CA1245367A - Gain controlled electronic ballast system - Google Patents

Abstract

GAIN CONTROLLED ELECTRONIC BALLAST SYSTEM

ABSTRACT

In one form of the invention, a gain controlled elec-tronic ballast system (100) is provided having a power source (112) for actuating a pair of gas discharge tubes (140 and 140'). The ballast system (100) includes a filter network (111) which is connected to the power source (112) for establishing a substantially constant voltage signal and suppressing harmonic frequencies generated by the electronic ballast system 100. An induction circuit (115) is coupled to the filter network (111) for generating a voltage across the gas discharge tube (140 and 140') responsive to the driving current. The induction circuit (115) includes an automatic gain control network (117) to maintain the driving current at a predetermined value and generates a switching signal. A switching circuit (113) is coupled to the induction circuit (115) for es-tablishing the driving current at a substantially constant and predetermined frequency responsive to the switching signal.

Description

124~;3~

GAIN CONTROL~ED ELECTRONIC BAL~AST SYSTEM
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BACKGROUND OF THE INVEN~ION
FIELD OF THE INVENTION

This invention is directed to electronic ballast systems for gas discharge tubes. In particular, this invention pertains to automatic gain controlled ballast systems for fluorescent tubes. In one embodiment of this concept, there is provided electronic ballast systems which are current driven and provide for automatic gain control. In another embodiment, the subject invention relates to an electronic ballast system having a toroidal transformer to provide a predetermined variable inductance for regulating a power output to a gas discharge or ~luores-cent tube. Further, this invention pertains to an electronic ballast system which is transistorized and where the current gain of various transistors range over a wide value from one system unit to another and this invention provides for electrical circuitry which will maintain the gas discharge tube light output fluctuation to a minimum.

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~2~L53~7 PRIOR ART
Electronic ballast systems for gas discharge tubes are known in the art. However, in some prior art elec-tronic ballast systems, no provision is made for frequency stabilization of the circuit. Thus, in such prior art electronic ballast systems, when a gas discharge tube is removed from the circuit, there is deleterious flick-ering of the remaining gas discharge tube or in some cases, a complete breakdown of the visible light from the re-maining gas discharge tubes.
In other prior a~t electronic ballast systems, the light output of the gas discharge tubes are highly depen-dent upon the gain of the transistors used in the circuit.
In such prior art systems where the transistor gains be-tween one unit and another vary over a large value, the light output from the gas discharge tubes fluctuates by large amounts. Thus, in such prior art systems, addi-tional circuitry must be added to maintain the light output fluctuation as constant as possible hetween differing units.

, -_3_ ~245~

SUM~ARY OF THE INVENTION
A gain controlled electronic ballast system having a power source for actuating at least one gas discharge tube with a driving current is provided. A filter cir-cuit is connected to the power source for both maintain-ing a substantially smooth direct current voltage signal and suppressing harmonic frequencies generated by the electronic ballast. Induction circuitry is coupled to the filter circuit and has a tapped primary winding for generating a voltage across the gas discharge tube respon-sive to the driving current. The induction circuit has a multiplicity of secondary windings where one of the secondary windings is a switching conkrol winding for generating a switching signal. A switching circuit is couplad to the induction circuit for establishing the driving current at a substantially constant and predeter-mined frequency responsive to the switching signal esta-blished by the switching control winding of the induction clrcuit.

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-4- ~245~7 BRIEF DESCRIPTION OF T~E DRAWINGS

FIG. 1 is an electrical schematic drawing of the current driven gain controlled electronic ballast sys-tem; and, FIG. 2 is an electrical schematic drawing of an em-bodiment of the gain controlled electronic ballast sys-tem.

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DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 2, there is shown current driven automatic gain controlled ballast system 100, and self-regulating electronic ballast system 10, respectively.
Thus there is shown electronic ballast systems 10 and 100 having power source 112 for actuation of at least one o a pair of gas discharge tubes 140 and 140'. Gas discharge tubes 140 and 140' may be of a standard fluores-cent type system having first and second filaments 142, 144, and 142' and 144', as is shown.
Power source 112 may be 210 to 240 volt, 50 hz. AC
power source for the embodiments herein described with the understanding that a particular power source designa-tion is used for illustrative purposes only, and may be an AC power source of any standardized voltage genexated at frequencies approximating 50.0 or 60.0 hz.
In broader concept, power source 112 may be a ~C
power electrical source applied internal or external to systems 100 and 10 in a manner well known in the art by .

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removal of predetermined portions of the overall circuitry.
Power for systems 10 and 100 is supplied by power source 112 to switch 114 which may be a commercially available standardized switch element such as a single pole, single throw switch mechanism.
Power is input through power line 116 to rectifica-tion circuit 118 which provides for a full wave rectification of the power source AC voltage. Rectification circuit 118 may be a full wave bridge circuit, as shown in the figures. Full wave bridge circuit 118 is formed by diode elements 120, 122 r 124 and 126 for providing rectification of AC voltage from power source 112. In the embodiments shown, diode elements 120-126 may be one of a number of standard diode elements, and in one form of ballast systems 10 and 100, have standardized designation of lN4005.
Bridge circuit 118 provides an output pulsating DC
voltage signal passing on output line 138, which pulsating signal is applied to filter network 111. Filter network 111 filters the aforementioned pulsating DC voltage passing -from rectification circuit 118. Filter network 111 is electrlcally coupled to bridge circuit 118 by output line 138.

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In order to provide for a substantially continuous smooth signal for operation of systems 100 and 10, filter circuit 111 includes smoothing filter 136 used ~or averag-ing the pulsating DC voltage signal. Rectification bridge circuit 118 is coupled to ground 130 in order to be the return path for the DC supply for opposing ends of bridge circuit 118, providing DC power input to filter network 111 .
Smoothing filter 136 o~ filter network 111 includes choke element 132 and shunt capacitor 134. Choke element 132 is coupled on a first end in series relation to recti-fication circuit 118 and is additionally coupled to shunt capacitor 134 on an opposing end. Shunt capacitor 134 is connected in parallel relation with the overall output vf ~ilter network 111, as is ~hown. Shunt capacitor 1~4 is coupled on a first end to choke element 132 and to filter output line 141, as well as being coupled on an opposing end to DC return 65 in FIG. 2 and ground 130 in FIG. 1.
In combination, shunt capacitor 134 with choke element 132 functions to substantially average out the 100.0 Hz pulsating DC voltage supplied by full wave bridge circuit ,~

