The simple digital circuit in Fig 1 allows a µP to control ac power precisely without using any digital-to-analog circuitry. In operation, binary-comparator IC₂ compares a data word held in latch IC₁to the output of a ripple counter, IC₃. The system's controlling computer loads the desired data word into IC₁.

A 1-MHz clock drives IC₃, providing very fine timing resolution for 50- or 60-Hz power. At the ac power's zero crossing, optoisolator IC₄ clears ripple counter IC₃, synchronizing the ripple counter with the ac line. At the zero crossing, the binary comparator's output also goes inactive. The binary comparator's output controls an isolated triac driver, IC₅.

After the next zero crossing, the counter proceeds to count up. The binary comparator holds its output inactive until the count exceeds the latched data. The triac then switches on for the duration of the ac half-cycle until the next zero crossing occurs.

You can add more ac power-control channels by replicating the latch, comparator, and triac driver. The clock's ripple counter can drive several channels at once. Inserting an EPROM or a PAL device between the ripple counter and the binary comparator allows you to generate exponential, logarithmic, or arbitrary waveforms. (DI #1562)