Patent Publication Number: US-6664742-B2

Title: Filament cut-back circuit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an operation of a lamp. The present invention specifically relates to filament cutback. 
     2. Description of the Related Art 
     When lamps are operated by a programmed-start ballast, which by definition requires heating,of the lamp filaments before lamp ignition, the lamp life is increased. The heating of the lamp filaments typically cease upon lamp ignition in order to reduce losses during normal operation of the lamp. Also, it is important not to exceed the lead current limits given by the lamp manufacturer. Hence, a good filament cutback circuit is necessary for improved lamp performance and reduced power loss at normal operation. 
     One filament cutback circuit as known in the art employs a capacitor in series with a filament winding and a filament to achieve a first-order cut back. A second filament cutback circuit as known in the art employs a parallel coupling of a capacitor and an inductor coupled in series between the filament winding and the filament. This circuit operates as a low impedance circuit during a preheating of the lamp filaments, and as an open circuit (i.e., infinite impedance) during normal operation of the lamp. A third filament cutback circuit as known in the art employs a series coupling of a capacitor and an inductor coupled in series between the filament winding and the filament. This circuit operates as a short circuit (i.e., zero impedance) during a preheating of the lamp filaments, and as a high impedance circuit during normal operation of the lamp. 
     The present invention is an improvement over the aforementioned prior art filament cut-back circuits. 
     SUMMARY OF THE INVENTION 
     The present invention is a filament cut-back circuit. Various aspects of the present invention are novel, non-obvious, and provide various advantages. While the actual nature of the present invention covered herein can only be determined with reference to the claims appended hereto, certain features, which are characteristic of the embodiments disclosed herein, are described briefly as follows. 
     One form of the present inventions is a filament cut-back circuit comprising a filament winding and an impedance circuit in electrical communication with said filament winding. The impedance circuit operates as a short circuit in response to a reception of an alternating voltage at a preheat frequency. The impedance circuit operates as an open circuit in response to a reception of an alternating voltage at an operating frequency. 
    
    
     The foregoing form as well as other forms, features and advantages of the present invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the present invention rather than limiting, the scope of the present invention being defined by the appended claims and equivalents thereof. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a first embodiment of a filament cut-back circuit in accordance with the present invention; 
     FIG. 2 illustrates a first embodiment of the FIG. 1 impedance circuit; 
     FIG. 3 illustrates a second embodiment of a filament cut-back circuit in accordance wit he present invention; and 
     FIG. 4 illustrates a third embodiment of a filament cut-back circuit in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a filament cut-back circuit  10  of the present invention. Circuit  10  comprises an alternating current (“AC”) voltage source V AC  coupled to a primary filament winding PFW whereby a secondary filament winding SFW provides an alternating winding voltage V W  in square wave form. Circuit  10  further comprises a new and unique impedance circuit  20  coupled in series between secondary filament winding SFW and a filament F. In response to alternating winding voltage V W , impedance circuit  20  operates as a short circuit during a pre-heat frequency of winding voltage V W  (typically 90 kHz), which is any time prior to an ignition of a lamp containing filament F. Subsequently, impedance circuit  20  operates as an open circuit during an operating frequency of winding voltage V W  (typically, 45 kHz), which is after the ignition of the lamp. Impedance circuit  20  further serves to provide a high impedance at a third and higher harmonic frequency, which is important in view of a significant third and higher harmonic content of winding voltage V W . With impedance circuit  20 , winding current (not shown) associated with winding voltage V W would be high during a pre-heat frequency of winding voltage V W and low during an operating frequency of winding voltage V W . This would give good preheat energy to filament F and reduce losses during normal operation of filament F. 
     FIG. 2 illustrates a filament cut-back circuit  10   a  including one embodiment of impedance circuit  20 . A capacitor C and an inductor L 1  are coupled in parallel. This parallel coupling of capacitor C and inductor L 1  is coupled in series between the filament winding FW and an inductor L 2 . Inductor L 2  is further coupled in series to the filament F. The impedance established by capacitor C, inductor L 1 , and inductor L 2  is in accordance with the following equation[1]:                Z        (   w   )       =       jw        [       (       L   1     +     L   2       )     -       w   2     ·     L   1     ·     L   2     ·   C       ]         1   -       w   2     ·     L   1     ·   C                 [   1   ]                         
     where j is square-root of −1, and w is the frequency of winding voltage V W  in radians/sec. 
     In order to operate as a short circuit during the pre-heat frequency of winding voltage V W , the capacitance of capacitor C, the inductance of inductor L 1 , and the inductance of inductor L 2  is in accordance with the following equation [2]: 
     
       
         [( L   1   +L   2 )− w   2   ·L   1   ·L   2   ·C ]:=0  [2] 
       
     
     In order to operate as an open circuit during the operating frequency of winding voltage V W , the capacitance of capacitor C and the inductance of inductor L 1  is in accordance with the following equation [3]: 
     
       
         (1 −w   2   ·L   1   ·C ):=0  [3] 
       
     
     FIG. 3 illustrates a filament cut-back circuit  11  of the present invention. Circuit  11  comprises AC voltage source V AC  coupled to primary filament winding PFW as previously described in connection with FIG.  1 . Circuit  11  further comprises a first series coupling of a secondary filament winding SFW 1 , an impedance circuit  20   1  and a filament F n  to a nth series coupling of secondary filament winding SFW n , an impedance circuit  20   n  and a filament F n . Each impedance circuit  20   1 - 20   n  operates in the same manner as an operation of impedance circuit  20  as described in connection with FIG.  1 . Additionally, each impedance circuit  20   1 - 20   n  can employ the embodiment of impedance circuit  20  as described in connection with FIG.  2 . 
     FIG. 4 illustrates a filament cut-back circuit  12  of the present invention. Circuit  12  comprises impedance circuit  20  coupled in series between AC voltage source V AC  and primary filament winding PFW. Impedance circuit  20  operates in response to a reception of an alternating source voltage (not shown) from AC voltage source V AC  in an analogous manner to the operation of impedance circuit  20  in response to a reception of alternating winding voltage V W  as described in connection with FIG.  1 . Additionally, impedance circuit  20  can employ the embodiment of impedance circuit  20  as described in connection with FIG. 2 in an analogous manner to the employment of the embodiment of impedance circuit  20  in circuit  10   a.    
     While the embodiments of the present invention disclosed herein are presently considered toe preferred, various changes and modifications cane made without departing from the spirit and scope of the present invention. The scope of the present invention is indicated in the appended claims, and all changes that come within the meaning and range of equivalents are intended to be embraced therein.