~ ~2~S3~7 118. The combination of shunt capacitor 134 and choke element 132 additionally maintains the current drawn by systems 100 and lO at an average value without creating an overall power factor which would be deleteriously lead-ing or in the alternative, lagging. Such deleterious lead or lag of the power factor may be found where large inductances were used in the overall circuit, when~ a large capacitance is used as the sole filtering mechanism for smoothing the pulsating DC voltage generated.
It is to be noted that in the event choke or series inductor element 132 were deleted from ballast system 100, shunt capacitor 134 would thus draw an increased current. This increased current is commonly referred to as a surge current and would be evident on each cycle as capacitor 134 began charging. Through the incorporation of series inductor 132, inductance 132 stores energy during each cycle to supply current for initial charging of shunt capacitor 134 which thus provides the smooth average curre~t as seen by power source 112.
Filter network lll which is coupled to power source 112 includes correction circuit 119. Correction circuit :
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119 is an electrical network where the elements have pre-. .
determined values selected in a manner to allow tuning of the network in order to substantially reduce harmonic oscillations which might otherwise be coupled back to power source 112. The tuning of correction circuit 119 is designed in a manner to provide significant reduction in the amplitude of the first five harmonic frequencies coupled into the DC supply of electronic ballast systems 10 and 100. It has been found that harmonic frequencies which aré multiples of the initial five harmonic frequencies are similarly reduced, as is typical in filters of this type.
Referring now specifically to the embodiment shown in Figure 1, fi.rst capacitor 123 may be approximately a 1.0 microfarad, 350.0 volt Mylar~ ype capacitor. Second capacitor 127 may ~e a 0.5 microfarad, 350.0 volt Mylar~
type element with first series resistor 125 having an . approximate value designation of 82.0 ohms, 1.0 watt resistor.
: Additionally, correction circuit 119 further includes second RC circuit 129 having second capacitor 133 coupled .

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in series relation to second resistor 131. Series com-bination of second capacitor 133 and second resistor 131 is connected in parallel relation to series inductor or choke element 132 in order to provide a low impedance path for any harmonic frequencies tG first RC circuit 119 which creates a low impedance path to ground 130.
~ current signal passing through power input line 141 responsive to actuation of power source 112 is inserted to bias resistor 152 as well as bias capacitor 154, Bias resistor 152 and bias capacitor 154 are connected in parallel relation each with respect to the other. The combination of bias resistor 152 and bias capacitor 154 are coupled to center tap line 160 of trigger control winding 143 of inverter transformer 178. As can be seen, trigger control winding 143 is coupled to both filter network 111 and switching circuit 113.
Center tap line 160 provides a center tap to trigger control winding 143 and establishes a switching control signal having opposing polarity when taken with respect to the center tap. Bias resistor 152 and bias capacitor 154 are used to establish a bias voltage for initiation .

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of an oscillation when electronic ballast system 100 is initially energized.
In the embodiment herein described, bias resistor 152 has a value app~oximating 220.0 X 103 ohms and bias capacitor 154 may have an approximate value of l.O micro-farads.
Current limiting resistor 156 and blocking diode 158 are connected in series combination and are coupled to center tap line 160. The series combination of current limiting resistor 156 and blocking diode element 158 pro-vides for return to ground 130 for the trigger signal generated in trigger control winding 143 when electronic ballast system 100 has passed into an oscdllation phase.
Although not important to the inventive concept as herein described, but provided for illustrative purposes, current limiting resistor 156 may have a value approximating 15.0 ohms with a dissipation range of approximately 1.0 watts. Blocking diode 158 may be a commercially available element having a common designation lN4001, and is coupled to a first end of current limiting resistor 156 and on an opposing end to ground 130.

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-12~ 5~7 Current driven gain controlled electronic ballast system 100 includes switching network 113 which is coupled to induction network 115. Switching network 113 includes a pair of transistors 170 and 170' connected in feedback relation to trigger control winding 143. Such coupling of transistors 170 and 170' to trigger control winding 143 allows switching a current signal responsive to a trigger signal produced.
Current enters trigger control winding 143 on center tap line 160, is divided and flows through both first transistor line 162 and second transistor line 164 to bases 172 and 172' of transistors 170 and 170'. First and second transistors 170 and 170' may be of the NPN
type which are commonly commercially available, and may have a designation of MJE135005.
In general, due to manufacturing considerations, one of first and second transistors 170 and 170' will . ~ , ~have a higher gain than the other. Thus, the transistor 170 or 170' having the higher gain will be turned "on"
or to a conducting state first. When either of flrst or second transistors 1/0 or 170' goes into a oonducting ., ~ . . ., ~, .; .. ~ -, , - ~ ' ., :

-13- ~4~3~7 mode, the other transistor 170 or 170' is held in a non-conducting state for the time interval during which the`
other transistor 170 or 170' is in the conducting or "on"
state.
Assuming that second transistor 170' enters a con-ducting state, the voltage level of second transistor collector 174' is then brought into the neighborhood of second transistor emitter 176' within approximately 1.0 volts.
~ s .is seen in the schematic of FIG. 1, emitter element 176' is electrically coupled to inverter transformer gain control secondary winding 181. Transformer gain control secondary winding lSl is also coupled to ground 130. Thus, the current path for the base drive current is completed.
Additionally, emitter element 176 o first transistor 170 is coupled to inverter transformer gain control second-ary winding 180 which in a similar fashion as the case of secondary winding 181, is coupled to ground 130.
Induction circuitry 115 includes inverter transformer 178 which is connected to switchins network 113 as has been previously described. Inverter transformer 178 -14- ~2~S3~7 includes a multi-tap p.rimary winding 182, as well as a multiplicity of secondary windings 202, 204, 206, trigger control winding 143, and inverter transEormer gain control secondary windings 180 and 181. A pair of coupling capa-citors 186 and 188 are coupled in series relation to a respective tap of primary winding 182, as well as gas discharge tubes 140 and 140'. Opposing ends of primary winding 182 are coupled to respective collector elements 174 and 174' o:E transistors 170 and 170' through lines no and 92.
Further included in induction circuit 115 is tuning capacitor 135 which is connected in parallel relation to the primary winding 182. Tuning capacitor 135 is connected between collector elements 174 and 174' of transistors 170 and 170' for protection of transistors 170 and 170' from excessive high voltages which may be produced i~E
one of gas discharge tubes 140 or 140' are electrically removed from the ballast system 100. Tuning capacitor 135 changes the oscillation frequency in the event that one of gas dischaxge tubes 140 or 140' is removed from the circuit 100. In this event, a lower voltage is induced in primary winding 182 which thus prevents damage to ,~

