Two-wire current transmitter with isolated transducer circuit

Claim 2

A two-wire current transmitter for connecting to a source of substantially constant DC voltage including a transducer circuit, DC/DC converter means connected between the source and the transducer circuit and providing a DC output to said transducer circuit, said transducer circuit including a sensor circuit which provides a DC signal which varies as a function of a condition being measured, a current controller providing control of current in the transducer circuit responsive to the sensor circuit DC signal, and a feedback resistance in series with the current controller and connected to the sensor circuit to provide a feedback signal in opposition to the DC signal to achieve a balance in said current controller so that the transducer current varies as a function of the condition being measured, said source providing power for the change in current in the transducer circuit through said converter means at said substantially constant DC voltage whereby the source current varies as a function of the condition being measured.

Claim 3

The combination as specified in claim 1 wherein said DC/DC converter means comprises a single transformer having input and output windings DC isolated from each other.

Claim 4

The combination as specified in claim 2 wherein the input circuit to said transformer includes switching means to provide polarity reversing currents in the primary winding of said transformer.

Claim 5

The combination specified in claim 2 wherein said transformer is a tape wound core toroid transformer.

Claim 6

The combination specified in claim 1 wherein said current controller comprises a differential input amplifier, and said sensor circuit includes a resistance bridge, said feedback resistance being connected into said bridge and causing a balancing of signals at the inputs of said amplifier.

Claim 7

The combination of claim 5 wherein said DC/DC converter means comprises a transformer having a pair of secondary windings, the secondary windings providing excitation voltage for said amplifier and for said bridge, the excitation voltages across said bridge being less than the excitation voltages for said amplifier.

Claim 8

A two-wire current transmitter for connection on two wires to a DC supply and load comprising a voltage regulator circuit energized by the two wires and providing a substantially constant DC voltage output, a transducer circuit, an electrical energy converter means having its input connected to the output of the voltage regulator and its output connected to the transducer circuit, said converter means having its output DC isolated from its input, said transducer circuit including a sensor circuit providing a DC signal which varies as a function of a condition being measured and further including a current controller providing a control of current in said transducer circuit responsive to the DC signal from said sensor circuit, a feedback resistance in series with the current controller, means connecting said feedback resistance to the sensor network, current changes responsive to said DC signal causing a feedback signal in said feedback resistor opposing said DC signal in a manner to stabilize the current output of the current controller so that the transducer circuit current varies as a function of the condition being measured, said transducer circuit being powered only through said voltage regulator and said converter so that the current flowing in said voltage regulator also varies as a function of the condition being measured.

Claim 9

A two-wire current transmitter for connection on two wires to a DC supply and load comprising a voltage regulator circuit energized by the two wires and providing a substantially constant DC voltage output, a transducer circuit, an electrical energy converter means having its input connected to the output of the voltage regulator and its output connected to the transducer circuit, said converter means having its output DC isolated from its input, said transducer circuit including a sensor circuit providing a DC signal which varies as a function of a condition being measured and further including a current controller providing a control of current in said transducer circuit responsive to the DC signal from said sensor circuit, a feedback resistance in series with the current controller, means connecting said feedback resistance to the sensor network, current changes responsive to said DC signal causing a feedback signal in said feedback resistor opposing said DC signal in a manner to stabilize the current output of the current controller so that the transducer circuit current varies as a function of the condition being measured, said transducer circuit being powered only through said voltage regulator and said converter so that the current flowing in said voltage regulator also varies as a function of the condition being measured.

Claim 10

The combination as specified in claim 7 wherein said electrical energy converter means comprises a single transformer having input and output windings DC isolated from each otHer to prevent the transmission of direct currents therebetween.

Claim 11

The combination as specified in claim 8 wherein said transformer is a tape wound core toroid transformer.

