THREE ELECTRODE SPARK GAP APPARATUS
United States Patent 3715614
A three electrode spark gap apparatus wherein three electrodes are arranged along the same axis with two of the electrodes concentrically arranged. The electrodes and the enclosure which houses the electrodes are generally cylindrical shaped and are dimensioned so that the radius of the enclosure is at least four times greater than the radius of the largest electrode located within the enclosure to prevent damage to the enclosure from the arc between electrodes.
US Patent References:
Gaseous electric discharge tubes and electrodes
Gates - April 1957 - 2790923
Radar transmit receive switch
Biondi - July 1957 - 2799804
Gaseous electric discharge tubes and electrodes
Gates - April 1957 - 2790923
Radar transmit receive switch
Biondi - July 1957 - 2799804
Application Number:
05/184381
Publication Date:
02/06/1973
Filing Date:
09/28/1971
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
313/634, 313/602
International Classes:
H01T2/02; H01T2/00; H01J17/30
Field of Search:
313/183,188,198 315/349
Primary Examiner:
Brody, Alfred L.
Claims:
Having described the invention, what is claimed is
1. A three electrode spark gap comprising:
2. The three electrode spark gap as recited in claim 1 wherein the surface area A3 terminates a maximum radial distance R3 from said central axis which is greater than R1.
3. The three electrode spark gap is recited in claim 2 wherein the surface area A3 is greater than the surface area A2.
4. The three electrode spark gap as recited in claim 1 wherein the arc discharge surface of said third electrode is spaced further from the arc discharge surface of said first electrode than said second arc discharge surface.
5. The three electrode spark gap as recited in claim 2 wherein the arc discharge surface of said third electrode is spaced further from the arc discharge surface of said first electrode than said second arc discharge surface.
6. The three electrode spark gap as recited in claim 3 wherein the arc discharge surface of said third electrode is spaced further from the arc discharge surface of said first electrode than said second arc discharge surface.
7. The three electrode spark gap as recited in claim 1 wherein said airtight enclosure includes:
8. The three electrode spark gap as recited in claim 2 wherein said airtight enclosure includes:
9. The three electrode spark gap as recited in claim 3 wherein said airtight enclosure includes:
10. The three electrode spark gap as recited in claim 4 wherein said airtight enclosure includes:
11. The three electrode spark gap as recited in claim 5 wherein said airtight enclosure includes:
12. The three electrode spark gap as recited in claim 6 wherein said airtight enclosure includes:
13. A three electrode spark gap as recited in claim 1 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
14. A three electrode spark gap as recited in claim 2 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
15. A three electrode spark gap as recited in claim 3 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
16. A three electrode spark gap as recited in claim 4 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
17. A three electrode spark gap as recited in claim 5 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
18. A three electrode spark gap as recited in claim 6 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
19. A three electrode spark gap as recited in claim 7 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
20. A three electrode spark gap as recited in claim 8 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
21. A three electrode spark gap as recited in claim 9 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
22. A three electrode spark gap as recited in claim 10 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
23. A three electrode spark gap as recited in claim 11 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
24. A three electrode spark gap as recited in claim 12 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
25. A three electrode spark gap as recited in claim 1 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
26. A three electrode spark gap as recited in claim 2 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
27. A three electrode spark gap as recited in claim 3 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
28. A three electrode spark gap as recited in claim 4 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
29. A three electrode spark gap as recited in claim 7 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
30. A three electrode spark gap as recited in claim 8 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
31. A three electrode spark gap as recited in claim 9 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
32. A three electrode spark gap as recited in claim 13 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
33. A three electrode spark gap as recited in claim 14 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
34. A three electrode spark gap as recited in claim 15 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
35. A three electrode spark gap as recited in claim 16 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
36. The three electrode spark gap as recited in claim 1 wherein the first base and second base of said enclosure are comprised of a material having a coefficient of linear thermal expansion in the range of 50 × 10-6 to 0.5 × 10-6 so that glass may be sealed to said bases without cracking.
37. The three electrode spark gap as recited in claim 7 wherein the first base and second base of said enclosure are comprised of a material having a coefficient of linear thermal expansion in the range of 50 × 10-6 to 0.5 × 10-6 so that glass may be sealed to said bases without cracking.
38. The three electrode spark gap as recited in claim 8 wherein the first base and second base of said enclosure are comprised of a material having a coefficient of linear thermal expansion in the range of 50 × 10-6 to 0.5 × 10-6 so that glass may be sealed to said bases without cracking.
39. The three electrode spark gap as recited in claim 9 wherein the first base and second base of said enclosure are comprised of a material having a coefficient of linear thermal expansion in the range of 50 × 10-6 to 0.5 × 10-6 so that glass may be sealed to said bases without cracking.
