GB2205457A - Electrical surge diverter unit - Google Patents

Abstract

The diverter comprises a three electrode gas discharge tube DT1 connected in parallel with zener diodes ZD1, ZD2 across input lines, the junction between zener diodes ZD1 ZD2 being connected via a diode D1 to earth and to an electrode of tube DT1. The input lines are connected to output lines via a capacitor C1 and a 250mA fuse FS1 having a resistance of 1 ohm or less. In an alternative embodiment, (Figs 2, 3), the components are assembled on a printed circuit board, the fuse FS1 is replaced by a direct connection, and the earth connection is established via a point contact between sections (26, 28) of printed conductor. The unit may be integrated into a conventional coaxial aerial socket box or formed as a device to be plugged into such a socket. <IMAGE>

Description

ELECTRICAL SAFETY BARRIER SPECIFICATION This invention relates to electrical safety barriers which provide protection against current surges, such as from lightning strikes. The invention is particularly concerned with barrier devices and circuits suitable for protecting VHF and UHF install-ations and apparatus, hi-fi equipment, telephone equipment and other electrical appliances against the effects of lightning strikes.

Television sets and hi-fi installations are particularly susceptible to the effects of lightning strikes, the energy of which is transmitted from the aerial, through the socket and cable to the equipment itself. In the event of a thunderstorm many people will disconnect their television sets or other equipment from the mains supply, but leave the aerial cable connected to the socket.

It is an object of the present invention to provide a safety barrier by means of which the energy of a lightning strike is dissipated within the socket or within an adaptor fitted to the socket, so that the television set or other apparatus is effectively shielded from the effect of the phenomenon.

According to one aspect of the invention there is provided an electrical safety barrier device for protecting electrical appliances against current surges, comprising electronic circuit means- comprising means to allow current flow from an input to an output, means to absorb the current flow in the event of a current surge, and means to dissipate the current surge other than through the output.

In accordance with one preferred embodiment of the present invention the safety barrier comprises a discharge tube arranged to absorb the energy of the strike, diode means connected in circuit with the discharge tube, and fuse means which allows signals to pass through the barrier under normal use but which will isolate equipment connected to the output side of the barrier in the event of a current surge.

It is an important feature of this embodiment of safety barrier that the fuse means should have an extremely low resistance, preferably less than 1 ohm.

A fuse having a rating of 250 mA and a 1 ohm resistance is preferred.

It is a particular advantage of a second embodiment of the present application that it can deal with multiple strikes without damage or degradation. The safety barrier of this embodiment does not require the replacement of a fuse in the event that the barrier is subjected to a current surge.

A conventional coaxial cable comprises an inner conductor wire which carries the signal, and an outer conductor in the form of a mesh which functions as a screen. Electrically insulating material separates the inner and outer conductors, and a tough sheathing layer is provided around the outer screen conductor. It is already known to connect the outer screen to earth or ground, but it has never heretofore been possible to earth the inner, signal-carrying conductor, because one would thereby lose the signal.

It is therefore an object of this embodiment of the present invention to provide a safety barrier in which, in the event of a current surge, both inner and outer conductors are protected by having a path to earth. It is considered to be broadly novel to earth both conductors of a coaxial cable in one unit.

In accordance with another aspect of the invention there is provided an electrical safety barrier device for connecting a coaxial cable to an output and to protect an electrical appliance connected to said output from current surges transmitted along said cable, the device comprising a pair of input terminals for connection to inner and outer conductors respectively of said cable, a pair of output terminals for connection to an appliance, means normally providing separate conductive paths from one of said input terminals to one of said output terminals and from the other of said input terminals to the other of said output terminals, and means operative to isolate both said output terminals from said input terminals in the event of a current surge on said cable.

It is crucial to the satisfactory operation of this embodiment of the safety barrier that the contact between said zones should be the smallest possible point contact, in order to minimise signal loss under normal operating conditions.

It will also be appreciated that both conductors of the coaxial cable are connected to earth at the same point on the printed circuit board.

The necessary point contact is advantageously achieved by having an element of the printed circuit tapering to a point, with said point being in minimum contact with the zone connected to earth.

Preferably, the tapering of the said element is symmetrical with respect to the axis of that element, so that the point of the element is central with respect to the element.