-15- ~53~7 transistor elements 170 or 170'.
As will be described in following paragraphs, primary winding 182 of inverter transformer 178 is tapped in a manner to provide an auto-transformer configuration. Par-ticularly, inverter transformer'primary winding 182 is tapped by high voltage output line 137 and high voltage output line 139 on re~spective oppQSing ends of primary winding 182. With center tap line 141 referencing the primary winding 182 to the DC power supply, a step down auto-transformer configuration is provided in each half of primary winding 182. Each half of primary winding 182 then functions as a primary winding on alternate half cycles of the oscillation produced.
Electronic ballast system 100 is current driven, which is in contradistinction to some prior art ballast systems where the saturating transformer i5 driven by the magnitude of a feedback voltage. In ballast system 100, during one half of the overall cycls, the collector current of first transistor 170 is in a feedback mode with inverter transformex 17~.
Current flows from powar source 112 through center tap line 141 and then through one half of primary wlnding , .

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1;~45;~i7 182 to transistor collector line 190 and finally to collector element 174 of transistor 170. Current passing through one half of primary winding 182 induces a voltage in trigger winding 143 which generates a base drive voltage. The base drive voltage is fed to base element 172 through line 162 which further reinforces the turning"on"of tran-sistor 170. Base and collector currents pass through emitter elements 176 through line 145 and then through gain control winding 180 and to ground 130.
In a similar manner, during alternate half cycles, collector current of second transistor 170' is in feedback coupling to inverter transformer 178 where again current flowing from the power source to center tap line 141 flows through the other half of primary winding 182 and then into collector line 192 and to collector element 174'.
. This current flow in the alternate half cycle induces .
a voltage in trigger control winding 143 which has a polar-ty opposite to that which was generated in the previous half cycle due^~to the flow direction of the current in~
primary winding 182. Such flows through line 164 to base : : eIement 172' and then both base and collector currents :

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, -17- ~2~ 77 flow through emitter element 176' through line 147 to gain control winding 181 and then to gxound 130.
After oscillation begins, both base voltages induced in gain control windings 180 and 181 are negative with respect to ground 130. However, the base voltage for the transistor 170 or 170' whichever is in the "on" state is less negative than its respective emitter voltage and therefore, properly biased. This bias voltage is pre-determined by the difference in turns in the respective windings 180 or 181 and 143 and therefore maintains a constant difference in potential which may occ~lr between base 172 or 172' and emitter 176 or 176', respectively.
Collector current which flows through inverter trans-former primary winding 182 during each half cycle generates a magnetic flux and induces voltages in all of the secondary windings of inverter transformer 178 while the~current is increasing towards its steady state value. As the current approaches its maximum value its rate of change diminishes and thus the induced secondary voltages are correspondingly reduced. When the steady state current is reached, no transformer action takes place, and the transistor 170 or 170' which was in the 7~011~ state no ~ ~ .

53~7 longer receives a base drive signal from trigger control winding 143 and therefore turns "off".
This sequence of events terminates the current from flowing in primary winding 182 which has the effect of reversing the direction of the magnetic flux. Thus there is induced an opposite polarity voltage in trigger control winding 143 turning the transistor 170 or 170' into an "on" state if such was previously in the "off" state.
A current is responsively driven in the opposite direction through primary winding 182 and induces a trigger base drive signal. Once again, the collector current reaches a steadv state value and the transformer action terminates and there is provided a repetitive process of oscillation whose frequency is determined by the in-ductance characteristics of inverter transformer 178.
In this manner, the frequency of oscillation is determined by the characteristics of the core, the number of turns of the primary~winding 182, and the current ~lowing through . ~
primary winding 182. Thus, the oscillation frequency is much less dependent on supply ~oltage than that which is known in prior art systems and produces a visible light output from discharge tubes 140 and 140' which is substan-.

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tially constant in having minimal flicker even with large variations in supply voltage.
As is known in classical transistor theory, the emitter current of a transistor is the combination of the base current and the collector current. In the operation of ballast system 100, the base current component of the emitter current, with reference to transistor 170 when it is in the "on" state, flows from ground 130 into block-ing diode 158 and through current limiting resistor 156 into tap line 160. Current flows through half of the winding of trigger control winding 143 to line 162 into base 172 and then through transistor emitter 176 into inverter transformer gain control secondary winding 180 and then to ground 130.
During a next consecutive half cycle, when second transistor 170i i9 in the "on" state, base current flows from ground 130 through blocking diode 158 and then through current limiting resistor 156 into center tap line 160 and trigger control winding 143.
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. -20- 12483~7 Current in trigger control winding 143 then passes through line 164 to base 172' of second transistor 170' and through base emitter junction 172', 176' to second inverter transformer gain control winding 181 and then to ground 130. A complete path for the base current is thus established during each half cycle when system lO0 is in oscillation.
Current driven automatic gain controlled ballast system 100 provides for a unique method of achieving gain control without the requirement for matching of transis-tors or the adjustment of gains with external componentsO
Ballast system 100 includes automatic gain controlled circuitry 117 which has a pair of windings 180 and 181 which are secondary windings of inverter transformer 178.
Inverter transformer gain control secondary windings 180 and 181 are coupled to emitter elements 176 and 176' of first and second transistors 170 and 170', respectively, ~ ~ , as is shown in FIG. l.
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As will bè detailed in following paragraphs, secon-dary windings 180 and 181 of automatic gain controlled circuit 117 are wound in a predetermined manner which .

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-~2~3~7 is in the same direction as the primary winding 182, to provide a negative feedback voltage to each of emitter elements 176 and 176' of first and second transistors 170 and 170'. When collector current flows through first section 194 of primary winding 182, an induced voltage is generated in first inverter transformer gain control secondary winding 180 and is phased in a manner such that winding 180 negatively biases emitter 176 of first tran-sistor 170 with respect to ground 130 to provide a negative feedback from first transistor 170.
A reference feedback voltage is provided which is proportional to the current drawn~through first section 194 of primary winding 182 and is the collector current of irst transistor 170. Similarly, on alternate half cycles,~.the collector current of second transistor 170' flows to second section 198 of primary winding 182 which provides negative feedback for second transistor 170'.
Due to the fact that collector currents of first and second transistors 170 and 170' are a function of the base current and the gain of the respective transistors ~ ~ .
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170 or 170', and assuming that the base current of each transistor 170 or 170' are substantially equal, the dif~e-rence in collector currents is proportional to the gain of each of transistors 170 and 170'.
By providing negative feedback voltage proportional to the collector current, the gain of each transistor 170 and 170' may be regulated to a predetermined value.
Since the negative feedback limits the gain of each tran-sistor 170 or 170' to a predetermined value which is less than the minimum gain of the transistor 170 or 170' as specified by the manufacturer, the gain of each transistor 170 or 170' as seen by the circuit, will be substantially the same.
Referring to the gain control, it i5 to be understood that the emitter currents flowing through gain control windings 180 and 181 effect the magnetization field within the core of inverter transformer 178. Obviously, this effect will either be additive or subtractive thus in-fluencing the base drive voltage induced in trigger winding 143 by shifting the operating point on the hy.steresis curve for the core material of inverter transformer 178.

