Abstract:
In a high-frequency electronic ballast, a fluorescent lamp is connected with and powered by way of a series-resonant LC circuit. A resistive load is connected with the LC circuit, thereby to constitute a load therefor before ignition of the fluorescent lamp or in case the fluorescent lamp were to fail to ignite.

Description:
RELATED APPLICATIONS 
     The present application is a Continuation-in-Part of Ser. No. 07/579,569 filed Sep. 10, 1990; which is a Continuation-in-Part of Ser. No. 06/787,692 filed Oct. 15, 1985; which is a Continuation of Ser. No. 06/644,155 filed Aug. 27, 1984, now abandoned; which was a Continuation of Ser. No. 06/555,426 filed Nov. 23, 1983, now abandoned; which was a Continuation of Ser. No. 06/178,107 filed Aug. 14, 1980, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to electronic ballasts for rapid-start fluorescent lamps, particularly where the lamps are powered via a series-resonant LC circuit. 
     2. Description of Prior Art 
     For a description of pertinent prior art, reference is made to U.S. Pat. No. 4,677,345 to Nilssen; which patent issued from a Division of application Ser. No. 06/178,107 filed Aug. 14, 1980; which application is the original progenitor of instant application. 
     Otherwise, reference is made to the following U.S. Pat. No. 3,263,122 to Genuit; U.S. Pat. No. 3,320,510 to Locklair; U.S. Pat. No. 3,996,493 to Davenport et el.; U.S. Pat. No. 4,100,476 to Ghiringhelli; U.S. Pat. No. 4,262,327 to Kovacik et al.; U.S. Pat. No. 4,370,600 to Zansky; U.S. Pat. No. 4,634,932 to Nilssen; and U.S. Pat. No. 4,857,806 to Nilssen. 
     SUMMARY OF THE INVENTION 
     Objects of the Invention 
     Objects of the present invention are those of providing for cost-effective electronic ballasts as well as compact screw-in fluorescent lamps. 
     This as well as other objects, features and advantages of the present invention will become apparent from the following description and claims. 
     Brief Description 
     The present invention is directed to providing improved inverter circuits for powering and controlling gas discharge lamps. The inverter circuits according to the present invention are highly efficient, can be compactly constructed and are ideally suited for energizing gas discharge lamps, particularly “instant-start” and “self-ballasted” fluorescent lamps. 
     According to one form of the present invention, a series-connected combination of an inductor and a capacitor is provided in circuit with the inverter transistors to be energized upon periodic transistor conduction. Transistor drive current is preferably provided through the use of at least one saturable inductor to control the transistor inversion frequency to be equal to or greater than the nature resonant frequency of the inductor and capacitor combination. The high voltages efficiently developed by loading the inverter with the inductor and capacitor are ideally suited for energizing external loads such as gas discharge lamps. In such an application, the use of an adjustable inductor permits control of the inverter output as a means of adjusting the level of lamp illumination. 
     According to another important form of the present invention, reliable and highly efficient half-bridge inverters include a saturable inductor in a current feedback circuit to drive the transistors for alternate conduction. The inverters also include a load having an inductance sufficient to effect periodic energy storage for self-sustained transistor inversion. Importantly, improved reliability is achieved because of the relatively low and transient-free voltages across the transistors in these half-bridge inverters. 
     Further, according to another feature of the present invention, novel and economical power supplies particularly useful with the disclosed inverter circuits convert conventional AC input voltages to DC for supplying to the inverters. 
