GB659880A - Improvements in or relating to electrical measuring bridge circuits - Google Patents

Abstract

659,880. Bridge circuits. DUBILIER CONDENSER CO. (1925), Ltd. (Bailey, J. T.). July 20, 1949, No. 19107. [Class 37] An A.C. bridge circuit includes a two-wayrectifier connected to the A.C. source and comprising a pair of grid controlled rectifier circuits and a common D.C. output circuit, a device for measuring the current in the output circuit, and means for controlling the rectifier circuits connected across the second diagonal of the bridge circuit. The bridge comprises a pair of ratio arms R4, R5, the unknown impedance in the form of a condenser Cx, and a standard impedance comprising one of a number of selectively connectible fixed condensers C1, C2, C3. Standard inductance coils L1, L2, L3 and standard resistances R1, R2, R3 are also provided for inductance and resistance measurements. The standard impedance is selected by means of a multiple switch A<SP>1</SP>. As shown, the impedance ratio between the arms R4, R5 is varied by means of an adjustable contact A2, or alternatively the standard impedance may be variable and the ratio between the arms R4, R5 maintained constant. The bridge is supplied from an A.C. source through lines. L and a plug P by way of series resistances R6, R7. The other diagonal points A1, A2 are connected to the cathode C and control grids G1, G2 of a double-triode amplifier valve T through a coupling condenser C4 and grid leak R8. The anodes P1, P2 of the valve T are connected to the A.C. supply through resistances R9, R10. The anode circuits are completed through a connection between the cathode C and the contact A2, a microammeter I being included in the circuit. The contact A2 is provided with a pointer moving over scales S1, S2, S3 calibrated in units of the respective impedances to be measured. When the bridge is balanced no potential is applied to either grid G1, G2 of the double-triode T, which acts as a double wave rectifier and allows unidirectional current to pass through the return circuit including the indicator I, the amplitude of this current being adjusted to a predetermined maximum value by adjusting the series resistances R9, R10. If the bridge is unbalanced an alternating voltage is impressed on the grids G1, G2 and a decrease in the meter current results. Contacts A2 is then adjusted until. the current through the meter I reaches its predetermined maximum value. In a modification the anodes P1, P2 are connected to the centre-tapped secondary of a transformer whose primary is energized from the A.C. source, the centre tapping providing the return path through a measuring instrument and a series impedance.