Relays are simple switching devices that control high-power electrical devices with low-power signals. Functionally, electromechanical relays and solid state relays (SSRs or solid-state relays) behave similarly. The difference lies in the fact that SSRs don’t require moving parts to perform the task of opening or closing contacts in the circuit.
Solid state relays are often seen as the poster child for the semiconductor industry. They use the electrical and optical properties of semiconductors such as silicon-controlled rectifiers (SCRs), triodes for alternating current (TRIACs), and switching transistor outputs to perform input to output isolation and switching functions.
What are some key advantages that might tip the scales in favor of SSRs over electromechanical relays?
- Long lifetime: Since there are no moving parts inside SSRs, they are not subject to any contact failure, mechanical strain, or corrosion. SSRs can operate for millions of cycles with no deterioration in performance and the lifetime is said to be 50 to100 times that of electromechanical relays. Hence solid state relays are ideal for repetitive applications and when low maintenance is preferred.
- Fast switching: Due to the absence of movable parts, SSRs have a rapid response time. This means power to a load can be applied and removed quickly and precisely, which is especially important for applications that require quick ON-OFF switching.
- Shock and vibration resistant: Without movable parts, SSRs will not bounce electrically or mechanically. This makes them highly effective against erratic conditions in demanding and high vibration environments. For instance, Schneider Electric’s SSD solid state relays comply with IEC 61373 for railway and rolling stock applications.
The above benefits make SSRs highly suited for applications involving heating control, lighting control, and motion control. Some of the main target segments include HVAC, packaging, furnace, and food and beverage.
Of course, nothing is perfect. There are some limitations of SSRs. But fret not — there are also solutions to overcome these limitations with Schneider Electric’s Harmony solid state relays.
- Thermal management: Semiconductors are never completely ON or OFF. In the ON state, the presence of substantial resistance in solid state relays can cause significant heat generation when current is flowing. As thermal stress can drastically reduce the lifetime of the SSRs, it is necessary to ensure the relay is protected against excessive heat. To ensure proper heat dissipation, Schneider Electric’s Harmony SSRs offer DIN-rail mount options, which are integrated with heatsink, as well as panel-mount options with a variation of heatsinks to select while considering the ambient temperature and load current of your application.
- Product size: In general, SSRs are bigger in size compared to electromechanical relays due to the heatsink. This becomes a concern faced by many technicians during installation as control panels are becoming more compact in size. Schneider Electric can help you save panel space with these products from their Harmony solid state range:
- Slim-design SSL range, which is only 6.2mm in width
- SSM2 range with two inputs and two outputs in a singular relay that is 18mm wide
- SSD range with high power density packed inside 22.5mm width (up to 35A) and 45mm in width (up to 60A) models
- Silent operation: This is a double-edged sword. With the absence of moving contacts, solid state relays don’t generate any noise, making it ideal in commercial and medical applications that call for noiseless operation. However, the “click” sound of contacts switching can be useful at times, especially to alert users of malfunctions, which is the case with electromechanical relays. As SSRs operate silently, it’s challenging to identify fault conditions unless manual testing is carried out.
To ease troubleshooting and avoid reactive maintenance, Schneider Electric has developed a smart diagnostic solid-state relay with embedded visual indication LEDs and a debugging test button to monitor the operation and provide immediate response to abnormalities.
There is no clear answer to the commonly asked question: Are electromechanical relays or solid state relays better? The answer is: neither. Each has its merits and demerits. The choice depends on your specific application requirements. If you’re considering the service of your equipment, SSRs will be the most cost-effective choice as you rarely need to worry about maintenance, repairing, or replacement after installation.
Schneider Electric’s Harmony range of SSRs boasts a wide selection of products to choose from. Need help finding the ideal solid state relay for your application? Contact your local RS account manager at 866-433-5722.