     Yet further, according to still another feature of the invention, a rapid-start fluorescent lamp is powered by way of a series-resonant LC circuit; while heating power for the lamp&#39;s cathodes is provided via loosely-coupled auxiliary windings on the tank inductor of the LC circuit. Alternatively, cathode heating power is provided from tightly-coupled windings on the tank inductor; in which case output current-limiting is provided via a non-linear resistance means, such as an incandescent filament in a light bulb, connected in series with the output of each winding. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front elevation of a folded fluorescent lamp unit adapted for screw-in insertion into a standard Edison incandescent socket; 
     FIG. 2 is a schematic diagram illustrating the essential features of a push-pull inverter circuit particularly suitable for energizing the lamp unit of FIG. 1; 
     FIGS.  3 A- 3 D is a set of waveform diagrams of certain significant voltages and currents occurring in the circuit of FIG. 2; 
     FIG. 4 is a schematic diagram of a DC power supply connectable to both 120 and 240 volt AC inputs; 
     FIG. 5 is a schematic diagram which illustrates the connection of a non-self-ballasted gas discharge lamp unit to the FIG. 2 inverter circuit; 
     FIG. 6 is a schematic diagram which illustrates the use of a toroid heater for regulation of the inverter output; 
     FIG. 7 is an alternate form of push-pull inverter circuit accordind to the present invention; 
     FIG. 8 is a schematic diagram showing the connection of a gas discharge lamp of the “rapid-start” type to an inductor-capacitor-loaded inverter according to the present invention; 
     FIG. 9 is a modification of FIG. 8, showing loosely-coupled auxiliary windings on the tank inductor; and 
     FIG. 10 is another modification of FIG. 8, showing nonlinear current-limiting means connected with the output of tightly-coupled auxiliary windings on the tank inductor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a screw-in gas discharge lamp unit  10  comprising a folded fluorescent lamp  11  suitably secured to an integral base  12 . The lamp comprises two cathodes  13 ,  14  which are supplied with the requisite high operating voltage from a frequency-converting power supply and ballasting circuit  16 ; which, because of its compact size, conveniently fits within the base  12 . 
     The inverter circuit  16  is connected by leads  17 ,  18  to a screw-type plug  19  adapted for screw-in insertion into a standard Edison-type incandescent lamp socket at which ordinary 120 Volt/60 Hz power line voltage is available. A ground plane comprising a wire or metallic strip  21  is disposed adjacent a portion of the fluorescent lamp  11  as a starting aid. Finally, a manually rotatable external knob  22  is connected to a shaft for mechanical adjustment of the air gap of a ferrite core inductor to vary the inductance value thereof in order to effect adjustment of the inverter voltage output connected to electrodes  13 ,  14  for controlled variation of the lamp illumination intensity. 
     With reference to FIG. 2, a power supply  23 , connected to a conventional AC input, provides a DC output for supplying a high-efficiency inverter circuit  24 . The inverter is operable to provide a high voltage to an external load  26 , which may comprise a gas discharge device such as the fluorescent lamp  11  of FIG.  1 . 
     The power supply  23  comprises bridge rectifier having four diodes  27 ,  28 ,  29  and  31  connectable to a 240 volt AC supply at terminals  32 ,  33 . Capacitors  34 ,  36  are connected between a ground line  37  (in turn directly connected to the inverter  24 ) and to a B+ line  38  and a B− line  39 , respectively. The power supply  23  also comprises a voltage doubler and rectifier optionally connectable to a 120 volt AC input taken between the ground line  37  and terminal  33  or  32 . The voltage doubler and rectifier means provides a direct electrical connection by way of line  37  netween one of the 120 volt AC power input lines and the inverter  24 , as shown in FIG.  2 . The bridge rectifier and the voltage doubler and rectifier provide substantially the same DC output voltage to the inverter  24  whether the AC input is 120 or 240 volts. Typical voltages are +160 volts on the B+ line  38  and −160 volts on the B− line  39 . 
     With additional reference to FIG. 4, which shows an alternate power supply  23 ′, the AC input, whether 120 or 240 volts, is provided at terminals  32 ′ and  39 . Terminal  39  is in turn connected through a single-pole double-throw selector switch  41  to terminal  37 ′ (for 120 volt operation) or terminal  33 ′ (for 240 volt operation). In all other respects, power supplies  23  and  23 ′ are identical. 
