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."
Omronís 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
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.
FTAís 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
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
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
Telecom signal relays typically operate from 0 to 3 A. Power relays are
also used in power supplies or for battery backup.
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
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.
Finally, relays need to be packaged to facilitate PCB or SMT assembly and
meet UL, CSA, etc. approvals at a minimum.