Note: Descriptions are shown in the official language in which they were submitted.
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LOW-VOLTAGE NON-THERMIONIC BALLAST-FREE
FLUORESCENT LIGHT SYSTEM AND METHOD
BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION
The present invention relates to fluorescent ~.ighting
systems and, more particularly, to low-voltage, non-
thermionic (no heated filament) ballast-free fluorescent
lighting systems which are more efficient, less expensive,
substantially free of RF emissions and which preferably use
conventional commercial, office and home-grade fluorescent
tubes.
In most commercial and home-grade fluorescent lighting
systems, the heart of the system is the ballast, which is
an inductance or transformer device that boosts the
incoming voltage to a higher voltage level to start the
fluorescent tubes and then, once the fluorescent tubes are
lit or ignited (gas-activated, ionized or "discharged"),
reduces the voltage to a level for normal continuous
lighting. Moreover, these prior systems often use
transformer filament windings to heat the filaments to
therefore provide thermionic emission for assisting in the
ignition phase. Heated filaments vaporize and form black
deposits at the end of each tube and limit tube life.
Also, sputtered material traps fill gases and reduces gas
pressure. Early ballasted fluorescent lighting systems are
shown in Figures lA and 1B. In Figure lA the ballast unit
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L is in series with filaments F and switch S, and in Figure
1B, glow switch GS which opens after the filaments are
heated to initiate a discharge.
Ballast transformers are often the most expensive part
of commercial fluorescent lighting systems. There have
been numerous past efforts to provide fluorescent lighting
systems which do not use ballast transformers.
Electronic ballasts of the type shown in Figure 1C are
common in the art and are disclosed in International
Rectifier Publication Application Notes AN-995, "Electronic
Ballasts Using the Cost-Saving IR2155 Driver". In this
circuit, two power switches Q1, Q2 are connected in a totem
pole topology with the tube circuits consisting of an LC
series resonant circuit with the lamp across one of the
reactances. The switches are power MOSFETS driven to
conduct alternately by windings on current transformer T.
In this circuit, the primary winding is driven by current
to the lamp circuit and operates at the resonant frequency
of L and C. A starting pulse is provided by a starting
circuit comprised of resistor R1 and capacitor C1 and DIAC
D1 connected to one of the gates of one of the power
switches. After oscillation is initiated, a high frequency
square wave (30-BO kHz) excites the LC resonant circuit.
The sinusoidal voltage across the reactance C is magnified
by the Q at resonance and develops sufficient amplitude to
strike the fluorescent lamp. In this system, the filaments
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of the lamp are heated and connected in series with the
series resonant circuit.
RF driven fluorescent lamps are known in the art. In
Piejak et al U.S. Patent No. 5,325,024, a fluorescent light
source includes multiple lamp tubes driven in parallel by
a single RF source. External and internal capacitive
couplings of the electrodes to the gas filling of the tubes
is used to induce the plasma-forming discharge period.
Godyak et al U.S. Patent No. 5,300,860 discloses
capacitive coupled fluorescent lamps with RF magnetic
enhancement.
Presz et al U.S. Patent No. 4,920,299 discloses a
push-pull fluorescent dimming circuit in which high voltage
DC pulses are used to ionize the mercury argon vapor lamp.
I5 Dual outputs from the digital timing circuit control a pair
of switches in the arc voltage power supply for alternately
reversing the polarity of the arc voltage supplies.
Kerwin U.S. Patent No. 4,973,885 discloses a low
voltage direct current powered fluorescent lamp in which a
stabilized blocking oscillator circuit provides hiQh
voltage alternating current for addition and operation of
a lamp as well as power for operating a filament or heaters
included in the lamp. In Kerwin, the blocking oscillator
circuit incorporates a transformer which steps up the
voltage for ignition of the lamps.
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In Roberts U.S. Patent No. 5,359,263, a resonant
voltage rise in an LRC ballasting circuit is utilized to
increase the cyclic crest voltage for lamp ionization.
