GB2172132A - Power line continuity monitoring circuit - Google Patents

Abstract

A device for detecting unauthorised disconnection of a power consuming apparatus from a power supply point is interposed between the power supply point (A,B) and the consuming apparatus (C,D), and comprises: (a) means (4) for detecting a flow of current from the device, through the power consuming apparatus, and back to the device (b) means (1,2) for regulating the flow of current from the supply point via the device to the consuming apparatus such that in a first condition the normal consumption of the apparatus is allowed and under a second condition a reduced current sufficient for continuity monitoring but insufficient to operate the apparatus is allowed (c) means (8) for supplying said reduced current (or a substitute therefor) from the device in the event of power failure at the supply point (d) means (9,10) for generating an alarm signal if the current measured in (a) above ceases. <IMAGE>

Description

SPECIFICATION Power line continuity monitoring circuit The present invention relates to a circuit for monitoring power supply line continuity for a piece of electrical or electronic equipment.

Incidents of theft of electrical and electronic equipment, both domestic and commercial, increases every year. The value of such equipment may be high and its protection is becoming increasingly important.

Security systems which detect the entrance of the unauthorised person into a defined volume or area, for example, a room or an entire building, are known. However, these systems are generally complex and expensive. Furthermore, such systems may be ineffective/inappropriate in for example, areas open to the public e.g. shops and exhibition during opening hours and hospitals.

Devices for monitoring specific items of equipment are known. Such devices detect the movement of equipment and have a built-in alarm. One such device for protecting video tape recorders, is adapted for insertion in the video tape housing socket of such recorders.

Movement detection devices of this kind have the disadvantage that they must be disarmed and removed to permit the equipment protected to be used or moved.

An object of a first aspect of the present invention is to provide a circuit capable of detecting unauthorised removal of electrical/electronic equipment requiring an external power source, irrespective of whether the equipment is in operation and which permits movement of that equipment within the limit imposed by its power supply cable.

According to the present invention there is provided a circuit for monitoring power supply line continuity for a piece of electrical/electronic equipment which circuit is adapted for connection to the power supply for said equipment and comprises means for detecting a flow of current through said equipment and means enabling supply of a current to the equipment which is insufficient to operate the equipment.

Thus, the present invention is capable not only of monitoring the continuity of a power supply line to the circuit but also that from the circuit to the equipment.

In a preferred embodiment, the production of current insufficient to operate the equipment is achieved by means of an impedance element connected in series with the power supply. This impedance element may be short circuited by means of a switch connected in parallel with it to permit operation of the equipment. Thus, switching on and off of the equipment is achieved while maintaining a flow of current through the equipment at all times.

In a preferred embodiment of the present invention, the circuit comprises an impedance for connection in series with the power source and the equipment to be protected. A voltage detector monitors the voltage drop across this impedance and compares this with a set value. Should this voltage drop below the set value, the voltage detector emits a signal to operate, for example, an audible alarm.

The nature of said impedance is such that during high current consumption i.e. when the equipment is in operation, the loss of power supply voltage is insignificant. Nevertheless, at low current levels i.e. that insufficient to operate the equipment, the voltage drop across this impedance must be sufficient to be measurable. In one embodiment of the present invention, this impedance comprises two diodes connected in parallel with each other for conduction in opposite directions, and with a resistor. In an alternative embodiment, the impedance consists of an inductor or a transformer which at the current levels required for operation of the equipment,is at magnetic saturation so that the power supply loss is insignificant.

In a preferred embodiment of the present invention, a signal generator is provided for supply of current to the power supply network. In the absence of power from the power supply, this injection of current permits detection of current flow through the equipment to continue and hence differentiation between a power failure and disconnection of the power supply to the equipment.

In a preferred embodiment of the present invention, means are provided to increase the sensitivity of current detection during power supply failure. This has the advantage of reducing the power supply necessary from a stand-by power source, for example, a rechargable battery for the signal generator.

The circuit preferably comprises switch means for disabling the circuit. Such switch means may be operable by means of a key or code unique to this switch or having a low probability of selection. The code may exist, for example, in the physical shape of a key or, alternatively, by the setting of switches or buttons in a predetermined combination or time sequence.

The circuit further comprises alarm means for producing an audible and/or visual alarm signals. The alarm generation means and associated drive circuitry, may be incorporated within the same housing as said circuit and/or may be provided at a remote location. In the latter case, signals from the circuit may be transmitted to the alarm device by means of radio frequency, infrared, microwave or ultrasonic radiation or by direct electrical connections or via mains cable wiring.

An object of a further aspect of the present invention is to provide a circuit capable of detecting unauthorised removal of electrical/electronic equipment requiring an external power source, which circuit is capable of differentiating between a power failure and disconnection of the equipment from the power source.

