GB2205459A

GB2205459A - Electric current monitoring

Info

Publication number: GB2205459A
Application number: GB08812402A
Authority: GB
Inventors: Geoffrey William Candy
Original assignee: MIDLAND AUTOMATION Ltd
Current assignee: MIDLAND AUTOMATION Ltd
Priority date: 1987-05-30
Filing date: 1988-05-25
Publication date: 1988-12-07
Anticipated expiration: 2008-05-25
Also published as: GB8712740D0; GB8812402D0; GB2205459B

Abstract

Apparatus for monitoring a change of current flow in an electric supply line L includes a current transformer CT, rectifier R and smoothing circuit for deriving an input signal representative of the current flowing in the supply line, a pair of capacitors C1, C2 which are alternately supplied with a voltage derived from the input signal by means of a clocked switching device S1, and a further switching device S2 whereby the voltages appearing on the capacitors are sampled at predetermined intervals of time and fed to a differential amplifier A4 which is arranged to produce an output signal in response to a change in the input signal. The signal may indicate failure of apparatus being supplied, or a requirement for an increased voltage, eg, to ensure proper striking of additional fluorescent lights. <IMAGE>

Description

Title: "Electric Current Monitoring" Description of Invention This invention relates to apparatus for monitoring electric current in a supply line and detecting a change in such current.

There are many circumstances in which a change in the magnitude of an electric current in a supply line may be used to trigger a control or other function. For example, for energy conservation purposes, fluorescent lighting may be run at a relatively low voltage after starting at normal mains voltage, and in the supply to such a lighting installation it is necessary to restore the supply voltage to its normal full value whenever additional lights on the same supply are switched on in order to ensure that they start properly, but to date no entirely reliable form of monitoring for this purpose appears to have been devised.

Other circumstances where a change in current flow may be signalled include the protection of electrical equipment from unauthorised use or adjustment.

In accordance with the invention, we provide apparatus for monitoring electric current in a supply line to provide an output signal indicative of a change in current flow, said apparatus comprising a current transformer, means for deriving from said current transformer a smoothed input signal representative of the current flowing in said supply line, sampling means for monitoring said input signal at predetermined intervals of time, comparator means for comparing successive values of said input signal and for producing an output signal whilst a difference between successive sample values of said input signal continues to be detected.

The current transformer may be connected to a load, across which a full wave rectifying and smoothing circuit is connected to provide said input signal. Preferably, a linear rectifier circuit is employed, together with R.C.

smoothing.

The input signal thus produced may be sequentially switched to one of two or more capacitors by said sampling means so that each capacitor in succession carries a voltage which is dependent on the value of the input signal, which in turn is representative of the current flowing in the associated supply line. The voltages appearing across each of two successive capacitors can then be applied to the comparator means to determine whether the value of the input signal changes during the sampling intervals.

Conveniently, the comparator means may take the form of a differential amplifier arranged to produce an output signal in response to inputs from the capacitors varying by more than a predetermined degree.

The output from the differential amplifier may be in the form of a pulse fed to any suitable output device and in particular may serve as a control signal to operate ancilliary equipment.

The invention will now be described by way of example, with reference to the accompanying drawing which shows a schematic circuit diagram of an apparatus in accordance with the invention.

The coil of a current transformer CT associated with a supply line L is connected across a load resistor Rl so as to produce across the resistor a sinusoidal voltage representative of the current flowing in the supply line L.

A rectifier R, preferably of the kind known as a linear rectifier, produces a full-wave rectified output which is applied to an R.C. smoothing circuit to produce a smoothed input signal which is applied to a unity gain amplifier Al, the output of which is applied alternately to capacitors CI and C2 at predetermined clock intervals.

For this purposes, the circuit includes an electronic clock 10 which, in the illustrated embodiment comprises four NAND gates interconnected in such a way as to produce the timing wave forms A, B, C and D as shown.

Wave forms A and B determine the sampling interval and wave forms C and D which effect sampling are supplied to a solid state switching device Sl to operate switching elements indicated diagrammatically at 11 and 12.

The arrangement is such that wave forms C and D briefly close switches I I and 12 alternately at the sampling interval corresponding to wave forms A and B. In this way, the voltage appearing across input resistor R2 at the instant of successive samplings is supplied from the unity gain amplifier A1 to the capacitors C1 and C2 in succession. Accordingly, the voltages appearing for the time being across these capacitors are representative of the voltages established across the input resistor R2 at the two most recent samplings.

The voltages appearing across the capacitors C1 and C2 are supplied by way of unity gain amplifiers A2 and A3 to a differential amplifier A4, whereby any difference in the voltage is amplified to produce an output which is supplied to a Schmitt trigger circuit A5.

A further solid state switching device 52 is incorporated into the circuit either between the capacitors Cl and C2 and the unity gain amplifiers A2 and A3, as shown, or between these amplifiers and the differential amplifier A4.

The device 52 incorporates four switching elements represented at 13, 14, 15 and 16 which operate as a two-pole change-over switch. The arrangement is such that switching elements 13 and 15 are closed when switching elements 14 and 16 respectively are open, whilst switching elements 14 and 16 are closed when switching elements 13 and 15 respectively are open. Switching between the two states is accomplished by means of wave forms A and 13, in such a manner that alternately first switching elements 13 and 15 are closed at the same time so as to connect C1 to A2 and C2 to A3 simultaneously and then switching elements 14 and 16 are closed at the same time so as to connect C2 to A2 and C I and A3 simultaneously.