Tao et al U.S. Patent No. 5,578,907 discloses a power
supply circuit for a fluorescent lamp in which a power
supply, such as a high-voltage square wave switching power
supply, drives a current regulating inductor and a starting
capacitor. The fluorescent tube is connected across the
starting capacitor.
In Nilssen U.S. Patent No. 5,512,801, a ballast for an
instant start parallel connected lamp is provided. In this
patent, a half bridge inverter is powered from a DC voltage
and provides a 30 kHz square wave like inverter output. As
in Figure 1C, the inverter output is applied to a series
resonant LC circuit and parallel connected across the
capacitor of the LC circuit is a plural series combination
which consists of an instant start lamp series connected
with a current limiting capacitor. The magnitude of the
high current frequency voltage across the tank circuit is
controlled by controlling the frequency of the inverter
output voltage. Prior to lamp ignition, the magnitude of
the high-frequency voltage controlled to a relatively high
level to provide for sufficiently forceful lamp ignition.
Hirschman Patent No. 4,959,591 discloses a rectifier
inverter circuit for operation of fluorescent lamps in
which lamp energy is supplied via a resonant LC circuit (as
in Figure 1C) which is driven at a frequency of between 10
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and 100 kHz and preferably at about 35 kHz from a push-pull
oscillator or frequency generator formed by two
transistors, each of which have bypass diodes connected to
the cross-linked current carrying path thereof. These
switching transistors receive control voltage through
feedback windings from a feedback transformer having a
primary winding serially connected with the lamp. A step-
up converter is coupled between a starting capacitor for
the push-pull generator, and the fluorescent lamp filaments
are connected in series with the LC resonant circuit.
In Williams U.S. Patent No. 5,408,162, the combination
of a switching regulator and a high-voltage inverter
produces a high sinusoidal ignition voltage of
approximately 1400 volts peak-to-peak with low RF
emissions.
Fahnrich et al U.S. patent No. 4,808,887 discloses a
low-pressure discharge lamp operating at a high frequency
with the low induction power network which has a push-pull
frequency generator and a series resonant LC circuit (as in
Figure 1C) with a harmonic filter connected in series with
the lamp so as to assure sinusoidal operation. A similar
system for suppressing harmonics is disclosed in Farnrich
et a1 Patent No. 4,782,268.
Williams Patent No. 5,548,189 discloses a DC to AC
converter coupled to a switching regulator which converts
low DC voltage into higher sinusoidal AC voltage for
driving a fluorescent lamp. This reference points out the
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geometric nature of the magnetic step-up transformers and
seeks to reduce the transformers size by using a special
piezoelectric acoustic transformer.
Nilssen U.S. Patent No. 5,581,161 uses a voltage
multiplying circuit to raise the voltages for operating a
fluorescent lamp to 1100 to 1500 volts.
THE PRESENT INVENTION
The basic objective of the present invention is to
provide improved fluorescent lighting systems.
Another object of the present invention is to provide
a more energy-efficient fluorescent lighting system.
Another object of the invention is to provide a more
energy-efficient fluorescent lighting system which is low
in cost and operates at low voltages.
Another object of the invention is to provide a low-
voltage (from about 2 to about 85 - 90 volts) fluorescent
lighting system having a square wave voltage in the
frequency range of about 75 kHz to about 3.5 - 4 MHz.
Another objective of this invention is to provide a
fluorescent lighting system wherein one or more
conventional fluorescent tubes is non-thermionical7v
operated and driven by a low-voltage, high-frequency square
wave source.
Another objective of this invention is to provide a
fluorescent lighting system wherein multiple fluorescent
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tubes are electrically connected in series and non-
thermionically driven by a low-voltage square wave voltage.
Another object of the invention is to provide a
fluorescent lighting system in which the light intensity is
variable from low-level illumination to high-level
illumination and from high-level illumination to low-level
illumination.
