US2920224A

US2920224A - Cold-cathode gas-discharge tubes

Info

Publication number: US2920224A
Application number: US603891A
Authority: US
Inventors: Back Raymond Richard
Original assignee: Hivac Ltd
Current assignee: Hivac Ltd
Priority date: 1955-08-25
Filing date: 1956-08-14
Publication date: 1960-01-05
Anticipated expiration: 1977-01-05

Description

Jan. 5, 1960 PULSE AMPLITUDE IN VOLTS.

R. R- BACK COLD-CATHODE GAS-DISCHARGE TUBES Filed Aug. 14, 1956 PULSE DURATION IN -MICROSECS.

irncs United States Patent signor to Hivac Limited, South Ruislip, England, a

British company Application August 14, 1956, Serial No. 603,891

Claims priority, application Great Britain August 25, 1955 3 Claims. (Cl. 313-197) This invention relates to cold-cathode gas-discharge tubes and is more particularly concerned with tubes of the so-called relay type in which the main discharge between a cathode and an anode can be initiated by a control potential applied to an associated trigger electrode. It is a characteristic of such tubes that, once the main discharge is initiated, the trigger electrode has no further control until the potential between the cathode and anode of the main discharge gap is suitably reduced.

In such tubes there are a number of important parameters which govern the utility of the tube in specific circuit applications. Among these parameters are: (a) the main gap breakdown voltage which is the value of the voltage necessary between anode and cathode of the main discharge gap to cause self-breakdown, i.e. without the aid of a control potential on the trigger electrode; (b) the trigger breakdown voltage which is the minimum value of the voltage between the trigger electrode and another electrode of the tube (usually the main discharge cathode) necessary to cause the initiation of the required main discharge; (c) the main gap maintaining voltage which is the minimum value of the voltage between the anode and cathode of the main discharge gap necessary to maintain the main discharge with a given value of discharge current flowing; (d) the dynamic trigger sensitivity value which is a measure of the time period between the instant of application of a suitable control potential to the trigger electrode and the instant of initiation of the consequential main discharge, and (e) the extinguishing or recovery time which is a measure of the time period between the instant of reduction of the voltage existing between the main anode and main main gap maintaining voltage and the subsequent instant when the trigger electrode regains control of the tube operation. Other important factors in the commercial design, manufacture and use of such tubes are: (i) the tube-to-tube variation of the above parameters occurring in a batch of otherwise identical tubes; (ii) the main discharge current rating which is the maximum permissible value of the main discharge current and (iii) the physical size of the tube.

One object of the present invention is to provide an improved form of such relay tube construction partic ularly adapted for use in applications such as electronic counting circuits and telephone switching systems. An-

other object is to provide a tube construction in which the tube-totube spread orvariation of parameters such cathode to a'value below that of the above-mentioned 2 annular rim which merges at its inner edge smoothly into the end of said tube, the anode is arranged to be of rod or wire-like form disposed to lie coaxial with said cathode tube but substantially displaced away from the end of said cathode tube provided with said annular rim while the trigger electrode is arranged to be of ring-like form located around and in close spaced relationship to the outer edge of the aforesaid annular rim of the cathode.

In order that the invention may be more readily understood a number of constructional embodiments thereof will now be described with reference to the accompanying drawings, in which:

Fig. l is a side elevational view of one form of tube construction, the envelope and other parts being shown partly in section.

Fig. 2 is a fragmentary side view of the electrode structure taken in a direction at right angles to that of Fig. 1. P Fig. 3 is a plan view of the electrode structure of Fig. 4 is a fragmentary part-sectional, part-elevational view of one alternative form of cathode construction.

Fig. 5 is a fragmentary part-sectional, part-elevational View of another alternative form of cathode construction, w ile Fig. 6 is a graph illustrating a certain characteristic of the tube as shown in Figs. 1, 2 and 3.

Referring first to Figs. 1, 2 and 3, the tube shown comprises a cathode 10, an anode 11 and a trigger electrode 12 located within a tubular shaped glass envelope 13.

