Abstract:
A lamp ignition circuit is provided which can initiate operation of a gas discharge lamp using a driving voltage which is similar in magnitude to the lamp operating voltage. The lamp ignition circuit is useful with a semi-resonant ballast and lamp circuit in which switching operations intrinsic to the lamp shock-excite a series-connected inductor and capacitor into semi-resonant operation corresponding to an energy exchange and transfer during each half-cycle of the alternating current source to drive the lamp to start and maintain operation of the lamp using line voltage. The ignitor circuit has a disabling function following ignition of the lamp which is operable when the operating voltage of the lamp is approximately the line voltage of the power source. The disabling function triggered by an increase in voltage across the ignition circuit following operation of the lamp.

Description:
This is a continuation-in-part application of prior U.S. patent application Ser. No. 08/968,093, originally filed Nov. 12, 1997, now U.S. Pat. No. 5,962,988, which is a continuation-in-part of U.S. application Ser. No. 08/556,878, filed Nov. 2, 1995 (now U.S. Pat. No. 5,825,139), both of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a lamp ignition circuit having a disabling function which operates with a low-wattage discharge semi-resonant ballast and lamp circuit. Further, the invention relates to a lamp ignition circuit, the disabling function of which does not require the lamp operating voltage to be considerably higher than the ballast open-circuit voltage during lamp run conditions. 
     BACKGROUND OF THE INVENTION 
     A low-wattage discharge lamp circuit which provides lamp-driven voltage transformation and ballasting is described in U.S. Pat. No. 5,825,139 (commonly assigned to Hubbell Incorporated). The lamp circuit described therein uses the discharge breakdown mechanism of the lamp itself at least once each half-cycle to excite a series-connected inductance and capacitance into ringing up to an instantaneous and root mean square (RMS) open circuit voltage (OCV) of approximately twice the input line voltage to drive the discharge lamp. This is in contrast with a conventional gas discharge lamp circuit which supplies higher voltage to the lamp to maintain operation. For example, a conventional gas discharge lamp circuit is typically provided with a semiconductor switching device to augment the source voltage to provide the required lamp ignition voltage. 
     The measured lamp operating voltage of the lamp circuit described in U.S. Pat. No. 5,825,139 is higher than the line voltage because the lamp itself facilitates its own driving voltage. The lamp circuit is advantageous because it does not require such switching circuits as the aforementioned semiconductor switching device and therefore requires fewer components. Instead, switching operations intrinsic to the lamp shock-excite the inductance and the capacitance into an energy exchange and transfer during each half-cycle at a higher frequency than the frequency of the AC source connected to the lamp circuit. The circuit values for the inductance and capacitance are chosen to allow this semi-resonant operation. In other words, these circuit reactors are different from self-resonant reactors because they are resonant when the switching lamp excites them and therefore are capable of being shocked by the switching action of the lamp. Accordingly, the lamp circuit described in U.S. Pat. No. 5,825,139 is hereinafter referred to as a semi-resonant ballast and lamp circuit. 
     A lamp starting circuit or ignitor is normally present in a lamp circuit and is typcially switched out of operation, or its influence on the lamp circuit is minimized, by the lamp entering a normal operating mode. Conventional ignitors do not function properly with the semi-resonant ballast and lamp circuit described in U.S. Pat. No. 5,825,139 because they depend upon the lamp operating voltage being considerably lower than the ballast OCV. A need therefore exists for an ignition circuit which can ignite a lamp in a semi-resonant ballast using substantially the line voltage. A need also exists for an ignition circuit which does not require an operational distinction such as the significant difference between the instantaneous OCV and the lamp operating voltage used to provide or withhold ignition pulses in conventional ignitor circuits. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a lamp ignition circuit is provided which can start and maintain operation of a gas discharge lamp using only line voltage as the activating electromotive force. 
     In accordance with an aspect of the present invention, a lamp ignition circuit for a semi-resonant ballast and lamp circuit is provided which does not require an operational distinction such as a significant difference between the instantaneous OCV and the lamp operating voltage to provide or withhold ignition pulses as do conventional ignitor circuits. 
     In accordance with another aspect of the present invention, a lamp ignition circuit is provided which has a disabling function triggered by an increase in voltage across the ignition circuit following operation of the lamp. 
     A discharge lamp circuit comprises: (1) a discharge lamp operable from an alternating current power source; (2) an inductor; (3) a first capacitor, the inductor, the lamp and the capacitor being connected in series; and (4) an ignitor circuit connected at one end thereof to a first node between the inductor and the lamp and connected at the other end thereof to a second node between the capacitor and the power source. Switching operations intrinsic to the lamp shock-excite the inductor and the capacitor into semi-resonant operation corresponding to an energy exchange and transfer during each half-cycle of the alternating current source to drive the lamp to start and maintain operation of the lamp using line voltage. The ignitor circuit has a disabling function following ignition of the lamp which is operable when the operating voltage of the lamp is approximately the line voltage of the power source. 
     In accordance with another embodiment of the present invention, an ignitor circuit for a semi-resonant ballast and lamp circuit is provided. The semi-resonant ballast and lamp circuit is operable to use switching operations intrinsic to a discharge lamp to shock-excite a series-connected inductor and capacitor into an energy exchange and transfer during each half-cycle of an alternating current source providing power to the semi-resonant ballast and lamp circuit to start and maintain operation of the lamp using line voltage. The ignitor circuit comprises: (1) a second capacitor; (2) a capacitor charging circuit for charging the second capacitor with an offset voltage; and (3) a pulse generator circuit for generating pulses via discharging of the second capacitor to ignite the lamp when combined with the offset voltage and line voltage from the power source. The pulse generator circuit is connected at one end thereof to a first terminal of the second capacitor. The second capacitor is connected at a second terminal thereof to a first node between the inductor and a first terminal of the lamp. The pulse generator circuit is connected at another end thereof at a second node between the capacitor and the power source. The pulse generating circuit is rendered ineffective for igniting the lamp when voltage across the first node and the second node increases during operation of the lamp. A disabling circuit is provided for the ignitor circuit which is triggered by a voltage corresponding to the root mean square voltage of the power source. 
     In accordance with yet another embodiment of the present invention, an ignitor circuit for a semi-resonant ballast and lamp circuit comprises: (1) a resistor and a second capacitor connected in a series circuit and across the lamp; (2) a transformer having a primary winding and a secondary winding; (3) a breakover device; and (4) third capacitor connected at one terminal thereof to respective first terminals of the primary winding and the secondary winding and at the other terminal thereof to a return path of the lamp to the power source, the breakover device having a terminal connected to the second terminal of the primary winding and another terminal connected to the series circuit, the second terminal of the secondary winding being connected to the supply side of the lamp. The second capacitor charges through the resistor until a breakover voltage corresponding to the breakover device is reached. The second capacitor discharges through the primary winding to allow the transformer to generate a pulse for igniting the lamp using substantially the line voltage. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The various aspects, advantages and novel features of the present invention will be more readily comprehended from the following detailed description when read in conjunction with the appended drawings, in which: 
     FIG. 1 is a schematic diagram of a semi-resonant ballast and lamp circuit having an ignitor circuit constructed in accordance with an embodiment of the present invention. 
     FIG. 2 is a schematic diagram of a semi-resonant ballast and lamp circuit having an ignitor circuit constructed in accordance with an embodiment of the present invention. 
     FIG. 3 is a schematic diagram of a semi-resonant ballast and lamp circuit having an ignitor circuit constructed in accordance with an embodiment of the present invention. 
    