     The inverter circuit  24  of FIG. 2 is a half-bridge inverter comprising transistors  42 ,  43  connected in series across the DC voltage output of the power supply  23  on B+ and B− lines  38  and  39 , respectively. The collector of trasistor  42  is connected to the B+ line  38 , the emitter of transistor  42  and the collector of transistor  43  are connected to a midpoint line  44  (designated “M”) and the emitter of transistor  43  is connected to the B− line  39 . The midpoint line  44  is in turn connected to the ground line  37  through primary winding  46  of a toroidal saturable core transformer  47 , a primary winding  48  on an identical transformer  49 , an inductor  51  and a series-connected capacitor  52 . The inductor  51  and capacitor  52  are energized upon alternate transistor conduction in a manner to be described later. 
     An external load  26  is preferably taken off capacitor  52 , as shown in FIG.  2 . The inductor  51 , preferably a known ferrite core inductor, has an inductance variable by mechanical adjustment of the air gap in order to effect variation in the level of the inductor and capacitor voltage and hence the power available to the load, as will be described. When the load is a gas discharge lamp such as lamp  11  in FIG. 1, variation in this inductance upon rotation of knob  22  accomplishes a lamp dimming effect. 
     Drive current to the base terminals of transistors  42  and  43  is provided by secondary windings  53 ,  54  of transformers  49 ,  47 , respectively. Winding  53  is also connected to midpoint lead  44  through a bias capacitor  56 , while winding  54  is connected to the B− lead  39  through an identical bias capacitor  57 . The base terminals of transistors  42  and  43  are also connected to lines  38  and  44  through bias resistors  58  and  59 , respectively. For a purpose to be described later, the base of transistor  42  can be optionally connected to a diode  61  and a series Zener diode  64  in turn connected to the midpoint line  44 ; similarly, a diode  63  and series Zener diode  64  in turn connected to the B− line  39  can be connected to the base of transistor  43 . Shunt diodes  66  and  67  are connected across the collector-emitter terminals of transistors  42  and  43 , respectively. Finally, a capacitor  68  is connected across the collector-emitter terminals of transistor  43  to restrain the rate of voltage rise across those terminals, as will be seen presently. 
     The operation of the circuit of FIG. 2 can best be understood with additional reference to FIG. 3, which illustrates significant portions of the waveforms of the voltage at midpoint M (FIG.  3 A), the base-emitter voltage on transistor  42  (FIG.  3 B), the current through transistor  42  (FIG.  3 C), and the capacitor  52  voltage and the inductor  51  current (FIG.  3 D). 
     Assuming that transistor  42  is first to be triggered into conduction, current flows from the B+ line  38  through windings  46  and  38  and the inductor  51  to charge capacitor  52  and returns through capacitor  34  (refer to the time period designated I in FIG.  3 ). When the saturable inductor  49  saturates at the end of period I, drive current to the base of transistor  42  will terminate, causing voltage on the base of the transistor to drop to the negative voltage stored on the bias capacitor  56  in a manner to be described, causing this transistor to become non-conductive. As shown in FIG.  3   c , current-flow in transistor  43  terminates at the end of period I. 
     Because the current through inductor  51  cannot change instantaneously, current will flow from the B− bus  39  through capacitor  68 , causing the voltage at midpoint line  44  to drop to −160 volts (period II in FIG.  3 ). The capacitor  68  restrains the rate of voltage change across the collector and emitter terminals of transistor  42 . The current through the inductor  51  reaches its maximum value when the voltage at the midpoint line  44  is zero. During period III, the current will continue to flow through inductor  51  but will be supplied from the B− bus through the shunt diode  67 . It will be appreciated that during the latter half of period II and all of period III, positive current is being drawn from a negative voltage; which, in reality, means that energy is being returned to the power supply through a path of relatively low impedance. 
