Use Solid State Relays in Factory Automation for High Reliability, Fast Switching, and Low EMI

To avoid expensive downtime, advanced diagnostics and predictive maintenance are increasingly used in factory automation across industries such as food and beverage production, automated assembly, and other continuous processing systems, as well as HVAC, water purification, and power generation machinery. Reliable relays for switching are crucial to these automated industrial processes. They must be able to switch rapidly, operate continually under challenging conditions with minimal contact wear, and not cause electromagnetic interference (EMI) that can affect wireless sensors and controls.

Solid-state relays (SSRs) meet the switching demands of factory automation using innovative technology that enhances reliability and durability to ensure consistent performance even in harsh environments.

This article briefly discusses the switching requirements of factory automation. It then introduces examples of SSRs from Littelfuse and shows how they can be used to address those requirements.

Defining and meeting factory automation switching requirements

The switching needs for factory automation include reliability at low cost, fast actuation times with no contact bounce or arcing, minimal EMI that can affect nearby circuits or the wireless sensors and networks that are increasingly used in factories, and high tolerance of mechanical shock and vibration.

SSRs employ semiconductor devices to perform the switching operation to meet these requirements. They can be actuated by either AC or DC voltages with distinct models for each type of excitation (Figure 1).

Figure 1: These functional block diagrams of SSRs show the critical components for both DC (top) and AC (bottom) actuated models. (Image source: Littelfuse Inc.)

DC-actuated SSRs (top) regulate the applied voltage. AC-actuated SSRs (bottom) utilize a full-wave bridge rectifier to convert the exciting signal to DC. Both types of SSRs optically isolate the actuating signal from the output. The active element of these SSRs is a pair of silicon-controlled rectifiers (SCRs). These SSRs include overvoltage protection in the form of a transient voltage suppression (TVS) diode connected between the gates of the SCRs to safeguard the SSR and prevent unexpected state changes in the presence of electrical transients on the network.

The switching response takes advantage of the fast switching times of the semiconductor devices and is controlled by the trigger circuit. The switching can be at zero crossings of the output voltage after applying the actuating signal or random (instantaneous) with the actuating signal (Figure 2).

Figure 2: The switching characteristic is selected to match the intended application of the SSR. (Image source: Littelfuse Inc.)

The zero-crossing turn-on is used for applications with high currents, such as industrial heaters, where it minimizes in-rush currents. Instantaneous turn-on is used where switching must occur at a high frequency. Instantaneous switching yields the highest possible switching frequency.

Examples of SSRs

Addressing the need for more reliable and longer-lasting power switching relays in industrial and commercial machine applications, Littelfuse Inc. has designed the SRP1 family of high-endurance SSRs. It is offered in two model lines: the SRP1-CB high-endurance bare SSR, and the SRP1-CR high-endurance all-in-one SSR with Touch-Safe and overvoltage protection (Figure 3, left and middle).

Figure 3: Shown are the SRP1-CB bare SSR (left), the SRP1-CR Touch-Safe SSR (center), and the alternative quick-connect terminals of the SRP1-CB…F (right). (Image source: Littelfuse Inc.)

Littelfuse designed proprietary semiconductors to minimize component degradation due to heat and deliver optimum performance under harsh conditions. Both model lines offer units with AC or DC actuation and output current ratings of 10 A, 25 A, and 50 A in either of two output voltage ranges, 24 V_AC to 240 V_AC or 48 V_AC to 600 V_AC. The models differ in that the SRP1-CR includes integrated protection and installation features, including an IP20 finger-safe cover, TVS diode overvoltage protection, and a pre-attached thermal pad. The SRP1-CB…F version (Figure 3, right) also features quick-connect terminals.

The relays are single-pole, single-throw (SPST) devices wired in series with the output load (Figure 4).

Figure 4: The SRP1 SSRs are wired in series with the output load; the input is driven by either an AC or DC actuating signal, depending on the model. (Image source: Littelfuse Inc.)

SSRs require a heatsink to operate at their specified ratings. Both Littelfuse models use the latest direct bonding technology to ensure maximum reliability and product longevity. The SRP1-CR relays include a built-in thermal pad for efficient heat dissipation that doesn’t need a thermal compound, allowing for a clean and easy installation. Thermal derating plots (Figure 5) show the maximum output current that can be supported for various ambient temperatures and heatsinks of various thermal resistances.

Figure 5: Shown are derating curves for the SRP1-CR series SSRs for various ambient temperatures and heatsinks. (Image source: Littelfuse Inc.)

Thermal resistance is specified in ˚C per watt (°C/W). A heatsink rated at 10°C/W will get 10°C hotter than the surrounding air for each 1 W of heat it dissipates. Heatsinks with lower thermal resistance are more efficient than heatsinks with higher thermal resistance and, hence, cool better.

The SRP1 SSRs are certified to comply with various health, safety, environmental, electromagnetic compatibility, and electrostatic immunity standards, including UL, CAN/CSA, IEC, CISPR, RoHS, and REACH. They are well-matched to applications for heating and motion control in industrial automation. They also work well in the food and beverage industry, controlling industrial ovens, packaging equipment, and conveyor systems. Heating, ventilation, and air conditioning systems use these relays for air handlers and compressors, as do large-scale lighting systems. All these applications require superior performance and ultra-high reliability, and the SRP1 series has options to meet various current, voltage, response time, and switching requirements.

For example, the SRP1-CBAZL-050NW-N is an SSR rated at 50 A with a 24 V_AC to 240 V_AC output. It accepts an input voltage of 90 V_AC to 280 V_AC and switches at the zero crossing of the output voltage. It offers a maximum on-state resistance of 6.3 milliohms (mΩ), which would result in only a 0.3 V drop across the relay at maximum current. The rated maximum voltage drop is 1.3 V. The relay turns on in less than 20 milliseconds (ms), including the waiting time for the zero crossing, and turns off in less than 30 ms.

The SRP1-CBDZL-010NF-N is an example of a DC-actuated SSR. It has an output current rating of 10 A with an output voltage range of 24 V_AC to 240 V_AC. The input voltage range is 4 V_DC to 32 V_DC. It also switches at the zero crossing of the output voltage, and has the same maximum on-state resistance of 6.3 mW. It differs from the standard SRP1-CB relays in that it features quick-connect terminals. Its turn-on time is equal to one-half the cycle of the output waveform.

The SRP1-CRARH-025TC-N is a Touch-Safe version of the SRP1 series and is rated at an output current of 25 A. This model is AC actuated using an input voltage of 90 V_AC to 280 V_AC. It differs from the other models because it has an instantaneous switching response and uses a high output voltage range of 48 V_AC to 600 V_AC. The relay turn-on time is less than 20 milliseconds (ms), and its turn-off time is less than 30 ms.

The fastest response time is achieved using a relay with a DC input and an instantaneous switching response. An example is the SRP1-CRDRL-010TC-N. This SSR has an output current rating of 10 A with an output voltage range of 24 V_AC to 240 V_AC. The input voltage range is 4 V_DC to 32 V_DC. Its turn-on time is 20 microseconds (µs), and its turn-off time is less than one-half the cycle of the output waveform, making it among the fastest relay cycle times.

Conclusion

To meet the switching performance, reliability, and international standards requirements of factory automation, designers can rely upon the SRP1 series of SSRs. This series uses advances in semiconductor technology to achieve long life, high switching speed, and minimal EMI across a wide range of output currents and input control options.