    
     Throughout the drawing figures, like reference numerals will be understood to refer to like parts and components. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In order for a lamp to strike, the lamp requires sufficient OCV from a ballast. FIG. 1 depicts a semi-resonant ballast and lamp circuit as described in U.S. Pat. No. 5,825,139 comprising a series-connected inductance  20 , lamp  24  and capacitance  22 . The semi-resonant ballast and lamp circuit is operated from, but not limited to, a 120 volt RMS line voltage and is therefore incapable of supplying sufficient OCV for the lamp to strike. An ignitor circuit is provided as an auxiliary circuit branch and comprises a resistor  38  and a diode  40  to charge the series resonant capacitor  22  with DC offset voltage. The value of the resistor  38  is selected such that the combination of the offset and the AC line voltage with ignitor pulses provides sufficient OCV for the lamp to ignite. In accordance with the present invention, ignitor circuits  34 ,  50  and  80  are described below in connection with FIGS. 1,  2  and  3 , respectively, to provide ignitor pulses for a semi-resonant ballast and lamp circuit in which the ignitor starts and the ballast maintains operation of a gas discharge lamp using line voltage, and which do not require an operational distinction such as a significant difference between the instantaneous OCV and the lamp operating voltage to provide or withhold ignition pulses, as in conventional ignitor circuits. 
     With reference to FIG. 1, the inductor  20  and series capacitor  22  are both selected to provide ballasting to operate the lamp as described in the U.S. Pat. No. 5,825,139 incorporated herein by reference. The instantaneous OCV of the lamp and ballast circuit arrangement depicted in FIG. 1 is the input voltage V 1 . An advantage of this semi-resonant ballast and lamp circuit is the ability to drive the discharge lamp  24  with a relatively low input voltage without the use of an autotransformer ballast, which can significantly improve the overall efficiency of the ballast circuit. Since conventional ignitors use the difference between the instantaneous OCV and the lamp operating voltage to provide or withhold starting pulses, their use presents problems in connection with a lamp and semi-resonant ballast circuit as shown in FIG.  1 . This is because the lamp voltage of the lamp and ballast circuit configuration in FIG. 1 is approximately the line voltage and therefore does not provide adequate means for making an operational distinction for use with a conventional ignitor. The semi-resonant ballast and lamp circuit of FIG. 1, however, presents a significance difference between the voltage across the ignitor (i.e., V AB ) during open circuit conditions and during operation of the lamp. For example, a 150 watt metal halide (MH) lamp circuit being operated from a 120 VAC power supply presents a 67 volt V RMS  difference between V AB  during open circuit and operating states. The semi-resonant ballast and lamp circuit in FIG. 1 is unique in that the voltage V AB  is higher during lamp operation, which is in contrast with the voltage being lower during lamp operating conditions in a standard ballast and ignitor configuration. 
     With continued reference to FIG. 1, the capacitor  26  is charged each half-cycle of the input voltage through a resistor  28 , a positive temperature coefficient (PTC) resistor  30  and a radio frequency choke (RFC)  32 . The resistor  28  sets the time constant for determining the number of pulses per half-cycle. The RFC  32  decouples the ignitor circuit  34  from the high frequency pulse that it is generating. When the capacitor  26  reaches an instantaneous voltage that is substantially equal to the breakover voltage of a sidac  36 , the sidac  36  conducts and discharges the energy stored in the capacitor  26 . This energy is transferred through the tapped ballast inductor  20  and appears across the lamp terminals in the form of a high voltage pulse. The high frequency impedance of the capacitor  22  is low and has nominal effect on the high frequency, high voltage ignitor pulse. The PTC  30  is chosen to have a trip current above the current required for ignitor operation during open circuit conditions. Unlike conventional lamp and ignitor circuits, when the lamp  24  has begun operating, the voltage across the ignitor circuit  34  rises and the current passing through the ignitor circuit  34  increases. If the PTC trip current is then exceeded, the PTC self-heats, causing the resistance therein to rise to a level where the capacitor  26  does not charge to the breakover level of the sidac  36 . Accordingly, the ignitor circuit  34  ceases to function. 
     By way of an example, for a 150 W MH, 120 VAC lamp and ballast circuit, the following circuit values in Table 1 are applicable. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Ignitor Circuit Components of FIG. 1 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 L = 118 millihenries 
               
