Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
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Field of the Invention
This invention pertains to lamp dimming and more parti-
cularly to providing HID lamps with lamp dimming control
without having to provide existing or conventional lamp-and-
ballast networks with additional wiring or components.
Description of the Prior Art
It has been discovered that providing HID lamps with
less than rated current, but at the same voltage level, such
lamps can operate very efficiently at lower light intensity
than their rated value. It is desirable, for instance, to
be able to turn down HID lamps in areas that are little
used. Turning off the lamps in such areas is often undesir-
able because having more than ambient light is frequently
preferred to having no light at all. Furthermore, if the
lamps were completely turned off, a relatively long warm-up
time is required to bring the lamps back to full illumination.
Examples of such use include a warehouse installation,
a parking lot installation, a tennis court installation
after hours of regular use, and a street lighting installa-
tion at late hours when there are virtually no automobiles
on the road.
Heretofore, it has been common to provide dimming by
having a ballast capable of at least partial current bypass
operation. When less than full current goes through the
lamp, because some of the current is bypassed around a
portion of the ballast reactor, then the lamp dims. The
amount of current bypass controls the amount of dimming.
The usual method or technique of providing this bypass
is to utilize a ballast having at least two separate inductors
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and to connect a yated semiconductor device, such as a gated
triac, around one of them~ The gating of the triac determines
the degree of bypass, and hence the amount of lamp current.
Many devices may be used to gate the bypass semiconductor.
Some of these devices use separate wiring to the semiconductor.
Others use superimposed signals to perform the gating. But,
in both instances there is a requirement to add to the
existing installation, either in the form of additional
wiring, additional electronic components, or both.
Therefore, it i~ a fea~ure of the present invention to
provide improved dimming of an HID lamp by a control system
utilizing the same ballast components, same wiring and same
voltage source used in a lamp network not having dimming as
a part thereof.
It i8 another featur~ of the present invention to
provide improve~ dimmlng of an HID lamp already installed
wlthout dlmming provl~lon wlthout having to add additional
oompon~nto or wlrlng to tho ~n~tallation.
SUMMARY OF THE INVF.NTION
_
In one broad aspect, the invention comprehends a
dimmer circuit for controlling the amount of current
through an HID lamp. The circuit includes variable
frequency means for producing an ac voltage at substantially
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constant amplitude and variable in frequency, which
variable frequency means includes a network for
producing a square-wave, ac voltage, and ballast means
connected to -the variable frequency means and to the
lamp. The ballast means is primarily inductive, and
a voltage from the variable frequency means at a
frequency greater than power line frequency increases
the effective impedance of the bàllas-t means, and
thereby reduces the effective current through the lamp
compared with the application of the same voltage level
at power line frequency.
In a preferred embodiment of the invention a variable
frequency generator i~ uged to provide power at a constant
voltage level and over a frequency range from about 60 Hz to
180 Hz. One convenien~ generator inclu(ies a switching
network for selectively providing first a positive voltage
value and then a comparable negative voltage value, the
resulting waveform being a square wave.
Following amplification, the variable frequency voltage
is applied to a lamp-and-halla~t network having a primarily
inductive component. A~ the frequency is increased, the
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current is decreased, for the same applied voltage level,
and, therefore, the lamp of the network is di~ ecl.
BRIEF DESCRIPTION OF TIII~ DRAWINGS
So that the manner in which the above-recited features,
advantages and objects of the invention, as well as others
which will become apparent, are attained and can be under-
stood in detail, more particular description of the invention
briefly summarized above may b~ had by reference to the
embodiments thereof which are illustrated in the drawings,
which drawings form a part of this specification. It is to
be noted, however, that the appended drawing~ illustrate
only typical embodiments of the invention and are therefore
not to be ~onslderod limiting of its scope, for the invention
may admit to other equally effective embodiments.
In ths Drawing~:
Fig. 1 is a ~impl~ied block diagram of a preferred
embodiment of the pres~nt invention.
Fig. 2 is a simplified schemat~c diagram of one inductive
ballast-and-lamp network in accordance with the pre~ent
invention.
Fig. 3 i~ a ~implified schematic diagram of a second
inductive ballast-and-lamp network in accordance with the
present invention.
