Distribution Control Systems, Inc.

Brian C Ramey

Sioe T Mak

An interface circuit (10) for use with electronic metering equipment (E) to provide a linear output response to an AC or analog input. An air core transformer (12) has a first coil or winding (12a) connected to a load (L). An AC current I.sub.LOAD is produced by the load and is coupled to a secondary coil or winding (12b) of the transformer. The transformer has an air core rather than a ferromagnetic core so to have no saturable core. Because no load is drawn by transformer coil, the voltage across a terminal of the coil equals the EMF induced in the coil. The EMF, in turn, equals the mutual inductance of the coils multiplied by the rate of change of magnetic flux linkages over time. The output from the transformer is applied as an input to an operational-amplifier (16) which has a high input impedance. Further, the amplifier is a low drift amplifier which performs a linear amplification of the input signal provided thereto. In certain embodiments, a pair of similar amplifiers are used and the secondary winding of the transformer is a tapped winding so to accommodate both a low range and a separate high range of inputs. The output from the operational amplifier is supplied to an analog-to-digital converter (18) so a digital output is supplied to the metering equipment. If the tapped coil, dual amplifier arrangement is used, the resolution requirements of the converter are lower than if the single amplifier circuit configuration is used. Use of the interface circuit eliminates the DC component of the sensed voltage and provides a linear response for the metered AC input to the measuring equipment.

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University City, MO, United States of America

Brian C Ramey