Abstract:
A ballast circuit for a gas discharge lamp, having the capability to shift frequency after starting to reduce electromagnetic interference (EMI). Embodiments of the circuit contain an oscillator circuit that generates and supplies an oscillating signal and a time delay circuit, which generates a time delay to signal the oscillator to shift frequency. In embodiments of the circuit, the frequency shift is achieved by selecting different passive components used to generate the oscillator frequency.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to ballast circuits for starting gas discharge lamps, and more particularly, to an improved, rapid start ballast circuit for a fluorescent lamp that switches from a first frequency to a second frequency for more efficient and reliable starting of the lamp. 
       BACKGROUND OF THE INVENTION 
       [0002]    A gas discharge lamp is a well-known light source that typically consists of a glass envelope containing a low-pressure gas such as argon, krypton, neon, or a mix of these gases, and a quantity of an ionizable material such as mercury. 
         [0003]    The lamp emits light by creating an electric arc passing through the gas. The arc is created by applying a large Alternating Current (AC) voltage across the cathodes of the lamp. 
         [0004]    A fluorescent lamp is a well-known type of gas discharge lamp. A typical fluorescent lamp consists of an elongate gas envelope having an interior wall coated with a suitable phosphor, and having a cathode at each end of the envelope for application of an AC voltage across the lamp. 
         [0005]    In operation, the gas discharge lamp appears as a negative impedance device; that is, the voltage drop across a gas discharge lamp will tend to decrease with increasing discharge current. Thus, a high voltage is required to create or strike the arc through the lamp followed by a lower voltage to maintain the arc once the arc is struck. 
         [0006]    A ballast circuit is normally used to provide a high starting voltage and to provide a positive series impedance for other current limiting mechanisms to maintain the arc voltage once the lamp is struck. In a typical ballast circuit, the ballasting function is generally provided by an inductor connected in series with the gas discharge lamp. A gas discharge lamp has a natural frequency; that is the lowest frequency at which the gas discharge lamp will resonate without the addition of any external inductance or capacitance. 
         [0007]    The purpose of the foregoing Abstract is to enable the public, and especially the scientists, engineers, and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection, the nature and essence of the technical disclosure of the application. The Abstract is neither intended to define the invention of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way. 
         [0008]    Still other features and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description describing only the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by carrying out my invention. As will be realized, the invention is capable of modification in various obvious respects all without departing from the invention. Accordingly, the drawings and description of the preferred embodiment are to be regarded as illustrative in nature, and not as restrictive in nature. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a block diagram of a ballast circuit for a gas discharge lamp. 
           [0010]      FIG. 2  is a schematic diagram of an embodiment of a ballast circuit for supplying electrical energy to a single gas discharge lamp. 
           [0011]      FIG. 3  is a schematic diagram of an embodiment of a ballast circuit for supplying electrical energy to two gas discharge lamps. 
           [0012]      FIG. 4A  is a front view of an embodiment of a droplight containing ballast circuits for a gas discharge lamp. 
           [0013]      FIG. 4B  is a side view of an embodiment of a droplight containing ballast circuits driving a single gas discharge lamp. 
           [0014]      FIG. 5A  is a front view of an embodiment of a droplight containing ballast circuits driving two gas discharge lamps. 
           [0015]      FIG. 5B  is a side view of an embodiment of a droplight containing ballast circuits driving two gas discharge lamps. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    While the invention is susceptible of various modifications and alternative constructions, certain illustrated embodiments thereof have been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims. 
         [0017]    In the following description and in the figures, like elements are identified with like reference numerals. The use of “or” indicates a non-exclusive alternative without limitation unless otherwise noted. 
         [0018]    A gas discharge lamp will start most easily when operated at its natural frequency. Therefore, ballast circuits are commonly designed to operate at the natural frequency of the gas discharge lamp. However, operation at this frequency often generates undesirable, harmonic signals, which then may radiate as Electro Magnetic Interference (EMI). Thus, it is often desirable to operate the lamp at a lower frequency after starting to reduce the radiation of undesirable harmonics. 
         [0019]    Referring to  FIG. 1 , a block diagram of a ballast circuit, ballast circuit  10  includes a filter  12 , which suppresses high frequency noise that may exist on the AC power input. The filtered signal is then supplied to a rectifier circuit  14  that converts the alternating current line signal to a continuous signal for use by the remaining components. The continuous current signal (i.e. direct current signal) is then supplied to an oscillator circuit  16  and a time delay circuit  18 . The oscillator circuit  16  provides a high frequency signal to a lamp or bulb driver circuit  20 , which in turn drives lamp  24  with a high frequency, high voltage signal. Lamp filter circuit  22  suppresses high frequency harmonics generated by lamp or bulb driver circuit  20 . 
         [0020]    Time delay circuit  18  switches the output frequency of oscillator circuit  16  from a first frequency to a second frequency upon the expiration of a time delay triggered by an event. In some embodiments of the ballast circuit, the event triggering the time delay is application of power to the oscillator circuit  16 . In other embodiments, the event may be provided, without limitation, by a user-manipulated switch. 
