ARC DISCHARGE LAMP CONTROL CIRCUIT
United States Patent 3700956
A high-frequency, lightweight, solid-state ac ballast for operating arc discharge lamps such as fluorescent lamps. An ac ballast in accordance with the invention includes a single transistor and a storage capacitor interconnected with a lightweight air-gap ferrite core transformer provide a blocking oscillator circuit capable of high-frequency ac operation. To initiate operation of the blocking oscillator circuit, low-frequency (e.g., 50-60 hertz) ac input voltage signals are converted to a dc voltage and applied to a starting circuit for producing a voltage for charging the storage capacitor. When the voltage developed across the capacitor reaches a predetermined value, the transistor is forward biased into conduction thereby initiating operation of the blocking oscillator circuit. After a predetermined period of time, the transistor is reversed biased by a voltage produced across a regenerative feedback winding of the air-gap transformer thereby terminating operation of the blocking oscillator circuit. During the operation of the blocking oscillator circuit through several cycles, high-frequency ac voltage signals (e.g., 20 kilohertz) are produced across a load winding of the air-gap transformer for operating a pair of fluorescent lamps. A modified version of the abovedescribed ac ballast includes a square hysteresis loop ferrite toroid in the blocking oscillator circuit, in addition to the air-gap transformer, for causing the transistor to switch more quickly between its conducting and non-conducting states.
US Patent References:
Transistor voltage converter
Wilting et al. - November 1961 - 3008068
Fluorescent lighting system
Bell - June 1968 - 3389299
High frequency ballast unit
Greene - February 1960 - 2923856
GASEOUS DISCHARGE LAMP CIRCUIT EMPLOYING A PULSER AND A SATURABLE REACTOR
Paget - July 1969 - 3457458
Controlled rectifier inverter circuit
Schultz - May 1962 - 3034015
Transistor voltage converter
Wilting et al. - November 1961 - 3008068
Fluorescent lighting system
Bell - June 1968 - 3389299
High frequency ballast unit
Greene - February 1960 - 2923856
GASEOUS DISCHARGE LAMP CIRCUIT EMPLOYING A PULSER AND A SATURABLE REACTOR
Paget - July 1969 - 3457458
Controlled rectifier inverter circuit
Schultz - May 1962 - 3034015
Application Number:
05/103582
Publication Date:
10/24/1972
Filing Date:
01/04/1971
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
331/112, 315/98, 315/DIG.007
International Classes:
H05B41/282; H05B41/28; H05B41/29
Field of Search:
315/101,108,98
US Patent References:
| 2964676 | Circuit arrangements for operating low pressure electric discharge lamps | December 1960 | Davies et al. |
Primary Examiner:
Saalbach, Herman Karl
Assistant Examiner:
Baraff C.
Claims:
What is claimed is
1. A control circuit for operating an arc discharge lamp, comprising:
2. A control circuit in accordance with claim 1 wherein the arc discharge lamp is a fluorescent lamp.
3. An ac ballast for operating an arc discharge lamp comprising:
4. A ballast in accordance with claim 3 further comprising a ballasting capacitance connected in series with the load secondary winding of the air-gap transformer.
5. A ballast in accordance with claim 4 further comprising a bypass capacitance connected between the collector and the emitter of the transistor.
1. A control circuit for operating an arc discharge lamp, comprising:
2. A control circuit in accordance with claim 1 wherein the arc discharge lamp is a fluorescent lamp.
3. An ac ballast for operating an arc discharge lamp comprising:
4. A ballast in accordance with claim 3 further comprising a ballasting capacitance connected in series with the load secondary winding of the air-gap transformer.
5. A ballast in accordance with claim 4 further comprising a bypass capacitance connected between the collector and the emitter of the transistor.
Description:
BACKGROUND OF THE INVENTION
The present invention relates to a control circuit for operating arc discharge lamps and, more particularly, to a single-transistor solid-state ac ballast for operating arc discharge lamps such as fluorescent lamps.