40. The three electrode spark gap as recited in claim 10 wherein the first base and second base of said enclosure are comprised of a material having a coefficient of linear thermal expansion in the range of 50 × 10-6 to 0.5 × 10-6 so that glass may be sealed to said bases without cracking.
41. The three electrode spark gap as recited in claim 11 wherein the first base and second base of said enclosure are comprised of a material having a coefficient of linear thermal expansion in the range of 50 × 10-6 to 0.5 × 10-6 so that glass may be sealed to said bases without cracking.
42. The three electrode spark gap as recited in claim 12 wherein the first base and second base of said enclosure are comprised of a material having a coefficient of linear thermal expansion in the range of 50 × 10-6 to 0.5 × 10-6 so that glass may be sealed to said bases without cracking.
43. A three electrode spark gap comprising:
44. The three electrode spark gap as recited in claim 1 wherein said arc discharge surfaces of said first and third electrodes are comprised of barium aluminate.
1. A three electrode spark gap comprising:
2. The three electrode spark gap as recited in claim 1 wherein the surface area A3 terminates a maximum radial distance R3 from said central axis which is greater than R1.
3. The three electrode spark gap is recited in claim 2 wherein the surface area A3 is greater than the surface area A2.
4. The three electrode spark gap as recited in claim 1 wherein the arc discharge surface of said third electrode is spaced further from the arc discharge surface of said first electrode than said second arc discharge surface.
5. The three electrode spark gap as recited in claim 2 wherein the arc discharge surface of said third electrode is spaced further from the arc discharge surface of said first electrode than said second arc discharge surface.
6. The three electrode spark gap as recited in claim 3 wherein the arc discharge surface of said third electrode is spaced further from the arc discharge surface of said first electrode than said second arc discharge surface.
7. The three electrode spark gap as recited in claim 1 wherein said airtight enclosure includes:
8. The three electrode spark gap as recited in claim 2 wherein said airtight enclosure includes:
9. The three electrode spark gap as recited in claim 3 wherein said airtight enclosure includes:
10. The three electrode spark gap as recited in claim 4 wherein said airtight enclosure includes:
11. The three electrode spark gap as recited in claim 5 wherein said airtight enclosure includes:
12. The three electrode spark gap as recited in claim 6 wherein said airtight enclosure includes:
13. A three electrode spark gap as recited in claim 1 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
14. A three electrode spark gap as recited in claim 2 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
15. A three electrode spark gap as recited in claim 3 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
16. A three electrode spark gap as recited in claim 4 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
17. A three electrode spark gap as recited in claim 5 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
18. A three electrode spark gap as recited in claim 6 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
19. A three electrode spark gap as recited in claim 7 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
20. A three electrode spark gap as recited in claim 8 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
21. A three electrode spark gap as recited in claim 9 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
22. A three electrode spark gap as recited in claim 10 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
23. A three electrode spark gap as recited in claim 11 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
24. A three electrode spark gap as recited in claim 12 wherein the pressure in said enclosure is less than 750 × 10-3 torr and includes a mixture of hydrogen and Argon.
25. A three electrode spark gap as recited in claim 1 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
26. A three electrode spark gap as recited in claim 2 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
27. A three electrode spark gap as recited in claim 3 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
28. A three electrode spark gap as recited in claim 4 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
29. A three electrode spark gap as recited in claim 7 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
30. A three electrode spark gap as recited in claim 8 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
31. A three electrode spark gap as recited in claim 9 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
32. A three electrode spark gap as recited in claim 13 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
33. A three electrode spark gap as recited in claim 14 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
34. A three electrode spark gap as recited in claim 15 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
35. A three electrode spark gap as recited in claim 16 wherein said arc discharge surfaces of said second and third electrodes are comprised of different materials and said first and third electrodes are comprised of the same materials.
36. The three electrode spark gap as recited in claim 1 wherein the first base and second base of said enclosure are comprised of a material having a coefficient of linear thermal expansion in the range of 50 × 10-6 to 0.5 × 10-6 so that glass may be sealed to said bases without cracking.
37. The three electrode spark gap as recited in claim 7 wherein the first base and second base of said enclosure are comprised of a material having a coefficient of linear thermal expansion in the range of 50 × 10-6 to 0.5 × 10-6 so that glass may be sealed to said bases without cracking.
38. The three electrode spark gap as recited in claim 8 wherein the first base and second base of said enclosure are comprised of a material having a coefficient of linear thermal expansion in the range of 50 × 10-6 to 0.5 × 10-6 so that glass may be sealed to said bases without cracking.
