US3702952A

US3702952A - Gas tube surge protective device and method for making the device

Info

Publication number: US3702952A
Application number: US188208A
Authority: US
Inventors: Glenn Cassidy; Donald A Hartman; Lyle J Hentz; William D Horn
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1971-10-12
Filing date: 1971-10-12
Publication date: 1972-11-14
Anticipated expiration: 1989-11-14
Also published as: DE2249523C3; BE789890A; DE2249523A1; IT966240B; JPS5718305B2; GB1389338A; FR2156255B1; ES407786A1; FR2156255A1; CH549882A; JPS4846850A; DE2249523B2; SE381775B; CA966191A

Abstract

A gas filled surge protective device having two conventional electrodes is provided with a starter electrode. This electrode is formed in place on the wall of the device by controlled sputtering of electrode material. The sputtered material is electrically connected to one electrode and surrounds a portion of the other electrode so that ionization starts in the annular space therebetween.

Description

United States Patent Cassidy et al. [451 Nov. 14, 1972 [54] GAS TUBE SURGE PROTECTIVE [56] References Cited DEVICE AND METHOD FOR MAKING I THE DEVICE v UNITED STATES PATENTS [72] Inventors: Glenn Cassidy Upper Black Eddy; 2,829,295 4/1958 Gastetal ..3l3/198 X 3,388,274 6/1968 Kawiecki et a1. ..3l3/l98 X flenemwn' 3 454 811 7/1969 Scudner 317/62 x Lyle J. Hentz, Whitehall; William D. Horn Nazareth of Primary Examiner-James D. Trammell [73] Assignee: Western Electric Company, Incor- Attorney-W. M. Kain et a].

porated, New York, NY. 22 Filed: Oct. 12, 1971 [57] ABSTRACT A gas filled surge protective device having two con- [211 Appl' 188208 ventional electrodes is provided with a starter electrode. This electrode is formed in place on the wall of [52] US. Cl ..317/62, 313/217 the device by controlled sputtering of electrode [51] Int. Cl. ..l-I02h 9/06 material. The sputtered material is electrically con- [58] Field of Search ..3 1 3/198, 210, 217; 317/62 nected to one electrode and surrounds a portion of the other electrode so that ionization starts in the annular space therebetween.

12 Claim, 5 Drawing Figures /0 l 22 1 13:, i /4 I/ 20 5 26 i 5 28 l 34 1; '32 a0 4 j /6 PATENTEDmv 14 I972 SHEET 2 BF 2 TI ME 5 5 EE EE m m TM MT ma mwwwmm 432 NO STARTER WITH STARTER ELECTRODE ELECTRODE GAS TUBE SURGE PROTECTIVE DEVICE AND METHOD FOR MAKING THE DEVICE BACKGROUND OF THE INVENTION sion line and ground. It inherently limits the voltage,

which subsequently appears across the line, to protect the line as well as associated vulnerable electronic equipment from electrical stress or overload damage such as might be caused by lightening or other induced over-voltage transients. The central office and subscriber equipment of a telephone system are typical of the apparatus which is so'protected.

A gas-tube, voltage surge protector is a two-state device which is nonconductive at its normal line voltage or conductive when a voltage transient causes the enclosed gas to ionize. The general requirements of such a protector include the ability: (1) to conduct many short-duration pulses (e.g., few microseconds),

moderate current pulses (e.g., 200 amps), and occasional short duration, large current pulses (e.g., 2,000 amps); (2) to limit the surge voltage appearing at the terminals of vulnerable equipment to safe values; (3) to breakdown at the same voltage or very nearly so each time breakdown occurs; and (4) to withstand normal voltages appearing on the transmission line without breaking down.

Gas-tube, voltage surge protectors generally comprise a tubular ceramic or glass housing closed at each end by a metal electrode to form a hermetically sealed discharge chamber for enclosing an ionizable gas mixture. There is an internal gap between the electrodes,

and one of the electrodes is typically connected to the transmission line while the other is connected to ground. When a surge of abnormally high voltage appears on the line, the gas tube breaks down; i.e., the gas ionizes, and conducts the abrupt increase in current from one electrode across the gap through the device to the other electrode and ground. The device, thereby, inherently limits the voltage which appears across itself, and therefore the equipment with which it is connected in parallel, to safe values.

