US 3855495 A
An injection-triggered xenon flash tube having an elongated glass envelope with an electrode at each end and a pair of end caps. Each end cap includes a shoulder portion comprising a tubular piece of insulating material coaxially secured about an end of the glass envelope by an insulating adhesive, and a tubular metal terminal portion secured to and coaxially projecting from the insulating shoulder and electrically connected to the electrode at that end of the envelope through a lead-in conductor.
Description (OCR text may contain errors)
United States Patent [191 Pappas et a1.
[ FLASH TUBE WITH INSULATOR END CAP  Inventors: John A. Pappas, Winthrop; Robert J. Cosco, Amesbury; Charles C. Kokinos, Lynn, all of Mass.
 Assignee: GTE Sylvania Incorporated,
22 Filed: Nov. 5, 1973 21 Appl. No.: 412,814
 US. Cl 313/318, 313/220, 313/331  Int. Cl. HOlj 5/56  Field of Search 313/217, 218, 219, 268, 313/311, 331, 334, 220, 318
 References Cited UNITED STATES PATENTS 2,657,325 10/1953 Homer et a1 313/218 X [111 3,855,495 [451 Dec. 17, 1974 3,363,133 1/1968 Harris et al 313/218 X Primary ExaminerPaul L. Gensler Attorney, Agent, or Firm-Edward J. Coleman [5 7] ABSTRACT An injection-triggered xenon flash tube having an elongated glass envelope with an electrode at each end and a pair of end caps. Each end cap includes a shoulder portion comprising a tubular piece of insulating material coaxially secured about an end of the glass envelope by an'insulating adhesive, and a tubular metal terminal portion secured to and coaxially projecting from the insulating shoulder and electrically connected to the electrode at that end of the envelope through a lead-in conductor.
6 Claims, 3 Drawing Figures FLASH TUBE WITH INSULATOR END CAP BACKGROUND OF THE INVENTION This invention relates generally to electric discharge lamps and, more particularly, to injection-triggered flash tubes having an elongated envelope, such as those employed in photocopying machines.
Flash tubes generally comprise two spaced apart electrodes within a sealed glass envelope having a rare gas fill, typically xenon, at a sub-atmospheric pressure. Such lamps are connected across a large capacitor charged to a substantial potential, which is, however, insufficient to ionize the xenon fill gas. Upon application of an additional pulse of sufficient voltage, the xenon is ionized, and an electric arc is formed between the two electrodes, discharging the large capacitor through the flash tube, which emits a burst of intense light, usually of short duration. In some cases the pulse voltage is applied between an external trigger wire wrapped around the envelope and the electrodes. However, in other cases an external wire is not flesible since it may result in undesirable arcing between the trigger wire and a proximate lamp reflector or else the high potential applied to the external trigger wire might be hazardous to operating personnel.
In those cases, the lamp may be internally triggered by applying the pulse voltage directly across the lamp electrodes; this is also referred to as injection triggering. Usually the voltage required is about 30 to 50 percent higher than that required to trigger the same lamp with an external trigger wire. This poses no particular problem in itself, since the lamp operating circuit can be designed to supply sufficient pulse voltage to the lamp.
With respect to the efficiency and reliability of lamp operation, however, it is desirable to substantially reduce the trigger voltage required to strike an arc in an injection-triggered flash tube. For example, a lower trigger voltage will shift the starting characteristic curve (a plot of triggervoltage vs. anode, or supply, voltage) to a lower level and thereby enlarge the reliable operating region. This permits the fill gas pressure to be increased (which shifts the characteristic curve upward) and yet still retains reliable starting characteristics. An increased fill pressure results in a brighter light output during flashing and, provides improved light output maintenance, as higher gas pressures enhance conduction of heat from the electrodes to keep them cooler during operation. A higher fill pressure also reduces peak current in the flash tube to thereby provide a longer operating life. Advantages of economy are also provided, as a reduction in the energy and peak voltage required to ignite the flash tube will mean less expensive regulating circuits in the power supply design.
SUMMARY OF THE INVENTION I Accordingly, it is an object of the present invention to provide an improved injection-triggered flash tube design.
It is a particular object to provide an injectiontriggered flash tube with improved starting characteristics.
