Capacitive lamp out detector

An electronic ballast includes a boost circuit, a low voltage control circuit, and a driven inverter in a half-bridge, push-pull, series resonant, parallel loaded configuration. The boost circuit includes a low voltage output for powering the control circuit. In the event of a fault, the control circuit shuts off the boost circuit and the inverter. A sense capacitor is in series with the lamps and the voltage across the capacitor prevents a timer in the control circuit from shutting off the ballast. The timer circuit waits a period longer than the time it takes for the lamps to start normally. The sense capacitor is either in series with the lamp across the resonant capacitor or is in series with the resonant capacitor.

BACKGROUND OF THE INVENTION 
This invention relates to electronic ballasts for fluorescent lamps and, in 
particular, to electronic ballasts which stop operating in response to a 
fault condition such as a defective lamp or a missing lamp. 
A gas discharge lamp, such as a fluorescent lamp, is a non-linear load to a 
power line, i.e. the current through the lamp is not directly proportional 
to the voltage across the lamp. Current through the lamp is zero until a 
minimum voltage is reached, then the lamp begins to conduct. Once the lamp 
conducts, the current will increase rapidly unless there is a ballast in 
series with the lamp to limit current. 
A resistor can be used as a ballast but a resistor consumes power, thereby 
decreasing efficiency, measured in lumens per watt. A "magnetic" ballast 
is an inductor in series with the lamp and is more efficient than a 
resistor but is physically large and heavy. A large inductor is required 
because impedance is a function of frequency and power lines operate at 
low frequency (50-60 hz.) 
An electronic ballast typically includes a rectifier for changing the 
alternating current (AC) from a power line to direct current (DC) and an 
inverter for changing the direct current to alternating current at high 
frequency, typically 25-60 khz. Since the frequency of the inverter is 
much higher than 50-60 hz., the inductors for an electronic ballast are 
much smaller than the inductor in a magnetic ballast. 
Converting from alternating current to direct current is usually done with 
a full wave or bridge rectifier. A filter capacitor on the output of the 
rectifier stores energy for powering the inverter. The voltage on the 
capacitor is not constant but has a 120 hz "ripple" that is more or less 
pronounced depending on the size of the capacitor and the amount of 
current drawn from the capacitor. 
Some ballasts include a boost circuit between the rectifier and the 
inverter. As used herein, a "boost" circuit is a circuit which increases 
the DC voltage, e.g. from approximately 180 volts (assuming a 120 volt 
input) to 300 volts or more for operating a lamp, and/or which provides 
power factor correction. "Power factor" is a figure of merit indicating 
whether or not a load in an AC circuit is equivalent to a pure resistance, 
i.e. indicating whether or not the voltage and current are sinusoidal and 
in phase. It is preferred that the load be the equivalent of a pure 
resistance (a power factor equal to one). 
If a lamp is not connected to an electronic ballast while power is applied 
to the ballast, the voltages and currents within the ballast can become 
extremely high, destroying the ballast. In addition, if a lamp is 
disconnected from a ballast, the person disconnecting the lamp is exposed 
to the high voltages of the ballast, e.g. by touching the terminals at one 
end of the lamp while the other end of the lamp is connected to the 
ballast. Many ballasts are designed to generate extra high voltages 
initially, to assure an instantaneous or a rapid start of a lamp, then to 
reduce the voltage when the lamp is conducting. When a lamp is removed, 
the circuitry within such ballasts reverts to a start-up mode and produces 
an extra high output voltage at the very time a person may be touching the 
terminals of the lamp. 
Some electronic ballasts include a transformer in the output stage to 
isolate the lamp circuit from electrical ground. An isolation transformer 
makes an electronic ballast heavy and expensive. This invention relates to 
electronic ballasts having what is known as a "direct coupled output", 
i.e. a path exists between the high voltage supply within the inverter and 
the output terminals of the inverter during each cycle of the AC output. 
U.S. Pat. No. 5,004,955 (Nilssen) discloses an electronic ballast having a 
direct coupled output. The ballast includes a half-bridge, triggered, 
series resonant, parallel loaded inverter in which lamp current is sensed 
by a transformer winding in series with the lamps. A lack of current is 
interpreted as a fault and the inverter is disabled. The inverter is 
triggered on or off using RC timing circuits and attempts a re-strike 
every couple of seconds. 
