Current transformer with active load termination

Abstract

A current transformer and transresistance amplifier are combined; the secondary winding of the current transformer being connected to the input of the transresistance amplifier and being virtually short-circuited because of the very low input impedance of the transresistance amplifier. The transresistance amplifier, nevertheless, supplies an output voltage which is proportional to current in the primary winding of the current transformer. An output voltage is developed from the current in the secondary winding. An important advantage, among others, of the subject combination is that a current transformer having considerably lower volt-ampere capacity may be employed. A current transformer employed alone for the same purpose would have to have a much larger volt-ampere capacity.

What is claimed is: 
     
       1. In combination: a current transformer including primary and secondary windings adapted for conducting primary and secondary currents, respectively; and, a transresistance amplifier for terminating said secondary winding and serving as an active load which virtually short-circuits said secondary winding while supplying an output voltage proportional to the primary current, if any..]. .Iadd.and, means coupled with said transresistance amplifier for supplying in response to said output voltage another output voltage phase-inverted with respect to said output voltage. .Iaddend. .[.2. The combination according to claim 1 further comprising means coupled with said transresistance amplifier for supplying in response to said output voltage another output voltage 
     
     
        phase-inverted with respect to said output voltage..]. 3. In combination: a current transformer comprising primary and secondary windings adapted for conducting primary and secondary currents, respectively; and an operational amplifier including first and second input terminal means, output terminal means, and a feedback resistance connected between said first input terminal means and said output terminal means, said operational amplifier having a high open-loop gain and a low input resistance, measurable between said first and second input terminal means, said secondary winding being connected between said first and second input terminal means and in parallel with said low input resistance whereby there is no substantial potential difference between said first and second input terminals even when said secondary winding conducts secondary current, said operational amplifier supplying, between said output terminal means and said second input terminal means, an output voltage 
     
     
        proportional to said primary current. 4. The combination according to claim 3 wherein said input resistance of said operational amplifier is defined as:   Ri ≈ Rf/Ao     where Ri is the input resistance, Rf is the magnitude of said feedback resistance, and Ao is the open-loop gain of said operational amplifier.   
     
     
           The combination according to claim 4 wherein Ri is not greater than 0.5 
     
     
        ohm. 6. The combination according to claim 3 wherein said output voltage is defined as:   Vo = - Is Rf = - Ip Rf/K     where Vo is the magnitude of the output voltage, Is is the magnitude of the secondary current, Rf is the magnitude of the feedback resistance, Ip is the magnitude of the primary current, and K = Ns/Np, and Ns and Np are the   
     
     
        number of turns on the secondary and primary windings, respectively. 7. The combination according to claim 3 wherein said output voltage is a varying voltage which may include a DC component and said combination according to claim 3 is further comprised of means connected between said output terminal means and said second terminal means for translating said 
     
     
        output voltage without said DC component. 8. The combination according to claim 3 further comprising inverting operational amplifier means, responsive to said output voltage, for supplying another output voltage which is phase-inverted 180° with respect to said output voltage. 
     
     
           In combination: a transformer including at least one secondary winding adapted for conducting secondary current; and, operational amplifier means serving as an active load for terminating said secondary winding in a virtual short circuit while supplying an output voltage proportional to said secondary current, if any.[...]. .Iadd., said operational amplifier means including input and output terminal means and a feedback resistance connected between said input and output terminal means, said output voltage being defined as:   Vo = Is Rf     where Vo is the output voltage, Is is the secondary current, and Rf is the feedback resistance. .Iaddend. .[.10. The combination according to claim 9, wherein said operational amplifier means includes input and output terminal means and a feedback resistance connected between said input and output terminal means, said output voltage being defined as:     Vo = - Is Rf     where Vo is the output voltage, Is is the seconday current, and Rf is the   
     
     
        feedback resistance..]. 11. The combination according to claim 9 wherein said output voltage is a varying voltage which may include a DC component and said combination according to claim 9 is further comprised of means coupled with said operational amplifier means for translating said output 
     
     
        voltage without its DC component. 12. In combination: a current transformer including primary and secondary windings; a high gain amplifier including first and second input terminals between which said secondary winding is connected so that said amplifier represents a very low impedance connected across said secondary winding, said amplifier including an output terminal; and, a feedback impedance connected between 
     
     
        said first input terminal and said output terminal. 13. The combination according to claim 12 wherein the primary and secondary windings are adapted for conducting primary and secondary currents, respectively, and the very low impedance of said amplifier virtually short-circuits said secondary windings so that no substantial difference of potential exists between said first and second input terminals while substantially all of the secondary current is conducted from said first input terminal through said feedback impedance to said output terminal so that between said output terminal and said second input terminal there exists a potential difference substantially equal to the product of the secondary current and 
     
     
        the feedback impedance. 14. The combination according to claim 13 wherein said potential difference between said output terminal and said second input terminal is a varying potential difference which may include DC component and said combination according to claim 13 is further comprised of means connected between said output terminal and said second input terminal for translating said varying potential difference without its DC component. .[.15. A current transformer including a current carrying primary and a current-carrying secondary, and a transresistance amplifier terminating said secondary in a virtual short circuit while said amplifier in response to the secondary current, supplies an output voltage proportional to primary current..].