This arrangement ensures that the same-amplifier (A3 as illustrated) always receives the voltage which appeared on resistor R2 at the instant of the most recent sampling, whilst the other amplifier (A2 as illustrated) always receives the voltage which appeared on resistor R2 at the instant of the previous sampling, regardless of which capacitor (CI or C2) is storing the most recent value. As a result, although the most recent value determined by successive samplings is stored alternately on capacitor Cl and capacitor C2, that value is first applied to amplifier A3 and then transferred to amplifier A2 at the instant of the next sampling.

When a constant current is flowing in line L, the voltage appearing across the input resistor R2 will remain substantially constant and capacitors Cl and C2 will both carry the same voltage so that the output of the differential amplifier A4 will be zero. The operational characteristics of the differential amplifier A4 can be chosen or varied to provide the required degree of sensitivity in order to produce an output signal from the Schmitt trigger only in response to a predetermined voltage difference at the capacitors Cl and C2 so as to allow for minor variations in the current in the supply line.

If the current in line L increases between two successive samplings, the output from amplifier A3 will increase at the instant of the next sampling and for the duration of the sampling interval the differential amplifier A4 will produce a positive output since the output of amplifier A2 will be at the previous output level of amplifier A3. If the current stabilises to a new equilibrium value within the sampling interval, the next value of voltage on resistor R2 will remain the same, and at the next sampling the output of the amplifiers A2 and A3 will again be equal, so that there is zero output from the differential amplifier A4. The positive pulse output from the differential amplifier, if of sufficient magnitude to activate the Schmitt trigger A5, will then produce an output signal indicative of the increase in current in line L.

Conversely, if there is a drop in current, with the circuit as illustrated, the differential amplifier A4 will produce a negative pulse output, which will not actuate the Schmitt trigger.

It will be appreciated that the output of the Schmitt trigger is of a binary nature, having one value when the trigger is set and another when it is reset. Thus, the presence of-the positive output signal signifies an increase in current flowing in line L without being dependent on, and without providing an indication of, the magnitude of that change. Any slight drift in the value of the current will not cause a response, because the difference between successive samplings of the voltage across the input resistor will be too small to cause the output of the differential amplifier A4 to deviate from its quiescent level to the extent necessary to trip the Schmitt trigger.

For example, the quiescent output of amplifier A4 may be set at about half line voltage. This may drift somewhat due to slow changes in the current as monitored by the current transformer, but when there is a significant change in the current, the output voltage from A4 will rise significantly and trip the Schmitt trigger.

Thus, as applied to the monitoring of an electric light installation, the increase in current resulting from additional lights being switched on will be established in less than the sampling interval (typically about 0.5 second) so that the first sampling which takes place after such change will produce the required output pulse, which can be utilised to cause an increase in the supply voltage on line L to be maintained for a predetermined period of time sufficient to ensure proper striking of any lights just switched on.

In the circuit as illustrated, the switches Sl- and S2 are so phased that an increase in the voltage appearing across resistor R2 results in a positivegoing output pulse from differential amplifier A4, so that the circuit monitors for an increase in current in line L. However, by changing the phasing of either switch Sl or switch S2, the differential amplifier A4 could be arranged to provide a positive-going output pulse in response to a decrease in the voltage appearing across resistor R2.

Where the apparatus is used to monitor the current supply to a set of fluorescent lights the output from the Schmitt trigger in response to an increase in current may be used to increase the line voltage for a predetermined period of time sufficient to ensure that when additional fluorescent lights are switched on, so causing the increase in current, a higher voltage will be maintained on the supply line for a sufficient period of time to enable such lights to strike properly.

However, the apparatus may also be used in other applications where it is required to monitor for a decrease in current in the line L, for example so as to produce a warning of failure of electrical apparatus drawing current through line L.

Morever, if the value of current flowing in line L continues to increase or decrease over a period of time corresponding to several samplings, the output pulse from differential amplifier A4 will be maintained whilst such change continues, so that the circuit can be used to monitor for such circumstances also.

Claims

CLAIMS:

I. Apparatus for monitoring electric current in a supply line to provide an output signal indicative of a change in current flow, said apparatus comprising a current transformer, means for deriving from said current transformer a smoothed input signal representative of the current flowing in said supply line, sampling means for monitoring said input signal at predetermined intervals of time, comparator means for comparing successive values of said input signal and for producing an output signal whilst a difference between successive sampled values of said input signal continues to be detected.

2. Apparatus according to Claim I wherein the current transformer is connected to a load, across which a full-wave rectifying and smoothing circuit is connected to provide said input signal.

3. Apparatus according to Claim 2 wherein a linear rectifier circuit is employed, together with RC smoothing.

5. Apparatus according to any one of the preceding claims wherein the input signal is sequentially switched to one of two or more capacitors by said sampling means so that each capacitor in succession carries a voltage which is dependent on the value of the input signal when sampled.

6. Apparatus according to Claim 5 wherein the voltages appearing across each of two successive capacitors are applied to the comparitor means to determine whether the value of the input signal changes during the sampling interval.

7. Apparatus according to Claim 6 wherein the comparitor means comprises a differential amplifier arranged to produce said output signal in response to inputs from the capacitors varying by more than a predetermined degree.

8. Apparatus according to any one of the preceding claims wherein the output from the differential amplifier is in the form of a pulse for feeding to an output device.

9. Apparatus according to Claim 8 further including means for supplying said output pulse as a control signal to operate ancilliary equipment.

10. Apparatus according to Claim 9 wherein said output signal is generated in response to a detected increase in the current flow in the supply line and is arranged to cause the voltage on the supply line to be increased for a predetermined period of time.

11. Apparatus according to Claim I and substantially as hereinbefore described with reference to and as shown in the accompanying drawing.