GB1500891A - Systems for transmitting information in an alternating current electricity supply system - Google Patents

Abstract

1500891 Signalling over power lines GENERAL ELECTRIC CO Ltd 16 July 1975 [18 July 1974] 31964/74 Heading H4R Signalling over power lines is effected by reducing the amplitude of selected cycles of the power waveform, throughout the same small portion of the cycle period including a voltage zero by drawing current from the system only throughout that portion. As described, the amplitude is reduced for a period not greater than one tenth of one cycle of the power supply at a negative-topositive going zero-crossing by firing a thyristor connected across the power supply. At the transmitter Fig. 5, the mains voltage across LN is differentiated and phase-shifted by 90 degrees in R21, C21 then squared I to produce positive pulses corresponding to positive mains half-cycles. The trailing edges of the pulses trigger a timer 2 producing further pulses of such duration that their trailing edges define the angle at which the thyristor is to be fired. The firing pulses are generated at 3 and gated to the firing transformer T1 when the code generator is operative and detectors 7, 8 confirm that the zero-crossing is in the proper direction and the waveform amplitude is below a predetermined value. The information code is such as to modify the first cycle for START, three selected odd cycles out of cycles 3 to 23 for ADDRESS and an odd cycle out of cycles 25 to 31 for DATA. At the receiver Fig. 7, the mains voltage is applied across R9, R10 to begin charging capacitor C4 when TR4 is turned off at each negative-topositive zero crossing. Diode D4 prevents overcharging. Capacitor C5 charges to the peak value held in C4 via D5 and TR5. Resistor R13 acts as a slow bleed to C5 and any charging current from C4 to C5 is monitored via TR5 and TR6 to provide an output G which lacks a pulse at the instant that the power waveform is modified. To control TR4, transistor TRI is off during negative half-cycles across LN and therefore produces a pulse train B whose amplitude equals the bias voltage + V. The trailing edges occur at negative-to-positive zero crossings and cause TR2 to turn off, allowing C2 to charge until TR2 turns on again. This therefore produces control pulses for driver TR3 to control TR4.