Relays & switches do more with less

DN Staff

November 3, 1997

9 Min Read
Relays & switches do more with less

Two technology trends are fueling the seemingly mundane market of relays and switches these days. One is the drive for greater miniaturization while pushing the current carrying capabilities of these devices. In the other direction is the increasing need to handle reliably lower "logic level" voltages and amperages common to microprocessor-driven applications.

"Better and better MOSFETs," according to Doug Lionetti, marketing manager for Aromat Precision Components, New Providence, NJ, "are enabling solid-state relay improvements." He cites their linear rely-like characteristics for true switching action. Every 12 to 18 months there seems to be a new generation of greater numbers of linear switches packed within a smaller surface area of silicon, while being able to handle higher currents. Such progress, Lionetti says, is related to the microelectronics fabrication methods that enable each succeeding wave of microprocessors to have more transistors in a given space. "It's a win-win situation. With less real estate taken up by relays, designers have more room to design."

Design changes that add barriers between coils and contact areas, and developments in insulating films, are parts of the reason why electromechanical relays are carrying more current in a given volume, says Dan Davis, product manager at Siemens Potter & Brumfield Products Division, Princeton, IN. "Another important trend is that the relay coil power required is becoming less and less over time," he adds. "By using springs and motors to help effect relay action, these smaller coil currents produce less internal heating and also lower driving-current requirements."

Keeping cool. Davis also cites higher-temperature electrical insulation systems leading to sustained relay use at higher temperatures. Such materials are currently capable of withstanding temperatures upwards of 155C, as opposed to 85C in the early 1980s. They allow wider input voltages, and thus less regulated power supplies, to be accommodated in addition to higher ambient temperatures. With less degradation over time while under temperature and voltage loads, these materials permit manufacturers to meet worldwide standards for input/output and open-contact electrical isolation, even with smaller units in harsh environments. And Aromat's Lionetti adds that regulating authorities have been flexible in meeting such spacing requirements by basing approvals on test results rather than traditionally rigid specifications.

Also materials driven, plated relay contacts have replaced iron ones, so corrosion is less of a problem. Siemen's Davis adds, "The trend is to sealed relays which also alleviates contamination concerns." He cites a standardized unit the company produces with a vent tab that can be broken off, if needed, to allow plastic outgassing or ionized gases from high-current arcing to escape.

Better materials, not just for contacts but also for springs, and improved "part control" methods result in more current in smaller relays, says Martin Hauser, chief engineer at Hasco Components International, Bellerose Village, NY. The company is using an alloy of copper, indium, and tin oxide to replace silver and silver cadmium for contacts. The copper facilitates heat dissipation and handling higher power, while the other constituents insure thermal expansion compatibility. Hauser cites Hasco relays with the new contact material capable of 200,000 operations carrying 20A at 120V. A 0.4 x 0.4 x 0.4-inch relay now available switches such current and can carry up to 40A. Hasco is working with metal suppliers for reed switch material as well.

Improved processes have driven relay costs down by any measure, says Hauser. "A DPDT relay, 2 x 3 x 3 inches operating at 30A, cost $5 in the 1960s. Today, the same capability can be had in a 1 x 3/4 x 3/4-inch package and costs $1.25." Automated assembly, adjustments, and testing is making possible such improved products and savings, he notes. Finally, Hauser adds that demand for electro-mechanical relays is growing, driven in part by the burgeoning need for circuit-board testing, "since complete isolation (1014 {OMEGA}) is needed between contact points, which you can't get in solid-state devices."

Electro-mechanical Relay Current Trend

Current density (amp/inch3)

Year

5

1970s

27

1990

45

1997

Switched on. Ganging multiple electro-mechanical switches (upwards of 40) together for activation by a single lever or other device is one trend noted by Galen Bertram, application engineering manager for Honeywell Micro Switch, Freeport, IL. The smaller the switches in question, the more efficiently limited space behind a panel can be used, as in vehicle or aerospace applications. Many of these are for logic-level (5-16V, 1-5 mA) uses while others can push, for example, up to 25A in a volume 3/8 x 7/8 x 1.5 inches which previously saw 5-10A.

