Abstract:
A battery operated fluorescent lamp is disclosed, which is operable from the battery while the battery is being recharged, comprising a tubular housing configured as a handle grip at one end and a cylindrical lens portion at the other end. The tubular housing lockably connects to a battery pack. The cylindrical 3030 lens portion encloses a miniature fluorescent bulb. The electrical circuitry, enclosed within the handle grip and alternately operable from either 120 VAC or 12 VDC, includes a converter circuit, a battery charging circuit, and a fluorescent lamp ballast circuit. The battery pack is electrically coupled to simultaneously receive charging current from an output of the charging circuit and to deliver DC supply voltage to the fluorescent lamp ballast circuit without the occurrence of a net discharge of the battery pack.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present U.S. patent application claims priority from earlier filed U.S. Provisional Patent Applications: Serial No. filed May 2, 2003 and entitled “Integrated Circuit For Task Light,” and Serial No. 60/467,981 filed May 5, 2003 and entitled “Electrical Circuit For A Portable Fluorescent Task Lamp.” 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention generally relates to battery operated lamps and, more particularly, to battery operated fluorescent lamps having built-in battery recharging capability and operable from either a 120 VAC or 12 VDC source of power.  
           [0004]    2. Description of the Prior Art  
           [0005]    Portable incandescent lamps, which operate by using an electric current to heat a filament, have been readily available for use as flashlights, task lights or work lights (e.g., ‘drop’ lights), camp lights, and the like. While generally reliable and reasonably durable, incandescent lamps are inefficient, whether operated from AC or DC voltage sources. Further, battery operated incandescent lamps are generally limited in the amount of light output because of the inefficiency of heated filament technology. Other disadvantages of incandescent lamps include the susceptibility of filaments to breakage and the heat produced, which can be uncomfortable when used in close quarters.  
           [0006]    Portable fluorescent lamps have also been readily available for use as flashlights, task lights or work lights (e.g., ‘drop’ lights), camp lights, and the like. As is well known, fluorescent lamps are relatively efficient compared to incandescent lights, but they require a ballast device of some type to provide both a high starting voltage to ionize the gas within the bulb and a current-limiting impedance to limit the current flowing between the lamp terminals after the gas becomes ionized and highly conductive. In conventional AC operated fluorescent lamps the ballast device is a relatively large, heavy inductor in series with the fluorescent bulb. The large inductor provides a high back EMF when the alternating supply current reverses in the inductor, which causes a high starting voltage to ionize the gas within the bulb. The large inductance also provides a substantial impedance to the flow of current through the bulb after the ionization takes place.  
           [0007]    In conventional portable fluorescent lamps, a small fluorescent bulb rated at, e.g., four watts, can be illuminated effectively with a battery voltage of 7.5 to 9.0 volts and a small step-up converter circuit to produce the relatively high starting voltage required. For such a low power rating, the inductance required to limit the current after ionization is correspondingly small enough to allow a practical battery operated fluorescent lamp that is not too bulky or heavy. However, fluorescent bulbs rated at four watts or even six watts do not provide much more light than a typical seven watt incandescent night light. Further, at 7.5 volts DC, the five large, C or D-cell alkaline batteries typically used in such lamps, which may provide up to one hour of illumination between battery replacement or recharging, causes the lamp to be bulky and heavy.  
           [0008]    There are higher rated fluorescent bulbs available, such as a 13 watt Compact Fluorescent Lamp (CFL) Bi-Pin bulb. Such a bulb provides much higher light output but requires that more power be delivered by the ballast circuit. With conventional technology, this requirement demands a larger ballast circuit and further limits the battery life. While battery technology is continually improving,  13  watt, battery powered, portable fluorescent lamps, to be practical to use, must rely on rechargeable batteries. Typically, the lamp, in order to keep the size and weight within practical limits, contains only the batteries, the bulb, and an electronic ballast circuit. After a relatively short duration of use, typically one hour, the batteries must be replaced or recharged on an external battery charger. A typical external battery charger may have substantial bulk and weight, especially if it operates from a standard wall outlet of 120 VAC. There is currently no known portable fluorescent lamp available that includes the batteries, ballast, and bulb that also includes a built-in AC-DC converter and battery charger in a compact, flashlight-sized, light-weight package.  
           [0009]    What is needed is a higher efficiency, 13 watt portable fluorescent lamp that includes a built-in battery charger and operates off of either 120 VAC or 12 VDC power, yet is compact and light weight, i.e., approximately the size and weight of a conventional flashlight powered by two or three “D” cells. Further, the portable fluorescent lamp must be as easy to handle as a flashlight—i.e., have all the electronics and the battery pack housed in an enclosure approximately the same size as the handle portion of a conventional “D” cell flashlight having two cells. The design must accordingly produce very little heat so that it may be comfortably held by the handle that encloses the electronics. The handle must be small enough in diameter to hold easily and securely in the average-sized person&#39;s hand. Further, the battery charger built-in to the handle must be efficient enough to recharge the battery pack in under 90 minutes while the portable lamp is in use.  
