Patent GB-1231004-A (GB)
IPC: G01R17/10
Filed: 1968-04-11
Abstract: 1,231,004. Measuring bridge circuits. ZAVOD ELEKTROIZMERITELNYKH PRIBOROV. April 11, 1968, No.1599/68. Heading G1U. A measuring bridge system which is selectively switchable between Wheatstone and Thomson bridge configurations incorporates two ganged comparison arms and two ratio arms, and an auxiliary circuit having several series connected resistance sections and switches for connecting resistors in a given section either in series or in parallel, the auxiliary circuit being initially adjustable so that the resistance of one section relates to the resistance of one or several further sections in the ratio 1:10<SP>n</SP> (n an integer or zero), to establish a standard ratio of resistance, for subsequent use in setting precise ratios of resistance between arms of the bridge system. The system also incorporates multi-position switches so arranged and connected that they are settable to (a) connect each section of the auxiliary circuit as the test resistance of a Wheatstone bridge during initial adjustment of the section, (b) connect the standard ratio of resistances established by the auxiliary circuit when properly adjusted in a bridge configuration with one comparison arm and one ratio arm and then with both ratio arms, so that these arms may be properly adjusted and (c) isolate the auxiliary circuit and switch one or both comparison arms and both ratio arms to form parts of either a Wheatstone or a Thomson bridge. In the circuit arrangement shown, the bridge comparison arms are R 1 , R 4  and the ratio arms are R 2  + R 6  and R 3  + R 5 . Ganged switches 1-5 have five settings I-V for switching between the various setting up and operating conditions. An independently operated double switch 6 ganged to the switches K 1 , K 2  enables the sections of the auxiliary circuit ABCD to be first adjusted as individual parts of a Wheatstone bridge, and the resulting standard reference ratio to then be switched into a circuit, for adjusting the bridge arms. With the ganged switches 1-5 in position I, the sections of the auxiliary circuit are adjusted as follows: - With switch 6 in position I, (to connect r 1  in a Wheatstone bridge with R 1 , R 2 , R 3 ), the reading on the meter 7 is noted. With switch 6 in position II resistor r 7  is adjusted to provide the same reading, so that AB: BC = 1:1. In position III, the switches K1 and K2 are closed to set r 2 , r 3  and r 4  + r 5  in parallel, and r 5  is adjusted to again provide the same reading. When the switches K1 and K2 are opened, these resistors will be in series and equivalent to 9r 1 . In positions IV and V, the leading wiper arm of switch 6 is connected to point C or point D respectively to enable either BC (=AB) to be connected into circuit, or BD (=10AB) to be connected into circuit. With the auxiliary circuit so adjusted to provide a standard resistance ratio of 1:1 or 1:10 at the prevailing ambient condition, the switches 1-5 are set to position II. This connects the standard resistance ratio across R1 and R2 + R6, to form a Wheatstone bridge which enables R6 to be adjusted to provide the same ratio, (the ratio arm R2 + R6 in fact consists of switched parallel resistors enabling different ratios to be set up). The switches 1-5 are then set to position III to form a bridge incorporating the resistance ratio and the arms R2, R3 + R5, in which R5 is adjusted to provide the required ratio. With the switches 1-5 in position IV, the reference ratio is switched out of circuit and a conventional Wheatstone bridge formed by connecting the unknown resistance to be measured across the terminals P1, P5. In position V part of a Thomson bridge is formed, the unknown resistance R x  being connected across P1, P2 and a standard R N  across P3, P4. A battery and low resistance link r are used to connect R x  and R N  in series, external to the system shown.