39. The three electrode spark gap as recited in claim 9 wherein the first base and second base of said enclosure are comprised of a material having a coefficient of linear thermal expansion in the range of 50 × 10-6 to 0.5 × 10-6 so that glass may be sealed to said bases without cracking.
40. The three electrode spark gap as recited in claim 10 wherein the first base and second base of said enclosure are comprised of a material having a coefficient of linear thermal expansion in the range of 50 × 10-6 to 0.5 × 10-6 so that glass may be sealed to said bases without cracking.
41. The three electrode spark gap as recited in claim 11 wherein the first base and second base of said enclosure are comprised of a material having a coefficient of linear thermal expansion in the range of 50 × 10-6 to 0.5 × 10-6 so that glass may be sealed to said bases without cracking.
42. The three electrode spark gap as recited in claim 12 wherein the first base and second base of said enclosure are comprised of a material having a coefficient of linear thermal expansion in the range of 50 × 10-6 to 0.5 × 10-6 so that glass may be sealed to said bases without cracking.
43. A three electrode spark gap comprising:
44. The three electrode spark gap as recited in claim 1 wherein said arc discharge surfaces of said first and third electrodes are comprised of barium aluminate.
Description:
BACKGROUND OF THE INVENTION
This invention relates to a gaseous conductor for controlling discharges of energy greater than 100 joules stored in capacitor. The invention is more particularly related to a gaseous conductor that includes three electrodes, one of which is a trigger electrode that renders the gaseous conductor conductive to discharge the capacitor.
In certain blasting operations such as those performed in tunnels and shaft mining, it is generally desirable to explode explosives with electric blasting caps. Typically, the electric blasting caps are detonated by electric energy which is received from a battery or ac power source. Obviously, the most important aspect in blasting is the safety of the people involved in the operations. Therefore, a most important safety feature of any blasting circuit is the electric or mechanical device that isolates the explosives from the electrical energy that will be used to detonate the explosives. Examples of electrical systems for firing explosive bridge wire devices or the like may be found in U.S. Pat. No. 3,417,306 to J. L. Kanak titled "Regulated Voltage Capacitor Discharge Circuit" and U.S. Pat. 2,950,419 to I. E. Linkroum titled "Ignition Apparatus." In all of the prior art blasting circuits, the reliability and the life of the electrical element that isolated the explosives from the electrical energy that detonated them was always a serious problem to work with. One device which is utilized to isolate the explosives from the electrical circuitry is a two or three electrode gaseous conductor called a "spark gap." In this device, one electrode triggers the gaseous conductor conductive and establishes an arc between the trigger electrode and cathode electrode that jumps to the anode electrode permitting the rapid discharge of the stored energy in the capacitor. It is important that this device operates (conducts) properly every time it is triggered. The spark gap includes a glass enclosure to allow for the observation of the arc discharge when it occurs. One problem with prior art spark gaps is the arc between the electrodes which blackens and melts the glass to a point where it is questionable whether or not the spark gap would operate or even worse, would operate after an unknown time delay. When dealing with explosives, this condition is too dangerous to permit.
SUMMARY OF THE INVENTION
This invention provides a three electrode spark gap device that is shaped and dimensioned to protect the glass enclosure around the electrodes from unwanted material sputtered from the cathode and heat from the arc between the electrodes both of which could damage the device and prevent proper operation of the spark gap.
The invention is a three electrode spark gap for capacitive discharge circuits that is characterized by the physical sizes and the locations of the components which optimize the life and operation of the device.
In one embodiment of the invention, the three electrode spark tap comprises: an air-tight enclosure comprised of a glass tube having a Kovar (T.M.) base mounted at each end of the tube to form the enclosure, the tube includes a central longitudinal axis and an inside radial cross-sectional area A4, the inside surface of the tube spaced from the central axis a minimum radial distance R4; an ionizable atmosphere, that includes argon, contained in the enclosure at a pressure less than atmospheric; a first elongated electrode mounted on one base and extending into the enclosure along the central axis, the free end portion of the first electrode including an arc discharge surface having a cross-sectional area A1, the surface terminating from the central axis a maximum radial distance R1, which is less than R4, and wherein the Ratio R4/R1 is greater than 4 and the ratio A4/A1 is greater than 3; the second elongated electrode mounted on the other base and extending into the enclosure along the central axis, the free end portion of the second electrode including an arc discharge surface which is spaced from and faces the arc discharge surface of the first electrode and which has a cross-sectional area A2, which is less than A1 and which terminates a maximum radial distance R2 from the central axis which is less than R1, the second electrode electrically isolated from the first electrode; and a third tubular electrode mounted on the said other base and extending into the enclosure along the central axis, the third tubular electrode electrically isolated from the first and second electrodes and mounted concentric with the second electrode, said free end portion of said third electrode including an arch discharge surface which is spaced from and faces the arc discharge surface of the first electrode and which has a cross-sectional area A3, which is greater than A2, but less than A1, and which terminates a maximum radial distance R3, from the central axis which is greater that R1, and wherein the ratio R4/R3 is greater than 4.
