This invention relates to glow-type starter devices used for starting discharge lamps and circuits employing such starters, and more particularly, to a starter and starting circuit for use in the operation of high intensity discharge lamps containing noble gases at high pressures, e.g., in excess of 100 torr.
High intensity discharge lamps, such as high pressure sodium lamps, commonly include noble gases at pressures below 100 torr. For example, the starting gas in the standard high pressure sodium lamp is xenon at 14 torr. Lamps containing noble gases at pressures below 100 torr can be started and operated by utilizing an igniter in conjunction with a lamp ballast. The igniter electronically provides high voltage, short duration pulses which assist in initiating discharge. The lamp ballast converts the AC line voltage to the proper amplitude and impedance level for lamp operation.
It has been found that the inclusion in high pressure sodium lamps of xenon as the noble gas at pressures well in excess of 100 torr is beneficial to lamp performance. However, the igniter described above does not produce reliable starting at xenon pressures above about 100 torr. A conductor wrapped around the discharge tube and connected to one of the electrodes is described as having been utilized in assisting the starting of a lamp containing xenon at pressures up to 300 torr in U.S. Pat. No. 4,179,640. The lamp is described as having been operated from a conventional ballast and starting pulse generator.
Another arrangement for starting high pressure discharge lamps is shown in U.S. Pat. No. 4,137,483. A switching circuit contained within the lamp induces a high voltage starting pulse. The high voltage pulse operates in conjunction with a conductor wrapped around the discharge tube to initiate discharge in the lamp. The igniter and the conventional ballast are not used.
Recent developments have indicated the desirability of including xenon at pressures in excess of 300 torr in high pressure sodium lamps. However, none of the starting arrangements described above are effective to reliably start lamps having xenon pressures in excess of 300 torr. For example increases in efficiency of approximately 20% can be obtained by increasing the xenon pressure to 400-500 torr, but the voltage necessary to start these lamps is approximately double that required for lamps having a fill pressure below 100 torr. Such high amplitude starting pulses, however, undesirably increase the dielectric stresses on auxiliary equipment.
An improved starting arrangement for overcoming the aforementioned problems with respect to the starting of high intensity discharge lamps with fill gas pressures in excess of 300 torr is described in a copending application Ser. No. 139,310, filed Apr. 11, 1980 and assigned to GTE Laboratories Incorporated. A high pressure sodium lamp including a discharge tube containing xenon at pressures in excess of 300 torr is reliably started by the combination of an igniter, a conductor wrapped around the discharge tube, and a switching circuit. A conventional lamp ballast provides ac power during starting and normal operation. The conductor about the discharge tube is a starting aid which intensifies the electric field within the xenon filled tube. The igniter provides periodic pulses of 2,500-4,000 volts with a duration of about one microsecond. The switching circuit provides a high voltage pulse having an amplitude about equal to the amplitude of the periodic pulses and a duration much greater than the duration of the periodic pulses; e.g., in the order of 100 microseconds. Hence, reliable starting is achieved by increasing the pulse duration rather than the amplitude in order to minimize the dielectric stresses on auxilliary equipment.
In the aforementioned copending application Ser. No. 139,310, the specific means disclosed for implementing the switching circuit comprises, in one instance, a manual switch in series with a current limiting resistor connected across the lamp. A second switch implementation disclosed in the copending application is illustrated schematically in FIG. 1 and comprises a thermal switch 54 including a heater resistor 58 and a bimetal switch 60 connected in series. Inputs A and A' from the lamp ballast and igniter are coupled through the heater resistor 58 to the electrodes of the discharge lamp 50. The bimetal switch 60 and a current limiting resistor 56 are coupled in series across the electrodes of the discharge lamp 50. In operation, the lamp ballast provides AC power to the points A and A' and the igniter provides periodic pulses of high amplitude and short duration to the points A and A' as described hereinabove. Also, the conductor 52 promotes the formation of an ionization path within the discharge lamp 50 as also described above. In a cold condition, the bimetal switch 60 is closed. Therefore, when power is applied to the points A and A', current flows through the resistor 58, the bimetal switch 60, and the resistor 56. The heater resistor 58 is placed in close proximity to the bimetal switch so that heat generated by current passing therethrough will heat the bimetal switch 60. After a predetermined time, the heat generated by the resistor 58 causes the bimetal switch 60 to switch to the open position and the current drawn from the ballast is rapidly decreased. The rapid decrease in current drawn from the ballast causes the highly inductive output of the ballast to generate a high voltage pulse which provides sufficient energy to initiate discharge in the lamp 50. The current drawn by the discharge lamp 50 through the resistor 58 causes the resistor 58 to remain heated and the bimetal switch 60 to remain in the open position. If for some reason, the discharge lamp 50 does not start when the bimetal switch 60 opens, no current is drawn through the resistor 58, and the bimetal switch 60 cools until it recloses. Heating of the resistor 58 again occurs, causing the bimetal switch 60 to open and another high voltage starting pulse is generated. Thus, the starting process is repeated until a discharge is initiated in the lamp 50.