GB2272301A

GB2272301A - Circuit for measuring current using temperature sensors

Info

Publication number: GB2272301A
Application number: GB9321847A
Authority: GB
Inventors: Andrew Ceri Davies
Original assignee: Smiths Group PLC
Current assignee: Smiths Group PLC
Priority date: 1992-11-05
Filing date: 1993-10-22
Publication date: 1994-05-11
Also published as: GB9321847D0

Abstract

The current to be measured is supplied to a first resistor 1 includes a first temperature sensor 3 responsive to the temperature of the resistor, an amplifier 4, 7 produces an output representative of the difference between the output of the first temperature sensor 3 and the output of a second temperature sensor 5 responsive to the temperature of a second resistor 6, the output of the amplifier 7 is connected to a square root unit 8, which provides a voltage that is representative of the square root of the difference between the first output and second output signals across the second resistor 6 and to a processor 9 that calculates the current through the first resistor 1 from the voltage across the second resistor 6. The square root unit avoids the problem of long setting times for low current values and instability at high currents. <IMAGE>

Description

Current Measurement This invention relates to current measurement circuits and to methods of measuring current.

Where it is necessary to measure current without introducing a large voltage drop, the current can be passed through a shunt resistor and measured by its heating effect on the resistor. One way in which this is done is to supply the voltage from a temperature sensor mounted close to the resistor to one input of a summing node. The other input of the summing node receives the voltage from a similar temperature sensor mounted close to a second resistor of the same value as the first resistor. The error signal from the summing node is supplied to a high gain amplifier, which produces a voltage across the second resistor. When the current being measured increases, this causes an increase in the output voltage from the first temperature sensor. This increases the error voltage supplied to the amplifier and hence increases its output voltage.An increase in the output voltage cause an increase in the current passing through the second resistor and hence an increase in its temperature. This in turn increases the voltage produced by its associated temperature sensor which results in a reduction in the error voltage applied to the amplifier until the temperature ofthe second resistor rises to that ofthe first resistor. This feedback action results in a system in which the error voltage is near zero and in which the temperature rise above ambient is nearly identical for the two resistors. The output voltage from the amplifier is the same as the RMS input voltage across the first resistor enabling the current to be calculated.

The problem with this system is that it can have a long settling time for low current values and can be unstable at high currents. The reason for this is that the temperature sensors measure power dissipation of the resistors, not the applied voltage. The power dissipation is proportional to the square of the voltage so that the feedback loop voltage gain is no longer linear but is proportional to the output voltage. The value of the feedback loop gain affects the settling time of the loop and, hence, the bandwidth of the circuit is proportional to the signal.

It is an object of the present invention to provide a temperature measuring circuit and method that can be used to alleviate the above-mentioned problem.

According to one aspect of the present invention there is provided a circuit for measuring current flowing in a conductor including a first resistor connected in series with the conductor, the circuit including a first temperature sensor mounted in thermal contact with the resistor such that the temperature sensor provides a first output that varies according to variation in temperature ofthe first resistor, a second resistor, a second temperature sensor mounted in thermal contact with the second resistor such that the second temperature sensor provides a second output that varies according to variation in temperature of the second resistor, amplifier means arranged to provide a third output in accordance with the difference between the first and second outputs, and square root means arranged to provide a current to the second resistor that is dependent on the square root of the third output from the amplifier and such that the output of the square root means is representative of the voltage across the first resistor.

In this way, a substantially constant feedback loop voltage gain can be produced that is independent of the output voltage.

The circuit preferably includes processing means arranged to divide the value ofthe voltage across the second resistor by its resistance so as to provide an output representative of current flowing through the second resistor and hence the first resistor. The circuit preferably includes a display arranged to provide a display representation ofthe current flowing through the first resistor.

According to another aspect of the present invention there is provided a method of measuring current flow including the steps of supplying the current to a first resistor, deriving a first output signal representative of the temperature of the first resistor, deriving a second output signal representative of the temperature of a second resistor, deriving a further output signal representative of the square root of the difference between the first output signal and the second output signal, supplying the further output signal to the second resistor, and deriving from the further output signal an indication of the current supplied to the first resistor.