Gas-tube surge protectors must maintain a high open-circuit resistance, typically ohms. Upon breakdown in normal usage, the ionized gas causes sputtering and/or pitting of the metal electrodes. The sputtered material accumulates on the walls of the glass or ceramic enclosure adjacent the gap, progressively spreads toward the junction of the electrodes and the housing, degrading the insulation resistance, and ultimately short circuits the electrodes. This greatly reduces the life time and integrity of the insulation. In addition, interelectrode continuity through .the sputtered material on the ceramic walls interferes with the fidelity of transmission because it is effectively an unstable resistance and, therefore, a source of noise.

The sputtered material is prevented from rapidly destroying the insulation resistance in prior art gastube, surge protectors, such as that disclosed by US. Pat. No. 3,454,81 l, issued to F. G. Scudner, Jr., on July 8, 1969, by providing a setback for each electrode. Each setback prevents the sputtered material from joining the adjacent electrode.

However, this leaves the sputtered material electrically floating on the wall of the enclosure where it affects the start of ionization but not in a controlled manner. As a result, the breakdown voltage is neither as low nor as consistent as it is when the material is not floating, i.e., is connected to one electrode. Further, these prior art devices locate the gap in the center of the enclosure so that they do not utilize the full potential inherent in the length of the housing for'insulating the electrodes from the sputtered material.

SUMNIARY OF THE INVENTION Accordingly, it is an object of the invention to provide a voltage surge protective device that will break down at a lower voltage value, and in a narrower range than heretofore, during repetitive surges, i.e., in a controlled manner.

It is another object of the invention to form a starter electrode by sputtering electrode material onto the walls of the enclosure in such a way that the sputtered material contacts and electrically connects to one electrode.

It is still another object of the invention to locate the main discharge gap at the opposite end of a discharge chamber from a setback to provide the longest possible insulation path and thus, to prolong the life of the surge protector.

With these and other objects in view, the gas-tube, voltage surge protective device of the present invention comprises a tubular ceramic housing having a coaxial counterbore at only one end.

A first metal electrode, having a shoulder complimentary to the counterbore but smaller so that the shoulder is spaced from the surface of thecounterbore, is disposed at the counterbore end of the housing. A cylindrical portion of the first electrode, smaller in diameter than the bore of the tubular housing, extends coaxially into the bore past the midpoint of its length. The annular space formed between the shoulder and the counterbore is set back from the annular space formed between the cylindrical portion of the electrode and the bore of the tubular housing.

A second metal electrode, having a truncated shoulder smaller than and coaxially protruding into the bore of the tubular housing, is disposed at the other end of the housing. The first and second electrode close the ends of the housing to form a chamber for ionizable gas.

The space between the faces of the cylindrical portion of the first electrode and the truncated shoulder of the second electrode is the primary or main gap of the device. Because the cylindrical portion extends beyond the rnidlength of the housing, this gap is formed near the second electrode away from the set-back annular space. The combination of the set-back space and distance of the gap from the setback maintains high open-circuit insulation resistance and prolongs the integrity of the device.

Electrode material is sputtered onto the wall of the housing to intentionally join with the second electrode and effectively extend it along the wall so that this elec trode surrounds a part of the cylindrical portion of the first electrode. The annular space between the cylindrical portion of the first electrode and the surrounding portion of the second electrode is by design, smaller than the main gap between the electrodes. The sputteredportion of the second electrode acts as a starter electrode to initiate breakdown in the space between it and the first electrode at a lower voltage than otherwise, and to produce breakdown at the same voltage each time. This starter electrode, therefore, exerts control over the breakdown voltage.