     When the inductor current reaches zero at the start of period IV, the current through the primary winding  46  of the saturable inductor  47  will cause a current to flow out of its secondary winding  54  to cause transistor  43  to become conductive, thereby causing a reversal in the direction of current through inductor  51  and capacitor  52 . When transformer  47  saturates at the end of period IV, the drive current to the base of transistor  43  terminates and the current through inductor  51  will be supplied through capacitor  68 , causing the voltage at midpoint line  44  to rise (period V). When the voltage at the midpoint line M reaches 160 volts, the current will then flow through shunt diode  66  (period VI). The cycle is then repeated. 
     As seen in FIG. 3, saturable transformers  47 ,  49  provide transistor drive current only after the current through inductor  51  has diminished to zero. Further, the transistor drive current is terminated before the current through inductor  51  has reached its maximum amplitude. This coordination of base drive current and inductor current is achieved because of the series-connection between the inductor  51  and the primary windings  46 ,  48  of saturable transformers  47 ,  49 , respectively. 
     The series-connected combination of the inductor  51  and the capacitor  52  is energized upon the alternate conduction of transistors  42  and  43 . With a large value of capacitance of capacitor  52 , very little voltage will be developed across its terminals. As the value of this capacitance is decreased, however, the voltage across this capacitor will increase. As the value of the capacitor  52  is reduced to achieve resonance with the inductor  51 , the voltage on the capacitor will rise and become infinite in a loss-free circuit operating under ideal conditions. 
     It has been found desirable to regulate the transistor inversion frequency, determined mainly by the saturation time of the saturable inductors  47 ,  49 , to be equel to or higher than the natural resonance frequency of the inductor and capacitor combination in order to provide a high voltage output to external load  26 . A high voltage across capacitor  52  is efficiently developed as the transistor inversion frequency approaches the natural resonant frequency of the inductor  51  and capacitor  52  combination. Stated another way, the conduction period of each transistor is desirably shorter in duration than one quarter of the full period corresponding to the natural resonant frequency of the inductor and capacitor combination. When the inverter  24  is used with a self-ballasted gas discharge lamp unit, it has been found that the inversion frequency can be at least equal to the natural resonant frequency of the tank circuit. If the capacitance value of capacitor  52  is reduced still further beyond the resonance point, unacceptably high transistor currents will be experienced during transistor switching and transistor burn-out will occur. 
     It will be appreciated that the sizing of capacitor  52  is determined by the application of the inverter circuit  24 . Variation in the values of the capacitor  52  and the inductor  51  will determine the voltages developed in the inductor-capacitor tank circuit. The external load  26  may be connected in circuit with the inductor  51  (by a winding on the inductor, for example) and the capacitor may be omitted entirely. If the combined circuit loading of the inductor  51  and the external load  26  has an effective inductance of value sufficient to effect periodic energy storage for self-sustained transistor inversion, the current feedback provided by the saturable inductors  47 , 49  will effect alternate transistor conduction without the need for additional voltage feedback. When the capacitor  52  is omitted, the power supply  23  provides a direct electrical connection between one of the AC power input lines and the inverter load circuit. 
     Because the voltages across transistors  42 ,  43  are relatively low (due to the effect of capacitors  34 ,  36 ), the half-bridge inverter  24  is very reliable. The absence of switching transients minimizes the possibility of transistor burn-out. 
     The inverter circuit  24  comprises means for supplying reverse bias to the conducting transistor upon saturation of its associated saturable inductor. For this purpose, the capacitors  56  and  57  are charged to negative voltages as a result of reset current flowing into secondary windings  53 ,  54  from the bases of transistors  42 ,  43 , respectively. This reverse current rapidly turns off a conducting transistor to increase its switching speed and to achieve inverter circuit efficiency in a manner described more fully in my co-pending U.S. patent application Ser. No. 103,624 filed Dec. 14, 1979 and entitled “Bias Control for High Efficiency Inverter Circuit” (now U.S. Pat. No. 4,307,353). The more negative the voltage on the bias capacitors  56  and  57 , the more rapidly charges are swept out of the bases of their associated transistors upon transistor turn-off. 