The use of ac ballasts for operating arc discharge lamps such as fluorescent lamps is well known to those skilled in the art. For example, it is commonly known to employ an inductive transformer ballast to supply a high starting voltage to one or more fluorescent lamps to ionize the gas in the lamps and thereby to start discharge and also to limit the current through the ionized lamps to a predetermined rated value. Although inductive transformer ballasts are presently the most common type of ballasts used in fluorescent lighting applications, particularly in high-power lighting applications, they nonetheless tend to be heavy and bulky, due to the large amounts of copper winding and iron employed by the transformers in such ballasts, thereby resulting in an undesirably high cost. In addition, such ballasts produce large amounts of unwanted heat and noise. Moreover, such ballasts are capable of only low-frequency operation, e.g., 50-60 hertz, thereby resulting in lamp operating efficiency which is less than optimum.
To overcome some of the problem associated with the above described inductive transformer ballasts, particularly weight problems, it has been proposed to replace such ballasts with inductive ballast arrangements including relatively lightweight transformers such as autotransformers. The control of the operation of these autotransformers to produce the desired mode of operation of a fluorescent lamp load is achieved by pulse generating, gating, or switching circuits interconnected with the autotransformers. Typically, these circuits include solid-state devices such as silicon controlled rectifiers, Diacs, and Triacs. While the abovementioned arrangements offer some improvement over the inductive transformer ballasts, they are still rather heavy and require a large number of components. As a result, such arrangements are generally too expensive to be considered at the present time for high-volume mass production. In addition, such arrangements are capable of only low-frequency operation and are susceptible to producing undesirable amounts of rfi noise.
SUMMARY OF THE INVENTION
Briefly, in accordance with the present invention, an arc discharged lamp control circuit is provided which avoids many of the shortcomings and disadvantages associated with prior art arc discharge lamp control arrangements such as described hereinabove. A control circuit in accordance with the invention includes an input terminal means adapted to receive an ac input voltage signal. An ac input voltage signal received at the input terminal means is converted by a converter means coupled to the input terminal means to a dc voltage signal. The dc voltage signal is applied to a starting means coupled to the converter means which operates in response to the dc voltage signal to produce a control voltage. The control voltage is employed in the control circuit of the invention for initiating operation of an oscillator circuit means.
The oscillator circuit means of the invention generally includes a transistor having a conducting condition and a non-conducting condition, an impedance means coupled to the starting means and to the transistor, and a transformer means coupled to the transistor and to an arc discharge lamp which is to be operated by the control circuit. The impedance means operates in response to the control voltage produced by the starting means to develop a voltage thereacross for initiating operation of the transistor in its conducting condition. When the transistor is initially operating in its conducting condition, the transformer means operates to cause the transistor to operate more fully in its conducting condition. When the transistor is operating more fully in its conducting condition, the transformer means then operates to cause the transistor to operate in its non-conducting condition. While the transistor is successively operating in its conducting and non-conducting conditions, the transformer means further operates to produce an ac operating voltage signal for operating the arc discharge lamp.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 illustrates a single-transistor, solid-state ac fluorescent lamp ballast in accordance with the invention; and
FIG. 2 illustrates a modified version of the solid-state ac fluorescent lamp ballast shown in FIG. 1.
GENERAL DESCRIPTION OF THE INVENTION -- FIG. 1
Referring now to FIG. 1, there is shown a solid-state ac fluorescent lamp ballast 1 in accordance with the present invention. The ballast 1 includes a pair of input terminals 2 to which an input signal is applied for operating the ballast 1. Typically, the input signal is an ac voltage signal having an amplitude of approximately 110-120 volts and a frequency of approximately 50-60 hertz. The input terminals 2 are connected to a conventional full-wave bridge rectifier circuit 4 employing four diodes d. The output of the full-wave rectifier circuit 4 is applied across a filter capacitor C1, for filtering the full-wave recitifed output of the rectifier circuit 4, and also across a starting circuit 6 comprising a series arrangement of a diode D and a resistor R1. As will be explained hereinafter, the starting circuit 6 serves to initiate operation of a blocking oscillator circuit 7 connected therewith.
The blocking oscillator circuit 7 generally comprises an npn transistor Q1, a storage capacitor C2, a bypass capacitor C3, a current-limiting resistor R2, and a lightweight ferrite core air-gap transformer T. The ferrite core air-gap transformer T includes a primary winding P, and a plurality of secondary windings S1-S5.