According to this further aspect of the present invention there is provided a circuit for monitoring the continuity of a power supply line for a piece of electrical/electronic equipment, which circuit is adapted for connection to the power supply line and comprises means for monitoring power supply from said power supply line and a signal generator for supply of current to the power supply network for detection in the absence of power from the power source.

A preferred embodiment of the present in vention ,comprises a unit adapted to be plugged into a mains socket and having a socket adapted to receive a plug connected to the power supply cable of the equipment to be protected. This embodiment permits simple connection and disconnection of the circuit of the present invention.

Embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings in which: Fig. 1 is a biock diagram of an embodiment of the present invention, and Fig. 2 is a circuit diagram of a preferred embodiment of the present invention.

Figure 1 shows a circuit having input terminals A and B for connection to a power supply source and output terminals C and D for connection to the equipment to be protected.

A switch 1 connected in parallel with an impedance element 2 is located in series with terminals A and C and provides for switching on and off of the equipment.

Impedance eiements 3 and 4 are connected in series with terminals D and B. Element 3 has a high impedance to low voltage signals and a low impedance to large supply currents so that during operation of the equipment, i.e.

with switch 1 closed, the voltage drop across this impedance element is small.

Impedance element 4 may have one of two impedance characteristics. In the presence of power supply voltage on terminal A the impedance of element 4 is such as to limit the voltage drop across this element. In the absence of such voltage supply, this impedance attains a higher value, as will be described later.

A voltage comparator circuit 9 filters the voltage across element 4 and compares this with a threshold value. Should this voltage not exceed the threshold value continuously or intermittently within a predetermined period, this circuit will emit an alarm signal which, in the present embodiment, is fed to an alarm indicator 10.

Circuit means 5 are connected to terminal A for detecting the presence/absence of supply voltage at terminals A and dictate the impedance characteristic of impedance element 4.

In the present embodiment, these means 5 are also connected for supply of charging current to a local power source 6, i.e. a rechargable battery, which is connected across power rails E and F for supply of power to the voltage comparitor circuit 9.

A signal generator 8 is also connected across power rails E and F. This generator supplies a "low power" stimulation voltage signal to terminal B downstream of impedance element 3.

A switch 7 is provided in power rail E which when closed connects power from the local source 6 across the generator 8 and circuit 9 thereby arming the circuit. Arming switch 7 may be, for example, a switch operated by a physical key or through the setting of a combination of switches or buttons and/or their operation in a specific time sequence.

Impedance element 2 has a characteristic such that when switch 1 is open, the current supply from terminal A to the equipment connected across terminals C and D is insufficient to produce an output from the equipment but sufficient to supply a current via terminal D to provide a voltage across element 4 greater than the threshold voltage. Where the power supply to be connected across terminals A and B is alternating, element 2 may comprise a capacitor, whereas for a DC power supply, this element may comprise a resistor. Thus, the combination of switch 1 and element 2 permits the equipment to be switch on and off at will while maintaining a current signal at all times across impedance element 4.

The operation of the circuit shown in Figure 1 will now be described.

When supply power voltage is present across terminals A,B, this is detected by circuit means 5 which determine the impedance presented by element 4 accordingly. Circuit 9 compares the voltage drop across element 4 with a threshold value. Should this voltage not exceed the threshold value continuously or intermittently within a predetermined period, as will occur in the case of a power line discontinuity between terminals C, D and the equipment, circuit 9 will send an output signal to operate alarm indicator 10.

In the absence of power supply voltage across terminal A + B, either through disconnection of the power supply from the circuit or a power failure, circuit means 5 will cause the impedance of element 4 to increase. Provided the power supply and equipment are still connected across terminals A, B and C, D respectively, i.e. in the case of a simple power failure, the low level stimulation voltage supplied by generator 8 to terminal B will cause current to flow from this terminal via impedance connected across the power supply source to terminal A, via switch 1 or impedance element 2 to terminal C, through the equipment being monitored to terminal D and from there to the local reference rail F via the impedance element 4.Since the impedance of element 4 has been increased, the voltage comparitor circuit 9 will detect a voltage in excess of the threshold value and no alarm signal is generated. However, if the connection between terminals A and B or terminals C and D has been broken, then the voltage detected across impedance element 4 will be below the threshold value causing circuit 9 to generate an alarm switching signal to operate alarm indicator 10.

Figure 2 shows a circuit diagram of a preferred embodiment of the circuit illustrated in Figure 1. This circuit is intended for connection at terminals L1, N1 and El to the live, neutral and earth terminals, respectively, of a 240 Volt AC mains supply and at terminals L2, N2 and E2 to the live, neutral and earth mains terminals, respectively, of the equipment to be protected. The line connecting the terminals El, E2 plays no part in the electrical operation of the circuit but is provided so as to maintain electrical safety.