Preferably, the element which is tapered to a point defines a rectilinear track to the point from its connection to the electrical circuitry. It has been established that if this element is bent or curved or in any way non-linear, then the barrier is rendered less effective and/or signal loss is increased.

Presently preferred embodiments of electrical safety barrier in accordance with the invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a schematic circuit diagram of a first embodiment of electrical safety barrier for use against the effects of lightning strikes; Fig. 2 is a schematic circuit diagram of a second embodiment of electrical safety barrier for use against the effects of lightning strikes; and, Fig. 3 shows a printed circuit board embodying the safety barrier of Fig. 2, and with a coaxial cable connected thereto.

The electrical safety barrier of Fig. 1 can either be integrated into a conventional aerial socket, or alternatively can be formed as a plug-in unit to be fitted to a conventional aerial socket.

As shown in Fig. 1, the safety barrier circuit is connected between an input I/P connected to an aerial and an output O/P connected to a television set, hi-fi equipment or the like. A gas discharge tube DT1 is connected across the input, and a pair of Zener diodes ZD1 and ZD2 is connected in parallel across the discharge tube. A conventional diode D1 is connected between the discharge tube and the junction between the back-to-back Zener diodes. A capacitance Cl is connected across the Zener diodes ZD1, ZD2 to act as a smoothing component to prevent degradation of the VHF or UHF signal.

In the transmission line between input and output there is connected a fuse FS1. Under normal use the fuse must allow the signals to pass through the safety barrier, but upon the occurrence of a surge at the input the fuse must operate to isolate the output from the input. Because of the nature of the VHF and UHF signals, it is necessary that the fuse FS1 should have an extremely low resistance, not greater than- 1 ohm.

Preferably a fuse having a rating of 250 mA and a resistance of not more than 1 ohm should be used.

The general circuit diagram of the electrical safety barrier of the second embodiment is very similar to that of the first embodiment. In the second safety barrier, as shown in Fig. 2, a gas discharge tube DT1 or its equivalent is connected, across the inner and outer conductors 10 and 12 respectively of the coaxial cable which bears the incoming signals. The output terminals 14 and 16 for the inner conductor and outer conductor respectively may take the form for example of a conventional wall-mounted socket into which a television set, hi-fi equipment, or the like is connected. A pair of Zener diodes ZD1 and ZD2 is connected in parallel across the discharge tube. A diode D1 is connected between the discharge tube and the junction between the back-to-back Zener diodes. A capacitance C1 is connected into the outer conductor circuit in order to prevent signal degradation or loss.

A 33 pF silver mica capacitor has been found to be particularly suitable. This particular component satisfactorily covers all existing European band widths. An earth connection, indicated by line 18, is connected to a point between the discharge tube and the diode D1. It is this earth connection 18 which is the crucial part of this embodiment of the invention and which is shown in more detail in Fig. 3.

Referring now to Fig. 3, this shows the printed circuit board which carries the components of the circuit of Fig. 2. The coaxial cable is indicated generally at 20. A clamp 22 which is secured to a zone 23 of the PCB by a screw has direct connection with the outer screen mesh conductor 12 of the cable. The inner conductor wire 10 is connected by a screw 24 to a different zone 25 of the PCB. On the PCB there is indicated at 14 the inner conductor output connection to the aerial socket. Similarly, at 16 are indicated two outer conductor output connections to the aerial socket.

The gas discharge device DT1 is connected between the PCB points A, B and C, shown in both Figs. 2 and 3.

From point B extends the element 26 of the printed circuit board which constitutes the earth connection.

This element 26 is a rectilinear leg which tapers symmetrically to a point at one end where it is in minimum point contact with a straight edge 28 of a zone 30 of the PCB which is connected to earth. By a minimum point is meant a contact having a width of for example a few thousandths of an inch. The lower limit will be set by manufacturing techniques. However, the aim is to enable current strikes to be directed to earth across this point contact in the event that the safety barrier is triggered, while providing the minimum conduction path for signals across this point contact in normal use. In practice, it has been found that a signal loss of less than 1.5 dB can be achieved with this configuration. It will be noted that there is just the one earthing point for both conductors of the coaxial cable, i.e. from leg 26 to zone 30.

It is important that the leg 26 should be rectilinear and provide a linear track from point B to the point contact with earth zone 30. Any curvature or bend or kink in this leg 26 makes the device function less satisfactorily.