     When a transistor base-emitter junction is reversely biased, it exhibits the characteristics of a Zener diode having a reverse breakdown voltage on the order of 8 to 14 Volt for transistors typically used in high-voltage inverters. As an alternative, to provide a negative voltage smaller in magnitude on the base lead of typical transistor  42  during reset operation, the optional diode  61  and Zener diode  62  combination can be used. For large values of the bias capacitor  56 , the base voltage will be substantially constant. 
     If the load  26  comprises a gas discharge lamp, the voltage across the capacitor  52  will be reduced once the lamp is ignited to prevent voltages on the inductor  51  and the capacitor  52  from reaching destructive levels. Such a lamp provides an initial time delay during which a high voltage, suitable for instant starting, is available. 
     FIG. 5 illustrates the use of an alternate load  26 ′ adapted for plug-in connection to an inverter circuit such as shown in FIG.  2 . The load  26 ′ consists of a gas discharge lamp  71  having electrodes  72 ,  73  and connected in series with a capacitor  74 . The combination of lamp  71  and capacitor  74  is connected in parallel with a capacitor  52 ′ which serves the same purpose as capacitor  52  in the FIG. 2 circuit. However, when the load  26 ′ is unplugged from the circuit, the inverter stops oscillating and the development of high voltages in the inverter is prevented. The fact that no high voltages are generated by the circuit if the lamp is disconnected while the circuit is oscillating is important for safety reasons. 
     FIG. 6 illustrates a capacitor  52 ″ connected in series with an inductor  51 ″ through a heater  81  suitable for heating the toroidal inductors  47 ,  49  in accordance with the level of output. The load  26 ″ is connected across the series combination of the capacitor  52 ″ and the toroid heater. The heater  81  is preferably designed to controllably heat the toroidal saturable inductors in order to decrease their saturation flux limit and hence their saturation time. The result is to decrease the periodic transistor conduction time and thereby increase the transistor inversion frequency. When a frequency-dependent impedance means, that is, an inductor or a capacitor, is connected in circuit with the AC voltage output of the inverter, change in the transistor inversion frequency will modify the impedance of the frequency-dependent impdance means and correspondingly modify the inverter output. Thus as the level of the output increases, the toroid heater  81  is correspondingly energized to effect feedback regulation of the output. Further, transistors  42 ,  43  of the type used in high voltage inverters dissipate heat during periodic transistor conduction. As an alternative, the toroid heater  81  can use this heat for feedback regulation of the output or control of the temperature of transistors  42 ,  43 . 
     The frequency dependent impedance means may also be used in a circuit to energize a gas discharge lamp at adjustable illumination levels. Adjustment in the inversion frequency of transistors  42 ,  43  results in control of the magnitude of the AC current supplied to the lamp. This is preferably accomplished where saturable inductors  47 ,  49  have adjustable flux densities for control of their saturation time. 
     FIG. 7 schematically illustrates an alternate form of inverter circuit, shown without the AC to DC power supply connections for simplification. In this Figure, the transistors are connected in parallel rather than in series but the operation is essentially the same as previously described. 
     In particular, this circuit comprises a pair of alternately conducting transistors  91 ,  92 . The emitter terminals of the transistors are connected to a B− line  93 . A B+ lead  94  is connected to the center-tap of a transformer  96 . In order to provide drive current to the transistors  91 ,  92  for control of their conduction frequency, saturable inductors  97 ,  98  have secondary windings  99 ,  101 , respectively, each secondary winding having one end connected to the base of its associated transistor; the other ends are connected to a common terminal  102 . One end of transformer  96  is connected to the collector of transistor  91  through a winding  103  on inductor  98  in turn connected in series with a winding  104  on inductor  97 . Likewise, the other end of transformer  96  is connected to the collector of transistor  92  through a winding  106  on inductor  97  in series with another winding  107  on inductor  98 . 