As indicated in FIG. 1, the storage capacitor C2 of the blocking oscillator circuit 7 is connected in parallel with the diode D. The top end of the primary winding P of the air-gap transformer T is connected directly to the collector of the transistor Q1 and the other (bottom) end is connected directly to one end of the resistor R1. The secondary winding S1 of the air-gap transformer T represents a regenerative feedback winding and is connected at one end directly to the base of the transistor Q1 and at the other end to the emitter of the transistor Q1 via the current-limiting resistor R2 and the storage capacitor C2. The bypass capacitor C3 is connected between the emitter and collector of the transistor Q1 and serves to protect the transistor Q1 from any switching transients occurring in the primary winding P of the air-gap transformer T by providing a current path for such transients around the transistor Q1.
The secondary winding S2 of the air-gap transformer T represents a step-up load winding for providing an operating voltage (e.g., 400-600 volts ac) for a pair of fluorescent lamps L1 and L2 and is connected in series with the fluorescent lamps L1 and L2 via a ballasting capacitor C4. The ballasting capacitor C4, in conjunction with the inductive leakage reactance of the air-gap transformer T, acts to limit current through the lamps L1 and L2 during operation thereof. The ballasting capacitor C4 also serves to provide a unity power factor load condition. Typically, the fluorescent lamps L1 and L2 are of the rapid-start, 40-watt T12 type. The secondary windings S3-S5 of the air-gap transformer T represent filament heating windings for supplying filament voltage to the filaments f of the fluorescent lamps L1 and L2.
OPERATION--FIG. 1
In the operation of the solid-state ac ballast 1 of FIG. 1, an ac voltage signal applied to the input terminals 2 of the ballast 1 is full-wave rectifed by the full-wave rectifier circuit 4 and applied across the filter capacitor C1. The full-wave rectified voltage is filtered by the filter capacitor C1 to provide a dc voltage across the starting circuit 6. Typically, in the interest of cost, the filter capacitor C1 is selected to have a small value (e.g., 50 microfarads). Therefore, the dc voltage produced by the filter capacitor C1 includes a high ripple content, a condition which is entirely acceptable in the present invention. In response to the dc voltage produced by the filter capacitor C1 across the diode D and the resistor R1, current flows through the resistor R1 into the storage capacitor C2. The storage capacitor C2 is gradually charged by this current and when the voltage developed across the storage capacitor C2 reaches the forward-bias voltage of the diode D, typically 0.6-0.7 volts, it is clamped to that value by the diode D. This voltage across the storage capacitor C2 (0.6-0.7 volts) causes the base of the transistor Q1 to be biased positive with respect to the emitter and is of sufficient amplitude to establish a small amount of base current in the transistor Q1. As a result, a small amount of initial collector current flow is established in the transistor Q1 and, thus, through the primary winding P of the air-gap transformer T.
In response to the initial current flow through the primary winding P, a positive voltage is initiated by the airgap transformer T across the load winding S2 for operating the fluorescent lamps L1 and L2. At the same time, a feedback voltage is initiated by the air-gap transformer T in the feedback winding S1 and coupled into the base of the transistor Q1. The polarity of the feedback winding S1 is established (as indicated by the conventional dot notation) so as to cause the base of the transistor Q1 to become more positive with respect to the emitter (positive feedback), thereby causing increased collector current flow into the primary winding P and, therefore, a consequential increase in the value of the voltage across the load winding S2 and an increase in the value of the feedback voltage induced in the feedback winding S1. The above action involving the transistor Q1 and the feedback winding S1 is regenerative in nature, and continues in a very rapid manner until the transistor Q1 operates in its saturation state. During the abovementioned regeneration action, as the voltage induced in the feedback winding S1 quickly increases in value, the diode D is reverse biased by the voltage induced in the feedback winding S1 (that is, the anode of the diode D becomes negative with respect to the cathode) and the voltage across the storage capacitor C2 is reversed from its previous, initial polarity.