A circuit for measuring current and its method of operation will now be described, by way of example, with reference to the accompanying drawing, which shows the circuit schematically.

The circuit includes a first resistor 1 with a value of 50Q connected in line 2 carrying a current of 11. A temperature sensor 3, such as a thermistor, is mounted directly on the resistor 1 so that it is in thermal contact with it. The voltage output of the sensor 3 is supplied to the positive input of a summing node 4 of a high gain amplifier 7. The negative input of the node receives the output from an identical temperature sensor 5 mounted in thermal contact with an identical 50Q resistor 6. The output of the summing node 4 is an error voltage Ve which is supplied to the input of the amplifier 7, which has an output connected to a square root unit 8.

The unit 8 provides a voltage at its output, which is the square root of the voltage at its input.

The voltage output Vout of the unit 8 is supplied across the second resistor 6 and to the input of a processor 9. The processor 9 carries out a division operation on its input to provide an output to a display or other utilization means 10 representative ofthe current 11. The display 10 gives a display representation of the current I1.

The output from the sensor 5 forms a feedback loop with the summing node 4, the amplifier 7 and the square root unit 8. An increase in current flowing through the resistor 1 will cause a temperature rise that is proportional to the square of the voltage across the resistor. This produces a corresponding rise in the voltage output ofthe sensor 3 and hence an initial increase in the error voltage at the output of the summing node 4 and of the amplifier 7.

The increase in the output ofthe amplifier 7 causes an increase in the current supplied to the second resistor 6 and hence an increase in its temperature. The temperature of the second resistor 6 will continue to rise until its temperature is the same as that of the first resistor 1 and the error voltage applied to the amplifier 7 drops to zero. The voltage across the second resistor 6 will, therefore, be the same as that across the first resistor 1. The processor 9 divides the value ofthe voltage Vout by that ofthe resistance (50Q) to produce a signal representing the current I1. This signal is supplied to the display 10 to provide a display representation of the current flowing on conductor 2. The purpose of the square root unit 8 is to counteract the squaring action of the resistors and temperature sensors so as to produce a constant overall feedback loop gain independent ofthe value ofthe output voltage Vout. This produces an optimum settling time for the loop for any value of the current I1.

Claims

1. A circuit for measuring current flowing in a conductor including a first resistor connected in series with the conductor, wherein the circuit includes a first temperature sensor mounted in thermal contact with the resistor such that the temperature sensor provides a first output that varies according to variation in temperature of the first resistor, a second resistor, a second temperature sensor mounted in thermal contact with the second resistor such that the second temperature sensor provides a second output that varies according to variation in temperature of the second resistor, amplifier means arranged to provide a third output in accordance with the difference between the first and second outputs, and square root means arranged to provide a current to the second resistor that is dependent on the square root of the third output from the amplifier and such that the output of the square root means is representative of the voltage across the first resistor.

2. A circuit according to Claim 1, wherein the circuit includes processing means arranged to divide the value of the voltage across the second resistor by its resistance so as to provide an output representative of current flowing through the second resistor and hence the first resistor.

3. A circuit according to Claim 1 or 2, wherein the circuit includes a display arranged to provide a display representation of the current flowing through the first resistor.

4. A circuit substantially as hereinbefore described with reference to the accompanying drawing.

5. A method of measuring current flow including the steps of supplying the current to a first resistor, deriving a first output signal representative of the temperature of the first resistor, deriving a second output signal representative of the temperature of a second resistor, deriving a further output signal representative ofthe square root ofthe difference between the first output signal and the second output signal, supplying the further output signal to the second resistor, and deriving from the further output signal an indication of the current supplied to the first resistor.

6. A method substantially as hereinbefore described with reference to the accompanying drawings.

7. Any novel feature or combination of features as hereinbefore described.