Abstract:
A triggerable ceramic gas tube voltage breakdown device, particularly adapted for use in an electrical circuit for controlling the light output of a photoflash lamp, includes means for reducing the attenuation of an electrical trigger pulse in the region of the electrode gap due to the ceramic spacer tube. The electric field intensity in the region of the electrode gap resulting from the trigger pulse may be enhanced by disposing annular conductive material in the region and by connecting that material to a source of the trigger pulses. Alternatively or in conjunction therewith, the configuration of the ceramic spacer tube may be altered by removing material from the ceramic spacer tube in the region of the electrode gap, thereby enhancing the electric field intensity in that region resulting from the trigger pulses.

Description:
BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The device of the present invention generally relates to gas tube voltage breakdown devices, often commonly referred to as surge arresters, and, more particularly, to a new and improved hermetically sealed gas tube voltage breakdown device having a ceramic insulating spacer and a trigger electrode and particularly adapted for repetitive use as a voltage breakdown device in an electrical circuit, for example, in an electrical circuit for controlling the light output of a photoflash lamp. 
     B. Description of the Prior Art 
     Hermetically sealed gas tube voltage breakdown devices, commonly known as and used as surge arresters, are old and well-known in the art. For example, pertinent prior art gas tube voltage breakdown devices or surge arresters are disclosed in U.S. Pat. Nos. 3,588,576; 4,084,208; and 4,287,548. Typically, such devices are used as surge arresters to protect electrical equipment from damage or destruction due to the presence of overvoltage surges. However, such devices also have been used in electrical circuits requiring a voltage breakdown device capable of conducting relatively high currents. 
     In the prior art, electrical circuits have been used to control the light output of photoflash lamps. Such circuits generally supply high voltage across a capacitor to store a charge for lighting a photoflash lamp. In addition, switching means in the form of a manual switch or a photoresistor are used to extinguish the photoflash lamp when sufficient illumination has been provided. Examples of circuit elements used to extinguish the photoflash lamp by electrically shorting a storage capacitor and/or the photoflash lamp are cold cathode thyratrons or hermetically sealed gas tube voltage breakdown devices or surge arresters utilizing glass spacer tubes. Glass spacer tubes used in such an application have included a trigger electrode disposed about the glass spacer tube in the region of the electrode gap to increase the electric field intensity in that region upon the presence of a trigger pulse, thereby to cause the sparkover or breakdown of the electrode gap and electrical current conduction through the voltage breakdown device. 
     Generally, the life expectancy of a voltage breakdown device with a glass spacer tube used in such an application is relatively short since the glass spacer tube tends to become embrittled. The use of a gas tube voltage breakdown device utilizing a ceramic spacer tube would result in a higher life expectancy since the ceramic would not become embrittled and deteriorate as rapidly as the glass spacer tube. However, due to the significantly higher dielectric constant of the ceramic spacer tube as compared to the glass spacer tube, the application of an external trigger pulse in the region of the electrode gap of a typical ceramic gas tube voltage breakdown device would have an insufficient effect upon the electric field intensity in that region and would thus be unsuitable for causing sparkover or gap breakdown and current conduction through the voltage breakdown device. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a new and improved gas tube voltage breakdown device or surge arrester. 
     Another object of the present invention is to provide a new and improved spacer tube for use in a gas tube voltage breakdown device or surge arrester. 
     Another object of the present invention is to provide a new and improved triggerable gas tube voltage breakdown device or surge arrester. 
     Another object of the present invention is to provide a new and improved triggerable hermetically sealed ceramic gas tube voltage breakdown device or surge arrester for use in an electrical circuit, such as an electrical circuit for controlling the light output of a photoflash lamp. 
