Patent GB-664658-A (GB)
Title: Improvements in or relating to electric circuits
IPC: H04Q1/18
Filed: 1949-08-08
Abstract: 664,658. Current and voltage measuring. NAAMLOOZE VENNOOTSCHAP PHILIPS' GLOEILAMPENFABRIEKEN. Aug. 8, 1949 [Aug. 11, 1948], No. 20588/49. Class 37. [Also in Group XL (c)] . Equality of two voltages, e.g. derived from wipers MS, GK, Fig. 1, in automatic signalling systems, is indicated by applying each to the grid of a pair of valves B<SP>1</SP>, B2 having a common cathode resistance R5 of high value, the valves controlling a relay T that is energized or de-energized when the two voltages 1, 2 are substantially equal. As shown in Fig. 1, the relay T has two windings and is released when the voltages are equal. The release of relay T stops the stepping of wiper GK. The circuit of Fig. 1 suffers from the disadvantage that if, initially, voltage 1 exceeds voltage 2 the relay T is energized in one direction, so that should voltage 2 later exceed voltage 1, the relay T is magnetized in the opposite direction and will be de-energized at the transition; to avoid this, voltages 1, 2 are applied over a Graetz rectifier circuit S1, S2, S3, S4, Fig. 2, so that valve B1 is always fed with the higher of the two voltages and the relay is always energized in the same direction. Alternatively, the relay T, Fig. 3, may be fed over suitably-poled rectifiers S1, S2, S3, S4. The resistance R5 may be replaced by a pentode B5, Fig. 5. so as to maintain a total constant current through valves B1, B2; this may be further ensured by providing negative feed-back for pentode B5; in the arrangement of Fig. 5, the degree of equality of voltages 1, 2 necessary to de-energize the relay T may be controlled by varying resistances R9, R10. Screen-grid valves may replace valves B1, B2, the screen grids being fed over constant potential devices (iron resistances or neon lamps) to maintain constancy of screen grid potential, Figs. 6, 7 (not shown); this makes the anode currents at equilibrium independent of the magnitude of the potentials 1, 2. In Fig. 8, further rectifiers S11, S12 shunt the relay circuit T so that for large differences the relay is not heavily magnetized. In Fig. 9, inequality between the anode currents of valves B1, B2 opens the rectifier circuits to allow A.C. from T1 to pass to T2 for amplification, rectification and control of a relay. The current applied to the relay in Figs. 1 ... 5 may be applied to a winding on an iron core to vary its permeability and thus alter the inductance of a second winding to vary the tuning of a tuned circuit and thus alter the frequency of an oscillator; the variable frequency is fed through a frequency distinguishing circuit to control a relay, Fig. 10 (not shown). In a further modification, Fig. 11, the anode of valve B1 is applied to a trigger circuit comprising valves B3, B4 arranged so that when valve B3 conducts at a specific grid potential, valve B4 is made non-conducting to de-energize relay T. Valve B3 may be fed from a rectifier network connected to both anodes of valves B1, B2 so that valve B3 is made conducting when either of the anodes of valves B1, B2 reach a specific potential, Fig. 12 (not shown). The control grid of valve B3 may be connected to a potential divider connected between the anode and cathode of valve B1, Fig. 13 (not shown); it may also be connected to a potential divider connected between the cathodes of valves B1, B2 and the common point of two reversely-connected rectifiers, each connected to an anode of valves B1, B2, Fig. 14 (not shown).