Patent Publication Number: US-2010109504-A1

Title: Starting coil for induction lighting

Description:
BACKGROUND OF THE DISCLOSURE 
     This application claims priority from U.S. provisional application Ser. No. 61/110,349, filed 31 Oct. 2008, the entire disclosure of which is hereby expressly incorporated herein by reference. 
     This application relates to a high intensity discharge (HID) lamp, and particularly to an electrodeless or induction HID lamp, and more particularly to an electrodeless or induction ceramic HID lamp. 
     In induction lighting, a helical electrically conductive starting coil is sometimes used to initiate a capacitive discharge then a toroidal plasma inside the lamp is maintained by a main coil surrounding the lamp. The starting coil must be positioned close to the lamp and as a result the temperature increases from ambient temperature to several hundred degrees Celsius when the lamp is operating. These temperature extremes, or thermal cycling, will ultimately cause the starting coil to lose mechanical strength and sag. If the individual turns of the helical starting coil were to touch each other, an electrically closed loop would be formed and a high current would be induced in the starting coil. High current may potentially damage the starting coil. An electrically closed loop in the starting coil will weaken the capacitive discharge and fail to initiate a toroidal plasma inside the lamp. 
     Another issue with the starting coil is that over time the coil is subject to oxidation. The high temperature associated with lamp operation will expedite the oxidation of the starting coil and reduce the useful working life of the starting coil. Unfortunately, this reduced life is directly at odds with one of the major benefits associated with induction lighting, i.e., long life. 
     Accordingly, a need exists to significantly improve the life of a starting coil of an induction lighting assembly. As noted above, significant improvement is required on at least two fronts, namely mechanical support to address the loss of mechanical strength and associated sagging, and reducing the oxidation issue. 
     SUMMARY OF THE DISCLOSURE 
     A primary advantage of the present disclosure resides in the ability to address the useful life of the starting coil. 
     A part of this advantage resides in the ability to adequately address the loss of mechanical strength associated with thermal cycling. 
     Another part of the advantage provided by the present disclosure relates to limiting oxidation of the starting coil. 
     Yet another advantage of the present disclosure resides in the limited impact on the light output of the lamp, while facilitating start-up or ignition of the main envelope. 
     Still another benefit is associated with the ease of assembly. 
     Still other benefits and advantages of the present disclosure will become apparent from reading and understanding the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of an electrodeless discharge lamp. 
         FIGS. 2-6  are views of different embodiments for improving the life of the starting coil. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning first to  FIG. 1 , there is shown a lamp assembly and, more particularly, an electrodeless high intensity discharge (HID) or ceramic HID lamp assembly  100 , that includes a main envelope or arc body  102 . The arc body preferably has an ellipsoidal or generally spheroidal portion  104  that encloses a similarly shaped cavity or main chamber  106  housing a desired main fill therein. The main chamber is hermetically sealed from the external or ambient environment after the main fill has been introduced or dosed into the arc body. The arc body is preferably made from a ceramic material that is light transmissive such as a polycrystalline alumina, although other materials may be used where conducive to the demands or needs of the electrodeless lamp. 
     The generally spheroidal portion  104  of the arc body has first and second polar regions  110 ,  112 . Extending outwardly from the first polar region  110  is an envelope extension or leg  114 . The leg is preferably hollow and thereby defines a cavity or starting chamber  116  that communicates with the main chamber  106 . The leg has a substantially smaller cross-sectional dimension than the spheroidal portion. 
     A radio frequency or RF coil  120 , sometimes referred to as the main coil, extends about an equatorial or median region  122  of the arc body. The coil is preferably a multi-turn assembly such as the illustrated coil  120  that includes first and second turns, although a greater number of turns could be used if so desired. The coil preferably has a low profile and desirably does not significantly impact or block the light emitted from the main chamber. The main RF coil is closely disposed adjacent a perimeter of the equatorial region  122  of the spheroidal portion in order to provide energy to the fill and continue to power the arc discharge (toroidal-shaped discharge) once ignition of the main fill occurs. 
     A high voltage conductor or wire  124  extends from a high voltage power source (not shown) and terminates closely adjacent the second polar region  112  of the arc body. In addition, a starting member, starting conductor, or helical starting coil  126  has a first end  128  disposed adjacent a first or distal end of the leg  114 . The helical starting coil preferably has a diameter closely dimensioned to the outer dimension or diameter of the leg distal end. In this arrangement, the starting coil  126  increases in diameter as the starting coil proceeds along the length of the leg toward the first polar region  110  of the arc body where a second end  130  of the starting coil abuts or is closely spaced from the first polar region  110  of the arc body. The first end  128  of the starting coil is connected to an LC resonant circuit which provides a start-up or ignition charge to the starting coil  128 . The operation of the circuit is well known in the art so that further discussion herein is deemed unnecessary to understanding the present disclosure. 
