-to-output isolation ranging from 2,500 to 5,000 Vac.

Bob Jesson, an applications engineer for Solid State Optronics (San Jose, CA) says small size and many features are the advantages of SSRs. And such characteristics drive laptop and palm top capabilities.

While the cost of SSRs has traditionally been substantially higher (up to several dollars per unit) than EMRs, Jesson notes this is coming down due to manufacturing automation and reduction in material costs from more timely production. Another SSR disadvantage, greater on-resistance (about 10 V vs. 0.1 V), is being alleviated with heat dissipation improvements and semiconductor technology advances. Future development of copper-based microcircuits may lead to further reductions.

While the materials performance of many SSRs allows them to meet prospective international isolation standards for voltage, current limiting is another matter. Such lightning-strike resistance is now stipulated for when, say, a modem is not operating. Future requirements will be for when the device is working. But being able to combine features in the circuitry should allow accommodating a current surge akin to the method of now generating a voltage rise up on detection, for dissipation within existing board circuitry.

The incredible shrinking relay. Omron's Harmon sees no end to EMR relay reduction, but new manufacturing methods are needed to facilitate future developments. "In order to make the next jump in size reduction, we will have to go to micromachining technology," he notes. This would allow fitting, say, 32 to 64 devices in the same space as two to four of the the smallest conventional EMRs. With molecular fabrication of devices "while the overall cost might be higher, new functions can be added and there is a tremendous savings in space," Harmon concludes.

FTA's Sato feels that micromachined devices cannot switch as much current or voltage as now required in many telecom applications, as in line card switching. Thus there are questions of meeting performance and standards in the future. Low-signal applications, as in automated test equipment and low-level digital telecom signals will probably be the first applications of such devices. But, he adds, that looking at what was state-of-the-art ten years ago, and then five years ago, "Every time out we kept saying 'How will we make things smaller in the future?'--but we did!"

For a table of Telecom Relay Suppliers, click here.

Pointers in selecting telecom relays

Omron Electronics' Terry Harmon offers some guidelines for specifying telecom relays:

Telecom signal relays typically operate from 0 to 3 A. Power relays are also used in power supplies or for battery backup.

  • Electromechanical relays (EMR) generally have low contact or On Resistance and lower cost, while solid state relays (SSR) have higher reliability and indefinite life because of no moving parts.

  • Terminal/mounting styles are PCB through-hole or surface-mount technology (SMT) gullwing or "inside-L" for close spacing.

  • Contact forms are: single-pole, single-throw (SPST) and double-pole, single-throw (DPST) with both Form A, meaning normally open until activated; SPDT and DPDT, both Form C, which switches from one connection to another upon activation. Not many SSRs are Form C due mainly to cost. (Form B is normally closed.)

  • For EMRs, types available are non-latching (which disengage when turned off) and single- and dual-coil latching.

  • Isolation requirements include surge withstand for lightning strikes (Bellcore and FCC standards); dielectric strength to accommodate longer-duration (1 min) power-line crossings of circuits (FCC standards); and European and North American standards for even longer surge duration (EN60950/IEC950/UL1950) that take effect in the year 2000.

  • With SSRs the driver is isolating between inputs and outputs. For EMRs coil-to-contact and contact-to-contact isolation are paramount.

  • Lifetime includes electrical life at rated load and, for EMRs, mechanical life.

  • EMR contact materials, under a layer of gold, include: AgPd (silver palladium) for inductive loads and voltage kickbacks AgNi (silver nickel) for high in-rush currents Ag (silver) for general purpose use.

  • Maximum operating temperatures to consider are 70C (158F) (adequate for "in-house" applications) and 85C (185F) (good for field or outdoor usage).

  • Dimensional concerns encompass profile (height) and real estate (length and width).

  • Power consumption for EMRs is determined by size and efficiency in needing less power to move the relay. For SSRs, this translates as current draw to turn it on.

  • Finally, relays need to be packaged to facilitate PCB or SMT assembly and meet UL, CSA, etc. approvals at a minimum.

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