FR1504926A - Improvements to data transmission devices - Google Patents

Abstract

1,122,068. Protective arrangements. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. 1 Dec., 1966, No. 52104/65. Heading H2K. [Also in Divisions G1 and H4] Light pulses modulated in accordance with the current variations on a high voltage line are transmitted via a light guide to a location at low (earth potential) where the pulses are detected for measurement or control purposes. The devices at high potential include a current 'ransformer coupled to the line, a generator of square pulses of constant width and repetition rate, and a light-emitting device, e.g. a gallium arsenide diode. Pulses from the generator are width modulated in accordance with variations in the line current and amplified before supplying the light source. At earth potential the light transmitted by the fibre optic guide is detected by a photo-diode and the amplified and demodulated signal used for measurement of the actual value, the r.m.s., or the instantaneous value of the line current or to the control of protective apparatus, e.g. a circuit-breaker. The circuit (Fig. 3, not shown), for modulating the square pulses from the generator is transistorized and supplied from the line. Square pulses of a frequency much higher than the A.C. line variations (in the case of an A.C. line) are applied at e i , charge up a capacitor (C 1 ) and cause transistors (T 1 , T 2 ) to conduct, producing an output voltage e 0 . The duration of conduction of the first transistor (T 1 ) after the end of a pulse e;, depends on the voltage at a point (V e ) which follows variations in the line current. The receiver circuit (Fig. 4, not shown), includes an amplifying transistor (T 3 ) and integrating and D.C. filter arrangements. Various constructions for the insulating structure, adapted to include a light guide, are described (Figs. 5-7, not shown).