         [0021]    The construction and operation of circuits for line filter circuit  12 , rectifier circuit  14  and lamp filter circuit  22 , are well understood by those skilled in the art. 
         [0022]      FIG. 2  is a circuit diagram of the ballast circuit shown in  FIG. 1 . Referring to  FIG. 2 , an embodiment of line filter  12  includes a first capacitor  40 , a second capacitor  42  in parallel with a transformer  44 . Line filters are well known to those skilled in the art and the ballast circuit  10  is not limited to the particular embodiment of the line filter shown. 
         [0023]    In a preferred embodiment, the value of capacitor  40  is 0.1 uF, the value of capacitor  42  is 0.1 uF, and the value of transformer  44  is 60 mH. 
         [0024]    Rectifier circuit  14  includes capacitors  46  and  48  connected to diodes  50  and  52  to form a full wave rectifier circuit. Again, rectifier circuits are well known to those skilled in the art and the ballast circuit is not limited to the particular rectifier circuit shown in  FIG. 2 . In a preferred embodiment of the rectifier circuit  14 , the value of capacitors  46  and  48  are 22 uF. 
         [0025]    An embodiment of oscillator circuit  16  includes a self-oscillating, half-bridge driver circuit in oscillator module  54 . In the embodiment shown, this function is provided by an IR2153 device, provided by International Rectifier®. While the use of an integrated circuit is particularly convenient, the ballast circuit  10  is not limited to the use of an integrated circuit oscillator, or a particular part supplied by International Rectifier®. For example, an oscillator comprising discreet components may be used. In one embodiment, the discreet components may parallel the internal components provided by the IR2153 integrated circuit. Other oscillators are well known to those skilled in the art. 
         [0026]    In the embodiment shown, the frequency of oscillation is set by discreet components: a resistor  56 , a first capacitor  58 , and a second capacitor  60 . The frequency of operation may be selected by examining the data sheet for oscillator module  54  in selecting the appropriate values of resistor  56  and capacitors  58  and  60 . Note that the oscillator will operate at a first frequency when the value for capacitor  60  is selected, and capacitor  58  is essentially removed from the circuit by the time delay circuit  18  in the manner described below. Oscillator module  54  will operate at a second frequency when capacitors  58  and  60  are in series, essentially adding their capacitance values. 
         [0027]    An embodiment of time delay circuit  18  includes a capacitor  62  in parallel with a zener diode  64 . Capacitor  62  and zener diode  64  are connected to resistor  66 , and transistor  68  is connected to zener diode  64 . In operation, capacitor  62  is charged by current passing through resistor  66 . When the voltage on capacitor  62  exceeds the breakdown voltage of zener diode  64 , zener diode  64  conducts current and turns on transistor  68 , which shorts first capacitor  58  and changes the operating frequency of oscillator module  54 . Resistor  70  is used to bias transistor  68 . In the embodiments shown, transistor  68  is an n-channel Field Effect Transistor (FET). However, persons skilled in the art will recognize that other transistors may be used with appropriate changes to bias circuitry, such as, without limitation, bipolar transistors a p-channel FETs. 
         [0028]    In a preferred embodiment of time delay circuit  18 , the value for resistor  66  is 510K ohms, the value for capacitor  62  is 4.7 uF, and diode  64  has a breakdown voltage of 8.2 volts. Time delay circuit  18  is set primarily by the values of capacitor  62  and resistor  66 . 
         [0029]    Referring again to  FIG. 2 , driver circuit  20  includes two driver transistors: transistor  72   a  and transistor  72   b . In a preferred embodiment, transistors  72   a  and  72   b  may each an n-channel FETs, with the gates driven by oscillator circuit  16 . While n-channel FETs are used in the embodiment shown, persons skilled in the art will recognize that other drivers may be used, such as bipolar transistors or p-channel FETs, with appropriate changes in bias circuits. 
         [0030]    Lamp filter circuit  22  includes capacitor  74  and capacitor  76  connected in parallel with transistors  72   a  and  72   b , respectively. Lamp filter circuit  22  may optionally include inductor  78  connected in series with lamp  24 . Lamp filter circuit  22  may also optionally include capacitor  80  connected in parallel with lamp  24 . 
         [0031]    In a preferred embodiment, capacitors  74  and  76  have values of 1000 pF. Inductor  78  has a value of 2.5 mH and capacitor  80  has a value of 0.01 uF. 
         [0032]    Focusing now on the operation of time delay circuit  18  and oscillator circuit  16 , when power is applied at the input to the ballast circuit, power will be applied to the oscillator circuit  16  and to the time delay circuit  18 . At this stage of operation, transistor  68  is off (non-conducting), and capacitors  60  and  58  are connected in series so that their capacitance values add and so that the frequency of operation depends on both their values. 
         [0033]    When voltage is applied to the time delay circuit  18 , current flows through resistor  66  and charging capacitor  62 . As capacitor  62  charges to a voltage greater than the breakdown voltage of zener diode  64 , zener diode  64  will conduct current through resistor  70 , applying a voltage to the gate of transistor  68 , turning transistor  68  on (i.e. conducting). As transistor  68  turns on, it essentially shorts capacitor  58  to ground, so that the frequency of oscillation depends on capacitor  60 . 