         SUMMARY OF THE INVENTION  
         [0010]    Accordingly, there is disclosed a 13 watt, battery operated, portable fluorescent lamp that is provided by the advancement in technology of the present invention. The lamp comprises a tubular housing configured as a handle grip portion at one end and a cylindrical lens portion at the other end. The tubular housing lockably connects to a compact battery pack. The cylindrical lens portion encloses a miniature, 13 watt fluorescent bulb. The electrical circuitry, enclosed within the handle grip and alternately operable from either 120 VAC or 12 VDC, includes a converter circuit, a battery charging circuit, and a fluorescent lamp ballast circuit. The compact battery pack is electrically coupled to the charger and ballast circuits and configured to simultaneously receive charging current from an output of the charging circuit and to deliver DC supply voltage to the fluorescent lamp ballast circuit during use of the lamp without the occurrence of a net discharge of the battery pack.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 illustrates a block diagram of one embodiment of a portable fluorescent lamp according to the present invention;  
         [0012]    [0012]FIG. 2 illustrates an electrical schematic diagram of one embodiment of a converter circuit that may be used in the portable fluorescent lamp of FIG. 1;  
         [0013]    [0013]FIG. 3 illustrates an electrical schematic diagram of one embodiment of a battery charging circuit that may be used in the portable fluorescent lamp of FIG. 1;  
         [0014]    [0014]FIG. 4 illustrates an electrical schematic diagram of an embodiment of an electronic ballast circuit that may be used in the portable fluorescent lamp of FIG. 1;  
         [0015]    [0015]FIG. 5 illustrates a pictorial view of one embodiment of a portable fluorescent lamp according to the present invention;  
         [0016]    [0016]FIG. 6 illustrates an exploded pictorial view of one embodiment of a battery pack for use with the portable fluorescent lamp of FIG. 5;  
         [0017]    [0017]FIG. 7 illustrates a pictorial view of an assembled battery pack for use with the portable fluorescent lamp f FIG. 5; and  
         [0018]    [0018]FIG. 8 illustrates a partially cut-away pictorial view of the interior of the embodiment of the portable fluorescent lamp of FIG. 5.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    Referring to FIG. 1, there is illustrated a block diagram of one embodiment of the electrical circuitry for a portable fluorescent lamp  10  according to the present invention. The principal components of the electrical circuitry for the lamp  10  include a converter circuit  12 , a battery charger circuit  42 , a battery pack  52 , an electronic ballast circuit  62 , and a miniature fluorescent bulb  72 . The battery charger  42  may be operated from either a 120 VAC voltage source or a 12 VDC voltage source. The converter circuit  12  receives the 120 VAC via lines  14 ,  16 , which may terminate in a receptacle (not shown) that mates with a matching plug of an AC line cord (not shown). The converter produces an output voltage of approximately 13 volts DC under load on lines  18 , 20 , which terminate at the terminals of one side of a DPDT switch  22 . When the wiper contacts of the switch  22  are in the “AC” position, the lines  18 ,  20  are connected to the lines  24 ,  26 , which connect to the +12 volt and the common (COM) input terminals respectively of the battery charger circuit  42 . Thus, in the “AC” position, the switch  22  couples the converter circuit  12  between the 120 VAC voltage source and the input to the battery charger circuit  42 .  
         [0020]    Alternatively, the battery charger circuit may be operated directly by a 12 VDC voltage source via lines  28 ,  30 , which may terminate in a receptacle (not shown) that would mate with a matching plug of a DC line cord (not shown), and connect to the terminals of the other side of the DPDT switch  22 . When the wiper contacts of the switch  22  are in the “DC” position, the lines  28 ,  30  are connected to the lines  24 , 26 , which connect to the +12 volt and the common (COM) input terminals respectively of the battery charger circuit  42 . Thus, in the “DC” position, the switch  22  couples the lines  28 ,  30  between the 12 VDC voltage source and the input to the battery charger circuit  42 . (Note: the 12 volt source rating is a nominal rating and may, in the case of an automotive battery, actually be in the range of 12.6 to 14.8 volts). A diode  32 , is inserted in series with the line  28  as a protective feature to prevent damage that may result from a reversed polarity DC voltage being applied to the electrical circuitry. The switch  22  is an optional feature. In some versions of the portable fluorescent lamp  10 , the lines  18 ,  24 , and  28  are tied together and the lines  20 ,  26 , and  30  are tied together. The control of which voltage source is used may then be determined by which line cord is connected between the voltage source and the portable fluorescent lamp  10 . Alternatively, the connections for an external 12 VDC source may be deleted, or, the connections for the 120 VAC source and the converter circuit itself may be deleted. Either alternate may be provided to accommodate particular product variations. It will also be appreciated that a portable fluorescent lamp having a built-in battery charger and battery pack in a small, light weight package is a combination not commonly found in the prior art.  
