Patent ID: 7525297

Claim:
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.