Wide dynamic range a.c. current sensor

• US 4,835,463 A
• Filed: 08/24/1987
• Issued: 05/30/1989
• Est. Priority Date: 08/24/1987
• Status: Expired due to Term

• Current Assignee: Terabeam Corporation (Proxim Wireless Corporation)
• Original Assignee: Metricom, Inc.
• Inventors: Baran, Paul, Knutson, Timothy J.
• Primary Examiner(s): Karlsen, Ernest F.
• Application Number:
  US07/088,931
• Time in Patent Office: 645 Days
• Field of Search: 324/123 R, 324/126, 324/127, 324/117 R, 323/356, 323/357
• US Class Current: 324/123.00R
• CPC Class Codes: G01R 1/203 Resistors used for electric measuring, e.g. decade resistors standards, resistors for comparators, series resistors, shunts resistors in general H01C; microwave or radiowave terminations H01P1/26; coupling devices H01R
• CPC Class Codes: G01R 15/183 using transformers with a magnetic core

Abstract

Various combinations of three elements are disclosed for use in accurate measurement of power through current measurement over a wide dynamic range, namely, a current shunt which is preferably an isothermal current shunt, a current transformer, which is preferably a high initial permeability current transformer, and a low-impedance burden load, which in the preferred embodiment includes an active negative impedance element which causes the removal of the effects of excitation current by canceling secondary winding resistance of the current transformer. In a specific embodiment of an isothermal current shunt according to the invention, a very linear device is achieved by construction out of copper in such a manner that the poor resistance versus temperature coefficient of copper does not affect the accuracy of measurement. Specifically, the shunt is constructed in an unbalanced isothermal bridge configuration, so that heat-induced variations are suppressed. Further, a very small current transformer is used having a core optimized for high initial permeability.

Claims

1. An apparatus for measuring a.c. current over a wide dynamic range in for example a power measuring system comprising:

a current-carrying means, including a current-dividing shunt, capable of carrying a large current without substantial heating;

a transformer having a primary and a secondary, said primary being connected in series with said shunt thereby to define a current transformer; and

a burden load, said burden load having a burden load impedance formed of a series connection of said secondary, a first electronic amplifying means and a second electronic amplifying means, said first electronic amplifying means having a first amplification factor at least in part determined by the value of winding resistance of said secondary, said second electronic amplifying means having a second amplification factor wherein the product of said first amplification factor and said second amplification factor equals unity so as to present to said secondary a near zero impedance load;

said secondary including a secondary winding having a termination coupled to said burden load.

2. The apparatus according to claim 1 wherein said first electronic amplifying means comprises a first operational amplifier and a first impedance means in a negative feedback loop for said first operational amplifier wherein amplification of said amplifying means is substantially equal to the ratio of the impedance value of said first impedance means and the impedance value of winding resistance of said secondary.

3. The apparatus according to claim 2 wherein said secondary has a first terminal and a second terminal and wherein said first electronic amplifying means is coupled to present as a load to said secondary at said first terminal a high impedance at a first input terminal of said first electronic amplifying means, and further wherein said first electronic amplifying means is coupled in series with a second input terminal of said second electronic amplifying means, the output terminal of said second electronic amplifying means being coupled to said second terminal, said second electronic amplifying means having an amplification factor to complement the amplification factor of said first electronic amplifying means without oscillation, thereby to present a negative impedance to said secondary removing effects of said secondary winding resistance.

4. The apparatus according to claim 3 wherein said second electronic amplifying means comprises a second operational amplifier and a second impedance means, said second impedance means being coupled in a negative feedback loop for said second operational amplifier, said second impedance means being formed of a material having a temperature to resistance coefficient equal to said secondary winding and is disposed in isothermal relationship to said secondary winding thereby to minimize heat-induced differences in resistance between said secondary winding resistance and said second impedance.

5. The apparatus according to claim 1 wherein said core is formed of hydrogen strain relieved supermalloy.

6. The apparatus according to claim 1, 3, or 5 wherein said current shunt comprises:

first and second solid conductive bars of equal length, said first and second bars being electrically and thermally coupled at a first or current input node and at a second or current output node; and

a third round solid conductive bar electrically and thermally coupled to said first bar at a first hole at a third node and to said second bar at a second hole at a fourth node and coupled in series with said primary of said current transformer, said third bar being a continuation of the primary of said current transformer and being constructed of the same material as said first and second bars,

the bar length between said input current node and said third node being different than the bar length between said input current node and said fourth node, and

the bar length between said output current node and said third node being equal to the bar length between said input current node and said fourth node,

said third round solid conductive bar being disposed in parallel with adjacent walls of said first bar and said second bar, and

wherein said current transformer comprises a core formed of an elongate toroid mounted around said third bar and between said first bar and said second bar;

thereby to form an unbalanced bridge with a current transformer at the shunt and being resistant to hum.

7. The apparatus according to claim 6 wherein said current transformer further comprises a core formed of a ferrite material having high initial permeability.

8. The apparatus according to claim 1 wherein said current transformer further comprises a core formed of a ferrite material having high initial permeability.