US 3882354 A
An oscillator including a single transistor is connected in circuit with the primary winding of a stepup transformer. The terminals of a fluorescent lamp are connected between the primary and secondary of the transformer. One filament of the lamp is connected in the base circuit of the transistor. The filament acts as a constant current device to limit the current flowing in the base circuit and thereby permit the inverter to operate over a wide range of input voltages.
Description (OCR text may contain errors)
United States Patent [191 May [451 May 6,1975
[ INVERTER BALLAST CIRCUIT FOR FLUORESCENT LAMP  Inventor: Randall L. May, Andover, Kans.
 Assignee: The Coleman Company, Wichita,
 Filed: July 23, 1973  Appl. No.: 381,858
 US. Cl. 315/101; 315/105; 315/206;
315/221; 315/DIG. 5; 3l5/DIG. 7  Int. Cl H051) 41/29; HOSb 41/392  Field of Search 315/94-98,
315/101, 105, 200, 206, 221, 278, DIG. 2, DIG. 5, DIG. 7, 177-179, 181; 331/112  References Cited UNITED STATES PATENTS 3,432,723 3/1969 Miller et al. 315/94 3,629,648 12/1971 Brown et a]. 315/98 Primary Examiner-R. V. Rolinec Assistant ExaminerE. R. LaRoche Attorney, Agent, or Firm-Dawson, Tilton, Fallon & Lungmus [5 7] ABSTRACT An oscillator including a single transistor is connected in circuit with the primary winding of a stepup transformer. The terminals of a fluorescent lamp are connected between the primary and secondary of the transformer. One filament of the lamp is connected in the base circuit of the transistor. The filament acts as a constant current device to limit the current flowing in the base circuit and thereby permit the inverter to operate over a wide range of input voltages.
2 Claims, 1 Drawing Figure INVERTER BALLAST CIRCUIT FOR FLUORESCENT LAMP BACKGROUND AND SUMMARY The present invention relates to an inverter ballast circuit for fluorescent lamps. There are many such circuits already known. and the purpose of these circuits is to convert a DC. voltage to an alternating voltage of suitable magnitude andifrequency to energize a fluores cent lamp.
One of the problems with prior inverter ballast circuit designs is that they donot operate well over a wide range of input D.C. voltages. One of the major uses of inverter ballast circuits is in recreational vehicles; and depending upon the vehicle design and the state or condition of the batteries of the vehicles, an inverter ballast circuit, to be practical, must operate with a range of input voltages extending from 8 or 9 volts up to 25 volts.
The present invention has as a principal object, the design of an inverter ballast circuit for operating fluorescent lamps which is capable of operation over a wide range of input D.C. voltages. The present circuit includes an oscillator having a single transistor as an active element. The transistor is connected in circuit with the primary winding of a step-up transformer. The terminals of a fluorescent lamp are connected between the primary and secondary windings of the transformer.
One filament of the lamp is connected in the base circuit of the transistor. preferably in series between the base terminal of the transistor and one terminal of the primary winding of the transformer. The filament thus acts as a constant current device to limit the gain of the transistor independent of the input voltage source. That is to say. with the filament of the lamp in series with the base circuit of the transistor. if the input voltage is increased. a slight additional average current through the filament will cause it to heat up. thereby increasing its resistance dramatically. and tending to stabilize the base current at a fixed value. Similarly, if the input voltage is decreased, a corresponding slight decrease in the base current will substantially reduce the resistance of the heater filament of the lamp, and thereby permit more base current to flow than would otherwise flow if the resistance of the heater filament were not so temperature dependent.
The light given off by a fluorescent lamp is a linear function of the applied voltage, and for slight changes in applied voltage, the change in light intensity is not very noticeable. Hence, the present invention provides a suitable inverter ballast circuit for fluorescent lamps over a wide range of input voltages.
Another feature of the present invention is that it produces a better starting circuit, especially in cold weather. The reason for this is that when the circuit is first turned on, the filament of the tube is cold. thereby providing a maximum of base current and. hence. a maximum voltage output from the transformer to start the lamp. The filament heats rapidly with a maximum of base current. and this has an advantage in that a tube with a hot filament starts easier than a tube with cold filaments.
