Trigger produces pulse for zero AC mains - using series-coupled quadrature low pass filters between zero detectors and mains

摘要

The trigger produces an output pulse when the ac mains supply waveform cuts the zero axis and is for synchronising clock generators in television equipment. The mains voltage or the output voltage of a mains transformer is passed over two low pass filters (26-29) to a null-detector (31, 34, 35). The low pass filters are coupled in series and shifted in phase by 90 deg. relative to one another. A coupling capacitor (40) connects the detector to the low pass filters. The detector output (48) is coupled to a sawtooth waveform generator (22, 23) whose own output feeds a variable gain amplifier (10) in the pulse-width controlled power supply.