>By their very nature, SSR's and Triac's will only turn off on a zero
>crossing of the AC power feed.  This is why they are non-applicable to DC
>circuits.  You can only control when they are turned on.

Unless you use a "Gate Turn Off" SCR where you do force the device to stop
conducting by a signal on the gate. I have not used any of these, but do
remember seeing datasheets for the early ones made by RCA. They did require some
hefty drive signal into the gate to turn them off. I believe they have improved
in gate turn off sensitivity since then, but do not have any figures.