Abstract:
A combination electronic ballast designed to operate a metal halide lamp and a high-pressure sodium gas discharge lamp is disclosed. The ballast includes a primary and secondary coil coupled to a magnetic core. The primary is adapted to be connected to an ac power source and the secondary is adapted to be connected to a metal halide lamp and a high-pressure sodium lamp. The primary and secondary coils are connected together using a capacitance circuit including two capacitors and a switch. The capacitance circuit is designed to connect both of the capacitors between the primary and secondary coil when a high pressure sodium lamp is connected to the ballast and to connect only one of the capacitors in parallel when a metal halide lamp is connected to the ballast. In an alternative embodiment, the primary is adapted to be connected to one of four different ac power sources.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to a ballast for gas discharge lamps. More particularly, this invention pertains to a ballast that may be used with both Metal Halide (MH) Lamps and High Pressure Sodium (HPS) Lamps. 
     Ballast devices for gas discharge lamps are well known in the art. For example, U.S. Pat. Nos. 3,599,037, 3,772,565, 3,873,910, 4,016,452, 4,162,428, 4,350,934, 4,501,994 disclose ballast devices that may be used to control various types of gas discharge lamps. None of these patents, however, disclose or suggest a ballast that may be used to control both a MH lamp and a HPS lamp. 
     Generally, a ballast designed to control a MH lamp (a MH ballast) may not be used to control an HPS lamp. An HPS lamp requires a starting aid, a low starting voltage, and a large current to operate properly. A MH ballast, on the other hand, does not include a starting aid and cannot be used to control an HPS lamp because an MH ballast provides a voltage that is higher than that required for the HPS lamp, as well as, a current that is lower than that required for the HPS lamp. 
     In addition, the voltage drop across an HPS lamp exhibits a wide range over the lifetime of the lamp. Thus, an HPS ballast is also designed to vary the current delivered to an HPS lamp in response to the voltage drop of the HPS lamp. As a result, the power delivered to the HPS lamp remains within a desired range over the entire range of voltages exhibited by the HPS lamp. A MH ballast does not provide the required variation in current. Furthermore, even if a starting aid is incorporated into a MH ballast, the HPS lamp will operate grossly under-wattage and exhibit wide variations in operating wattage as the HPS lamp exhibits its customary swings in voltage. 
     For similar reasons, an HPS ballast may not be used to control a MH lamp. If a standard MH lamp is used with an HPS ballast, the MH lamp will operate in an over-wattage state. 
     As a result of the incompatibility between HPS ballasts and MH ballasts, end users of MH and HPS lamps are required to purchase both types of ballasts, that is MH ballasts and HPS ballasts. The costs associated with purchasing both types of ballasts are undesirably high and end users have indicated a desire for a single ballast capable of operating both a MH lamp and an HPS lamp. 
     What is needed, then, is a combination ballast that may be used to operate both MH and HPS lamps. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a combination ballast for operating both MH lamps and HPS lamps. 
     Another object is to provide a combination ballast capable of delivering a current to a HPS lamp that is dependent on the voltage drop across the HPS lamp. 
     A further object of the present invention is to provide a combination ballast having a capacitance circuit with a capacitance and a means for varying the capacitance of the capacitance circuit. 
     These and other objects are provided by a ballast including a magnetic core, a primary coil wrapped around the magnetic core, a secondary coil wrapped around the magnetic core, a capacitance circuit having a capacitance connected between the primary coil and the secondary coil, a switch included in the capacitance circuit for varying the capacitance of the capacitance circuit, and a starting aid for lamp starting. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of one embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, one embodiment of the present invention includes a magnetic core  20 , a primary coil  22 , a secondary coil  24 , a pair of shunts,  26  and  28 , a capacitance circuit  30 , and a starting aid  32 . Magnetic core  20  is commonly referred to as a T-L magnetic core; that is, a magnetic core formed out of a T-shaped magnetic portion  36  and two L-shaped portions,  38  and  40 . Magnetic core  20  is designed to provide a magnetic circuit for the ballast as is well known in the art and may vary in size according to the wattage of lamps that are connected to the ballast. In one embodiment, designed to operate a 400 watt HPS lamp and a 400 watt MH lamp, the magnetic core  20  is 5.75 inches long, 4.250 inches wide, and 2.32 inches high. 
