Patent GB-824416-A (GB)
Title: Voltage divider calibrating apparatus
IPC: G01R1/28
Filed: 1957-04-24
Abstract: 824,416. Calibrating potentiometers: SIEGEL, H., and ADISE, H. H. April 24, 1957 [July 31, 1956], No. 13067/57. Class 37. Apparatus for calibrating or adjusting voltage dividers or potentiometers by comparing successive portions of the uncalibrated resistance element with a given standard resistance element, including a source of current for both elements, and having an arrangement for establishing a current flow through the resistance elements in a selected ratio, and an indicator, is characterized in that the arrangement is such as to maintain the ratio constant during calibration or adjustment without manual aid and independent of a change in the total resistance of the uncalibrated resistance, and a measuring circuit for connecting the indicator to selectable points on each element for comparing the voltage drop across corresponding portions of the elements, whereby a potentiometer can be calibrated or adjusted in accordance with a preselected law of proportionality independent of the difference between the total resistance of the two resistance elements. A potentiometer 10 under test comprises resistance element 12 (Fig. 1) connected to terminals 16, 18, and a slider 14 connected via switch 38 to an indicator 42, responsive to current or potential difference between its terminals, which is also connected to a voltage divider 46 comprising standard resistors 48 to 52 respectively, series connected over contacts 54, 56, 58, 60, 62. To find the resistance of element 10 the switches 24, 38 are as shown and slider 14 and contact 70 are moved to the end of element 12 adjacent terminal 16 and to contact 62 respectively while the current source 26 is adjusted until indicator 42 reads zero. The switches are then reversed and keeping the source 26 in its adjusted position a variable standard resistance 32 is adjusted until the indicator 42 again reads zero, the resistance of resistor 32 then equalling that of divider 46 and so that of element 12. To calibrate tapping points on element 12 the switches 24 and 38 are as shown and contact 70 moved into engagement with one of the contacts 54, 56, 58 or 60, the slider 14 is then adjusted until indicator 42 reads zero and this position establishes the tap corresponding proportionally to the selected tap on the standard voltage divider and may be indicated on scale 72. By suitable choice of resistors 48 to 52 any desired spacing of tapping points on potentiometer 10 may be obtained. The voltage divider 46 may be replaced by a calibrated variable potentiometer, Fig. 2 (not shown). The potentiometer 90, Fig. 3, comprises separate variable resistors 92, 94, 96, 98, 100 series connected through taps 102, 104, 106, 108, 110 selected by slider 112. If the total resistance is to be X ohms then with variable resistor 32 adjusted to X ohms and with switches 24 and 38 reversed from the position shown, the contact 70 on the standard divider 46 is moved to contact 62, the source 26 is then adjusted until indicator 42 reads zero and the voltage drop across divider 46 and across variable resistor 32. are equal, so establishing the current ratio for the two dividers. With the switches 38 and 24 as shown, slider 112 engaging contact 102 and contact 70 engaging contact 54, resistor 92 is adjusted until indicator 42 reads zero. Slider 112 and 70 are then moved to the next contacts 104 and 56 for adjustment of resistor 94 and so on until on adjustment of the last portion 100 so that potential at contact 110 equals that of contact 62 the total resistance of divider 90 equals the preselected resistance of X ohms of the resistor 32. In an arrangement for use when the adjustable source of current is not available the standard divider 46 and the divider 90 under test are arranged in series as shown in Fig. 4, both energized by current source 126, with an adjustable resistance 128 in parallel with the divider 46. The zero end of divider 46 is at the junction terminal 120 and not at earth as in previous arrangements and to compare the two voltage drops of opposite plurality the following apparatus is provided: a pair of equal resistors 134, 136 are connected between terminal 120 and output terminals 138, 140 which are connected to fixed contacts 142, 146 associated with relay armatures 144, 148 respectively, which are oscillated in unison by A.C. energized coil 150 between opposite contacts 142, 152 and 146, 156. With selected calibration base X ohms determined by resistor 128 in parallel with divider 46 across terminals 130, 132 the currents through dividers 90 and 46 will bear a preselected ratio with respect to one another, namely i s  and i t  being the current through the dividers 46 and 90 respectively and r s , r e  to resistances of divider 46 and resistor 128 respectively. With contacts 70 and 112 positioned to engage contacts 54 and 102, the resistance portion 92 is then adjusted until the voltage drop between contacts 102 and terminal 120 equals that between contact 54 and terminal 120 and so on for each pair of contacts. The current in dividers 90 and 46 will vary proportionally during calibration so that the ratio i s /i t  remains constant. In the indicating circuit, due to the oscillation of armatures 144 and 148 the accumulated voltage in condenser 158 is alternatively compared with voltages e 1  and e 2  as shown, if an inequality exists a charging or discharging current will flow to render the voltage on condenser 158 equal to e 1  or e 2  and the resultant A.C. output voltage at terminals 138, 140, which may be amplified and indicated, represents the difference between e 1  and e 2  in magnitude and phase sense.