DESCRIPTION OF THE DRAWINGS The invention, together with its various features and advantages, can be understood from the following more detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 isa cross-sectional view of a generally cylindrical voltage surge protective device, which is suitable for individual gas filling, embodying the invention; FIG. 2 is a cross section of a generally cylindrical voltage surge protective device, which is suitable for mass gas filling and processing, embodying the invention;

FIG. 3 is a circuit for forming the starter electrode of the invention;

FIG. 4 is a schematic diagram showing the no-current voltage pulses of the circuit of FIG. 3; and

FIG. 5 is a chart showing the changes in level and range of breakdown voltage for both slow and fast rise voltage surges.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown a voltage surge protective device having a tubular insulative housing 12. The housing 12 is preferrably made of ceramic, such as aluminum oxide.

Seal surfaces

14 and 16 of the housing 12 are metallized to provide thin adherent coatings (not shown) on the surfaces to which electrodes may subsequently be brazed. The metallization is typically molybdenum-manganese. However, the housing 12 may be constructed of glass with suitable metal end pieces having a coefficient of expansion matching that of the glass, as is well known in the art. In addition, the bore 18 of the housing 12 is enlarged coaxially at one end to form a counterbore 20 with its cylindrical surface setback from the bore 18. Typically, the dimensions in inches of the housing may be 0.280 O.D., 0.134 ID. and 0.150 long with a 0.170 diameter 0.025 deep counterbore.

A first electrode 22 is brazed to the counterbored end of the housing 12 by methods well known in the art. Preferrably, the electrode 22 is oxygen-free copper having a shoulder 26 and a cylindrical member 28. Advantageously, brazing material 24 is the eutectic alloy of copper and silver, initially in the form of a thin washer which is assembled between to electrode 22 and the housing 12 prior to brazing. However, other brazing materials may be used.

The shoulder 26 is coaxially disposed on the electrode 22 so that it is concentric with and compliments the counterbore 20 but is distant from the surfaces thereof. The dimensions of the shoulder in inches, forexample, may be 0.168 diameter by 0.022 long. This provides an annular space of about 0.001 inches, between the counterbore 20 and the shoulder 26 of the electrode 22, which is set back from the bore 18 of the housing 12.

The cylindrical portion or member 28 of the electrode 22 extends coaxially into the bore 18. A face 30 of the member 28 forms one side of a main gap 32 of the device 10. The length of the cylindrical portion or member 28 is such as to position the face 30 beyond the midpoint or midlength of the bore 18 near the end of the housing 12 opposite the counterbore 20. The diameter of the cylindrical member 28 is chosen so that the distance across anannular space 34 is appreciably smaller than (less than half) the distance across the gap 32. Typically, the cylindrical member 28 is 0.1 17 inches in diameter by 0.092 inches long.

A second oxygen-free copper electrode 36 is brazed to the noncounterbored end of the housing 12 with brazing material 24 in washer form in the same way that the first electrode 22 is brazed to the counterbored end. A conical frustrum portion 38 extends concentrically into the bore 18 and a face 40 thereof forms the other side of the gap 32. Typically, the conical portion 38 has a base diameter of 0.124 inches, a height of 0.020 inches and a small diameter of 0.117 inches in diameter.

The first electrode 22, in the first embodiment of the invention shown in FIG. 1, is provided with a passage 41 and a vacuum tight tubulation 43 so that the enclosure or chamber formed by the electrodes and housing may be evacuated and back-filled with gas.

By techniques well known in the art, the chamber is evacuated and outgassed through the tubulation 43 to a pressure of 5 X l0" torr and then back-filled with an ionizable gas mixture, preferrably 10 percent hydrogen and percent argon, to a pressure that will yield the desired electrical characteristics, about torr in this case. Finally, the tubulation is pinched closed to hermetically seal the voltage surge protective device 10.

The device 10 may be made without the passage 41 and tubulation 43, as shown in the embodiment of FIG. 2. In this embodiment, a multiplicity of devices 10 are assembled with suitable brazing washers and placed in a bell jar apparatus where they are degassed by evacuating the entire bell jar. The pressure in the jar is maintained at less than l X 10 torr while the peak temperature is held at 500 C. When outgassing is complete, the apparatus is back filled with a 10 percent hydrogen-90 percent argon gas mixture at an elevated pressure which will result in the desired pressure, about 150 torr in this case, when the devices 10 are cooled to 25 C. Finally, the temperature of the devices 10 is raised just enough to melt the brazing solder washers which were assembled between the

electrodes

22 and 36 and the ceramic housing 12. Upon cooling, hermetically sealed chambers forming surge protective devices 10 are produced without tubulations but containing the gas mixture at the desired pressure.

