Solid-State Relays Enhance Reliability

May 9, 2008
SSRs and PLCs Have Changed Over the Years. Find Out How

By Don Talend

With the lines between solid-state relay (SSR) and programmable logic controller (PLC) capabilities blurring in recent years, SSR manufacturers see a major role for relays in maintaining equipment functions and reliability.

Actually, what has changed more than SSR designs over the past few years is the breadth of industrial applications that use SSRs, says Tom Edwards, senior technical adviser with Opto 22. “There are two basic functions,” he states. “Solid-state relays use the technology of optical isolation to protect the control source from the nasty real world. The other function is what we call ‘zero crossing.’ It never applies power to the load when the AC power is at its peak—only when it’s at its minimum, and it reduces electrical noise and shock to the system being powered. So for devices like medical equipment, copiers and electronic drives, the solid-state relay is the product of choice and that hasn’t changed.”

What has changed is the equipment in which SSRs are used, continues Edwards. “Now they’re in your iPod and they’re small enough to fit in your pocket. There are new applications coming online all the time in production, packaging, heat control—basic industrial control areas.”

Mike Garrick, lead product marketing specialist, power supplies, relays and cabling solutions for Phoenix Contact agrees with the protection that SSRs can provide control devices. “SSRs isolate the PLC from dangerous field devices such as inductive loads,” notes Garrick. “They also convert low-voltage, low-cost DC control to a high voltage that might be required by an AC field device. Finally, SSRs can convert high-density, low-current control to activate higher-current field devices. Using a PLC with its normal I/O cards—which are, for the most part, solid state—marries well with solid-state relays, especially when reliability is of the utmost importance.”

Whether or not a PLC design uses SSRs is dependent on the trade-offs between positive and negative characteristics of SSRs, says Garrick. “The beneficial SSR characteristics include very reliable operation due to a theoretically infinite lifetime; no interference; shock and vibration resistance; high-frequency switching capability; compatibility with most PLC platforms; and an ability to drive higher voltage or current than what is available in a standard I/O card,” he says.

But SSRs do have some shortcomings too, says Garrick. These include output current leaks in the off state, heat generation, easy de-rating of output current with temperature and cost. “Cost always will be a negative characteristic, but it can be justified when a system must operate reliably,” points out Garrick. “The money spent on troubleshooting and maintenance easily can offset the initial expense of using solid-state relays.”

Because the capabilities of programmable SSRs and PLCs—particularly microPLCs—are dovetailing, the natural inclination is to consider using a PLC instead of an SSR. “SSRs still offer more simplicity in many cases,” says Jeff Pinegar, product manager, automation, Phoenix Contact. “While the capabilities of programmable relays approach those of microPLCs, microPLCs generally still are physically larger, more expensive and more complex to apply. Just as programmable relays have added more capability that makes them more complex, microPLCs now approach the low end of the traditional PLC class in capability and complexity.”

The performance characteristics of SSRs hold the key to their enhancement of control device reliability, whether on their own or working in conjunction with a PLC, says Edwards. “Control devices are relatively limited in the amount of power they can switch,” he says. “Ours can switch 3 A, which is about 300 W—that’s not very much. The choices would be to have the PLC control a mechanical relay to control the amount of power they can switch or to use a solid-state relay. The reason why SSRs are better is no moving parts: that translates to a very long life. Further, they’re hermetically sealed so they can go into any environment, including high vibration. A third factor is the zero-crossing feature that allows the solid-state relay to essentially turn on the device smoothly without shocking the device. They truly protect the device from noise and the kind of shock that shortens its life.”

When a PLC’s outputs are not optically isolated, an SSR can add a layer of protection to the control device, says Edwards. “The optical isolation buys a level of protection that, for reasons that aren’t always known when you do it, can save you a ton of money and ton of grief later because the equipment is much more survivable with the SSR in between,” he says.

Sometimes a PLC performs better without the presence of a relay. “The dividing line is that if you’re switching more than 3 A of power, which is probably most of the control applications for solenoids and small actuators and little heaters, then an interposing SSR makes no sense at all; it would be completely redundant,” says Edwards. “On the other hand, if you need to switch more power, the SSR is the logical choice.”

Edwards adds that the affordability of SSRs has recently increased. “The price of the solid-state relay, compared to its cost to build, dropped dramatically in the last decade,” he points out, adding that SSRs actually are less expensive for some applications than their mechanical counterparts are. First designed in the now-familiar hockey-puck style by Opto 22 in the early 1970s, the SSR is a mature product whose unit cost to produce is much lower than when it was first developed. “The dimensions are about 4 squ. in.—1 7/8 in. by 2 1/8 in.—that is a size that gives you a heat sink that’s appropriate to get rid of the amount of energy that a solid-state relay generates,” says Edwards. “It became the de facto standard immediately, and it hasn’t changed to date.”

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