Claim 12

A two-wire current transmitter for connection to a power source of substantially constant DC voltage, comprising a converter circuit having a DC input and a DC output and including a transformer providing DC isolation between a primary circuit and a secondary circuit of the converter, said primary circuit including switching means for connection to the source to provide alternating current to the transformer, said secondary circuit including rectification means to provide a secondary direct current output, means to maintain the direct current in the primary circuit proportional to the secondary direct current, a transducer circuit energized by said secondary direct current, said transducer circuit including a sensor network providing a first electrical signal which varies as a function of a condition being measured, a DC amplifier having an input connected to the sensor network, a current controller having its input connected to the output of the DC amplifier and including means providing control of the magnitude of the secondary direct current, and a feedback resistance in series with the current controller, and said feedback resistor being connected into the sensor network to provide a second electrical signal in opposition to the first electrical signal to achieve a balance in the output of the current controller so that the secondary direct current varies as a function of the condition being measured.

Claim 13

A two-wire current transmitter for connection to a power source of substantially constant DC voltage, comprising a converter circuit having a DC input and a DC output and including a transformer providing DC isolation between a primary circuit and a secondary circuit of the converter, said primary circuit including switching means for connection to the source to provide alternating current to the transformer, said secondary circuit including rectification means to provide a secondary direct current output, means to maintain the direct current in the primary circuit proportional to the secondary direct current, a transducer circuit energized by said secondary direct current, said transducer circuit including a sensor network providing a first electrical signal which varies as a function of a condition being measured, a DC amplifier having an input connected to the sensor network, a current controller having its input connected to the output of the DC amplifier and including means providing control of the magnitude of the secondary direct current, and a feedback resistance in series with the current controller, and said feedback resistor being connected into the sensor network to provide a second electrical signal in opposition to the first electrical signal to achieve a balance in the output of the current controller so that the secondary direct current varies as a function of the condition being measured.

Claim 14

A two-wire current transmitter for connection to a source of power maintained at substantially constant DC voltage whereby changes in power therefrom result in changes in current flow in said source, including a transducer circuit, electrical converter means connected to the source on said two wires and providing a DC output to said transducer and being the power source for said transducer circuit, said transducer circuit including a control circuit and a current controller means to regulate current flow in said transducer circuit responsive to signals from said control circuit, said converter including means to change power required from said source as a function of current changes in said transducer circuit, a feedback resistance connected to the current controller output, and means connecting said feedback resistance to the current controller to provide a feedback signal in opposition to the signal from the control circuit to achieve balance in said current controller so that the transducer current varies as a function of the signals from the control circuit, said source providing power required for the change in current in the transducer circuit through said converter means at said substantially constant DC voltage whereby the current in said source is proportional to the control circuit signal.

Claim 15

A two-wire current transmitter for connection to a source of power maintained at substantially constant DC voltage whereby changes in power therefrom result in changes in current flow in said source, including a transducer circuit, electrical converter means connected to the source on said two wires and providing a DC output to said transducer and being the power source for said transducer circuit, said transducer circuit including a control circuit and a current controller means to regulate current flow in said transducer circuit responsive to signals from said control circuit, said converter including means to change power required from said source as a function of current changes in said transducer circuit, a feedback resistance connected to the current controller output, and means connecting said feedback resistance to the current controller to provide a feedback signal in opposition to the signal from the control circuit to achieve balance in said current controller so that the transducer current varies as a function of the signals from the control circuit, said source providing power required for the change in current in the transducer circuit through said converter means at said substantially constant DC voltage whereby the current in said source is proportional to the control circuit signal.

Claim 16

The combination as specified in claim 11 wherein said converter comprises a single transformer having an input winding and an output, switching means to provide pulsating DC current from said source to said input winding, and rectifier means on the output of said transformer to provide DC power to the transducer circuit.

Claim 17

The transmitter of claim 11 further characterized in that said control circuit comprises a sensor responsive to a condition being measured so that said control signal changes as a function of the sensor value.

Claim 18

The transmitter of claim 11 wherein said current controller includes a differential input amplifier having an output connected to current control elements, said differential inputs to said amplifier sensing changes in the control signal and said feedback signal to control current as a function of said signals.