~29~53~

Thus, in the event that the transistor gain is above a desired value, the operating point on the hysteresis curve will decrease, resulting in a decrease of.the base drive voltage induced in winding 143. In opposition, if the transistor gain is less than the desired value, the.collector and emitter currents will be reduced and the operating point on the hysteresis curve will increase simultaneously increasing the base drive voltage in order to regulate system operation.
The base current flowing through current limiting resistor 156 and center tap line 160 follows a symmetrical path through each of the transistor circuits and therefore, the base currents for all intents and purposes, will be substantially identical, and since the gain is held at a predetermined value, the collector currents will also be substantially identical.
The apparent transistor gain will be substantially the same for both transistors 170 and 170'. Additionally, the transistor gain is automatically controlled by the negative feedback generated in first inverter transformer ., :,.

-24- ~2 ~ S 3 67 gain control secondary winding 180 and second inverter -gain control winding 181.
During the "off" time, both the base.voltages and the emitter feedback voltage are positive with respect~
to ground potential, however, the difference in voltage :~
between them is such that the bases 172 or 172' is biased negatively by approximately 2.~ volts with respect to its corresponding emitter 176 or 176'. This provides for a fast fall time and a short storage timé, and there-fore, a low dissipation is provided in transistors 170 and 170'. As the DC voltage applied to power line 141:
increases with an increase in input AC voltage from power ~ :
source 112, both of the base voltages and the emitter feedback voltage increase in magnitude, however, their relative difference remains constant at Dpproximately 0.7 volts for the particular transistors and power output herein described.
Referring now in detail to inverter.transformer:`l78,. :.
current flows through primary section'l98 into collector~
174' of transistor 170' which for the purposes of this:~
current description, is in`a conducting state. When ~: :
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-25- ~2~ 7 switching takes place, transistor 170' goes to an "off"
condition, or non-conducting state, which then causes a rapid change in current and produces a high voltage in primary section 198. The high voltage is then seen at second coupling capacitor 188 which is coupled by line 139 to primary section 198.
In a similar manner, a high voltage having opposing polarity is induced in primary section 194 similar to the voltage value of primary section 198. This is applied to gas discharge tube 140 and to first coupling capacitor 186 which is connected to first section 194 by coupling or tapping line 137.
Voltage induced in first section 194 of primary wlnding 182 when first transistor 170 is switched to an "off"
state, is substantially equal in magnitude, but opposite in polarity, to that induced in second section 198 of primary winding 182 when second transistor 170' is switched to an "off" state.
Thus, an alternating voltage is generated at the predetermined frequency established by the saturation of inverter transformer 178. In a similar manner, the ~

~26- ~2~53~

voltage induced in second section 198 of primary winding 182 is also alternating at the predetermined frequency and approximately 180 out-of-phase with a voltage gener-ated in first section 194 of primary winding 182. This is due to the fact that each winding is on opposite sides of the center tap, and only one transistor 170 or 170' is in an "on" or "off" state during one time interval.
First and second coupling capacitors 186 and 188 are coupled to respective taps on inverter transformer primary winding 182 of inverter transformer 178. Capa-citors 186 and lR8 are also coupled to first filaments 142 and 142' of gas discharge tubes 140 and 140', respec-tively for discharging the induced voltage signal.
Secondary filament heater windings 202 and 206 are coupled in series relation to each of the first and second coupling capacitors 186 and 188 for discharging the in-duced voltage in primary sections 194 and 198 of primary windings 182 into gas discharge tubes 140 and 140'. As is clearly seen, secondary filament heater windings 202 and 204 of inverter transformer 178 are coupled to fila-ments 142 and 144 of gas dsicharge tube 140. In like ` `I~

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manner, secondary filament heater windings 204 and 206 of inverter transformer 178 heat filaments 144' and 142' of gas discharge tube 140'.
The induced voltage which is dischargedin fluores cent tubes 140 and 140' cause a current to flow from fila-ments 142 and 142' to filaments 144 and 144', respec-tively. Both filaments 144 and 144' are coupled to ground 130 through filament lead 208. Second filaments 144 and 144' of gas discharge tubes 140 and 140' are coupled in parallel each to the other through lines 208 and 210.
Secondary filament heater winding 204 is connected in parallel relation to both second filaments 144 and 144' of gas discharge tubes 140 and 140'. Similarly, filament heater secondary windings 202 and 206 are connec ted in parallel to first filaments 142 and 142', respec-tively. Thus, first filaments 142 and 142' are heated by ~ilament heater windings 202 and 206 and second filaments 144 and 144' share heater current from heater filament ,~ ~
secondary winding 204 which is coupled to ground 130 to provide a current path for the induced discharge current.

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~ ' -28- ~4~3~7 Referring now to the self-regulating electronic ballast system 10 of FIG. 2, there is shown harmonic filter circuit ll9 which includes harmonic filter capacitor 28 which is coupled in series relation to harmonic filter resistor 30. Harmonic filter capacitor 28 is connected on a first end to power output line 138 and on the opposing end to harmonic filter resistor 30. Harmonic filter resistor 30 is connected on its first end to filter capacitor 28 and on the opposing end to return line 130. Thus, harmonic filter circuit 119 has harmonic filter capacitor 2~ connected in series relation to harmonic filter resistor 30 and the series combination is connected in shunt relation to bridge circuit 118.
In the embodiment shown in FIG. 2, harmonic filter capacitor 28 is approximately 1.0 microfarad, 400.0 volt Mylar~ type capacitor, and harmonic filter resistor 30 is approximately a 240.0 ohm 2.0 watt resistor.
Self-regulation control circuitry 17 is coupled between return line 130 and in~erter networ~ 115. Self-regulation control circuitry 17 includes first capacitor 54, toroid transformer 56, and current limiting resistor 58. Current . .

