US3409793A - Gas-filled discharge device having a grid with an element particularly spaced from the cathode - Google Patents

Description

3,409,793 LEMENT AN E 2 Sheets-Sheet 1 A TTORNEY Nov. 5, 1968 P. w. STUTSMAN GAS FILLED DISCHARGE DEVICE HAVING A GRID WITH PARTICULARLY SPACED FROM THE CATHODE Filed May 22, 1951 1 I l 4. 1 I L q Q J v Illvlll I I 1 I I II I n 8 A A Q Q //y// 0fi/////,/// 6 2 F. W. STUTSMAN Nov. 5, 1968 3,409,793 GAS FILLED DISCHARGE DEVICE HAVING A GRID WITH AN ELEMENT E PARTICULARLY SPACED FROM THE CATHODE 2 Sheets-Sheet 2 Filed May 22, 1951 LOAD FIG. 4-

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United States Patent Olfice 3,409,793 Patented Nov. 5, 1968 Claims. (Cl. 313-193 This is a divisional of my copending application Ser. No. 101,279, filed June 25, 1949, now Patent No. 3,004,192.

This invention relates to electron discharge devices, and more particular-1y to means whereby oscillations inherent in a cold cathode gas discharge tube may be controlled and utilized in the operation of the tube.

In gaseous discharge devices, particularly of the cold cathode type, when a voltage is applied between a grid adjacent the cathode, and the cathode, oscillations are generated in the gas space between the cathode and the electrode. One explanation of the cause of said oscillations is as follows.

In order to efiectively control the biasing potential between the grid and cathode, the grid is fed from a positive voltage through a relatively high resistance, and, as a result, current variations between the cathode and grid result in voltage variations between the cathode and grid. Let us assume that, because of random electron and ion production in the gas, the electron supply to the anode is momentarily decreased. The decrease in electrons results in a decrease in tube current, which, in turn, results in an anode voltage rise. This anode voltage rise accelerates any free electrons in the gas, causing a small surplusof electrons to be formed by collision. The free supply of electrons thus formed, plus the higher anode voltage, permits an increase in tube current. The increase in tube current causes a drop in anode voltage, which reduces the number of electron-ion-molecule collisions, which, in turn, decreases the electron supply to the anode. Thus a cyclic variation in tube current and voltage occurs and the tube oscillates.

Applicant has discovered that by the use of a filter network connecting the grid structure through a low impedance path to ground, these oscillations may be reduced to a point where they will not fire the tube at an undesired time. Furthermore, applicant has discovered that these oscillations are lowest when the distance between the grid structure and the cathode is at minimum breakdown distance.

As used throughout the specification and claims, the term minimum breakdown distance is defined as being the distance between two electrodes in a gaseous medium whereat the smallest voltage between said electrodes is required to create a discharge between said electrodes. It may be noted that this distance will vary with the pressure and type of gas surrounding the electrodes and electrode configuration. When the grid is positioned at the minimum breakdown distance from the cathode, the oscillations produced in the tube prior to breakdown of the tube will be less than those produced with other spacings of the electrodes, since the oscillations generated between the grid and cathode in general vary directly as a function of the voltage applied between said grid and cathode. Thus, by positioning the grid at the minimum breakdown distance from the cathode, the maximum stability of the device is achieved.

Furthermore, applicant has discovered that by the use of an external damping circuit with a suitable biasing potential applied between the grid and the cathode of the gas discharge device whereby the oscillations between the grid and the cathode are reduced to a point where the tube is not fired, the tube may be fired by varying the components of the damping network such that the damp ing of the oscillations is reduced whereby the oscillations increase to a point where they will fire the tube.

The tube may also be fired by varying the capacitance of a charge condenser in the grid circuit whereby the current flow in said condenser, due to the variations thereof, is arranged to produce a voltage pulse across a resistor in the grid circuit of the tube.

The particulardetails of specific embodiments of this invention are more particularly pointed out hereinafter, reference being had to the accompanying drawings, wherein:

FIG. 1 illustrates a longitudinal, cross-sectional view of a gas discharge device utilizing this invention, taken along line 11 of FIG. 2;

FIG. 2 illustrates a transverse, cross-sectional view of the device shown in FIG. 1, taken along line 2-2 of FIG. 1;

FIG. 3 illustrates a circuit utilizing the gas discharge device, and having one form of damping network therein;

FIG. 4 illustrates another circuit utilizing the gas discharge device, and showing one method for varying the damping circuit; and

FIG. 5 illustrates an operating characteristic of a gas dsicharge device of the type illustrated in FIGS. 1 and 2.