In either of the above embodiments, the positioning of the gap 32 near the second-electrode end of the device 10 provides two advantageous conditions: first, material sputtered from the

faces

30 and 40 onto the bore 18 of the housing 12 must extend a long distance along the bore before it can provide a leakage path to or short out the first electrode 22; and second, the sputtered material will quickly build up and join with the second electrode 36 to form an extension of that electrode and make it virtually cup shaped. This extension is a shell which may be thought of as a starter portion 42 or starter electrode 42.

The first condition, in conjunction with the setback, prevents the sputtered material along the bore 18 from reaching the first electrode 22 for a long time, and thus yields the advantage of greatly lengthened life of the device 10. The second condition causes ionization to start in the space 34 at nearly the same voltage each time. The ionization then spreads to and triggers breakdown and conduction in the main gap 32, thus, yielding better reproducibility of the breakdown voltage than in those devices where the sputtered material is electrically floating, i.e., where the sputtered material is not connected to either electrode.

The gas-tube surge protective device should be aged before actual use. This conditions the device 10 and is done by dc. pulsing the device at some predetermined current level, pulse duration, and polarity. The aging cleans the electrodes by ion bombardment from the ionized gas. The material displaced by bombardment, i.e., the material sputtered from the first electrode, deposits on the bore 18 of the housing 12. By intentionally prolonging the aging, as in the present invention, so that more material is displaced, the material is built up sufficiently to contact the second electrode 36 and form a portion 42 of the second electrode in situ. It is to be noted that the portion 42 can be deposited by other techniques such as applying conductive metal paste and sintering or vapor depositing conductive material. However, whatever the method not only is start of ionization aided but a surge protective device 10 having a lower breakdown voltage and better reproducibility of the breakdown voltage is obtained.

Sputtering the material from the first electrode 22 to the wall of the housing 12, i.e., bore 18, is to be preferred and may be carried out by pulsing with the circuit shown schematically in FIG. 3 and in the manner illustrated in FIG. 4 which shows the voltage variation under no-current conditions.

A relatively high voltage is needed to initiate ionization but once conduction starts a lower voltage will sustain it. This fact is used as shown in FIGS. 3 and 4. An ionization circuit 44 provides a high voltage ionization initiation pulse 45, shown solid in FIG. 4. This pulse leads by 90 a lower voltage, current-sustaining pulse 47, shown dashed'in FIG. 4, which is provided by a sputtering circuit 46. Under no-current conditions, the ionization pulse 45 is about 1,500 peak volts and is obtained from a step-up transformer 48 and a l 17 volt, 60 I-Izac. source 49 by rectification. The sustaining pulse 47 is about 290 peak volts and is obtained by rectification from a 208 volt (rms), 60 Hz, a.c. source 50.

The current in the ionization circuit 44 is limited to the milliampere range by the resistors 52 A and B, which are about 10,000 and 100,000 ohms, respectively, to hold down the size and cost of the step-up transformer 48.

The sustain current in the sputtering circuit 46 is controlled by means of a thyratron rectifier 54, an external timing circuit (not shown) which actuates a relay switching contact 56. The grid of the thyratron rectifier 54 is normally biased negatively by a source 58 of bias voltage and no current flows through the rectifier. However, when the timing circuit actuates the relay and closes the contact 56, the negative grid bias is overcome by a source 60 of positive bias of greater magnitude than the negative bias and the rectifier 54 conducts. The current drawn passes through the surge protector 10, which is inserted in the, circuit so that its first electrode 22 is negative and second electrode is positive, and a resistor 62. The resistor 62 is small, of the order of 10 ohms, so that the current is limited only by the rectifier 54.

The external timing circuit closes the contact 56 for six cycles, i.e., 0.1 seconds, each second. This is repeated 10 times, i.e., for a total of 10 seconds. At the end of this time the starter electrode 42 is formed on the walls of the housing 12 and becomes an integral part of the second electrode 36.