-29- 12~ 7 limiting resistor 58 is coupled on a first end to return line 6S and on a second end to first winding 55 of toroid transformer 56. First winding 55 of toroid transformFr 56 is coupled on a first end to current limiting resistor 58 and on a second end to base coupling capacitor 54.
Base coupling capacitor 54 is coupled on one end to first winding 55 of toroid transformer 56 and on the opposing end to base driving winding 48 of induction circuit 115.
Although not important to the inventive concept as herein described, current limiting or damping resistor 58 may have a value approximating 2.0 - 3.0 ohms and having a dissipation rating of approximately 0.25 watts. Toroid transformer 56 may have a first windinq 55 with 16 turns of number 28 wire, a second winding 57 of a `single turn formed by DC power input or filter output line 141 passing through the axis of the toroid core. Base coupling capa-cltor 54 may be a 0~15 microfarad, lO0.0 volt Mylar~ ype capacltor.
The~series combination of current limiting resistor 58,~first~winding 55 of toroid transformer 56, and base coupling capacitor 54 provides for return paths for the :
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-30~ 5~7 base drive signal of switching circuit 113 subsequent to self-regulating electronic ballast system 10 going into an oscillation phase~
Self-regulating electronic ballast system 10 further includes switching network 113 which is feedback coupled to induction circultry 115 for establishing a regulated current. As will be seen in following paragraphs, switching network 113 includes a regulation mechanism for maintain-ing the power output of gas discharge tube 14~at = pre-determined and substantially constant value.
Switching network 113 includes transistor 72 connected in feedback relation to bias control winding 48 of inverter transformer 40. This coupling allows switching of a current signal responsive to a bias signal produced. Referring to bias~control winding 48 of inverter transformer 40, current entering the first end of bias control winding 48 passes through winding 48 to base element 78 of tran-, sistor 72. Transistor 72 may be of the NPN type which - ~ is commercially avail=ble and ln one commerci=lly purch=sed transistor, has a designation of MJE 13005.

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., -:31~ 53~7 It is to be understood that self-regulating electronic ballast system lO is designed to provide a consistency in visual light output, as well as power input to gas discharge tube 140 by maintaining the current of collector element 74 substantially constant independent of the current gain of a particular transistor 72 used in electronic ballast system 10. It has been determined that the li~ht output should not fluctuate more than plus or minus 3.0%
while the current gain of transistors 72 used in system lO may vary in the e~treme between lO.0 and 60Ø It is to be further understood that although system lO has been shown in the illustrated embodiment of FIG. 2 to operate a single gas discharge tube 140, the principle as herein described is general in concept and may be used in duaI systems since in such cases, transistor current gains would not necessarily have to be matched by palrs.
Initially a positive voltage provided to base element 78 by resistor el.ement 53 assures a small but sufficient initiating current flow through base element ?8 for ini-tiation of conduction through transistor 72.: A value of 1.2 megohms has been used successfully for resistor 53.

-:32- ~2~S~67 When transistor 72 goes into a conducting or "on"
state, current from DC output line 141 is coupled to primary winding 42 of inverter transformer 40 and passes through the axis of the core of toroidal transformer 56. Such current passes through first section 46 of primary winding 42 to tap line 25 which is coupled to collector element 74 of switching transistor 72.
Current flows through transistor 72 from collector 74 to emitter element 76 and then from emitter element 76 through return line 65. The increasing collector current established by switching transistor 72 induces a voltage in bias control winding 48 which is coupled to base element 78 of transistor 72. Base current flows from base element 78 to emitter element 76 in transistor 72 to return line 65.
In completion of the circuit, the current flows through current limiting resistor 58, first winding 55 of toroid transformer 56, and base coupling capacitor 54. The series combination of elements as aforementioned creates a pulse type base drive for switching transistor 72 "on" and "off"
for predetermined periods of time.

~2~S367 The pulse wh.ich drives transistor 72 controls the frequency of operation of the self-regulating electronic ballast system 10 shown in FIG. 2. At the terminating point of this pulse, transistor 72 goes to an "off" s~ate since the pulse differentiation through capacitor 54 supplies a negative signal to base element 78 which is limited in value magnitude by diode 38. The energy stored in pri.mary winding 46 of inverter transformer 40 then dis charges to coupling capacitor 60 and to fluorescent or gas discharge tube 140. This current induces a voltage in bias control winding 48 which then switches transistor 72 back to an "on" state in order that the cycle may be repetitive.
Primary winding 42 of inverter transformer 40 is a tapped winding which is connected in an auto-transformer configuration such that the voltage induced in primary winding second section 44 is coupled in series relation and adds to the voltage across primary winding first section 46. The total voltage across primary winding 42 is coupled to coupling capacitor 6Q which is connected in series relation with primary winding 42. Obviously, as seen in ~: ,...

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FIG. 2, coupling capacitor 60 is coupled on a first end to primary winding 42 of inverter transformer 40 and is further coupled on a second end to first filament 1~2 ~.
of gas discharge or fluorescent tube 140, as well as to a first end of protection capacitor 62.
Protection capacitor 62 is coupled in parallel rela-tion with gas discharge tube 140 and in series relation to output coupling capacitor 60 in order to prevent a generation of excessive voltage values from the circuit of system lO. For purposes of the embodiment herein des-cribed, capacitor 62 may be a 0.003 microfarad, 1.0 kilo-volt Mylar~apacitor.
Inverter transformer 40 includes secondary windings 50 and 52 which provide a filament voltage for fluores-cent tube 140. First filament drive winding 50 is coupled ln parallel relation to first filament 142 of fluorescent tube 140 and second fllament drive winding 52 is coupled in parallel relation to second filament 144 of gas dis-charge tube 140. A first end of second filament drive winding 52 is coupled to return line 55.

' _35_ 1Z~5367 Thus, when coupling capacitor 60 couples the dis-charge voltage from primary winding 42 to first filament 142 gas discharge tube 140, a current is able to flow through gas discharge tube 140 from first filament 142 to second filament 144 and then through return line 65 back to power source 112.
The generation of induced voltages occurs as the collector current is increasing towards its maximum value, and as is cleax, as the current reaches the maximum value, the rate of change is substantially zero, and thus, the induced secondary voltages are correspondingly reduced to substantially zero.
When the maximum current is reached, no transformer action takes place, and transistor 72 which was in the "on" state no longer receives base drive signals from base drive winding 48 of inverter transformer 40 and there-fore transistor 72 turns to an "off" condition.
When transistor 72 is turned to an "off" state, the collector current which was flowing through first section 46 of inverter transformer primary winding 42 terminates abruptly. The rapid change in collector current induces ~:.

