Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Reexamination Certificate
1997-11-25
2001-02-06
Le, Dinh T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
C327S362000, C327S369000, C327S514000, C363S021030
Reexamination Certificate
active
06184725
ABSTRACT:
TECHNICAL FIELD
This invention relates to a circuit arrangement for making isolated voltage and/or current measurements on a transmission line with a go conductor and a return conductor for supplying power to electrical apparatus.
BACKGROUND OF THE INVENTION
Circuit arrangements for the isolated sensing of binary electric signals of different pulse heights at a transmission line are known, for example, from EP 0 398 456 B1 or from a prospectus of Hewlett Packard entitled “High-Linearity Analog Optocouplers”, 1993, pages 1 to 16.
In networks with central powering, both the supply voltage and the supply current can generally be measured at the terminal station. This was done formerly using simple analog pointer-type instruments which were tied to the supply potential.
Since the introduction of operator terminals, however, it must be possible to process the two measured values digitally. This involves difficulties with electrical isolation, because the potentials of the voltages on the transmission lines may be up to 2 kV above ground potential, with voltage and current values occurring between the go conductor and the return conductor in the ranges of 0 V to approximately 300 V and 0 A to approximately 70 mA, respectively.
One possibility of making an isolated measurement is to use an analog-to-digital converter (ADC) which is tied to the potential of the transmission line and whose output can be taken at a potential near ground potential with the aid of an optocoupler interface. This, however, requires a separate power supply which is tied to the potential of the transmission line. The supply current from the transmission line cannot be used because in the event of an interruption no current flows there and only a very small current may be consumed in a path parallel to the line. Another problem is that any fairly linear, temperature-independent measurement using optocouplers can only be implemented with complex compensation circuits. In addition, if active components, such as transistors, which necessarily form part of an ADC circuit, are used at an elevated potential, the so-called latch-up problem arises, which, if the supply voltages rise above or fall below the rated values even by small amounts, may result in thermal destruction of the components.
CH 676 393 A5 discloses a circuit arrangement for converting an analog measurement voltage into an isolated digital signal in which the measured voltage is coupled out near ground potential via an optoelectronic coupling element consisting of a light-emitting diode (LED) and a phototransistor tied to the potential of the transmission line. The voltage measurement itself is made by means of a capacitor and a circuit which controls the charging and discharge of the capacitor. Such a capacitor, however, is necessarily quite voluminous. In addition, its discharge curve follows an exponential function, which impairs the accuracy of the measurement.
The above-cited EP 0 398 456 B1 discloses a circuit arrangement for the isolated sensing of binary electric signals of different pulse heights which includes an optocoupler that is connected in series with an electronic circuit module to the go and return conductors of the circuit arrangement. This circuit arrangement can be operated without clock pulses but has the disadvantage of being unsuitable for measuring analog signals because of the optocoupler's high degree of nonlinearity. It also requires a high-potential constant-current source for the optocoupler, which causes the above-mentioned latch-up problem.
The above-cited prospectus of Hewlett Packard also describes circuit arrangements with pairs of optocouplers in which the analog measured values are obtained near ground potential by comparing the two signals provided by the two optocouplers, since the individual optocouplers themselves deliver only analog signals. This, however, presents new problems because of the different aging rates of the two optocouplers.
SUMMARY OF THE INVENTION
The object of the invention is to improve a circuit arrangement of the above kind by the simplest possible means in such a way that as few active components as possible, particularly no analog-to-digital converters and no power supplies, are necessary at the potential of the transmission line, and that the measured voltage and current values are related to voltages and currents on the transmission line as linearly as possible.
According to the invention, this object is attained with respect to the voltage measurement by connecting a series combination of a transformer, a first optically controllable, clocked switching element, and a first resistor into a shunt branch between the go conductor and the return conductor.
With respect to the current measurement, the object of the invention is attained by connecting a second resistor into the go conductor or the return conductor and shunting the second resistor by a series combination of a transformer and a second optically controllable, clocked switching element.
The circuit arrangements according to the invention solve the above problems by “chopping” the voltages by means of the respective optically controllable, clocked switching elements. The clock frequency from a grounded clock generator which is optically coupled to the high-voltage side will, as a rule, be of the order of 1 kHz. In the secondary of the transformer, an alternating voltage proportional to the respective measurement signal is thus induced which can be taken as an isolated voltage, for example at ground potential. It is only necessary to provide suitable isolation between the primary and secondary of the transformer.
An analog-to-digital converter is not required in the circuit arrangements according to the invention. A power supply tied to the potential of the transmission line is not necessary, either. Thus, by minimizing the active components on the high-voltage side, the danger of latch-up, among other things, is significantly reduced. Finally, the voltage measurement is possible with a smaller load than in the prior art, because a simple resistor is used instead of a network.
In a preferred embodiment of the circuit arrangement according to the invention, an isolated voltage measurement and an isolated current measurement can be performed simultaneously.
In a preferred embodiment of this combined circuit arrangement, the same transformer is provided for the voltage measurement and for the current measurement. In this manner, the need for one of the two expensive high-voltage-isolated transformers is eliminated.
To be able to take off a DC signal for the desired measured quantities, in a further embodiment of the invention, a rectifier device is provided at the secondary of the transformer.
In a development of this embodiment, the rectifier device comprises at least one diode, in the simplest case a diode connected between the two secondary outputs of the transformer.
In a preferred development of the above embodiments, a constant-current source is provided at the primary of the transformer for linearizing the characteristic of the rectifier circuit.
In this manner, a greater dynamic range can be opened for the measurement. While in the prior art, voltages between the go conductor and the return conductor could only be taken off in the range between 50 V and 350 V, with the use of a constant-current source at the primary of the transformer, the voltage values taken from the secondary may vary between 20 V and 350 V.
In a particularly preferred embodiment of the combined circuit arrangement according to the invention for making isolated voltage and current measurements, instead of the second optically controllable, clocked switching element connected in parallel with the second resistor, a switch is provided which can connect the end of the first optically controllable, clocked switching element remote from the transformer to the end of the first resistor facing the transformer in a first switch position or to the end of the second resistor remote from the transformer in a second switch position. This eliminates the need for
Alcatel
Le Dinh T.
Ware Fressola Van der Sluys & Adolphson LLP
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