The cathode 10 is in the form of a cylindrical tube 15 provided with an annular rim 14 projecting radially outwards from its lower end, the inner edge of such rim merging smoothly into the main tubular portion 15. The upper end of the cathode cylinder may be inturned slightly as shown to increase its stiffness. I

The tubular portion 15 is disposed with its axis coin cident with that of the related anode 11 which is in the form of a rod or wire 16 located within the bore of a recess 17 formed in a stem 18 of glass which, although initially separate, is subsequently fused into integral relationship with the envelope 13. As will be seen from the drawing, the end of the anode nearest to the cathode is displaced a substantial distance away from the end of such cathode provided with the annular rim l4.

The stem 18 has a platform or button 19 integral with its upper end, the bore of the recess 17 passing through said button and terminating at its upper surface. The button 19 serves as an anchorage for a number of support wires which are fused therein, such support wires includng two wires 20 for supporting the cathode 10 and two turther wires 21 which support the trigger electrode 12.

The trigger electrode 12 is of ring-like form and compr ses a short, nearly complete,'cylinder 22 disposed coaxial with the cathode 10. The inner radius dimension of the cylinder 22 is only slightly greater than that of the I .outer edge of the rim 14 so as to provide a narrow annular trigger-to-cathode discharge gap. The upper edge of the cylinder 22 is preferably so disposed that it lies approximately in the mid-thickness plane of the annular rim 14 of the cathode.

The inner surface of the cathode 10 is coated with suitable emissive material having a low work function from a level as shown at 23 on the tubular portion 15 down to and around the under or anode-facing surface of the rim 14. The cathode is secured in position by attachment to a transverse mounting plate 26 whose opposite ends are connected, as by spot welding, to the support wires 20. p

The enclosing glass envelope 13 is of tubular form and is constructed with a pinch region 24 at its lower end through which the various connecting leads 25, 26, 27

from the cathode 10, the anode 11 and the trigger electrode 12 respectively pass outwardly in the usual way. A gettering member 28 is located in the tube space below the button 19, said gettering member being secured to one of the connecting wires.

The gas filling usedfor the tube may be any suitable combination of rare gases, chosen and adjusted in their proportions and filling pressure in accordance with well known. practice, to suit the particular type of tube characteristics required. A preferred gas filling for many purposes, including that of a discharge tube suitable for electronic counter operation, is one embodying neon and argon, for example, a mixture of 99%. neon and 1% argon. With the latter gas a convenient filling pressure is 40 mm. of mercury. Other gaseous mixtures may. however, be helium and argon or, a combination of neon, helium and argon.

An alternative form of cathode construction is shown in Pig. 4. This also has a cylindrical shaped main tubular portion 15a but the lower annular projecting rim 14a is made of approximately semi-circular cross-section. with the inner edge merging smoothly into the main tubular portion as before. As in the example shown in Figs. 1, 2 and 3, the under-surface of this curved rim is coated with emissive material contiguously with the lower surface portion of the inside of the cylinder.

A further alternative form of cathode construction is shown in Fig. where the main portion is in the form of a hollow truncated cone 15b. The lower edge of this cone is shown merging into a rim 14b of semi-circular cross-section like that shown in Fig. 4 but such outwardly projecting rim may be of other broadly similar shape, for instance, substantially of flat annular form as shown in Figs. 1, 2 and 3.

The trigger electrode 12 may also be modified in various ways without departing from the scope of the invention. It may, for example, be arranged in the form of two C-shaped elements disposed in diametrically opposed relationship one on each side of the cathode. Alternatively, and in some cases preferably, the trigger electrode may be in the form of a complete ring but this can lead to difiiculty during manufacture by absorbing an undue amount of the RF. energy used for heating the cathode during processing of the tube in its manufacture. The trigger electrode may also be in the form of a member made of circular section wire instead of flat metal strip as shown.

The arrangement according to the invention emplo ing a short tubular cathode with an outwardly directed rim in conjunction with an anode displaced axially away the rim end of the cathode and with the trigger electrode immediately surrounding the outer rim of the cathode provides a construction in which the anode can always see the initial trigger-to-cathode discharge and serves also to provide an arrangement in which the dynamic trigger sensitivity value is high and in which, moreover, the spread of the various tube parameters as previously discussed is materially reduced.