Referring now to FIGS. 1 and 2, there is shown a gas discharge device comprising an envelope 10, which may be made of any desired insulating material such as glass, said envelope shown here by way of example as being tubular and having at one end a glass seal 11' through which extends a plurality of lead 12 for connecting the elements of the device to any desired circuit.

Inside envelope 10, there is positioned a cathode 13 which may be of any desired type and as illustrated herein is a hollow cylindrical member positioned coaxial with envelope 10, the inner surface of said cathode cylinder being coated with electron emissive material. As shown here, the lower end of the cylindrical cathode 13 is covered by a wire mesh 14 which becomes activated by being coated by electron emissive material during the processing of the tube. Mesh 14 thus acts as an auxiliary cathode from which keep-alive current is drawn in the standby condition of the tube.

Cathode 13 is supported on two rods 15 which extend from the upper end of the cylinder 13 along opposite sides of its inner surface parallel to the axis thereof and down the length of the envelope 10 to the lower end thereof Where they are connected to two of the lead-in members 12 which extend through the glass press 11. The rods 15 are rigidly attached to the cathode cylinder 13 as, for example, by being welded thereto, and are surrounded from a point immediately below the bottom of the cathode cylinder 13 by glass tubes 16 which extend downward to the glass press 11 and are fused thereto.

Extending upward from the press 11 coaxial with envelope 10 and cylinder 13 is a third glass tube 17 which contains therein an anode rod 18 attached to a lead 12 extending through the press 11. The anode rod 18 extends a small distance, for example, a distance equal to the diameter of said rod beyond the end of the glass tube 17 which terminates some distance below the bottom of the cathode cylinder 13.

The anode rod 18 is shielded from the cathode 13 by a cup-shaped grid 19 which is inverted and placed over anode rod 18 and glass tube 17 not touching anode rod 18. Grid 19 is rigidly positioned with respect to the other elements of the tube by being attached, as by welding, to a strap 20 extending around glass tubes 16 and 17, strap 20 being, in turn, attached, as by welding, to a lead 12 extending through glass press 11 whereby potentials may be applied to grid 19. A loop 2]. is positioned such that one position thereof-comes in close proximity withthe screen 14 of the cathode 13,-one end of said loop being connected to the uppermostpoint of grid 19, and the other end thereof being connected, as by welding, to strap 20.

The distance between the nearest portion of loop 21 and screen 14 is substantially equal to the minimum breakdown distance for the particular gas and gas pressure used in the device. By so positioning loop 21 with respect to auxiliary cathode 14, the minimum voltage is required between said loop and said cathode to produce a'discharge therebetween and, therefore, at the voltages iust below those required to produce the discharge, oscillations generated in the gas between loop 21 and screen 14 are at a minimum, as previously described.

It may be seen that due to the shield 19 surrounding the anode 18, a high potential may be applied between rod 18 and cathode 13 without discharging the tube. This is due to the fact that the grid 19 has a high work function and, therefore, few electrons are available therefrom for ionizing the space between grid 19 and anode 18, and also due to the fact that the distance between grid 19 and anode 18 is made different from the minimum breakdown distance for those two electrodes. While, as shown here, the distance between grid 19 and anode 18 is greater than the minimum breakdown distance, it is to be clearly understood that the same result may be accomplished by making said distance less than the minimum breakdown distance, since the breakdown voltage between two electrodes in a gaseous medium increases for both an increase or a decrease of their spacing from the minimum breakdown distance.

Referring now to FIG. 3, there is shown a circuit utilizing the discharge device illustrated in FIGS. 1 and 2 wherein the cathode 13 is connected to ground. The anode 18 is connected to a source of B+ through a load 22 which may be, for example, a relay or a fuse. The grid structure 19 and 21 is connected together and connected to said B+ through a resistor 23 which may be, for example, on the order of 10,000 megohrns and through a condenser 24 which may be, for example, micromicrofarads, to a damping circuit comprising a condenser 25 which may be, for example, 100 microfarads and a resistor 26 in parallel with said condenser which may be, for example, 100 megohrns, said damping circuit, in turn, being connected through an input signal source to ground.

