It’s always tempting to replace a legacy hardware technology with its younger, hipper (read: semiconductor-based) counterpart. That bulky, shockingly heavy magnetic power supply now sitting upon your lateral file has become something of a relic, replaced with the spiffy and highly-efficient switch-mode power supply, the one that fits in your shirt pocket. Those clunky mechanical push-buttons on your front panel? Pushed aside in favor of touch sensors. And don’t even get me started on light bulbs.
So, what about your relays? Is it time to migrate from those large-footprint switches to something a little smaller? Something a little quieter? Something a little more solid state, like the cool kids use? Maybe. But, as with anything else, it all depends upon the application. Here are five questions you should ask yourself before making the switch:
1. Does the failure mode of the switch carry safety implications?
Electromechanical relays have one key safety advantage over solid state relays: they can reliably fail open, the air gap between contact points prohibiting current flow from source to load. This prevents minor catastrophes—the dishwasher flooding the kitchen, for example—as well as life-threatening ones. Though there is a chance of contacts welding shut due to excessive current or other factors, this happens rarely and is most often credited to the circuit designer and not to the component. By contrast, in the event of severe electrical over-stress solid state relays tend to fail as a short. In certain applications this can have devastating consequences.
2. How many actuation cycles are you designing for?
Though careful circuit design and component selection can maximize the lifetime of an electromechanical relay, the operating life of a solid state relay is typically orders of magnitude longer. If millions of cycles are anticipated, an SSR is worth considering.
3. How demanding is the application, electrically?
This questions covers a lot of territory, but there are some key electrical characteristics to consider when contemplating a migration to solid state relays. The circuit’s susceptibility to voltage surge, EMI, and ESD is an important concern. Because SSRs are semiconductor devices with relatively sensitive internal structures, overvoltage and ESD can cause accidental triggering. EMRs don’t share this susceptibility, making them a popular choice especially wherever punishing transients or surges tend to occur.
4. How demanding is the application, physically?
EMRs and SSRs have different sensitivities to their physical environment. For instance, the performance of an SSR wanes in high-temperature environments, unlike that of its mechanical counterpart. The SSR’s vulnerability to ambient temperature and thermal rise during conduction periods necessitates not only performance derating but also proper heatsinking and the mounting considerations associated with it. For this reason, EMRs can be a more cost-effective solution. On the other hand, applications prone to physical shock and vibration can benefit from a switch that has no moving parts, while designs indicating smaller form factors benefit from the SSR’s smaller footprint.
5. How are you operating the switch?
If extremely short switch-and-settle time is a design imperative, then a solid state solution may be desirable. Where an EMR might require 20ms to switch the load, an SSR can get the job done in a fraction of the time without the contact chatter associated with electromechanical solutions. Switching frequency is another consideration. Though some EMRs perform better than others against this criterion, SSRs are generally better-suited for applications involving rapid switching. SSRs are also advantageous in applications requiring precise or line-synchronized operation and low-noise switching.
About the Author:
Cliff Ortmeyer is the Technical Marketing Manager for Newark element14, a division of Premier Farnell. He has over 20 years’ experience in the semiconductor and electronics design industry in various capacities including FAE, Market Development and Technical Marketing. Cliff also holds multiple patents in both analog and digital design.
For more information, visit www.newark.com.