               
                   
                 C1 = 27 microfarads 
               
               
                   
                 D = 2000 V, 0.25A (rectifier) 
               
               
                   
                 R1 = 33,000 ohms 
               
               
                   
                 C2 = 0.22 microfarads 
               
               
                   
                 S = 150 V (sidac) 
               
               
                   
                 R2 = 2000 ohms 
               
               
                   
                 PTC = 180 ohms @ 25 C, I TRIP  = 70 milliamps 
               
               
                   
                 RFC = 55 millihenries 
               
               
                   
                   
               
             
          
         
       
     
     The semi-resonant ballast and lamp circuit in FIG. 2 illustrates another ignitor circuit which uses the significant difference between V AB  during lamp run and open circuit conditions to disable the ignitor circuitry. The semi-resonant ballast and lamp circuit operate as described in the aforementioned U.S. Pat. No. 5,825,139. The semi-resonant ballast and lamp circuit has an ignitor circuit  50  which is more advantageous than the ignitor circuit  34  depicted in FIG. 1 because it does not rely on the thermal characteristics of a single component as does the ignitor circuit  34 . The ignitor components including the diode  40 , the resistor  38 , the capacitor  26 , the resistor  28 , the RFC  32  and the sidac  36  operate in the same manner as described in connection with FIG.  1 . 
     With continued reference to FIG. 2, a bi-directional thyristor  52  and series resistor  54  are provided across the capacitor  26 . A thyristor trigger circuit is also provided which comprises zener diodes  56  and  58 , a resistor  60 , the capacitor  62  and another sidac  64 . During a non-operating lamp condition, the voltage V AB  is approximately 125 V RMS , which is not adequate to cause zener diodes  56  and  58  to conduct. When the lamp  24  begins to operate, however, the voltage V AB  increases to approximately 213 V RMS , which is sufficient to turn on the zener diodes  56  and  58 . Under this higher voltage condition for V AB , the capacitor  62  charges through the resistor  60  until the voltage across the capacitor  62  reaches the breakover voltage of the sidac  64 . The sidac  64  then conducts, which activates the bi-directional thyristor  52 . The thyristor  52  then discharges energy stored in the capacitor  26  through the resistor  54 . The overall result is that the capacitor  26  does not store enough energy to activate the sidac  36 ; therefore, no high voltage ignitor pulses are generated when the lamp begins to operate based on the difference between the voltage V AB  during open circuit and lamp operating conditions. Table 2 provides exemplary values for the ignitor circuit depicted in FIG.  2 . 
     
