Electronic – What does it mean to soft switch a relay

There are a few problems with replacing electromechanical relays with solid state relays.

1) Most SSR's cannot control an AC signal. They are only rated for DC. They will usually conduct all the time with a reverse biased drain-source current, due to the body diode of the MOSFETs.

2) SSR's can be damaged by exceeding the maximum current through the photodiode, which is usually no more than 10-20mA. A relay will take 100's of mA through its coil. So they aren't a drop in replacement.

3) They are faster than electromechanical relays, which can cause problems for some circuits (the circuit may be designed to reset in the time the relay is transitioning between the two states, for example.)

4) They are sensitive to ESD, electromechanical relays are not.

5) And finally, most importantly, they do not click when they change, which makes them kind of boring. I love hearing my scopes' relays click when you change the attenuation. Don't underestimate how awesome this is.

The 6cpm is indeed one switch over per 10 seconds, this is the effective advised maximum operating speed under a load, because making and breaking an active current will create a thermal (and even ionic in quite a few cases) effects in and around the contacts.

Going faster under load will seriously decrease the life expectancy, because the contacts are not made for those stresses.

If your load is lighter, or generally more friendly than the rated load, you may be able to go faster and still get a close to similar life time, in relays they generally only specify no load and rated load conditions, sometimes a middle figure, and leave the rest to maths and experience on the engineer using them.

The second table underlines the difference between the life time of the mechanics and its capabilities and the life time of the contacts themselves. If you put a rated load on, it will last "only" 100000 operations at maximum 6cpm, less long at a higher speed, somewhat longer at lower speeds. Same goes for the load, if the instantaneous turn-on and turn-off power on the contacts are halved, you may get a statistical average of 150000 operations at 6cpm, for example.

But, you can see, that with no load to damage the contacts, suddenly the thing can be expected to operate for up to 5000000 operations (50 times more) at a speed 30 times faster, because that's what the mechanical system supports.

As to your extra question in the comments to your question: Can you operate it at 100ms switch time? If you look at the datasheet, you see it's operating time is 15ms and its release time is 5ms, so yes, you probably could use it for switching over at 10Hz. You have to be aware though, that 20ms of every 100ms period the contacts will be "floating in air" between positions.

You also need to be aware that the relay is not in any way rated or tested for that. The 180cpm is pretty much the limiting factor they assumed in testing, because that's the upper limit they envisioned, so you may be putting on stresses it cannot handle in the long term. But it is likely to work for at least a while.