GB527103A - Circuit arrangements applicable to signalling systems utilising currents obtained from elements such as photo-electric cells - Google Patents

Abstract

527,103. Valve amplifiers; copying telegraphy. STANDARD TELEPHONES & CABLES, Ltd. March 31, 1939, No. 10116. Convention date, May 17, 1938. [Classes 40 (iii) and 40 (v)] Direct-current impulses, such as those produced from a photoelectric cell in telegraphy, are converted into alternating currents with a minimum of transients, and means are provided (a) for limiting the amplitude of the output, and (b) for preventing the passage of waves below a predetermined value, for instance when transmitting images without half-tones. Light from a lamp 1 passes through an aperture or " chopper " 3 on to a photo-electric cell 2 connected to a valve 5 which is resistance-capacity coupled to an amplifier 9. The impedance of the condenser 7 is made much greater than that of the resistance 6, and the time-constant of the condenser 7 and resistance 8 is made small relatively to the signal frequency. The charging of the condenser 7 then produces an alternating potential across resistance 8, and the valve 5 acts as a converter (not an amplifier) and transients are reduced. Fig. 2 shows a limiter circuit for the output. So long as the peak-potential induced by the amplifier 91 across the secondary winding 12 is less than the difference of potential across the terminals of resistances 13, 14, the diodes 16, 17 serve to prevent current from passing through the winding. A higher voltage peak, however, opens the diodes and, in effect, short-circuits the winding 12, thus preventing the production of waves of excessive amplitude in the primary winding 11, which feeds the working relay. The maximum voltage to be passed is regulated by adjusting the resistances 15. A similar limiting arrangement is described for a Class-B amplifier (Fig. 3 not shown). The circuit shown in Fig. 5 is arranged to prevent the passage of current of less than a predetermined value. Two diodes 46, 47 are normally biased to open-circuit the transformer coupling 44, 45 between the amplifiers 42, 43 for weak currents, thereby cutting-out impulses produced by a toofeeble illumination of the photo-electric cell.