As the transistor Q1 is caused to operate near saturation by the voltage induced in the feedback winding S1, current in the primary winding P of the air-gap transformer T increases very little, and the voltage across the load secondary winding S2 and, thus, across the lamps L1 and L2, becomes reduced from its peak positive value and approaches zero value. At the same time, the voltage induced in the feedback winding S1 rapidly decays and, through regenerative action, causes the base drive for the transistor Q1 to become rapidly reduced thereby causing the transistor Q1 to rapidly operate in its non-conducting state and to provide a minimum value of collector current to the primary winding P. It is to noted that during the abovementioned regeneration action, as the voltage in the feedback winding S1 becomes reduced, the capacitor C2 is charged toward its initial polarity by virtue of current again flowing through the resistor R1. The value of this voltage approaches the value of the forward-bias voltage of the diode D. As current flow in the collector circuit of the transistor Q1 and, therefore, through the primary winding P of the air-gap transformer T, assumes the minimum value, as mentioned hereinabove, the voltage across the load secondary winding S2 increases in the negative direction to its maximum negative value and then back in the positive direction. A feedback voltage is also induced in the feedback winding S1 of the air-gap transformer T having a polarity opposite to that previously induced in the feedback winding S1. This feedback voltage serves momentarily to keep the base of the transistor Q1 negative with respect to the emitter and thereby to keep the transistor Q1 in its non-conducting state. Although a voltage is present across the storage capacitor C2 at this time tending to make the base of the transistor Q1 positive with respect to the emitter, the combined value of this voltage and the feedback voltage is insufficient to cause the transistor Q1 to operate in its conducting state.
When the storage capacitor C2 is again charged by current flow through the resistor R1 to a value equal to the forward-bias voltage of the diode D (that is, 0.6-0.7 volts), a new cycle of operation of the blocking oscillator circuit 7 commences. With repeated operation of the blocking oscillator circuit 7 through several cycles, high-frequency ac voltage signals (e.g., 20 kilohertz) are caused to be produced across the load secondary winding S2 of the air-gap transformer T for operating the fluorescent lamps L1 and L2 at a high frequency, thereby resulting in improved lamp operating efficiency.
MODIFIED BALLAST--FIG. 2
Referring now to FIG. 2, there is shown a solid-state ac ballast 10 representing a modified version of the ac ballast 1 shown in FIG. 1. As will be apparent hereinafter from a listing of the particular components employed in the ballasts of FIGS. 1 and 2 and from typical values for the parameters of such components, the ballast 10 of FIG. 2 is the same as that of FIG. 1 with the exception that a lightweight ferrite toroid T1 having a square hysteresis loop is employed in the base circuit of the transistor Q1, a different value of resistance is employed for the current-limiting resistor R2, and a different number of turns is employed for each of the primary winding P, the feedback winding S1, and the load winding S2. As indicated in Fig. 2, the ferrite toroid T1 includes a primary winding PW connected in series with the current-limiting resistor R2 and the feedback winding S1, and a secondary winding SW connected at one end to the base of the transistor Q1 and at the other end to one end of the storage capacitor C2. The ferrite toroid T1, due to its square hysteresis loop, becomes saturated and unsaturated very quickly and serves more quickly to switch the transistor Q1 between its conducting and non-conducting states. As a result, the possibility of the air-gap transformer T becoming saturated is minimized.
Some typical values for the parameters of the components employed in the ballasts 1 and 10 are as follows:
d lN4003 D 1N4003 Q1 2N3902 C1 50 microfarads C2 0.44 microfarads C3 0.012 microfarads C4 0.022 microfarads R1 68 kilohms R2 (FIG. 1) 27 ohms R2 (FIG. 2) 32 ohms AIR GAP 0.028 inches P 76 turns (FIG. 1); 86 turns (FIG. 2) S1 (FiG. 1) 4 turns S1 (FIG. 2) 7 turns S2 (FIG. 1) 120 turns S2 (FIG. 2) 140 turns S3 11/2turns S4 11/2turns S5 l1/2turns Core (T) EI-40-GP3, Nippon Industrial Ferrite Corp. Core (T1) TO62H101A, Allen Bradley Corp. PW 41 turns SW 37 turns
While there has been shown and described what are considered preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as called for in the appended claims.
The present invention relates to a control circuit for operating arc discharge lamps and, more particularly, to a single-transistor solid-state ac ballast for operating arc discharge lamps such as fluorescent lamps.
