Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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T~NSIENT CONTROL CIRCUIT FO~
~LUO~ESCENT LAMP SYSTEMS
Field of the Invention
The present invention relates to control systems or
fluorescent lamps and, more particularly, to an improved start-
ing method and system for such lamps which reduces the effect
of transients and extends the life of the lamp cathodes.
Background of the Invention
It is generally recognized by those skilled in the art of
electrical dimming control systems for fluorescent lamps that
the externally heated cathodes of rapid start type fluorescent
lamps must be heated to a temperature that permits the required
level of thermionic electron emission to be achieved. For this
reason, such fluorescent lamp dimming control systems usually
provide for initially turning the lamps "full-on~ so that the
rated arc current flows, before dimming, i.e., reduction of the
arc current, is undertaken. Such full-on ignition of the lamps
is generally accomplished by applying full rated line voltage
to the standard transformer-ballast u~ually employed as the
lamp driver. This approach is described, for example, in U.S.
patent 3,350,935 (see column 12 lines 27-35) and ~.S. patent
3,352,045 (see column 7 starting at line 5). When full rated
A.C. line voltage is applied to the ballast driving the ~luor-
escent lamp load, the cathode heating voltaye a3 well as the
necessary arc striking voltage appear at the lamp electrodes at
the speciiied nominal magnitudes. After a short heating peri-
od, the cathode begins to emit electrons, and the arc thereaf-
ter i~nites and extinguishes one or more times before the cath-
ode reaches the temperature at which the thermionic emission
provided is capable of sustaining the arc at the rated cur-
.
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rent. This initial arc-on/arc-off operation causes the cathode
to ~sputter~ which substantially contrlbutes to cathode wear.
The term "sputtering~ as used here refers to the actual physical
emission or giving off of cathode material from the remainder of
the cathode caused when arc current flows to the c~hode prior to
the temperature of the cathode reaching a value which insures
sufficient electron emission. ThuS, the cathode is, in effect,
operating in a temperature-limited mode rather than in a space-
charge-limited mode as intended.
Cathode wear is the primary determinant of the life of
a fluorescent lamp because when the cathode is finally consumed,
insufficient emission electrons are available to ignite or main-
tain the arc. Nevertheless, this ignition wear phenomena is
accepted in the prior art. The lamp manufacturing industry
15, generally rates a standard 40 watt lamp as having a 20,000 hour
Mean Time Between Failure (MTBF) life based on a test cycle of
three hours "on" and twenty minutes "off". It is also well known
that lamp operating life will be e~tended when longer "on" peri-
ods are provided between the starting events which cause the
cathode wear.
This invention concerns a novel apparatus for providlng
efficient, long-life operation of the class of fluorescent lamp
control system based on the power control techniques disclosed ln
my U.S. patent 4,352,045, issued on September 28, 1982, and Cana-
dian patent application Serial No. 480,648 filed on May 2, 1985.
In partlcular, the invention is applicable to systems which com-
prise an ~.C. voltage source for supplylng pow0r to an ele~trlcal
load devlce comprlsing a transform~r-ballust
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driving a ~luorescent lamp or lamps having externally heated
cathodes, and which use the power control methodology disclosed
in the above-identified patent and patent application. Al-
though reference is made to the patent an~ patent application
~or a more complete description of this rnethodology, a key ele-
ment thereo~ concerns the control of a capacitive synchronous
switch, i .e., a synchronously operated switch such as a tran-
sistor having a capacitor connected in shunt thereacross.
~ n object of the invention is to reduce the cathode wear
discussed above and thus extend lamp life, as well as reduce
any deterministic or probabilistic excursions of electric cir-
cuit variables which exceed the normal steady state values of
system components due to changes in the operating state of the
system, e.g., excursions (transients) produced by s~itching of
the branch circuit used to implement the A.C. voltage source.
