AC mains isolation resistance monitoring circuit - measures DC voltage developed by constant current generator

摘要

A circuit arrangement for monitoring the isolation of an alternating current mains network involves superimposing an auxiliary electrical parameter on the network. Two constant current generators (Q1, Q2) supply equal currents (Ic1, Ic2). The current fed from the first source (Q1) flows through a choke (Dr) and the AC mains network with its isolating resistance (Riso) and results in a voltage appearing at the non-inverting input of a differential amplifier (V1). The current from the second source (Q2) flows through a compensating choke (Lk) to earth. The DC voltage drop across the compensating choke is fed to the inverting input of the differential amplifier. The output voltage of the differential amplifier (Um) depends linearly on the isolation resistance of the network. The choke and compensating coil are thermally coupled so that the circuit is not temp. sensitive.