The use of ac ballasts for operating arc discharge lamps such as fluorescent lamps is well known to those skilled in the art. For example, it is commonly known to employ an inductive transformer ballast to supply a high starting voltage to one or more fluorescent lamps to ionize the gas in the lamps and thereby to start discharge and also to limit the current through the ionized lamps to a predetermined rated value. Although inductive transformer ballasts are presently the most common type of ballasts used in fluorescent lighting applications, particularly in high-power lighting applications, they nonetheless tend to be heavy and bulky, due to the large amounts of copper winding and iron employed by the transformers in such ballasts, thereby resulting in an undesirably high cost. In addition, such ballasts produce large amounts of unwanted heat and noise. Moreover, such ballasts are capable of only low-frequency operation, e.g., 50-60 hertz, thereby resulting in lamp operating efficiency which is less than optimum.
To overcome some of the problem associated with the above described inductive transformer ballasts, particularly weight problems, it has been proposed to replace such ballasts with inductive ballast arrangements including relatively lightweight transformers such as autotransformers. The control of the operation of these autotransformers to produce the desired mode of operation of a fluorescent lamp load is achieved by pulse generating, gating, or switching circuits interconnected with the autotransformers. Typically, these circuits include solid-state devices such as silicon controlled rectifiers, Diacs, and Triacs. While the abovementioned arrangements offer some improvement over the inductive transformer ballasts, they are still rather heavy and require a large number of components. As a result, such arrangements are generally too expensive to be considered at the present time for high-volume mass production. In addition, such arrangements are capable of only low-frequency operation and are susceptible to producing undesirable amounts of rfi noise.
SUMMARY OF THE INVENTION
Briefly, in accordance with the present invention, an arc discharged lamp control circuit is provided which avoids many of the shortcomings and disadvantages associated with prior art arc discharge lamp control arrangements such as described hereinabove. A control circuit in accordance with the invention includes an input terminal means adapted to receive an ac input voltage signal. An ac input voltage signal received at the input terminal means is converted by a converter means coupled to the input terminal means to a dc voltage signal. The dc voltage signal is applied to a starting means coupled to the converter means which operates in response to the dc voltage signal to produce a control voltage. The control voltage is employed in the control circuit of the invention for initiating operation of an oscillator circuit means.
The oscillator circuit means of the invention generally includes a transistor having a conducting condition and a non-conducting condition, an impedance means coupled to the starting means and to the transistor, and a transformer means coupled to the transistor and to an arc discharge lamp which is to be operated by the control circuit. The impedance means operates in response to the control voltage produced by the starting means to develop a voltage thereacross for initiating operation of the transistor in its conducting condition. When the transistor is initially operating in its conducting condition, the transformer means operates to cause the transistor to operate more fully in its conducting condition. When the transistor is operating more fully in its conducting condition, the transformer means then operates to cause the transistor to operate in its non-conducting condition. While the transistor is successively operating in its conducting and non-conducting conditions, the transformer means further operates to produce an ac operating voltage signal for operating the arc discharge lamp.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 illustrates a single-transistor, solid-state ac fluorescent lamp ballast in accordance with the invention; and
FIG. 2 illustrates a modified version of the solid-state ac fluorescent lamp ballast shown in FIG. 1.
GENERAL DESCRIPTION OF THE INVENTION -- FIG. 1
Referring now to FIG. 1, there is shown a solid-state ac fluorescent lamp ballast 1 in accordance with the present invention. The ballast 1 includes a pair of input terminals 2 to which an input signal is applied for operating the ballast 1. Typically, the input signal is an ac voltage signal having an amplitude of approximately 110-120 volts and a frequency of approximately 50-60 hertz. The input terminals 2 are connected to a conventional full-wave bridge rectifier circuit 4 employing four diodes d. The output of the full-wave rectifier circuit 4 is applied across a filter capacitor C1, for filtering the full-wave recitifed output of the rectifier circuit 4, and also across a starting circuit 6 comprising a series arrangement of a diode D and a resistor R1. As will be explained hereinafter, the starting circuit 6 serves to initiate operation of a blocking oscillator circuit 7 connected therewith.
