GB1352216A - Isolation guarding measuring instrument - Google Patents

Abstract

1352216 Measuring impedance; distinguishing electric signals SYSTOMATION Inc 22 July 1971 [27 July 1970] 34318/71 Heading G1U An arrangement for measuring the impedance of an in-circuit component provides two test leads PL - 2 - 4, PL - 2 - 5 to connect the unknown impedance as part of a feedback amplifier network, and further guarding leads, e. g. PL - 21, which are maintained at the potential of one of the two test leads and which in use are connected to nodes in circuit paths which are in parallel with the component under test. The guarding leads prevent current in such parallel paths disturbing the measurement. A high gain amplifier 1C6, 1C7, is used in the amplifier network, one test lead and the guarding leads being connected to inputs of the amplifier so that they are all maintained at approximately earth potential. According to the position of a mode selector S4, the unknown component is connected as the feedback (if it is resistive or inductive) or input (if it is capacitive) impedance of the amplifier 1C6, 1C7, the input or feedback impedance respectively being provided by a standard resistor R48-R55 selected according to the range required. A reference signal source Fig. 1A, (not shown), and (1C4, 1C5) provides selectably a regulated A.C., positive D.C. or negative D.C. voltage (at TP - C) for use as an input to the feedback amplifier network. The magnitude of the network output is, after amplification (1C8, 1C9) indicated by a digital meter PM - 1 whose decimal point determining circuitry 26 is coupled to the range and mode selecting switches S3, S4. The network output is also passed to an A. C. / D. C. distinguishing and polarity indentifying circuit 27. If the network output is D. C., its polarity is indicated by the lighting of an appropriate lamp L9, L10; if A. C. the lamps will flicker. A standard resistor R56, connectible as though it were an unknown impedance, enables the calibration to be checked. Further positions of the mode selector S4 enable currents and voltages to be measured by being applied, directly or through a standard input resistance R57, R58, to the feedback amplifier network. A modification operating on similar principles is described Figs. 2A-D, (not shown), the guarding leads here being connected directly to earth, the output indicator being an analogue meter (M1), and the A. C. /D. C. distinguishing and polarity identifying arrangement being a centre zero meter (M2) shunted by two back-toback diodes in series with the network output.