GB2195846A

GB2195846A - Detecting disconnection of electrical apparatus

Info

Publication number: GB2195846A
Application number: GB08624245A
Authority: GB
Inventors: Michael John Walker
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1986-10-09
Filing date: 1986-10-09
Publication date: 1988-04-13
Also published as: GB8624245D0

Abstract

An electrical apparatus which has a lead for connection to a remote station is provided with a detector to ascertain when the lead is disconnected. An oscillator, based on an operational amplifier (20), includes a positive feedback capacitor (21) and a tuned circuit (22, 23) connected across the amplifier inputs. A transformer (22, 24) forming part of this tuned circuit couples the oscillator output to the lead through a capacitor 25. With this arrangement, changes in the effective impedance of the lead cause corresponding variations in the frequency and/or amplitude of the oscillator output. A window detector (30, 34, 32) measures amplitude variations across the tuned circuit (22, 23) so as to indicate, eg. by an audible alarm, disconnection of the lead. <IMAGE>

Description

SPECIFICATION Electrical apparatus having a lead This invention relates to an electrical apparatus having a lead through which the apparatus receives power, or through which the apparatus receives or sends signals.

There is often a requirement to be able to determine automatically whether the lead is connected to an associated power supply, e.g. a battery, a d.c. supply, or an a.c. supply, or to determine whether the lead is connected to some other piece of equipment.

For example, it may be desired to give a warning if a piece of medical life support equipment is disconnected from its power supply accidentally, even though the equipment contains its own back-up battery, and it is desirable for the automatic warning device to be able to distinguish between such accidental disconnection and, for example, a mains supply failure which will normally be temporary.

Similarly there are situations in which it is required to detect whether a lead connecting a computer to a peripheral is properly connected.

It is an object of the invention to provide an electrical apparatus which incorporates a detector which can distinguish between disconnection (or severance) of a lead connected to the apparatus and other fault conditions.

It has been discovered by the applicant that if an oscillatory signal is injected into the lead via a suitable coupling means it is possible to measure effects resulting from the change in lead impedance which arise when the lead is disconnected or severed.

Accordingly, the invention provides an electrical apparatus having a lead for connection of the apparatus to a remote station, and including an oscillator for producing an oscillating signal, coupling means coupling the output of the oscillator to said lead, said coupling means providing d.c. isolation between the lead and the oscillator and being such that when the oscillator is operating, changes in the effective impedance of the lead result in corresponding changes in the amplitude of the oscillatory signal injected into the lead and impedance sensitive means responding to such changes in the effective impedance of the lead.

In its simplest form, the invention may be a fixed frequency, fixed amplitude oscillator and the coupling means would then include a series impedance. The actual amplitude of the signal injected would then be measured by said impedance sensitive means.

Alternatively the oscillator and coupling means may be such that changes in the effective impedance of the lead causing corresponding changes in the frequency and/or amplitude of the oscillator output signal. This corresponding change would be measured by the impedance sensitive means.

The oscillator and coupling means may be such that the oscillator does not operate when the effective impedance of the lead is below a predetermined limit. The impedance sensitive means would then be sensitive to the operating state of the oscillator.

Preferably, the oscillator is constituted by an operational amplifier which has a positive feedback capacitor connected between its non-inverting input terminal and its output terminal and a parallel tuned circuit connected across its inverting and non-inverting input terminals, the tuned circuit including a transformer which also forms part of the coupling means.

When this arrangement is employed, the impedance sensitive means may comprise a voltage window detector connected to detect when the voltage across the tuned circuit is between upper and lower predetermined levels (one of which is preferably adjustable), an averaging circuit connected to the output of the window detector and a voltage comparator for comparing the output of the averaging circuit with a reference level. This arrangement provides excellent noise rejection.

In the accompanying drawings: Figure 1 is a circuit diagram of a simple example of an electrical apparatus in accordance with the invention; Figure 2 is a circuit diagram of an oscillator and coupling means suitable for use in examples of the invention; Figure 3 is a circuit diagram of another example of the invention; Figure 4 is a detailed circuit diagram of an oscillator used for tests described herein; Figure 5 is a graph showing the results of such tests; Figure 6 is a graph like Fig. 5, but showing test results for a slightly modified oscillator; and Figure 7 is a circuit diagram showing another embodiment of the invention.

Referring firstly to Fig. 1, the apparatus is designated by reference numeral 10. This apparatus may be any electrical apparatus, such as a television set, burglar alarm, computer or any device which has to be connected to the a.c. mains, to a d.c. power supply or to some other piece of equipment by a lead 11. Accordingly no internal details of the apparatus are shown, but only the elements which are added to the apparatus in accordance with this example of the invention.

