This article was posted on 02/10/2012

A tale of two relays

When solid-state relays (SSRs) took off commercially in the 1980s, it was generally assumed that electromechanical relays (EMRs) would soon lumber off into the sunset like a dinosaur. This scenario, however, has not come to pass. In fact, while SSRs have experienced enormous growth since their introduction, both technologies have continued to develop and thrive.

EMRs are still a reliable workhorse product used in a variety of applications. They are available in several configurations and can accommodate a wide range of voltage and current levels. The devices have an electrically rugged design that is better able to withstand voltage spikes, current surges, and temperature variations. Their output also offers true electrical isolation — a physical break of the load circuit, which is a must in applications where safety or system protection is an issue. Their flexibility and generally lower cost make them an attractive option.

The primary drawbacks of EMRs are that they require more power to operate, have slower switching speeds, and have a larger physical footprint (relative to an equivalent SSR). Internal moving parts also make them susceptible to physical wear. This shortens operating life, especially in high-cycle applications. During operation, EMRs can emit audible noise and voltage arcs can occur across the contact gap. This can make them unusable in some environments.

The more expensive SSRs are semiconductor devices that use an optical coupling to provide electrical isolation between the input and output circuits. The output typically uses a MOSFET, SCR, or TRIAC to connect or disconnect the load circuit. These devices have inherently fast switching speeds, which can be an order of magnitude faster compared to EMRs — a distinct advantage.

No moving parts also mean that they can operate without wear, giving them an exceptionally long operational life. This is ideal for high-cycle switching. SSRs are more resistant to vibration and provide noise and bounce free operation. They also use less power to operate and save on board space with their small, compact size.

However, as with most semiconductors, SSR performance can be affected by high temperatures — especially heat generated from the device itself. In this instance, an attached heat sink is often required for dissipation. Additionally, voltage transients may affect operation by causing physical damage to the unit or by triggering false switching signals.

The SSR output is polarity dependent (for dc loads) and can be subject to leakage currents, the latter indicating that the output circuit may never be totally off which can affect load operation. When in failure mode, SSRs also tend to short and fail closed which in many applications is not an option.

Today, a complementary balance has developed between EMRs and SSRs: each device comfortably fitting into applications based on their own inherent design strengths. With current trends calling for increased miniaturization, surface-mount technology, and integrated modular design, it will be compelling to watch how these two veteran technologies continue to evolve and compete.

Michael J. Kawa

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