Tag Archives: SSR

What is Industrial Connectivity?

Engineers include any component involved in the path of delivering control signals or power for doing useful work as part of industrial connectivity. Typically, components such as terminal blocks, connectors, motor starters, and relays are part of industrial connectivity.

Engineers divide industrial connectors into four categories depending on the environments in which they operate—commercial, industrial, military, and hermetic. Commercial applications do not consider temperature and atmosphere as critical operating factors affecting performance. Industrial applications require connectors capable of handling more rugged environments involving hazards such as sand, dust, physical jarring, vibration, corrosion, and thermal shock.

Most general connectors use low-cost materials to merely maintain electrical continuity. However, designers have a large variety of materials from which to choose for making connectors. These include brass, beryllium copper, nickel-silver alloys, gold, gold-over-silver, gold-over-nickel, silver, nickel, rhodium, rhodium-over-nickel, and tin.

No wire preparation is necessary for use in terminal blocks. The user only needs to strip the insulation and install the wire using a screwdriver. One can use a wide range of wire sizes with terminals that provide an easy way to hookup wires from different components, ensuring fast connection/disconnection during troubleshooting and maintenance.

Manufacturers make terminal bodies from a copper alloy with the same expansion coefficient as the wire it connects. This prevents uneven expansion from causing loosening between the connector screws and the wire, avoiding an increase in contact resistance. Using similar metals also avoids corrosion, usually with two different metals in contact, as a result of electrolytic action between them.

SSRs or Solid-State Relays control load currents passing through them. For this, they use power transistors, SCRs, or silicon-controlled rectifiers, or TRIACs as switching devices. Engineers use isolation mechanisms such as optoisolators, reed-relays, and transformers for coupling input signals to the switching devices to control them.

To reduce the voltage transients and spikes that load-current interruptions typically generate, engineers use zero-crossing detectors and snubber circuits, incorporating them within solid-state relays.

Semiconductor switches generate significant amounts of waste power, and engineers must minimize their operating temperature using heat sinks attached to solid-state relays. SSRs can operate in rapid on/off cycles that would wear out conventional electromechanical relays quickly.

Electromechanical relays physically open and close electrical contacts for operating other devices. In general, they cost much less than equivalent electronic switches. They also have some inherent advantages over solid-state devices. For instance, the input circuit in electromechanical relays is electrically isolated from the output circuits, and one relay can have more than one output circuit, each electrically isolated from the others.

Furthermore, the contact resistance offered by electromechanical relays is substantially lower than that offered by a solid-state relay of a similar rating. The contact capacitance is lower as well, benefitting high-frequency circuits. Compared to solid-state relays, electromechanical relays are far less sensitive to transients and spikes, not turning on as frequently as SSRs do. Brief shorts and overloads also damage electromechanical relays to a far less extent than the damage they cause to SSRs.

Improved manufacturing technology is now making available electromechanical relays in small packages suitable automated soldering for PCB mounting and surface mounting.

Thermal Protection Prevents SSR Failure

Solid State Relays (SSR) are replacing conventional electromagnetic relays for load control applications in the industry, as they hold several advantages over the latter. However, SSRs often face overheating causing them to fail. Newer designs now come with integrated thermal protection that improves longevity, efficiency, and system safety by preventing overheating and failure of SSRs.

Machinery driven by large motors requires a system to switch off the power supply to the motor on sensing higher than normal heat, thereby preventing expensive damage. Usually, this is accomplished by an electrical relay accomplishes this by interrupting the power supply to the motor. Presently, the industry uses two main types of electrical relays for the purpose—an electromagnetic relay (EMR) or a solid-state relay (SSR). Although EMRs are the tried and trusted solution for load circuit management, SSRs are now making successful inroads into their market share.

One of the major drawbacks of EMRs is their limited life span, and their susceptibility to external influences such as shock, vibration, and magnetic noise, among others. This causes wear and reduces the life cycle. On the other hand, the all-solid-state construction of the SSR, without any moving parts, makes them highly tolerant of external disturbances. As there is no wear to reduce accuracy, SSRs enjoy longer life cycles and offer predictable operation. For instance, while an EMR may work reliably for hundreds of thousands of cycles, an SSR continues to perform satisfactorily even after five million cycles of operation.

SSRs carry a several-fold entry price hike over their similarly rated electromechanical counterparts, which are priced considerably lower. Therefore, unless the application demands exclusive seclusion from positioning, vibration, shock, and/or magnetic interference, using an EMR is often more economical. SSRs are more suited to harsh operating environments, and their longer lifespan soon provides their return on investment.

Unlike EMRs, SSRs generate heat when conducting current. Unless managed by a thermal component, overheating can damage an SSR, resulting in an outage of the manufacturing system or assembly line, leading to expensive repair expenses.

To address the challenge of overheating, designers now integrate a thermostat within the SSR. This prevents the device from overheating and ensures the relay always operates within its safe operating area (SOA). Furthermore, it protects the operation of the system and components from potential outages and/or damage.

The user can set the maximum operating temperature depending on the application. If the internal temperature of the SSR crosses the set threshold, the integrated thermostat embedded within cuts off power to the input circuit. The internal power-switching device mounts a metal plate, whose temperature the thermostat constantly monitors. If the temperature of the metal plate exceeds the normal range, the power-switching device signals the SSR to turn off the power.

By providing a trip during overheating conditions, the built-in thermal protection ensures   near-absolute equipment damage. This translates into reduced maintenance expenses and production downtimes. The user can choose to turn on power automatically when the temperature has returned to normal, or opt for an inspection before switching on the power manually. The second option helps to troubleshoot design issues in the system.