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Current-Sampling Transformer Circuit

IP.com Disclosure Number: IPCOM000042843D
Original Publication Date: 1984-Jun-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 3 page(s) / 63K

Publishing Venue

IBM

Related People

Commander, RD: AUTHOR [+2]

Abstract

A current-sampling transformer circuit is described for use with a bridge-driven motor. The transformer is pulsed at high frequency, gives high voltage isolation between a sensing circuit and the motor drive circuit, and senses both DC and AC motor current. A current source is pulsed into the primary of the transformer, and the secondary of the transformer is connected through a diode to a sensing resistor through which the motor current passes. Two applications of this circuit are described to prevent motor-switching transients affecting the accuracy of DC measurement or damaging the transformer: the first injects current into the primary and the second shorts the primary both during the off period of motor drive. Fig.

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Current-Sampling Transformer Circuit

A current-sampling transformer circuit is described for use with a bridge-driven motor. The transformer is pulsed at high frequency, gives high voltage isolation between a sensing circuit and the motor drive circuit, and senses both DC and AC motor current. A current source is pulsed into the primary of the transformer, and the secondary of the transformer is connected through a diode to a sensing resistor through which the motor current passes. Two applications of this circuit are described to prevent motor-switching transients affecting the accuracy of DC measurement or damaging the transformer: the first injects current into the primary and the second shorts the primary both during the off period of motor drive. Fig. 1 shows the basic transformer circuit in which a current source I1 is pulsed into the primary of transformer T. The secondary of transformer T is connected through diode D2 to current-sensing resistor R2 in series with the motor current. When the motor current is in its forward direction, junction a is positive with respect to junction b. When current I1 is on, secondary current will flow through diode D2, clamping the secondary voltage at one diode voltage drop above the voltage across R2. When transformer T has unity turns ratio, and forward voltage differences in diodes D1 and D2 may be ignored, capacitor C1 will charge to a voltage V0 which is equal to the voltage across R2, thus sensing the motor current. R1 is chosen so that C1 will discharge faster than the maximum decrease rate of the motor current. Thus, voltage V0 follows variations in motor current. When I1 turns off, T flyback voltage is clamped by diode D3 at a negative voltage -VFB. Increasing this negative voltage decreases transformer T recovery time. Resistor R4 has a high value to damp ringing at flyback. Two modifications of the basic circuit are given to prevent inductive flyback transients resulting from switching motor current from affecting the accuracy of DC measurement and from damaging transformer T when the circuit is used in series with a bridge-driven motor. The circuits are used to sense when the motor current reaches a predetermined limit value, when the bridge is turned off for a time set by a single-shot. During the single-shot the motor current passes in the reverse direction through the sense resistor via flyback diodes in the bridge, and decays to a lower value at a rate set by the motor inductance and the bridge supply voltage. Fig. 2 shows a first modified circuit with waveforms illustrated in Fig. 3. This circuit injects current into transformer T primary during the off period of the bridge drive arm. An additional current sourc...