         [0021]    Continuing with FIG. 1, the battery charger circuit  42  produces a DC voltage suitable for charging the battery pack  52 . In the illustrative embodiment described herein, the output voltage is approximately 7.2 volts DC for charging a battery pack  52  containing six 1.2 volt, rechargeable nickel-metal-hydride (NiMH) cells. In the illustrated embodiment, the six NiMH cells are AA size, rated at 2200 milliAmpere-hours capacity, to provide sufficient power (approximately 15.8 watts) to drive a 13 watt miniature fluorescent lamp bulb to full brightness. This battery configuration was chosen for its compactness, and persons skilled in the art will appreciate that the portable fluorescent lamp  10  of the present invention operates with an efficiency exceeding 80%. The reasons for this high efficiency will become apparent in the detailed description which follows. It will also be understood that other battery configurations are certainly feasible and are contemplated for other similar applications. In the illustrated embodiment, the 7.2 volts output voltage is applied to the lines  44 , 46 , which couple the output of the battery charger circuit  42  to the battery pack  52  via terminals  48 ,  50  for charging the battery pack  52 , and to the input terminals of the electronic ballast circuit  62 . A switch  54 , connected in series with the line  44 , functions as an ON-OFF switch for the portable fluorescent lamp  10 . The terminals  48 ,  50  may be separate contacts located on the housing (not shown in FIG. 1) of the portable fluorescent lamp  10  or they may be incorporated into a connector mounted on the housing of the portable fluorescent lamp  10 .  
         [0022]    Continuing with FIG. 1, it is appreciated that the electronic ballast circuit  62  operates on the same voltage, in this case approximately 7.2 volts, that is applied to the battery pack  52 . The fluorescent ballast circuit produces a high voltage waveform output of approximately 400 Volts AC and approximately 30 KHz for “starting” the fluorescent bulb  72  via lines  64 ,  66 , which couple to terminals  68 ,  70 . The fluorescent bulb  72  is plugged into the terminals  68 ,  70 . After ionization of the gas within the envelope of the fluorescent bulb  72 , the electronic ballast circuit  62  limits the current flowing through the fluorescent bulb  72 . In an optional feature, a pair of normally open (NO) contacts  74 ,  76  are connected, via lines  80 ,  82 , in series with the positive voltage line  44  from the battery pack  52  or the battery charger circuit  42 , as will be described herein below. The contacts  74 ,  76  are closed whenever a fluorescent bulb  72  is plugged into the terminals  68 ,  70  by the action of the barrier  78  on the pin base of the fluorescent bulb  72 . The terminals  68 ,  70  may be part of a receptacle connector. These contacts provide a safety feature that limits access to the high voltage that may be present at the terminals  68 ,  70 , when a bulb  72  is not plugged into the terminals  68 ,  70 .  
         [0023]    Referring to FIG. 2, there is illustrated an electrical schematic diagram of one embodiment of a converter circuit  100  that may be used in the portable fluorescent lamp of FIG. 1. The converter circuit  100  is configured as a feed forward converter that operates at approximately 50 KHz. and provides a DC output voltage of approximately 13 (±1) Volts under load from an input of 120 VAC at 50/60 Hz. The converter converts the low frequency 120 VAC input voltage to a high frequency AC voltage, steps down the AC voltage to a low voltage in the transformer  116 , and then rectifies and filters the low voltage to produce the low voltage DC output. The circuit is very efficient because the circuit losses are much smaller at the higher frequency. In FIG. 2, the 120 VAC input is applied to input terminals  102 ,  104  to a bridge rectifier  106 . A series resistor  108  between terminal  102  and the bridge rectifier  106  acts as a fuse. The rectified DC output voltage appears at a positive node  110  and a negative node  112  which is also the return node. A filter capacitor  114  is connected across the DC output at nodes  110 ,  112 . This rectifier circuit supplies approximately 170 VDC to the rest of the converter circuit to be described.  
         [0024]    The 170 VDC output of the rectifier is applied across a primary winding  118  of an isolation transformer  116  and a transistor switch  126  in series. In the illustrative embodiment, the transistor switch  126  is a type IRF740 N-channel MOSFET rated at 400 Volts, 6.3 Amps, and having an Rds(on) of &lt;0.55 Ohms. This device is available from STMicroelectronics. One side of the primary winding  118  having the polarity symbol (a dot) is connected to node  110 , the positive output of the rectifier bridge  106 . The other side of the primary winding  118 , at node  124 , is connected to the drain terminal of the transistor switch  126 . The source terminal of the transistor switch  126  is connected to the return node  112 . During operation, the transistor switch  126  is turned on and off at a 50 KHz rate, which periodically charges the primary winding  118  with a pulse of current to produce a 170 Volt peak-to-peak square wave. According to the turns ratio of the transformer  116 , a smaller, stepped-down replica of the pulse waveform produced across the primary  118  of transformer  116  appears across the secondary winding  122  of transformer  116 . The transistor switch  126  is caused to turn on and off by a pulse control signal applied to the gate terminal of the transistor switch  126  that is supplied from the “Q” output at pin  3  of an integrated circuit timer (timer IC)  140  operated as an a-stable multivibrator or oscillator. The timer IC  140  used in the disclosed embodiment is a standard  555  type timer IC available from a variety of manufacturers. The control signal has a duty cycle of approximately 50%. In the description which follows, the term “integrated circuit” may be abbreviated as “IC.” 
