Thermal display

Prevent Solid-State Relay Failure with Thermal Protection

Solid-state relays provide robust solutions for electronic switching in load control applications and hold many advantages over electromechanical relays. New, emerging solid-state relay designs offer integrated thermal protection that can prevent overheating and improve system safety, efficiency and longevity.

Most large motor driven machinery require a system attached to the motor’s power supply to sense overheating and turn off the motor to prevent this damage. Most of the time, this device is an electrical relay that turns power on and off. Two main types of these relays exist: electromechanical relays (EMRs) and solid-state relays (SSRs). EMRs have been the tried and ready solution for managing load circuits. However, in the last 30 years, SSRs have taken a great deal of the market share.

EMRs vs. SSRs

The life span of EMRs and SSRs are one of the major differences between them. EMRs are mechanical based and have moving parts, making them susceptible to magnetic noise, vibration, shock, and other external influences, which can affect wear and life cycle. SSRs on the other hand offer a durable, all-solid-state electronic construction with no moving parts to affect wear or accuracy, thereby offering predictable operation and longer life. The average lifespan of electro-mechanical relays is in the range of hundreds of thousands of cycles, compared to 5 million hours of three-phase solid-state relays.

These are the advantages of solid-state switching technology.

In addition to a longer lifespan, SSRs provide faster switching than EMRs, making them adaptable to a wider range of high power load applications. They operate silently with low-input power consumption and produce little electrical interference. Both shock- and vibration-resistant, SSRs can withstand harsh environments and continue to operate accurately and reliably, whereas EMRs may need frequent replacement in harsh conditions.

SSRs are compatible with control systems, immune to magnetic noise, and encapsulated to protect critical components. Their solid-state design makes them position insensitive and provides design engineers more flexibility to mount SSRs anywhere within an application—whether sideways or upside-down. SSRs can be installed in places where there is heavy vibration with no interference in performance, whereas mechanical-based EMRs are very sensitive to positioning, shock, and vibration, thereby restricting design options.

SSRs do carry a heavy price point. EMRs, as a longstanding technology, have a lower price point of entry. As such, when deciding between SSRs and EMRs, one needs to consider the conditions they will be used for. For situations that do not require heavy shielding from harsh environments, shock, or vibration applications, an EMR may be suitable. SSRs have shown their benefit and eventually, due to their longer lifespan, will provide their return on investment, especially in harsh operating environments.

The Thermal Management Challenge

Modern SSRs have problems dealing with excess heat as SSRs generate heat when conducting current. Like the motors they control, there is a thermal management component to their operation. Should overheating occur, diagnosing and replacing a damaged SSR can take time while the assembly line or manufacturing system is down and out of service, running up even more costs.

Solid-state relays like Sensata’s 53TP Series offer long operational life of over 5 million hours and can withstand harsh environments found in many industrial systems.

To illustrate how an SSR operates, consider the product’s use in commercial refrigeration applications in the building equipment market. In a refrigeration application, the SSR’s function is to turn the compressor on or off to keep the system temperature within a specified range. A buffer is engineered into the circuitry using a variety of components to ensure that the desired tripping action occurs. 

The SSR generates internal heat when it conducts the load current. Failure to adequately protect the solid-state relay can cause damage to the relay or to the load.  

Next-Generation Solid-State Relays

To address the overheating challenge, new SSRs integrate a thermostat into the device, ensuring that the relay always operates in a safe or protected mode. This prevents the SSR from overheating, thus protecting component and system operation from potential damage or shutdown.

The new SSR cuts off input circuit power when the internal temperature goes beyond the specified maximum, as determined by the application requirements by means of an integrated thermostat embedded inside. The thermostat senses the internal temperature of a mechanical interface with a metal plate where the internal power-switching device is mounted. If the heat exceeds the normal range, it sends a signal to the SSR to turn off the power.

This built-in thermal protection completely prevents overheating conditions by providing a trip before equipment damage can occur, which can help reduce maintenance costs and production downtime. Power is automatically turned on again when the temperature has cooled down to within the normal operating range.


Next-generation SSRs incorporate thermal protection via an embedded thermostat within the SSR, preventing overheating conditions.

In addition to preventing overheating, the integrated thermal protection function can troubleshoot design issues in the system. It can help identify incorrect heat sinking capacity in the SSR or system; poor installation resulting in insufficient heat sinking contact; heat dissipation efficiency of the system; and other issues, providing a preventive maintenance feature for the engineer.

New SSR design can be adapted to other industrial and manufacturing applications. For example, consider a conveyor belt application where a motor could stick and cause overload and potential damage to the system. In this case, the SSR with integrated thermal protection would prevent overheating from occurring by shutting down the conveyor belt as soon as a pre-determined heat threshold was met within the SSR’s thermostat. 

In injection molding applications where limited space can cause the temperature in the cabinet to rise, thermal protection prevents the SSR from overheating if the heatsinking is not adequate, thus avoiding costly repairs. For heating systems, the thermally protected SSR can help shut down the heating element if there is a problem with the temperature controller that causes a temperature runaway, thereby protecting the entire system.

New “smart SSRs” are equipped with technology that protects them thermally. The technology incorporates a microcontroller with firmware specific to the desired internal trip temperature that activates a decision from the pre-programmed software settings. This provides the SSR with an automated decision-making capability inside the SSR package, protecting motors and systems from over-heating and breakdown.

Sensata Technologies supplies sensing, electrical protection, control, and power management solutions with operations and business centers in 13 countries. For more information, visit the company’s website.


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