The blocking oscillator circuit 7 generally comprises an npn transistor Q1, a storage capacitor C2, a bypass capacitor C3, a current-limiting resistor R2, and a lightweight ferrite core air-gap transformer T. The ferrite core air-gap transformer T includes a primary winding P, and a plurality of secondary windings S1-S5.
As indicated in FIG. 1, the storage capacitor C2 of the blocking oscillator circuit 7 is connected in parallel with the diode D. The top end of the primary winding P of the air-gap transformer T is connected directly to the collector of the transistor Q1 and the other (bottom) end is connected directly to one end of the resistor R1. The secondary winding S1 of the air-gap transformer T represents a regenerative feedback winding and is connected at one end directly to the base of the transistor Q1 and at the other end to the emitter of the transistor Q1 via the current-limiting resistor R2 and the storage capacitor C2. The bypass capacitor C3 is connected between the emitter and collector of the transistor Q1 and serves to protect the transistor Q1 from any switching transients occurring in the primary winding P of the air-gap transformer T by providing a current path for such transients around the transistor Q1.
The secondary winding S2 of the air-gap transformer T represents a step-up load winding for providing an operating voltage (e.g., 400-600 volts ac) for a pair of fluorescent lamps L1 and L2 and is connected in series with the fluorescent lamps L1 and L2 via a ballasting capacitor C4. The ballasting capacitor C4, in conjunction with the inductive leakage reactance of the air-gap transformer T, acts to limit current through the lamps L1 and L2 during operation thereof. The ballasting capacitor C4 also serves to provide a unity power factor load condition. Typically, the fluorescent lamps L1 and L2 are of the rapid-start, 40-watt T12 type. The secondary windings S3-S5 of the air-gap transformer T represent filament heating windings for supplying filament voltage to the filaments f of the fluorescent lamps L1 and L2.
OPERATION--FIG. 1
In the operation of the solid-state ac ballast 1 of FIG. 1, an ac voltage signal applied to the input terminals 2 of the ballast 1 is full-wave rectifed by the full-wave rectifier circuit 4 and applied across the filter capacitor C1. The full-wave rectified voltage is filtered by the filter capacitor C1 to provide a dc voltage across the starting circuit 6. Typically, in the interest of cost, the filter capacitor C1 is selected to have a small value (e.g., 50 microfarads). Therefore, the dc voltage produced by the filter capacitor C1 includes a high ripple content, a condition which is entirely acceptable in the present invention. In response to the dc voltage produced by the filter capacitor C1 across the diode D and the resistor R1, current flows through the resistor R1 into the storage capacitor C2. The storage capacitor C2 is gradually charged by this current and when the voltage developed across the storage capacitor C2 reaches the forward-bias voltage of the diode D, typically 0.6-0.7 volts, it is clamped to that value by the diode D. This voltage across the storage capacitor C2 (0.6-0.7 volts) causes the base of the transistor Q1 to be biased positive with respect to the emitter and is of sufficient amplitude to establish a small amount of base current in the transistor Q1. As a result, a small amount of initial collector current flow is established in the transistor Q1 and, thus, through the primary winding P of the air-gap transformer T.
In response to the initial current flow through the primary winding P, a positive voltage is initiated by the airgap transformer T across the load winding S2 for operating the fluorescent lamps L1 and L2. At the same time, a feedback voltage is initiated by the air-gap transformer T in the feedback winding S1 and coupled into the base of the transistor Q1. The polarity of the feedback winding S1 is established (as indicated by the conventional dot notation) so as to cause the base of the transistor Q1 to become more positive with respect to the emitter (positive feedback), thereby causing increased collector current flow into the primary winding P and, therefore, a consequential increase in the value of the voltage across the load winding S2 and an increase in the value of the feedback voltage induced in the feedback winding S1. The above action involving the transistor Q1 and the feedback winding S1 is regenerative in nature, and continues in a very rapid manner until the transistor Q1 operates in its saturation state. During the abovementioned regeneration action, as the voltage induced in the feedback winding S1 quickly increases in value, the diode D is reverse biased by the voltage induced in the feedback winding S1 (that is, the anode of the diode D becomes negative with respect to the cathode) and the voltage across the storage capacitor C2 is reversed from its previous, initial polarity.