In getting to the latter high-power uses, Bertram also cites attention to heat dissipation in design. "In arcing applications, with the low heat dissipation inherent at the contacts, stationary terminals are no longer staked with a rivet. Designs are moving toward resistance-welded silver contacts on brass terminals." Welding expedites heat removal by forming a more substantial heat path in addition to using silver's higher thermal conductivity. This design eliminates a manufacturing step, as well as any loosening of the rivet joint. If needed for applications up to 15A, silver plating of other components further increases heat dissipation. Similarly, copper terminal materials are favored over brass for uses to 25A. Also, switch cases and covers require plastics with higher electrical insulative properties and no dielectric breakdown.

Adroit mechanical design also boosts switch performance. Bertram says quicker activation gives longer switch life. Newer designs insure that the plunger moves quickly, cutting the time the contact exerts lower force, which is when resistance heating is the greatest.

'Pentafurcated' fingers on Allen-Bradley PenTUFF contact blocks progressively blanket a rounded stationary contact, eliminating bounce effects. The range of finger flexure also counters intermittent opening due to shock and vibration.

Low power. In logic-level use, smooth, spherical contacts are being replaced by serrated (grooved), projection (bumps), or bifurcated configurations to thwart particle and chemical contamination. Without any high-voltage arcing present to clean these contacts of contamination, such "multiple-hit" configurations, along with a "brushing" action, minimize the detrimental effect a single contamination particle can produce. Even if not completely removed, particles will drop into "valleys" for less contact surface contamination.

"Corrosion resistance is a major consideration in today's low-voltage industrial environments," according to Brian Delfosse, marketing supervisor for Rockwell Automation/Allen-Bradley, Bloomington, MN. "Humidity, hazardous chemicals and gases, and dirt and debris all decrease the reliability and life expectancy of a switching device."

Constructing every component of the device with non-corrosive materials is one way to combat this problem, Delfosse notes. Gold and palladium alloys have excellent conductivity and corrosion resistance, good for moveable contact (spanner) construction. Stainless steel works well for base and terminal design.

Another low-power (24V and 24 mA or less) switching consideration is highlighted by Delfosse. He notes "contact bounce" may occur with single contacts "one or more times for a few milliseconds when they are closed. This may send false signals to logic controllers."

For such PLC uses, Allen-Bradley has developed pentafurcation technology "which divides the ends of the contact movable spanner into five individual fingers," says Delfosse. The multiple, split spanner creates independent and sequential contact. Because the spanner is more flexible, each finger bounces at a different frequency to allow at least one available current path. The design also protects switches from loss of contact due to shock and vibration.

Increased safety in the workplace is also driving the need for low-energy products, adds Delfosse. A 24V rating improves safety because the lower-voltage devices powered at this level are considered inherently safe. At 24V, people can work with circuits on-line, without danger of physical harm.

While the advent of electronic switches and relays may have had many thinking that the days of such electro-mechanical devices were numbered, new materials and designs, miniaturization, and higher current densities, mean there are more places today where both types can be used.


What smaller relays and switches mean to you

  • More real estate for additional components.

  • Meeting electrical-isolation standards within small volumes.

  • Pushing higher power through smaller packages.


Which switch (or relay) to pick

Some helpful considerations offered by the experts include accounting for relay "in-rush" loads as well as steady state conditions, according to Siemens' Dan Davis. As an example, he notes a 30A motor will have a lock-rotor rating (before it gets up to speed) typically six times higher than the steady-state current. For tungsten lights, such effects can be 10 times as great. Another point he makes: At operating temperature, the resistance of relay coils is greater, reducing coil current. Thus, more voltage is needed to produce the ampere-turns necessary to generate a specified contact pulling force.

Martin Hauser of Hasco says many young engineers automatically try to use solid-state relays from the outset on a design project. He cites a designer who was recently developing a piece of military equipment that had 1,500V across the contacts. Trying to do it with a solid-state solution, there was no way the input/output (line isolation from ground) requirement could be met. The eventual solution: an electromechanical DPDT relay costing under $1.

Hauser feels the lower cost, ease of use, and reliability of electromechanical relays should be considered first, unless contact bounce is a problem. To which Honeywell Micro Switch's Galen Bertram adds, electro-mechanical devices tend to be "designed out" by electronic sensors that do multiple functions via programming, and that offsets their higher cost.

"There is a need for young people to be educated in relays at the university level," says Hauser. "And industry associations could do more to that end and in educating industry users as well."

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