Depending on who you ask, solid state relays are the superior power switching solution, but others insist that electromechanical relays are the obvious choice. Who is right and for which reasons? To answer these questions, let's explore the differences between electromechanical and solid state relays and how they operate as well as compare their performance specifications on a number of levels.
What is a relay?
A relay is a power switching solution that can be used to allocate power without manually opening and closing the switch. To switch power on and off, a relay simply requires a small electrical signal. This signal is a metaphorical “gatekeeper” for a much larger electrical signal. The ability to have low-power control over a high-power signal is what has made relays so prominent throughout the history of electronics.
What are the differences between electromechanical and solid state relays?
Electromechanical Relay (EMR)
An electromechanical relay uses a physical moving part to connect contacts within the output component of the relay. The movement of this contact is generated using electromagnetic forces from the low-power input signal, allowing the completion of the circuit that contains the high-power signal. The physical component within the electromechanical relay commonly makes a “click” sound, which can actually be useful in some situations, though it can lead to internal arcing and takes a relatively large amount of time to move.
Solid State Relay (SSR)
A solid state relay just might be the poster child of the semiconductor industry. SSRs use a low power electrical signal to generate an optical semiconductor signal, typically with an octocoupler, that transmits and energizes the output signal. When activated, the input optical signal acts as the “switch” that allows a high voltage signal to pass through the SSR’s output component. There are several ways of doing this, but the common theme between them all is the lack of moving parts, thus making them solid-state.
Figure 1 – A typical EMR (Electromechanical Relay), and an EMR block diagram depicting its moving parts.
Figure 2 – Panel Mount Solid State Relay – from Crydom and a diagram showing the optotransistor mechanism. Diagram courtesy of Wikipedia.
Both technologies can be used for applications in heating, lighting, motion control, and more. However, solid state relays are superior to electromechanical in most comparative categories. Electromechanical relays are a relatively old technology that use a simple mechanical design approach, whereas solid state relays are much newer and advanced—and yes, more complex. One can argue that something complex is not necessarily better than a comparable simpler product that does the same task. However, the more complex SSR might just win you over when it comes to performance.
What are the advantages of a solid state relay vs electromechanical relay?
Solid state relays come with a rather large and perhaps intimidating upfront price tag when compared to an electromechanical relay. However, integrating an SSR rather than an EMR may result in an equal or even lower overall cost depending on the particular application, something we will discuss a little later. In addition, SSRs often outperform EMRs in several areas. Let’s compare the two:
As you can see, SSRs are in general more dynamic in their performance and they features they offer when compared to EMRs. There is one area, however, where EMRs often have the advantage: thermal management. SSRs can have power dissipation orders-of-magnitude larger than electromechanical relays simply because of the physics they utilize. This generally means that component designers must integrate heat sinks and fans into their designs, which does increase the overall upfront cost you incur by using the SSR.
Before you write off SSRs based on upfront cost alone, however, let’s talk more about total cost of ownership as well as the individuals determining what “cost” really means.
Comparing Costs: How Solid State Relays Can Still Be Worth It in the Long Run
Engineers tend to focus on performance, and better performance generally means higher costs. Supply Chain Managers, on the other hand, are much more concerned with initial pricing of parts and delivery times, which are referred to as procurement costs. Business and Marketing Managers spotlight warranty costs, which include life expectancy and associated maintenance costs such as downtime, travel time, replacement and repair time, and manpower. Of all these costs, only the initial pricing of parts can be equated to the aforementioned “one-time upfront cost.”
Therefore, just because electromechanical relays have a lower initial price tag does not mean they “cost less” given the aggregate of all the other hidden costs that come into play later on down the road. All too often in today’s businesses, managers are demanding “faster-better-cheaper” solutions, which are frequently competing priorities – cheaper doesn’t necessarily mean faster and/or better. However, using Crydom solid state relays and contactors, you can indeed achieve faster, better, and cheaper when considering total cost of ownership. Their solid state relays are reliably performing solutions that contribute to application longevity, so you rarely have to worry about servicing, repairing, or replacing them once they’ve been installed.
Comparing Total Ownership Cost of Solid State Relays and Electromechanical Relays
To help you analyze the total cost of ownership (TCO) for SSRs vs. EMRs, Crydom has developed an easy-to-use TCO Calculator that can be found on their website under the “Tools” tab. The calculator considers information related to both direct and indirect costs, and it also assumes both switching solutions have been property selected to match the load and the power systems. Additionally, the calculator incorporates costs mainly associated with technology, as they are common amongst various applications.
The calculator's cost per unit is the acquisition cost of the switching solution. There are also for adding heat sinks (for SSRs) and sockets (for EMRs) if either is required. Life expectancy, given in number of cycles or operations, is also taken into account, as are specific requirements for a given application. These might include projected cycles per minutes or type of load (e.g. resistive, inductive, transformer/capacitor, or ballast).
The type of load is required for estimating life expectancy adjustments for EMRs, and the calculator automatically selects proper derating based on the chosen load: 80% derating for motors, coils or solenoids; 75% derating for transformers or capacitors; and 70% for ballasts. The warranty period, also included in the calculation, is used for a time reference in the estimated results of the total cost. It can be adjusted to show different time frames, from one month to a year or more. Cost per service, another variable, must also be considered, as it changes between applications or from one business model to another. Figure 3 shows Crydom’s TCO calculator.
Figure 3 – Crydom's TCO calculator.
When you're choosing between electromechanical relay technology and solid state relay technology, you may have a hard time getting past the upfront cost of SSRs. However, while the initial cost of the EMR is less than the SSR, as the cycle count increases so does the cost of maintaining, repairing, and/or replacing the EMR. Therefore, once you have worked through the total cost of ownership of both SSRs and EMRs, you will soon realize the lifetime cost of SSRs is the same or less than that of EMRs in many cases. Given the advantages SSRs have over EMRs with regards to features and performance, it isn’t difficult to justify integrating an SSR into your design.
See related product
See related product
Want to know more? Why not read up on What's Inside a Solid State Relay?