GB1506633A - Electrical isolating circuits - Google Patents

Abstract

1506633 Voltage measuring N SENGER 21 May 1975 [21 May 1974 19 Nov 1974 14 Feb 1975 24 Feb 1975] 22016/75 Heading G1U [Also in Division H2] An electrical isolating circuit has an input port for connection to one circuit and an output port for connection to another circuit and comprises a rectifier connected to the input port, a chopper for chopping the rectified output to produce an A.C. output signal of chopper frequency, a filter circuit connected to the chopper output for passing signals at the chopper frequency while preventing signals of a lower frequency from passing, and another rectifier for rectifying signals passed through the filter for delivery to the output port, the arrangement being such that for signals of the lower frequency, the impedance seen looking into one of the ports is substantially independant of any external impedance connected across the other of the ports. The embodiment shown in Fig. 3 may be connected to the input and/or output of a conventional capacitive voltage transformation circuit, Fig. 1 (not shown) to isolate circuits connected to this voltage transformer from the impedance changes occurring when the capacitors switch between series and parallel. In Fig. 3 alternating input voltage U e of frequency f 1 is applied to rectifier 240, whose output is chopped in circuit 241 at a frequency f 2 much greater than f 1 . The chopped output is applied to filter circuit 244, the values of whose components C1 ... C4 and L together with the frequency f 2 , are chosen so that the impedance seen looking into the input or output of the isolating circuit is substantially independent of any impedance connected across the output or input respectively. The output of filter 244 is applied to rectifier 245, which feeds a polarity correcting circuit (or commutating switch arrangement) 246, whose output is applied to a low pass filter 247 which removes the chopping frequency f 2 and its harmonies to produce an output signal whose frequency f a is the same as f 1 . The theoretical basis of the isolating circuits is explained assuming a very small impedance of capacitors C1 ... C4 at the frequency f 2 and an inductance L which provides a short circuit to earth at the frequency f 1 . If the input voltage U e is a uni-directional one (U c ), members 246, 247 are omitted and smoothing capacitors (250, 251) are connected across the outputs of rectifiers 240, 245, Fig. 4 (not shown).