These added elements comprise an oscillator 12, coupling means 13 and a detector 14.

Coupling means is connected to inject oscillatory signals into the lead and, as shown, includes a transformer with a primary winding 15 and a secondary winding 16. The primary winding 15 is connected to the oscillator output and the secondary winding is connected by a capacitor 17 and a resistor 18 in series to the lead 11. The oscillator is designed to produce an oscillating signal of a frequency which is several orders of magnitude higher than the frequency of signals normally likely to be present on the lead 11. For example, if the lead 11 is an a.c. mains supply lead, the os cillator could operate at a frequency in the range of, say 50KHz to 10MHz. The capaci tance of capacitor 17 is chosen appropriately to pass the injected signals from the oscilla tors but to block d.c. and mains frequency a.c.The transformer provides complete isola tion of the oscillator from the lead 11.

The-effect of the resistor 18 is to provide a high source impedance so that the amplitude of the oscillatory signals injected into the lead will be dependent on the effective impedance of the lead. In the case of an a.c. supply lead this effective impedance will result from a combination of effects including other equip ment connected to the lead, capacitive and inductive coupling of the lead and ground, the load represented by the power source, and power distribution devices such as meters, fuses etc.By careful setting of the level at which the detector 14 operates it can be ar ranged that whenever the lead is connected to the a.c. mains system, the effective impe dance of the lead is sufficiently low to ensure that the detector does not operate, even if there is no power actually on the mains because a remote switch or fuse is open circuit, whilst ensuring that the detector will operate if the lead is disconnected, the impedance then rising to a level determined exclusively by the apparatus itself and the lead.

Although the embodiment described above is described in relation to the detection of the connection of an a.c. mains lead, it is to be understood that it is also applicable to other leads, and in fact, it is more suitable for uses in a "clean" environment, i.e. an environment in which little or no noise is present on the lead.

Turning now to Fig. 2, the oscillator and coupling means shown therein are particularly suitable for use in embodiments of the present invention. The oscillator comprises an operational amplifier 20 which has a positive feedback capacitor 21 connected between its output terminal and its non-inverting input terminal. A parallel circuit is connected between the inverting and non-inverting inputs of the amplifier. This tuned circuit consists of a transformer winding 22 in parallel with a capacitor 23. The inverting input terminal is connected to one end of the winding 22 and the non-inverting input terminal is connected to a tapping on the winding. The secondary winding 24 of the transformer is connected by a series capacitor 25 to the lead.

With an oscillator of this configuration, both the frequency and the amplitude of the oscillator output (which may be regarded as being the voltage across the tuned circuit) vary in accordance with the effective impedance of the lead. When this impedance is reduced the amplitude and the frequency will both be reduced, so that the amplitude of the signal injected into the lead is reduced.

By'choice of suitable component values it can be arranged that the oscillator does not operate at all when the effective impedance of the lead is below a certain level. This feature can usefully be employed in applications where it is desirable to avoid injecting signals into the lead when the lead is properly connected. When a connection is severed the oscillator starts to run and this can be detected either by detecting the presence of the oscillatory signal at the oscillator output, or by detecting the increase in current flowing to the oscillator when it starts to run. This mode of operation is, however, subject to noise interference and can only be employed in "clean" environments.

The preferred use of the oscillator/coupling means is shown in Fig. 3. Here a lead impedance sensitive circuit is used which makes use of a voltage window detector, an average circuit and a voltage comparator. The voltage window detector includes two voltage comparators 30 and 31. One end of the tuned circuit 22, 25 is connected to the non-inverting input of comparator 30 and to the inverting input of comparator 31. A resistor chain R1, R2, R3, R4 is connected between the positive and negative d.c. supply rails for the circuit and provides a series of bias voltages.

The highest bias voltage, that at the junction of resistors R1 and R2, is applied to the noninverting input of comparator 31. The voltage at the junction of resistor R2 and R3 is applied to the inverting input of comparator 20 and that at the junction of resistor R3 and R4 is applied to the non-inverting input of a further voltage comparator 32. The resistor R4 is a potentiometer and its slider is connected to the inverting input of comparator 30. The outputs of the two comparators are connected to an AND gate 33. The output of the gate 32 is connected to an R-C averaging circuit 34, 35 the output of which is connected to the inverting input of the comparator 32.

