Patent Publication Number: US-6906462-B1

Title: Gas discharge lamp with ignition assisting electrodes, especially for automobile headlights

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a 35 USC 371 application of PCT/DE 00/00910 filed on Mar. 28, 2000. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention is directed to gas discharge lamps, and more particularly to gas discharge lamps for motor vehicle headlights. 
   2. Brief Description of the Prior Art 
   One prior art gas discharge lamp is known from German Patent Disclosure DE 196 10 387 A1. This gas discharge lamp has a burner vessel, with a void into which two electrodes of opposed polarity protrude, between which an electric arc develops in operation. A filling of gas and solid substances is disposed in the void. Also disposed in the void is at least one auxiliary electrode, which serves to improve the ignition performance of the gas discharge lamp. The at least one auxiliary electrode is disposed in such a way that compared to the burning path between the two electrodes, a shorter ignition path is brought about, which facilitates the ignition of the gas discharge lamp. It can prove difficult to assure the requisite separation between the electrodes and the at least one auxiliary electrode and the separation of the burning path and the ignition path. 
   SUMMARY OF THE INVENTION 
   The gas discharge lamp of the invention has the advantage over the prior art that it is possible to dispose the at least one auxiliary electrode on the outside of the burner vessel in a simple manner. As a result of the high-frequency electrical voltage applied to both the auxiliary electrode and the counterelectrode, dielectrically hindered discharges, so-called silent discharges or barrier discharges, are generated in the void of the burner vessel. As a result, the gas in the filling of the void is energetically excited. Upon ignition of the gas discharge lamp by the application of high voltage to the electrodes, the electrical breakdown through the gas of the filling in the void takes place faster and/or sooner, at a lower voltage, than would be the case without the auxiliary electrode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention will become apparent from the detailed description contained below, taken in conjunction with the drawing, in which: 
       FIG. 1  shows a gas discharge lamp in a longitudinal section; 
       FIG. 2  shows a detail, marked II in  FIG. 1 , of the gas discharge lamp of a first exemplary embodiment on a larger scale; and 
       FIG. 3  shows the detail II of the gas discharge lamp in a second exemplary embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A gas discharge lamp shown in  FIGS. 1-3  is intended in particular for use in vehicle lighting devices, such as headlights. The gas discharge lamp has a base  10 , shown highly simplified, which can be embodied in multiple parts and by way of which it can for instance be disposed in an opening in a reflector of the headlight. A burner  12 , which has a discharge vessel  14 , is connected to the base  10 . The discharge vessel  14  has a void  15 , into which at least two electrodes  16 ,  18  protrude and which contains a filling that comprises a noble gas, preferably xenon, and mercury, and optionally metal halides as well. The discharge vessel  14  has tubular extensions  20 ,  21 , adjoining the void  15 ; a lead line  22  to the electrode  16  is disposed in the extension  20  toward the base  10 , and a line  23  connected to the other electrode  18  is disposed in the extension  21  remote from the base  10 . The discharge vessel  14  with the extensions  20 ,  21  is of glass, preferably quartz glass. A metal fuel  24  which preferably comprises molybdenum connects lines  22 ,  23  to the electrodes  16 ,  18 , respectively. The burner  12  can moreover have a glass tube  26 , at least partly surrounding the discharge vessel  14  with its extensions  21 ,  21 , and this tube can serve on the one hand to protect the discharge vessel  14  and on the other to shield against UV radiation occurring in operation of the gas discharge lamp. A return line  28  extending outside the burner  12  is connected to the line  23  and leads along the burner  12  to the base  10 ; it can be provided with an insulator  29 . The lead line  22  to the electrode  16  and the return line  23  from the electrode  18  are each connected to the base with a respective plug contact, not shown. 
   For the operation of the gas discharge lamp, an electrical choke device is provided, which has an ignition part and a control part. The ignition part can be disposed separately from the gas discharge lamp, or it can be integrated with the base  10  thereof. The ignition part serves to ignite the gas discharge lamp, and it generates high voltage, which is applied to the electrodes  16 ,  18  in order to bring about a breakdown between them. Depending on the state of the gas discharge lamp, especially in the warm state, a very high voltage is required to ignite the gas discharge lamp. As an example, the ignition part can be constructed on the principle of resonance ignition. In operation of the gas discharge lamp, an electric arc develops between the electrodes  16 ,  18  in the void  15  of the discharge vessel  14 . 
   In a first embodiment of the invention, at least two auxiliary electrodes  40 ,  42  are provided, which are disposed outside the discharge vessel  14  in the region of its void  15 . 
   In  FIG. 2 , the auxiliary electrodes  40 ,  42  are shown in accordance with a first exemplary embodiment, in which they are formed by an electrically conductive coating of the outside of the discharge vessel  14  in the region of the void  15 . The coating for forming the auxiliary electrodes  40 ,  42  can for instance comprise a metal with adequate heat resistance and can be applied to the discharge vessel  14  by vapor deposition, painting, or in some other suitable way. 