Fig. 4 is a simplified schematic diagram of a variable
frequency, square-wave generator in accordance with the
present invention.
Fiq. 5 is a simpli~ied block and schematic diagram of
a switchi~g network ~e~ul in th~ network shown in Fig. 4~
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I)ESCRIPTION OF PR13FEFlREl:) FMBODIMEN'l`S
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Now referring to the drawin~s and first to Fic~. 1, a
dimming circuit is shown in accordance with the present
invention. Lamp 10 is connected to a ballast network 12
comprising both an inductive and a capacitive component for
operational purposes not involved directly wi~h this invention,
as is well known in the art. ~he drive voltage which is
normally applied to the network is the power line voltage at
a nominal 60 Hz~ In the circuit shown, power is provided
from variable frequency generator 14 including a square-wave
voltage forming network 16 for reasons hereafter explained.
The output from generator 14 is applied to a voltage amplifier
18 to produce an output level suitable for powering the
lamp~and-ballast network.
In operation, the output from variable frequency generator
14 o~n be beRt under~tood as being irst at a predetermined
level applled to the lamp-and-ballast network at a nominal
60 Hz. The lamp and balla6t components present a complex
inductive load under quiescent operating conditions that
establish a nominal full brightness current. When the
variable frequency generator is adjusted to increase the
frequency, the voltage level remaining the same, then the
inductive load becomes greater and, hence, the current
becomes Qmaller. In notational language, E=IZ, wherein E
equals the applied voltage level; I equals the current
through the lamp and inductive ballast; and Z equals the
impedance of the total load. The inductive component of
impedance can be further represented by j2~fL, wherein j
indicate~ that this component i~ 90 degrees out of phase
from the resistance component; 2~ and L are constants for a
given inductor; and f eguals frequency. Hence, when E
remains constant and f is increased, I then decreases.
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decreasing current through lamp 10 reduces th~ light intellsity
therefrom. Ilence, an increase in fre(3~ ncy r~snlts in
dimming of the lamp.
Two possible arrangements of all inductive load in coln-
bination with lamp 10 are shown in l`i(Js. 2 and 3. In each
case inductor 20 is in series with the lamp. In I`i(~. 2,
capacitor 22 is in series with inductor 20 and lamp 10. In
Fig. 3, capacitor 24 is connected across lamp 10.
Fig. 4 is a simplified schematic diagram of a network
suitable for development of a square-wave signal for operatiny
in accordance with the present invention. The applied line
voltage at 60 Hz is applied to bridye 32, which is illustrated
in simplified form. The output thereErom for the operation
of a typical HID lamp network is a resultant 400-600 dc
voltage, capacitor 34 acting as a filter component for the
bridge output.
A capacitor divider aomprising identical capacitors 36
and 38 (which alternately can be equalized or balanced)
establishes a mid-point 37 therebetween at zero vclts.
Theae capacitors have low impedance values for all operatiny
frequencies of interest and provide low impedance buffering with
regard to voltage changes. Switches 40 and 42 are electronic
switches which open and close in alternate fashion to first
present an output of first polarity at terminal 39 with
respect to the zero-volt terminal 37 and then an output of
second polarity with respect thereto~ If the total dc level
is 600 volts, then the peak of each polarity would be 300
volts~ The switching rate of switches 40 and 42 determines
the frequency of the square wave output.
Additional low pass filters can be provided to filter
out high frequency harmonics, switching transients and the
like, since the cirGuit is frequency sensitive. The pro-
duction of a square wave 1n the manner described eliminates
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the effect of line distortions and other superimpeded high
frequencies that may be included on the power line. Alternative
to the two-transistor switch network is a fu11 bridge.
Fi~. 5 illustrates a network suitable for operating ~s
the electronic switching network of Flg. 4 just described.
It is understood, of course, tha-t t:he Fig. 4 network illustrates
a preferred embodiment only; however, there are many alternate
circuits of establishing a square wave voltage. This network
includes a multivibrator 50 having two alternately produced
output pulses, one of which is connected to the base of npn
transistor 52 and the other of which is connected to the
base of npn transistor 54. These two transistors are
connected so that the emitter of transistor 52 is connec~ed
to the collector of transistor 54.
