Solid-state relays (SSRs) have been around for a long time. Although they haven't necessarily been a dominant force in relays, several factors, including some novel applications, are continuing to increase their usage in the marketplace.
SSRs may be replacing conventional electromechanical relays (EMRs) in some general applications, but they're carving their own niche in high-reliability and long-life applications. Here, where reliability and longevity concerns dictate their use, SSRs have made the largest impact.
The trend is to make SSRs smaller, run cooler, and come closer in price parity to EMRs. When viewed from the perspective of three of the most important operating parameters that any relay must meet—input power, electrical life, and cost parity—SSRs stand up well to EMRs.
Input power: While most relays require between hundreds of milliwatts to a few watts to operate, SSRs only take microwatts to a few milliwatts to function. In an EMR, an electromagnetic field must be driven in order to physically move a contact assembly. In an SSR, the driving power alone is enough to drive an LED of an optical coupler. This provides tremendous savings in operating power.
Also, the cost of relay drivers may be reduced, if not eliminated, because the SSR can be driven by direct TTL instead of by an individual driver. For example, a 20-A or more SSR may have an input power of 20 mW or less, whereas the same product as an EMR might take 900 mW or more. This could result in better power management, especially when several SSRs must operate at the same time. Additionally, the power supply controlling the relay inputs can be reduced in size and cost. In short, where input power is at a premium, the SSR is better suited.
Electrical life: An SSR may have about ten times the electrical life of an equivalent EMR, continuing its trend of use in high-reliability applications.
The stress in an SSR is opposite that of the EMR. The damage in an EMR occurs during the switching of the relay, where carrying the current is easy. An SSR, however, can switch current very well, but it generates heat during the current-carrying process. This isn't a major issue, though, because SSRs are usually derated to handle carry current. The SSR can switch with little degradation, so the electrical life is quite high, typically five to 10 times higher than in EMRs.
Plus, life is rated in terms of current-carrying hours, rather than cycles, because the mitigating restriction is current. A typical number is 100,000 hours, or over 11 solid years of "on-state" operation.
Cost parity: Over the past five years, lower-power SSRs have dropped in price to the point where they're much closer than their EMR equivalents. The cost parities have come closest in industries like telecom, datacom, test and measurement, instrumentation, and industrial outputs. While SSRs rated above 3 A still cost three to four times more than equivalent EMRs, pricing for SSRs under 3 A is generally no more than one-and-a-half times higher.
Another cost-analysis function is the life versus replacement cost. Although an EMR is priced lower than an SSR, it may require three replacements during the product's life. For example, an EMR might have a life of 100k operations, whereas an SSR can possibly function for more than 500k operations. If the application calls for 300k operations, the EMR will have to be replaced three times, whereas the SSR will last the whole life of the product. Therefore, the SSR costs less overall, which is why SSRs are popular in commercial equipment and industrial controls, where very long product lives are necessary.