According to the invention, a low voltage (under 85 -
90 volts, preferably in the range of 7 - 12 volts at about
100 kHz), non-thermionic, ballast-free, fluorescent
lighting system comprises at least one fluorescent lighting
lamp or tube (constituted by Uv-responsive phosphor-coated
envelope confining a gaseous discharge medium at a
predetermined pressure between a pair of electrodes) and a
low-voltage square wave power supply. The square wave
power supply incorporates a solid state switch which is
operated to generate a substantially square wave
alternating current wave at the lamp or tube electrodes
such that the voltage supplied to the electrodes reverses
polarity more rapidly than the pattern of electron and ion
density in the tube can shift so that electrons throughout
the length of the tube are continually accelerated and
will, through several cycles of said square wave, create
free electrons and ions throughout the tube's volume, in
steady state operation and illuminate the fluorescent
lighting lamp.
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According to a preferred embodiment of the present
invention, a fluorescent lighting system comprises at least
one fluorescent tube with electrodes (which may be
conventional filaments or not) immersed in a gaseous
discharge medium, such as argon gas and mercury vapor; and
the fluorescent tubes are non-thermionically driven with a
low-voltage, high-frequency square wave voltage. In the
preferred embodiment, the fluorescent lamp driver circuit
includes an inverter circuit using two solid state
switching devices which are connected across a direct
current supply. The gate electrode of each switch
transistor is connected in circuit with a primary winding
for each switch device and a primary winding of the
transformer. A starting circuit to start the oscillator is
utilized to provide a positive turn-on pulse to the gate
electrode of one of the transistor switches. When one of
the transistor switches turns on, its voltage is rapidly
switched to ground which starts the circuit in oscillation.
In the preferred embodiment, the oscillating frequency is
set at about 100 kHz, but the range of successful operation
runs from about 75 kHz through about 4 MHz. Since there
are no high voltages in the driver circuit, safe operation
is assured. Illumination or luminosity levels or dimming
can be achieved by varying the voltage ( or energy level )
from the direct current supply. In the preferred
embodiment, care is taken to assure that there are no spike
voltages due to inductive kick and the like since voltage
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spikes are not required for starting or operation and, if
present, may reduce operating efficiency. Since the
fluorescent lamps are non-thermionically driven, the
luminous efficiency is significantly improved. Moreover,
at the preferred high frequency, power supply components
can be smaller.
The invention has the following further features:
(1) Being Non-Thermionic
You can intermingle tubes of different ratings or
configuration, like the new "watt miser" 32 watt or the new
25 watts "energy savers" with the "standard" 40 watts (four
footers). The light output essentially remains the same
regardless of the tube rating. Today's usual shop lights
can only use 40 watt regular tubes due to the shortcomings
of the ballast as well as the use of chains to hang them
because they can be a fire hazard. Tubes that are "non
operative" on standard systems will operate (function).
(2) Being Ballast-Free
The fixture weight and operating temperature are
substantially reduced, eliminating the need for chain
hanging. The system is not a temperature driven fire
hazard.
( 3 ) Since the system is ballast-free, there is no need for
a sound rating because the system is silent. The greatly
reduced heat and weight will allow the use of a plastic
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housing, eliminating the "electric shock hazard" as well as
the need for grounding.
(4) Being of Reduced Heat.
The system can be mounted in any orientation and in
contact with standard combustible surfaces (wood,
wallpaper, etc.).
(5) The fluorescent tubes, if filamented, will keep
emitting normal light even in the event that one or both
filaments are inoperative.
14 ( 6 ) Most fluorescent arrays or multiple tube units consist
of identical tubes in parallel. The plural or multiple
tube array systems can comprise identical or different
rated tubes in series.
(7) Standard 1-1/4 and 1-1/2 inch diameter four-feet or
long fluorescent tubes filled with conventional mercury
vapor and/or argon gases and simple non-filamentary~
electrodes, and even conventional tubes with non-working or
burned out filaments can be successfully used in the
practice of this invention.
(8) Flexible plastic tubing, such as used in surgical gas
transport systems, having UV responsive phosphors
incorporated on the walls therein, Lexan~' type hard
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plastic, shatter-proof gas retention vessels with simple
discharge electrodes in the gas and fluorescent coatings on
the walls can be driven in accordance with the invention.
In such cases, the darkening of the plastic due to UV
bombardment with time can be advantageous, or the darkening
can be prevented with a Uv blocking coating.