: .
., ~24~;3~7 voltages again in second section 44 of invexter transformer primary winding 42 and the corresponding secondary windings 50, 52 and 48. As is known from classical theory, the polarity of the voltages induced by the rapid collapse of the collector current is such that transformer 40 attempts to maintain the direction of the original current in winding 46. Due to the direction of current flow in windings 46 and 48 as indicated by nomenclature dots 77, the voltage induced in bias control winding 48 of inverter-transformer 40 is of the oppo~ite polarity as previously described when the collector current was flowing. Thus, a negative signal on base element 78 with respect to emitter 76 lS
generated and transistor 72 is switched to an "off" con-dition.
As has been described in previous paragraphs, this allows for a repetitive cycle with a collector current ~ waveform which closely approximates a sawtooth, where :: there is a substantially linearly increasing period followed by a rapid decrease to substantially a zero value and then a substantially linearly increasing current back to the peak value.

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~37~ ~2~53~7 '~

The frequency of oscillation is determined by the combined characteristics of the core, the number of turns of first section 46 of primary winding 42, and the current flowing through first section 46 of primary~winding 42.
Thus, oscillation frequency is much less dependent on supply voltage than that which is known ln the prior art and produces a visible light output from gas discharge tube 140 which is substantially constant and having a minimization of visual flicker even when there exists substantially large variations in supply voltage.
In one working and operable embodiment of self-regulating electronic ballast system lO, inverter trans-former has a ferrite core wlth a 0.125 millimeter gap to reduce the likelihood that inverter transformer 42 will go into a saturating mode. Primary winding 42 is formed of 123.0 turns of number 24 wire and secondary windings 50, 52 and 48 are each 1.0 turns of number 24 wire.
Self-regulating electronic ballast system lO makes ~- use of the concept of a variable inductance in the form _ .

, ., .. A.. =~ ..... ' .. ` ' '' ` ~ . ;
, ' ~ " ~ ~ ' '''' ' ' ' ' ' ' ~ ' ' ' , ' :

" ~: :

~2~5367 of toroidal core 27 wound with 16.0 turns in which the base current passes. Line 141 passes through the axis of the toroidal core 27 which carries the collector current of transistor 72. The direction in which current flows through the two windings is such that their respective magnetic fields are additive within toroidal core 27 of toroidal transformer 56.
Therefore, the inductance which is seen in first winding 55 of toroidal transformer 56 is a function of both the base current and the collector current multiplied by the respective turn ratios and the permeability of magnetic core 27 depends on the base and current.
In actual practice, the inductance variations of second winding 57 of toroidal transformer 56 may be neglected since second winding 57 is formed of only a single turn and winding 57 inductance is relatively low as well as coupled in series with the inductance o first section 46 of primary wlnding 42. The inductance of second winding 57 has been found not to be significant when compared with the inductance of first section 46 of primary winding 42 which is substantially larger in absolute value.

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~39~ ~2~5367 In order to insure oscillation within self-regulating electronic ballast system 10 of switching transistor 72, bias control winding 48 is speclfically designed to supply sufficient voltage to turn "on" transistor 72 of the lowest gain which may be expected to be obtained from a manufac-turer of these systems. In this manner, it is assured that txansistor 72 will go to an "on" state and reach saturation and thus, the base to emitter voltage will be at least the 0.7 volts required to switch transistor 72 to the saturation state.
Regardless of the gain of transistor 72 used in self-regulating electronic ballast system 10, the collector voltage and collector circuit impedance is substantially the same and thus, the substantially same collector current will flow whether a transistor with a gain of 10.0 or 50.0 is being utilized. Therefore, since the base current is a function of the collector current divided by the gain of transistor 72, it is seen that the base current must change if a transistor 72 of different gain value is to be used and function properly in self-regulating electronic ballast system 10. Where the base current changes, then an electronic element in the base circult . -40- ~2~S3~7 must change its impedance value which is the function of self-regulating circuit 17 and primarily first winding 55 of toroidal transformer 56.
In order to achieve self-regulation, the design of toroidal transformer 56 is such that the maximum permeability of core 27 is reached with a transistor whose gain is at a maximum expected value. Likewise, the inductance of first winding 55 of toroidal transformer 56 will there-fore be at a maximum and hence a minimum current will flow through the base circuit for transistor 72.
The impedance of a winding having a magnetic core is related to the number of turns of the winding and current flowing therethrough as well as inversely to the length of the magnetic path in the core. The point of operation may be adjusted hy either changing the size of the toroid or by inserting parallel resistor 51 in parallel relation with toroid first winding 55 for adjustment of the corres-ponding exciting field. A value of 270.0 ohms has been successfully used for parallel resistor 51.
Thus, with first winding 55 of toroidal transformer 56 being at a maximum value of inductance, its impedance ~2~1~i3~7 is significantly larger than the in~pedance of current limiting resistor 58 and base coupling capacitor 54 such that the controlling factor is limiting the current to base element 78 of transistor 72. With transistor 72 having a maximum gain value, little current is needed and for example, if the gain or beta of transistor 72 is 50.0, then it is seen that the base current is 1/50th of the collector current.
EIowever, the voltage induced in base drive winding 48 has been designed to turn "on" a transistor of lower gain and therefore, there is excess energy to be dissipated in the base circuit of transistor 72. The excess energy is st.ored in first winding 55 of toroidal transormer 56. This im~edance of first winding 55 is primarily in-ductive as opposed to resistive, and there is little dissi-pation in the form of heat, and thus, there is provided an efficient means of dissipating the excess energy which is liberated when transistor 72 is in an "off" state.
In opposition, when a transistor of low gain is used in self-regulating electronic ballast system 10, the base current obviously must increase and the permeability of . ..

.. . .
.

-42- 1 Z ~ 53 67 core 27 of toroidal transformer 56 shifts in a downward direction to a lower value than would be measured for a high gain transistor and the inductance is less than was seen with a high gain transistor. Thus, a series impedance is reduced which allows a greater base current to flow and compensates for the lower gain transistor 72 being used in system 10.
Hence, there is provided a variable inductance in first winding 55 of toroidal transformer 56 which is essen-tially the self-regulating element and allows sufficient base current to switch transistor 72 to an "on" state regardless of the gain or beta of transistor 72. In this manner, the output of self-regulating electronic ballast system 10 remains relatively constant within a ~3.0% margin when comparing one system to another with extreme effi-ciency and without unnecessary dissipation of excess~heat.
~ A.lthough this invention has been described in connec-tion with speclfic forms and embodiments thereof, it will be appreciated that ~arious modifications other than those discussed may be resorted to without departing from the ~.

