| 摘要 |
Control signals are transmitted through a.c. mains supply lines to control operation of electrical equipment powered from the supply lines. A near short-circuit condition is introduced across the a.c. mains supply lines, for example, by firing a thyristor connected in series with a fuse across the supply lines, for the final portion of positive half cycles of the mains supply voltage, thus holding the voltage to a near zero value for this length of time. A receiver coupled to the mains supply lines includes a zero crossing detector circuit which responds to the near zero voltage conditions across the supply lines to perform a particular control operation on an associated appliance connected to the receiver. Different control operations may be effected by defining the control signals over a group of half cycles of the a.c. supply mains and selecting one or more half cycles of the group during which the supply voltage is reduced to the near zero level, thereby producing a sequence of selected and non-selected half cycles within the group so that by changing the sequence the particular control operation can be changed accordingly.
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