App note from Fairchild Semiconductors about their zero voltage crossing isolators. Link here (PDF)
The zero-cross family of optically isolated triac drivers is an inexpensive, simple and effective solution for interface applications between low current dc control circuits such as logic gates and microprocessors and ac power loads (120, 240 or 380 volt, single or 3-phase).
This paper describes the operation of a basic driving circuit and the determination of circuit values needed for proper implementation of the triac driver. Inductive loads are discussed along with the special networks required to use triacs in their presence. Brief examples of typical applications are presented.
