GB2237114A

GB2237114A - Transient signal isolator

Info

Publication number: GB2237114A
Application number: GB8921492A
Authority: GB
Inventors: Emmanuel Cohen
Original assignee: MANCHESTER LASERS
Current assignee: MANCHESTER LASERS
Priority date: 1989-09-22
Filing date: 1989-09-22
Publication date: 1991-04-24
Also published as: GB8921492D0

Abstract

A transient signal isolator comprises a magnetic field sensor (2) in a gap in a core (1), a coil connected to a transmission line (not illustrated) carrying measurement signals to be monitored, and a further coil (4) connected to an AC current supply. The AC current signal (in coil 4) is effective to wipe out any remanent magnetism in the core gap resulting from transient voltages which are to be isolated e.g. due to lightning strikes on the transmission line. Thus the isolator does not cause the decalibration of the system of which it forms a part. A filter (6) filters out signals at the frequency of the AC current. <IMAGE>

Description

TRANSIENT SIGNAL ISOLATOR The present invention relates to a transient signal isolator for isolating an electrical circuit from potentially damaging transient voltages.

Sensitive electrical equipment is often connected to signal transmission lines on which low amplitude slowly varying signals are to be monitored by that equipment. If the transmission lines are exposed for example to electromagnetic effects resulting from lightening strikes, short rise time and high voltage transients can be induced on the lines. In such cases, it is necessary to be able to isolate the sensitive electrical equipment from transients on the transmission lines without impairing the ability of the equipment to monitor a low amplitude low rise time signal of interest.

It is known to provide isolators which rely upon the use of a Hall element positioned in a magnetic circuit provided with an energising coil to which signals to be monitored are applied. Such an arrangement provides electrical isolation between an input signal applied to the coil and an output voltage developed by the Hall element. Many examples of such circuits have been proposed, for example those described in British Patent Specification GB-A-1,488,775 and GB-A2,098,340. GB-A-1,488,775 specifically refers to the use of a Hall element and an associated magnetic circuit to provide electrical isolation between an input current and an output voltage, it being stated that such isolation is sometimes an advantage.

One application for such devices is in the water industry. In that industry, water levels and the like are remotely sensed to provide managers of the system with the information necessary to control correctly the distribution of water from a number of sources which might be many miles apart. Transducers are provided for example at individual reservoirs to monitor the water level therein. The transducers provide slowly varying DC output signals which are transmitted over long underground or overheated transmission lines to a central control station. If lightening strikes the ground in the vicinity of such a line, large short duration voltage transients can be induced in the line. If these transients are not isolated from monitoring equipment positioned at the control station the system can be disabled for prolonged periods.It is accordingly necessary to provide some means for isolating the equipment at the control station from such transients to ensure continuous effective water management.

British Patent Specification GB-A-2,213,943 describes the use of a Hall element in an isolator suitable for the purposes outlined above. This outlines the benefits that can be obtained using such a transient isolator. The conventional circuit described in GB-A-2,213,943 is of a very simple nature however and although it provides the required transient isolation it is not satisfactory in many circumstances as the output of the Hall element can be decalibrated by the transient signals which it is intended to isolate. The problem is that transient signals of relatively high voltage but short duration can drive the magnetic core of the device into saturation and as a result the remanent magnetism in the gap of the magnetic circuit after the transient has died away can result in the output of the Hall element being decalibrated.Essentially the remanence causes the output of the Hall element to be displaced so that the calibration of the device is altered. The output of the Hall element is thus no longer a true measure of the signal transmitted to the isolator by the relevant transducer and accordingly the system operator cannot rely upon the output of the Hall element as a basis for system management decisions.

It is an object of the present invention to provide a transient signal isolator which obviates or mitigates the problems outlined above.

According to the present invention there is provided a transient signal isolator comprising a magnetic core defining a gap, a magnetic field sensor positioned in the gap, a coil wound on the core, the coil being connected to a transmission line carrying measurement signals to be monitored, and means for monitoring the output of the magnetic field sensor to provide an output representative of signals on the transmission line, wherein a further coil is wound on the core, and an AC current supply circuit is connected to the further coil to drive an AC current therethrough, the amplitude and frequency of the AC current being such that an AC flux is generated in the core which is greater than the coercive force of the material from which the core is fabricated.

The AC current signal is effective to wipe out any remanent magnetism in the core gap resulting from transient voltages which are to be isolated. Thus the isolator of the present invention does not cause the decalibration of the system of which it forms a part.

Preferably a filter is provided on the output of the Hall element to remove signals at the frequency of the AC current supply circuit. Thus the core can be demagnetised without any superimposed AC signal being apparent at the output of the isolator.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic illustration of an embodiment of the present invention; Fig. 2 is an illustration of the remanent magnetic effects which can be expected with a conventional core material; and Fig. 3 is an illustration or a circuit in accordance with the present invention which can overcome remanence effects.