         [0025]    Operating voltage Vcc for the timer IC  140  is applied to pin  8 . Pin  4  of the timer IC  140  is also connected to pin  8 . The operating voltage at pin  8  is produced by a dropping resistor  150  and a 12 Volt zener diode  152  connected in series across the 170 VDC output of the rectifier at nodes  110 ,  112 . Capacitor  154  provides some high frequency filtering of the DC voltage supplied by the action of zener diode  152 . This simple power supply provides the starting voltage for operating timer IC  140 . At other times, the operating voltage for timer IC  140  (Vcc) is provided by a rectified output from a secondary winding  120  of transformer  116  connected between node  156  and the common node  112 . The voltage across the secondary winding  120  is rectified by diode  158 , filtered by capacitor  154 , and applied to pin  8  of the timer IC. The frequency of the a-stable oscillator is set by resistor  142  and capacitor  144 . Resistor  142  is connected between pin  3  of the timer IC  140  and pins  2  and  6  of the timer IC  140  tied together. Capacitor  144  is connected between pin  6  of the timer IC  140  and the common terminal  112 . A bypass capacitor is connected between pin  5  of the timer IC  140  and the common terminal  112 .  
         [0026]    Continuing with FIG. 2, the low voltage output across the secondary winding  122  at nodes  170 ,  176  of transformer  116  is rectified by rectifier  172  connected in series with the node  170  of the secondary winding  122 . The rectified output voltage is filtered by capacitor  178  connected between a positive node  174  and a negative (common) node  180 . The node  180  is connected to the node  176  of the secondary winding  122 . Persons skilled in the art will appreciate that the DC output voltage of the converter  100  is unregulated, and thus subject to variation as the AC input voltage varies. However, the regulation of the actual DC charging voltage applied to the battery pack during charging is regulated by another part of the electrical circuitry in the portable fluorescent lamp  10 .  
         [0027]    Referring to FIG. 3, there is illustrated an electrical schematic diagram of one embodiment of a battery charging circuit that may be used in the portable fluorescent lamp of FIG. 1. The battery charging circuit  200  is essentially a DC-to-DC switching regulator controlled by a battery charging controller IC  210  responsive to a feedback signal from the DC voltage output. The switching regulator is driven by an a-stable timer IC oscillator  260  operating at 50 KHz, similar to that used in the converter circuit  100  described herein above. The output of the oscillator applied to the gate of an N-channel FET is gated by a logic circuit  280  controlled by the battery charging controller. The battery charging circuit  200  in the illustrative embodiment of FIG. 3 operates from a 12 to 14 VDC input and provides an output voltage of approximately 7.2 Volts while delivering a charging current of up to approximately 1.5 Amperes to the battery pack  52  of FIG. 1. The input voltage may be supplied from a converter operating from a 120 VAC voltage source as illustrated in FIG. 2 or from a 12 to 14 Volt battery such as an automotive battery.  
         [0028]    The 12 VDC input is applied across the positive terminal  202  and the negative (common) terminal  204 , which correspond respectively to nodes  206 ,  208 . Connected in series between node  206  and a positive output terminal  222  are, in order, a P-channel MOSFET transistor switch  250 , a rectifier diode  252 , node  254 , and inductor  256 . The transistor switch  250  in the illustrative embodiment is a type FQB11P06 P-channel MOSFET rated at −60 Volts, −8.05 Amps, and having an Rds(on) of &lt;0.175 Ohms. This device is available from Fairchild Semiconductor. Node  206  is connected to the source terminal of the transistor switch  250 . The anode of diode  252  is connected to the drain terminal of transistor switch  250  and the cathode of the diode  252  is connected to node  254 . The negative (common) output terminal  224  is connected to node  208 . Another rectifier diode is connected between node  254  (cathode) and node  208  (anode). The three integrated circuits of FIG. 3, 210,  260 , and  280 , are each connected between node  206 , the Vcc supply, and node  208 , the Vss common terminal.  
         [0029]    Continuing with FIG. 3, the circuit of the battery charging controller  210 , will now be described. The battery charging controller IC  210 , in the illustrative embodiment, is a type bq2002C, a “NiCd/NiMH Fast-Charge Management IC” manufactured by Unitrode Corporation, a subsidiary of Texas Instruments, Dallas, Tex. In FIG. 3, a resistor  212  is connected between node  206  and pin  6  (the Vcc terminal) of the battery charging controller IC  210 . Pin  5  (a temperature sense input) of controller IC  210  is connected to pin  6  of controller IC  210 . Connected between pin  6  of controller IC  210  and node  208  are a 5.1 Volt zener diode  214 , a bypass capacitor  216  and a first resistor  218  in series with a second resistor  220 . The zener diode  214  sets the Vcc voltage for controller IC  210  at 5.1 Volts DC. The junction between the two resistors  218 ,  220 , which form a resistive voltage divider, is connected to pin  1  of controller IC  210  to set the operating mode of the battery charging controller IC  210  (“charge timer, top-off, voltage termination mode, trickle rate,” etc.). Pin  7  (the Vss terminal) of controller IC  210  is connected to the common node  208 .  