     The B+ terminal is connected to terminal  102  through a bias resistor  108 . A bias capacitor  109  connects terminal  102  to the B− lead  93 . This resistor and capacitor serve the same function as resistors  58 ,  59  and capacitors  56 ,  57  in the FIG. 2 circuit. 
     The bases of transistors  91 ,  92  are connected by diodes  111 ,  112 , respectively, to a common Zener diode  113  in turn connected to the B− lead  93 . The common Zener diode  113  serves the same function as individual Zener diodes  62 ,  64  in FIG.  2 . 
     Shunt diodes  114 ,  116  are connected across the collector-emitter terminals of transistors  91 ,  92 , respectively. A capacitor  117  connecting the collectors of transistors  91 ,  92  restrains the rate of voltage rise on the collectors in a manner similar to the collector-emitter capacitor  68  in FIG.  2 . 
     Inductive-capacitive loading of the FIG. 7 inverter is accomplished by a capacitor  118  connected in series with with an inductor  119 , the combination being connected across the collectors of the transistors  91 ,  92 . A load  121  is connected across the capacitor  118 . 
     FIG. 8 illustrates how an inverter loaded with a series capacitor  122  and inductor  123  can be used to energize a “rapid-start” fluorescent lamp  124  (the details of the inverter circuit being omitted for simplication). The lamp  124  has a pair of cathodes  126 ,  127  connected across the capacitor  122  for supply of operating voltage in a manner identical to that previously described. In addition, the inductor  123  comprises a pair of magnetically-coupled auxiliary windings  128 ,  129  for electrically heating the cathodes  126 ,  127 , respectively. A small capacitor  131  is connected in series with lamp  124 . 
     FIG. 9 illustrates the very same circuit arrangement as that of FIG. 8 except that the auxiliary windings  128 ,  129  are only loosely coupled to the inductor  123 , thereby providing for a manifest limitation on the amount of current that can be drawn from each auxiliary winding in case it were to be accidentally short-circuited. 
     FIG. 10 also illustrates the very same circuit arrangement as that of FIG. 8 except that the cathodes  126 ,  127  are connected with their respective auxiliary windings  128 ,  129  by way of nonlinear current-limiting means  132  and  133 , respectively. 
     In FIG. 10, the non-linear current-limiting means  132 ,  133  are shown as being two (small) incandescent lamps. However, other types of non-linear resistance means could be used as well. 
     Both the FIG. 9 circuit and the FIG. 10 circuit serve the same basic purpose; which is that of preventing damage to the ballast circuit (such as that if FIG. 2) in case the leads used for connecting to one of the lamp cathodes  126 ,  127  were to be accidentally shorted. This damage prevention is accomplished by providing for manifest limitation of the maximum amount of current that can be drawn from each one of the auxiliary windings  128 ,  129 . In the circuit of FIG. 9, this manifest limitation is accomplished by having the auxiliary windings  128 ,  129  couple sufficiently loosely to the main inductor  123 —such as by providing a magnetic shunt between inductor  123  and the auxiliary windings—thereby correspondingly limiting the degree of impact resulting from an accidental short circuit. Such a short circuit would result in a net reduction in the effective inductance value of the tank inductor  123 ; which net reduction in inductance may in turn cause a precipitous increase in the magnitude of the reactive current drawn from the inverter by the series-connected inductor  123  and capacitor  122 , thereby causing damage to the inverter. 
     Additional Explanations and Comments 
     (a) With reference to FIGS. 2 and 5, adjustment of the amount of power supplied to load  26 ′, and thereby the amount of light provided by lamp  71 , may be accomplished by applying a voltage of adjustable magnitude to input terminals IP1 and IP2 of the Toroid Heater; which is thermally coupled with the toroidal ferrite cores of saturable transformers  47 ,  49 . 
