Patent GB-1008482-A (GB)
Title: Improvements in potentiometric measuring circuits
Inventor: WIGAN EDMUND RAMSAY
IPC: G01R1/28
Filed: 1962-11-09
Abstract: 1,008,482. Vectoral voltage measurement; potentiometers:Photoangle measurement. GRIFFEN & GEORGE Ltd. Jan.21, 1964 [Nov. 9, 1962], No. 42533/62. Heading G1U. An A.C.potentiometer comprises two impedance networks across a pair of conductors; each network comprising three impedances each of which is electrically matched to its counter-part in the other network while two impedances have significantly different characteristic phase angles and are either (a) series connected in their respective networks with the third impedance shunted across one impedance in one network and across the other in the other network; the networks being parallel across the conductors or (b) parallel connected in their respective networks with in one network one of the two impedances shunted by the other in series with the third impedance and in the other network with the counter-part of the other impedance of the two shunted by the counter-part of the one impedance in series with the third impedance; the networks being in series across the conductors and in either case the third impedance of each network being transformer coupled thereto. In Fig.1, an A.C. energized transformer T 1  having an earthed centre tapped secondary energizes RC networks A and B having equal capacitance and equal resistance in each network. Network A comprises equal capacitances C A1  and C A2  in series with resistance R A  coupled by transformer T A  to resistive potentiometer 11, while network B comprises equal resistences R B1  R B2  in series with capacitance K B  coupled by transformer T B  to resistive potentiometer P B . R B1  = R B2  = 1/2 R A  and C A1  = C A2  = 2 CE  So that the capacitative and resistive components are equal. Centre taps of the secondaries of equal transformers T A1 ,T B  are connected over switch S 2  to a balance detector (not shown), and an unknown voltage to be measured is applied between the sliders of P A , P B . A multicontact switch S1 operating on banks of capacitors selects capacitance values for C A1 , C A2  , C B  suitable for the working frequency. A ganged changeover switch S 2  connects the networks into the operating conditions as shown, and alternatively as a Wien bridge for adjustment of the capacitors for balance at the working frequency. In operation, the output of potentiometer P A  represents the real component and that of potentiometer P B  the imaginary component of voltage, so that an unknown voltage of the same frequency is applied to 12, 15 and the potentiometers adjusted for null reading of the balance indicator; the potentiometer sliders being calibrated in magnitude and phase angle. In an asymmetrical modification (Fig. 2) networks A and B across the output supply on to transformer T 1  comprises respective capacitors C A , C B  in series with respective resistors R A , R B , the appropriate capacitor values for working frequency being selected by switch S 1 . Potentiometer P A  is coupled by transformer T A  across R A  and potentiometer P B  is coupled by an equal transformer T B  across C B . R A  is variable for matching, and a balance indicator is transformer coupled between the secondary centre taps of T A , T B . Switch S 2  enables the networks to be interconnected, as before, into a Wien bridge for adjustment of the capacitors for balance at the working frequency, and an unknown voltage at similar frequency is applied over terminals 12, 15 to the sliders of P A , P B  which are calibrated to indicate magnitude and phase angle. For direct interpretation of the slider calibration in cartesian or polar co-ordinates a phase shift circuit F is energized across T 1  and comprises a bank of capacitors 20 selectable with C A , C B  by switch S 1 , in series with a variable resistance 22 and with a potentiometer 21 connected to a tap on T 1  The potentiometer energizes the test circuit from which the unknown voltage is derived, over transformer T 3 . Alternatively networks A, B are energizable direct from the phase shift circuit over a switch (not shown) and the phase shift is directly dependent on the setting of potentiometer 21 provided that the value of capacitance 20 is varied inversely to frequency with C A  and C B . Initially on balancing a first unknown voltage, phase shift circuit F is adjusted to give readings of e.g. 100 in the magnitude potentiometer P A  and +j 0 on the quadrature potentiometer P B , and it is thereafter left unaltered so that the relative values of magnitude and phase angle of subsequent voltages may be read directly. In a further modification (Fig. 4) network A comprises a capacitance C A  shunted by a resistance R A  in series with the primary of transformer T A  whose secondary is shunted by potentiometer P A , while network B comprises resistor R B  equal to R A  shunted by capacitance C B  equal to C A  in series with the primary of transformer T B  whose secondary is shunted by potentiometer P B . The transformers are equal and the networks are series connected across A. C. energized transformer T 1  while the capacitors are switchably variable by S 1  in accordance with the working frequency. Centre taps of the secondaries of transformers T A , T B  are transformer coupled to a balance indicator G, while the sliders of P A , P B  are connected to test probes 12,15 exploring unknown voltages of a test circuit K energized from the A.C. supply through a transformer 24 and a phase shift circuit F, wherein the transformer secondary is earthed through a resistor 22, and a capacitance 20 switchably variable with capacitances C A , C B , while the centre tap is earthed through a potentiometer 21 whose slider is connected to K. For polar co-ordinate measurements the secondaries of T A , T B  may energies opposed diagonals of an equal armed resistance bridge (Fig. 5 not shown); opposed arms having sliders ganged to move symmetrically in unison to derive an output potential for comparison with the output voltage, in which where p is defined by the position of the sliders on their corresponding resistances, and the sliders may be calibrated to 90 degrees. The network for # from 0 to 90 degrees may be switchably rotated through 90 degrees for a calibration from # = 90 degrees to # = 180 degrees, and a high impedence correction potentiometer is connectable across the sliders with its own slider ganged thereto so that the comparison potential derived therefrom is compensated for the non-linear law of the modulus by reducing it at each end of the slider travel to 0.707 of its value at the midpoint (Fig. 6 not shown). Alternatively for potential comparison the balance detector coupling transformer is connected between one slider and earth while the unknown is connected between the remaining slider and earth (Fig.7 not shown). In a further modification (Fig. 8 not shown) an unknown potential of which neither side is earthed is measurable in polar co-ordinate terms; the comparison voltage being derived from profiled potentiometer networks ganged in pairs to vary the modulus and the argument of the comparison potential.