         [0030]    Also connected between pin  6  of controller IC  210  and node  208  is a network of light emitting diodes (LEDs) including resistor  232 , LED  234 , LED  236  and resistor  238 , all connected in series. The junction of LEDs  234  and  236  is connected to pin  2  of controller IC  210 . Pin  2  is the charging status output, which indicates whether the battery is being charged at a fast charge rate (steady red LED  234 ), or at a trickle rate (blinking red LED  234 ) or that the battery is fully charged (steady green LED  236 ). Pin  3  of controller IC  210 , the battery voltage input, is connected through a resistor  226  to the positive output terminal  222 . A resistor  228  and a bypass capacitor  230  are connected in parallel between pin  3  of controller IC  210  and the common node  208 . Bypass capacitor  230  prevents the termination of charging on noise that may be present on the output terminal  222 . Pin  8  of controller IC  210 , the charge control output terminal, is connected to a node  240 . A pull-up resistor  242  is connected between node  240  and node  206 . The output signal at pin  8  of controller IC  210  is a logic high for fast charging, pulsed for trickle charging, and logic low when charging is not occurring.  
         [0031]    Timing for the switching regulator circuit of the battery charging circuit  200  is provided by timer IC  260 , a type  555  timer IC available from a variety of manufacturers. Vcc pin  8  of timer IC  260  is connected to node  206  and also to the Reset pin of timer IC  4  of U 3   260 . Vss pin  1  of timer IC  260  is connected to the common node  208 . Timing resistor  262  is connected between the Q output pin  3  of U 3   260  and the TR pin  2  of timer IC  260 , which is also tied to the CV pin  6  of timer IC  260 . The timing capacitor  264  is connected between pins  2 , 6  of timer IC  260  and the common node  208 . Pin  5  of timer IC  260  is connected to the common node by capacitor  266 . The timer IC  260 , connected as an a-stable oscillator, provides a 50 KHz, 50% duty cycle pulse train at pin  3  for driving the transistor switch  250 .  
         [0032]    The pulse train signal from pin  3  of the timer IC  260  is gated to the transistor switch  250  by logic circuit  280  under the control of the charge control output from pin  3  of the battery charging controller IC  210 . The logic circuit  280  may be a four stage NAND gate IC such as a type CD4093, which is available from a variety of manufacturers. Two stages of logic circuit  280 , NAND gates  282  and  284 , are connected in series with their inputs (respectively  1 ,  2  and  12 ,  13 ) tied together and the input (pins  1 ,  2 ) of NAND gate  282  tied to the output (pin  11 ) of NAND gate  284 . This configuration provides an inverter/driver for the pulse train signal for the transistor switch  250 . The output of NAND gate  282  at pin  3  is coupled to one input, pin  6 , of NAND gate  286  of logic circuit  280 , and also to pins  8 ,  9  of NAND gate  288  of logic circuit  280 , whose output pin  10  is left floating. The other input of NAND gate  286  at pin  5  of logic circuit  280  is connected to the node  240 , which is the charge control output of the battery charging controller IC  210 . Thus, a logic high signal at node  240  (logic circuit  280  pin  5 ) enables the pulse train signal from NAND gate  282  at pin  3  to be coupled to the gate of the transistor switch  250 .  
         [0033]    Under the control of the 50 KHz, 50% duty cycle pulse train applied to the gate terminal of the transistor switch  250 , the transistor switch  250  turns ON, and charging current flows through diode  252  and inductor  256  into the positive terminal of the battery pack connected to the positive output terminal  22  (see the battery pack  52  in FIG. 1). Also during this period, the charging current charges the inductor  256 , building a magnetic field around the inductor  256 . In the next period of the pulse train signal, the transistor switch  250  turns OFF, and current ceases to flow through diode  252 . At this instant, the magnetic field surrounding the inductor  256  collapses, causing current to flow in the opposite direction through the inductor  256 . At this time, the diode  258  is forward biased and the inductor delivers charging current through the diode  258  and into the negative terminal of the battery being charged, which is connected to the negative terminal of the battery charging circuit  200 . In this way, charging current is delivered to the battery pack during both periods of the pulse train signal, when the transistor switch  250  is alternately in its ON and OFF states. Thus, the battery charging circuit  200  is operating “full time” to charge the battery pack.  
         [0034]    Continuing with FIG. 3, a modification may be made to the battery charging circuit if it is intended to operate from an external DC power source such as a automotive storage battery the typically supplies 12.6 to 14.8 volts, depending on the state of charge and the load connected to the battery. The aforementioned battery voltage available is somewhat lower than the voltage provided by the converter circuit of FIG. 2. The modification, which provides a way to increase the duty cycle of the switching regulator, consists of connecting resistor  262  to the Vcc terminal, pin  8  of the timer IC  260  instead of to pin  3  of the timer IC  260 , and adding a resistor from the junction of the resistor  262  and capacitor  264  to pin  7  of the timer IC  260 . The value for this additional resistor is selected according to the duty cycle that is desired—the ratio of resistor  262  to the added resistor determines the duty cycle.  