Accordingly, it is an object of this invention to provide a three electrode spark gap for firing an explosive bridge wire device or the like that is more reliable and has longer life than prior art devices.
Another object of this invention is to provide a device for triggering explosive devices.
Another object of this invention is improve the safety features of existing capacitor discharge devices for blasting applications.
It is still a further object of this invention to provide a trigger device for a capacitive discharge circuit that electrically isolates energy stored in the capacitor from a detonating device connected to explosives.
The above and other objects and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings and claims which form a part of this specification.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 illustrates a three electrode spark discharge device that incorporates the principles of this invention.
FIG. 2 is a cross-sectional view of the device shown in FIG. 1 along the lines II--II.
FIG. 3 is an enlarged cross-sectional view of the concentrically arranged electrodes shown in FIG. 1.
FIG. 4 is a side view of the electrodes shown in FIG. 3 taken along the lines IV--IV.
FIG. 5 is a partial electrical schematic diagram of the spark discharge device utilized in a capacitor discharge circuit.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, FIG. 1 illustrates a spark discharge device for triggering a capacitor discharge circuit. The spark gap device shown is used in blasting machine circuits and serves as a switch for discharging the energy stored in the capacitor through blasting caps. This device has been found to be greatly superior to mechanical switches in this application because the contacts of mechanical switches burn away after relatively few operations because of the energy passing through them. Ordinary gaseous conductors are inappropriate for use in blasting machine applications because the discharge energy is high (greater than 100 joules). Therefore, the dimensions and materials used in a spark gap device are very important.
FIG. 1 shows a spark gap device which comprises: an enclosure formed by base plates 5 and 6, mounted on the ends of the glass tube 4, and electrodes 1, 2 and 3 mounted on the base plates 5 and 6 along the central or longitudinal axis X of the tube 4. The tubing 4 is comprised of a transparent material such as glass so that when an arc is initiated in the enclosure, it may be observed. Because of the high temperatures associated with the arc and for sealing purposes, the preferred glass is borosilicate glass which has a thermal expansion coefficient of about 5 × 10 - 6 . The base plates 5 and 6 are comprised of Kovar (a nickel, cobalt and iron alloy) which has a linear thermal expansion coefficient also of about 5 × 10 - 6 . Since the thermal expansion coefficients of both the glass 4 and the bases 5 and 6 are about the same the glass 4 may be melted onto the bases 5 and 6 and cooled without cracking to form an air-tight enclosure. The enclosure may be evacuated by a tubulation in the glass or by the tubulation 51 shown attached to the base 5. The sealed enclosure is evacuated to a pressure below 1 × 10 - 3 torr and then backfilled with an ionizable atmosphere to a pressure below atmospheric. A preferred ionizable atmosphere contains argon and is a mixture of 80 percent hydrogen and 20 percent argon filled to a pressure of about 350 × 10 - 3 torr.
The cathode electrode 1, the anode electrode 3, and the trigger electrode 2 are comprised of materials chosen for their electrical and physical characteristics at high currents, voltages and temperatures. Examples of acceptable electrode materials are molybdenum, tungsten, thoriated tungsten and tungsten with a low work function (less than 4.5 electron volts) metal such as thorium, aluminum, barium and mixtures thereof. In the preferred embodiment shown, the cathode electrode 1 is tungsten with barium aluminate, the anode 3 is comprised of tungsten with barium aluminate, and the trigger electrode 2 is molybdenum. Because of the high energy associated with this arc gap discharge, tungsten is the electrode material for the anode and cathode because of its high temperature characteristics (high melting temperature). Barium aluminate is added to the tungsten to improve the electrical characteristics of those electrodes as barium aluminate increases the emissivity of electrodes. The trigger electrode 2 is comprised of molybdenum because of its electrical and thermal expansion properties.
For a detailed discussion of gaseous conductors see "Vacuum Tube and Semi-Conductor Electronics," Chapter 12 "Electrical Discharges in Gases" by Jacob Millman, published by McGraw Hill and "Gaseous Conductors," by James Cobine published by Dover Publications.
