Zero Crossing Detector Circuits

Digitally Controlled Dimmable Lighting
To implement both the described control methods, the zero crossing point of the mains input must be detected. The circuit diagram for the controller is shown above. However the zero crossing circuit actually used is shown below. It is possible to buy IC’s with zero crossing detectors, but the components used were those available at the time this unit was built.

The transistor base is supplied with the full wave rectified sine wave. When VBE < vc =" 5V." vc =" 0V.">

Zero Crossing Detectors and Comparators
This circuit has been around (almost) forever, and it does work reasonably well. Although it has almost zero phase inaccuracy, that is largely because the pulse is so broad that any inaccuracy is completely swamped. The comparator function is handled by transistor Q1 - very basic, but adequate for the job. The circuit is also sensitive to level, and for acceptable performance the AC waveform needs to be of reasonably high amplitude. 12-15V AC is typical. If the voltage is too low, the pulse width will increase.

A Unique Discrete Zero-Crossing Detector
A zero-crossing detector delivers an output pulse that synchronizes other circuitry to the transitions through zero volts of a sinusodial source for both polarity excursions. This detector, which was developed to operate from the ac power line, includes a unique negative-voltage detector/level shifter.

VRS Zero Crossing discrete detector interface
This circuit was born out of much tinkering. I have gone through several revisions to get what you see here. The circuit has the minimum number of components that I could think of. I have experimented with different components and values for every part shown in the schematic, these are the best component values that I could put together. For the true electronic purists, you will note that this circuit isn't a true zero crossing detector. It approximates a zero crossing detector.

Zero Crossing Detector Simultion