Patent GB-683919-A (GB)
Title: Circuit for the comparison of the potentials of two d.c. sources
IPC: H03K5/24
Filed: 1950-06-16
Abstract: 683,919. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. June 16, 1950 [July 13, 1949], No. 15108/50. Class 40 (iv). [Also in Group XXXVI] To indicate when the potentials of two direct current sources are equal, each is applied to one of two circuits which pass current in one direction only, the two circuits being connected by high resistances to a source of potential outside the range of potentials to be compared, so that when these latter are unequal only one of the circuits carries current, while when the potentials are equal, both circuits carry current; the current flow is substantially constant under all conditions. Development of the invention from the prior art. The invention is developed from the circuit of Fig. 1 in which the A.C. transferred from Ti to To is dependent on the difference between the potentials Vt, Vm; when Vt-Vm is negative, the output Vs is zero, and when Vt-Vm is positive the output Vs reaches a maximum. It is known to use two such circuits in parallel, Fig. 3, one being poled inversely to the other to obtain a characteristic of Fig. 4a or Fig. 4b according as the outputs are added or subtracted. It is also known to provide two circuits of the type of Fig. 1 in series, Fig. 5a, one being inversely poled with respect to the other, so as to obtain a narrower peak to the characteristic, Fig. 6. The circuits may be poled in the same direction but inversely connected to Vt and Vm, Fig. 5b. Embodiments of the invention. The circuit of Fig. 7 is.used to avoid biasing batteries; Vo is a negative potential far outside the range of Vt and Vm, the potentials to be compared. If Vt is higher than Vm, current flows from A to point 1, divides through rectifiers a, b and unites at 2 to make S slightly lower than A; the potential of point 3 is thus above that of B and no A.C. passes from Ti to To. When the potentials Vt, Vm are equal, point 1 is at a higher potential than point 2 and point 3 is at a lower potential than point 4, and A.C. is passed to the detector. If Vt is below Vm, rectifiers a, b become non- conducting and no A.C. passes from Ti to To. The transformer Tm may be replaced by an inductance whose centre tap is connected through R to Vo, Fig. 8 (not shown), or by a high resistance whose centre tap is connected through R to Vo, Fig. 9 (not shown). The rectifiers a, b, c, d may be reversely poled and Vo made positive, Fig. 11 (not shown). The slopes of the characteristic, Fig. 6, may be made steeper by employing rectifiers e, f, g, h, Fig. 10, connected in opposition as a shunt across the resistance S ; in this arrangement, if Vt=Vm the rectifiers e, f, g, h are non-conductive and do not shunt any of the A.C. passing to the detector, but if Vt is greater or less than Vm, one or other of the pairs of rectifiers e, f, or g, h, become conductive to shunt the A.C. from passing to the detector. In another embodiment of the invention the potentials Vt, Vm, Fig. 14a, are applied over reversely poled rectifiers a, b to a comparator such as a galvanometer or a circuit such as shown in Fig. 3, and to equal resistances R connected to a negative source Vo. At equality of Vt and Vm no current flows through the comparator, but if Vt is not equal to Vm, current flows through the comparator. In a modification, Vo is made positive, and the rectifiers a, b are reversed, Fig. 14b (not shown). The comparator may be of the symmetrical kind, e.g. a differential moving-coil instrument, and the common lead from S, Fig. 15a, may be taken to the source Vo. Again Vo may be changed in sign if the rectifiers a, b are reversed, Fig. 15b (not shown). Application to automatic telephony. A finder wiper FB moves over a bank of contacts at potentials V1, V2 ... Vm until it meets a contact of the same potential as that marked by marker wiper MS. Then, A.C. passes from Ti to To to five cold cathode tubes CC to operate relay Tr to release relay P which de-clutches wiper FB; the operation of relay Tr also releases relay Sr and prepares the circuit for a fresh operation initiated by energizing relay Sr by means not shown. In Fig. 16, when FB reaches a contact of potential equal to Vx, no A.C. passed from Trl to cold cathode tube CCl, and the tube CCl therefore ceases to conduct so that condenser C discharges to cause tube CC2 to conduct and operate relay Tr to de-energize the clutch relay P. Specification 654,159 is referred to.