Discharge lamp ballast circuit

#The present invention relates to discharge lamp operating circuits, and more particularly concerns direct current ballast circuits for such lamps.

#It is a general object of the invention to provide an improved direct current operating circuit for loads, especially gaseous discharge lamps.

#It is a particular object of the invention to provide an improved direct current ballast circuit for gaseous discharge lamps which avoids lamp de-energization due to de-ionization of the lamp during operation, and reduces lamp re-ignition voltage.

#The invention will be better understood from the following description taken in conjunction with the accompanying drawings, in which:

#FIG. 1 is a circuit diagram of a direct current operating circuit for a gaseous discharge lamp in accordance with an embodiment of the invention;

#FIG. 2 is a circuit diagram of a modification of the FIG. 1 circuit;

#Referring now to the drawings, and particularly to FIG. 1, there is shown a circuit diagram of a typical embodiment of the invention comprising terminals 1 of a source of alternating current, and induction coil L.sub.1 connected at one side to one of the source terminals and at the other side to an input terminal of full wave bridge rectifier 2, which comprises diodes D1, D2, D3 and D4 arranged in conventional manner as shown, the other input terminal of bridge rectifier 2 being connected to the other source terminal 1. Auxiliary induction coil L.sub.2 is inductively coupled to main induction coil L.sub.1, such as by arrangement of the two coils on a common magnetic core on opposite sides of a magnetic shunt. Such an arrangement of inductively coupled coils is shown, for example, in the patent to Willis, U.S. Pat. No. 3,873,910, assigned to the same assignee as the present invention, and the disclosure thereof is accordingly incorporated herein by reference. Auxiliary induction coil L.sub.2 is connected at opposite sides respectively to the input terminals of another full wave bridge rectifier 3 similar to bridge rectifier 2 and comprising diodes D5, D6, D7 and D8. The output terminals of bridge rectifier 3 are connected to the output terminals of bridge rectifier 2 which in turn are connected across lamp load 4. Lamp 4 is typically a high intensity gaseous discharge lamp, such as a mercury vapor lamp, metal halide lamp or a high pressure sodium vapor lamp. Due to this arrangement, the direct current supplied to lamp 4 by main induction coil L.sub.1 via bridge rectifier 2 is substantially out of phase with the direct current supplied to the lamp by auxiliary coil L.sub.2 via bridge rectifier 3. As a result, the average current through the lamp and the voltage across the lamp is substantially increased over the average magnitude of current and voltage which would be applied in the absence of auxiliary coil L.sub.2 and its associated rectifier circuit, and therefore the tendency of the lamp to drop out because of de-ionization at current zero is largely prevented, and at the same time the lamp re-ignition voltage is substantially reduced so that starting of the lamp is facilitated. As will be understood, the phase shift provided by the described circuit is achieved by the approximate 180.degree. phase shift occurring in coil L.sub.2 with respect to coil L.sub.1 due to the described inductive coupling, combined with the approximate 90.degree. lead in current of the output of coil L.sub.2 due to the voltage across L.sub.1 leading the current through L.sub.1. The resultant phase shift caused by the operation of both coil circuits thus is approximately 90.degree. at lamp starting. After the lamp has started, the degree of phase shift is somewhat reduced from 90.degree..

#In a typical case, the turns ratio of coils L.sub.1 and L.sub.2 is about 1:1, but this ratio may be about 1:4 or 1:5 to provide for increased line power factor where desired.

#FIG. 5 illustrates a typical current wave form characteristic of the lamp current during lamp operation. As will be seen, the minimum points of the wave form are raised substantially above current zero, thereby avoiding lamp de-ionization during the operation of lamp with the unfavorable results therefrom as previously described.