     T-shaped portion  36  includes a slot  42  positioned at one end of T-shaped portion  36  for controlling the secondary magnetizing reactance of the ballast. The present invention contemplates that the size of slot  42  may be varied as necessary in order to optimize the operation of the ballast. In one embodiment, slot  42  has a width of 0.156 inches and a length of 1.125 inches. 
     Magnetic core  20  also includes first and second windows,  44  and  46 , separated by T-shaped portion  36 . Although a T-L magnetic core is described above, other types of magnetic cores known in the art, such as an E-I magnetic core, may be used as well. 
     Secondary coil  24  is wrapped around the T-shaped magnetic portion  36  over slot  42  and includes a MH lamp connector tap  48 , a secondary capacitance circuit tap  50 , a HPS lamp connector tap  52 , and a starting aid tap  54 . The MH lamp connector tap  48  is adapted to be connected to one end  56  of a MH lamp  58 , the capacitance circuit tap  50  is connected to the capacitance circuit  30 , the HPS lamp connector tap  52  is connected to one input  116  of starting aid  32  and designed to be connected to one end  112  of an HPS lamp  114 , and starting aid connector  54  is connected to a second input  120  of the starting aid  32 . 
     MH lamp connector tap  48  and HPS lamp connector tap  52  should be positioned on secondary coil  24  so that the voltage necessary for operating the MH lamp appears at the MH lamp connector tap  48  and the voltage necessary for operating the HPS lamp appears at HPS lamp connector tap  52 . This is accomplished by positioning MH lamp connector tap  48  and HPS lamp connector tap  52  on secondary coil  24  so that a sufficient number of turns are included between the MH lamp connector tap  48  and the capacitance circuit tap  50  and the HPS lamp connector  52  and capacitance circuit tap  50 . In a similar manner, starting aid tap  54  should be positioned on secondary coil  24  so that the voltage necessary for operating the starting aid  32  appears at the starting aid tap  54 . Thus, starting aid connector tap  54  should be positioned on secondary coil  24  so that a sufficient number of turns are included between starting aid connector tap  54  and capacitance circuit tap  50 . 
     For example, in one embodiment designed to operate a 400 watt MH lamp and a 400 watt HPS lamp, secondary coil  24  includes 458 turns of Number 15½ Copper wire between MH lamp connector tap  48  and capacitance circuit tap  50 , HPS lamp connector tap  52  is positioned at 297 turns, and starting aid tap  54  is positioned at 268 turns. In an alternative embodiment, Number 15 Copper wire may be used for the secondary coil  24  in order to reduce thermal heating in the secondary coil. 
     Second end  122  of MH lamp  56 , second end  124  of HPS lamp  114 , and third input  126  of starting aid  32  are designed to be connected to a common terminal  100 . Although FIG. 1 shows both MH lamp  58  and HPS lamp  114  connected to the ballast, the present invention contemplates that only one of the lamps will be connected at any given time. Thus, either MH lamp  58  or HPS lamp  114  may be connected to the present invention at any given time. 
     Primary coil  22  is wrapped around T-shaped portion  36  and separated from secondary coil  24  by magnetic shunts,  26  and  28 . The embodiment shown in FIG. 1 is a multi-volt embodiment of the present invention. As such, primary coil  22  includes first, second, third, and fourth ac power source connector taps  62 ,  68 ,  70 ,  72 , a common terminal tap  64 , and a primary capacitance circuit tap  66 . In an alternative embodiment where the ballast will not be required to operate with multiple ac power sources, the primary coil may include only one ac power source connector tap for connection to an ac power source. 
     First, second, third, and fourth ac power source connector taps  62 ,  68 ,  70 , and  72  are designed to be connected to positive terminals  76 ,  78 ,  80 , and  82  of ac power sources  84 ,  86 ,  88 , and  90 . In one embodiment, ac power sources  84 ,  86 ,  88 , and  90  are  277 ,  240 ,  208 , and  120  volt ac power sources, respectively. Although FIG. 1 shows all four ac power sources connected to the ballast, the present invention contemplates having only one ac power source connected at a time. Thus, only ac power source  84 ,  86 ,  88 , or  90  would be connected at any given time. Alternative embodiments may include different ac power source voltages. 