FIG. 2 is a cut-away view taken along lines II--II of FIG. 1. The cathode electrode 1 includes a generally flat arc discharge surface 11. The discharge surface 11 has a total surface area A1, which extends a predetermined distance R1 from the central axis. Similarly, the inner diameter borosilicate glass tubing 4 defines a radial cross-sectional area A4 that extends a predetermined distance R4 from the central axis X. To adequately protect the borosilicate glass from melting or receiving sputtered cathode material during the presence of an arc discharge, the radial distance R4 of the inner wall of the glass tubing 4 is at least 4 times greater than the radial distance R1, which is the terminal point of the discharge surface 11 of the cathode electrode 1. In this preferred embodiment wherein the enclosure is cylindrical and the electrodes are cylindrical and arranged concentric with the central axis of the tubing, it is preferred that the ratio of the radial area A4 within the tube 4 be at least three times greater than the discharge surface area A1 of the cathode electrode 1.
FIG. 3 is an enlarged view of the anode electrode 3 and trigger electrode 2 mounted concentric with the central axis X on the base plate 6. Unlike ordinary gaseous conductors which have fragile probes located within the enclosure to trigger the device, this device employs a generally cylindrical molybdenum electrode having a cross-sectional area sufficient to handle currents associated with discharges greater than 100 joules. The trigger electrode 2 is isolated from the anode electrode 3 by an insulating sleeve 22. The insulating sleeve 22 is brazed with copper base brazing material 23 to the trigger electrode 32 to make the connection air tight. So that the starting characteristics of the spark discharge are optimized, the trigger discharge surface 21 extends beyond the end of the anode surface 31. As can be seen from the drawing, the anode electrode 3 is generally tubular shaped and mounted concentrically with the trigger electrode 2. The free end portion of the anode electrode 3 is comprised of a tungsten material with barium aluminate. The anode 3 is also brazed to the insulator 22 by a copper based alloy 33. The member 35 that extends from the anode 3 is an electrical conducting member for attaching the anode to a capacitive circuit. To assure the anode electrode 3 is mounted in air-tight relationship to the base 6, it is brazed to the base 6 with a copper base alloy 33. FIG. 4 is an end view of the electrode shown in FIG. 3 taken along lines IV--IV. This view shows the physical relationships between the anode electrode 3 and the trigger electrode 2. The trigger electrode 2 is mounted concentric with the central axis X and terminates in an arc discharge surface 21. The discharge surface 21 has a predetermined cross-sectional area A2, which extends a radial distance R2 from the central axis X. The anode electrode 3 is mounted concentric with the central axis X and the trigger electrode 2 and terminates in an arc discharge surface 31. The anode arc discharge surface 31 has a predetermined surface area A3, which extends a radial distance R3 from the central axis. The radius R3 is dimensioned so that the ratio R4/R3 is greater than 4. This assures that the envelope 4 (FIG. 2) will be adequately protected from the heat generated by an arc between the electrodes.
FIG. 5 shows schematically how the trigger discharge device is used. The spark discharge device has its anode 3 connected to a source of stored energy such as a positive terminal of a charged capacitor, its anode electrode 1 connected to a load, and the trigger electrode 2 connected to a means for receiving a trigger pulse 71, 72 and a resistor 73.
In operation, the spark gap discharge device would operate as follows: When a trigger pulse is applied to the trigger electrode 2 an electric arc is initiated between the trigger electrode 2 and the cathode 1. The value of the trigger pulse necessary to initiate this arc is a function of the voltage between the electrodes the electrode materials, the spacing between electrodes, the gas in the enclosure, and the pressure of the gas within the enclosure. Once an arc is established between the trigger electrode 2 and the cathode 1, the energy stored in the storage capacitor begins to discharge through the arc and to the load. However, the current limiting resistor 73 begins to limit the current through the trigger electrode so that the arc, looking for the path of least resistance, jumps to the arc discharge surface 31 of the anode electrode 3. The energy in the storage capacitor is then discharged through the blasting caps which explode the dynamite.
The following information summarizes the pertinent details in assembling a preferred embodiment of the invention.
EXAMPLE
I Part Size Material ____________________________________________________________ ______________ (4) Tubing ID 2.025 Borosilicate OD 2.250 Glass (5) (6) Bases OD 2.030 Kovar (11) Cathode discharge surface OD .2666 Tungsten combined with a low work function metal (21) Anode ID .430 Tungsten with a low work function material discharge surface OD .4375 (31) Trigger discharge surface OD .125 Molybdenum ____________________________________________________________ ______________
Spacing between cathode and trigger: 0.135
Spacing between cathode and anode: 0.165
Gas Fill: 80% H and 20% A to 350 × 10 - 3 torr
Breakdown voltage (cathode-trigger): 3700 - 3800 volts DC
Electrodes mounted on bases with copper brazing material
Trigger voltage:3KV
While a preferred embodiment of the invention has been disclosed, it will be apparent to those skilled in the art that changes may be made to the invention as set forth in the appended claims, and, in some cases, certain features of the invention may be used to advantage without corresponding use of other features. For example, the electrodes and enclosure may take shapes other than cylindrical so long as the maximum radial distance which the discharge surfaces of the electrodes extend from the central axis is in the proper ratio to the minimum radial distance that the inner wall of the enclosure is spaced from the central axis. Accordingly, it is intended that the illustrative and descriptive materials herein be used to illustrate the principles of the invention and not to limit the scope thereof.
