Two wire current transmitter responsive to a resistive temperature sensor input signal

• US 4,016,763 A
• Filed: 09/27/1974
• Issued: 04/12/1977
• Est. Priority Date: 02/24/1970
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Grindheim, Earl A.
• Primary Examiner(s): Queisser, Richard C.
• Assistant Examiner(s): Shoon, Frederick
• Application Number:
  US05/510,026
• Time in Patent Office: 928 Days
• Field of Search: 73/362 AR, 323/75 H, 323/75 N, 340/186, 340/210
• US Class Current: 374/173
• CPC Class Codes: G01K 7/25 for modifying the output characteristic, e.g. linearising
• CPC Class Codes: G01L 1/225 Measuring circuits therefor
• CPC Class Codes: G01L 9/04 of resistance-strain gauges
• CPC Class Codes: G08C 19/02 in which the signal transmitted is magnitude of current or voltage G08C19/36, G08C19/38 take precedence

Abstract

Circuitry including a resistance sensor is excited by an external power source. A reference voltage is established, the voltage developed across the sensor resistance is compared at an amplifier input with a reference voltage signal and the voltage across a feedback resistor, and the total current drawn through the circuitry adjusts as a function of the sensor resistance to give a balanced amplifier input. Total current may be made a linear function of sensor resistance or a non-linear function by proper selection of certain resistors.

Claims

1. A resistance-to-current converter system comprising:

a. a sensing resistor responsive to a variable physical condition to be indicated,

b. a bridge including said resistor, said bridge having input and output junctions and being balanced only when said resistor has a predetermined value, whereby the bridge voltage developed at said output junctions is a function of the deviation of the resistor from said predetermined value as a result of a change in said physical condition,

c. a D-C amplifier whose input circuit is connected to the output junctions of said bridge whereby the impedance in the output circuit of the amplifier varies as a function of said bridge voltage,

d. a direct-voltage source providing an operating voltage both for said bridge and said amplifier,

e. a load connected in series with said source and having an output current flow therethrough resulting from variations in said resistor and hence indicative of said condition,

f. a voltage regulator,

g. a pair of wires connecting said series-connected source and load through said regulator across both the output circuit of said amplifier and the input junctions of said bridge, whereby the operating voltage of said amplifier is stabilized despite variations in said output current flow through said load resulting from variations in the value of said resistor,

h. a feedback resistor connected in series with said load to produce a feedback voltage, and

i. means to feed said feedback voltage into the arm of said bridge containing said sensing resistor in series opposition to the voltage produced by the sensing resistor to rebalance the bridge, whereby the output current is accurately proportional to changes in said resistor.

2. A converter system as set forth in claim 1, further including a Zener diode connected across said input junctions to stabilize the voltage thereacross.

3. A system as set forth in claim 1 wherein said regulator is constituted by a transistor having its emitter and collector in series with one of said wires, a reference voltage being applied to the base of said transistor.

4. A system as set forth in claim 1, wherein said resistor is a temperature-responsive element, and said load is a read-out device to indicate the temperature sensed by said element.

5. A system as set forth in claim 4 wherein said resistor is made of platinum.

6. A system as set forth in claim 4 wherein said resistor and amplifier are located in a first location, whereas the load and D-C source are located at a remote second location.

7. An electrical current transmitting system comprising a pair of current terminals adapted for connection to a load and power source that are in series;

a resistance network means connected in series with said current terminals to be powered and excited solely therethrough and including a first resistor which varies in resistance value as a function of temperature, for providing a first variable D.C. voltage signal which varies as a function of the temperature to be measured;

a feedback resistance means connected to said resistance network means;

means to be powered solely through said terminals connected to the feedback resistance means and to the current terminals to establish a current flow path between said terminals and to provide a known portion of current flowing between said terminals through said feedback resistance means to provide a second D.C. voltage signal which is a function of the current through said current terminals including an amplifier means having a voltage input portion and an output portion providing an output signal which is a function of the voltage at said voltage input portion, and current control and adjustment means coupled to said output portion to receive the output signal and to adjust the current in said current flow path between said terminals as a function of said output signal; and

means to couple said resistance network means and said feedback resistance means to said voltage input portion to provide a voltage at said input portion of said amplifier means which is a function of said first and second voltage signals.

8. The combination of claim 7 further characterized in that a voltage reference means is connected to said resistance network means in series with said feedback resistance means between said terminals to provide a reference voltage for said resistance network means.

9. The combination of claim 8 wherein said voltage reference means includes a fixed resistance and a semiconductor reference element in series with said fixed resistance, the series combination of said reference element and fixed resistance being connected across said first resistor so that an increase in excitation voltage across said first resistor is provided as a result of an increased current through said voltage reference means.

10. The combination as specified in claim 7 wherein said feedback resistance means includes a second resistor, said resistance network means further includes third and fourth resistors, said first, second, third and fourth resistors being connected in a symmetrical bridge arrangement having one end of said first resistor connected to one end of said third resistor to provide a first voltage terminal, and one end of said second resistor being connected to one end of said fourth resistor to provide a second voltage terminal, and wherein the other ends of said first and second resistors are connected to one of said current terminals and the other ends of said third and fourth resistors are connected together to the other current terminal, said feedback resistance means further including fifth and sixth resistors, and a voltage reference means for providing a reference voltage for said resistance network means including a voltage reference element having one end connected to the junction of said third and fourth resistors and the other end connected through said fifth resistor to the other of said current terminals and through said sixth resistor to said second voltage terminal.

11. The current transmitting system of claim 7 wherein said feedback resistance means comprises a second resistor, said resistance network means further including third, fourth and fifth resistors, all of said resistors being connected in a closed series loop having the first resistor connected between the second and fourth resistors and the third resistor connected between the second and fifth resistors, and a voltage reference element and a sixth resistor connected in series between the junction of said fourth and fifth resistors and the junction of said second and third resistors, one of said current terminals being connected to the junction of the fourth and fifth resistors to carry a first portion of the current to said resistance network means and the junction of the voltage reference element and the sixth resistor being connected to said one current terminal to carry a second portion of the current from said terminal to the resistance network means, said last-mentioned junction being connected through portions of said resistance network means to the other current terminal.