SSRs Are Useful, but Take Another Look at Modern Electromechanical Relays
So, you’re starting to work out the system blocks for your latest project which involves, among other requirements, having a low level DC voltage switch some power rails on and off. No problem: you note the control signal voltage and current drive along with the specifications of the load to be switched (again, voltage and current), and begin picking out a suitable solid state relay (SSR).
But, things soon get complicated. The same low level DC signal must simultaneously turn one AC line off and another one on, and it also needs to switch a 48 VDC line. The number of SSRs on the BOM is growing, and ensuring a solid drive to all of them is becoming a challenge.
Then an old-timer comes by and remarks, “You’ve probably never considered it, but maybe a single electromechanical relay (EMR) can solve all your problems at once?” Well, the old-timer is right that you’ve never even considered it, but an EMR sounds so, well… dated, if not downright archaic.
Think again. Despite the many well-known and impressive virtues of SSRs (no need to repeat them here), tens of millions of EMRs are still sold every year. While some are for replacement requirements, a large fraction is for totally new design-ins.
Why would any designer choose an electromechanical device – with all the “baggage” it presumably brings – when a solid state equivalent is available, and at comparable cost? Here’s why: while the EMR is functionally similar in the broadest sense, it has many unique characteristics and virtues compared to the SSR.
Like the SSR, the relay coil and its contacts are electrically (galvanically) isolated from each other with a multi-megaohm resistance path, but the EMR can do many things which an SSR cannot. Some of the special attributes of the EMR include:
- The relay contact forms a basic switch closure, and current through it can be AC or DC, independent of the coil drive; the contact resistance is in the milliohm range, so the voltage drop across the contacts is very close to zero, while the open contact resistance is an air gap and therefore in the multi-megaohm range.
- The EMR is a completely passive device, without active components such as an LED or phototransistor. This has implications for ruggedness and reliability. It is electrically and mechanically robust (partially due to its mechanical and thermal mass), and resists spikes, transients, and EMI which might momentarily trip or even damage an SSR. Most EMRs are rated for millions of operating cycles, while the sealed reed relay (a type of EMR) has a rating into the tens of millions.
- Despite the metal frame of most relays, neither coil nor contacts closure are grounded or connected to circuit common, so the relay can be placed anywhere in a circuit; that can be difficult due to the active nature of the SSR with some circuit topologies.
- While basic relay contacts are normally open (NO) when not energized, there are standard relays with contacts which are normally closed (NC) when not energized – and many ones that have both, using combined NO/NC contact pairing.
- The relay can be a multipole device, with more than one NO or NC contact pair; three, four, or even more independent NO and NC contacts are available, with DPDT being the most common (Figure 1). Even more flexible, these multiple contacts do not have to be carrying the same type and rating of loads, which is another benefit; some contacts can be rated for low level signals while others can be for power.
Figure 1: Shown are some of the many available contact configurations for the EMR (and some apply to the SSR as well), in industry standard designations. (Image source: Wikipedia)
For example, the AGQ200A4HX from Panasonic Electric Works is a pc board surface mount EMR. Though designed for telecom applications, there’s no reason it can’t be used in others. Its DPDT contacts (designated as 2 Form C arrangement in industry terminology) are independently rated at 2 amperes (A) and 125 volts AC or 110 volts DC, while its coil requires just 4.5 volts DC at 31 milliamps (mA) (Figure 2).
Figure 2: The AGQ200A4HX SMT relay from Panasonic is typical of low voltage DC input relays with DPDT contacts, each of which can switch 125 volts AC or 110 volts DC at 2 A; the same unit can handle both AC and DC loads simultaneously. (Image source: Panasonic Electric Works)
- Relays can be designed for coil currents as low as 10 or 20 mA or as high as tens of amps, with contacts rated to handling just a few tens of mA and a few volts all the way to several orders of magnitude greater for both parameters.
- EMR contacts are signal “agnostic;” as long as you stay within the voltage and current maximum ratings, it is irrelevant whether it’s a power signal, data signal, or mix across multiple contacts. Further, the load does not have to be well known or defined, it just has to be within the design limits; this is useful in cases where the load may have uncertain or hard to control characteristics.
- The most common failure mode of an EMR, by far, is the coil not becoming energized, so a NO contact fails “open” while an NC contact fails “closed” – which one is preferred or necessary may be a safety issue in the application. In contrast, SSRs tend to fail with a short circuit at their output, which may not be acceptable.
- There are standard EMRs available called “latching” relays which maintain their energized contact position even if coil power is removed or fails (a separate coil and signal unlatches them); this is a nice feature in some situations and a vital one in some safety related ones.
- The relay is very easy to troubleshoot; all that is needed is an ohmmeter to measure the unpowered coil continuity and DC resistance, and a simple AC or DC power source to energize the coil.
- Finally, and this is a personal factor for some designers, there’s the viscerally satisfying “click” which the EMR makes when the relay pulls in or drops out. Some engineers (myself included) love to hear the “click-click-click”, and even use it to monitor system activity.
So, the next time you are faced with an SSR challenge, don’t assume you need more or different SSRs. Modern versions of the venerable electromechanical relay – which has been around for about 150 years and is now highly refined and mature – may actually be the component to solve your problems with the most acceptable tradeoffs.