         [0035]    Referring to FIG. 4, there is illustrated an electrical schematic diagram of one embodiment of an electronic ballast circuit  300  that may be used in the portable fluorescent lamp of FIG. 1. The ballast circuit  300  converts the 7.2 volts DC, supplied by battery pack  52  to the positive input terminal  302  and negative (common) input terminal  304 , to a high frequency, high voltage AC signal. This high voltage signal, a 30 KHz square wave having a peak-to-peak amplitude of approximately 400 volts, is applied to the fluorescent bulb  370  to ionize the gas within the fluorescent bulb  370 . The ballast circuit  300  includes a current limiting feature to limit the current in the bulb after the gas is ionized and the fluorescent bulb  370  begins producing light.  
         [0036]    Connected between the positive input terminal  302  and a node  306  is a series-connected SPST switch  308  that is used to turn the fluorescent lamp ON and OFF. Switch  308  applies power to the ballast circuit  300 . The negative input terminal is connected to a common node  310 . A transformer  312  is configured to provide operating currents to a two-transistor, a-stable multivibrator or oscillator circuit and to step up the oscillator output voltage square wave to a value needed to start the ionization of the gas within the fluorescent bulb  370 . Transformer  312  includes a center tapped primary winding  314 A- 314 B, which is connected between nodes  316  and  318 . Node  316  connects to the collector of bipolar transistor  330 , which forms one side of the multivibrator circuit. Node  318  connects to the collector of an identical bipolar transistor  332 , which forms the other side of the multivibrator circuit. A capacitor  320 , which, in part, determines the operating frequency of oscillation of the a-stable multivibrator circuit, is connected between the nodes  316  and  318 . The center tap of the primary winding  314 A- 314 B, defined as node  322 , is connected through an inductor to node  306 . This inductor acts to prevent current spikes from the multivibrator when the transistors change states.  
         [0037]    Continuing with FIG. 4, a second primary winding  334  of transformer  312  is connected between nodes  336  and  338 . Nodes  336  and  338  connect to the supply voltage at node  306  through resistors  340  and  342  respectively. Nodes  336  and  338  provide bias current into the base terminals of transistors  330  and  332 , respectively. The emitters of the bipolar transistors  330  and  332  are connected to the common node  310 . Transistors  330  and  332 , which are type KSD 1691G available from Fairchild Semiconductors, are chosen for their very high gain, hfe, and very low saturation voltage, Vsat. As is well known in the art, when voltage is applied to the input terminals  302 ,  304  of the multivibrator circuit, the imbalance between the two transistors&#39; characteristics causes one of them to conduct current more quickly than the other, thus starting the oscillations of the a-stable multivibrator.  
         [0038]    The output of the multivibrator  330 ,  332  is taken from the secondary winding  350  of transformer  312 . The output signal is essentially a square wave having a frequency of approximately 30 KHz and a duty cycle of approximately 50%. The amplitude of the signal across the secondary winding  350  is approximately 400 volts peak to peak. One leg of the secondary winding is connected via a series capacitor  352  to a node  354 . The other leg of the secondary winding  350  is connected to a node  356 , which is also connected to the common node  310 . Nodes  354  and  356  are respectively connected to the terminals  358 , 360  of the receptacle for the bi-pin fluorescent bulb  370 . The fluorescent bulb  370  includes a base  372  containing the bi-pin terminals that plug into the receptacle terminals  358 ,  360 .  
         [0039]    It is well known that once the gas within a fluorescent bulb has become ionized, the bulb presents a negative impedance characteristic to the external circuitry connected to the terminals of the bulb. That is, once the bulb begins to conduct, the current will continue to increase without bound until the bulb is destroyed unless the current is limited to a safe value. In a conventional fluorescent lamp that is controlled by a conventional ballast, the ballast provides a large inductive impedance to the alternating current flowing in the lamp. In the illustrative ballast circuit of the present invention, the transformer  312  is designed with an air gap in the core so that a substantial inductive impedance appears in series with the current flowing in the secondary winding  350  and the fluorescent bulb  370 .  
