One of the control circuit’s triacs selects the tap on main transformer T1, which provides the proper, preregulated voltage to the secondary regulator. T2 and its associated components comprise the second-ary regulator.


The MOC3021is optically isolated triac driver devices. These devices contain a GaAs infrared emitting diode and a light activated silicon bilateral switch, which functions like a triac. They are designed for interfacing between electronic controls and power triacs to control resistive and inductive loads for 240 VAC operations.

The ADO 0804, IC1, digitizes a voltage-feedback signal from the secondary regulator’s output. The MC1415 demultiplexer, IC2, decodes the digitizer’s output. IC2, in turn, drives T1’s optoisolated triacs via the 74LS240 driver chip, IC3, and associated optoisolators.

Transformer T3 samples the circuit’s current output. The auxiliary, 12 V winding on T1 ensures no-load starting. The combination of op amp IC5 and the inverting transistor, Q1, square this current signal. The output of Q1 is the CLK signal, which triggers one-half of the one shot, IC4A, to begin the circuit’s A/D conversion. The one shots’ periods are set to time out within 1/2 cycle of the ac input.

Upon completion of its A/D conversion, IC1’s INTR output triggers the other half of the one shot, IC4B, which enables the converter’s data outputs. The rising edge of the CLK signal resets the one shot and latches the new conversion value into IC2. The latch, associated driver, and optoisolator trigger a selected triac according to the latest value of the voltage-feedback signal, VO.

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