         [0034]    In a preferred embodiment, the values of capacitor  60  and  58  are chosen so that oscillator circuit  16  starts oscillating at the natural frequency of lamp  24 . In the embodiment shown, the natural frequency is around 33 kilohertz. After a suitable time delay allowing the lamp  24  to start conducting and emitting light, the conduction by transistor  68  changes the frequency to a lower frequency, 25 kilohertz. At the lower frequency, less noise and fewer harmonics are generated by the driver circuit  20  and thus, less electromagnetic interference (EMI) is emitted by the ballast circuit. In the preferred embodiment shown, the value of resistor  56  is 28K ohms, the value of capacitor  58  is 3300 pF, and the value of capacitor  60  is 1000 pF. 
         [0035]      FIG. 3  shows a schematic diagram of a ballast circuit driving two lamps. To drive two lamps, the second lamp is essentially connected in parallel with the first lamp. In  FIG. 3 , similar components are numbered the same as in  FIG. 2 . Thus, a second capacitor  80   a , a second conductor  78   a  and a second lamp  24   a  are connected to the output of the driver circuit  20 . 
         [0036]    Ballast circuit  10  may be implemented as a circuit board serving as a mounting surface for the various components of ballast circuit  10 . The proper material of the circuit board and manner of mounting electrical components thereon are both well known to those skilled in the art. 
         [0037]    While ballast circuit  10  is useful for driving many types of gas discharge lamps in many types of applications, it is particularly useful in a fluorescent droplight.  FIGS. 4   a  and  4   b  show front and side views of a portable fluorescent droplight  400 . 
         [0038]    Droplight  400  comprises a case  401  that forms a handle  402  and a light emitter cavity  404 . Case  401  is preferably formed of high-impact plastic and may be split or constructed in two halves for ease of assembly. Case  401  also encloses various electrical components in droplight  400 , including ballast circuit  10 . Handle  402  may include ridges or a gripping structure  403  to assist the user in securely gripping droplight  400 . Cavity  404  has an opening to project light emitted by lamp  414  onto a work surface or object selected by the user. Cavity  404  may further include a reflector constructed of generally reflective material located generally behind lamp  414 . 
         [0039]    Droplight  400  may also comprise an electrical jack  406 . While a three—prong jack for 15 A, 120V service is shown; other styles of outlets may be used depending on country and current requirements. Electrical jack  406  makes the electrical power supplied to the portable fluorescent droplight  400  available to other devices that can be connected to the electrical outlet  406  in a manner well known in the art. 
         [0040]    The portable fluorescent droplight  400  may also comprise an electrical plug  408 , a power cord  410 , and an optional strain relief  412 . Strain relief  412  may be affixed to case  401  to retain a fixed end of power cord  410  in a well-known manner. Strain relief  412  alleviates tensile and lateral forces that arise between power cord  410  and case  401  due to movement of droplight  400  during use. In some embodiments, power cord  410  may be a three twisted conductor 16 AWG power cable of a type well known in the art. Similarly, plug  408  may be a grounded three prong male connector of a type well known in the art. Plug  408  is physically and electrically connected to a free end of power cord  410  in a well-known manner. The fixed end of power cord  410  is physically and electrically connected to the electrical jack  406  and to ballast circuit  10 . 
         [0041]    Gas discharge lamp  414  is electrically and physically connected to bulb socket  416 . Bulb socket  416  also physically locates the lamp  414  within light emitter cavity  404  and supplies lamp  414  with regulated electrical power generated by ballast circuit  10 . 
         [0042]    Case  401  also supports a switch assembly  418  for controlling electrical power to ballast circuit  10  and lamp  414 . An optional clear lens  422  may be used to protect lamp  24  during use. Lens  422  may be constructed of polyethersulfone (PES) or other suitably clear and durable material. Lens  422  may be supplied with optional vents  424  to dissipate heat produced by internal electrical components. Lens  422  may also be constructed in two layers: an inner layer may be used to prevent conductive heat transfer to an outer layer that is accessible to the user. An optional rotatable hook  420  may be supplied so that the user may hang droplight  400  for use. Rotatable hook  420  may be constructed of plastic, steel, or any other suitably strong material. 
         [0043]    Case  401  may include internal structures to support droplight components, including jack  406 , strain relief  412 , lamp  24 , bulb socket  416 , switch assembly  418 , and ballast circuit  10 . Screws or snap fitting may be used to support each of the components. 
         [0044]      FIGS. 5A and 5B  show an alternative embodiment of droplight  400 , employing two lamps,  414   a  and  414   b , and two rotatable hooks,  420   a  and  420   b.    
         [0045]    The exemplary embodiments shown in the figures and described above, illustrate, but do not limit the invention. It should be understood that there is no intention to limit the invention to the specific form disclosed; rather, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims. For example, while embodiments of the present invention were developed for fluorescent droplights, the invention is not limited to use with fluorescent droplights and may be used with other gas discharge lamps. Hence, the foregoing description should not be construed to limit the scope of the invention that is defined in the following claims.