Mains current flows to the equipment being monitored from terminal L1 via either a manually operable switch SW1 or capacitor C1 to terminal L2 and from the equipment via terminal N2 and networks 4 and 3 to terminal N1.

Switch SW1 is closed for supply of normal mains current to the equipment and this flows through the circuit with only an insignificant voltage drop. During this period the voltage across network 4 is monitored and compared with a threshold value. When the equipment being monitored is not in use, switch SW1 is open and capacitor C1 appears in series with the mains supply to the equipment, reducing the mains current to a low value. This low value of current is insufficient to operate the equipment, but sufficient to enable the voltage detected across network 4 to be in excess of the threshold value.

A diode D1 and resistors R1, R2 form a half wave battery charging circuit which maintains a trickle charge to a rechargeable battery 6 via networks composed of diode D2, transistor TR1 and resistor R3, and transistors TR2, diode D3 and resistor R4. D2 is a light emitting diode which can be by-passed by transistor TR1 and which when lit, indicates to the operator that the battery 6 is charging and when flashing, that the circuit is armed to monitor power line continuity. A network consisting of a resistor R3 and transistor TR2 forms a detector circuit which indicates the presence of a mains supply voltage.Whilst the mains supply is present, transistor TR2 is turned on which causes a transistor TR3 and hence transistor TR4 to turn on effectively placing resistor R11 across the remainder of network 4 i.e. diodes D9, D10, resistor R12 and capacitor C3. A network comprising diode D3 and resistor R4 is present to prevent the battery 6 discharging via resistor R3 and transistor TOR 1 when the monitoring circuit is not in use.

Network 4 constitutes a variable impedance across which a voltage is generated in response to passage of current through the equipment being monitored. When mains power is present, transistor TR4 is on and resistor R11 will determine the detection sensitivity. Diodes D9 and D10 provide a low impedance path for large mains currents and prevent a significant drop in the mains voltage. In the absence of mains power, transistor TR4 is off and resistor R12 now establishes the detection threshold i.e. impedance appropriate to a low value stimulation current which is induced in the mains supply network to produce a voltage across network 4 in excess of thepredetermined threshold. Capacitor C3 provides a low impedance path for high frequency currents which could otherwise cause false continuity indication.

Network 3 comprises two pairs of series connected diodes D5, D6, D7, D8 arranged in parallel for conduction in oppoisite directions.

This network provides a low impedance path for high mains supply currents whilst allowing the injection of a low voltage stimulation signal in series with the mains supply. Such signal injection is employed during mains power failure to cause a current to flow in the cable for detection at network 4.

A diode D11 and transistor TR5 form a voltage detector which monitors the voltage across network 4. Transistor TR5 is periodically switched on during the presence of a mains current supply. When turned on, transistor TR5 turns on transistor TR6 and discharges capacitor C4. So long as a mains current is periodically detected, the voltage across capacitor C4 will be at a low value. If the power supply line either from the power supply or to the equipment is broken, resistor R13 will charge capacitor C4 to the voltage of the battery 6.

Transistors TR7 and TR8 and diode D12 and resistors R14, R17, R16, R18, R15 and R20 form a voltage comparitor circuit which causes transistors TR7 and TRS to turn on if the voltage across capacitor C4 exceeds a given threshold value and will remain latched on even if the voltage across capacitor C4 subsequently falls to a low value.

A network comprising transistors TR11, TR12 and capacitors C6, C7 and resistor R25, R26, R27, R28 forms a square wave oscillator of frequency approximately 5Hz. This oscillator has three functions. Firstly, to provide a waveform which is passed as a stimulation signal via transistors TR14, TR15, TR16 and diode D15 and resistors R29, R30, R31 and capacitor C8 to terminal N1 for the purpose of inducing a small current in the mains cable when mains power is absent at terminal L1.

Secondly, to provide a means to switch the light emitting diode D2 on and off via transistors TR13, TR1 and resistors R32, R33.

Thirdly, to cause an audible alarm to be switched on and off by means of diodes D13, D14 and transistors TR9, TR10 and resistors R21 and R22.

In the event of a loss of supply line continuity, transistors TR7, TR8 will become latched on and transistors TR9, TR10 will turn on and off at approximately 5 Hz. The collector of transistor TR10 feeds a secondary power rail Z which is alternately switched to the battery voltage Y and allowed to float to a local zero volt rail X at 5Hz.

