Abstract:
In a lighting system  10 , a voltage pulse clamping device  12  is connected across a ballast coil  20 . The clamping device  12  regulates the ignitor output voltage pulse applied to high intensity discharge lamps during startup. The ignitor pulse is developed by an ignitor pulse circuit connected to the lamp  14 . The voltage clamping device  12  ensures that the ignitor output voltage does not exceed the maximum voltage rating of the lamp  14  being used, regardless of component tolerances or variations in the power supply  24  line voltage. Lamp  14  performance can be increased by the application of the appropriate voltage pulse to the lamp  14 , thereby avoiding degradation of the lamp ballast due to excessively high ignitor pulses.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the control of high intensity discharge lamps, and particularly to the ignitor pulse starting circuit, required for the initial start up and operation of high intensity discharge (HID) lighting systems. 
     2. Discussion of the Art 
     Conventionally, several steps are involved in the start up and sustained operation of an HID lamp. A first step is inherent to lamp design and involves the reduction of arc tube internal gas pressure with respect to atmosphere. A second step, includes the application of a preselected voltage to a set of electrodes to initiate and sustain an arc discharge in the constituent gas of an HID arc tube. A third step, involves using an appropriate gas mixture, which is typically a combination of at least two gases, one of which makes up only about 1% of the total volume and is called the minor constituent. The minor constituent aids in the arc discharge and subsequent thermionic emission of the primary gas mixture, called the major constituent. In the process of operating an HID lamp, commonly, an ignitor circuit is used to generate high voltage pulses to ionize the gases and initiate the arc discharge. As an alternative, a starter electrode may be used to apply either a heating effect, or a high voltage, which aids in the generation of the arc discharge during start up. It is to be noted that a starter electrode is not commonly used with ignitors. 
     A problem encountered with the use of ignitor circuits, is an inherent wide variation of ignitor pulse heights. The variation exists due to differing component tolerances. Another obstacle is a lack of interchangeability between ballast types. There will, for example, be different inductive and capacitive loading of the ignitor pulse by ballasts from different manufacturers, as well as different pulse specifications and different ballast designs. 
     BRIEF SUMMARY OF THE INVENTION 
     An ignitor pulse variable reduction method and apparatus, of the present invention overcomes the limitations of the prior art starting mechanisms by providing a universal ignitor for the different types of lamps and allows interchangeability of lamps, ignitors and ballasts between different lighting systems and manufacturers. While it is possible to match some components of a system to obtain better operational performance, the drawbacks include tighter manufacturing tolerances across all of these various components, and the associated increase in manufacturing costs. 
     The ignitor pulse variable reduction invention substantially reduces the requirement for tighter component tolerances by adding a voltage pulse clamping device which will ensure that the ignitor output voltage does not exceed the maximum lamp voltage rating, regardless of component tolerances or variations in supply line voltage. If the population of ignitor voltages is then set higher than the clamping voltage, all of the ignitors would then have approximately the same voltage level, due to the clamping device. The population of ignitor voltage levels, therefore, becomes a function of tolerance variations within the population of the clamping devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit schematic of an HID lighting system circuit and shows the original embodiment of the ignitor pulse variable reduction clamping device. 
     FIG. 2 is a circuit schematic of an HID lighting system and shows an improvement on the original embodiment of the ignitor pulse variable reduction clamping device. 
     FIG. 3 is a further embodiment according to the present invention. 
     FIG. 4 is a graph of test data and illustrates the effects of the clamping device on ignitor peak voltage levels as a function of the number of turns on the winding of the coil. 
     FIG. 5 is a graph of test data and illustrates the effects on the HID lighting system without the clamping device. 
     FIG. 6 is a graph of test data and illustrates the effects of a 1000 VAC rated, metal oxide varistor, clamping device. 