It has been found that voltage surge protective devices 10 made as disclosed herein have extremely desirable lower average (of forward and reverse) and narrower range of breakdown voltages, for both slow rise and fast rise surges, than protectors made without the starter electrode 42. The improved performance is best illustrated by the chart of FIG. 5 wherein the results for a slow-rise voltage surge of 200 volts per second are shown by dashed lines and the results for a fast-rise voltage surge of 450 volts per microsecond are shown by solid lines. The chart shows that the breakdown voltage is lower and the variation less (range narrower) for both fast-rise and slow-rise voltage surges.

Although cup-shaped electrodes, i.e. electrodes having the starter portion 42, and their benefits are known in the art, such electrodes are costly to form. However, by taking advantage of the fact that some of the electrode material naturally will be sputtered on the inner wall, i.e., bore 18 of the housing 12, the starter portion 42 can be formed in situ and the surge protective device 10 thereby endowed with the superiority of the cup-shaped electrode at little or no extra cost.

What is claimed is:

l. A surge protective device which comprises:

a. an insulative housing having a bore extending therethrough;

b. a first electrode disposed at one end of the housing and having a,member extending into the bore of the housing;

c. a second electrode disposed at the other end of the housing bore in spaced relation to the first electrode to form a gap therebetween and closing the housing bore to form a chamber for ionizable gas; and

d. a starter electrode deposited on the wall of the housing bore and joined to the second electrode, said starter electrode extending along the wall of the housing bore to circumscribe a portion of the member of the first electrode to initiate the ionization of the gas and provide reproducibility of breakdown voltage.

2. A surge protective device, as recited in claim 1,

wherein:

a. the insulative housing is ceramic having a counterbore at one end of the bore; and

b. the first electrode has a shoulder substantially concentric, complimentary with and spaced from the counterbore to provide a setback shielded from sputtered material to prevent shorting of the electrodes.

3. A surge protective device, as recited in claim 2, wherein the member extending into the bore also extends past the midlength of the bore so that the gap is located remote from the counterbored end to lengthen the life of the device.

4. A surge protective device, as recited in claim 1, wherein the first and second electrodes are copper.

5. A surge protective device, as recited in claim 1, wherein the starter electrode is other electrode material sputtered on the wall of the housing bore.

6. A surge protective device which comprises:

a. a tubular insulative housing having a counterbore at one end to form a setback;

b. a first copper electrode disposed at the counterbored end of the housing,

said first electrode having shoulder spaced substantially concentrically from and complimentary with the counterbore to shade the setback and prevent sputtered material on the inner wall of the tubular housing from contacting the first electrode,

said shoulder having a cylindrical portion extending coaxially therefrom into the bore of the tubular insulative housing past the midpoint and in spaced relation to the inner wall thereof,

the face of said cylindrical portion being distant from the counterbored end of the housing;

c. a second copper electrode disposed at the other end of the housing,

said second electrode having a conical frustrum portion extending coaxially into the bore of and in spaced relation to the inner wall of the tubular insulative housing, the face of the conical frustrum portion of the second electrode being spaced from the face of the cylindrical portion of the first electrode to form the device are gap,

said first and second electrodes and housing forming an enclosed chamber for a gas capable of rapid ionization upon application of a surge voltage to the electrodes; and d. a starter electrode joined to said second electrode, said starter electrode being electrode material sputtered onto the inner wall of the housing to form a shell electrically joined to the second electrode and circumscribing at least a part of the cylindrical portion of the first electrode to form an annular space between said starter electrode and cylindrical portion, said space being smaller than said device gap, and said starter electrode initiating discharge in the ionizable gas at a uniform level.

7. In combination with a surge protective device of the type wherein a tubular insulative housing is closed at one end by a first electrode having a cylindrical member extending into the bore of the housing and at the other end by a second electrode in spaced relation to the first electrode to form a discharge gap within a chamber for ionizable gas, the improvement, which comprises:

a starter electrode deposited on the inner wall of the housing and joined to the second electrode to initiate ionization of the gas and maintain the initiation at a uniform level.

8. A surge protective device, as recited in claim 7,

wherein the starter electrode is other electrode materialgputtered on the inner wallof the housing A surge protective device, as recite in claim 8,

wherein the device includes a setback at one end and the discharge gap at the other.

