GB2200243A

GB2200243A - Overvoltage protection device

Info

Publication number: GB2200243A
Application number: GB08701757A
Authority: GB
Inventors: David John Mellor
Original assignee: English Electric Valve Co Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1987-01-27
Filing date: 1987-01-27
Publication date: 1988-07-27
Anticipated expiration: 2007-01-27
Also published as: GB2200243B; GB8701757D0; EP0278612A1; EP0278612B1; DE3860668D1

Description

0 24 3 2- PROTECTION DEVICE This invention relates to a protection device

of the kind which is arranQeQ to become conductive when a transient excess voltage appears between two electrodes which form part of it. Typically, such a device is to withstand a large potential difference without breaking down into conduction between the two electrodes, but it can be brought rapidly into conduction when required so that typically a current of several thousand amps can flow between the two electrodes for a short period. A device of this kind can be used to protect relatively delicate electronic equipment or the like from the effect of transient over-voltages which can be induced onto the conductors by external events.

present invention seeks to improve the v of such devices to over-voltages and to enhance the speed at which they can change from a non conductive state into a conductive state so as to provide the necessary degree of protection very rapidly.

According to a first aspect of this invention a protection device includes a sealed chamber having associated with i cathodic source electrons into t that a rise in conductors above The sensitivit ts interior two conductive electrodes; a of free electrons arranged to release he chamber in a controlled manner, such potential difference between the two a threshold value causes electrical - 2 breakdown to occur between them utilising the free electrons.

According to a se-,ond aspect of protection device includes a sealed first conductive electrode projecting into it, constituting another conductor; and a region connecting with the first chamber and having an electron emissive electrode, the two chambers being such that in operation free electrons from the second chamber are released into the first chamber whereby a rise in potential difference between the two conductors above a threshold value causes electrical- breakdown to occur between them.

According to a third aspect of this invention a protection device includes a sealed chamber having a first conductor projecting into it; a second conductor which constitutes part of the wall of the chamber, the first conductor being tapered so that it has a reducing cross section within the chamber, and the second conductor being shaped so that the two conductors together constitute an electrical transmission line having a substantiallv constant characteristic impedance and so that the distance between the two conductors is least in the region of smallest The this invention a chamber having a a wall portion separate chamber cross section of the first conductor. source of free electrons is such that into that realon of the electrons are released chamber where electrical discharge between the two conductors is to be 1 achieved at a reasonably slow rate so as not to initiate breakdown below the desired threshold value. However, the release of electrons should be sufficiently large to ensure that there is always at least one free electron in the vicinity of the discharge region so as to promptly initiate discharge the moment an over-voltage occurs.

The invention is further described by way of reference to the accompanying drawings, in which:

Figure 1 illustrates a transient protection device in accordance with the present invention, and Figure 2 shows a modification.

Referring to the drawing, a first envelope consists of an outer cylindrical wall 1 formed of an electrically conductive material carrying a circular disc 2 at each end, these discs being formed of an electrically insulating material such as ceramic and being sealed around their outer peripheral edges to the wall 1 to enclose a chamber 4. An inner solid conductor 3 passes through the chamber 4 so formed and is sealed to the two end discs 2 where it p-asses through them. The central inner conductor 3 is a relatively robust and substantial conductor but it is of tapered section within the chamber 4, tapering smoothly to a mid- point 5 which is adjacent to an opening 6 in a second chamber 7. The inner walls of the outer conductor 1 are provided with a shaped portion 8 which is dimensioned to form a relatively small gap 9 in the-region of the opening 6. A typical dimension for gap - 4 9 is 0.5 mm.

The second chamber 7 constitutes a source of free electrons and it consisis of a conductive outer wall 10 having an inner central electrode 11 which is connected via a lead 12 to a d.c. power supply 13.

The two chambers 4 and 7 are, of course, interconnected via the opening 6 but both are hermetically sealed from the atmosphere. The chambers are initially evacuated to a fairly high level of vacuum, e.g. 10 -4 Torr after which the chambers are filled with an inert gas, such as argon or neon or a mixture of these. The gas type and pressure are chosen to determine the breakdown voltage, but the pressure is expected to be in the range 20 Torr to an excess of 760 Torr.

During operation, it is assumed that a relatively modest potential difference exists between the two conductors I and 3 which is such so as not to cause electrical breakdown in the region 9. Electrons are generated at the second chamber 7, particularly in the region of the opening 6 where an electron plasma exists. Free electrons from this plasma are randomly distributed in the'region of the opening 6 and the gap 9, but it is arranged that the electron density is very low indeed, in principle it being sufficient to ensure that at least one free electron exists within the gap at any one time. The electron plas-ma is generated by striking a glow discharge between the electrodes 10 and 11 under the action of the 5 ir power supply 13. The resulting ions as well as the electrons -assist in the rapid electrical breakdown. The d.c. current in the glow discharge is typically of the order of a few tens of microamps.

