Triac/Optoisolator combo - why won't it turn off?

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I bought a solid state relay (made of triac with zero crossing detected optocoupler), it turned on/off sine wave AC of Line by 5 volt dc. But it didn't turned off UPS voltage(square wave), it remains always on.
 

But it didn't turned off UPS voltage(square wave), it remains always on.
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Yes, this sounds reasonable. A ZC detector SSR doesn't work with square wave. You need at least a sufficient low impedance snubber to reduce dv/dt. But it may not work at all.
 

I tasted it in AC Line of pure sine wave 50Hz, 220V, but when it off, it gave 40V ac on load, why? i tested a triac in same line, but it's ok. (And load current is too small <1A. So di/dt is not problem here)
 

The measurement setup isn't very clear. A residual voltage with high impedance or no load may be caused by the snubber circuit.
 

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