The advantages provided by the invention include a longer oper-
ating li~e for the lamps or lamps used and/or for other system
components, a more efficient system operation, and an ability
to employ relatively low cost semiconductor devlces in the im-
plementation of the power controller. In addition, there are
energy savings provided by the ~dimrning-up" operation provided
by the system of the invention wherein the illumination pro-
duced is gradually brought Up to the desired level, as con-
trasted with pri.or ark ~ys~eln~ which ~rovide kllLI-or) Lr~i~ial
operatioll and then provide dilnlnlny dOWtl to th~ desired level.
It is noted that this latter mode o dimrnlng or visual pur-
poses even has negative psycholoyical e~ects, whicll are elilni-
nated with the system of the inventlon.
In accordance with a preerred embodirnent o~ the inven-
tion, a system is provided or controlling the A.C. power sup-
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~ :6557~~lied fLoln an ~.C. source to an electrical loa~ comprising at
least one transformer ballast and at least one fluorescent lamp
deiven by the transformer ballast and including externally
heated cathodes, the system including a switch connected be-
tween the A.C. source and the load, a power controller for con-
trolling swltching of the switch in timed relation to the A.C.
source voltage wave so as to control the power supplied to the
load, a capacitor connected in shunt across the switch, and
control means, connected to the power controller system, for,
responsive to energization of the system, controlling the
switching operation of the switch provided by said power con-
troller so as to initially limit the arc current supplied to
the load and thereby provide for ignition of the arc of the at
least one fluorescent lamp at an arc current level less than
that provided during full on operating condltions while also
providing heating of the externally heated cathodes prior to
the ignltion of the arc, and so as to thereafter provide gradu-
ally increasing arc current up to a predetermined value which
produces the de.sired illumination level.
In an exemplary embodiment, the power controller com-
~rises a control circuit producing a square wave output for
controlling switching of the switch and the control means ini-
tially inhibits the square wave output o the control circuit
and thereater controls the duration o~ the ~uare wave pulses
produced by the control circuit so as to provide a gradual in-
crease in the duration of these pulses with time. Advanta-
geously, the control circuit includes an operational amplifier
and the control means comprises means for supplying a gradually
increasing voltage to one input o the operational amplifier.
In a specific preferred embodiment, the voltage supplying means
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comprises a resistor-capacitor circuit and the gradually increas-
ing voltage is produced by charging of the capacitor of the
resistor-capacitor circuit.
other features and advantages of the present invention
will be set forth, or apparent from, the detailed description of
the preferred embodiments which follows.
The present inventlon will be further illustrated by
way of the accompanying drawings, in which:-
Figure 1 is a block diagram of the basic system inwhich the present invention is incorporated;
Figure 2 is a schematic representation of the waveforms
associated of the operation of the inventlon; and
Figure 3 ls a schematlc circuit diagram of a lighting
control system incorporating the invention.
Referring to Figure 1, which ls a schematic block dia-
gram similar to that in my copending Canadian application No.
480,648, there is shown the baslc units or components of a system
of the general type to which the invention is applicable. The
system of Figure 1 includes a power source which is implemented
by a branch circuit A.C. voltage source 10 and a branch circuit
switchlng devlce 12, a two-port ~lnput and output) power con-
troller 14 and an electrical load 16. The controller 14 requires
three wires, with the common wire being either the "hot" or the
neutral wire of the branch circuit.
As indlcated ln Figure 1, the electrical load comprises
a transformer ballast 18 and a fluorescent lamp 20 havlng a cath_
ode heater lndlcated at 22. The primary winding 18a of the
transformer ballast 18 is coupled to a low voltage wlnding 18a
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which provides the current necessary to externally heat the elec
trodes of lamp 20. It will be appreciated that these elec-
~ 2~55~6;rodes operate alternately as cathodes and anodes at the line
fre~uency of the A.C. voltage source lO(usually 60Hz in the
United States), and that the heater pins of these electrodes
are represented schematically by cathode heater 22. It will
also be understood that the showing in Figure 1 is highly sche-.
matic and that the transformer ballast secondary winding 18c is
connected in a conventional manner to the lamp load. Further,
a plurality of transformer ballasts and lamps can be obviously
employed.