A boost circuit or the inverter, or both, can be self-oscillating, 
triggered, or driven. A driven circuit requires a source of pulses for 
operation and the pulses are provided by a timer circuit or a more 
complicated integrated circuit designed for ballasts or electronic power 
supplies. A triggered circuit typically incorporates a small pulse 
generator for starting the circuit into oscillation. A capacitor charging 
up to the firing voltage of a diac or other semiconductor switch is 
typically used in such circuits, e.g. the Nilssen patent. The pulse 
generator may or may not be disabled when the ballast is operating 
normally. A self-oscillating circuit is constructed in such a way that the 
applied voltage causes the circuit to begin oscillation and typically 
includes a resistor having a high resistance to provide a temporary bias 
for initiating oscillation. 
U.S. Pat. No. 5,111,114 (Wang) discloses an electronic ballast having a 
direct coupled output. The ballast includes a half-bridge, triggered, 
series resonant, parallel loaded inverter in which lamp current is sensed 
as the voltage drop across an inductor in series with the lamps. Excess 
voltage is interpreted as a fault and the inverter is disabled. The 
inverter is triggered into oscillation using an RC/diac timing circuit in 
which the capacitor is discharged through a parallel transistor if a fault 
is sensed. An RC circuit controlling the parallel transistor has a longer 
time constant than the time constant of the RC/diac circuit. The Wang 
patent also discloses a large capacitor in series with the lamps, "large" 
being defined as at least ten times the capacitance of the resonant 
capacitor. 
There are many other circuits described in the prior art for automatically 
shutting off a ballast in the event of a fault. There remains a need for 
an efficient ballast having automatic shut off capability, i.e. a ballast 
which dissipates little power in normal operation and in a shut down mode. 
In view of the foregoing, it is therefore an object of the invention to 
provide an electronic ballast including a capacitive sensor for detecting 
a missing or defective lamp. 
A further object of the invention is to provide an electronic ballast which 
automatically reduces the output voltage in the event of a fault without 
dissipating a large amount of power. 
Another object of the invention is to provide an electronic ballast 
including automatic shut-off circuitry which dissipates very little power 
either during a fault condition or during normal operation of the ballast. 
SUMMARY OF THE INVENTION 
The foregoing objects are achieved in an electronic ballast including a 
boost circuit, a low voltage control circuit, and a driven inverter in a 
half-bridge, push-pull, series resonant, parallel loaded configuration. 
The boost circuit includes a low voltage output for powering the control 
circuit. In the event of a fault, the control circuit shuts off the boost 
circuit and the inverter. A sense capacitor is in series with the lamps 
and the voltage across the capacitor prevents a timer in the control 
circuit from shutting off the ballast. The timer circuit waits a period 
longer than the time it takes for the lamps to start normally. In one 
embodiment, the sense capacitor is in series with the lamps across the 
resonant capacitor and in another embodiment the sense capacitor is in 
series with the resonant capacitor.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates the major components of an electronic ballast for 
connecting fluorescent lamp 10 to an AC power line, represented by 
waveform 11. The electronic ballast in FIG. 1 includes boost circuit 12, 
energy storage capacitor 13, and inverter 14. Boost circuit 12 increases 
the DC voltage from the rectifier and stores it on capacitor 13. Inverter 
14 is powered by the energy stored in capacitor 13 and provides a high 
frequency, e.g. 30 khz, alternating current to lamp 10. 
AC input 15 includes bridge rectifier 16 having DC output terminals 
connected to capacitor 17 by rails 18 and 19. If rectifier 16 were simply 
connected to capacitor 13, then the maximum voltage on capacitor 13 would 
be equal to approximately 1.4 times the r.m.s. value of the applied 
voltage. Instead, the voltage on capacitor 17 is increased to a higher 
voltage by a boost circuit including inductor 21, transistor Q.sub.1, and 
diode 23. When transistor Q.sub.1 is conducting, current flows from rail 
18 through inductor 21 and transistor Q.sub.1 to rail 19. When transistor 
Q.sub.1 stops conducting, the field in inductor 21 collapses and the 
inductor produces a high voltage which adds to the voltage from bridge 
rectifier 16 and is coupled through diode 23 to capacitor 13. Diode 23 
prevents current from flowing back to transistor Q.sub.1 from capacitor 
13. 
