simple zero crossing detector

4111110.004N g simple zero crossing detector Zero crossing detectors are often con- tained in rather complex circuits, or they are part of an integrated circuit, the rest of which is not required. Basically, such a detector is required to give a pulse every time the mains voltage passes through the zero potential. The detector proposed here is very simple indeed: the mains voltage is transformed down, rectified in D1, and smoothed by C1to give a direct voltage of 17 V. Part of the mains voltage is taken from across R2and used to drive transistors T1. ..T3. Dur- ing positive half cycles Ti conducts and T2and T3are off, whereas negative half cycles switch on T2and T3, while T1is off. When the momen- tary voltage across R2lies between +0.6 V and —0.6 V, none of the tran- sistors conducts, so that the output voltage is high. In this way, a short positive pulse is produced every time the mains voltage passes through zero potential. Since operation is direct from the mains, there is n...