     Briefly, the device of the present invention comprises a new and improved hermetically sealed gas tube voltage breakdown device or surge arrester particularly adapted for use as a voltage breakdown device in an electrical circuit for controlling the light output of a photoflash lamp. The device includes a trigger electrode disposed about the region of the electrode gap and means for reducing the attenuation of an electrical trigger pulse or signal due to the ceramic spacer tube of the device. The ceramic spacer tube may be formed from two elongated cylindrical spacer tube halves joined together by annular conductive material in the region of the electrode gap that serves as an integrally formed trigger electrode. Alternatively or in conjunction therewith, the configuration of the ceramic spacer tuve may be altered by removing material from the ceramic spacer tube in the region of the electrode gap to reduce the attenuation of the trigger pulse in that region caused by the thickness of a ceramic spacer tube. By connecting the integrally formed trigger electrode to a trigger pulse source or alternatively by disposing a non-integral trigger electrode about the region of the electrode gap and connecting that trigger electrode to a trigger pulse source, the electric field intensity in the region of the electrode gap due to the trigger pulse is sufficient to initiate sparkover or gap breakdown and current conduction through the device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above and other objects and advantages and novel features of the present invention will become apparent from the following detailed description of the embodiments of the invention illustrated in the accompanying drawing wherein: 
     FIG. 1 is a schematic view of an electrical circuit for controlling the light output of a photoflash lamp utilizing a triggerable hermetically sealed gas tube voltage breakdown device constructed in accordance with the principles of the present invention; 
     FIG. 2 is a cross sectional view of the voltage breakdown device of FIG. 1 taken along line 2--2 of FIG. 1; 
     FIG. 3 is a transverse cross sectional view of the device of FIG. 2 taken along line 3--3 of FIG. 2; 
     FIGS. 4-6 are cross sectional views of alternative embodiments of hermetically, sealed, triggerable gas tube voltage breakdown devices constructed in accordance with the principles of the present invention; 
     FIG. 7 is a transverse cross sectional view of the device of FIG. 6 taken along line 7--7 of FIG. 6; 
     FIGS. 8 and 10-12 are cross sectional views of alternative embodiments of hermetically sealed, triggerable gas tube voltage breakdown devices constructed in accordance with the principles of the present invention; 
     FIG. 9 is an exploded perspective view of a spacer tube for a hermetically sealed, triggerable gas tube voltage breakdown device constructed in accordance with the principles of the present invention; and 
     FIG. 13 is a fragmentary cross sectional view of an alternative embodiment of a hermetically sealed, triggerable gas tube voltage breakdown device constructed in accordance with the principles of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawing and initially to FIGS. 1--3, there is illustrated a new and improved hermetically sealed, triggerable gas tube voltage breakdown device 20 (FIGS. 1-3) particularly adapted for use as a voltage breakdown device in an electrical circuit, for example, in an electrical circuit (FIG. 1) for controlling the light output of a photoflash lamp L. In accordance with an important feature of the present invention, the device 20 includes a ceramic insulating spacer tube or spacer 22, preferably formed from alumina; a pair of generally cup-shaped metal gap electrodes 24 and 26; and a metal trigger electrode 28. In the embodiment of FIGS. 1-3, the trigger electrode 28 is in the form of an elongated metal band or ring disposed in a generally U-shaped elongated recess 30 formed in and along the outer periphery of the spacer 22 in the region of an electrode gap 32 defined by the most closely spaced portions 34 and 36 respectively of the electrodes 24 and 26. Radially outwardly extending annular shoulder portions 38 and 40 of the electrodes 24 and 26, respectively, are sealed to the opposite longitudinal ends of the spacer 22 to form the hermetically sealed device 20. 
     In order to maintain a high direct current voltage breakdown characteristic of the gap 32 for use in the electrical circuit of FIG. 1, the spacing between the electrodes 24 and 26 forming the gap 32 preferably should be greater than or equal to two millimeters; and the device 20 should be internally pressurized with an inert gas. In addition, in order to operate over an acceptable number of repetitive duty cycles when used as a voltage breakdown device in the electrical circuit of FIG. 1, the portions 34 and 36 of the electrodes 24 and 26 should be coated with a low work function coating, consisting of metal and/or metal-salts, that functions as a getter for absorbing or chemically binding the non-inert gases that are released from the surfaces of the electrodes 24 and 26 due to the high currents flowing through the device 20. 