It has been common practice to provide metal end caps as the mounting terminals at each end of the elongated tubular glass envelope of a flash tube. Each end cap is electrically connected to the electrodes at its end of the lamp by means of a lead-in conductor and includes a shoulder portion which coaxially fits about the end of the glass envelope and is secured thereto by an insulating adhesive. The metal shoulder of the end cap imparts additional strength to the flash tube structure so that it may be snapped into and removed from its fixture without damage.
Quite unexpectedly, we have discovered that the metal end cap, and particularly the metal shoulder thereof, has an inhibiting effect on lamp starting. We believe that the electrical field gradient about the electrode being pulsed plays a significant role in the ignition of a flash tube by means of injection triggering. It appears that any conductors in the vicinity of the electrode, even the metal of the end cap, disturbs the electric field about the electrode by causing a distribution of the field which weakens it and, thus, inhibits lamp starting. This effect is generally not recognizable due to the wash out effect of ground planes typically associated with many flash tube applications. Further, the effect would not ordinarily be anticipated as the end cap is theoretically not a ground plane.
In accordance with the present invention, therefore, we have significantly reduced the starting voltage (trigger pulse) requirements of an injection triggered flash tube by employing an end cap having a shoulder portion which is completely formed of an insulating material. In this manner, the strengthening characteristics of a shouldered base are maintained while reducing the inhibiting effects on lamp starting caused by the proximity of conductive metal in the end cap.
BRIEF DESCRIPTION OF THE DRAWINGS This invention will be more fully described hereinafter in conjunction with the accompanying drawings, in which:
FIG. 1 illustrates an injection-triggered flash tube in accordance with the invention;
FIG. 2 is an enlarged sectional view of an end cap region of the flash tube of FIG. 1 according to one embodiment of the invention; and
FIG. 3 is a simplified enlarged sectional view of an end cap region of the flash tube of FIG. 1 according to an alternative embodiment of the invention.
DESCRIPTION OF PREFERRED EMBODIMENT Referring to FIG. 1, the flash tube comprises an hermetically sealed, light transmitting envelope 2 formed of an elongated piece of hard glass tubing (e.g., Corning No. 7740 glass) and having a cathode electrode 4 sealed within one end of the envelope and an anode electrode 6 sealed within the other end. The envelope is filled with a rare gas, such as xenon, at a subatmospheric pressure (e.g., 35 Torr) and is constricted to define an exhaust tip 8. At each end of the envelope is secured an end cap 10, which in accordance with the invention comprises a shoulder portion 12 of insulating material and a metal terminal portion 14 which is electrically connected to the electrode at that end of the lamp.
A preferred embodiment of an end cap according to the invention is best shown in FIG. 2. The shoulder portion 12 is formed of a substantially tubular piece of insulating material (e.g., nylon, Teflon, ceramic, a glass filled epoxy, etc.) which is coaxially disposed about the exterior of a respective end of the glass envelope 2 and secured thereto by an insulating adhesive 16 (e.g.,
Sauereisen, Silastic, etc.). A hole 18 may be provided in the side of the tubular shoulder 12 to facilitate filling the adhesive into the void between the shoulder and envelope.
Attachment of the metal terminal portion 14 to the insulating shoulder is facilitated by forming the insulating material to further include a coaxially extending tubular portion 20 of substantially smaller diameter than the shoulder and having an axial bore 22 through which passes the respective lead-in conductor 24.
The exterior terminal portion 14 is formed of substantially tubular metal conductor material (e. g., nickel plated brass) and contains a cylindrical cavity 26 within which the extended tubular portion 20 of the insulating material is fitted. The terminal 14 also includes an axial bore 28 which is aligned with bore 22 in the insulating material and through which passes the lead-in conductor 24. Conductor 24 is electrically secured to terminal portion 14, such as by soldering or brazing 30, at the end of the bore 28. In this manner, the metal terminal portion 14 is electrically connected to its respective electrode 4 (or 6) by means of the lead-in conductor 24, which is sealed through the respective end of glass envelope 2 and connected at one end to the electrode and at the other end to the metal terminal 14.
Terminal 14 may be mechanically secured to the insulating shoulder by a number of methods, and in FIG. 2 this is accomplished by a plurality of crimp indentations 32 disposed symmetrically about terminal portion 14 and engaging the extended tubular portion 20 of the insulating material.
Shoulder 12 is dimensioned to provide the desired handling strength, while terminal 14 is dimensioned to mate with the clip-in socket on the corresponding lamp fixture.