Fig. 6 illustrates, by way of example, the improve ment obtained for the dynamic trigger sensitivity value in a tube according to the invention (full line curve) as compared with a closely equivalent tube of substantially equivalent rating (dotted line curve). Thus, whereas with the existing tube using an 80 volt triggering pulse, the triggering time was of the order of 225 microseconds, that of a tube according to the invention was only 55 microseconds. In addition, whereas the spread of the parameter (b) given above was from 65-85 volts, the equivalent figures for a tube. according to the invention were of the order of 6274 volts only. The parameter (0) given above showed a spread of 6878 volts for a tube according to the invention as compared with a spread of 69-85 volts for the existing type of tube. The spread of the dynamic trigger sensitivity parameter (d) was between 35 and 90 m croseconds for a tube according to the invention as compared. with between 58 and 450 micro,-

seconds for the existing tube. Similarly, the parameter (e) discussed above showed a variation of 400-700 microseconds for a tube according to the invention as compared with a variation of 300-2000 microseconds for the existing tube. Atthesame time, the main gap break-down voltage parameter (a) was increased from 180 volts minimum for the existing tube to 210 volts minimum for the tube according to the invention.

In the particular, sub-miniature, tube according to the invention. he overall bulb size was approximately'35 mm. length by 10.5 mm. diameter and the cathode to anode. spacing dimension a, Fig. l, was 5.75 mm. The anode recession dimension b, was approximately 2 mm. The tube current rating was 1 ma.

I claim:

1. A cold-cathode gaseous discharge. tube of the relay type comprising a cathode, an anode and a separate trigger electrode by which a discharge between said cathode and an n b init at by h p a i n. of Su t bl control potential thereto, said cathode having the. shape of a short metal tube provided at one end with an outwardly directed integral annular rim which merges into the end of said tube with a curve of appreciable radius, said cathode having an electron-emissive surface coating upon the inner surface of said tube and extending around the contiguous surface of said flange, said anode being in the form of a rod disposed to lie coaxial with the axis of said cathode tube and sunk within an insulating recess with the end of said red at a position substantially displaced away from but facing towards that end of said cathode which is provided with said annular rim and said trigger electrode having the form of a short length cylinder of internal diameter slightly greater than the peripheral diameterof said rim and being disposed coaxial with the axis of said cathode and located immediately around and in close spaced relationship to the outer edge of the aforesaid annular rim of the cathode with the greater part of the length of said trigger electrode cylinder extending from said rim towards said anode.

2. A cold-cathode gaseous discharge tube of the relay type comprising a cathode, an anode and a separate trigger electrode by which a discharge between said cathode and anode, can be initiated by the application of a suitable control potential thereto, said cathode having the shape of a short metal tube provided at one end with an outwardly directed integral annular rim which merges into the end of said tube with a curve of appreciable radius, said cathode having an electron-emissive surface coating upon the inner surface of said tube and extending around the contiguous surface of said flange, said anode being in the form of a rod disposed to lie coaxial with the axis of said cathode tube at a position substantially displaced away from but facing towards that end of said cathode which is provided with said annular rim and said trigger electrode having the form of a short length axially gapped cylinder of internal diameter slightly greater than the peripheral diameter of said rim and being disposed coaxial with the axis of said cathode and located immediately around and in close spaced relationship to the outer edge of the aforesaid annular rim of the cathode with the greater part of the length of said trigger electrode cylinder extending from said rim towards said anode.

3. A cold-cathode gaseous discharge tube of the relay type comprising a cathode, an anode and a separate trigger electrode by which a discharge between said cathode and anode can be initiated by the application of a suitable control potential thereto, said cathode having the shape of a short metal tube provided at one end with an outwardly directed integral annular rim which merges into the end of said tube with a curve of appreciable radius, said cathode having an electron-emissive surface coating upon the inner surface of said tube and extending around the contiguous surface of said flange, said anode being in the form of a rod disposed to lie coaxial with the axis Qt said. athode ube a a Past ies substantia ly diseased away from but facing towards that end of said cathode which is provided with said annular rim and said trigger electrode having the form of two electrically interconnected C-shapcd halves of a short length cylinder disposed coaxial with the axis of said cathode and located immediately around and in close spaced relationship to the outer edge .of the aforesaid annular rim 'of the cathode with the greater part of the length of each of said halfcylinders extending from said rim towards said anode.

References Cited in the file of this patent UNITED STATES PATENTS Mendenhall Nov. 9, 1937 Ekstrand .i Feb. 3, 1942 Martin Sept. 7, 1943 Ingram Oct. 12, 1943 Slack et al. Mar. 23, 1948 Stutsrnan July 29, 1952