     The high voltage conductor  124  provides approximately 10 kv of the required high voltage to ionize the fill in the main envelope. When the starting coil  126  voltage increases to about 2.5 kv via the LC resonant circuit, capacitive discharge is initiated and a toroidal plasma inside the lamp is started. Further power required for maintaining the discharge is then provided by the RF coil and as controlled by the resonant circuit  140 . 
     The helical starting coil of  FIG. 1  increases in diameter from the top to the bottom, and the starting coil is relatively closely spaced to the leg. As noted in the Background, the high temperature and thermal cycling to which the starting coil is exposed indicates that increased support and protection is required.  FIG. 2  is a first preferred arrangement in which a physical or mechanical support is provided for the starting coil. Although selected portions of the lamp assembly of  FIG. 1  have been removed for ease of illustration and understanding, like reference numerals in the two hundred series (“ 200 ”) will be used to refer to similar components in  FIG. 2 , e.g., generally spheroidal portion  104  in  FIG. 1  is now referenced as generally spheroidal portion  204  in  FIG. 2 . Likewise, new components will be referenced by new reference numerals. Support  250  is preferably a light transmissive material such as glass, quartz, or ceramic so that light from the lamp will not be adversely blocked by the support. However, in some applications the top surface of the lamp has a layer of coating to direct the light to a specific direction. In these applications, support  250  can be non-light transmissive material or low light absorptive material since the coating will reflect the light. The support has a generally cylindrical conformation with an inner diameter  252  slightly greater than the outer diameter of the leg  214  to define an annular gap or space  254 . Although a cylindrical conformation is preferred to conform to the cylindrical conformation of the leg, it will also be appreciated that the support can adopt still other shapes or conformations, such as a cone shape with increasing diameter from a distal end of the leg toward the main envelope, without departing from the scope and intent of the present disclosure. A first or lower end  256  of the support either abuts or is positioned adjacent the interconnection between the leg and the spheroidal portion of the arc body. A second or upper end  258  of the support preferably terminates at or adjacent a terminal or distal end of the leg  214 . 
     The support preferably includes means for mechanically supporting the starting coil  226 , which is shown in  FIG. 2  as having a helical, generally constant diameter along its longitudinal extent. Means for supporting the starting coil is a groove  270  in this preferred arrangement provided along the inner diameter  252  of the support where the groove is dimensioned to substantially conform to the diameter of the starting coil  226 . As will be appreciated, if the starting coil  226  has a generally helical conformation, then the support means or internal grooves  270  in this embodiment likewise adopts a continuous, generally helical conformation along the inner surface of the support. Preferably, the groove has a generally C-shaped cross-sectional conformation to provide support over an extent greater than 180° of the outer surface of the starting coil. The inner surface  252  of the support and likewise the depth of the groove  270  are important to closely position the inner diameter of the starting coil adjacent but without contacting the outer surface of the arc body leg. Moreover, the support is also preferably formed from a material that can withstand the elevated temperature and thermal cycling associated with the lamp environment. Thus, the support is preferably a high temperature material (glass, quartz, or ceramic being preferred), and the support features along the inside surface of the support prevent the starting coil from mechanically sagging. To control the distance between the coil support  250  and lamp leg  214 , orientation projections  280  can be added on the inner surface of the coil support such that the coil support  250  can move along the axis of the lamp leg  214  but not in the radial direction of the lamp leg  214 . Also, the bottom surface of the coil support  250  can be shaped to conform to the top surface of the lamp. Although not believed to be as conducive to manufacturing, one skilled in the art will appreciate that the orientation projections may alternatively be formed on an external surface of the lamp leg. Likewise, the surface of the first polar region  210  can alternatively be shaped to conform to the bottom surface of the coil support  250 . 
