GB2069205A - A device for use in an electrical circuit and comprising two separable parts - Google Patents

Abstract

A device for use in an electrical circuit comprises a first part 1 including a switch 4 and a Zener diode 15 (or some other device the impedance of which falls greatly when the voltage across it exceeds a threshold value) connected in series with the switch between two conductors 16 and 17. The device also comprises a second part 2 including a circuit element 23, for example a heat or smoke detector, which is connected between said conductors 16 and 17 when the first and second parts 1 and 2 are assembled together, the switch 4 then being open. When the first and second parts 1 and 2 are separated, the circuit element 23 becomes no longer connected between said conductors 16 and 17 and the switch 4 closes. The switch may be, as shown, a jack switch 4 or a transistor switch. Several such devices may be connected in parallel and to a monitoring circuit which produces a signal when the first and second parts 1 and 2 of any one of the devices are separated, to indicate a fault condition. <IMAGE>

Description

SPECIFICATION Devices constructed for electrical circuits According to a first aspect of the invention, a device constructed for inclusion in an electric circuit comprises first and second separable parts, the first part including first and second electrical conductors interconnected by a current path including, in series with one another, a switch and a first circuit element the impedance of which falls greatly when the voltage across it rises above a critical value, the second part of the device including a second circuit element, the device being such that when the first and second parts are assembled together the switch is open and the second circuit element is connected to said conductors and when the first and second parts are separated the second circuit element is disconnected from said conductors and the switch is automatically closed so that if a sufficiently high voltage is present between said conductors, the first circuit element adopts its low-impedance state.

According to a second aspect of the invention, there is provided a monitoring circuit comprising a plurality of devices according to the first aspect of the invention with their first conductors connected together and with their second conductors connected together and means for producing a signal all the time any one of the first circuit elements is in its low-impedance state.

In the accompanying drawings: Figures land 2 show diagrammatically two different devices in accordance with the first aspect of the invention, and Figure 2 shows a monitoring circuit of a fire-alarm system, in accordance with the second aspect of the invention.

Figure 1 shows a device comprising a first part 1 and a second part 2. A first conductor 16 in the first part 1 interconnects terminals 5 and 6 and a second conductor 17 in the first part 1 interconnects terminals 7 and 8. Conductors 18 and 19 of a cable are connected to the terminals 5 and 7 and, indirectly, to the positive and negative terminals, respectively, of a battery. Conductors 20 and 21 of another cable are connected to the terminals 6 and 8 and to terminals 5 and 7 of another device like that shown in Figure 1. A switch 4 in the first part is in series with a Zener diode 15 in a current path which interconnects the terminals 5 and 7 and thus the conductors 16 and 17.

The switch is a jack socket and spigot arrangement, of which contacts 13 and 14 are held apart by a spigot 3 projecting from a base member 22 of the part 2 when the parts 1 and 2 are assembled together and this prevents the voltage between the conductors 18 and 19 being applied across the Zener diode, which therefore remains in its high-impedance state irrespective of the voltage between the conductors 18 and 19. When the parts 1 and 2 are assembled together, a circuit element 23, diagrammatically represented by the rectangle, in the second part 2, is connected, via conductors 24 and 25, pins 11 and 12 and sockets 9 and 10 to the terminals 6 and 8 and thus to the conductors 16 and 17.When the parts 1 and 2 are separated, the circuit element 23 is disconnected from the conductors 16 and 17 and the switch 4 is automatically closed so that the voltage between the conductors 18 and 19 is applied across the Zener diode 15 which adopts its low-impedance state if the voltage is above the critical voltage of the Zener diode.

Figure 2 shows a somewhat similar device, of which parts corresponding to those in Figure 1 are correspondingly numbered. Instead of a mechanical switch 4 in series with the Zener diode, there is an electronic switch in the form of a transistor 27, the base-collector path of which is bridged by a one megohm resistor 26 in the first part of the device.