     In a specific embodiment of the present invention, the wall thickness along the length of the spacer tube 22 outside of the recess 30 is in the range from about 0.040 inch to about 0.045 inch; and the reduced wall thickness of the spacer tube 22 along the length of the recess 30 may be in the range of from about 0.015 inch to about 0.030 inch and, preferably, is in the range of from about 0.020 to about 0.025 inch. The reduced wall thickness of the spacer tube 22 along the recess 30 in the region of the gap 32 significantly reduces the attenuation of an electrical trigger pulse or signal in that region caused by the relatively high dielectric constant of the ceramic spacer tube 22. Thus, a trigger pulse applied to the trigger electrode 28 is capable of causing sparkover or breakdown of the gap 32 and subsequent current conduction through the device 20. 
     Except for the use therein of the voltage breakdown device 20 with a ceramic insulating spacer 22, the electrical circuit of FIG. 1 is conventional per se. Essentially, a source S of direct current voltage, for example, 250 volts, charges a main storage capacitor C1 through a resistor R1 and a trigger capacitor C2 through the resistors R1, R2, and R3. When a switch S1 is closed, the photoflash lamp L is illuminated by the charge stored in the capacitor C1. When a switch S2, which may be a manual switch but more conventionally is a photoresistor, is closed, the trigger capacitor C2 is discharged through the low voltage winding of the transformer T to generate a high voltage electrical trigger pulse or signal in the high voltage winding of the transformer T, which trigger pulse is directed to the trigger electrode 28 of the device 20. The application of the trigger pulse to the trigger electrode 28 results in a greatly increased electric field intensity in the region of the gap 32, resulting in the sparkover or breakdown of the gap 32 and current conduction through the device 20, thereby discharging the capacitor C1 and electrically short circuiting and extinguishing the photoflash lamp L. 
     FIGS. 4-7 and 13 depect alternative embodiments of the device 20 in which the wall thickness of the spacer 22 is reduced in the region of the electrode gap 32 by various different physical modifications to the spacer 22. For example, a voltage breakdown device 50 (FIG. 4) includes a V-notch or groove 52 formed in and about the periphery of the spacer 22 to accommodate a round wire trigger electrode 54 and to provide a reduced wall thickness in the region of the electrode gap 32 in the size ranges referred to hereinabove with respect to the recess 30 (FIGS. 1-3). A voltage breakdown device 60 (FIG. 5) includes a generally U-shaped elongated recess 62 formed about the inner periphery of the ceramic spacer tube 22 to provide a reduced wall thickness in the region of the electrode gap 32 in the size ranges referred to hereinabove with respect to the recess 30 (FIGS. 1-3). 
     A voltage breakdown device 70 (FIGS. 6 and 7) includes an elongated flattened wall portion or surface 72 formed along a portion of one side of the spacer 22 to provide a reduced wall thickness of the spacer 22 in the region of the electrode gap 32 in the size ranges referred to hereinabove with respect to the recess 30 (FIGS. 1-3). 
     A voltage breakdown device 80 (FIG. 13) utilizes a thin walled ceramic spacer tube 22 having a uniform wall thickness in the size ranges referred to hereinabove with respect to the recess 30 (FIGS. 1-3) to thereby enable a trigger pulse applied to the trigger electrode 28 to sufficiently increase the electric field intensity in the region of the electrode gap 32 to cause a sparkover or breakdown of the gap 32 and current conduction through the device 80. 
     As opposed to the embodiments of FIGS. 8 and 10-12, in each of the devices 20, 50, 60, 70 and 80 (FIGS. 1-7 and 13) the spacer 22 electrically insulates the trigger electrode 28 (54 in FIG. 4) from the region of the electrode gap 32. The voltage breakdown devices 90, 100, 110 and 120 of FIGS. 8 and 10-12, respectively, are formed from a pair of elongated, cylindrical, ceramic spacer tube halves 22A and 22B and include annular conductive material or an integrally formed trigger electrode 130 disposed therebetween. The longidutinal ends of the spacer tube halves 22A and 22B are fixedly secured together to form a unitary spacer 22 with the trigger electrode 130 disposed in the region of the electrode gap 32. The annular conductive material 130 may be a suitable brazing material, such as a silver alloy washer (FIG. 9), for brazing together the metallized ends of the spacer tube halves 22A and 22B. 