       
         
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Ignitor Circuit Components of FIG. 2 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 R1 = 33,000 ohms 
               
               
                   
                 R2 = 2,500 ohms 
               
               
                   
                 R3 = 10 ohms 
               
               
                   
                 R4 = 1,500 ohms 
               
               
                   
                 L = 118 millihenries 
               
               
                   
                 C1 = 27 microfarads 
               
               
                   
                 C2 = 0.22 microfarads 
               
               
                   
                 C3 = 0.1 microfarads 
               
               
                   
                 S1 = 150V (sidac) 
               
               
                   
                 S2 = 150V (sidac) 
               
               
                   
                 T = 400 V, 6A (triac) 
               
               
                   
                 D = 2000 V, 0.25A (rectifier) 
               
               
                   
                 RFC = 55 millihenries 
               
               
                   
                 Z1 = 200 V (zener diode) 
               
               
                   
                 Z2 = 200V (zener diode) 
               
               
                   
                   
               
             
          
         
       
     
     Another embodiment for a low wattage ignitor circuit for a semi-resonant ballast and lamp circuit will now be described with reference to FIG.  3 . The semi-resonant ballast and lamp circuit comprises an inductor  82  and a series connected capacitor  84  with a lamp  24 . The inductor  82  and the capacitor  84  are operable to be semi-resonant at a frequency higher than the frequency of the AC power source such that, after the lamp has been ignited, the lamp  24  switches and causes a semi-resonant energy exchange with the reactances of components  82  and  84  thereby maintaining the lamp  24  at a stable operating condition up to full rated wattage, as described in the aforementioned U.S. Pat. No. 5,825,139. 
     With continued reference to FIG. 3, a back-charge is created on a capacitor  86  from the charging of the series capacitor  84  in the semi-resonant ballast circuit via the resistor R 2  and the diode D 1 . This back-charge provides the capacitor  88  with the ability to be charged through the resistor  90  so that the sidac  92  can breakover in both the positive and negative half-cycles over the standard input voltage range. When the sidac  92  breaks over, the charge stored in the capacitor  88  is discharged through the primary winding of the transformer  94 . The transformer  94  transforms this current pulse into a high voltage pulse. The capacitor  96  decouples low frequency AC and DC voltage from passing through the secondary winding of the transformer  94 . This transformation can occur several times per half-cycle of the 60 hertz line voltage. The high voltage pulses generated via the ignitor circuit  80  are of sufficient magnitude to ionize the arc tube of the gas discharge lamp  24 . This provides the ability to start and maintain operation of a gas discharge lamp using the line voltage. 
     The inductors L 1  and L 2  in FIG. 3 are used to subdue the loading effect of the resistor  90  and the capacitor  88  have on the high-voltage pulse. Both of the inductors L 1  and L 2  are used in order to overcome current limitations of the component. The inductors L 1  and L 2  divide the total current from the resistor  90  and the capacitor  88  so that each inductor can handle their respective amounts of current without overheating. The capacitor  86  is depicted as being attached to both the primary and the secondary common leads of the pulse transformer  94  due to the internal component connection. The transformer, however, can be a 3-lead or a 4-lead transformer without affecting circuit operation. 
     The ignitors  34  and  50  in FIGS. 1 and 2, respectively, are preferably used with an inductor-lamp-capacitor circuit configuration. Further, the leads of the ignitor are preferably provided across the lamp  24  and the capacitor  22  in order to obtain the voltage of both the lamp  24  and the capacitor  22 . The ignitor  80  in FIG. 3 is preferably used with either an inductor-lamp-capacitor circuit configuration or a capacitor-lamp-inductor circuit configuration. 
     Although the present invention has been described with reference to preferred embodiments thereof, it will be understood that the invention is not limited to the details thereof. Various modifications and substitutions have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. All such substitutions are intended to be embraced within the scope of the invention as defined in the appended claims.