US 3165668 A
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3,165,668 Patented Jan. 12, 1965 3,165,665 AP?ARATUS EMPLGYHNG NEGATHVE BR55181 ANCE DEVTQE GPERATKNG ELEQTRTQ DISCHARGE LAP/E 1 5 Theodore it. "piey, fianviiie, 113., assignor to General Electric 'Company, a corporation oi New York Filed Dec. 17, 1962, Ser. No. 245,122
This invention relates to apparatus for operating electric discharge lamps and more particularly to such an improved apparatus employing solid state devices.
In conventional ballasts an impedance element is usually provided in series with the electric discharge lamp in order to limit the lamp current to a proper value after the lamp is started. Where the electric discharge lamp is operated from an alternating power source, the impedance element may introduce into the lamp circuit an inductive reactance or the combination of inductive and capacitive reactance to provide the ballasting action. In applications where an electric discharge lamp is operated with direct current, the current supplied to operate the lamp is generally limited by placing a resistor in series circuit with the electric discharge lamp.
In an application Serial Number 192,231, filed on May 31, 1962, in the name of Walter F. Powell, Jr., and assigned to the same assignee as the present invention, there is described and claimed an arrangement for operating electric discharge lamps wherein the electric discharge lamps are ballasted by repetitively switching the current supplied to the lamps between controllable levels. The present invention relates to an improved arrangement employing one transistor and tunnel diode or the equivalents thereof in the lamp operating circuit. The ballasting of the lamp is accomplished by repetitive switching action wherein the energy delivered to the lamp and stored in the magnetic field of an inductor is controlled.
Accordingly, a general object of the present invention is to provide an improved ballast apparatus for operating electric discharge lamps wherein the ballasting action is achieved by a repetitively switching action to control the energy delivered to the lamp and stored in an inductor.
It is another object of the invention to provide an improved apparatus for operating electric discharge lamps employing relatively few solid state devices wherein current control is achieved by the fast switching action of the solid state devices.
In accordance with one form of my invention I have provided an improved apparatus for supplying energy from a source of potential to operate one or more electric discharge lamps. The apparatus includes a first semiconductor switching means such as 'atransistor, a second semiconductor means characterized by a negative conductance, for example, a tunnel diode, and an energy storage element such as an inductor, all of which are connected in series circuit relation with the lamp.
The lamp is ballasted by the repetitive switching action of the first semiconductor means, which causes the lamp current to be maintained within predetermined levels.
The switching action is controlled by the second semiconductor means which causes the first semiconductor means to be activated from a conductive to a nonconductive state as the second semiconductor means shifts from a high voltage to a low voltage state. To insure that energy is supplied to the lamp during the nonconductive interval of the first semiconductor means, energy stored in the energy storage element is delivered to the lamp during this interval. In a prefered embodiment of the invention a diode is connected in parallel circuit relation with the lamp and in circuit with the inductive energy storage element, to provide a path for the energy stored in the magnetic field of the element to be delivered to the lamp when the first semiconductor means is nonconductive.
The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. My invention, however, as to organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:
FIGURE 1 is a schematic circuit diagram of a ballast apparatus embodying my invention; and
FIGURE 2 is a schematic diagram of an apparatus embodying the improved arrangement of my invention wherein a starting circuit and filament transformer are provided.
Referring more particularly to the schematic circuit diagram shown in FIGURE 1, the improved apparatus 16 for supplying an operating potential to an electric discharge lamp 11, is shown enclosed in a dashed rectangle 12 which schematically represents the housing means for the components of the apparatus 10. is adapted for operation from a DC. source, input lead 13 being adapted for connection to the positive side of the source and input lead 14 being adapted for connection to the negative side. I
To provide the fast switching action required for controlling the operation of the lamp 111, a transistor Q is provided in a circuit with lead 14, a tunnel diode TD and inductor L and an output lead 15 connected in circuit with one end of the lamp 11. It will be seen that the other end of lamp 11 is connected in circuit with the positive side of the power source by means of lead 13. The transistor Q, the tunnel diode TD and the inductor L which serves as an energy storage element, are con nected in series circuit relation with lamp 11. i
A path for the inductive decay current from inductor L when the transistor Q; is switched to a nonconductive state, is provided by a unidirectional conducting device, such as a diode D which is connected to the lead joining the emitter of transistor Q with the tunnel diode TD and to input lead 13; The diode D is poled so that it does not conduct when'power is being supplied from the power supply to lamp 11. Depending upon the state of the tunnel diode TD the transistor Q is rendered conductive by a bias signal from potential source 16, which is coupled with the transistor Q and the tunnel diOde TD1.