Referring to Fig. 1, the illustrated arrangement comprises a toroidal core 1 of high permeability materials such as 3%Si-Fe grain oriented strip wound material with a high saturation flux density. The core defines a gap within which a Hall element 2 is positioned and supports a first winding 3 and a second winding 4. The first winding 3 is connected to a source of signals which are to be transmitted through the device, for example a transmission line extending to a remote transducer. The second core 4 is connected to an AC voltage source.

The Hall element 2 provides an output signal to an amplifier 5 the output of which is applied to a filter 6 which filters out signals at the frequency of the AC signal applied to the coil 4. The filtered output is in turn applied to a further amplifier 7 the output of which is delivered to suitable monitoring equipment (not shown).

The AC signal applied to the coil 4 has a frequency and amplitude sufficient to generate within the core 1 an AC flux which is greater than the coercive force of the core material.

Thus remanence effects resulting from AC transients applied to the coil 3 are suppressed. The output of the Hall element 2 does of course include a signal appropriate to the current passing through the coil 3 upon which is superimposed a signal corresponding to the AC current through the coil 4. The filter 6 is effective however to remove signal components resulting from the current through the coil 4 and therefore the output of the amplifier 7 is a true representation of the signal applied to the coil 3.

By way of further explanation, Fig. 2 illustrates remanence effects which are intended to be removed in accordance with the present invention. Curves 8 and 9 illustrate hysteresis effects which could be expected in a complete (ungapped) core. Because the core 1 is provided with a gap the hysteresis loop is sheared to the curves 10 and 11 in Fig. 2. Thus the desired high degree of linearity between the exciting current and the resultant flux is obtained even with very low DC currents applied to the coil 3 of for example a few milliamperes. The curves 10 and 11 do however indicate that DC transients applied to the coil 3 will, depending upon their direction, affect the remanent magnetic field in the gap once those transients have decayed. The remanent magnetism is indicated by the spacing between the lines 10 and 11 and the origin of the graph shown in Fig. 2.The present invention is concerned with the removal of this remanent magnetism.

Referring now to Fig. 3, this illustrates a circuit which can be connected to the Hall element 2 and the coil 4 of Fig. 1 to achieve the desired effects. The circuit of Fig.

3 can be considered in block form as an input section 12 incorporating a Hall element, an amplifier 13, a filter 14, a threshold setting circuit 15 the output of which is independent of the polarity of the output of the Hall element, a voltage to current converter 16, a power supply 17, an oscillator 18, and an output amplifier 19 the output 20 of which is connected to the coil 4 of Fig. 1. The voltage to current converter 16 provides an output 21 which corresponds to the output representative of the current passing through the coil 3 of Fig. 1.

In more detail, an amplifier 22 supplies current to the Hall element 2 such that an appropriate Hall element output is generated which is applied to an amplifier 23. An amplifier 24 and associated circuitry constitutes a Sallen Key active filter. The output of the amplifier 24 is applied to the threshold circuit which comprises amplifiers 25 and 26.

The output of the amplifier 26 is converted to a current signal by amplifier 27 and transistor 28. The required output signal is thus developed at output 21. The circuit elements 18 and 19 constitute respectively a conventional oscillator circuit and a conventional power amplifier. The frequency of the oscillator may be selected to be for example 50Hz.

During normal operation, a magnetic field is created in the gap within which the Hall element 2 is positioned which is representative of the current signal applied to coil 3 (Fig. 1). If a high voltage, rapid rise transient strikes the transmission line connected to coil 3, a high amplitude current can flow for a short time through the coil 3 in addition to the normal relatively small current which is to be monitored. The physical separation of the coil 3 and the Hall element 2 ensures that the transient signal does not damage any sensitive equipment connected to the Hall element. The transient current through the coil 3 could produce a very high output voltage from the Hall element but this will not occur providing the core 1 is of high permeability as the magnetic field is limited due to saturation of the core.

After the passage of a transient the magnetic state of the core is not neutral but is of some non-zero value corresponding to the remanence. The effects of remanence are however removed by the superimposed AC magnetic field caused by the AC signal applied to the coil 4. Thus excursions from zero of the signal appearing at output 21 will always be relative to zero flux density.

Claims

CLAIMS:

1. A transient signal isolator comprising a magnetic core defining a gap, a magnetic field sensor positioned in the gap, a coil wound on the core, the coil being connected to a transmission line carrying measurement signals to be monitored, and means for monitoring the output of the magnetic field sensor to provide an output representative of signals on the transmission line, wherein a further coil is wound on the core, and an AC current supply circuit is connected to the further coil to drive an AC current therethrough, the amplitude and frequency of the AC current being such that an AC flux is generated in the core which is greater than the coercive force of the material from which the core is fabricated.

2. A transient signal isolater according to claim 1, comprising a filter connected to the output of the Hall element to remove signals at the frequency of the AC current supply circuit.

3. A transient signal isolater substantially is hereinbefore described with reference to the accompanying drawings.