     FIG. 7 is a graph of test data and illustrates the effects of a 150 VAC rated, metal oxide varistor, clamping device applied across a tapped portion of the reactor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As is well known in the art, starting techniques for an HID lamp relies on electrical circuit properties which generate a preselected high voltage pulse of a specified width, or time duration measured in microseconds. This high voltage pulse, or ignitor pulse, generates free electrons from the electrode metal and initiates the arc discharge by having these free electrons collide with, and impart energy to, the gas atoms in the lamp. The actual voltage pulse imparted to the lamp, can vary, with the amount of variability being dependent on the variabilility of component tolerances. A problem associated with this variability is that some circuits can impart an ignitor pulse level which is too high, and is therefore detrimental to lamp operation and longevity, causing degradation of system components, such as ballast insulation deterioration. 
     Solving the problem of variability through the use of tighter tolerance on the components is one obvious solution, but there are manufacturing and economic problems with this. The present invention substantially reduces the requirement for tighter tolerances on these components by the addition of a voltage pulse clamping device which ensures that the ignitor output voltage pulse does not exceed the specified starting voltage for a particular HID lamp system. By setting the ignitor output voltage pulse higher than the recommended lamp starting voltage, then adding a voltage pulse clamping device to the circuit, the pulse can be clamped, or limited to a preselected voltage level. The variability of component tolerances will no longer be a major factor and the population of ignitor voltage pulses will be dependent on the clamping device tolerances. Clamping devices which can be used include, but are not limited to, Transorbs, zener diodes, and metal oxide varistors (MOVs). 
     With reference to FIG. 1 of the drawings, an HID lighting system  10 , according to the present invention is depicted. The system  10  may also include a fail safe capacitor  11 , a clamping device  12 , a lamp  14 , a firing capacitor  16 , a break-over device  18 , such as, but not limited to a SIDAC, a coil  20 , a resistor  22 , and an alternating current (A/C) power source  24 , which may be a 60 Hz power supply. The firing capacitor  16 , break-over device  18 , coil  20 , and resistor  22 , are components of an ignitor pulse circuit ( 16 ,  18 ,  20 , and  22 ,) which is used as part of ballast  26  to control power supplied to lamp  14  from power supply  24 . The fail safe capacitor  11 , limits the current flow through the clamping device  12 , in the event that the clamping device fails to a shorted condition, and can reduce the probability of clamping device  12  failure in the first place. It is to be understood that the ignitor circuit, while shown as part of the ballast, may also be separate from the ballast. The fail safe capacitor may be a 0.1 Mfd value, or a lesser rated value capacitor. 
     In a configuration (not shown) in which the clamping device  12  is not used in the circuit, the power source  24  charges the firing capacitor  16  through the coil  20  and the resistor  22 . As is well known to those skilled in the art, the power source  24  of the circuit causes the firing capacitor  16  to charge until the rated break over voltage of the break-over device  18  is exceeded, whereupon the break-over device SIDAC  18  changes rapidly from a non-conducting state, to a conducting state, and imparts an ignition pulse on lamp  14 . By this operation, the ignitor pulse circuit ( 16 ,  18 ,  20 ,  22 ) performs the start up of an HID lighting system. Without the clamping device  12  in the circuit, the actual voltage pulse delivered to the lamp  14  can vary from a desired or preselected value. The amount of variation being dependent on the various component tolerances of the firing capacitor  16 , the SIDAC  18 , the coil  20 , the resistor  22 , and any fluctuations of the power source  24 . 
     With continuing reference to FIG. 1, in one embodiment of the present invention, the fail safe capacitor  11 , and the clamping device  12 , are placed in serial connection in the lighting system  10 , as shown, and are in electrical connection across the entire coil  20 . The power source  24  charges the firing capacitor  16  through the coil  20  and the resistor  22 . In this embodiment the firing capacitor  16 , charges until the rated break over voltage of the clamping device  12  is exceeded, whereby it changes to a conducting state, and imparts an ignition pulse to the lamp. The clamping device  12 , along with the transformer action of coil  20 , substantially determines the operating characteristics of the ignition pulse. For example, when the clamping device  12  used is a 1000 VAC rated MOV, it can clamp at approximately 2000 volts peak (Vp). Moreover, two of these clamping devices connected in series can be used to clamp at approximately 4000 Vp. However, it is to be appreciated that the entire pulse path will also include system capacitance  25 , such as parasitic capacitance of line input, power factor corrections, and/or transformer capacitance. 
