mains zero-crossing detector

FE 124 July/August 1986 10a mains zero-crossing ‘01 detector Both safe and remarkably simple to construct, this circuit detects the zero crossing moments of the mains voltage, in order to provide other cir- cuitry with timing information about the correct instant for switching mains-connected loads; in other words, when the least possible switching dissipation is involved, and, therefore, least interference is induced on the mains lines. The proposed circuit operates direct off the mains, while comprising no more than two opto-couplers and two resistors. It is seen that photodiodes Di and D2 are connected in anti- parallel while being fed with the mains voltage via a resistor, which limits the current through the rel- evant diode to about 2 mA as it con- ducts (i.e. lights) during the negative or the positive half wave (D2 or Di respectively) of the mains sinewave; in either case, the circuit output voltage is low, since the associated phototransistor conducts and draws current ...