Patent GB-620595-A (GB)
Title: Improvements in and relating to sensitive direct current measuring apparatus
IPC: G01R19/20
Filed: 1946-07-10
Abstract: 620,595. Current and voltage measurements. BRITISH THOMSON-HOUSTON CO., Ltd. July 10, 1946, No. 20628. Convention date, July 11, 1945. [Class 37] [Also in Group XXXVIII] Apparatus for producing an alternating voltage or current proportional to the magnitude of a unidirectional current and having a phase relation which is indicative of its polarity, comprises a monode, i.e. a resistance having a substantial temperature coefficient, means for applying to the monode the unidirectional current, and means for intermittently heating the monode and allowing it to cool in regularly repeated succession. As shown in Fig. 1, the monode consists of a small temperature-sensitive resistance filament 1 enclosed in a bulb 2 filled with hydrogen at atmospheric pressure. The direct current or voltage to be measured is applied at terminals 3 and passes through the filament 1 by way of choke coils 4, 5 and a negative feedback control resistance 6. The filament 1 is also fed from a high-frequency oscillator 7 modulated at a low-frequency, e.g. 60 cycles, from a source 8. The filament temperature does not follow the individual cycles of the high-frequency current, but follows their envelope so that it is heated and cooled at the 60-cycle rate and its resistance is varied accordingly, but no 60-cycle voltage appears across the terminals of the filament 1 unless a D.C. is also flowing therethrough. When a D.C. is flowing, the D.C. voltage drop across the monode pulsates with change of resistance of the monode at the 60-cycle rate by an amount and in a direction dependent on the magnitude and polarity of the current or voltage to be measured. This is picked up and amplified by a circuit 9 tuned to its frequency and an amplifier sensitive to the phase of the 60-cycle current resulting in a direct current of measurable amplitude proportional to the D.C. input and of the same polarity. This operates an indicating and control instrument 10 or a recording instrument 11. The oscillator passes current only during the positive half-cycles of the 60-cycle wave, as shown in Fig. 2. The oscillator output is fed to the filament 1 through an air core transformer 13, condenser 14 and adjustable resistance 15, the circuit being tuned to the high-frequency used. The condenser 14 prevents D.C. from the source 3 passing through the secondary winding of the transformer 13, and the resistance 15 permits adjustment of the high-frequency heating current. The filament 1 may have either a positive or negative temperature coefficient. In order to prevent thermo-couple effect the filament and lead-in wires should be made of the same material, the lead-in wires being made from the same adjacent piece of wire to avoid slight differences in purity &c. The curve 17, Fig. 2, represents the variation in temperature and resistance of the monode filament. The high-frequency choke 5 and a byepass condenser prevent passage of high-frequency current to the transformer 20. When a small D.C. voltage is applied to the terminals 3, a direct current flows through the coils 4, 5, the filament 1 and the negative feedback resistance 6, but is prevented from passing into the transformers 13, 20 by the condensers 14, 21. A D.C. voltage is thereby superimposed on the highfrequency voltage across the monode filament 1. Due to the 60-cycle change in resistance of the monode the D.C. voltage drop across the monode has a 60-cycle variation as shown at 22, Fig. 2. Where it is not necessary to indicate the polarity of the D.C. voltage or current, a high sensitivity A.C. voltmeter may be connected in place of the primary winding of the transformer 20. Where it is desired to indicate the polarity the output of the transformer 20 is fed to a two-stage amplifier 24 supplied from a D.C. source 25. The output of the amplifier 24 is applied to the third grid of a pentagrid converter valve 26, the first grid of which is connected to a secondary winding 27 of the transformer 12, which supplies a phase reference voltage. The output voltage of the valve 26 is proportional to the magnitude of the control voltage on its third grid and has the phase relation of this voltage with respect to the reference voltage applied to its first grid. A valve 28 includes a rectifier on its left side and an amplifier on its right side. The rectifier is supplied with A.C. voltage from a secondary winding 29. The rectifier voltage is compared to the plate voltage of the converter valve 26 through resistances 30, 31 and a voltage proportional to the average of their voltages is supplied to the control grid of the amplifier through a connection 32, the instruments 10, 11 being connected in the cathode circuit. A small negative current supplied by the rectifier is introduced in the circuit 6, 10, 11 so that the instrument current is proportional to the D.C. voltage across the terminals 3. Current flows from the terminals 3 through the resistance 6 in such a direction as to oppose predominating current through the instrument 10 and serves as a negative feedback to stabilize the system. In a modification, the monode may be replaced by four monodes in bridge connection, two having positive temperature coefficients forming two opposite arms and two with negative coefficients forming the other two opposite arms. The high-frequency and D.C. connections are made across one diagonal, the tuned circuit 9 being connected across the other diagonal. In another arrangement, the monode is replaced by two monodes in series, one having a positive and one a negative temperature coefficient. A 60-cycle tuned circuit is connected across each of these monodes, these circuits feeding two primary coils of the transformer 20, to which they are reversely connected to correspond to the inverse variation of voltage across the monodes, so as to produce additive transformer action. In another modification, two monodes having similar temperature coefficients are connected in series and are heated alternately by separate oscillators during reverse half-cycles of the modulating frequency. The tuned output circuits feed reversely connected primary coils of the output transformer.