DESCRIPTION OF THE DRAWINGS
The above and other objects, advantages and features
of the invention will become more apparent when considered
with the following specification and accompanying drawings
wherein:
Figure lA is a circuit diagram of prior art
transformer ballasted fluorescent lighting systems,
Figure 1B is prior art ballasted fluorescent lighting
systems with a glow switch starter, Figure 1C is a
circuit diagram of an electronic ballasted fluorescent
lighting system,
Figure 2A is a general block diagram of the
fluorescent lighting system incorporating the
ZO invention and_Figure 2B illustrates the various shapes
of fluorescent tubes or fluorescent gas discharge
devices to which the invention is applicable,
Figure 3 is a general block diagram of a
fluorescent lighting system incorporating a preferred
embodiment of the invention,
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Figure 4 is a general block diagram of a
fluorescent lighting system showing the same driver
system driving at least three fluorescent lamps in
series,
Figure 5 is a detailed circuit diagram of a
preferred embodiment of the invention,
Figure 6 is a circuit diagram illustrating a
further preferred embodiment of the invention, and
Figure 7 is a diagrammatic illustration of a
further embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is based on the discovery that
using a rapidly repetitive low-voltage square wave
alternating voltage, ignition may take place in fluorescent
gas discharge tubes gradually at lower voltages and power.
Since the half-cycle period of the square wave alternating
voltage power according to the invention is very short (of
the order of 5 microseconds for 100 kHz), there is very
little opportunity for decay of the plasma between half-
cycles. At start-up, ambient free electrons in the gas
increase in energy in a half-cycle more than they lose
energy due to collision processes. According to the
invention, during one half-cycle, an electron will move in
a roughly constant electric field. During each interval
between collisions with neutral atoms, or ions, its kinetic
energy will increase if its previous collision left it
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traveling with a component of velocity in the direction of
the acceleration produced by the electric field. It will
decrease if its previous collision left it moving without
a component of velocity opposed to the field's
acceleration. According to the invention, the square wave
alternating supply voltage serves principally to raise the
effective electron energy (or temperaturey. The current
flowing consists of electrons flowing to the instantaneous
anode and positive ions flowing to the instantaneous
cathode where they recombine with electrons and are
released as neutral atoms. Total gas pressure in the tube
is sufficient to make the mean free path considerably less
than the tube diameter and much less than its length. Most
electrons and ions separate and recombine, in a small
fraction of the overall length of the tube, rather than
flowing as continuous streams along its axis. The
important thing for ionization of the light emitting gas
atoms is that the electron energy continues to increase
even though the field reverses periodically.
The operation of RF discharged at such low field
strengths that the electrons cannot accelerate to
ionization energy level in one-half cycle demonstrates that
the process is cumulative over many half cycles.
The biggest problem in 60-Hz lamps is that ion and
electron densities essentially virtually go to zero at the
end of each half-cycle. To achieve light output again
after a few milliseconds requires an active supply of
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electrons (the filament) with high heating power for that
filament. But, then, if the lamp system of the present
invention starts at voltage levels far below that usually
associated with plasma ~~breakdown«, why does an equally low
voltage applied constantly across a single tube not result
in the same glowing plasma?
This can be explained in terms of the natural tendency
of particles of a plasma subject to a static external field
to move so as to create a space charge pattern and field
that counteracts the applied field. The result of applying
a voltage between two electrodes is to induce positive
charge on the positive electrode and negative charge on the
negative electrode, the absolute amount of charge depending
on course on the capacitance between the two.
If free electrons and ions fill the space between
these electrodes, the electrons are pulled toward the
anode, and the positive ions toward the cathode, until in
the space between there is no longer a field and therefore
no means to cause further movement of the particles; a
voltage drop, that is, region of high field, will exist
very close to each of the two electrodes. The electrons
(and ions) in the main part of the tube will not be further
affected by the field; when electrons reach the high field
region near the anode, they will probably be accelerated to
half the applied voltage within less than one mean free
path of the anode's surface and hence will be unlikely to
produce ionization.