It is also important that the point at the end of the leg 26 should be central and symmetrical in relation to the leg itself. For example, if the leg 26 is rectangular and has a thin parallel-sided connecting strip extending between it and the edge 28 of the earth zone 30, this does not function satisfactorily. It is necessary that the surge passing along the leg should be "directed" towards the point and concentrated at the point, so that when the discharge tube fires the current surge has a minimum resistance path to follow.

Because the protection offered by the safety barrier in this embodiment is not dependent upon a fuse, but rather is based upon the configuration of the printed circuit board, the safety barrier is capable of dealing with multiple current surges without any need for attention to, or replacement of, the unit or any part of it.

Claims (21)

1. An electrical safety barrier device for protecting electrical appliances against current surges, comprising electronic circuit means comprising means to allow current flow from an input to an output, means to absorb the current flow in the event of a current surge, and means to dissipate the current surge other than through the output.

2. A device according to claim 1, wherein the dissipation means comprises earthing means.

3. A device according to claim 2, wherein the absorption means comprises a discharge tube.

4. A device according to claim 3, wherein the electronic circuit means includes a pair of Zener diodes connected in parallel across the discharge tube.

5. A device according to claim 4, wherein the electronic circuit means includes diode means connected between the discharge tube and the junction between the back-to-back Zener diodes.

6. A device according to claim 5, wherein the earthing means is connected to a point between the discharge tube and the diode.

7. A device according to any of claims 3 to 6, wherein the electronic circuit means includes capacitance means to act as a smoothing component to minimise degradation of normal signal throughput.

8. A device according to any preceding claim wherein the electronic circuit means comprises printed circuit board means and wherein the dissipation means further includes a first conductive earthing zone on the printed circuit board means arranged to be connected to earth and the absorption means further includes further conductive zone means arranged to be connected directly or indirectly to the input, wherein the two zones have minimum point contact with each other.

9. A device according to claim 8, wherein the point contact is achieved by the earthing means comprising an earthing element tapering to a point, with said point being in minimum contact with the zone connected to earth.

10. A device according to claim 9, wherein the point of- the earthing element is central with respect to the element.

11. A device according to claim 9 or 10, wherein the earthing element defines a rectilinear track to its tapered point from its connection to the circuit.

12. A device according to any of claims 8 to 11, wherein the input comprises a coaxial cable and the absorption means is connected across the inner and outer conductors of the cable via the conductive absorption zone means, which comprises a second conductive zone connected with the outer conductor and a third conductive zone connected with the inner conductor, wherein the first, second and third conductive zones are spaced apart.

13. A device according to claim 12, wherein the earthing element is connected by means of wires to the discharge tube and the second and third zones.

14. A device according to claim 12 or 13, wherein the normal output of the device comprises an inner conductor output connection on the third zone and a pair of outer conduction output connections on a fifth conductive zone on the printed circuit board means which is wired to the second zone via the smoothing capacitor.

15. A device according to any of claims 1 to 7, wherein the electronic circuit means includes a fuse connected in series between the input and output.

16. A device according to claim 15, wherein the fuse has a resistance not greater than 1 ohm.

17. A device according to claim 16, wherein the fuse has an amperage of 250 mA.

18. An electrical safety barrier device for connecting a coaxial cable to an output and to protect an electrical appliance connected to said output fron current surges transmitted along said cable, the device comprising a pair of input terminals for connection to inner and outer conductors respectively of said cable, a pair of output terminals for connection to an appliance, means normally providing separate conductive paths from one of said input terminals to one of said output terminals and from the other of said input terminals to the other of said output terminals, and means operative to isolate both said output terminals from said input terminals in the event of a current surge on said cable.

19. A device according to claim 18, which includes a printed circuit board having a zone thereof which is arranged to be connected to earth, and a further zone thereof which is arranged to be connected directly or indirectly to both said conductors, wherein said two zones have minimum point contact with each other, thereby to minimise the conduction path from said one zone to said further zone under normal operation but permitting both conductors to be connected to earth across said point in the event of a current surge.

20. A device according to claim 18 or 19, wherein both conductors are connected to earth at the same point on the printed circuit board.

21. An electrical safety barrier device for protecting electrical appliances against current surges, substantially a,s hereinbefore described with reference to Figure 1 or Figures 2 and 3 of the drawings.