Resistance measuring ohms converter circuit employing a constant current source

摘要

An ohms converter circuit for measuring the resistance of an unknown resistor Rx which monitors the actual output current of the current source employed therein to ensure its accuracy. The ohms converter circuit employs a highly stable current source which is protected against high voltage overloads. A current of known magnitude Iref is passed without branching through a series connection of Rx and a reference resistor Rref. Relays are utilized to switch between measurements of the voltage across Rx and the voltage across Rref. The voltage across Rref is checked periodically to verify the magnitude of the current of predetermined value Iref. The ohms converter establishes a loop circuit in which the current of known magnitude Iref passes through a scaler resistor to develop a voltage drop thereacross, which is connected with a floating potential in a loop circuit. A differential amplifier is coupled to differentially measure the voltages across the scaler resistor and the floating potential, and provides an output indicative thereof, which determines Iref. In this arrangement, the voltage drop across the scaler resistor equals the voltage drop across the floating potential, and the current Iref through the scaler resistor remains constant at a known magnitude. The current of known magnitude Iref also passes serially through the unknown resistor Rx, and a voltage measuring circuit measures the voltage across Rx to provide a precise determination thereof.