In use the output of the gate 33 is high only when the voltage across the tuned circuit is more than the voltage across the resistor R2, but less than the voltage across the series combination of resistor R3 and the upper part of the resistor R4. In fact, in many applications, the resistor R2 can be omitted (replaced by a link), so that comparator 31 operates as a simple zero-crossing detector. The fractional time in each oscillation cycle during which the output of comparator 31 is high is then independent of amplitude and frequency and equal to half the cycle period. The fraction time for which the output of comparator 30 is high is dependent on amplitudes since the voltage across the tuned circuit varies sinusoidally with time.As the amplitude increases the waveforms from the outputs of the two comparators 30, 31 become almost identical antiphase waveforms, so that two short pulses are produced by gate 33 in each cycle. At smaller amplitudes the duration of these pulses increases and at very small amplitudes the two pulses in each cycle may merge since the output of comparator 30 may not go low at all.

The voltage at the output of the averaging circuit thus rises as the amplitude of the oscillatory signal across the tuned circuit falls as a result of falling lead impedance.

In use, therefore, the potentiometer can be set so that the output of the averaging circuit is low enough to drive the output of comparator 32 high when the lead is disconnected but high enough to drive the output of comparator 32 low when the lead is properly connected irrespective what other loads are connected to the mains or of whether a remote fuse has blown or a remote switch has been opened.

Referring now to the circuit shown in Fig. 4, the oscillator shown corresponds closely to that included in both Figs. 2 and 3. Additional components shown include a load resistor R5 connecting the output of amplifier 20 to the +V rail (the amplifier 20 is of the type having an open collector output). A smoothing capacitor circuit 40, 41 connects the inverting input of amplifier 20 to ground and two diodes D1, D2 are connected in parallel in opposite directions across the transformer secondary winding. These diodes limit the effect of spikes etc. on the mains on the oscillator, while not substantially affecting the relatively low level output of the oscillator.

Fig. 5 is a graph in which the solid line depicts the effect of variations of the mains impedance (at the operating frequency of the oscillator) on the amplitude of the signal at the point A in Fig. 4 (the left hand scale being appropriate to this line). The dotted line shows the effect of mains impedance variation on the frequency of the oscillator output.

It will be noted that the oscillator ceased to operate when the mains impedance fell below about 8 ohms. Increasing the mains impedance gradually, causes the oscillator to restart at about 30 ohms impedance. Both the frequency of the amplitude vary over substantially the whole mains impedance range and either can therefore be used to detect changes in the mains impedance.

For the test, the results of which are shown in Fig. 6, a resistor of 82k ohms resistance was included in series with the feedback capacitor 21. The effect of this is to reduce the amplitude of the oscillator output by a factor of about eight at all impedance levels, while changing the frequency curve so that this is substantially flat at mains impedances about 500R and falls steadily below that impedance.

With this arrangement the oscillator stops if the mains impedance falls to about 30R and restarts when the impedance is above about 60R.

It has been found in practice that, for an oscillator frequency in the range 400-500 kHz, the mains impedance can vary between about 50R (when there is a heavy load) to about 600R (when there is only a very light load). Typically values of 200 to 300R are encountered.

Turning now to Fig. 7, the embodiment of the invention shown therein includes a rather different form of oscillator. This makes use of a "555" type CMOS timer integrated circuit 60 instead of an operational amplifier. This change permits the oscillator to be more distant from the transformer. A resistor R,o connects the "lower" end of the transformer primary winding to the OUTPUT pin of the integrated circuit 60, and another resistor R" (of approximately half of the resistance value) connects the OUTPUT pin to the TRIGGER and DISCHARGE pins, both of which are grounded by a capacitor.

The "top" end of the primary winding 22 is connected to the junction of two diodes 62, 63 in series between the supply rails. The signal at this "top" end of the primary winding is a substantially sinusoidal wave with a frequency of about 450kHz and a swing between about 0.2V and 6V, when the oscillator is running and the secondary winding 24 of the transformer is open circuit. The amplitude of the signal falls as the impedance of the mains circuit at this frequency falls. The output of the OUTPUT pin of the i.c. 60 is a square wave with an amplitude of about 6V and a 1:1 mark:space ratio.

The "top" end of the primary winding is connected, as mentioned above to the junction of the diodes 62, 63 and this junction is connected, in turn to the non-inverting input of an operational amplifier 64. This input is connected by two resistors R,2, R,3 and a variable resistor R,4 in series to the ground rail and the junction of the resistors R,2, R,3 is connected to the non-inverting input and is also connected by a capacitor 65 to ground.

Another operation amplifier 66 has its inverting input connected to the junction of two resistors R,5, R,6 connected in series between the supply rails, a capacitor 67 being connected in parallel with resistor R,6. The noninverting input of amplifier 66 is connected to the OUTPUT pin of circuit 60.

