SSR or EMR? Select the Right Relay

Solid-state and electromechanical relays are not necessarily interchangeable. Evaluate your application before deciding which to use.

Solid-state relays (SSRs) are replacing electromechanical relays (EMRs) in many applications across industry. There are several reasons why, including their long life, low noise, compact size, lack of moving parts, and total absence of arcing. These advantages make SSRs a popular choice for applications involving repetitive operations or fast turn-on/turn-off times, or in areas that require minimal electrical noise.

So, what types of SSR or EMR relays are right for the various applications in your plant? Automation professionals at Opto 22 (opto22.com), in Temecula, CA, provide some selection guidelines.

Use SSRs in applications that require:

Repetitive operation cycles. Such applications include lights and electric heaters. SSRs have no mechanical components to wear out and no failure mode related to the number of operation cycles.

Minimal electrical noise. SSRs greatly minimize electrical noise because they turn on and off when voltage is zero in the AC cycle. Conversely, most EMRs turn on and off at any point in the AC cycle, which means they can generate significant voltage spikes, causing electrical noise that can affect other devices in the area.

High-speed timing. SSR turn-on times are highly predictable, while times for a mechanical relay vary based on the nature of the device and the environment.

Consider EMRs in applications that require:

High starting loads. Such applications include motors and transformers. SSRs are more sensitive to voltage transients than EMRs. If a relay gets hit hard enough a sufficient number of times, even SSRs with good transient protection will degrade or fail. This makes SSRs less ideal for driving highly inductive electromechanical loads, such as some solenoids and motors.

Operation in high-temperature environments. SSRs become less efficient as the relay temperature rises. The current rating for an SSR is de-rated, or reduced, based on the ambient temperature. EMRs are not affected in the same way.

Zero leakage current. In the “off” state, an SSR will exhibit a small amount of leakage current—typically a few mA. Because EMRs are mechanical, they do not leak current.

Special SSR Concerns

According to Opto 22’s (Temecula, CA) automation experts, in the use of solid-state relays (SSRs), two factors inherent to semiconductor-based relays require special attention:

Leakage current. When in the “off” state, an SSR will exhibit a small amount of leakage current, typically a few mA. It’s slight, but this current can keep some loads from turning off, especially in high-impedance applications such as small solenoids or neon lamps, that have relatively small “hold in” currents. When SSRs that switch high voltages are electrically open, leakage current can still cause their circuits to produce potentially troublesome voltages on the outputs. These issues can usually be addressed by placing a power resistor, sized for 8 to 10 times the rated maximum leakage current for the SSR, in parallel with the load.

Operational-temperature limits. Semiconductor-based relays become less efficient as their temperature increases. Thus, the current rating for an SSR is de-rated, or reduced, based on the ambient temperature. Since SSRs also generate heat in the “on” position, heat management is vital.