The transistor Q used in the exemplification of the invention was a power PNP transistor connected in. a common emitter circuit configuration. The PNP transistor is comprised of three zones of semiconductive material forming two PN junctions, the base being joined to the N-zone, and the emitter and collector being joined to the outer P-zones. The transistor Q is switched from a conductive to an essentially nonconductive state by cancelling the bias signal which provides the base drive for transistor Q The tunnel diode ,TD used in the exemplification of the invention Was a two terminal semiconductor device employing a single PN junction. The electrode joined to the N-layer is referred to herein as the cathode and is represented schematically in the drawing by the channel shaped symbol. The electrode joined to the P-layer of the tunnel diode TD is referred to herein as the anode and is represented schematically in the drawing by the arrow symbol.
When the current level through the tunnel diode TD, is less than the predetermined peak point value, the tunnel diode TD, exhibits a low resistance and may be considered to be in a low impedance or low voltage state. insofar as the tunnel diode TD is concerned, the circuit The apparatus 10 has constant current properties; that is, the impedance of the tunnel diode TD does not at any time influence the lamp current except that the voltage drop across it is used as an indication of the lamp current. This voltage drop is used to produce. the switching action in the transistor Q by a process of non-linear current feedback or by emitter degeneration of the transistor Q Thus, each time the tunnel diode TD switches, it does so under conditions of constant current.
It will be appreciated that the apparatus of the present invention is intended to operate the electric discharge lamp 11. If the transistor Q had a higher voltage rating, a D0. supply of 200 volts could have been used to start and operate the fluorescent lamp 11 of the illustrated embodiment of the invention. Since the apparatus 1-0 was operated from a 60 volt D.C. supply, a starting circuit was used to initially fire the lamp. In FIGURE 2 I have shown an arrangement for starting an electric discharge lamp 11 by placing it initially across a source of alterhating current. A resistor R was provided to limit the current during the starting condition. Since lamp 29 was a hot cathode type of fluorescent lamp, a filament transformer T having a primary P and a pair of heating windings H and H was employed to supply a heating current to the lamp filaments.
It will be seen that the lamp operating circuit shown in 'FIGURE 2 is essentially the same as the operating circuit of FIGURE 1. Accordingly, I have used the same reference symbols to identify the corresponding parts of the operating circuit. The input leads 13, 14 in the schematic circuit diagram of FIGURE 1 are shown connected in circuit with a source of direct current potential, which is schematically shown as a battery 17. In order to switch lamp 20 overto the starting circuit, a double pole', double throw switch 18 was employed.
When the switch 18 engages the contacts 21, 22, the lamp 20 is placed in circuit across an alternating current source, such as a 60 cycle, 120 volt supply, to which leads 25, 26 are normally connected. As will hereinafter he more fully described the 24 inch T-12 High Output fluorescent lamp 20 used in the illustrated embodiment of the invention could be started directly from the 120 volt supply. It will be understood, of course, that longer lamps will usually require a higher source of starting potential.
When switch 18 is placed, in position so that the contacts 23 and 24 are engaged, the lamp 20 is connected in circuit with the operating circuit and is disconnected from the alternating current source except for the cathode heating current supplied by transformer T It will be seen that in either position of the switch 18, the filaments of fluorescent lamp 20 are supplied with heating current, andthe filaments of lamp 20 are connected by leads 27, 28, 29 and 30 with the filament heating windings H and H By way of illustration the apparatus shown in FIG- URE l was reduced to practice.v The following circuit components were used to operate a 24 inch T-12 High Output fluorescent lamp:
Transistor Q 2N1906 PNP power transistor.
