Patent GB-612333-A (GB)
Title: Improvements in measuring circuits for a.c. voltages
IPC: G01R19/22
Filed: 1946-04-05
Abstract: 612,333. Current and voltage measuring apparatus; power meters; impedance measurements. BRITISH THOMSON-HOUSTON CO., Ltd., and KEITLEY, R. April 5, 1946, No. 10568. [Class 37] A circuit arrangement for the indication or measurement of A.C. voltages comprises a pair of barrier layer rectifiers connected in series for the same direction of current flow and arranged in parallel with a D.C. measuring or indicating instrument, the voltage to be measured being applied between the common connection of the rectifiers and the common connection between a pair of condensers connected in series across the instrument, whereby the charges built up on the two condensers by rectification of the opposite half-cycles of the A.C. voltage produce a continuous current through the instrument. As shown in Fig. 1, a pair of barrier layer rectifiers R1, R2 are arranged in parallel with a D.C. instrument M. Connected in series with one another and in parallel with the instrument M are condensers C1, C2. The voltage V to be measured is applied between the points a, b through a resistance RS, the value of which is determined by the current to be passed through the instrument. Alternate half-cycles of the applied voltage are passed in the direction of the full line arrow through the rectifier R1 and build up a positive charge on the condenser C1, while the remaining half-cycles flow in the direction of the dotted arrows to the rectifier R2 and develops a positive charge on the condenser C2. These charges cause a current to flow through the instrument M, the average charge on the condensers being maintained at a value proportional to the voltage V. By adding a further pair of rectifiers R3, R4, Fig. 2, the sum of two voltages VI, V2 may be measured. If the polarity of the rectifiers R3, R4 is reversed, the difference of the voltages V1, V2 is measured. Further voltages may be added by connecting across the instrument M another pair of rectifiers for each of the voltages. Fig. 5 shows an arrangement for measuring the power consumed in an impedance Z connected to a circuit across which the voltage V is developed. Equal resistances 10, 11 are connected in series with the impedance Z and resistances 12, 13 are connected between the junction of the resistances 10, 11 and the other end of the impedance Z. Across each of the resistances 10, 11 is developed a voltage proportional to the current through the impedance Z and across the resistor 13 is developed a voltage proportional to the voltage across the impedance. The voltage across the points e, f is the sum of these voltages, while the voltage across the points f, g is their difference. The rectifier and circuit constants are so adjusted that the component rectified currents are proportional to the square of the voltages producing them, so that the instrument indicates the difference of the squares of the sum and difference of the voltage, across the resistors 10, 13, i.e. to the power absorbed by the impedance Z. Fig. 6 shows an arrangement for indicating zero voltage across the diagonal of an impedance bridge formed by impedances Z1, Z2, Z3, Z4. In the connection across the diagonal x, y is a switch S which, when closed, short-circuits the diagonal and enables any out-of-balance current in the rectifying circuit to be indicated. On opening the switch S balance of the bridge is indicated when the deflection of the instrument M is the same as with the switch closed. The rectifying circuit indicates the difference of the voltages between x, o and y, o and thus indicates when x, y are at the same potential. For adjustment the rectifier R2 may be connected to the lower end of.a potentiometer P, Fig. 7, the upper end of which is connected to the earthed point o through a condenser C3. The resistance RS may be reduced by inserting resistanees r, Fig. 8, in series with the instrument M. Condensers C4 may be inserted at the terminals of the instrument M to ensure that it is effectively operated at earth potential. Fig. 9 shows a modification wherein, by the use of two pairs of condensers, full wave rectification can be obtained with the summation or subtraction of two voltages. Additional condensers C5, C6 are employed so that the half of the circuit below the earth connection constitutes a mirror image of the half above the earth connection. The arrangement shown is such that the current flowing through the instrument M is proportional to the sum of the voltages V1, V2. By altering the polarity of the rectifiers R3, R4 the difference between these voltages may be indicated. The barrier layer rectifiers may employ silicon, germanium or selenium as the barrier layer.