         [0040]    Referring to FIG. 5, there is illustrated a pictorial drawing of one embodiment of a portable fluorescent lamp  400  according to the present invention. The portable fluorescent lamp  400  includes a tubular housing  432  having a handle grip ( or body) portion  402  at the lower end and a cylindrical lens portion  404  at the upper end. The cylindrical lens portion may be fabricated of a material that readily transmits light, and may further be configured to transmit light in all directions—i.e., 360 degrees—surrounding the longitudinal axis of the cylindrical lens portion  404 . Enclosed within the cylindrical lens portion  404  is a bi-pin fluorescent bulb  406  that is plugged into a receptacle base  408  inside the cylindrical lens portion  404 . Along the back side of the cylindrical lens portion  404  is a tubular spine  410 , which mechanically connects the handle grip portion  402 , the cylindrical lens portion  404  and an end cap  412  together. The tubular spine, which may have a somewhat flattened oval or rectangular cross-section, strengthens the structure of the portable fluorescent lamp  400  assembly to prevent breakage if the lamp  400  is dropped. The spine  410  serves to provide the additional stiffness to the lamp  400 , which is required because of the 8 to 10 degree offset of the cylindrical lens portion  404  relative to the handle grip portion  402  of the lamp  400 . The offset is built in to the tubular housing  432  so that when the lamp  400  is stood on its battery pack  500 , which serves as a base, the illumination from the lamp is directed downward toward the work surface. The tubular spine also provides space for circuitry to accommodate additional features such as a flashing light circuit, a circuit to drive indicator lights showing the status of the electrical circuitry and/or the batteries, etc.  
         [0041]    The battery pack  500 , which will be described in detail herein below, is secured to the lamp  400  by a pair of opposing mandible jaws, of which the jaw release button  506  of one of the mandible jaws is shown in FIG. 5. As the battery pack is brought into position against the bottom of the handle grip portion  402 , the jaws, having some built-in resilience to allow bending from a rest position, are inserted into slots in the handle grip portion  402  and snapped into place. The resilience is a property of the plastic material used to fabricate the handle grip portion  402  and the housing of the battery pack  500 .  
         [0042]    It will be appreciated that the battery pack  500 , when attached to the tubular housing  432  acts as a substantial base for the portable fluorescent lamp  400 , because of its mass (due to the batteries) and because the bottom of the battery pack  400  may be flat to provide a stable base. Alternatively, the bottom of the base may also be configured as a dual-plane surface. In this case, the bottom surface may comprise two separate planes, joined at a central location on the bottom surface, and which differed angularly from each other, enabling the lamp  400  to be positioned upright at two different angles. For example, one angle could be set slightly downward for greater illumination near the lamp and the other angle, which differed by only 5 to 10 degrees or so, would be useful for illuminating broader areas. Persons skilled in the art will further realize that the angle of illumination may be varied in other ways, such as incorporating a pivot, e.g., near the midpoint of the structure of the portable fluorescent lamp. Also shown in FIG. 5 along the back of the handle grip portion  402  is a receptacle  424  for an AC line cord (not shown) to be used when operating the lamp  400  from an AC voltage source.  
         [0043]    In an alternate embodiment not illustrated in FIG. 5, a receptacle for connecting a power cord to connect the lamp  400  to a DC voltage source such as an automotive battery supply may be included on the handle grip portion  402  of the tubular housing. It is feature of the portable fluorescent lamp  400  of the present invention that the inclusion of a battery charging circuit operative from a nominal 12 VDC supply enables the lamp  400  to be operated from a 12 VDC source as readily as from a 120 VAC source. The selection of voltage source, 120 VAC or 12 VDC, the selection may be made by merely changing the AC line cord or the DC power cord, or by an extra switch is described in conjunction with FIG. 1, which may be installed on the handle grip portion  402  of the tubular housing  432 .  
         [0044]    Referring to FIG. 6, there is illustrated an exploded pictorial view of one embodiment of a battery pack  500  for use with the portable fluorescent lamp of FIG. 5. The battery pack  500 , fabricated of molded plastic material, includes a bottom pan  502  having a pair of opposing mandible jaws  504  (“jaws  504 ”) molded integral with the bottom pan  502  and on opposite sides of the base  502 . The jaws  504  are oriented in a vertical direction, perpendicular to the bottom pan  502  and configured such that they are resilient when bent during installation or removal of the battery pack  500  onto or from the tubular housing  432  of the portable fluorescent lamp  400  of FIG. 5. The outer surface of the jaws  504  include a ridged button  506  for use in deflecting the jaws  504  to remove the battery pack  500  from the portable fluorescent lamp  400  as will be described further herein below.  
         [0045]    The bottom pan  502  of the battery pack  500  is further configured to receive a plurality of batteries assembled as a cell pack  510 . Disposed above the cell pack  510  is a retainer plate  512  for securing and positioning a pair of battery terminals  514 . The terminals  514  are installed in recesses  516  molded into the retainer plate  512 . One terminal  514  may be designated a positive terminal and connected to the positive terminal of the cell pack  510  and the other would be designated a negative terminal  514  to be connected to the negative terminal of the cell pack  510 .  
         [0046]    The battery pack  500  further includes a top cover  520  that includes a docking plate  530 , wherein the top cover fits over and encloses the cell pack  510  and retainer plate  512  when installed and secured to the bottom pan  502  using the resilient locking tabs  522  disposed near each corner of the bottom pan  502 . The top cover  520  includes openings  524  disposed on two opposite sides of the top cover  520  through which pass the opposing mandible jaws  504 . The top cover  520  also includes two contact openings  526  disposed in the docking plate  530  to expose and permit access to the positive and negative terminals  514  connected to the cell pack  510 . The contact openings  526  function to locate the positive and negative terminals  514  such that they make contact with corresponding terminals in the lower end of the handle grip portion of the tubular housing  432  containing the electrical circuitry when the battery pack  500  is assembled to the tubular housing  432  of the portable fluorescent lamp  400 .  