This invention relates to a gaseous conductor for controlling discharges of energy greater than 100 joules stored in capacitor. The invention is more particularly related to a gaseous conductor that includes three electrodes, one of which is a trigger electrode that renders the gaseous conductor conductive to discharge the capacitor.
In certain blasting operations such as those performed in tunnels and shaft mining, it is generally desirable to explode explosives with electric blasting caps. Typically, the electric blasting caps are detonated by electric energy which is received from a battery or ac power source. Obviously, the most important aspect in blasting is the safety of the people involved in the operations. Therefore, a most important safety feature of any blasting circuit is the electric or mechanical device that isolates the explosives from the electrical energy that will be used to detonate the explosives. Examples of electrical systems for firing explosive bridge wire devices or the like may be found in U.S. Pat. No. 3,417,306 to J. L. Kanak titled "Regulated Voltage Capacitor Discharge Circuit" and U.S. Pat. 2,950,419 to I. E. Linkroum titled "Ignition Apparatus." In all of the prior art blasting circuits, the reliability and the life of the electrical element that isolated the explosives from the electrical energy that detonated them was always a serious problem to work with. One device which is utilized to isolate the explosives from the electrical circuitry is a two or three electrode gaseous conductor called a "spark gap." In this device, one electrode triggers the gaseous conductor conductive and establishes an arc between the trigger electrode and cathode electrode that jumps to the anode electrode permitting the rapid discharge of the stored energy in the capacitor. It is important that this device operates (conducts) properly every time it is triggered. The spark gap includes a glass enclosure to allow for the observation of the arc discharge when it occurs. One problem with prior art spark gaps is the arc between the electrodes which blackens and melts the glass to a point where it is questionable whether or not the spark gap would operate or even worse, would operate after an unknown time delay. When dealing with explosives, this condition is too dangerous to permit.
SUMMARY OF THE INVENTION
This invention provides a three electrode spark gap device that is shaped and dimensioned to protect the glass enclosure around the electrodes from unwanted material sputtered from the cathode and heat from the arc between the electrodes both of which could damage the device and prevent proper operation of the spark gap.
The invention is a three electrode spark gap for capacitive discharge circuits that is characterized by the physical sizes and the locations of the components which optimize the life and operation of the device.
In one embodiment of the invention, the three electrode spark tap comprises: an air-tight enclosure comprised of a glass tube having a Kovar (T.M.) base mounted at each end of the tube to form the enclosure, the tube includes a central longitudinal axis and an inside radial cross-sectional area A4, the inside surface of the tube spaced from the central axis a minimum radial distance R4; an ionizable atmosphere, that includes argon, contained in the enclosure at a pressure less than atmospheric; a first elongated electrode mounted on one base and extending into the enclosure along the central axis, the free end portion of the first electrode including an arc discharge surface having a cross-sectional area A1, the surface terminating from the central axis a maximum radial distance R1, which is less than R4, and wherein the Ratio R4/R1 is greater than 4 and the ratio A4/A1 is greater than 3; the second elongated electrode mounted on the other base and extending into the enclosure along the central axis, the free end portion of the second electrode including an arc discharge surface which is spaced from and faces the arc discharge surface of the first electrode and which has a cross-sectional area A2, which is less than A1 and which terminates a maximum radial distance R2 from the central axis which is less than R1, the second electrode electrically isolated from the first electrode; and a third tubular electrode mounted on the said other base and extending into the enclosure along the central axis, the third tubular electrode electrically isolated from the first and second electrodes and mounted concentric with the second electrode, said free end portion of said third electrode including an arch discharge surface which is spaced from and faces the arc discharge surface of the first electrode and which has a cross-sectional area A3, which is greater than A2, but less than A1, and which terminates a maximum radial distance R3, from the central axis which is greater that R1, and wherein the ratio R4/R3 is greater than 4.
Accordingly, it is an object of this invention to provide a three electrode spark gap for firing an explosive bridge wire device or the like that is more reliable and has longer life than prior art devices.
Another object of this invention is to provide a device for triggering explosive devices.