       FIG. 3  shows another preferred support where like components are referenced by like reference numerals in the three hundred series (“ 300 ”), e.g. a generally spheroidal portion  104  of  FIG. 1  is referenced as generally spheroidal portion  304  in  FIG. 3 . The support  350  is again preferably formed of a high temperature material such as glass, quartz, or ceramic. Dimensioning of an inner surface or diameter  352  of the support is selected to provide an annular space or gap  354  around the outer surface or outer diameter of the arc body leg  314 . The support has a first or lower end  356  that either abuts or rests on the generally spheroidal portion of the arc body at the upper polar region  310  or may be slightly spaced from the arc body if the support is otherwise held in place relative to the arc body. A second or upper end  358  of the support preferably terminates adjacent the distal end of the leg  314 . Means for mechanically supporting the starter coil  326  is preferably an external groove  370 . Since the starting coil  326  is a helix, then the groove  370  is preferably a continuous helical groove dimensioned to receive the starting coil therein. Again, the groove preferably has a cross-sectional conformation that provides the needed support for the starting coil. For example, a generally C-shaped conformation may be desired since that conformation provides support over slightly greater than one-half the outer surface of the starting coil. Of course, it will be recognized that the support groove  370  could adopt other shapes, although preferably the groove closely conforms to the outer surface of the starting coil to provide the desired mechanical support and prevent the starting coil from sagging. Similarly, to control the distance between the coil support  350  and lamp leg  314 , orientation projections  380  can be added on the inner surface of the coil support (or alternatively can be added to an external surface of the lamp leg) such that the coil support  350  can move along the axis of the lamp leg  314  but not in the radial direction of the lamp leg  314 . Also, the bottom surface of the coil support  350  and the top surface of the lamp can be shaped to conform to one another. 
     Another preferred embodiment of the support or means for supporting the starting coil is shown in  FIG. 4 . Again, for ease of reference, and brevity, reference numerals are provided in the four hundred series (“ 400 ”) to identify like components. In this embodiment, mechanical support  450  again adopts a generally cylindrical conformation where an inner surface or diameter  452  is slightly greater than that of the outer surface or outer diameter of the leg  414  to define an annular space or gap  454 . The longitudinal extent of the support from a first or lower end  456  to a second or upper end  458  closely conforms to a longitudinal extent of the leg from the upper polar region  410  of the arc body  404 . Again, preferred materials for the support include glass, quartz, or ceramic because of the light transmissive properties of these particular high temperature materials. A primary distinction of this embodiment relative to the supports described in  FIGS. 2 and 3  is that the starting coil  426  is embedded or encased inside the support so that the starting coil is isolated from the ambient environment, i.e., air. This isolation reduces or eliminates concern with oxidation of the starting coil, while simultaneously providing the desired mechanical support that is necessary in this environment. Similarly, to control the distance between the coil support  450  and lamp leg  414 , orientation projections  480  can be added on the inner surface of the coil support (or alternatively can be added to an external surface of the lamp leg) such that the coil support  450  can move along the axis of the lamp leg  414  but not in the radial direction of the lamp leg  414 . Also, the bottom surface of the coil support  450  and the top surface of the lamp can be shaped to conform in to one another. 
     The embodiment of  FIG. 5  bears similarities to the advantages offered by the  FIG. 4  embodiment in that the starting coil  526  is not only mechanically supported by the support  550 , but the starting coil is also protected against oxidation by being embedded or encased within the support material. A primary distinction is that the support of  FIG. 5  is preferably bonded to the lamp leg  514 . Stated another way, there is no gap between the support and the arc body leg as in the previously described embodiments. Rather, in  FIG. 5  a thin layer of support material is provided over the leg  514  and the starting coil. Alternatively, the starting coil may be a thin layer of conductive material that is deposited on the lamp leg to form the helical conductor, i.e., the starting coil is formed or deposited in situ, instead of using a pre-formed helical metal wire joined to the leg with the layer of support material. However, in either event the support has sufficient thickness to mechanically support the starting coil  526  therein, and is also preferably thick enough to provide complete encapsulation and thereby protection from oxidation with the ambient environment. 
     The embodiment of  FIG. 6  provides a slightly different variation on the concept of providing mechanical support and protection against oxidation of the starting coil. Here, the starting coil  626  has a generally helical, extended pitch conformation and is shown as having a generally constant diameter as the starting coil extends along the length of the leg  614 . The support  650  is a high temperature material that provides a thin coating about the entire surface of the starting conductor. The high temperature materials preferred for the support may be glass, quartz, ceramic, or similar light transmissive materials that are able to withstand the high temperatures associated with lamp operation. Alternatively, if the material is sufficiently thin coated, and since the starting coil is not a light transmissive material, consideration may be given to using a different (low light absorptive or non-light transmissive material) since the extended pitch provides openings between respective turns of the starting coil. Thus, alternative high temperature materials that are not light transmissive may be useful in this embodiment for non-coated lamp application, whereas non-light transmissive materials may not be as desirable for the previously described embodiments where the support is a structure that extends the entire height of the leg. This is not to suggest, however, that light transmissive, high temperature materials could not be used as the thin coating or support  650  around the starting coil in  FIG. 6  that still leaves a substantial gap between the respective turns of the conductor to allow light to be transmitted therethrough. In coated lamp applications, support  650  can be low light absorptive material or non-light transmissive material since the coating will reflect the light. 
     The disclosure has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the disclosure be construed as including all such modifications and alterations.