A pulse signal must be provided to the gate of transistor Q.sub.1 in order 
to periodically turn Q.sub.1 on and off to charge capacitor 13. Inductor 
26 is magnetically coupled to inductor 21 and provides feedback to the 
gate of transistor Q.sub.1, causing transistor Q.sub.1 to oscillate at 
high frequency, i.e. a frequency at least ten times the frequency of the 
AC power line, e.g. 30 khz. 
Resistor 27, in series with the source-drain path of transistor Q.sub.1, 
provides a feedback voltage which is coupled to the base of transistor 
Q.sub.2. When the voltage on resistor 27 reaches a predetermined 
magnitude, transistor Q.sub.2 turns on, turning off transistor Q.sub.1. 
Resistor 27 typically has a small value, e.g. 0.5 ohms. Zener diode 31 
limits the voltage on the gate of transistor Q.sub.1 from inductor 26 and 
capacitor 32 and resistor 33 provide pulse shaping for the signal to the 
gate of transistor Q.sub.1 from inductor 26. 
In inverter 14, transistors Q.sub.3 and Q.sub.4 are series connected 
between rails 18 and 19 and conduct alternately to provide a high 
frequency pulse train to lamp 10. Inductor 41 is series connected with 
lamp 10 and is magnetically coupled to inductors 42 and 43 for providing 
feedback to transistors Q.sub.3 and Q.sub.4 to alternately switch the 
transistors. The oscillating frequency of inverter 14 is independent of 
the frequency of boost circuit 12 and is on the order of 25-50 khz. The 
arrangement of inverter 14 is known in the art as a half-bridge, push-pull 
inverter. 
As illustrated in FIG. 1, neither boost circuit 12 nor inverter 14 is 
self-starting, triggered, or driven. Circuitry for starting oscillation is 
omitted for clarity in describing the basics of one type of ballast. In 
its simplest form, such circuitry is a high resistance connected between 
the high voltage rail and the base of transistor Q.sub.1 and a high 
resistance connected between the high voltage rail and the base of 
transistor Q.sub.3. These two resistors would make boost circuit 12 and 
inverter 14 self-starting. 
FIG. 2 illustrates a boost circuit constructed in accordance with a 
preferred embodiment of the invention in which the boost circuit provides 
both low voltage, e.g. five volts, for powering other components of the 
ballast, and high voltage, e.g. 300 volts, for powering one or more lamps. 
Some elements in FIGS. 2 are drawn in heavier line to facilitate reading 
the schematic. 
Inductor 51 is magnetically coupled to inductors 21 and 26. The voltage 
induced in inductor 51 therefore includes a high frequency component from 
the operation of transistor Q.sub.1 and a low frequency component from the 
ripple voltage. The voltage from inductor 51 is coupled to a ripple 
detector including diode 53 and capacitor 55. The rectified voltage on 
capacitor 55 is coupled to the control electrode of transistor Q.sub.2 by 
resistor 56. This portion of the circuit significantly improves power 
factor and harmonic distortion. 
The boost circuit also includes diode 61 connected to inductor 51 and 
capacitor 62 connected between diode 61 and rail 19. The junction between 
diode 61 and capacitor 62 is brought out on line B. The output from 
capacitor 62 is a filtered, DC voltage, e.g. 5 volts, for powering other 
components within the ballast. 
Resistor 64, connected between high voltage rail 65 and the gate of 
transistor Q.sub.1, provides a DC path through the boost circuit for 
causing the boost circuit to begin oscillation. Resistor 64 has a high 
resistance, e.g. 660,000 ohms, and is of negligible effect once the boost 
circuit is oscillating. The boost circuit oscillates during each half 
cycle of the rectified input voltage, i.e. the boost circuit restarts 120 
times per second with the bias provided from resistor 64. Line A is 
coupled to the base of transistor Q.sub.2 through diode 67 and resistor 
68. As more fully described with FIG. 4, a high voltage on line A turns on 
transistor Q.sub.2 and quenches oscillation of the boost circuit. 