     The spacer 22 (FIGS. 8 and 10-12) may be formed with a uniform wall thickness (FIG. 8) or with a reduced wall thickness in the region of the electrode gap (FIGS. 10-12). For example, the voltage breakdown device 100 (FIG. 10) includes a V-notch or groove 132 formed about the inner periphery of the spacer 22 to provide a reduced wall thickness in the region of the electrode gap 32 in the size ranges referred to hereinabove with respect to the recess 30 (FIGS. 1-3). The voltage breakdown device 110 (FIG. 11) includes a generally U-shaped elongated recess 134 disposed about the inner periphery of the spacer 22 to provide a reduced wall thickness in the region of the electrode gap 32 in the size ranges referred to hereinabove with respect to the recess 30 (FIGS. 1-3). Similarly, the voltage breakdown device 120 (FIG. 12) includes a U-shaped elongated recess 136 formed in and about the outer periphery of the spacer 22 to provide a reduced wall thickness in the region of the electrode gap 32 in the size ranges referred to hereinabove with respect to the recess 30 (FIGS. 1-3). 
     If desired, the devices 90, 100, 110 and 120 may each include a flat band or ring of conductive material, essentially identical to the trigger electrode 28 (FIGS. 1-7 and 13), disposed about trigger electrode 130 and along the outer periphery of the ceramic spacer tube 22 in the region of the electrode gap 32 for connecting the trigger electrode 130 to a source of trigger pulses. Alternatively, the trigger electrodes 130 may be directly electrically connected to a source of trigger pulses. 
     Contrary to the device depicted in U.S. Pat. No. 4,287,548 in which an exterior conductive layer or electrode 2 overlaps an interior conductive strip 3 and in which the layer or electrode 2 is directly physically and electrically connected to the gap electrode 5 and the conductive strip 3 is directly physically and electrically connected to the gap electrode 4 such that the same voltage appears across the layer or electrode 2 and the strip 3 as the voltage across the gap electrodes 4 and 5, the devices 20, 50, 60, 70, 80, 90, 100, 110 and 120 all have one of the trigger electrodes 28, 54 and 130 that are physically separated from and electrically insulated from the two gap electrodes 24 and 26 such that the trigger electrodes 28, 54 and 130 are adapted to receive a trigger voltage signal from the transformer &#34;T&#34; in the electrical circuit of FIG. 1 that is other than or electrically distinct from the voltage across the two gap electrodes 24 and 26. In addition, contrary to the devices depicted in FIG. 4 of U.S. Pat. No. 3,989,985 and in FIG. 3 of U.S. Pat. No. 4,410,831 which include three electrodes and two electrode gaps, a first electrode gap between the first and second electrodes and a second gap between the second and third electrodes, in the devices 20, 50, 60, 70, 80, 90, 100, 110 and 120 disclosed herein, there are two and only two gap electrodes 24 and 26 each having a single electrode gap 32 therebetween, which electrode gap 32 is the only electrode gap in those devices. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described hereinabove. The term &#34;voltage breakdown device&#34; as used herein is intended to include within its scope hermetically sealed gas tube voltage breakdown devices, often referred to in the industry as surge arresters, functioning either as a surge arrester for conducting transient overvoltage surges therethrough to protect associated electrical equipment from damage or destruction due to such surges or as a voltage breakdown device in electrical circuits for conducting currents therethrough during the normal or steady state operation of such circuits. The term &#34;ceramic&#34; with reference to the spacer 22 is used herein in the European sense to designate a spacer 22 formed at a high temperature from nonmetallic, inorganic, earthy or clay material, other than glass.