Another advantage of the present invention, and one that is considered quite important, is that if the fluorescent lamp is dislodged from its socket, the circuit is shut off--that is, completely prevented from operating-- -because the base circuit of the transistor is interrupted. In many prior circuits, if the fluorescent lamp fell out of its socket, not only would the inverter circuit continue to operate. but it would generate an excessive voltage because such circuits were designed to produce a high voltage across the tube terminals until the lamp was lit. and the voltage would then drop as the lamp began to conduct. If the lamp did not conduct or became dislodged, the circuits continued to generate an excessively high voltage. Under these circumstances, the circuit also continues to draw excess current which will destroy the transistor and associated circuitry on the inverter.
The present invention also has the advantage that one or more fluorescent lamps can be connected in series with each other and excited by the same inverter ballast circuit. In such an arrangement. a No. I lamp would have its filament connected in series with the base circuit of the transistor in the oscillator. Thus. if any single tube goes out, it is simply replaced; but if all of the lamps go out. the No. l lamp is replaced because it is known that if the No. l lamp burns out. it will inhibit operation of the other lamps because the inverter ballast circuit will not be capable of operating.
Other features and advantages of the present invention wil be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the drawing.
THE DRAWING The single FIGURE is a circuit schematic diagram of an inverter ballast circuit incorporating the present invention.
DETAILED DESCRIPTION Referring to the drawing. reference numeral 10 generally designates a fluorescent lamp having a pair of terminals 11 at one end and a similar pair of terminals 12 at the other end. A filament I3 is connected between the terminals 11, and a similar filament I4 is connected between the terminals 12 is not used in the illustrated embodiment.
A source of input DC. voltage. such as a storage battery. has its positive terminal connected to an input terminal 16 of the circuit, and its negative terminal connected to the circuit terminal 17 which may be ground.
The terminal 16 is connected by means ofa diode 18 to the collector ofa transistor generally designated 19. A capacitor is connected from the collector of the transistor 19 to ground.
The emitter of transistor 19 is connected to a terminal 21 of a transformer T which has a primary winding generally designated 22 and a secondary winding 23. The primary winding 22 includes a first series of turns 24 and a second series of turns 25 connected in series. The terminal 21. previously mentioned, consists of a tap between the windings 24, 25.
A capacitor 27 is connected across the entire primary 22, to suppress transients, and the lamp 10 is connected between the primary and secondary windings, in series with a current limiting capacitor 28. That is. one of the terminals 11 is connected to the ungrounded terminal of the primary winding 22, and one of the terminals 12 is connected to the ungrounded terminal of the secondary 23 via capacitor 28.
The other terminal 11 of the lamp I0 is connected in series with a capacitor 30 to the base terminal of transistor 19. A diode 31 is connected in the polarity shown between the base and emitter terminals'of the transistor 19; and a leakage resistor 32 is connected between the base and collector terminals of the transistor 19.
OPERATION With the lamp and circuit connected as shown in the drawing, and a source of DC. voltage connected to the terminals l6, 17 in the polarity shown, the circuit operation will now be described. It will be appreciated that a manually operated switch is normally incorporated into the circuit for turning the lamp on and off, and such a switch may preferably be located in series with the diode 18.
When power is applied to the transistor 19, it will be appreciated that the base is biased positively relatively to the emitter, and this will initially cause the transistor to conduct. The resulting current flows through the section 24 ofthe primary winding 22 of the transformer T. Thus, the terminal 21 will begin to go positive, as will the corresponding dotted terminals of the section 25 of the primary 22 and the secondary winding 23. When the dotted terminal of the section 25 of the primary 22 goes positive, the transistor 19 becomes more forward biased, thereby resulting in additional emitter current, the final value of which will depend upon the base current which, in turn, is dependent upon the value of resistance in the filament l3. Initially, the filament 13 is cold, so the initial current energizing transformer T is large, and the secondary voltage is also large, thereby supplying a large voltage to start the tube 10.
Another way to view this operation is that when the filamenflli is cold, its resistance is low, so the base current is high, thereby reducing the voltage across the collector-emitter terminals of the transistor 19. Since the input voltage is constant, the remaining voltage appears directly across the section 24 of the primary winding 22.
