Spark gap devices

Abstract: A spark gap device includes planar electrodes 1 and 2 which in operation a discharge occurs. The electrodes 1 and 2 are spaced apart by a ceramic member 3 and are substantially parallel to one another. A trigger electrode 5, 7 is included for switching the device into conduction. The device has a low inductance structure which is compatible with stripline circuits.

Description: This invention relates to spark gap switching devices for use in stripline circuits. A spark gap device is one in which a discharge occurs between two electrodes. A trigger electrode is included so that the device may be used as a switch and the discharge is triggered by applying a suitable potential to it. In presently known spark gap switching devices, the electrodes are rod-like and contained within a cylindrical envelope. French Patent No. 2,097,418 provides a spark gap device having a pair of spaced apart planar parallel electrodes which overlap and have a discharge between them triggered by a third electrode, however it is proposed only to use this as an overvoltage limiter. The present invention seeks to provide an improved spark gap switching device. According to the invention, there is provided a spark gap switching device in a stripline circuit comprising planar electrodes located in different substantially parallel planes and having insulating material located between them, part of one electrode overlapping part of the other between which, in operation, a discharge occurs, and including a trigger electrode, the planar electrodes being formed as an extension of conductors in the stripline circuit. The planar electrodes are most conveniently flat but they could be curved for example. The degree of overlap determines the amount of current which a spark gap device is capable of passing. Typically, a current of several thousand amperes can flow between the two electrodes. Such a spark gap device presents a low inductance because of its configuration, thereby enabling fast switching speeds to be achieved compared to those generally possible using a conventional device. A typical value of the inductance of a device in accordance with the invention may be in the region of nanohenries, whereas that of a conventional spark gap used for similar applications would have an inductance of some microhenries. Thus, the current rise time is small and there are very low arc losses. The width of the electrodes is chosen according to the application in which the device is used. A spark gap device in accordance with the invention may be made extremely compact in one dimension by enclosing the electrodes within a planar envelope. It may also be made particularly robust, being highly shock-resistant and able to withstand greater vibration than a conventional spark gap device. The spark gap device can also be surface-mountable, which may be particularly advantageous in some applications. Another advantage of a device in accordance with the invention is that it can be cheap to fabricate. The gap between the electrodes may be hermetically sealed and the medium between the electrodes may be a vacuum, or a suitable gas or liquid. By suitably choosing the pressure of gas within the device, its characteristics can be changed without modifying its physical configuration. Electrically insulating material is included in the spark gap device, being located between the electrodes, which may, for example be brazed to the insulating material. In a preferred embodiment of the invention, the insulating material is absent from a cylindrical region where the discharge is arranged to occur, that is, the overlapping parts of the electrodes are circular. In a particularly advantageous configuration of the trigger electrode, it comprises a projecting portion arranged between the overlapping parts of the planar electrodes and a planar portion. It is preferred that insulating material is located between the planar portion of the trigger electrode and a planar electrode, and that preferably the planar portion of the trigger electrode and the planar electrode are located in respective different, substantially parallel, planes. The spark gap device in accordance with the invention may thus have a low profile and occupies a small volume. Advantageously, a protective coating may be included to encapsulate the device, thus preventing or reducing the tendency for external breakdown to occur. In one embodiment of the invention, the spark gap device is constructed and arranged to act as a light source. Each time the discharge is struck across the electrodes, light may be produced which may be sufficiently bright to be used, for example, for stroboscopic purposes, Preferably, at least some insulating material which surrounds the electrodes is transparent to the light but, for example, an optical fibre might be introduced into the space between the electrodes to conduct light from the device. Some ways in which the invention may be performed are now described by way of example with reference to the drawings in which: Figure 1 is a sectional view of a spark gap device in accordance with the invention; and Figure 2 is an exploded, perspective view of the device shown in Figure 1; With reference to Figures 1 and 2, a spark gap device in accordance with the invention comprises two planar electrodes 1 and 2, the first electrode 1 being connected to act as a cathode and the other electrode 2 as an anode. The electrodes 1 and 2 are arranged substantially parallel to one another, there being some overlap between them. A ceramic member 3 spaces the electrodes 1 and 2 apart and is brazed to them. The ceramic member 3 includes a circular central aperture which defines the overlapping areas of the two electrodes 1 and 2 between which, in operation, a discharge may occur. The spark gap device includes a trigger electrode indicated generally at 4. The trigger electrode comprises a cylindrical portion 5 which is arranged to project through an aperture 6 in the cathode electrode 1, and a planar portion 7. The planar portion 7 is arranged substantially parallel to the electrodes 1 and 2, being spaced from the cathode electrode 1 by a ceramic member 8 which is similar to the ceramic member 3 between the electrodes 1 and 2. The gap between the electrodes 1 and 2 is hermetically sealed and the pressure of the air within the device is chosen to provide desired operational characteristics. The whole device is encapsulated by a protective layer (not shown in Figure 2). To switch the device into conduction, a trigger pulse is required which is typically of the order of 5 kV, the trigger pulse being usually derived from a trigger transformer.

Claims: A spark gap switching device in a stripline circuit comprising planar electrodes (1,2) located in different substantially parallel planes and having insulating material (3) located between them, part of one electrode overlapping part of the other between which, in operation, a discharge occurs, and including a trigger electrode (4), the planar electrodes being formed as an extension of conductors in the stripline circuit. A spark gap device as claimed in claim 1 wherein the insulating material is absent from a cylindrical region (6) where the discharge is arranged to occur. A spark gap device as claimed in claim 1 or 2 wherein the insulating material (3) is ceramic to which the electrodes (1,2) are brazed. A spark gap device as claimed in claim 1 wherein the trigger electrode (4) comprises a projecting portion (5) and a planar portion (7). A spark gap device as claimed in claim 4 and including insulating material (8) located between the planar portion (7) of the trigger electrode (4) and a planar electrode (1). A spark gap device as claimed in claim 4 or 5 wherein the planar portion (7) of the trigger electrode (6) and the planar electrodes (1,2) are located in respective different, substantially parallel, planes. A spark gap device as claimed in any preceding claim and including a protective coating encapsulating the device. A spark gap device as claimed in any preceding claim, the device being constructed and arranged to act as a light source. A spark gap device as claimed in claim 8 wherein, when insulating material is included, at least some of the insulating material is transparent to light. A spark gap device as claimed in any preceding claim wherein the gap between the electrodes is hermetically sealed.

Inventors: MCCANN GARRY PAUL, MCCANN, GARRY PAUL
Applicants: EEV LTD, EEV LIMITED
Application year: 1988
Main classification: H01T2
IPC codes: H01T2