Another object of this invention is improve the safety features of existing capacitor discharge devices for blasting applications.
It is still a further object of this invention to provide a trigger device for a capacitive discharge circuit that electrically isolates energy stored in the capacitor from a detonating device connected to explosives.
The above and other objects and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings and claims which form a part of this specification.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 illustrates a three electrode spark discharge device that incorporates the principles of this invention.
FIG. 2 is a cross-sectional view of the device shown in FIG. 1 along the lines II--II.
FIG. 3 is an enlarged cross-sectional view of the concentrically arranged electrodes shown in FIG. 1.
FIG. 4 is a side view of the electrodes shown in FIG. 3 taken along the lines IV--IV.
FIG. 5 is a partial electrical schematic diagram of the spark discharge device utilized in a capacitor discharge circuit.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, FIG. 1 illustrates a spark discharge device for triggering a capacitor discharge circuit. The spark gap device shown is used in blasting machine circuits and serves as a switch for discharging the energy stored in the capacitor through blasting caps. This device has been found to be greatly superior to mechanical switches in this application because the contacts of mechanical switches burn away after relatively few operations because of the energy passing through them. Ordinary gaseous conductors are inappropriate for use in blasting machine applications because the discharge energy is high (greater than 100 joules). Therefore, the dimensions and materials used in a spark gap device are very important.
FIG. 1 shows a spark gap device which comprises: an enclosure formed by base plates 5 and 6, mounted on the ends of the glass tube 4, and electrodes 1, 2 and 3 mounted on the base plates 5 and 6 along the central or longitudinal axis X of the tube 4. The tubing 4 is comprised of a transparent material such as glass so that when an arc is initiated in the enclosure, it may be observed. Because of the high temperatures associated with the arc and for sealing purposes, the preferred glass is borosilicate glass which has a thermal expansion coefficient of about 5 × 10 - 6 . The base plates 5 and 6 are comprised of Kovar (a nickel, cobalt and iron alloy) which has a linear thermal expansion coefficient also of about 5 × 10 - 6 . Since the thermal expansion coefficients of both the glass 4 and the bases 5 and 6 are about the same the glass 4 may be melted onto the bases 5 and 6 and cooled without cracking to form an air-tight enclosure. The enclosure may be evacuated by a tubulation in the glass or by the tubulation 51 shown attached to the base 5. The sealed enclosure is evacuated to a pressure below 1 × 10 - 3 torr and then backfilled with an ionizable atmosphere to a pressure below atmospheric. A preferred ionizable atmosphere contains argon and is a mixture of 80 percent hydrogen and 20 percent argon filled to a pressure of about 350 × 10 - 3 torr.
The cathode electrode 1, the anode electrode 3, and the trigger electrode 2 are comprised of materials chosen for their electrical and physical characteristics at high currents, voltages and temperatures. Examples of acceptable electrode materials are molybdenum, tungsten, thoriated tungsten and tungsten with a low work function (less than 4.5 electron volts) metal such as thorium, aluminum, barium and mixtures thereof. In the preferred embodiment shown, the cathode electrode 1 is tungsten with barium aluminate, the anode 3 is comprised of tungsten with barium aluminate, and the trigger electrode 2 is molybdenum. Because of the high energy associated with this arc gap discharge, tungsten is the electrode material for the anode and cathode because of its high temperature characteristics (high melting temperature). Barium aluminate is added to the tungsten to improve the electrical characteristics of those electrodes as barium aluminate increases the emissivity of electrodes. The trigger electrode 2 is comprised of molybdenum because of its electrical and thermal expansion properties.
For a detailed discussion of gaseous conductors see "Vacuum Tube and Semi-Conductor Electronics," Chapter 12 "Electrical Discharges in Gases" by Jacob Millman, published by McGraw Hill and "Gaseous Conductors," by James Cobine published by Dover Publications.
FIG. 2 is a cut-away view taken along lines II--II of FIG. 1. The cathode electrode 1 includes a generally flat arc discharge surface 11. The discharge surface 11 has a total surface area A1, which extends a predetermined distance R1 from the central axis. Similarly, the inner diameter borosilicate glass tubing 4 defines a radial cross-sectional area A4 that extends a predetermined distance R4 from the central axis X. To adequately protect the borosilicate glass from melting or receiving sputtered cathode material during the presence of an arc discharge, the radial distance R4 of the inner wall of the glass tubing 4 is at least 4 times greater than the radial distance R1, which is the terminal point of the discharge surface 11 of the cathode electrode 1. In this preferred embodiment wherein the enclosure is cylindrical and the electrodes are cylindrical and arranged concentric with the central axis of the tubing, it is preferred that the ratio of the radial area A4 within the tube 4 be at least three times greater than the discharge surface area A1 of the cathode electrode 1.
