Patent GB-686075-A (GB)
Title: Improvements in circuits for use in automatic telephone-exchange and like systems
IPC: H04Q1/18
Filed: 1950-06-09
Abstract: 686,075. Automatic exchange systems. PHILIPS ELECTRICAL. INDUSTRIES, Ltd. June 9, 1950 [June 14, 1949], No. 14444/50. Class 40 (iv). In a system in which selectors K with multipled terminal banks have identification resistances Rx distinguishing the outlets, and a switch is positioned by comparing resistances Rx with a comparison resistance Rm previously selected by a marker switch MS, an electronic test device responds only to a potential VI common to free outlets and in its conductive condition alters the potential of the tested outlet to prevent double connections. Two groups of embodiments, Figs. 1 to 4 and Figs. 5 to 8, relate to the use of gas tubes and a trigger pair of vacuum tubes respectively; Fig. 8 also features an arrangement whereby breaking in upon an existing connection may be effected. As shown in Fig. 1, a switch K is started at drive magnet D by operation of differential relay T over winding T2. A gas tube B fires at each free outlet due to the potential difference between VI and V2, but remains cut off at outlets exhibiting earth potential. Engaged outlets are marked at contact r, while outlets under test derive earth potential from the voltage drop in resistance Rx due to the current taken by the test device. By using earth potential at the test outlet the effect upon the test of earth leakage (shown by resistance Re) is neutralized. Relay T compares the resistances Rx and Rm over its differential windings T1, T2 and is disabled when the outlet required is tested. In a modification, resistances Rm, Rx identifying each outlet have complementary values and are connected in series with the relay winding T1 during the test, a comparison current being applied to winding T2. In a second embodiment, Fig. 2, a high-resistance voltage comparison device NW of the type described in Specification 662,215, [Group XXXVI], is used. This arrangement permits simultaneous testing of Rx by two or more test devices. As shown, the comparison device NW comprises the balance arm of a Wheatstone bridge constituted by resistances Rm and Rx, gas tube B1 and fixed resistances R1, R2. The gas tube fires at all free outlets and the comparison device NW responds when the required outlet is found, as indicated by equipotentials at points 1 and 2. Firing of the gas tube varies the potential at point 1 so that the tubes associated with other switches are cut off at this outlet and hence this potential is not disturbed by parallel connections. Since, however, the potential at point 1 is dependent on the value of Rx, the identity of the outlet can be tested by the high-resistance comparison devices associated with other switches. An alternative arrangement, Fig. 3, does not, however, permit simultaneous testing. The resistances Rm and Rx, together with a gas tube B2, constitute two sides of a bridge and at free outlets the potential deviation at point 3 is compared with a fixed potential at point 4. By using a grid, the striking voltage of the tube may be adjusted. In a further arrangement, Fig. 4, which is independent of the running voltage of the gas tube, the circuit is similar to that shown in Fig. 3 except that the comparison device NW compares a fixed potential V3 with the potential at a tapping 5 on a high-resistance potentiometer R5, R6 which shunts the gas tube B2. When the tube becomes conductive, the tested outlet preferably acquires earth potential. This arrangement does not permit simultaneous testing. A resistance Rc connected to potential V1 is also provided in each test device so that the potential at point 1 when the gas tube is cut off approximates to the potential of a tested outlet; this potential prevents disturbance over the multiples of the characteristics of an outlet under test by the current through resistances R5, R6 of a further test device. A trigger-pair of triode vacuum tubes B3, B4 is used in the embodiment shown in Fig. 5, batteries Ba1, Ba2 being included in the crossconnections. The grid of tube B3 assumes a negative potential (- 40 v.) when tube B4 is conductive and hence tube B3 remains cut off when an earth potential marking characterizing busy outlets or outlets under test is encountered. Free outlets assume potential V1 (- 60 v.) so that tube B3 conducts and causes B4 to cut off. The identification resistance Rx characterizing the outlet under test is then compared with resistance Rm by differential relay T. The embodiments described in respect of Figs. 6 to 8 dispense with the use of batteries in the cross-connections. In Fig. 6 each control grid of the trigger pair is associated with a potentiometer R9, R10 or R11, R12 connected between the anode circuit of the other tube and a fixed potential V4. High resistances R5, R6 connected to a comparison device NW are shunted across the tube B3. A further triode B5 controlled from the test circuit by the current through resistance 20 and back-coupled over a rectifier g to the grid of B3 neutralizes the effect of line leakage resistance by maintaining the cathode lead in the conductive condition of tube B3 at earth potential for all values of Rx. A further embodiment, Fig. 7, includes an arrangement for reducing the delay in the valve trigger action due to line capacitance C. The circuit comprises a trigger pair B3, B4 and an auxiliary tube B6, tubes B3 and B6 having screen grids which are coupled to the cathodes over resistances R5, R6. The potential at point 5 is compared with a fixed potential V3 by NW. A resistance Rc is provided and functions in the manner previously described in respect of Fig. 4. In operation, when the switch engages free outlets, the line capacity is negatively charged through Rx. Tubes B3 and B6 become conductive and tube B4 cuts off so that the control grids of B3 and B6 acquire earth potential. This causes a strong current through tubes B3 and B6 (especially the latter) which rapidly discharges the line capacitance. The anode current of tube B6 ceases when its cathode attains earth potential, and since a high resistance R7 is included in the anode circuit of B3, the test current between Rx and Rm passes mainly between the cathodes and screen grids. In a further arrangement, Fig. 8, the identity of a busy outlet may be tested, breaking in facilities being provided under the control of a switch S. With switch S in its normal position and tube B3 cut off, the grid assumes potential V5 ( - 40 v.). Busy outlets are marked by a potential V7 ( - 25 v.) over the multiple. Tube B3 thus remains cut-off at busy outlets but becomes conductive at free outlets (characterized by a potential VI of - 60 v.), causing tube B4 to cut off and rectifiers g2, g4 to pass current so that the grid of B3 assumes earth potential. In the alternative position of S, the grid of B3 has a potential V6 ( - 20 volts) and consequently the tube conducts at busy outlets. Since the cathode of B3 in the conductive condition is at earth potential, rectifier g3 is cut off and prevents the potential V7 from affecting the identity test of the outlet. In this arrangement the resistances Rm, Rx comprise arms of a Wheatstone bridge with the device NW connected to the balance points. Specification 660,376 also is referred to.