Patent GB-826758-A (GB)
Title: Improved arrangement in automatic telephone and telegraph systems
IPC: H04M3/40
Filed: 1956-09-14
Abstract: 826,758. Automatic exchange systems; automatic volume control. VIGREN, S. D. Sept. 14, 1956 [Sept. 17, 1955; Nov. 9, 1955; Nov. 21, 1955], No. 28167/56. Class 40 (4). In an automatic exchange each link circuit and each register is provided with an individual direct current source which is connected in series with a direct current source common to all the link circuits and registers, and current control devices individually associated with the individually associated with the individual direct current sources maintain the current delivered from the respective individual direct current sources in series with the common source to a line connected to the respective link circuit or register at a substantially constant value independent of the resistance of the line. If desired, the control devices may cause the line current to increase slightly as the line resistance increases in order to compensate for the increased attenuation of high resistance lines. The individual control device may consist of relays, a saturable core reactor or a transistor, and may, if desired, also control the gain of an amplifier. Arrangements using relays as control device.- In the embodiment shown in Fig. 3, the relays R5, R6, and R7 control the current delivered from common battery B1 in series with individual battery B2 by connecting different points of the potential divider r3, r4, r5, connected across the latter, to the line conductor, and also control the gain of the negative impedance amplifier F1. When the handset at Ta is lifted R1 pulls up and completes a circuit for R3, during the operate time of which the line current flows in windings I of R5, R6, R7, and a local circuit is also completed for windings III of these relays. The relays R5, R6, R7 are remanence relays which are normally operated (in which condition they are shown in Fig. 3) and which release when the flux in winding III exceeds that in winding I. If the line resistance is very high, none of these relays releases, and as the resistance decreases R7, R6, and R5 release in turn. So when R3 operates the lower line wire is connected to the tap on the potential divider appropriate to the line resistance. The release of R7 connects r2 in parallel with F1, that of R6 connects r1 in parallel with F1, and that of R5 short-circuits the input to F1, the gain of F1 thus being progressively reduced to zero. Another embodiment using relays is described with reference to Fig. 4 (not shown). Arrangements employing saturable core reactors.-In the embodiment shown in Fig. 2 a saturable core reactor TR is operated with substantially constant direct current premagnetization and its rectified output is connected in series with the common battery B. The current delivered by the combination is substantially independent of the line resistance. In the Fig. 5 arrangement the saturable core reactor TR1 which has A.C. windings I, II and III, and D.C. windings IV and V, is fed from a common transformer T3. A negative impedance amplifier F2 is provided, and the common battery is indicated at B1. The rectified output from windings I and II of TR 1 is applied in series opposition with that of B1. When the handset at Ta is lifted the link shown is connected to the line in known manner over contacts K1 and the line current flows through the windings of relay R1 and winding IV of TR1. Assuming winding IV to have a relatively small number of ampere turns and that the current through the bias winding V is constant, the ampere turns of the A.C. windings, and hence the line current, is substantially constant, and independent of the line resistance. If desired, the ampere turns of winding IV may be so chosen that the line current increases slightly with increase in line resistance. In this case the voltage delivered by winding III will decrease with increase of line resistance, so that the rectified control voltage applied to the transistors of amplifier F2 will also decrease, thus increasing the gain of F2. Arrangements using transistors as control devices.-As the collector current of a junction transistor tends to equal the emitter current a transistor may be used in much the same way as a saturable core reactor by using a circuit giving substantially constant emitter current which supplies its collector current to the line.