FIG. 3 is an enlarged view of the anode electrode 3 and trigger electrode 2 mounted concentric with the central axis X on the base plate 6. Unlike ordinary gaseous conductors which have fragile probes located within the enclosure to trigger the device, this device employs a generally cylindrical molybdenum electrode having a cross-sectional area sufficient to handle currents associated with discharges greater than 100 joules. The trigger electrode 2 is isolated from the anode electrode 3 by an insulating sleeve 22. The insulating sleeve 22 is brazed with copper base brazing material 23 to the trigger electrode 32 to make the connection air tight. So that the starting characteristics of the spark discharge are optimized, the trigger discharge surface 21 extends beyond the end of the anode surface 31. As can be seen from the drawing, the anode electrode 3 is generally tubular shaped and mounted concentrically with the trigger electrode 2. The free end portion of the anode electrode 3 is comprised of a tungsten material with barium aluminate. The anode 3 is also brazed to the insulator 22 by a copper based alloy 33. The member 35 that extends from the anode 3 is an electrical conducting member for attaching the anode to a capacitive circuit. To assure the anode electrode 3 is mounted in air-tight relationship to the base 6, it is brazed to the base 6 with a copper base alloy 33. FIG. 4 is an end view of the electrode shown in FIG. 3 taken along lines IV--IV. This view shows the physical relationships between the anode electrode 3 and the trigger electrode 2. The trigger electrode 2 is mounted concentric with the central axis X and terminates in an arc discharge surface 21. The discharge surface 21 has a predetermined cross-sectional area A2, which extends a radial distance R2 from the central axis X. The anode electrode 3 is mounted concentric with the central axis X and the trigger electrode 2 and terminates in an arc discharge surface 31. The anode arc discharge surface 31 has a predetermined surface area A3, which extends a radial distance R3 from the central axis. The radius R3 is dimensioned so that the ratio R4/R3 is greater than 4. This assures that the envelope 4 (FIG. 2) will be adequately protected from the heat generated by an arc between the electrodes.
FIG. 5 shows schematically how the trigger discharge device is used. The spark discharge device has its anode 3 connected to a source of stored energy such as a positive terminal of a charged capacitor, its anode electrode 1 connected to a load, and the trigger electrode 2 connected to a means for receiving a trigger pulse 71, 72 and a resistor 73.
In operation, the spark gap discharge device would operate as follows: When a trigger pulse is applied to the trigger electrode 2 an electric arc is initiated between the trigger electrode 2 and the cathode 1. The value of the trigger pulse necessary to initiate this arc is a function of the voltage between the electrodes the electrode materials, the spacing between electrodes, the gas in the enclosure, and the pressure of the gas within the enclosure. Once an arc is established between the trigger electrode 2 and the cathode 1, the energy stored in the storage capacitor begins to discharge through the arc and to the load. However, the current limiting resistor 73 begins to limit the current through the trigger electrode so that the arc, looking for the path of least resistance, jumps to the arc discharge surface 31 of the anode electrode 3. The energy in the storage capacitor is then discharged through the blasting caps which explode the dynamite.
The following information summarizes the pertinent details in assembling a preferred embodiment of the invention.
EXAMPLE
I Part Size Material ____________________________________________________________ ______________ (4) Tubing ID 2.025 Borosilicate OD 2.250 Glass (5) (6) Bases OD 2.030 Kovar (11) Cathode discharge surface OD .2666 Tungsten combined with a low work function metal (21) Anode ID .430 Tungsten with a low work function material discharge surface OD .4375 (31) Trigger discharge surface OD .125 Molybdenum ____________________________________________________________ ______________
Spacing between cathode and trigger: 0.135
Spacing between cathode and anode: 0.165
Gas Fill: 80% H and 20% A to 350 × 10 - 3 torr
Breakdown voltage (cathode-trigger): 3700 - 3800 volts DC
Electrodes mounted on bases with copper brazing material
Trigger voltage:3KV
While a preferred embodiment of the invention has been disclosed, it will be apparent to those skilled in the art that changes may be made to the invention as set forth in the appended claims, and, in some cases, certain features of the invention may be used to advantage without corresponding use of other features. For example, the electrodes and enclosure may take shapes other than cylindrical so long as the maximum radial distance which the discharge surfaces of the electrodes extend from the central axis is in the proper ratio to the minimum radial distance that the inner wall of the enclosure is spaced from the central axis. Accordingly, it is intended that the illustrative and descriptive materials herein be used to illustrate the principles of the invention and not to limit the scope thereof.