When a high voltage transient appears so as to-raise the potential difference between the conductors 1 and 3 above a threshold value, electrical discharge in the region of the gap 9 occurs, and this breakdown is initiated by the presence of free electrons. It is the contribution and effect of these free electrons that ensures a reliable and very prompt breakdown. In the absence of any free electrons, electrical breakdownwould occur eventually, is large enough, rate.

In order to ensure that the electrical breakdown occurs very rapidly indeed it is necessary that the gap 9 is small, and since the chamber 4 is evacuated the spacing between the two conductors can be made much smaller within the chamber than it is possible to space the two conductors 1 and 3 in a normal atmosphere. Consequently, in order to permit the conductors to carry high frequency signals, the inner conductor 3 is also tapered soas to provide a constant impedance transmission -line.

By ensuring that the relative diameters of the conductor 3 and the conductor 1, as modified by the extension portion 8, is -constant, then the characterisitc impedance is provided that the potential difference but at a much slower and indeterminate 6 preserved. A typical characteristic impedance is 500, with the maximum diameter of the conductors I and 3 being lomm and 2.3mm re spectively, reducing to 1.84mm and 0.8mm respectively in the region of the gap 9. The provison of the tapered section greatly enhances the reliability and speed of operation and for certain applications the free electon source may not always be necessary. The two conductors 1 and 3, together with the portion 8,. therefore constitute an electrical transmission line having a characteristic impedance which does not alter at different points along the conductors. This the protection device to handle the transmission frequency signals without causing signal reflections which would result from variations in the line impedance. The benefit Of the tapered conductor sections to give a constant characteristic impedance can also be obtained with a conventional radioactive source of electrons and/or ions, such as Tritium or other radioactive isotope.

Instead of the gap 9, one of the insulating discs be placed at the point where the separation between two electrodes is least. Such an arrangement is shown Figure 2, in which two insulating discs 20 and 21 provided to define the walls of a gas filled chamber Electrical breakdown occurs across the surface 23, and cathodic electron source can be included (in a manner similar to Figure 2) to initiate electrical breakdown, or enables of hiah can the i n are 22.

- 7 t a radioactive isotope can be included in the gas which fills the chamber 22.

8 -

Claims

1. A protection device including a sealed chamber having associated with its interior two conductive electrodes; a source of free electrons arranged to release electrons into the chamber in a controlled manner, such that a rise in potential difference between the two conductors above a threshold value causes electrical breakdown to occur between them utilising the free electrons.

2. A protection device including a sealed first chamber having a conductor electrode projecting into it, a wall portion constituting another conductor; and a separate chamber region connecting with the first chamber and having being second a rise in potential difference between the two conductors abov-e a threshold value causes electrical breakdown to occur between them.

3. A protection device including a sealed chamber having a first conductor projecting into it; a second conductor which constitutes part of the wall of the chamber; the first conductor being tapered so that it has a reducing cross section within the chamber, and the second conductor being shaped so that the two conductors together constitute an electrical transmission line having a substantially constant characterisic impedance and so that the distance between the two conductors is least in the an electron emissive electrode, such that in operation free electrons from the chamber are released into the first chamber whereby the two chambers ..

0 9 - Q region of smallest cross section of the first conductor. 4. A protection device substantially as illustrated in and described with reference to Figure 1 or 2 of the accompanying drawings.

-io- Amendments to the claims have been filed as follows CLAIMS 1. A protection device including a sealed chamber having a first conductor projecting into it; a second conductor which constitutes part of the wall of the chamber; the first conductor being tapered so that it has a reducing cross section within the chamber, and the second conductor being shaped so that the two conductors together constitute an electrical transmission line having a substantially constant characteristic impedance and so that the distance between the two conductors is least in the region of smallest cross section of the first conductor.

2. A protection as claimed in claim 1, in which the first conductor has a seco,nd reducing cross-section within the chamber meeting the first reducing crosssection at the region of smallest cross-section of each.

3. A protection device is claimed in claim 1 or claim 2, in which there is provided a radioactive source to provide free electrons and/or ions in the chamber.

4. A protection device as claimed in any one of claims 1 to 3, in which there is provided a separate chamber region connecting with the first free chamber and having an electrode to provide a source of free electrons, the two chambers being such that in operation free electrons from the second chamber are released into the first chamber.

5. A protection device as claimed in any one of claims 1 to 4, in which an insulating disc is placed at the point i 4 _i - j 1 where the distance between the two conductors is least.

6. A protection device substantially as herein described with reference to the accompanying drawings.

1 Published 1988 a, The Patent Office. State House. 66'71 High Holborn, London W01R 4T?. Further copies may be obtained from The Patent Office. Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multdplex techniques Itd. SL Mary Crky, Kent. Con. 1/87.