Patent GB-672099-A (GB)
Title: Improvements in or relating to a.c. measuring devices
Inventor: WALL PETER KENNETH
IPC: G01R19/22
Filed: 1950-05-31
Abstract: 672,099. Current and voltage measuring apparatus. STANDARD TELEPHONES & CABLES, Ltd. May 31, 1950, No. 13660/50. Class 37. A rectifying circuit for measuring alternating currents comprises a plurality of rectifiers, a D.C. meter for indicating the rectified current, a bias source connected so as to render all the rectifiers conductive, and means for preventing the bias current from passing through the meter. In the half-wave rectifier meter circuit shown in Fig. 1, a triangular mesh is formed by two principal rectifiers 1, 2 and a D.C. current meter 3 having in series with it a third rectifier 4. A.C. to be measured is applied to terminals 5, 6 and a D.C. bias source for the rectifier circuit is connected to terminals 5, 7. A blocking capacitor 10, resistor 12, and by-pass capacitor 13 are connected as shown. The rectifiers are so connected that 1, 2 are in conducting condition when the corner 11 is positive to the corner 8, while the rectifier 4 is blocked in this condition, so that bias current does not pass through the meter 3. The measuring characteristic is shown by the line 14, Fig. 3, in which ordinates represent currents indicated by the meter 3 and abscissµ represent the value of the A.C. applied at terminals 5, 6 averaged over the positive or negative half cycles of the current wave. No current flows through the meter 3 until it exceeds the bias current 12 corresponding to the point 15, thereafter giving a linear response. By making the resistor 12 adjustable the point 15 may be shifted to cause the meter to begin to read at any desired value of I. In a modification, Fig. 2, additional rectifiers 16, 17 are provided, making a full-wave rectifier bridge 8, 9, 11, 18, the input being applied through terminals 6, 19 and blocking capacitors 10, 20 to the diagonal points 9, 18. The bias is applied from terminals 5, 7 through resistors 12, 21 to the other diagonal points 8, 11. In the arrangement shown in Fig. 4, two differently biased halfwave rectifying circuits are used, the meter 22 measuring the difference between the rectified currents produced thereby. The two rectifying circuits comprise principal rectifiers 23, 24, 25, 26, blocking rectifiers 27, 28, and by-pass capacitors 29, 30. One corner 31 is common to both circuits, the meter 22 being connected between this point and the junction point of the rectifiers 27, 28. The A.C. to be measured is supplied to the input terminals 32, 33 connected to the corners 34, 35 through blocking capacitors 36, 37. The. bias source is connected to terminals 38, 39, the latter being connected through high value resistors 40, 41 to the corner 42. The terminal 38 is connected through a switch 45 and a high value resistor 44 to the corner 43. The two rectifying circuits are thus supplied with bias current in series. In order that the bias current for the rectifiers 23, 24 may be less than that for the rectifiers 25, 26, two high value resistors 46, 47 are connected in series between the corner 31 and terminal 38, thus shunting the rectifiers 23, 24. Inductors 48, 49 prevent the bias current circuits from shunting the A.C. circuits. On account of the resistors 46, 47 the left-hand circuit begins to rectify before the right-hand circuit as shown by the line 14, Fig. 3, a parallel line 51 commencing at a point 52 corresponding to a larger bias current 14 representing the rectifying characteristic of the right-hand circuit. The meter reads zero until the value 12 corresponding to the point 15 is reached. The reading then follows the line 14 to the point 53 having the same abscissa as the point 52, where the right-hand circuit begins to rectify, and then remains constant as indicated by the horizontal line 54, since after the point 53 the difference between the two rectified currents remains constant. The corner 43 may be disconnected from the bias circuit by the switch 45, thereby reducing the bias current for the left-hand rectifying circuit to zero. The meter readings then start from zero and follow the line 55, Fig. 3. A shunt 57 may be connected across the meter 22 by a switch 56, the meter then indicating in accordance with the line 58, Fig. 3. A third switch 61, operating at the same time as the switches 45, 56, may be provided to short-circuit the resistor 40 while the switch 45 short-circuits the resistor 46. The right-hand rectifying circuit then comes into operation at the point 60, Fig. 3, and prevents overload of the meter when indicating according to the line 58. A modification is described in which two fullwave rectifier bridges are employed instead of the half-wave rectifying circuits shown in Fig. 4.