Resistive lamp ballast with re-ignition circuit

#This invention relates to ballasts for discharge lamps operated from alternating current line sources. More particularly, the present invention relates to a resistive lamp ballast operating together with means for sensing lamp voltage and means for applying the output of an oscillator circuit to the lamp to effect lamp re-ignition.

#For proper operation, discharge lamps require a current limiting or regulating device electrically connected between the lamp and the electrical energy source for the lamp. The most ubiquitous and available source of lamp energy is the conventional 50 or 60 cycle alternating current voltage present at conventional electrical outlets. Discharge lamps conventionally "see" this electrical power source as a source of constant voltage. Moreover, once lit, the resistance exhibited by a discharge lamp falls off with increasing lamp current. Thus, such lamps are not self-ballasting like conventional incandescent lamps whose resistance increases with filament current. Accordingly, for discharge lamps a current limiting device is required. This ballasting device may be an inductor, capacitor, resistor or an appropriately designed electronic circuit. However, the resistive ballast has a significant advantage of having a very low cost and is, accordingly, desirable for many discharge lamp applications.

#Furthermore, because of the wide-spread availability of 50 or 60 Hz alternating current power, it is, accordingly, desirable to be able to operate discharge lamps directly from such an electrical energy source. However, when operated on alternating current directly, discharge lamps extinguish twice each cycle at those points in time at which the lamp current goes to zero. Inductive ballasts aid in the re-ignition of discharge lamps operated under alternating current conditions, since they provide a voltage spike each time the lamp current drops to zero. This voltage is phased so that it adds to the line voltage. However, resistive ballasts are generally considered undesirable for alternating current operation since they cannot provide voltage spikes which may be employed for lamp re-ignition. Therefore, in resistive ballast circuits operating from alternating current lines, lamp re-ignition voltage is limited by the available line voltage. However, the re-ignition voltage of certain lamps, such as miniature arc lamps, may be higher than the available line voltage. One such miniature arc discharge lamp is the Halarc lamp which is currently preferably operated from a direct current power supply since this eliminates the need to re-ignite the lamp twice during each alternating current cycle. However, direct current operation in such lamps can create cataphoretic effects which may have an adverse effect on lamp color as a function of lamp operating position. Therefore, it is also desirable to be able to operate this lamp and other discharge lamps directly from alternating current sources. Additionally, as discussed above, it is also desirable to operate such lamps using a resistive ballast. However, in such situations, means must be provided to ensure lamp re-ignition during conditions of zero current.

#The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

#FIG. 1 is a schematic diagram illustrating one embodiment of the present invention; and

#FIG. 2 is a schematic circuit diagram more particularly illustrating a possible embodiment for the control circuit of FIG. 1.

#The block diagram schematic shown in FIG. 1 is particularly useful for understanding the operation and construction of the present invention. In this circuit, discharge lamp 10, comprising, for example, a miniature arc discharge lamp, is resistively ballasted with the combination of filament 12 and resistor R17. Discharge lamp 10, filament 12 and resistor R17 are connected in series. R17 may be eliminated if filament 12 is constructed so as to have an appropriate resistance. Filament 12 typically comprises a conventional incandescent lamp filament and operates to provide visible light output during the time that discharge lamp 10 is warming up. Accordingly, filament 12 and discharge lamp 10 are typically found contained within the same envelope as part of an integral lamp structure. Additionally, the secondary winding of transformer T1 is also connected in series with discharge lamp 10, resistor R17 and filament 12. It is through the secondary winding of T1 that re-ignition energy is supplied to discharge lamp 10 during zero current transition times. The frequency of the electrical re-ignition energy waveform is generally selected to be significantly in excess of the alternating current line input voltage which is typically either 50 or 60 Hz. In particular, successful experiments of the present invention have been carried out using a re-ignition energy frequency of approximately 200 KHz. Accordingly, transformer T1 may be constructed with a primary winding comprising about 50 turns of No. 30 wire around a ferrite core such as core 2616/A160/3B7 as supplied by the Ferroxcube Corporation of Saugerties, N.Y. Additionally, the secondary winding of transformer T1 may comprise approximately 300 turns of No. 30 insulated wire also wound around the ferrite core. Additionally, capacitor C5 may be disposed across lamp 10 and the secondary winding of transformer T1. Capacitor C5 operates as a high frequency bypass capacitor and is selected to have a capacitive value such that its impedance at the re-ignition energy frequency is relatively low, while at a frequency of approximately 50 or 60 Hz it is relatively high. In particular, a capacitor with a capacitance value of approximately 0.5 microfarads may be employed.

#During times at which the current through discharge lamp 10 is zero, the voltage that appears across lamp 10 is generally almost equal to the input line voltage. It is at these times that re-ignition energy is supplied to lamp 10 by the circuit of the present invention. In order to determine the timing for this re-ignition energy to be applied, it is necessary to provide a circuit which responds either to the voltage or current conditions across or through, respectively, lamp 10. For example, re-ignition energy may be applied to lamp 10 whenever the voltage across lamp 10 exceeds a predetermined threshold value. Such a circuit is shown in FIG. 1. In the embodiment illustrated in FIG. 1, it is the voltage condition which is used to determine the timing for triggering the application of a re-ignition voltage to the lamp. In particular, control circuit 14 operates to connect oscillator 16 to amplifier 18 under appropriate lamp conditions. Amplifier 18 then supplies a relatively high frequency signal along shielded cable 20 to the primary winding of transformer T1. Control circuit 14 operates to provide a higher voltage supply for the arc tube for a preset period of time. This preset period is typically selected to be less than 1/2 cycle of the power line frequency, so that the re-ignition cycle is repeated each half cycle of the input power waveform, as required.

#It should be particularly noted that the operation of the present invention offers several significant advantages for lamp operation. In particular, it should be noted that in its contemplated mode of operation, the ballast of the present invention observes the lamp voltage during each half cycle of applied alternating current voltage to determine during each half cycle whether or not it is necessary to apply a re-ignition voltage to the discharge lamp. It should also be noted that several types of discharge lamps which are employable with the present invention exhibit a characteristic warm-up time of between about 5 and about 20 seconds, generally depending on lamp size, during which time re-ignition voltages are particularly useful in reestablishing a discharge current through the ionizable medium. Furthermore, certain lamps exhibit characteristics during their first few hours of operation in which certain lamp impurities are "cleaned up" or neutralized. Depending upon lamp size, lamp construction, electrode material, and the specific nature of the ionizable medium, various lamps exhibit different needs for this early clean-up cycle. However, the ballast of the present invention is particularly advantageous in that requisite re-ignition voltages are applied to lamps requiring this clean-up, only as is necessary during lamp warm-up or clean-up cycles. Lastly, it is not uncommon for the alternating current power line supplying the lamp to exhibit voltage fluctuations. During such fluctuations when the line voltage is lower than normal, lamp re-ignition can be a particularly difficult problem. However, this problem is significantly mitigated in the present invention since re-ignition voltages would be applied during such conditions of low line voltage. Thus, the ballast and lamp of the present invention would continue to operate as long as tolerable levels of power line voltage were present. Accordingly, the automatic compensation advantages of the present ballast and lamp should therefore be particularly noted.