         [0047]    Referring to FIG. 7, there is illustrated a pictorial view of an assembled battery pack for use with the portable fluorescent lamp of FIG. 6. The reference numbers for the figure are the same as those of FIG. 6 (or a lower numbered figure) and they refer to the same structures. The battery pack includes a bottom pan  502  assembled to a top cover  520  with the pair of opposing mandible jaws  504  protruding through the openings  524  in the top cover  520 , and exposing the ridged buttons  506  to view. The ridged buttons  506 , disposed on opposite sides of the battery pack  500 , are pressed toward each other to release the opposing mandible jaws  504  from corresponding jaw catches (not shown) inside the lower end of the handle grip portion  402  of the tubular housing  432 . In FIG. 7, the assembled battery pack  500  further illustrates the docking plate  530  having the contact openings  526  and the positive and negative terminals  514  of the cell pack  510  visible therethrough.  
         [0048]    Referring to FIG. 8, there is illustrated a partially cut-away pictorial view of the interior of the embodiment of the portable fluorescent lamp of FIG. 5. The illustration depicts a half shell  600  of the tubular housing  432  of the portable fluorescent lamp  400  of FIG. 5, and includes one half of the handle grip portion  402 , the lens portion  404 , the fluorescent bulb  406 , the receptacle  408  for the fluorescent bulb, the tubular spine  410 , and the end cap  412 . The space above the lens portion  404  but within the end cap  412  is designated as reference number  414 . This space is available for additional features of the lamp  400 , which may include, for example, individual light indicators, spotlights or flashing lights, a hook for hanging the lamp  400 , a switch for an added electrical function, a magnet for supporting the lamp  400 , and the like.  
         [0049]    Further, the cut-away view of FIG. 8 illustrates one arrangement of substrates such as printed circuit boards for the electrical circuitry (See FIGS. 1-4) used in the illustrative embodiment. For example, a first circuit board  602  may contain and support the circuits of FIGS. 2 and 4, the 120 VAC converter and fluorescent ballast circuits respectively. Similarly, a second circuit board  604  may contain and support the battery charging circuit of FIG. 3. Other configurations are certainly possible, depending upon the particular architecture of the portable fluorescent lamp  400  of the present invention. Also shown in FIG. 8 are the receptacle  424  for the AC line cord (not shown) and the ON/OFF switch  426  for the lamp  400 . The receptacle  424  and the switch  426  and a battery pack terminal  428  are also shown in FIG. 5.  
         [0050]    While the invention has been shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit thereof. For example, the compact, efficient architecture of the portable, rechargeable fluorescent lamp  400  disclosed herein is readily adaptable to higher power fluorescent bulbs with relatively little increases in size and weight of the end product. Further, the lamp design permits use with interchangeable battery packs and/or battery chargers. Moreover, as described previously, the lamp may be configured for operation from both AC and DC power sources, or from either one alone. In an AC operated lamp, the AC line cord may be replaced with an AC line plug designed to fit a standard 120 VAC wall outlet. In this configuration the portable fluorescent lamp  400  of the present invention may then be used as a power failure emergency light that would remain fully charged and provide auxiliary lighting, either while plugged in to the outlet or while carried around as a portable lamp.  
         [0051]    Additional features may be included or modifications made in designs adapted to specific needs. As examples, the cylindrical lens portion  404  may be transparent or translucent. Translucent versions may be colored white or any of several other colors according to particular uses contemplated for the portable fluorescent lamp  400 . In an alternative embodiment, the cylindrical lens portion  404  may be configured to be interchangeable so that different colors or illumination properties may be conveniently provided. In yet other embodiments, the lens portion  404  may have cross-sections other than cylindrical, being, for example, square or rectangular, pentagonal or hexagonal, and so on. Reflectors may be incorporated within or outside the lens portion  404  to direct the light from the fluorescent bulb in predetermined directions or to shape or focus the light in particular predetermined ways. Such reflectors may further be interchangeable.  
         [0052]    It is further contemplated that the handle grip portion  402  may have other shapes or other surface finishes to permit other kinds of gripping features than the illustrative embodiment described herein above. The handle grip portion  402  or other parts of the tubular housing  432  may include eyelets to enable supporting the portable fluorescent lamp from a lanyard or hook or other tether device. Certain applications may include structural features to make the tubular housing  432  gas tight or water tight and/or to incorporate other features such as buoyant means to enable the portable fluorescent lamp  400  to float in water or to be used while immersed, as in marine applications. The tubular spine  410 , being hollow, includes space for additional circuitry or for relocating the electrical circuitry from the handle grip portion  402  of the tubular housing  432 . In the latter case, the batteries may then be located in the handle grip portion of the lamp, enabling a reduction in the size of the lamp. The implementation of all such features and modifications are well within the skills of persons skilled in the art, as will readily be appreciated.