In FIG. 3, transistors Q.sub.5 and Q.sub.6 are connected in series between 
high voltage rail 65 and ground rail 19. One side of inductor 71 is 
connected to the junction of transistors Q.sub.5 and Q.sub.6. Capacitor 72 
is connected between the other side of inductor 71 and ground, forming a 
series resonant LC circuit. Lamp 73 is connected in parallel with resonant 
capacitor 72. Inductors 74 and 75 are magnetically coupled to inductor 71 
and provide power for the filaments of lamp 73. Transistors Q.sub.5 and 
Q.sub.6 alternately conduct at a frequency determined by a driving circuit 
(FIG. 4) which is magnetically coupled to transistors Q.sub.5 and Q.sub.6 
by inductors 78 and 79. 
Capacitor 81 is connected in series with lamp 73 across resonant capacitor 
72. The voltage drop across capacitor 81 is coupled to line D by diode 86 
and resistor 87. Line D is connected to the low voltage or control portion 
of the ballast, illustrated in FIG. 4. When lamp 73 is connected to the 
ballast and the ballast is operating normally, the voltage across 
capacitor 81 is approximately one-half the voltage between rail 65 and 
rail 19. In the absence of a lamp, or if a lamp is defective, then the 
voltage across capacitor 81 is considerably lower or zero. This low 
voltage is detected by the control circuit and the ballast is shut-off. 
Diode 86 and resistor 87 draw a small direct current through lamp 73, 
causing a slight asymmetry in the current through the lamp and causing the 
D.C. voltage on capacitor 81 to be slightly less than one-half the voltage 
between rail 65 and rail 19. This is not detrimental but is an advantage 
because the small direct current prevents striations when dimming the lamp 
to low light levels. Providing alternating current and direct current 
while dimming a lamp is known in the art; see, for example, U.S. Pat. Nos. 
3,621,331 (Barron) and 4,862,042 (Herrick), the latter having been 
published on Nov. 6, 1986, as PCT application No. PCT/US86/00581. 
Resistor 82 is in series with transistors Q.sub.5 and Q.sub.6 and converts 
the current through transistor Q.sub.6 to a voltage which is coupled to 
line C by diode 84 and resistor 85. Line C is also connected to the 
control portion of the ballast and an excessively high voltage across 
resistor 82 causes the ballast to shut off. 
FIG. 4 is a schematic of the low voltage or control portion of a ballast 
constructed in accordance with a preferred embodiment of the invention and 
illustrates the circuitry powered from line B (FIG. 2). Lines A and B of 
FIG. 4 correspond to lines A and B of FIG. 2. Lines C and D of FIG. 4 
correspond to lines C and D of FIG. 3. 
In FIG. 4, driver circuit 91 is powered from line B and produces a local, 
regulated output voltage which drives rail 92 to approximately 5 volts. In 
one embodiment of the invention, driver circuit 91 was a 2845 pulse width 
modulator circuit. Pin 1 of driver circuit 91 is indicated by a dot and 
the pins are numbered consecutively clockwise. The particular chip used to 
implement the invention included several capabilities which are not 
needed, i.e. the invention can be implemented with a much simpler 
integrated circuit such as a 555 timer chip. 
Pin 1 of driver circuit 91 relates to an unneeded function and is tied 
high. Pins 2 and 3 relate to unneeded functions and are grounded. Pin 4 is 
the frequency setting input and is connected to the junction of resistor 
93 and capacitor 94. Pin 5 is electrical ground for driver circuit 91 and 
is connected to rail 19. Pin 6 of driver circuit 91 is the high frequency 
output and is coupled through capacitor 96 to inductor 97. Inductor 97 is 
magnetically coupled to inductor 78 and to inductor 79 (FIG. 3). As 
indicated by the small dots adjacent each inductor, inductors 78 and 79 
are oppositely poled, thereby causing transistors Q.sub.5 and Q.sub.6 to 
switch alternately at a frequency determined by resistor 73, capacitor 74, 
and the voltage on rail 92. 
Pin 7 of driver circuit 91 is connected to line B, the low voltage output 
of the boost circuit in FIG. 2. Pin 8 of driver circuit 91 is a voltage 
output for providing bias to the frequency determining network including 
resistor 93 and capacitor 94 which are series-connected between rail 92 
and rail 19. Pin 8 is connected to rail 92 to provide voltage for the rest 
of the circuitry illustrated in FIG. 4. 