     Returning to FIG. 1, first, second, third, and fourth ac power source connector taps  62 ,  68 ,  70 , and  72  should be positioned on primary coil  22  so that the voltage necessary for operating the MH lamp  58  appears at the MH lamp connector tap  48 , the voltage necessary for operating the HPS lamp  114  appears at the HPS lamp connector tap  52 , and the voltage necessary for operating the starting aid  32  appears at the starting aid tap  54 . The necessary voltages may be produced by including a sufficient number of turns between each ac power source connector tap and the common terminal tap  64 . 
     For example, in one embodiment designed to operate a 400 watt MH lamp and a 400 watt HPS lamp, primary coil  22  includes 339 turns of Number 19 Copper wire between first ac power source connector tap  62  and common terminal tap  64 , second ac power source connector tap  68  is positioned at 294 turns, third ac power source connector tap  70  is positioned at 252 turns, and fourth ac power source connector tap  72  is positioned at 147 turns. Common terminal tap  64 , as well as negative terminals  92 ,  94 ,  96 , and  98 , are designed to be connected to terminal  100 . 
     Capacitance circuit  30  includes a first capacitor  31  having a first end  60  and a second end  102 , a second capacitor  33  having a first end  104  and a second end  106 , and a switch  35  having a first end  108  and a second end  110 . Secondary capacitance circuit tap  50  is connected to first end  60  of first capacitor  31  and second end  110  of switch  35 . Primary capacitance circuit tap  66  is connected to second end  102  of first capacitor  31  and first end  104  of second capacitor  33 . Finally, second end  106  of second capacitor  33  is connected to first end  108  of switch  35 . 
     Capacitance circuit  30  is designed to vary the capacitive reactance of the ballast. As is known in the art, a MH lamp requires a ballast having a leakage reactance of approximately 0.75 to 1.0 times the value of the capacitive reactance of the ballast. Another way of stating this relationship is that the capacitive reactance is 1.0 to 1.33 times the value of the leakage reactance. HPS ballasts, on the other hand require a leakage reactance in the range of 2 to 4 times that of the capacitive reactance of the ballast. Or alternatively, the capacitive reactance should be 0.25 to 0.5 times the leakage reactance. The switch  35  varies the capacitive reactance by placing the second capacitor  33  in parallel with the first capacitor  31 . By proper selection of the first and second capacitors, the capacitance reactance of the ballast may be varied from 0.25 to 0.5 times the value of the leakage reactance to 1.0 to 1.33 times the value of the leakage reactance. 
     For example, in one embodiment, designed to operate a 400 watt HPS lamp and a 400 watt MH lamp, first capacitor  31  is a 29 μF capacitor and second capacitor  33  is a 26 μF capacitor. When a MH lamp is connected to the present invention, switch  34  is open and only first capacitor  31  is connected between the primary and secondary windings. When an HPS lamp is connected, switch  35  is closed and both first capacitor  31  and second capacitor  33  are connected between the primary and secondary windings. Thus, the capacitance of the capacitance circuit  30  may be varied by opening and closing switch  35 . Those skilled in the art will recognize that first capacitor  31  and second capacitor  35  may be varied to optimize the operation of the ballast. 
     Magnetic shunts,  26  and  28 , are designed to control the leakage reactance between the primary and secondary coils,  22  and  24 , as is well known in the art. The size of the magnetic shunts should be selected so that the ballast has the desired leakage reactance for a given capacitive reactance. Since the value of the leakage reactance and capacitive reactance are proportionally related the desired leakage reactance will depend on the value of the capacitive reactance of the ballast and, accordingly, the capacitive reactance will depend on the value chosen for the leakage reactance. In one embodiment, magnetic shunts,  26  and  28 , are both 2.5 inches long, 0.75 inches high, and 0.670 inches wide. When positioned within windows,  44  and  46 , an air gap of 0.024 inches remains around each shunt. 
     Thus, although there have been described particular embodiments of the present invention of a new and useful Combination MH/HPS Ballast, it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.