Tunnel diode TD; STD-612 tunnel diode (General Electric).
Diode D 4JA10D diode (General Electric).
Inductor L Ferrite core inductor, 45
Supply potential source..- 60 volts D.C.
Bias voltage source 0.5 volt D.C.
The lamp 11 was started initally by connecting the lamp directly across a 120 volt, 60 cycle power source as shown in FIGURE 2. The small filament transformer T having its primary P connected to the power source, was used to supply cathode heating current. After the lamp 11 was ignited, the lamp 11 was operated from the 60 volt D.C. supply at a frequency of 1000 cycles per second.
Referring now to FIGURES 1 and 2, the operation of the operating circuit will be more fully described. Referring more specifically now to FIGURE 2, the lamp 20 was ignited by switching it across the leads 25, 26 which were connected to an alternating current source. After the lamp was ignited, it was switched to the DC. source 17. Until the current through the lamp 20 reaches the peak point value of the tunnel diode TD it will be understood that the voltage drop across the tunnel diode is negligibly small. For the tunnel diode used in the exemplification of the invention, the voltage drop was approximately 60 millivolts. However, when the lamp current reaches the peak current value of the tunnel diode TD it switches to the high voltage state. In the high voltage state the voltage drop across the tunnel diode TD, increases in magnitude. For the tunnel diode TD, used in the exemplification of the invention, this voltage drop was approximately 0.5 volt. Since the polarity of the voltage drop across the tunnel diode TD is opposite to the bias potential, the effect of the 0.5 volt bias potential is thereby effectively cancelled out. Consequently, the transistor Q is rendered nonconductive.
For this condition of the circuit, the polarity of the voltage across the inductor L reverses, and the potential at point A shown in FIGURE 1 is now more positive than the potential at input lead 13. Diode D is forward biased and provides a path for the inductive decay current from the inductor 1. When the lamp current reaches the valley current value of the tunnel diode TD tunnel diode TD switches to the low voltage state. The voltage drop across the tunnel-diode TD falls off to a negligibly small value, and the bias potential again becomes effective to drive the transistor Q Thus, transistor Q is rendered conductive, and diode D reverts to a blocking state. Cur rent is now supplied to the lamp 20 from the DC. power source 17, and another cycle of operation begins.
The current supplied to the lamp from the power source is effectively limited since the transistor Q is switched off to block any excursion of current from the power source when the lamp current reaches the peak point value of the tunnel diode TD During the interval in which the transistor Q is switched oil, the lamp 11 is supplied with a decaying current from the inductor L connected in series with the lamp. The lamp current is not allowed to drop below the valley point value of the tunnel diode TD since the transistor Q is again switched on when electric discharge lamp 1]. is effectively operated by a chopped current that oscillates between the peak point and valley point values of the tunnel diode TD Referring to the circuit diagram of FIGURE 1, it will be seen that during the conductive state of transistor Q1, the path of current flow is essentially from the positive input 'lead 13, to lamp 11, the inductor L the tunnel diode TD the transistor Q and to the negative input terminal lead 14. When the lamp current reaches the peak point value of the tunnel diode TD the tunnel diode TD switches to its high voltage state, and the voltage drop across the tunnel diode TD cancels the potential across the source 16 to render transistor Q nonconductive. During the high voltage state of the tunnel diode TD the continuity of the supply of energy to the lamp 11 is maintained by the delivery of the energy stored in the magnetic field of the inductor L When the lamp current decays to the valley point value of the tunnel diode TD the tunnel diode TD switches to its low voltage state to thereby cause transistor switch Q to be again rendered conductive. This switching cycle is repeated to provide alternate excursions of energy from the power source and the inductor L the current level switching effectively limiting the lamp current.
From the foregoing description of the exemplification of my invention, it will be apparent that a solid state apparatus utilizing relatively few components such as a tunnel diode and power transistor can be employed to operate electric discharge lamps. An important advantage of the ballast apparatus of the invention is that the fluorescent lamp 11 can be operated with a relatively high efficiency. The only losses in the apparatus are the small losses in the semiconductor devices D Q and TD and the copper and radiation losses in the inductor L It will be appreciated that where the lamp 11 is operated at a frequency above 100 kilocycles, the self-inductance of the arc in the fluorescent lamp 11 would serve as a suflicient energy storage means, and the external inductor L would not be required.