_43_ 12453~7 spirit or scope o~ the invention. For example, equivalent elements may be substituted for those specifically shown and described, certain features may be used independently of other features, and in certain cases, particular loca-tions of elements may be reversed or interposed, all without departing from the spirit or scope of the invention as defined in the appended Claims.

. .

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Claims (57)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A self-regulating electronic ballast system having a power source for actuating at least one gas discharge tube with a regulated current to maintain said gas discharge tube output and input power at a predetermined value, comprising:

(a) filter means connected to said power source for (1) maintaining a substantially smooth direct current voltage signal, and (2) suppressing harmonic frequencies generated by said electronic ballast system;

(b) induction means coupled to said filter means and having a tapped primary winding providing an auto-transformer configuration for generating a voltage across said gas discharge tube responsive to said regulated current, said induction means having a multiplicity of secondary windings, said multiplicity of said secondary windings including a trigger control winding for generating a control current; and (c) switching means being feedback coupled to said induction means for establishing said regulated current, said switching means including regulation means for maintaining said power output of said gas discharge tube at a predetermined and substantially constant value, said regulation means including a toroidal transformer having (1) a first winding coupled in series relation with said trigger control winding and said switching means for modifying said control current, and, (2) a second winding coupled to said tapped primary winding of said induction means and said filter means in series relation for feedback to said first winding of said toroidal transformer.

2. The self-regulating electronic ballast system as recited in claim 1 where said switching means includes transistor means for said regulated current, said transis-tor means including a base element, a collector element, and an emitter element coupled to said power source.

3. The self-regulating electronic ballast system as recited in claim 2 where said toroidal transformer second winding is connected to said induction means tapped primary winding and said emitter element of said trans-mitter means in series relation each with respect to the other.

4. The self-regulationg electronic ballast system as recited in claim 3 where said toroidal transformer provides a predetermined variable inductance for regulating a power output to said gas discharge tube.

5. The self-reguating electronic ballast system as recited in claim 4 where said regulation means includes a base coupling capacitor connected on opposing ends thereof to said toroidal transformer first winding and said trigger control winding of said induction means for substantially blocking a direct current component signal.

6. The self-regulating electronic ballast system as recited in claim 4 where said regulation means includes a current limiting resistor coupled on opposing ends thereof in series relation with said toroidal transformer first winding and said emitter element of said transistor means.

7. The self-regulating electronic ballast system as recited in claim 6 where said current limiting resistor limits a current value input to said base element of said transistor means when said toroidal transformer first winding inductance is at a substantially minimum value.

8. The self-regulating electronic ballast system as recited in claim 5 where said trigger control winding of said induction means generates a switching control signal, said trigger control winding being coupled to said base coupling capacitor on a first end and to said base element of said transistor means on a second end thereof.

9. The self-regulating electronic ballast system as recited in claim 8 where said induction means includes an output coupling capacitor coupled on opposing ends thereof to said gas discharge tube and said primary winding of said induction means for blocking said direct current voltage signal from said filter means while simultaneously passing therethrough a pulsating induced signal from said primary winding of said induction means.

10. The self-regulating electronic ballast system as recited in claim 9 where said induction means includes a protection capacitor coupled in parallel relation with said gas discharge tube and in series relation with said output coupling capacitor, said protection capacitor for preventing generation of excessive voltage values when said gas discharge tube is removed from said electronic ballast system.

11. The self-regulating electronic ballast system as recited in claim 9 where said output coupling capacitor is of a predetermined capacitive value for discharging said voltage from said auto-transformer configuration of said primary winding to said gas discharge tube.

12. The self-regulating electronic ballast system as recited in claim 9 where said tap of said primary winding is coupled to said collector element of said transistor means.

13. The self-regulating electronic ballast system as recited in claim 8 where said toroidal transformer of said regulation means provides a predetermined variable inductance to regulate said power input to said gas discharge tube.

14. The self-regulating electronic ballast system as recited in claim 13 where said toroidal transformer includes a toroid core configuration of ferrite material for varying the inductance in said first winding of said toroidal transformer responsive to a particular gain value of said transistor means.

15. The self-regulating electronic ballast system as recited in claim 14 where said first and second windings each of which having a predetermined number of turns are wound in a manner that the respective magnetic flux of said first and second windings is additive within said toroid core, said first and second winding magnetic fluxes being generated by said base element curret and said collector element current.

16. The self-regulating electronic ballast system as recited in claim 15 where said first winding of said toroidal transformer has a greater number of turns than said second winding of said toroidal transformer.

17. The self-regulating electronic ballast system as recited in claim 8 where two of said multiplicity of said secondary windings of said induction means are connected to opposing filaments of said gas discharge tubes.

18. The self-regulating electronic ballast system as recited in claim 13 where said second winding of said toroidal transformer couples a variable inductance control current signal to said first winding of said toroidal transformer responsive to a predetermined value of a magnetic flux component in said toroidal core of said toroidal transformer.

19. The self-regulating electronic ballast system as recited in claim 18 where said first winding of said toroidal transformer couples a variable inductance control current signal to said toroidal core of said toroidal transformer.

20. The self-regulating electronic ballast system as recited in claim l where said filter means includes:
(a) harmonic filter means for substantially reducing harmonic frequencies generated by said induction means; and (b) smoothing filter means coupled in parallel relation to said harmonic filter means for maintaining said substantially smooth direct current voltage signal.

21. The self-regulating electronic ballast system as recited in claim 20 where said smoothing filter means includes a series inductor coupled in series relation to said power source and said induction means.

22. The self-regulating electronic ballast system as recited in claim 21 where said smoothing filter means includes a shunt capacitor coupled to said series inductor and said power source, said shunt capacitor being coupled in parallel relation with an output of said filter means.

23. The self-regulating electronic ballast system as recited in claim 21 where said harmonic filter means includes a harmonic filter capacitor, said harmonic filter capacitor being coupled to said series inductor and said power source.

24. The self-regulating electronic ballast system as recited in claim 23 where said harmonic filter means includes a harmonic filter resistor being coupled in series relation to said harmonic filter capacitor.

25. The self-regulating electronic ballast system as recited in claim 1 where said switching means includes a transistor element having a collector, base and emitter, said transistor element being coupled to said power source, said regulation means being coupled to said induction means and said transistor element,

26. The self-regulating electronic ballast system as recited in claim 25 where said switching means includes a diode coupled in parallel relation to said transistor element base and said emitter.