As shown, power controller 14 comprise~ a switch 24 hav-
ing a capacitor 26 connected in shunt thereacross and a syn-
chronous switch control (SSC) circuit 28 for controlling
switching of switch 24. ;For shorthand purposes switch 24 and
capacitor 26 will be referred to collectively as a capacitive
synchronous switch (CSS) which is denoted 30.
An important purpose of the invention is to supply at
least a minimum heater voltage, denoted Vh, to the cathode
heater pins 22 of lamp 20 which is sufficient to provide ex-
ternal heating thereof to a design temperature which provides
for the level of thermionic emission required for long lamp
lie as discussed above. To this end, the CSS 30 is operated
under the control of SSC 28 to maintain the RMS (heating) value
of the heater voltage Vh above the minimum required to pro-
vide long lamp lie throughout all operating ~ates o CSS 30
rorn full "oE" (i.e, the Gwitch open condition) where capacl-
tor 26 is connected in serieq with the prirnary winding 18a of
transformer ballast 18 to full "on" (the switch closed condi-
.tion) whereill the Eull line voltage VAc is applied to pr.irnary
winding 18a. It is noted that.for the full ~off~ state referr-
ed to above, the RMS voltage applied to the transformer-ballast~
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:imary winding 18a would be near the rated ~alue and this re-
quires selecting an appropriate value for capacitor 26 of CSS
30. Typically, a capacitive value of 3 microfarads is useful
with a standard 120 volt, 0.8 ampere high power factor trans
former-ballast driving two standaed F40 type, 4U watt rapid
start fluorescent lamps. The value of capacitor 26 can be de-
termined empirically by ad~ing'series capacitance to the bal-
last primary 18a until the RMS voltage across the primary wind-
ing 18a approaches that of thè A.C. line or the voltage at the
cathode heater 22 approaches a nominal 4.0 volts without firing
of the lamp arc, this value dropping towards 3.0 volts with
lamp loading.
' A characteristic of the power control methodology dis-
closed in my previous applications is that switching from the
full ~off" state to full ~on" state within a half cycle of the
line voltage produces a large transient line current~ ~his is
the consequence of the inability of the ferromagnetlc core of
the transformer ballast 18 to readily accomrnodate the sudden
polarity or phase reversal produced by this off-on switchin~.
Further, if, in addition, there is asynchronous operation, such
as is the case during initial turn-on, there will be additional
stressing or burdening o the semiconductor device or devices
represented by switch 24. These effects canno~ be avoided and
thus the conse~uences ~hereo~ must b~ ~imited or eliminated.
A further property or characteristic of the power'control
method with which the invention is concerned is that a step
change itl the state o the CSS 30 requires a finite number of
power line cycles before the resultant line current ~ransient
caused by this change subsides to zero and before the'line cur-
rent reaches the new steady state value thereof. The minimim
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time constant of the lag represented by this finite number of
cycles is dependen~ upon the parasitic resistance and induc-
tance of the'ballast transformer 18 when thefcore material is
at or near the saturated flux state thereof. The rnechanism
providing the decay of the transients is the asymmetries in the
positive and negative instantaneous line current waveforms dur-
ing a half cycle o~ the operation of CSS 30 acting with the
aforementioned parasitics to bring the circuit operating state
to the new symmetrical A.C. (Vdc=O) steady state value.