Transistors TR18, TR19 and capacitors C10, Cii and resistors R38, R39, R40, R41, VR1 form a square wave oscillator of frequency approximately 3 KHz. This circuit is fed from the secondary power rail Z so as to cause the 3 KHz oscillator to be pulsed on and off at 5 Hz. A network formed by capacitor C12 and resistors R42, R43, R44, R45, R46 and transistors TR20, TR21 TR22 is connected to the collector of transistor To19.

During one phase of the oscillation anarrow pulse is generated causing a transistor TR22 to be turned off for approximately 30,z secs.

A network comprising a capacitor C9 and resistors R34, R35, R36, R37 and transistors TR17, TR23 is connected to the collector of transistor TR18. During the alternate phase of the oscillation, a narrow pulse is generated causing a transistor TR23 to be turned on for approximately 50 it secs.

While transistor TR22 is on, the current through an inductor L1 and resistor R48 will grow to a value of approximately 100 mA.

When transistor TR22 is turned off, the energy in inductor L1 will be transferred via diodes D16, D17 to a network consisting of capacitor C13 and piezo electric sounder S1, with the result that the voltage change across sounder S1 will be approximately 100 volts.

When the transistor TR23 is briefly pulsed on, the network of capacitor C13 and sounder S1 will be discharged. Consequently, the sounder will receive a square wave signal of approximately 100 volts peak to peak amplitude at approximately 3 KHz and the audible signal so generated provides an alarm signal indicating a break in the continuity of the supply line to the equipment and hence unauthorised removal of the equipment being monitored.

A switch SW2 provided in the secondary power rail Z permits disconnection of power supply from battery 6 to this rail, allowing disarming of the monitoring circuit for disconnection of the equipment. The switch SW2 is operated by a coded key so that only keyholders may operate it.

Claims (18)

1. A circuit for monitoring power supply line continuity for a piece of electrical/electronic equipment, which circuit is adapted for connection to the power supply for said equipment and comprises means for detecting a flow of current through said equipment and means enabling supply of a current to the equipment which is insufficient to operate the equipment.

2. A circuit as claimed in claim 1, wherein said means enabling supply of a circuit comprise an impedance element connected in parallel with a short circuit switch for connection in series with the power supply.

3. A circuit as claimed in claim 1 or 2, wherein the current flow detection means comprise impedance means for connection in series with the power source and the equipment to be protected and a voltage detector for monitoring the voltage drop across said impedance means.

4. A circuit as claimed in claim 3, wherein the current flow detection means are adapted for switching between two sensitivity levels in dependence on whether power supply voltage is present at the input of the circuit.

5. A circuit as claimed in claim 3,wherein said impedance means comprise a resistor connected in parallel with two parallel connected diodes arranged for conduction in opposite directions.

6. A circuit as claimed in claim 5, wherein said impedance means further comprise a second resistor and a transistor connected in series with each other across the first mentioned resistor, the second resistor having a resistance lower than that of the first and said transistor being conductive in the presence of supply power voltage across the input to the circuit.

7. A circuit as claimed in any preceding claim, comprising a signal generator for supply of current to the power supply network for detection by the current flow detection means.

8. A circuit as claimed in any preceding claim further comprising coded switch means for disabling the circuit.

9. A circuit for monitoring the continuity of of a power supply line for a piece of electrica I/electronic equipment, which circuit is adapted for connection to the power supply line and comprises means for monitoring power supply from said power supply line and a signal generator for supply of current to the power supply network for detection in the absence of power from the power source.

9. A circuit as claimed in any preceding claim, further comprising alarm indicator means.

10. A circuit as claimed in any preceding claim adapted to provide an alarm signal at an output in the event of a loss of supply line continuity, for transmission to remote alarm indicator means.

11. A circuit as claimed in claim 10 wherein said circuit is adapted to generate at said output, radiation of one of the following frequencies: radio; infrared; microwave, and ultra sonic.

12. A circuit as claimed in any of claims 1 to 8 adapted for supply of an alarm signal to remote alarm indicator means in the event of a loss of supply line continuity, via mains cable wiring.

13. A circuit for monitoring power supply line continuity for a piece of electrical/electronic equipment substantially as herein described with reference to Figure 1 with or without reference to Figure 2 of the accompanying drawings.

14. A circuit for detecting the unauthorised removal of a piece of electrical/electronic equipment, comprising the circuit as claimed in any preceding claim.

15. A unit for monitoring power supply line continuity for a piece of electrical/electronic equipment, comprising a circuit according to any of claims 1 to 13.

16. A unit as claimed in claim 15 adapted to be plugged into a mains supply and having a socket adapted to receive a plug connected to the power supply cable of the equipment.

17. A unit as claimed in claim 15 or 16, comprising integral alarm indicator means.

18. A device for detecting the unauthorised removal of a piece of electrical/electronic equipment, constituted by the unit as claimed in any of claims 15 to 17.