The base of the transistor is also connected, via a socket 33 in the first part and a pin 34 in the second part and via the conductor 25, the pin 12, the socket 10 and the terminal 8, to the conductor 17. When the parts 1 and 2 are assembled, the transistor is non-conductive and when the parts 1 and 2 are separated the transistor is rendered conductive by the change in its base potential and the Zener diode adopts its low-impedance state if the voltage between the conductors 18 and 19 is high enough.

The circuit element 23, in each of the two examples described above, could be a device which receives an electrical signal from the conductors 18 and 19 and in response to it produces, at the device comprising the parts 1 and 2 or near it, a sensible indication, for example a visible and/or an audible warning signal. The circuit element 23 could instead be a device that develops an electrical signal, for example a voltage or a change of its impedance, that is transmitted to a distant signalling means where it produces a sensible indication, for example a visible and/or an audible warning signal. Examples of the latter kind of circuit element 23 are a smoke-detector and a heat-detector.

Instead of a Zener diode, there could be employed a voltage-dependent resistor (for example of the silicon type) or a neon gas-filled lamp or any other device the impedance of which falls greatly when the voltage across it rises above a critical value.

Instead of the pin and socket connections, there could be connections via leaf springs between the parts 1 and 2.

The jack socket and spigot arrangement of Figure 1 could be replaced by a micro-switch, a reed relay and magnet or any other moving-contact switch.

Figure diagrammatically shows three devices 40, 41 and 42, each as shown in Figure 1 or Figure 2, connected to each other as described above and each including a smoke-detector or a heat-detector as its circuit element 23. There would usually be far more than three such devices in the monitoring circuit. The conductors 18 and 19 of the device 40 are connected to terminals 51 and 52 and the conductors 20 and 21 of the device 42 are connected to opposite terminals of a resistor 55, for example of 2,700 ohms, which could, however, be replaced by a Zener diode.

A positive terminal 53 of a 24-volt battery is connected, via a voltage regulator VR and a resistor R1, for example of 47 ohms, to the terminal 51 and the negative terminal 54 of the battery is connected via a resistor R2, for example of 330 ohms, to the terminal 52. The terminal C of the voltage regulator is always at a constant potential with respect to the terminal 54 and normally a constant current flows in the path R1, 18,55, 19, R2, giving a constant voltage drop across the resistor R2. This voltage drop causes the transistor TR1 to be in the conductive state, which causes the transistor TR2 to be in the non-conductive state, which prevents warning lamps L1 being energised.

The circuit elements 23 in their quiescent state are of very high resistance and so they do not signficantly affect the conditions in the circuit. If the resistor 55 becomes disconnected, or if the conductive path between it and the terminal 51 or 52 is broken, the voltage drop across the resistor R2 becomes zero, transistor TR1 is rendered non-conductive, this ren- ders transistor TR2 conductive and so lamps L1 are energised.

In the event of the part 2 of one of the devices 40, 41 and 42 becoming separated from its part 1, the Zener diode 15 of that device receives a voltage higher than that required to put it in the lowimpedance state (for example 18 volts compared to 15 volts) so that it goes into that state. The constant voltage between terminals C and 54 is now redistributed, the voltage between terminals 51 and 52 falling and the voltage drop across the resistor R2 rising to such a value that the Zener diode Z1 is caused to conduct and base current for the transistor TR3 flows through the Zener diode Z1 and the resistor R3.The transistor TR3 becomes conductive and allows current to pass through diodes D7 and D8 and contacts 61 and 62 and lamps L1, although a separate lamp could be energised instead so that energisation of that lamp always signifies the separation of the parts 1 and 2 of one of the devices 40,41 and 42, as distinct from the disconnection of the resistor 55 or a break in one of the conductive paths between it and the terminals 51 and 52. In the illustrated case, when the transistor TR3 conducts a light-emissive diode LED1 emits light. If the parts 1 and 2 of one or both of the other devices 40,41 and 42 are separated, its Zener diode or their Zener diodes does not or do not change to the lowimpedance state, so there is substantially no effect on the part of the circuit shown to the left of terminals 51 and 52.The fault indication persists all the time any one or more of the devices 40,41 and 42 has its parts 1 and 2 separated.