The present invention is concerned with providing a con-
tinuous, gradual change in the switching time between the ~ull
off and on states of the CSS 30'in a manner such that the
transient line currentsjproduced by the polarity'(or phase) re-
versals from half cycle to half cycle are limited to a prede-
termined value below that which could be harmful to the semi-
conductor device(s) used to implement switch 26 of CSS 30. The
nvention provides for gradually increasing the 80n" time of
the switch 24 until a level is reached where the larnps fire,
while providing a prior voltaye which is always sufficient to
provide full heating of the lamp cathodes, thereby ameliorating
the effects of the current transients and asynchronous opera-
tion, while peoviding the require~ cathode heating. This ap-
proach preserves the fundam~ntal op~ratincJ charac~eri~ticY o
the power con~rol techni~lues oE my earli~r applLcation ~nd pa-
tent while providing lamp catllode heating at or above the re-
quired minimum for all operating states, i.e~, for both transi-
ent (upon starting) and,steady state operation. This mode of
operation provided by the invention is indicated in a highly
schematic manner in Figure 2 in which the output with time of
the SSC circuit 28 used in controlling switch 24 is shown as
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increasing gradually from a zero value at an initial time ~To) to
a value at which the lamps fire ~TF) and thereafter to a desired
operating value tTD). It should be noted that Figure 2 is highly
schematic and a large number of cycles would normally occur
before the arc is struck.
Referring to figure 3, a schematic circuit diagram of a
light control system incorporating the lnve~ti~n is illustrated.
The circuit shown is basically very similar to that disclosed in
my U.S. Patent No. 4,352,045 and my copending Canadian applica-
lo tion No. 480,648, and the following description thereof will belargely limited to the portions of the clrcuit used in implemen-
ting the invention. The CSS 30 of Figure 1 is baslcally cons-
tituted by transistors Q4 and Q5 and the dlode bridge formed by
diodes D13, D14, D15 and D16 (corresponding to switch 24 of Fig-
ure 1), and capacitor C8 ~corresponding to capacitor 26 of Figure1). It is also noted that detection of the voltage on the switch
formed by transistor Q4 Q5 and the diodes, used in inhibiting
closing of the switch by control circuit as provided for in Cana-
dian Serial No. 480,148 is implemented in this embodiment by the
connection to the diode bridge which includes resistors Rll, Rs
and R10 and a Zener diode Z2 connected in shunt with resistor
R10.
In order to effect the aforementloned slow turn-on of
the synchronous switch formed by transistors Q4 and Q5 and the
full wave brldge dlodes, a resistor-capacitor network, comprising
a series resistor ~7 and a shunt capacitor C3, is connected to
the lnput of an operatlonal ampllfler ~1 of the power controller
so as to lnhlblt the s~uare wave output of the operatlonal ampli-
fier Ql durlng the time after th0 inltial energlzation of thesystem that ls requlred for capacitor C3 to charge to the
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steady state level thereof. (It is noted that dual operational
amplifiers Ql are employed in this speciflc embodiment and re-
ference will be made to the first operational amplifier of the
dual in the discussion which follows). Initially, capacitor C3
will provide a short circuit, thereby holding the base of oper-
ational amplifler Ql to zero volts, and as capacitor C3
charges, operational amplifier Ql will begin produce a time
limited square wave output, the duration of which gradually in-
creases as discussed above. As explained previously, and is
shown schematically in Figure 2, after operational amplifier Ql
first produces a s~uare wave output, the duration of the square
wave will gradually increase with time until the voltage pro-
duced is such as to provide ignition of the arc and to esta-
blish equilibrium. This time period from initial energization
to arc ignition is typically one or more seconds.
The invention has been described above relative to the
application thereof to rapid start fluorescent lamps, but it is
to be understood that the invention is also useful in connec-
tion with other Eluorescent lamps such as so-called ~preheatn
lamps, and that the transient amelioration and upward dimming
features of the invention have application to even instant
start i-lUoresCen~ lamps.
Although the inventioll has been de~cribe~ relatlve to ex-
emplary embodiments thereof, it will be understood by those
skilled in the aet that variations and modi~ications can be ef-
Eected in the exemplary embodiments without departing from the
scope and spirit of the invention.
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