Referring now to FIG. 5, there is shown a graph illustrating the operation of this device. Along the abscissa of the graph is plotted the voltage between the grid 21 and the cathode 13 in volts, and along the ordinate is plotted the current drawn from cathode 13 to grid 21 in amperes, When the current of the discharge is 10 amperes or less, as shown by the area labeled Townsend discharge, the voltage across the discharge varies directly as a function of the current such that when the voltage increases, the current increases. When the voltage is increased to a point beyond that required to produce 10 amperes, as shown by point 28, the current will then increase with a decrease in voltage thereby entering the region labeled normal glow. Since the grid is fed through a relatively large resistor, the grid current, upon firing of the tube, will not pass beyond the normal glow region because the voltage drop across the resistor would then lower the grid voltage below that reqiured for maintaining the discharge between the grid and cathode. Thus, it may be seen that once a suflicient voltage is applied to increase the current along curve 27 to point 28, the current will then rapidly increase to form a glow discharge between the grid and cathode.

Some electrons from the glow discharge will move through the grid section 19 and be accelerated to the anode 18 and will ionize the space therebetween, to thereby establish cathode to anode conduction and fire the tube.

However, if the voltage between the electrodes is maintained below that required to produce the current 4. at point 28, this voltage being in this case by way-of example approximately volts, the current will not increase to form an arc. Thus, for example, if the grid voltage =were to be maintained at 178 volts above cathode, the current would be on the order of 10- amperes,.as shown by point 29 on the curve. A positive signal voltage of two volts applied to the grid will be sufiicient to fire the tube thereby producing a circuit having extremely great sensitivity-.'While these particular points have been taken by way of example, it is in practice possible to operate nearer to point 28 than two volts, and indeed it is possible to produce stable tubes wherein a grid signal of less than one volt will fire the tube.

In order to maintain the voltage between the grid.21 and the cathode 13, as shown in FIG. 3, the grid 21 is supplied with a positive voltage through a resistor 23. The value of this resistor is suchthat, in the absence-of an input signal, the current drawn by grid 21 will produce a voltage drop across resistor 23 sufiicient to drop the voltage supplied to grid 21 from 13+ to the required operating voltage of grid 21. For example, if the 8+ voltage is 200 volts and the value of resistor 23 is 10,000 megohrns, the voltage that must be dropped across resistor 23 must be slightly greater than 200 volts minus the potential of point 28 taken here by way of example as 180 volts or a drop of somewhat more than 20 volts. This then requires that current slightly greater than two times 10- be drawn by grid 21 or. the tube would be biased to operate at a point slightly above point 29 on the curve 27.

By varying the value of resistor 23 this point may be shifted along curve 27 to any desired bias potential. This biasing method is self-regulatory since, if the current increases, the drop .across resistor 23 increases, thus lowering the potential of the grid 21 which, in turn, reduces the current. Also, if the B+ voltage is varied, due, for example, to fluctuations in the power supply, the resistor 23 may be varied to produce the correct operating conditions and furthermore will automatically maintain the desired operating potential for snbstanial variations of the power supply.

"If the grid is being operated at a steady bias of, for example, the voltage of point 29, and oscillations between the cathode and grid are generated at the gaseous media, positive peaks of these oscillations will add to the steady bias of point 29, thus raising its potential instantaneously to a point where it may, and in most cases does, fire the tube. However, if these oscillations are damped by means of the circuit shown in FIG. 3 comprising a condenser 25 and resistor 26, the oscillations will be limited in their amplitude to a point where they will not fire the tube and at the same time be of a substantially uniform amplitude whereby their effect may be compensated for by decreasing the bias potential by an amount substantially equal to their amplitude. Indeed, if the filter system loaded the grid sufficiently heavily, these oscillations might be damped out altogether.

Referring now to FIG. 4, there is shown a circuit utilizing these oscillations to produce a voltage for triggering the tube whereby no external electrical signal is required. There is shown a tube similar to that illustrated in FIGS. 1, 2 and 3, like parts being referred to by like reference numerals. This comprises an envelope 10, an anode 18, cathode 13, grid section 21, adjacent to cathode 13, and grid section 19 shielding the anode 18 from the cathode 13, grid sections 19 and 21 being connected together. The anode 18 is connected through a load 22 to B+, said load being any desired current-operated device such as a relay. It is to be clearly understood that in both FIGS. 3 and 4, this'load could be placed in the cathode circuit rather than the anode circuit. The grid structure 19 and 21 is connected to B+ through a grid bias resistor 23 and to a filter circuit through a coupling condenser 25. The filter circuit comprises a resistor 26 and a variable capacitor 30 whose values are such that they are low impedance at the frequency of the oscillations generated in the tube. Resistor 26 is connected to ground, and condenser 30 is connected to B|.

