Schmitt Trigger - zero crossing circuit


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I'm not sure whether I've put my topic in the subject... Just want to have an explanation about the topic for my final year project. I'm doing "speed control using a thyristorised dc drive via microcontroller". If anybody can help me, do email me.
Thyristors, and SCR's, need a zero-crossing so that the devices can turn off (stop conducting). The current must drop to a certain value before the device will switch off. This usually happens at the end of the sine wave, when the voltage is lower (around 150 degrees). The turn- off should occur on the same cycle as when the gate firing signal is removed.