According to one specific implementation of a flash tube according to the invention, the length of the horizontal portion of the tubular glass envelope in FIG. 1 is approximately 21 inches; the length of each of the two vertical sections of the envelope is about 7.5 inches; the axial length of shoulder 12 is about 0.320 inch; the diameter of the shoulder is about 0.460 inch; and the length and diameter of terminal 14 are 0.500 inch and 0.280 inch, respectively. The lamp is filled with xenon at 35' Torr pressure; the shoulder 12 is formed of nylon, and terminal 14 is a nickel plated brass. The operating voltage range is from 2,000 to 3,500 volts DC; this has also been referred to as the anode or supply voltage. The minimum peak trigger voltage in this injection mode is about 10,000 volts DC (open circuit), with the trigger supply primary voltage being about 400 volts DC. This compares with a trigger voltage of 21,000 volts DC, with a trigger primary voltage of 850 volts DC, when the same type lamp is operated with the end caps being of the conventional metalshouldered type. Test data also indicates that the trigger primary voltage for flash tubes employing metalshouldered end caps can be reduced by increasing the distance between the electrode tip and the metal shoulder; however, maximum reduction of the inhibiting effects upon flash tube ignition caused by the end cap is achieved by use of the non-condutive shoulder.
FIG. 3 illustrates an alternative embodiment of the end cap wherein the metal terminal portion 34 includes a flared rim 36 at one end, and the corresponding insulating shoulder portion 38 contains an internal coaxial recess 40 within which the flared rim 36 is seated and secured, e.g., by an adhesive cement 42. Only the metal terminal portion 34 contains a bore 28 for accommodating lead-in wire 24, which is brazed to the terminal at junction 30 at the outer end of the bore.
Hence, although the invention has been described with respect to a specific embodiment, it will be appreciated that modifications and changes may be made by those skilled in the art without departing from the true spirit and scope of What we claim is: invention.
1. An injection-triggered flash tube comprising: an elongated, light-transmitting envelope which is hermetically sealed; a rare gas in said envelope; a pair of electrodes in said envelope, one disposed at each end thereof; a respective lead-in conductor sealed through each end of said envelope and connected to the electrode thereat; and an end cap at each end of said envelope, each of said end caps having a shoulder portion comprising a substantially tubular piece of insulating material coaxially disposed about and secured to the exterior of a respective end of said envelope, and an exterior terminal portion comprising a substantially tubular metal conductor secured to and coaxially projecting from said insulating shoulder, the piece of insulating material from which said shoulder is formed further including a coaxially extending tubular portion of substantially smaller diameter than said shoulder and having an axial bore through which said respective lead-in conductor passes, said metal terminal portion containing a cylindrical cavity within which the extended tubular portion of said insulating material is fitted and secured, and said terminal portion having an axial bore aligned with the bore in the extended tubular portion of said insulating material and through which said respective lead-in conductor passes and is electrically secured to said metal tenninal portion, whereby said terminal portion is electrically connected to the electrode disposed within said respective end of the envelope.
2. A flash tube according to claim 1 wherein said insulating shoulder portion of each end cap is secured to the exterior of an end of said envelope by an insulating adhesive.
3. A flash tube according to claim 1 wherein said respective lead-in conductor is electrically secured to said metal terminal portion at the outer end of said bore, and said terminal portion is mechanically secured to the extended tubular portion of said insulating material by crimp indentations about said terminal portion.
4. A flash tube according to claim 1 wherein said rare gas is xenon.
5. A flash tube according to claim 4 wherein said envelope is formed of glass tubing.
6. An injection-triggered flash tube comprising: an elongated, light-transmitting envelope which is hermetically sealed; a rare gas in said envelope; a pair of electrodes in said envelope, one disposed at each end thereof; a respective lead-in conductor sealed through each end of said envelope and connected to the electrode thereat; and an end cap at each end of said envelope, each of said end caps having a shoulder portion comprising a substantially tubular piece of insulating material coaxially disposed about and secured to the exterior of a respective end of said envelope, and an exteriorterminal portion comprising a substantially tubular metal conductor secured to and coaxially projecting from said insulating shoulder, said metal terminal portion having a flared rim at one end, said insulating shoulder containing an internal coaxial recess within terminal portion, whereby said terminal portion is elecwhlch the flared of Pomon fitted trically connected to the electrode disposed within said and secured, and said terminal portion containing an axial bore through which said respective lead-in conductor passes and is electrically secured to said metal 5 respective end of the envelope.