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In the fluorescent lamp system of this invention, the
applied square wave voltage is alternated rapidly enough
that the charged particles cannot move enough to accumulate
near cathode and anode during a half-cycle of the applied
voltage. Thus, the field remains almost continuously
active in accelerating electrons within the main body of
the tube.
Figure 2A is a schematic block diagram of a
fluorescent lighting system incorporating the invention.
A direct ~ current ( DC ) power supply 200 is protected by a
fast-acting fuse 201 and/or a crowbar circuit 202 which
provides fast-acting protection of the circuit in the event
of a fault. The DC voltage is applied to square wave
inverter circuit 203 which converts the DC voltage to a
low-voltage (between about 3 to about 20 volts RMS), AC
square wave voltage having a high-frequency (between about
75 kHz and about 4 MHz) which is applied to electrodes 204
and 205 of a fluorescent gas discharge device 208. The
current is very low so in comparison with light output
equivalent to a conventional 60 Hz, thermionically operated
fluorescent tube or lamp, the luminous efficiency is
significantly improved. Moreover, the fluorescent lamp
or tube can be straight, folded or looped as indicated in
Figure 28. A rheostat 2008 can be used to adjust or vary
the voltage or energy level from the source 203 to gas
discharge device 208 and thereby dim or vary the level of
luminosity from the lamp. Since the system does not depend
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on a large ignition voltage level, the luminosity can be
varied from low to high and back to low. In contrast,
most conventional dimming circuits for fluorescent lamps
require starting with a relatively high luminosity or level
of illumination and then reducing the level to a desired
point.
Figure 3 is a schematic block diagram illustrating two
fluorescent tubes 301 and 302 driven by low-voltage, high-
frequency square wave inverter circuit 303. Note that the
tubes 301 and 302 are connected in series so that while the
square wave inverter circuit 303 can be of the same
capacity as the square wave inverter circuit 202, if tubes
301 and 302 have the same length and diameter as
fluorescent gas discharge device 206, the volume of gas is
essentially doubled. Note that the devices 301 and 302 are
non-thermionically driven, even though the tubes may
incorporate conventional filaments (not shown).
Figure 4 is a schematic block diagram illustrating
three or more fluorescent tubes 401, 402...40N driven by a
low-voltage, high-frequency, square wave inverter circuit
404. In this case, the serially connected fluorescent
tubes 401, 402...40N, the middle tubes 402...are not
directly connected to the output terminals of the inverter
circuit 404. The total volume of gas driven is the sum of
the volumes of gas in the individual of fluorescent gas
discharge devices, none of which is thermionically
energized or excited. If all of the fluorescent tubes are
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identical or essentially the same they all illuminate with
equal intensity. However, tubes of different lengths,
diameter and gas pressures have been successfully operated
in series.
A preferred embodiment of the system for driving two
FT40 fluorescent tubes in series is illustrated in Figure
5. The component values and component types are merely
exemplary. This produces a square wave voltage at about
100 kHz with sharp transitions such that the voltage
supplied to the lamp L1 electrode LE1 and lamp L2 electrode
LE2 reverses polarity more rapidly than the pattern of
electron and ion density in the gaseous volume can shift so
that electrons throughout the length of the tube are
continually oscillated and will, through several cycles of
the square wave, create ions throughout the tubes, gaseous
volume, in steady state operation.
In this embodiment of the invention, alternating
current (120 VAC for example) is applied through a fast-
acting fuse 10 to terminals 11 and 12 of full wave bridge
rectifier 13 which provides DC voltage which is filtered by
an electrolytic capacitor 15. In this embodiment fast-
acting fuse 10 or a crowbar circuit at the output is
required to prevent damage to the circuit if the lamp is
removed from the circuit. High-frequency filter capacitor
16 is connected across the AC input to the bridge rectifier
13. Other sources of direct current voltage, such as
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batteries, solar cells, etc., may be used to provide
operating energy.
The fluorescent lamp driver comprises an oscillator
circuit using two solid state switching devices or
transistors Q3 and Q4 (HEXFET~S, IRF624). The switching
transistors Q3 and Q4 are connected in totem-pole-fashion
across the direct current supply lines 17 (+) and 18 (- or
ground). The gate electrode G circuit of each switch
driver Q3 and Q4 is connected in circuit with a primary
winding PW1 (twenty-five turns) for switch device Q3 and
primary winding PW2 (twenty-five turns) for switch device
Q4.