     (b) With commonly available components, inverter circuit  24  of FIG. 2 can be made to operate efficiently at any frequency between a few kHz to perhaps as high as 50 kHz. However, for various well-known reasons (i.e., eliminating audible noise, minimizing physical size, and maximizing efficiency), the frequency actually chosen is in the range of 20 to 40 kHz. 
     (c) The fluorescent lighting unit of FIG. 1 could be made in such manner as to permit fluorescent lamp  11  to be disconnectable from its base  12  and ballasting means  16 . However, if powered with normal line voltage without its lamp load connected, frequency-converting power supply and ballasting circuit  16  is apt to self-destruct. 
     To avoid such self-destruction, arrangements can readily be made whereby the very act of removing the load automatically establishes a situation that prevents the possible destruction of the power supply and ballasting means. For instance, with the tank capacitor ( 52 ) being permanently connected with the lamp load ( 11 )—thereby automatically being removed whenever the lamp is removed—the inverter circuit is protected from self-destruction. 
     (d) At frequencies above a few kHz, the load represented by a fluorescent lamp—once it is ignited—is substantially resistive. Thus, with the voltage across lamp  11  being of a substantially sinusoidal waveform (as indicated in FIG.  3   d ), the current through the lamp will also be substantially sinusoidal in waveshape. 
     (e) In the fluorescent lamp unit of FIG. 1, fluorescent lamp  11  is connected with power supply and ballasting circuit  16  in the exact same manner as is load  26  connected with the circuit of FIG.  2 . That is, it is connected in parallel with the tank capacitor ( 52 ) of the L-C series-resonant circuit. As is conventional in instant-start fluorescent lamps—such as lamp  11  of FIG.  1 —the two terminals from each cathode are shorted together, thereby to constitute a situation where each cathode effectively is represented by only a single terminal. However, it is not necessary that the two terminals from each cathode be shorted together; in which case—for instant-start operation—connection from a lamp&#39;s power supply and ballasting means need only be made with one of the terminals of each cathode. 
     (f) with respect to the circuit arrangement of FIG. 9, in situations where the tank inductor  123  includes a ferrite magnetic core having an air gap, one particularly cost-effective way of accomplishing the indicated loose coupling between the tank inductor  123  and the auxiliary windings  128 ,  129  is that of arranging for the auxiliary windings to be placed in the air gap in such a manner that they each couple only with part of the magnetic flux crossing the air gap. 
     (g) in FIG. 1, the compact screw-in fluorescent lamp has a longitudinal central axis penetrating through the center of the bottom of base  19  (i.e., at the point where lead  18  is connected), passing up centrally between the two legs of lamp  11 , and emerging at the center of the very top of lamp  11 . 
     (h) In FIG. 1, as a skilled artisan would perceive by direct inspection, the visible parts are drawn to scale. Thus, for instance: 
     (i) the height and width (i.e., diameter) of screw-base  19  are in proper proportion to those of an actual screw-base on an ordinary household incandescent lamp; 
     (ii) the diameter of the individual straight legs of the folded fluorescent lamp  11  are shown in proportion to the diameter of the screw-base; 
     (iii) the diameter of the bent portion connecting the top parts of the two straight lamp legs is shown in proper proportion to the diameter of the lamp legs; and 
     (iv) the distance between the two straight lamp legs is shown in proper proportion to the diameter of those lamp legs. 
     Of course, for a screw-in fluorescent lamp to have maximum utility, it is imperative that it has dimensions sufficiently compact to permit it to be conveniently used in most places where an incandescent lamp would ordinarily be used. Thus, it is important that its maximum diameter not be any larger than those of an ordinary household incandescent lamp (whose maximum diameter is typically about twice that of its screw-base). The screw-in fluorescent lamp depicted in FIG. 1 clearly satisfies those requirements.