Are Contact or Solid-State Relays Better for Automation?

Relays are switching devices that enable machines to respond to control signals with precision, serving as the essential link between low-voltage control logic and high-voltage operations. Whether an automated system needs to manage high-power motor loads or coordinate programmable logic controllers (PLCs), choosing the right type of relay can dictate certain aspects of efficiency and durability. This blog will focus on comparing contact and solid-state relays, touching on their different modes of operation, advantages, challenges, and suitable uses to help you make the best decision for your automation requirements.

Contact Relays

Contact relays, also referred to as electromechanical relays (EMRs), operate by opening and closing electrical contacts through an internal electromagnetic coil. When current flows through the coil, it creates a magnetic field that moves the armature toward a fixed core, completing the circuit. Meanwhile, their spring-loaded mechanism ensures the contacts return to their default position when the coil is de-energized.

A notable variant of contact relays, reed relays contain thin, magnetic reed switches enclosed within a glass tube filled with inert gas. When the coil surrounding the tube is energized, the reeds are magnetically drawn together, closing the circuit. Reed relays are further distinguished by benefits like their:

Compact size and simpler design, creating faster responses and benefitting densely packed electronic systems
Lower power consumption
Superior signal integrity for switching low-level signals

Benefits in Automation

Useful for general switching applications, including motor controls and safety circuits, standard EMRs are often favored for their:

High current switching capabilities
Clear physical disconnection, which provides better electrical isolation
Low initial cost and widespread availability
Ability to support both AC and DC loads

Challenges and Considerations

Contact relays come with drawbacks like:

Mechanical wear and tear, especially with high switching frequency
Slower switching speed due to the time it takes for contacts to physically move
Contact bounce that can result in unreliable signal transmission, particularly in fast-paced control systems
Arcing, which is especially present in high-voltage applications
An audible clicking sound when switching

For systems that are expecting long lifespans with minimal maintenance, these aspects must be properly mitigated through design and protection features.

Solid-State Relays (SSRs)

Instead of mechanical contacts, SSRs use semiconductor components like thyristors, triacs, or transistors to switch a circuit electronically. The input signal activates an optical isolator or a trigger circuit, which in turn initiates the switching semiconductor. Some notable variants include:

Zero-Crossing SSRs: These relays are designed to activate only when the AC load voltage crosses the zero-voltage point of the sine wave. By delaying switching until this exact moment, zero-crossing SSRs significantly reduce electrical transients and electromagnetic interference (EMI).
Random Turn-on SSRs: Random turn-on SSRs can switch the load at any point in the AC cycle as soon as the control signal is applied, making them highly effective in phase control applications.

Benefits in Automation

In general, solid-state relays serve rapid or repetitive operations well, such as conveyor systems, packaging machines, temperature management devices, and PLC-based control circuits. They boast:

High-frequency switching and faster response times
A longer operational life due to no contact wear or mechanical degradation
Resistance to shock and vibration
Silent switching
A lightweight and compact design

Challenges and Considerations

The limitations that SSRs can introduce include:

A higher initial cost, although long-term savings can be achieved through their reduced maintenance demands
More excess heat generation, requiring heat sinks or additional thermal management
Limited overcurrent protection, which means they do not tolerate short circuits well without added circuitry
Even in the “off” state, some current may leak through

Choosing the Right Type of Relay for Automation

All in all, both contact and solid-state relays offer distinct advantages and limitations, neither type being universally “better.” The best choice depends on the requirements of a particular automated system.

Choose contact relays when:

High power switching at lower frequencies is needed
Mechanical isolation is preferred
Building more cost-effective systems

Meanwhile, choose solid-state relays when:

High-speed, frequent switching is required
Long lifespan and minimal maintenance are priorities
Vibration or noise reduction is a design concern

Secure Relays for Automated Setups on ASAP Industrials

Automation is integral to processes in so many industries, so selecting dependable, high-performance relays can have a significant impact on operational uptime. ASAP Semiconductor is here to help you secure relays and automation components through our procurement platform ASAP Industrials.

On this website, thousands of products from trusted manufacturers and those suitable for varied industrial requirements are ready for purchase, all of which will be shipped out alongside all documentation to prove their authenticity and compliance with relevant industry standards. Furthermore, with competitive pricing, prompt lead times, and curated solutions, we stand ready to support even your most complex fulfillment needs. If you want to know more about our parts and services, take a look at our database or connect with a representative of ours via phone or email.

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