In the event of one of the circuit elements 23 detecting heat or smoke, whether or not one or both of the other elements 23 has or have come out of circuit due to the separation of the parts 1 and 2 of the device or devices concerned, the impedance of the circuit element which has detected the heat or smoke falls to such an extent that the current which then passes through the path R1, 18, the circuit element 23 that has detected the heat or smoke, 19, R2 is higher than the current that passes through the resistor R2 in any of the circumstances described above, so the voltage drop across the resistor R2 becomes higher than it is in any of the circumstances described above and is high enough to render Zener diode Z2 conductive, which causes transistor TR4 to conduct. This causes a relay RL1 to operate to disconnect the negative potential from the emitters of the transistors TR1, TR2 and TR3, by opening of the contacts 61 and 62, so that any indications of separation of the parts 1 and 2 of one of the devices 40,41 and 42 or of disconnection of the resistor 55 are cancelled. The relay RL1 closes its contacts 62 and 64, so that the relay coil remains energised through diode D3, and devices L2 are energised, through diode D2, to indicate a fire.

Claims (12)

1. A device constructed for inclusion in an electrical circuit, the device comprising first and second separable parts, the first part including first and second electrical conductors interconnected by a current path including, in series with one another, a switch and a first circuit element the impedance of which falls greatly when the voltage across it rises above a critical value, the second part of the device including a second circuit element, the device being such that when the first and second parts are assembled together the switch is open and the second circuit element is connected to said conductors and when the first and second parts are separated the second circuit element is disconnected from said conductors and the switch is automatically closed so that, if a sufficiently high voltage is present between said conductors, the first circuit element adopts its low-impedance state.

2. A device according to Claim 1, in which the first circuit element is a Zener diode.

3. A device according to Claim 1 or 2, in which the switch is a jack socket and spigot arrangement, two contacts of which are held apart by a spigot projecting from said second part when said first and second parts are assembled together but automatically close when said first and second parts are separated.

4. A device according to Claim 1 or 2, in which the switch is a transistor the base of which is directly connected to the emitter via said second part when said first and second parts are assembled together but adopts a potential different from that of the emitter, making the emitter-collector path conductive, when said first and second parts are separated.

5. A device according to any preceding claim, in which said second circuit element is one which, in response to an electrical signal which it receives, produces a visible andior an audible warning signal.

6. A device according to any one of Claims 1 to 4, in which said second circuit element is a smokedetector or heat-detector which develops an electrical signal, for transmission to a distant signalling means, when it detects smoke or heat.

7. A device constructed forinclusion in an electrical circuit, the device comprising first and second separable parts of which the first part includes first and second conductors interconnected by a switch and a first circuit element, the device being substantially as described above with reference to Figure 1 or Figure 2 of the accompanying drawings.

8. A monitoring circuit comprising a plurality of devices according to any preceding claim with their first conductors connected together and with their second conductors connected together and means for producing a signal all the time any one of the first circuit elements is in its low-impedance state.

9. A monitoring circuit according to Claim 8, including a series circuit comprising a source of direct current, a resistor and said devices connected in parallel with each other and with a high impedance device, there being means for monitoring the voltage drop across said resistor and for producing said signal at a warning device when the voltage drop rises due to one of the first circuit elements changing to its low-impedance state.

10. A circuit according to Claim 9, wherein in the event of said high impedance device becoming disconnected or said series circuit being otherwise broken, so that said voltage drop falls to zero, a signal is produced at said warning device or at another warning device.

11. A circuit according to Claim 9 or 10, in which in the event of the second circuit element in one of said devices exhibiting a sufficiently great fall in impedance, the current through said resistor is higher than it is when the first and second parts of said device are separated and the resultant higher voltage drop across said resistor results in a signal at a further warning device.

12. A monitoring circuit substantially as described above with reference to Figure 3 of the accompanying drawings.