Resistor 20 and capacitor 21, with DIAC 22 form a
starting circuit for the lower transistor switch Q3. In
this embodiment, when the DIAC 22 reaches about 35 volts,
a positive turn on pulse is applied to gate G1 of the lower
switch device Q3. When switch Q3 switches on, the drain
voltage is rapidly switched to ground which starts circuit
oscillation. Current flowing through the two turn primary
winding PW3 provides gate drive voltages for switching the
switch devices Q3 and Q4. This causes the circuit to
oscillate at about 100 kHz. Primary winding PW3 speeds up
switching of the switches Q3 and Q4 by an order of
magnitude. This is caused by a feedback switching action
speeding up the switching operation of switches Q3 and Q4.
Figure 6 illustrates a low-voltage square wave
inverter circuit requiring a minimum of five components
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(the electrolytic filter capacitor C1 is deemed to be a
part of the DC power source or supply). Switch 57 couples
DC voltage (7.2 volts for example) from a battery to the
low-voltage square wave inverter drive circuit 701 via
dimmer resistor 702 and filter capacitor 701. This driver
circuit includes an oscillation transformer 702 having a
center tapped primary winding 704 having primary winding
705 and 706 with the center top 707 connected to gate
electrode 708 of oscillating diode transistor 709. The
opposing ends of oscillating diode D1 are connected to the
upper and lower ends of the primary windings 705 and 706.
A capacitor shunts the oscillating transistor/diode 709.
The exemplary circuit components are as follow:
Fluorescent tube FT6
Resistor R1 1500 Ohms
Capacitor C1 47UF lOV Electrolytic
Transistor diode 709 5609/6BC/ECB
Capacitor 711 2A562K
Capacitor 712 2A22K
The output to the fluorescent tube is about 1.4 volts
RMS at 3.9 MHz open circuit and 1.7 MHz, square wave at the
tube. Thus, the system has no ballast transformer, no
thermionic heating of filaments, no starter circuit, and
produces light in a more energy-efficient way.
Figure 7 diagrammatically illustrates a
transformerless square wave inverter circuit. Here, the
positive (+) and negative (-) terminals of a direct current
source are alternately connected to opposing electrodes of
the fluorescent lamp ( s ) . In this case, when switches S3 and
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S4 are closed simultaneously or at the same time
( preferably by the same signal from controller CONT, the
positive terminal (+) is connected to electrode 8-1 and the
negative terminal (-) is connected directly to electrode
8-2. When the switches Sl and S2 are simultaneously closed
(and switches S3 and S4 are open) by controller CONT, the
positive terminal (+) is connected directly to lamp
electrode 8-2 and the negative terminal (-) is connected to
fluorescent lamp electrode 8-1. Controller CONT can operate
the switches in the range of about 75 kHz to about 3.9 MHz
and preferably operates the switches to cause the square
wave applied to lamp electrodes 8-1 and 8-2 to be at a
frequency of about 100 kHz.
In this invention, the magnitude of the alternating
I5 voltage at the electrodes is of small significance in
initiating the discharge reaction, allowing the capability
to start the production of visible light at a low or high
intensity -- since the light generated is in direct
proportion to the total energy input . ( There is no need
for a large "starting strike" voltage to ionize the gas.)
Experiments with a transparent Phillips mercury vapor
electric discharge lamp model H39KB-175 (175 watts)
connected to the 1.2 watt driver (shown in Figure 6) shows
the same behavior and characteristics of the fluorescent
application. It is believed that the reaction starts at
one end of the tube and rapidly extends to the other or far
end and then gets stable. Experiments connecting only one
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electrode supports this theory. This is why several lamps
in series can be used, because the field reversal is
achieved before the original distal reaction in the tube
collapses.
While preferred embodiments of the invention have been
described and illustrated, it will be appreciated that
other embodiments, adaptations and modifications of the
invention will be readily apparent to those skilled in the
art.
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