Diode 56 ls connected across the collector-emitter of
tran8istox 52 and diode 58 is connected across the collector-
emittor of tran~i~tor 54. ~he collector of transistor 52 is
connected to a positive bia~ voltage and th~ emitter of
transistor 54 ~s connected to a negative bias. The output
i8 connected to the junction between the diodes. Alternate
conduction to saturation of transistors 52 and 54 causes the
creation of the square-wave output previously described.
It is`readily apparent that although the description of
the exemplary network has been with respect to the develop-
ment of a square-wave voltage, a variable frequency voltage
of any other configuration maintaining a predetermined rms
level would satisfactorily operate in the manner just
described. A square wave is readily generated and variable
and hence its selection in the preferred embodiment. For
example, a pulse width modulator may readily be used to
change the nominal sine wave of the power line to a suitable
square wave.
Moreover, the discussion of the preferred embvdiment
indicates a preference for e primarily inductive ballast and
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operating from a full bright 60 ~Iz fr(clu~ncy to ~ hi(~her
frequency, where "full dim" operation occurs. 'I'he 60 llz
frequency would be a fre~uency above thc reson.ln~ fre(luenc~y
for the LC circuit (where the lamp current would be greatest:)
and a relatively large capacitor would be errlployed. As t~e
frequency is increased, operation would be further away from
the resonant peak current. A full dim current is typically
about one-third of a full bright current, so the full dim
frequency would be about 180 Hz.
However, ~he same principle would apply to any frequellcy
range of operation, 60 Hz only being selected for discussion
purposes because of convenience. When the circuit operates
with respect to higher ~requencies, however, the components
are smaller in size. If operating in the kHz range, it is
better to operate above the acoustic frequency range for the
lamp(s) to ensure ~table, and apparent noise-free operation.
An autotransformer connection can also be connected in
con~unction with capacitor 24 and provides ready connection
for higher open circuit voltage ~or startin~ purposes or for
operating metal halide HI~ lamps.
The same principles discussed above would also apply to
a primarily capacitive ballast connection. This would be
because operation would be with respect to operating frequencies
below the resonant frequency for the LC circuit. A 60 Hz
frequency would be farther away than a 180 Hz frequency in
this case. Or, as the frequency increases, operation is
more and more toward the peak, hence achieving brighter
operation. A relatively small capacitor is used in this
case.
It is not practical to h~ve the operating current
below 60 Hz since to go much lower than that would cause a
visible flicker condition and even shut-off of ~he lamp.
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Therefore, for a primarily capacitive ~allast, it would be
advisable to select the component values such th~t full
bright operation is at a voltage oper.ltion apE~lied at a
frequency of 180 Hz and full dim operation is at a voltage
operation applied at a frequency oï 60 Hz, assuming that the
resonant frequency for the circuit is greater than 180 Hz.
Other frequency selections can be made operating in accordance
with the principles just discussed.
Although inductive and capacitive ballast arrangements
have been discussed above, it is apparent that any type of
reactive type ballast can be used. For example, a lead peak
regulating ballast is a popular type of ballast with which
the present invention is useful.
One primary advantage of the present dimrning system
just described over dimmlng systems in the prior art is that
dimminy control can be provided without additional wiring to
the lamp-and-ballast or lamp structure for the sole purpose
of accommodating to providing lamp dimming. This means that
existlng lamps can be readily provided lamp dimming by
merely disconnecting the power factor capacitors to prevent
a load from being connected across the switching transistors
and by modifying the voltage devel~pment networ~ applying
the power to the lamp or lamps in the manner discussed.
While particular embodiments of the invention have been
shown, it will be understood that the invention is not
limited thereto, since many modifications may be made. One
example is that variable frequency generator 14 does not
have to be set manually to achieve dimming, but may be part
of an automated system. For example, it may be desirable to
dim street lights when no automobiles are being sensed along
a particular stretch of road~ A sensor network could provide
the variable frequency voltage for effecting dimming and a
return to full brightne~ when an approaching vehicle is
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sensed. Moreover, the above discussion has beell with reg~rd
HID lighting. Similar operation of fluorescent lamps,
incandescent l.amp~ and LPS lamps can be provided. Ot.her
modificati.ons also may be made and will. become apparent to
those skilled in the art~
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