GB1403818A - resistance measuring circuit - Google Patents

Abstract

1403818 Measuring resistance COMMONWEALTH SCIENTIFIC & INDUSTRIAL RESEARCH ORGANIZATION 26 Sept 1973 [28 Sept 1972] 45102/73 Heading G1U An unknown resistance R x (e.g a resistance thermometer) is measured by passing through it a current whose value is determined by the variable ratio 3 of an A.C. supply 2 and comparing this current with that from a constant current source whose magnitude is determined by the A.C. supply voltage and at least one standard resistor. Amplifier A 2 is arranged to have substantially unity gain and thus the voltage at point. A follows the output voltage from the resistive or inductive voltage divider 3. When detector 4 indicates null, point B is at earth potential and thus the current through R x is determined by the divider ratio. The current from the constant current source is to a good approximation -e/R 4 and thus R x can be found. In order to remove quadrature phase signals due to the inductive nature of any of the resistances, a quadrature signal may be subtracted from the circuit output to counteract any error Fig. 2 (not shown) In Fig. 3 (not shown) only a single standard resistor (R s ) is used to supply the constant current, although this necessitates a floating D.C. power supply.