Linear alternating current interface for electronic meters
First Claim
1. An interface circuit for use with electronic metering equipment which meters alternating current (AC) parameters comprising:
- means for coupling AC current flowing through a load to the electronic metering equipment used to perform the metering, said coupling means being a "no load" air core transformer having a primary coil to which the load is connected and a secondary coil whereby a voltage impressed across input terminals of the primary coil equals a voltage induced in the secondary coil, where the mutual inductance of the transformer is determined in accordance with the expression
space="preserve" listing-type="equation">M.sub.s =lN/5 ln ((a-R.sub.1)/R.sub.1)μ
H.and where a coil of the transformer winding is a thin wire, square coil equidistantly positioned between parallel, cylindrical conductors, the plane of the coil coinciding with that of the conductors with current flowing through one conductor in one direction and in the other conductor in the opposite direction, the coil having N number of turns and sides of length "1", the distance between the longitudinal centerline of the conductors being "a", and the distance from the longitudinal centerline of each conductor to the adjacent edge of the coil being R1 ;
amplifier means to which an AC signal developed by the secondary coils of said means for coupling is supplied, said AC signal being linearly proportional to the AC current flowing through the load, the amplifier means performing a linear amplification of the input provided thereto to produce an AC output signal; and
,conversion means responsive to an output signal from the amplifier means for converting the AC signal from the amplifier means to a digital signal supplied to the electronic metering equipment used for metering purposes.
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Accused Products
Abstract
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 ILOAD 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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Citations
8 Claims
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1. An interface circuit for use with electronic metering equipment which meters alternating current (AC) parameters comprising:
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means for coupling AC current flowing through a load to the electronic metering equipment used to perform the metering, said coupling means being a "no load" air core transformer having a primary coil to which the load is connected and a secondary coil whereby a voltage impressed across input terminals of the primary coil equals a voltage induced in the secondary coil, where the mutual inductance of the transformer is determined in accordance with the expression
space="preserve" listing-type="equation">M.sub.s =lN/5 ln ((a-R.sub.1)/R.sub.1)μ
H.and where a coil of the transformer winding is a thin wire, square coil equidistantly positioned between parallel, cylindrical conductors, the plane of the coil coinciding with that of the conductors with current flowing through one conductor in one direction and in the other conductor in the opposite direction, the coil having N number of turns and sides of length "1", the distance between the longitudinal centerline of the conductors being "a", and the distance from the longitudinal centerline of each conductor to the adjacent edge of the coil being R1 ; amplifier means to which an AC signal developed by the secondary coils of said means for coupling is supplied, said AC signal being linearly proportional to the AC current flowing through the load, the amplifier means performing a linear amplification of the input provided thereto to produce an AC output signal; and
,conversion means responsive to an output signal from the amplifier means for converting the AC signal from the amplifier means to a digital signal supplied to the electronic metering equipment used for metering purposes. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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Specification