When power is applied to the ballast, the boost circuit produces both a 
high voltage output and a low voltage output. The low voltage output is 
coupled by line B to driver circuit 91 which powers rail 92. Initially, 
transistors Q.sub.7, Q.sub.8, and Q.sub.9 are non-conducting. As soon as 
driver circuit 91 begins operation and produces a voltage on rail 92, 
current flows through a first timer circuit including resistor 110 and 
capacitor 116. Capacitor 116 charges to a voltage determined by the 
voltage divider including series connected resistors 110 and 112 and this 
voltage is sufficient to turn on transistor Q.sub.8. When transistor 
Q.sub.8 turns on, transistor Q.sub.7 is turned on. 
Resistor 101 and transistor Q.sub.7 are series-connected between rails 92 
and 19. When transistor Q.sub.7 is nonconducting, resistor 101 is 
connected in parallel with resistor 93 through diode 103. When resistor 
101 is connected in parallel with resistor 93, the combined resistance is 
substantially less than the resistance of resistor 93 alone and the 
frequency of operation of driver circuit 91 is substantially higher than 
the resonant frequency of the LC output circuit. At this point, the output 
voltage across the resonant capacitor is not high enough to start lamp 73 
(FIG. 3). However, power flowing through inductor 71 is coupled to the 
filaments of lamp 73 by inductors 74 and 75. After the lamp has warmed up 
a predetermined length of time, e.g. 0.75 seconds, transistor Q.sub.7 
conducts, thereby reverse biasing diode 103 and disconnecting resistor 101 
from resistor 93. When diode 103 is reverse biased, the current into 
capacitor 94 is substantially reduced, the frequency of driver circuit 91 
decreases to approximately the resonant frequency of inductor 71 and 
capacitor 72, and the output voltage across capacitor 72 increases. 
when transistor Q.sub.8 conducts, current flows through a shut-off timer 
including capacitor 120 and series connected resistor 121. If no lamp is 
connected to the ballast, capacitor 120 charges to a voltage determined by 
the voltage drop across resistor 133, turning on transistor Q.sub.10. When 
transistor Q.sub.10 turns on, line A is coupled to rail 92 and current 
flows through diode 141 to the base of transistor Q.sub.9, turning on 
transistor Q.sub.9. 
Referring to FIG. 2, line A is coupled through resistor 68 and diode 67 to 
the base of transistor Q.sub.2. The positive voltage on line A from rail 
92 turns on Q.sub.2, thereby turning off Q.sub.1 and shutting off the 
boost circuit. With the boost circuit shut off, the voltage on line B 
decays and driver circuit 91 ceases operation, shutting off the inverter 
(FIG. 3). Since driver circuit 91 is turned off, the voltage on rail 92 
collapses, shutting off SCR 130. 
The control circuit does not require a holding current for SCR 130 to 
prevent the inverter and the boost circuit from operating. On the 
contrary, the operating voltage is removed from SCR 130, turning off the 
SCR and preventing the SCR from latching on. Cascaded timer circuits 
prevent the ballast from turning on immediately. The shut-off mechanism is 
entirely within the low voltage, low power portion of the ballast, further 
reducing power consumption when the ballast is shut off. 
Referring to FIG. 4, the shut-off circuit including transistor Q.sub.10 is 
prevented from turning off the ballast by an opposing current from line D. 
As illustrated in FIG. 3, line D is coupled through diode 86 and resistor 
87 to the junction between lamp 73 and capacitor 81. With lamp 73 in place 
and operating normally, capacitor 81 charges to approximately half the 
voltage between rail 65 and rail 19. 
Resistor 87 (FIG. 3) and diode 86 provide a resistive current path from 
sense capacitor 81 to capacitor 120 (FIG. 4). The current from line D 
opposes and is greater than the charging current through Q.sub.8, causing 
the net voltage across capacitor 120 to forward-bias diode 131 as 
capacitor 120 charges from line D. When diode 131 is forward-biased, then 
the base-emitter junction of transistor Q.sub.10 is reverse-biased, 
transistor Q10 is rendered non-conducting, and the shut-off circuit is 
reset. Since transistor Q.sub.10 is non-conducting, the gate of SCR 130 is 
not coupled to rail 92, the SCR remains non-conducting, and the inverter 
continues to operate. The net voltage across capacitor 120 determines 
whether or not the ballast is turned off. 