As the" current flowing in the secondary changes slopepolarity, the voltage across the primary section 25 and secondary 23 will also diminish, and the reduced voltage at the primary section 25 will reduce the base corrent in transistor 19. The oscillations in the secondary will thus drive the transistor, but the magnitude of base current remains substantially constant.
As the temperature of the filament 13 increases, its resistance increases dramatically, thereby reducing the base current until a stable, equilibrium position is reached.
The operation thus far described has been only for the starting and continuous operation of the circuit for a given voltage source. One of the principal features of the present invention is that if the voltage source changes in magnitude, or if the circuit is used with a different voltage source of different magnitude, the ballast circuit readily accommodates itself to the change in input voltage magnitude while supplying the lamp 10 with substantially the same terminal voltage for operation.
For example, if the input voltage across the terminals l6, 17 is reduced, the average current flowing in the filament 13 is correspondingly reduced and this will cause the filament to cool somewhat, thereby decreasing its resistance in a nonlinear fashion so that the resulting decrease in base current is almost negligible. ln other words, I have found that the filament of the fluorescent tube, in the circuit illustrated, acts as a substantially constant current source. If the voltage across the terminal l6, 17 increases, the resulting base current increase will raise the temperature of the filament 13, thereby dramatically increasing its resistance value, and permitting only a negligible increase in base current.
It will also be observed that if the tube 10 becomes dislodged from the circuit, the transistor 19 cannot oscillate because the base circuit is disconnected from the regenerating section 25 of the primary 22 of the transformer T. That is to say. there is nothing to turn the base circuit off and cause it to oscillate.
In the embodiment shown, the ratio of the turns of the windings 22, 25, 23 is as follows: 13/9/250. This turns ratio is useful in energizing a single lamp. When it is desired to use more than one lamp, they may be connected in series and the voltages adjusted accordingly. When such an arrangement is used, only the No. 1 tube has its filament connected in series with the base of the transistor 19, and the remaining tubes are connected in series. Thus, if any of the tubes other than the No. 1 tube burns out, it is simply replaced, but if the No. 1 tube is burned out, this fact will be made known because none of the tubes will then burn.
Having thus described in detail a preferred embodiment of the present invention, persons skilled in the art will be able to modify certain of the structure which has been illustrated and to substitute equivalent elements for those disclosed while continuing to practice the principle of the invention; and it is, therefore, intended that all such modifications and substitutions be covered as they are embraced within the spirit and scope of the appended claims.
1. An inverter ballast circuit for energizing a fluorescent lamp from a source of direct voltage comprising: a transistor having a collector, an emitter and a base terminal, said collector being connected to one terminal of said source; a transformer having a primary winding and a secondary winding, said primary winding including an input section and a feedback section; means for connecting said lamp between said primary and secondary winding; means for connecting said feedback winding section of said transformer in series with a filament of said lamp and with the base-emitter junction of said transistor, the emitter junction of said transistor being further connected in circuit with said input winding section of said primary winding of said transformer such that current flowing through said transistor will energize said input winding section to induce a voltage in said secondary and in said feedback winding section, said input winding and said transistor being arranged in an oscillatory circuit, whereby as the current in said input winding section increases, said filament will limit the input current to said transistor, said filament being characterized as being a substantially constant current element over its operating range, thereby to control the output voltage of said transistor.
2. In an inverter ballast circuit for energizing a fluorescent lamp from a source of direct voltage with an oscillating circuit having an active element and controlled by an input current, the improvement comprising: transformer means including a primary winding and a secondary winding, said primary winding including an input winding section and a feedback winding.
section; means connectingsaid lamp between said secondary and said primary windings; and means connect-.
ing said feedback winding section, a filament of said LII keep the input current to said active element substantially constant whereby a substantially constant voltage is applied and energizes said lamp independent of the magnitude of said voltage source over a given range.
UMTED STATES PATENT OFFICE QERTHEATE 9F CURRECTION Patent No. 3 882 354 InVentor(s -1' L May Dated May 6, 1975 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
assignees name to:
The Coleman Company, Inc.
RUTH C. MASON 'Arlc'sling Officer gigmd and fiealzd' this fif h Day of August1975 C. MARSHALL DANN ('ununissinncr of Parents and Trarlwnurks