     The clamping effect on the ignitor pulse ensures that the ignition pulse imparted to the lamp  14  does not exceed a preselected ignition pulse level and will be of a nearly equal value across a population of clamping devices. Any variation in the ignitor pulse, therefore, becomes a function of component tolerance of the clamping device  12 . It is noted in FIG. 1, clamping device  12  is not depicted as part of ballast  26 . However, FIG. 1 is drawn in this manner for convenience, and clamping device  12  may be considered a separate element as shown in FIG. 1 or alternatively, part of ballast  26  as is depicted in FIG.  2 . 
     With reference to FIG. 2 of the drawings, a preferred embodiment of the invention is implemented with a clamping device  12 , and a fail safe capacitor  11 , inserted into the circuit in a manner which preselects the number of coil  20  windings or turns used, and through transformer action affects the clamping operation. The connection of the clamping device  12 , along with the transformer action of coil  20 , substantially determines the clamping device  12  operating parameters, although system variables, including system capacitance  25  will also be part of the current pulse path. In the preferred embodiment of FIG. 2, a power source  24  supplies energy to the circuit and charges the firing capacitor  16  through the coil  20  and the resistor  22 . The firing capacitor  16  charges until the rated break over voltage of the clamping device  12  is exceeded through transformer action, whereupon the clamping device  12  changes rapidly from a non-conducting, to a conducting state, and imparts an ignition pulse on lamp  14 . Furthermore, in this preferred embodiment, the clamping device  12 , can be of a much lower voltage rating, such as a 120 VAC rated MOV for example. The selection on the number of windings of coil  20  used, along with the action of the clamping device  12 , yields a reduction in the variation of the ignitor pulse voltage levels across a population of HID lighting systems. 
     It is also to be noted that clamping device  12  may be a device with variable or adjustable clamping values. Use of such a device allows a user to adjust the clamping action. In yet another embodiment, tap  27 , may be configured as an adjustable tap whereby the number of turns used, and which determines the clamping device  12  connection, may be adjustable within a single system. These foregoing embodiments increase the operating range, and usefulness of the clamping circuit. In laboratory testing, the bobbin flange from an output coil of various ballasts was removed and insulation removed by scrapping it off of the wire windings to create an electrical connection point or tap  27 , at the end layers of the coil  20  windings. For testing purposes the tap  27  was positioned at 60, 120, 180, 240, 300, and 347 turns of the coil  20  windings. Tests were then performed with three separate MOVs, rated at 150 VAC, 250 VAC, and 300 VAC respectively. 
     Turning to FIG. 3, a further embodiment according to the present embodiment is illustrated. In this embodiment, at least some turns of coil  20  are placed around the outside of ballast  26 , and clamping device  12  is connected to these turns. In this embodiment, the core of the ballast  26  is metal (e.g. steel), and does not respond to the high frequency pulse. By the design, a true transformer is configured, as opposed to an autotransformer. It is to be understood that all of the coil may be placed outside of ballast  26 , and the clamping device  12  may be internal (with external connections), or may be external to the ballast  26 . 
     With reference to FIG. 4, the test data yielded three curves, solid lines ( 28 ,  30 ,  32 ), for the three different rated MOV clamping devices. The test data corresponds to the peak clamping voltage graphed as a function of the number of turns on the coil  20  windings, where tap  27  is positioned on the windings. Curve  28  is representative of the data for the 150 VAC rated MOV. Curve  30  is representative of the data for the 250 VAC rated MOV. Curve  32  is representative of the data for the 300 VAC rated MOV. Analysis of the graphed test data reveals that as the number of turns on tap  27 , of coil  20  decreases, the peak clamping voltage increases in an approximately linear fashion. Using linear regression statistical analysis yields a set of curves, dashed lines ( 34 - 36 ), and a corresponding linear equation which is used to calculate the number of turns on the coil  20  windings which yields a desired peak clamping voltage for a particular type of an HID lamp. 