As the transistor Q1 is caused to operate near saturation by the voltage induced in the feedback winding S1, current in the primary winding P of the air-gap transformer T increases very little, and the voltage across the load secondary winding S2 and, thus, across the lamps L1 and L2, becomes reduced from its peak positive value and approaches zero value. At the same time, the voltage induced in the feedback winding S1 rapidly decays and, through regenerative action, causes the base drive for the transistor Q1 to become rapidly reduced thereby causing the transistor Q1 to rapidly operate in its non-conducting state and to provide a minimum value of collector current to the primary winding P. It is to noted that during the abovementioned regeneration action, as the voltage in the feedback winding S1 becomes reduced, the capacitor C2 is charged toward its initial polarity by virtue of current again flowing through the resistor R1. The value of this voltage approaches the value of the forward-bias voltage of the diode D. As current flow in the collector circuit of the transistor Q1 and, therefore, through the primary winding P of the air-gap transformer T, assumes the minimum value, as mentioned hereinabove, the voltage across the load secondary winding S2 increases in the negative direction to its maximum negative value and then back in the positive direction. A feedback voltage is also induced in the feedback winding S1 of the air-gap transformer T having a polarity opposite to that previously induced in the feedback winding S1. This feedback voltage serves momentarily to keep the base of the transistor Q1 negative with respect to the emitter and thereby to keep the transistor Q1 in its non-conducting state. Although a voltage is present across the storage capacitor C2 at this time tending to make the base of the transistor Q1 positive with respect to the emitter, the combined value of this voltage and the feedback voltage is insufficient to cause the transistor Q1 to operate in its conducting state.
When the storage capacitor C2 is again charged by current flow through the resistor R1 to a value equal to the forward-bias voltage of the diode D (that is, 0.6-0.7 volts), a new cycle of operation of the blocking oscillator circuit 7 commences. With repeated operation of the blocking oscillator circuit 7 through several cycles, high-frequency ac voltage signals (e.g., 20 kilohertz) are caused to be produced across the load secondary winding S2 of the air-gap transformer T for operating the fluorescent lamps L1 and L2 at a high frequency, thereby resulting in improved lamp operating efficiency.
MODIFIED BALLAST--FIG. 2
Referring now to FIG. 2, there is shown a solid-state ac ballast 10 representing a modified version of the ac ballast 1 shown in FIG. 1. As will be apparent hereinafter from a listing of the particular components employed in the ballasts of FIGS. 1 and 2 and from typical values for the parameters of such components, the ballast 10 of FIG. 2 is the same as that of FIG. 1 with the exception that a lightweight ferrite toroid T1 having a square hysteresis loop is employed in the base circuit of the transistor Q1, a different value of resistance is employed for the current-limiting resistor R2, and a different number of turns is employed for each of the primary winding P, the feedback winding S1, and the load winding S2. As indicated in Fig. 2, the ferrite toroid T1 includes a primary winding PW connected in series with the current-limiting resistor R2 and the feedback winding S1, and a secondary winding SW connected at one end to the base of the transistor Q1 and at the other end to one end of the storage capacitor C2. The ferrite toroid T1, due to its square hysteresis loop, becomes saturated and unsaturated very quickly and serves more quickly to switch the transistor Q1 between its conducting and non-conducting states. As a result, the possibility of the air-gap transformer T becoming saturated is minimized.
Some typical values for the parameters of the components employed in the ballasts 1 and 10 are as follows:
d lN4003 D 1N4003 Q1 2N3902 C1 50 microfarads C2 0.44 microfarads C3 0.012 microfarads C4 0.022 microfarads R1 68 kilohms R2 (FIG. 1) 27 ohms R2 (FIG. 2) 32 ohms AIR GAP 0.028 inches P 76 turns (FIG. 1); 86 turns (FIG. 2) S1 (FiG. 1) 4 turns S1 (FIG. 2) 7 turns S2 (FIG. 1) 120 turns S2 (FIG. 2) 140 turns S3 11/2turns S4 11/2turns S5 l1/2turns Core (T) EI-40-GP3, Nippon Industrial Ferrite Corp. Core (T1) TO62H101A, Allen Bradley Corp. PW 41 turns SW 37 turns
While there has been shown and described what are considered preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as called for in the appended claims.