27. The self-regulating electronic ballast system as recited in claim 26 where said switching means diode includes a polarity opposite to a polarity of a junction of said transistor element base and transistor element emitter.

28. The self-regulating electronic ballast system as recited in claim 1 where said power source is an AC
power source.

29. The self-regulating electronic ballast system as recited in claim 28 including rectification means for providing full wave rectification of said power source AC
voltage, said rectification means being coupled to said AC power source and said filter means.

30. The self-regulating electronic ballast system as recited in claim 29 where said rectification means includes a full wave bridge circuit.

31. A current driven gain controlled electronic ballast system having a power source for actuating at least one gas discharge tube, comprising:
(a) filter means connected to said power source for (1) establishing a substantially constant voltage signal, and, (2) suppressing harmonic frequencies generated by said electronic ballast system;
(b) induction means coupled to said filter means for generating a voltage across at least one of said gas discharge tubes responsive to a driving current, said induction means including automatic gain control means for maintaining said driving current at a predeter-mined value and having trigger means for generating a switching signal, said filter means including a correction network for substantially reducing harmonic frequencies generated by said induction means and smoothing filter means coupled in parallel relation to said correction network for maintaining said substantially constant voltage signal, said smoothing filter means including a series inductor coupled in series relation to said power source and said induction means; and (c) switching means coupled to said induction means for establishing said driving current at a substantially constant and predetermined frequency responsive to said switching signal established by said trigger means, said driving current being passed through. said induction means for generating said voltage across said gas discharge tube substantially independent of fluctuations, in a voltage signal of said power source.

32. The electronic ballast system as recited in claim 31 wherein said smoothing filter means includes:
a shunt capacitor coupled to said series inductor and said power source, said shunt capacitor being coupled in parallel relation with an output of said filter means.

33. The electronic ballast system as recited in claim 32 wherein said correction network includes:

(a) a first resistor connected in series to a first capacitor having predetermined values for substan-tially reducing predetermined harmonic frequencies, said first series resistor and capacitor combination being coupled in parallel relation to said power source;
(b) a second capacitor having a predetermined value coupled in parallel relation to said first series resistor and capacitor combination; and (c) a second resistor connected in series with a third capacitor to provide a low impedance path for predetermined harmonic frequencies, said second series resistor and capacitor combination being coupled in parallel relation to said series inductor of said smoothing filter means.

34. The electronic ballast system as recited in claim 31 where said switching means includes a pair of transistors defining a first transistor and a second transistor, said pair of transistors being coupled to said induction means.

35. The electronic ballast system as recited in claim 34 where each of said first and second transistors have a base element, a collector element, and an emitter element, each of said emitter elements of said first and second transistors being connected to said automatic gain control means.

36. The electronic ballast system as recited in claim 31 where said induction means includes:

(a) an inverter transformer coupled to said switching means and said filter means, said inverter trans-former having a tapped pair of primary windings and a multiplicity of secondary windings;
(b) a pair of coupling capacitors, each of said coupling capacitors connected in series relation to a tapped portion of one of said respective primary windings and one of said gas discharge tubes; and (c) a tuning capacitor coupled between collector elements of a pair of first and second transistors respectively;
said tuning capacitor for preventing generation of excessive voltages when one of said gas discharge tubes is removed from the circuit.

37. The electronic ballast system as recited in claim 36 where said primary windings of said inverter transformer are tapped in a manner to provide a step down auto-transformer configuration.

38. The electronic ballast system as recited in claim 36 where one of said pair of said primary windings of said inverter transformer passes a current on alternate half cycles of said predetermined frequency relative to the other of said primary windings of said inverter transformer.

39. The electronic ballast system as recited in claim 36 where said switching means includes said first and second transistors, each of said first and second transistors having a respective base element, a collector element and an emitter element, said collector elements of said first and second transistors being coupled to a respective first end of one of said primary windings of said inverter transformer.

40. The electronic ballast system as recited in claim 39 where said first and second transistor base elements are coupled to one of said multiplicity of said secondary windings of said inverter transformer.

41. The electronic ballast system as recited in claim 40 where said coupled secondary winding is connected on opposing ends thereof to said transistor base elements for establishing a trigger signal.

42. The electronic ballast system as recited in claim 41 where said coupled second winding is center tapped, said center tap being coupled to said filter means.

43. The electronic ballast system as recited in claim 41 where said secondary winding is wound in the same direction as said primary winding.

44. The electronic ballast system as recited in claim 36 where each of said primary windings of said inverter transformer have a second end, said second ends of said primary windings being coupled to said filter means.

45. The electronic ballast system as recited in claim 44 where each of said pair of coupling capacitors have a predetermined capacitive value for discharging said voltage from one of said taps of said primary windings of said inverter transformer across a respective one of said gas discharge tubes.

46. The electronic ballast system as recited in claim 36 where said inverter transformer is a ferrite core transformer.

47. The electronic ballast system as recited in claim 3 where said automatic gain control means is comprised of one pair of said multiplicity of secondary windings of said inverter transformer.

48. The electronic ballast system as recited in claim 47 where one of said pair of secondary windings comprising said automatic gain control means is coupled to said emitter element of said first transistor and said other of said secondary windings is coupled to said emitter element of said second transistor.

49. The electronic ballast system as recited in claim 48 where said pair of secondary windings of said automatic gain control means are wound in a predetermined manner to provide a negative feedback voltage to each of said emitter elements of said first and second transistors.

50. The electronic ballast system as recited in claim 49 where said pair of secondary windings are wound in the same direction as said primary windings.

51. The electronic ballast system as recited in claim 48 where said pair of secondary windings of said automatic gain control means are wound in a predetermined manner for shifting the operating point on the hysteresis curve of said inverter transformer to regulate the gain of said first and second transistors.

52. The electronic ballast system as recited in claim 51 where said pair of secondary windings are wound in the same direction as said primary windings.

53. The electronic ballast system as recited in claim 36 where at least two of said inverter transformer secondary windings are connected to opposing filaments of at least one of said gas discharge tubes.

54. The electronic ballast system as recited in claim 31 where said power source is an AC power source.

55. The electronic ballast system as recited in claim 54 including rectification means for providing full wave rectification of said power source AC voltage, said rectification means being connected to said AC power source and said filter means.

56. The electronic ballast system as recited in claim 55 where said rectification means includes a full wave bridge circuit.

57. The electronic ballast system as recited in claim 55 including secondary filter means for filtering a pulsating DC voltage from said rectification means, said filter secondary means being coupled to said rectification means.