It will be apparent to those skilled in the art that semiconductor devices other than those described in the exemplification of the invention can be adapted to perform the functions in accordance with the present invention and that many other modifications of the invention may be made. Therefore, I intend by the appended claims to cover all such modifications that fall within the true spirit and scope of the invention.
What I claim as new and desire to secure by Letters Patent of the United States is:
1. An apparatus for supplying energy from a source of potential to an electric discharge lamp, said apparatus comprising a first semiconductor means having a number of zones of semiconductive material forming at least a pair of PN junctions, said means being rendered conductive by a bias signal, a second semiconductor means characterized by sharp impedance shift when operated under predetermined current conditions, said second means switching to a high voltage state in response to a first predetermined value of current passing through said second means and switching to a low voltage state when the current through said second means decreases to a second predetermined value, bias means coupled with said first and second means for applying a bias signal to rendersaid first means conductive, an inductive energy storage means, and circuit means for connecting said first and second semiconductor means and said storage means in circuit with the lamp and the potential source, said circuit means including a unidirectional conducting device to provide a path for the delivery of energy stored in the inductive energy storage means to the lamp when said first means is rendered nonconductive, said first semiconductor means being rendered conductive by said bias means until the current through said second semiconductor means reaches said first predetermined value whereby said second means is switched to a high voltage state and cancels the bias signal to render said first means nonconductive,
2. An apparatus for operating an electric discharge lamp from a power source, said apparatus comprising a semiconductor means, a tunnel diode connected in series circuit relation with said semiconductor means, a potential source coupled with said semiconductor means for rendering said semiconductor means conductive, said potential source being connected in circuit with said tunnel diode, said potential of said source being cancelled when said tunnel diode is switched to a high voltage state and being applied at said semiconductor means when said tunnel diode reverts to a low voltage state, said supply of energy from the power source to the lamp being interrupted when said semiconductor means is rendered nonconductive, an energy storage element, and circuit means for connecting said semiconductor means, said tunnel diode and said energy storage element in circuit with said lamp, said circuit means providing a path for the storage of energy in said element during the supply of operating potential from the source to the lamp and for delivery of said energy stored in said element to the lamp when said supply of operating potential from the source to the lamp is interrupted by said semiconductor means.
3. An apparatus for operating a fluorescent lamp from a source of potential, said apparatus comprising a transistor, a tunnel diode, an inductor, circuit means for connecting said transistor, said tunnel diode and inductor with the source of potential in circuit for operating the lam said means including a unidirectional conducting device connected in circuit with said inductor and said tunnel diode to provide a path for energy released from said inductor when the transistor is rendered nonconductive, means coupled with said transistor to apply a bias potential thereto, said tunnel diode being switched to a high voltage state when the current therethrough reaches a first predetermined level thereby cancelling said bias potential and rendering said transistor nonconductive, said tunnel diode switching to a low voltage state when the current therethrough decreases to the second predetermined level, whereby said apparatus operates said lamp by repetitively causing the current supplied to the lamp to vary between said first and second predetermined levels.
4. An apparatus for operating an electric discharge lamp from a source of direct current potential, said apparatus comprising a transistor, a bias potential source coupled with the transistor for rendering said transistor conductive, a tunnel diode, an inductor and circuit means including an input for connection with the source of potential and connections for placing said transistor, said tunnel diode and inductor in series circuit relation with the lamp, said circuit means including means to provide a path for the supply of energy from the inductor to the lamp when said transistor is rendered nonconductive, said tunnel diode causing the bias potential to be cancelled when the current therethrough reaches a first predetermined level thereby rendering the transistor nonconducting and said tunnel diode being switched to a low voltage state when the current therethrough decreases to a second predetermined level thereby rendering said transistor conductive.
References Cited in the file of this patent UNITED STATES PATENTS 3,060,348 Todd Oct. 23, 1962