Referring to FIG. 3, the current through transistors Q.sub.5 and Q.sub.6 is 
converted into a voltage by series resistor 82. This voltage is coupled to 
line C by resistor 85 and diode 84. Line C is directly connected to the 
gate of SCR 130 and an excess voltage across resistor 82 causes SCR 130 to 
conduct, shutting off the ballast. This portion of the circuit protects 
the ballast when a lighted lamp is removed from an operating ballast. When 
the lamp is removed, the output voltage of the inverter rises swiftly and 
the corresponding voltage across resistor 82 triggers SCR 130, raising the 
operating frequency instantly, before the shut-off circuit can operate, 
thereby lowering the output voltage to a safe level. 
As used herein, shutting off the ballast means reducing the output voltage 
either to a safe level or to zero volts. When SCR 130 conducts, 
transistors Q.sub.9, Q.sub.8, and Q.sub.7 cause the frequency of the AC 
output to increase, thereby reducing the voltage drop across the resonant 
capacitor. Even if the boost circuit is turned off, the charge on 
capacitor 62 (FIG. 2) is sufficient to power driver circuit 91 for forty 
milliseconds or so. Thus, the frequency is raised to protect someone 
coming in contact with the ballast. Driver circuit 91 is turned off to 
prevent SCR 130 from latching. When driver circuit 91 turns off, the 
output voltage from the ballast goes to zero. After a predetermined delay, 
the start-up sequence begins. 
In FIG. 4, when SCR 130 conducts, diode 141 is forward-biased and current 
flows through capacitor 143 and resistor 145. The voltage drop across 
resistor 145 causes the base-emitter junction of transistor Q.sub.9 to 
become forward-biased and transistor Q.sub.9 conducts, connecting the base 
of transistor Q.sub.8 to rail 19 and discharging capacitor 116. Even after 
driver circuit 91 ceases operation, capacitor 143 keeps transistor Q.sub.9 
conducting, thereby preventing the ballast from restarting for a period 
determined by the RC time constant of a second timer circuit including 
capacitor 143 and resistors 145 and 146. Because the timing circuits are 
cascaded, the periods defined by the first and second timing circuits are 
consecutive and add up to a delay in excess of one second. 
After capacitor 143 has discharged, transistor Q.sub.9 turns off, 
permitting capacitor 116 to begin charging. If the AC input voltage has 
not been interrupted, the boost circuit will attempt to restart the driver 
circuit by producing a voltage on line 8. However, because transistor 
Q.sub.9 is not conducting, transistor Q.sub.7 is not conducting and the 
output frequency of the inverter is considerably higher than the resonant 
frequency of inductor 71 and capacitor 72 (FIG. 3). Thus, the output 
voltage across capacitor 72 is quite low. 
When transistor Q.sub.9 stops conducting, capacitor 116 can begin to 
charge, thereby turning on transistor Q.sub.8 and transistor Q.sub.7, 
reducing the frequency of drive circuit 91 approximately to the resonant 
frequency of the LC output and increasing the voltage across capacitor 72. 
If the fault condition still exists, the ballast shuts off again within 25 
milliseconds, attempts a re-strike after about 1.5 seconds and the cycle 
continues until the fault is corrected or the AC input voltage is 
interrupted. 
FIG. 5 illustrates an alternative embodiment of the invention in which the 
sense capacitor is connected in series with the resonant capacitor. 
Capacitor 81 is connected between resonant capacitor 72 and rail 19. The 
output voltage from capacitor 81 is coupled by resistor 87 and diode 86 to 
line D as previously described. The operation of the embodiment of FIG. 5 
is similar to that of FIG. 3 except that a smaller voltage is produced 
across capacitor 81. 
The invention thus provides an improved electronic ballast in which fault 
sensing is accomplished with very little power dissipation either during a 
fault condition or during normal operation of the ballast. Unlike some 
ballasts of the prior art in which a triggered SCR must be furnished with 
a sustaining current, the invention relies on the charge stored in 
capacitors to shut off a low voltage, low power portion of the ballast, 
thereby disabling the entire ballast. Cascaded timing circuits provide the 
desired turn-on and turn-off characteristics to provide safe operation. 
Having thus described the invention, it will be apparent to those of skill 
in the art that various modifications can be made within the scope of the 
invention. For example, the invention can be used with more than one lamp 
in series. While described in connection with a self-oscillating boost 
circuit, it is understood that a triggered or driven boost circuit can be 
used instead. The timing circuits can be changed to suit a particular 
application or to suit changing governmental or quasi-governmental 
regulations.