The filter circuit 30 and 26 is adjusted such that it does not damp the oscillations completely but merely loads them to a point where they are steady and uniform. By decreasing the value of the capacitor 30, the impedance of the filter circuit to the oscillation is increased, thereby reducing a loading on said oscillations whereupon the oscillations will increase in amplitude. With a suitable bias applied through resistor 23 such that the oscillations do not fire the tube, it may be seen that by decreasing the value of the capacitor 30, the oscillations may be caused to build up to a point where the tube Will fire.

It is to be clearly understood that the filter circuits shown here are merely by way of example, and any known filter circuits which would accomplish the desired filtering might be used. Furthermore, to vary the impedance of the filter circuit, other elements thereof besides a capacitor may be varied, for example, the resistor 26 could be decreased to accomplish the desired result. The value of resistor 26 has been found to be somewhat critical for best results in oscillation damping. The resistor acts as an absorber of the oscillations, and if it is either increased or decreased, the amplitude of the oscillations will in general be increased.

Since the condenser 30 is connected to B+, the plates thereof will be charged. If the capacitance of this condenser is varied, current will flow in the condenser 30 to produce a charge proportionate to the new value of capacitance. For example, if the capacitance is increased, electrons will flow from ground through resistor 26 to condenser 30. This will produce a positive voltage across resistor 26 for the duration of the charging period. This positive voltage may be used to fire the tube by being applied to the grid 21 through condenser 25.

This completes the description of the embodiments of the invention illustrated herein. However, many variations thereof will be apparent to persons skilled in the art without departing from the spirit and scope of this invention. For example, any number of grid sections could be used in the current structure of the tube. Other forms of bias might be used than the simple grid biasing resistor. The oscillation damping network could be returned to B+ rather than ground, and various voltages could be used for the anode and grid voltages other than those specifically recited herein. Therefore, applicant does not wish to be limited to the specific embodiments of the invention, as described herein, except as defined by the appended claims.

What is claimed is:

1. An electron discharge device comprising a gas-filled envelope containing an anode, a cathode, and a grid structure interposed between said anode and said cathode, said grid structure shielding said anode from said cathode and having an additional element thereof positioned at substantially the minimum breakdown distance from said cathode.

2. An electron discharge device comprising a gas-filled envelope containing an anode, a cold cathode, and a grid structure interposed between said anode and said cathode, said grid structure shielding said anode from said cathode and having an element thereof positioned at substantially the minimum breakdown distance fromsaid cathode.

.3. An electron discharge device comprising a gas-filled envelope containing an anode, a cold cathode, and a grid structure, said grid structure having a first element there of positioned at substantially the minimum breakdown distance from said cathode and a second element of said grid connected to said first element and shielding said cathode from said anode.

4. An electron discharge device comprising a gas-filled envelope containing an anode, a cold cathode, and a grid structure, said grid structure having a first element thereof positioned at substantially the minimum breakdown distance from said cathode and a second element thereof connected to said first element and shielding said cathode from said anode, said second element being positioned a greater distance from said cathode than said first element.

5. An electron discharge device comprising a gas-filled envelope containing an anode, a cathode, and a grid structure, said grid structure having a first element thereof positioned at substantially the minimum breakdown distance from said cathode and a second element thereof connected to said first element and shielding said cathode from said anode, said second element being positioned a greater distance from said cathode than first element and a distance from said anode structure substantially different from the minimum breakdown distance therebetween.

References Cited UNITED STATES PATENTS 1,628,045 5/1927 Hendry 313-199 1,893,887 1/1933 Girad 313-193 1,999,649 4/1935 Brett 313-206 2,003,012 5/1935 Sashoff 313-193 2,084,725 6/1937 Dallenbach 313-199 2,119,855 6/1938 Depew 313-193 2,295,569 9/1942 Depp 313-196 2,444,962 7/1948 Stutsman 313-214 JAMES W. LAWRENCE, Primary Examiner.

Claims (1)

1. AN ELECTRON DISCHARGE DEVICE COMPRISING A GAS-FILLED ENVELOPE CONTAINING AN ANODE, A CATHODE, AND A GRID STRUCTURE INTERPOSED BETWEEN SAID ANODE AND SAID CATHODE, SAID GRID STRUCTURE SHIELDING SAID ANODE FROM SAID CATHODE AND HAVING AN ADDITIONAL ELEMENT THEREOF POSITIONED AT SUBSTANTIALLY THE MINIMUM BREAKDOWN DISTANCE FROM SAID CATHODE.

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