Patent GB-635848-A (GB)
Title: Electrical circuit for the detection and measurement of small changes of a.c. voltages and for the measurement of a.c. voltages
IPC: G01R19/165
Filed: 1947-08-14
Abstract: 635,848. Voltage and impedance measurements; determining dimensions of materials; locating conductors. URMENYI, L. Aug. 14, - 1947, No. 22634. [Class 37] A circuit for detecting and measuring small changes in A.C. voltages consists of a source of D.C. voltage, a rectifier, and a conductor having an appreciable impedance, the D.C. voltage and the rectifier being so connected that the D.C. voltage acts as a bias to the rectifier and the conductor is in series with the rectifier. The conductor may form part of an indicating device. As shown in Fig. 1, a diode or other two-terminal rectifier 2 is connected in series with a source 1 of A.C. voltage, small changes of which are to be measured or detected. In series with the diode 2 is a battery 3, in parallel with a potentiometer 4, and a conductor 5 such as an indicator meter. Instead of the battery 3 and potentiometer 4 any variable source of D.C. voltage may be used or alternatively, if the A.C. voltage is variable, any fixed source of D.C. voltage. The variable voltage is adjusted so that the D.C. voltage is a high fraction of the A.C. peak voltage, e.g. 99 per cent, so that a small increment of A.C. voltage may be easily detected. If the D.C. bias voltage is measured the instrument 5 may be used for the measurement of the A.C. voltage. The diode 2 may be replaced by a bridge rectifier unit or by a triode arranged as an anode bend rectifier. Fig. 4 shows the application to a circuit used in metal detectors, crack detectors, gauges, &c. An oscillator 1 feeds the primary coil 2 of a transformer having two secondary coils 3, 4 connected in opposition and tuned by a condenser 5 to the oscillator frequency. This tuned circuit is connected in series with a coil or resistor 7 and a D.C. bias voltage is obtained from a tapping on a potentiometer 9 connected across the coil 2, the A.C. voltage tapped therefrom being rectified by a rectifier 10, and passed through a filter circuit 11, 12, 13 and a resistor 14. The coils 2, 3, 4 are so adjusted that the voltages in the coils 3, 4 are slightly different in amplitude and approximately opposite in phase and the biassing D.C. voltage which appears across the condenser 12 is adjusted by varying the tapping on the potentiometer 9 until the D.C. voltage is less than the peak A.C. voltage appearing across the condenser 5. The A.C. component of the voltage across the coil 7 is amplified by a valve 18 and applied to the grid of a valve 24. The grid of a similar valve 25 receives a comparison voltage from the potentiometer 9 which is adjusted until zero current is obtained in a galvanometer 34. On bringing metal near the coil 4 the coupling between the coils 2, 4 is decreased by the proximity of non- ferrous metal and increased by the proximity of ferrous metal, the galvanometer 34 deflecting to the right in one case and to the left in the other. Fig. 5 shows a thickness gauge in which primary coils 2, 3 are fed from an oscillator 1. Secondary coils 4, 5 are in series opposition and tuned by a condenser 6 to the oscillator frequency as described in Specification 476,613. The foil of which the thickness is to be measured is inserted between the coils 3, 5 and the voltage in the coil 4 is adjusted by a variable resistor 7 across the coil 2. The phase relation between the voltages across the coils 4, 5 may be improved by connecting a condenser 8 across the coil 3 or by placing a piece of foil 9 between the coils 2, 4. In series with the coil 4 are connected a rectifier 10, a conductor 11 which may be a coil or a resistor, and a source of D.C. voltage provided by tapping an A.C. voltage from a potentiometer 12, rectifying in a rectifier 13, and filtering by condensers 14, 15 and choke or resistor 17. The A.C. component of the voltage across 11 is then amplified by a valve 19. The resistor 7 or the potentiometer 12 is provided with 'a scale, which may be calibrated in terms of foil thickness. Fig. 6 shows a gauge in which an A.C. source 1 supplies a bridge comprising inductance arms 2, 3 and resistance arms 4, 5, the inductance of one or both of the coils 2, 3 being influenced by the dimension to be measured. Across the diagonal 6, 7 are connected a rectifier 8, a variable D.C. source 9 and a conductor 10, the voltage across which is amplified by a valve 11.