| 摘要 |
PURPOSE:To perform phase control securely, by a method wherein clock pulse of an oscillator in synchronization with AC power source is supplied to clock input of a counter, and output pulse of a zero-cross detector is supplied to clear input. CONSTITUTION:Zero cross point of AC power source frequency is detected by a zero-cross detector 3, and its output is supplied as clear input of a first counter 7, oscillation stop input of an oscillator 6 and input of a frequency divider 8. In the first counter 7, output values are forcibly set to zero at every half-wave frequency of the AC power source by output pulse of the zero-cross detector 3, and then clock pulse from the oscillator 6 is counted. The second counter 9 is supplied with output of the frequency divider 8 as clock input. A value coincidence circuit 10 transmits output to a firing circuit 11 when count values of the first and second counters 7, 9 coincide.
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