10. A method of making a gas-filled surge protective device of the type wherein a tubular housing is closed at one end by a first electrode having a cylindrical member extending into the bore of the housing and closed at the other end by a second electrode spaced therefrom to'form a gap therebetween and to form a chamber for ionizable gas, which comprises:

depositing conductive material on the bore of the housing, said material contacting the second electrode and extending therefrom along the bore to circumscribe a portion of the cylindrical member and to form a starter electrode.

11. A method of making a gas-filled surge protector, as recited in claim 10, wherein conductive material is deposited by sputtering.

12. A method of making a gas-filled surge protector, as recited in claim 11, wherein the cylindrical member extends into the bore past the midlength to locate the gap in proximity to the end of the housing.

Claims

1. A surge protective device which comprises: a. an insulative housing having a bore extending therethrough; b. a first electrode disposed at one end of the housing and having a member extending into the bore of the housing; c. a second electrode disposed at the other end of the housing bore in spaced relation to the first electrode to form a gap therebetween and closing the housing bore to form a chamber for ionizable gas; and d. a starter electrode deposited on the wall of the housing bore and joined to the second electrode, said starter electrode extending along the wall of the housing bore to circumscribe a portion of the member of the first electrode to initiate the ionization of the gas and provide reproducibility of breakdown voltage.

2. A surge protective device, as recited in claim 1, wherein: a. the insulative housing is ceramic having a counterbore at one end of the bore; and b. the first electrode has a shoulder substantially concentric, complimentary with and spaced from the counterbore to provide a setback shielded from sputtered material to prevent shorting of the electrodes.

6. A surge protective device which comprises: a. a tubular insulative housing having a counterbore at one end to form a setback; b. a first copper electrode disposed at the counterbored end of the housing, said first electrode having shoulder spaced substantially concentrically from and complimentary with the counterbore to shade the setback and prevent sputtered material on the inner wall of the tubular housing from contacting the first electrode, said shoulder having a cylindrical portion extending coaxially therefrom into the bore of the tubular insulative housing past the midpoint and in spaced relation to the inner wall thereof, the face of said cylindrical portion being distant from the counterbored end of the housing; c. a second copper electrode disposed at the other end of the housing, said second electrode having a conical frustrum portion extending coaxially into the bore of and in spaced relation to the inner wall of the tubular insulative housing, the face of the conical frustrum portion of the second electrode being spaced from the face of the cylindrical portion of the first electrode to form the device arc gap, said first and second electrodes and housing forming an enclosed chamber for a gas capable of rapid ionization upon application of a surge voltage to the electrodes; and d. a starter electrode joined to said second electrode, said starter electrode being electrode material sputtered onto the inner wall of the housing to form a shell electrically joined to the second electrode and circumscribing at least a part of the cylindrical portion of the first electrode to form an annular space between said starter electrode and cylindrical portion, said space being smaller than said device gap, and said starter electrode initiating discharge in the ionizable gas at a uniform level.

7. In combination with a surge protective device of the type wherein a tubular insulative housing is closed at one end by a first electrode having a cylindrical member extending into the bore of the housing and at the other end by a second electrode in spaced relation to the first electrode to form a discharge gap within a chamber for ionizable gas, the improvement, which comprises: a starter electrode deposited on the inner wall of the housing and joined to the second electrode to initiate ionization of the gas and maintain the initiation at a uniform level.

8. A surge protective device, as recited in claim 7, wherein the starter electrode is other electrode matErial sputtered on the inner wall of the housing.

9. A surge protective device, as recited in claim 8, wherein the device includes a setback at one end and the discharge gap at the other.

10. A method of making a gas-filled surge protective device of the type wherein a tubular housing is closed at one end by a first electrode having a cylindrical member extending into the bore of the housing and closed at the other end by a second electrode spaced therefrom to form a gap therebetween and to form a chamber for ionizable gas, which comprises: depositing conductive material on the bore of the housing, said material contacting the second electrode and extending therefrom along the bore to circumscribe a portion of the cylindrical member and to form a starter electrode.