The amplifiers 64, 66 have their open-collector outputs connected together and by a resistor R,,which effectively serves the same purpose of the AND gate in Fig. 3. These command outputs are connected by a resistor R,8 and a diode 68 in parallel to the non inverting input of an output operational amplifier 69, such input being also connected by a capacitor 70 to ground. The inverting input of the amplifier is connected to the junction of resistors R,s and R,6.

The circuit described operates in a manner very similar to that shown in Fig. 3. Thus when the outputs of amplifiers 64 and 66 are simultaneously high for a sufficient fraction of the oscillator cycle the output of the output amplifier goes low, signifying "disconnection".

The amplifiers 64 and 66 are both d.c. connected to the output of the timer i.c. 60 and tracking errors which can occur in the Fig. 3 embodiment are thereby avoided.

Mains transient protection is provided by the diodes 62, 63. Resistors R,8 and capacitor 70 serve both to provide a smoothing function and to delay a "disconnected" output being produced until about 5 seconds after disconnection occurs, thereby avoiding spurious triggering by intermittent disconnection. The diode 68 charges capacitor 70 rapidly at switch-on.

An inhibiting input to the non-inverting input of amplifier 69 is provided via a resistor R20 for connection to a rectified a.c. supply within a piece of equipment being monitored. This inhibits the warning device when the equipment is actually operating.

Many variations of the basic principle of the invention are possible. For example the coupling transformer may be constructed to permit its secondary winding to be connected inseries with the mains lead, rather than across it. The oscillator/coupling means/detector combination may be built into a power consuming device, such as a television set, or into a power supplying device, such as a time switch or d.c. power supply. The combination may also be built into a completely independent piece of apparatus with which the lead is connected, such apparatus serving soley to detect if one or other end of the lead is disconnected.

In every case, of course, the lead impedance sensitive means may be connected to an audible alarm device which is powered by a battery built into the apparatus. Then, whenever the battery powered circuit is energised, the audible alarm device is actuated when the lead is disconnected.

Claims

1. An electrical apparatus having a lead for connection of the apparatus to a remote station, and including an oscillator for producing an oscillating signal, coupling means coupling the output of the oscillator to said lead, said coupling means providing d.c. isolation between the lead and the oscillator and being such that when the oscillator is operating, changes in the effective impedance of the lead result in corresponding changes in the amplitude of the oscillatory signal injected into the lead and impedance sensitive means responding to such changes in the effective impedance of the lead.

2. Apparatus as claimed in claim 1 in which said oscillator is a fixed frequency, fixed amplitude oscillator and said coupling means includes a series impedance, said impedance sensitive means comprising detector means for detecting the amplitude of the signal injected into said lead.

3. Apparatus as claimed in claim 1 in which the oscillator and the coupling means are such that changes in the effective impedance of the lead cause corresponding changes in the frequency and/or amplitude of the oscillator output signal, such corresponding changes being detected by said impedance sensitive means.

4. Apparatus as claimed in claim 1 in which said oscillator and said coupling means are such that the oscillator does not operate if the effective impedance of the lead is below a predetermined limit, said impedance sensitive means detecting whether or not the oscillator is operating.

5. Apparatus as claimed in claim 1 in which said oscillator comprises an operational amplifier which has a positive feedback capacitor connected between its non-inverting input and its output and a parallel tuned circuit connected across its inverting and non-inverting inputs, the tuned circuit including a transformer which also forms a part of said coupling means.

6. Apparatus as claimed in claim 5 in which the impedance sensitive means comprises a voltage window detector connected to detect when the voltage across the tuned circuit is between upper and lower predetermined levels, an averaging circuit connected to the output of the window detector circuit and a voltage comparator for comparing the output of the averaging circuit with a reference level.

7. Apparatus as claimed in claim 6 in which one of said predetermined levels is adjustable.

8. Apparatus as claimed in claim 1 in which said oscillator comprises an integrated circuit timer device connected to provide a square wave output, a tuned circuit including a capacitor and a transformer primary winding, which is included in the coupling means, to which said square wave output is applied so as to provide a substantially sinusoidal wave form across said tuned circuit, the amplitude of which is dependent on the effective impedance of the lead.

9. Apparatus as claimed in claim 8 in which said impedance sensitive means comprises means for synchronously detecting levels of the square wave output of the sinusoidal waveform.

10. Electrical apparatus substantially as hereinbefore described with reference to Figs.

1 and 2 of the accompanying drawings.

11. Electrical apparatus substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.

12. Electrical apparatus substantially as hereinbefore described with reference to Fig. 4 of the accompanying drawings.

13. Electrical apparatus substantially as hereinbefore described with reference to Fig. 7 of the accompanying drawings.