   The two auxiliary electrodes  40 ,  42  are disposed, at least approximately diametrically opposite one another, on the discharge vessel  14 . It is also possible for more than two auxiliary electrodes  40 ,  42  to be provided; then for instance each two auxiliary electrodes are disposed facing one another. The coatings that form the two auxiliary electrodes  40 ,  42  are separate from one another, and each extends over part of the circumference of the discharge vessel  14  around the void  15 . A respective conductor track  44  extending along the discharge vessel  14  can be connected to the auxiliary electrodes  40 ,  42 , and these tracks lead to the base  10  of the gas discharge lamp. Alternatively, the auxiliary electrodes  40 ,  42  can be contacted in an arbitrary other way, such as by means of cables connected to them. By means of the electrical choke device, such as its ignition part, a high-frequency high voltage is applied to the auxiliary electrodes  40 ,  42 , and the two auxiliary electrodes  40 ,  42  have opposite polarity from one another. It can be provided that the auxiliary electrodes  40 ,  42  are supplied with high-frequency high voltage for only a predetermined period of time prior to and optionally during the ignition of the gas discharge lamp, or alternatively, it can be provided that the auxiliary electrodes  40 ,  42  are supplied with high-frequency high voltage constantly while the gas discharge lamp is off. The high voltage can amount to more than 1 kV, and the frequency can amount to more than 10 kHz. 
   If the auxiliary electrodes  40 ,  42  are supplied with high-frequency high voltage, dielectrically hindered discharges, so-called silent discharges or barrier discharges, are generated in the void  15  of the discharge vessel  14  of the gas discharge lamp. As a result, the filing of the void  15 , and in particular the noble gas, is energetically excited. Upon ignition of the gas discharge lamp by means of the ignition part of the choke device by the application of high voltage to the electrodes  16 ,  18 , the dielectrical breakdown through the noble gas in the void  15  between the electrodes  16 ,  18  takes place faster and/or at a lower ignition voltage than would be the case without the auxiliary electrodes  40 ,  42 . The ignition part of the choke device can thus be designed as correspondingly weaker, which has cost advantages. The gas discharge lamp can also be embodied more simply in its overall design in terms of insulation provisions, since only lesser ignition voltages are required. Furthermore, the gas discharge lamp can attain a longer service life because of the reduced ignition load. 
   The auxiliary electrodes  40 ,  42  are disposed between the outside of the discharge vessel  14  and the glass tube  26 , so that the electrodes cannot be touched, and no risk emanates from them. As an alternative to the embodiment described above, the auxiliary electrodes  40 ,  42  can also be formed by a conductive coating of the inside of the glass tube  26 , in the region of the glass tube  26  surrounding the void  15  of the discharge vessel  14 . Then the contacting of the auxiliary electrodes  40 ,  42  can again be done via conductor tracks on the inside of the glass tube  26 , or in some arbitrary other way. 
   In  FIG. 3 , the gas discharge lamp is shown in fragments in accordance with a second exemplary embodiment. The fundamental construction of the gas discharge lamp is the same as in the first exemplary embodiment, but the embodiment of the auxiliary electrodes is modified. The auxiliary electrodes  40 ,  42  in the second exemplary embodiment are disposed as separate parts on the outside of the discharge vessel  14 , in the region of its void  15 . The auxiliary electrodes  40 ,  42  comprise electrically conductive material, especially metal, and can be adapted in their shape to the outside of the discharge vessel  14  in the region of the void  15 , or in other words can for instance be embodied in curved fashion. The auxiliary electrodes  40 ,  42  can be connected to the discharge vessel  14  by means of a clamp or detent connection, by adhesive bonding, or in some arbitrary other way. The auxiliary electrodes  40 ,  42  can also be clamped in place between the discharge vessel  14  and the glass tube  26 . Connected to the auxiliary electrodes  40 ,  42  are electric lines  46  that lead to the base  10  of the gas discharge lamp and serve to connect the auxiliary electrodes  40 ,  42  to the high-frequency high voltage, as in the first exemplary embodiment. 
   In another embodiment of the invention, only one of the auxiliary electrodes  40 ,  42 , which can be embodied as described above, is provided on the outside of the discharge vessel  14 . Acting as the counterelectrode to this one auxiliary electrode  40  or  42  is one of the electrodes  16 ,  18  protruding into the void  15 . As in the first embodiment, a high-frequency high voltage of opposite polarity is applied to the auxiliary electrode  40  or  42  and to the counterelectrode  16  or  18 , so that between the auxiliary electrode  40  or  42  and the counterelectrode  16  or  18 , an electrical field develops, which penetrates the void  15  and facilitates the ignition of the gas discharge lamp. 
   The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.