US 7,525,297 B2
Current transformer with impedance compensation and associated methods
Umberto Gibellini, Coral Gables, Fla. (US)
Assigned to FALCO Ltd., Hong Kong (Hong Kong Special Administrative Region of the People's Republic of China, The)
Filed on Dec. 01, 2005, as Appl. No. 11/291,750.
Prior Publication US 2007/0126411 A1, Jun. 07, 2007
Int. Cl. H01F 38/28 (2006.01)
U.S. Cl. 323—357  [323/356; 324/127] 9 Claims
OG exemplary drawing
 
1. An impedance-compensated current transformer comprising:
a primary winding;
a power source electrically coupled through said primary winding;
a secondary winding magnetically coupled to said primary winding, said secondary winding having a secondary winding magnetizing inductance and a secondary winding resistance;
an impedance-compensation circuit electrically coupled across said secondary winding;
wherein said impedance-compensation circuit actively reflects to said secondary winding a virtual impedance which is equal in magnitude and opposite in polarity to said secondary winding resistance such that the current-driving voltage across said secondary winding magnetizing inductance is driven to zero independently of the value of said secondary winding magnetizing inductance;
wherein said impedance-compensation circuit includes means to vary said virtual impedance of said impedance-compensation circuit in direct proportion to variations in the temperature of said secondary winding; and
wherein said impedance-compensation circuit comprises an operational amplifier having a positive input, a negative input, an output, a positive supply and a negative supply; a first resistor having a resistance approximately equal to said secondary winding resistance; a thermistor with a negative temperature coefficient having an average resistance which is much greater than the resistance of said first resistor; and a second resistor having a resistance which is much greater than the resistance of said first resistor; wherein said first resistor is electrically coupled across said operational amplifier negative input and said operational amplifier output; said thermistor is electrically coupled across said operational amplifier positive input and said operational amplifier output and is exposed to approximately the same temperature as said secondary winding; and said second resistor is electrically coupled across said operational amplifier positive input and ground.