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I know solid state relays where only briefly discussed but one thing I noted the presenter stated the AC SSR could be used for DC applications as well. That is not the case for many types of SSR. Most AC SSR used in DC application may not turn off when the control signal is removed. The output circuitry is different and it is best to use the correct SSR(AC or DC) for your application.
This isn't something I've personally struck, but I can see what you mean, something about the input voltage needing to swing back through zero for the relay to switch off, regardless of the input trigger signal?
Personally, all the ones I've used have been fine for AC and DC, but will keep in mind for sure! Cheers.