     The test data for the 150 VAC rated MOV clamping device, and a particular ballast, yields curve  34 , dashed line in FIG. 4, of which the corresponding linear equation, of the form y=mx+b, is given by: 
     
       
           y =−5.6019 x +2664.4  EQ. 1 
       
     
     The test data for the 250 VAC rated MOV, and the 300 VAC rated MOV, for a particular ballast, were combined to yield curve  36 , dashed line in FIG. 3, of which the corresponding linear equation is given by: 
     
       
           y =−7.171 x +3490.5  EQ. 2 
       
     
     These equations correspond to the ignitor output voltage pulse y as a function of the number of windings x to which the clamping device  12  is connected via tap  27 . These same equations are then useful for calculating the number of turns on the winding x of coil  20  which yield a preselected ignitor output voltage pulse y. The electrical operation of the clamping device  12  and the transformer action of coil  20  determine the waveform of the ignitor output voltage pulse. It is therefore possible to regulate the ignitor pulse voltage level applied to an HID lamp by properly selecting a clamping device  12  and calculating the number of turns required to obtain the desired ignitor output voltage pulse for a specific type of HID lamp and ballast system. This allows for optimization of the pulse peak and width for a particular lamp and ballast combination. Similarly, test data for other types of lamps and ballasts will yield a linear equation of the form y=mx+b, which is then used to calculate tap  27  placement of the clamping device. The linear equation is derived by application of statistical analysis, linear regression to the test data. 
     Additional testing to determine the feasibility of the concept was performed, and circuit voltage waveform data on lamp  14  was obtained using an oscilloscope. With reference to FIG. 5, a voltage waveform  38  was generated by the ignitor circuit without the clamping device  12  in the lighting system circuit. The vertical axis of the graph is in increments of 500 volts (0.5 kilovolts), the peak of the voltage waveform has a maximum of approximately 3600 Vp, without the clamping device  12  in the circuit. The horizontal axis is in 1 microsecond increments, and a major portion of the ignitor pulse energy is imparted to lamp  14  in approximately 5 microseconds, during an ignitor pulse duration  40 . 
     With reference to FIG. 6, depicted is a voltage waveform  42  generated by a 1000 VAC rated clamping device across the entire coil  20  of the lighting system circuit. The vertical axis of the graph is in increments of 500 volts, the peak of the voltage waveform has a maximum of approximately 2600 Vp, with the clamping device  12  in the circuit. The horizontal axis is in 1 microsecond increments, and a major portion of the ignitor pulse energy is imparted to lamp  14  in approximately 6 microseconds, during an ignitor pulse duration  44 . 
     With reference to FIG. 7, a voltage waveform  46  was generated with a 150 VAC rated clamping device across a predetermined number of windings of coil  20 , in the lighting system circuit. The vertical axis of the graph is in increments of 500 volts, the peak of the voltage waveform has a maximum of approximately 2500 Vp, with the clamping device  12  in the circuit. The horizontal axis is in 1 microsecond increments, and a major portion of the ignitor pulse energy is imparted to lamp  14  in approximately 5 microseconds, during an ignitor pulse duration  48 . 
     Thus, the present invention uses fail safe capacitor  11 , and a single, small, and inexpensive clamping device  12 , which controls ignition pulse levels within a much narrower tolerance band than would otherwise be possible using several components. This allows for lower manufacturing costs and a less bulky solution for the mechanisms required to control the ignition pulse for start up of an HID lighting system. 
     Inasmuch as the present invention is subject to variation, modifications and changes in detail, it is intended that all matter described throughout this specification and shown in the accompanying drawing be interpreted as illustrative only and not in a limiting sense. Accordingly, it is intended that the invention be limited only by spirit and scope of the hereto attached claims.