Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DISCHARGE LAMP STARTING AND OPERATING CIRCUIT
TECHNICAL FIELD
This invention relates to discharge lamp starting and
operating circuits, and more particularly to an improved
circuit for efficiently starting and operating low wattage,
high intensity discharge lamps.
BACKGROUND ART
United States Pa~ent Number 4,329,621, ( Applic~ti~n
fil~ e~ ~Yr 15t 1980) ~ n~-to the present a~ n~e~
C describes a lamp starting circuit including an input ballast
circuit that typically comprises an inductor, capacitor and
resistor, and a circuit shorting switch, referred to therein as
a glow-starter device. The circuit shorting switch operates in
a controlled gas atmosphere as described in
1~ United States Patent Number 4,329,621,
and assigned to the present assignee. The shorting switch
starter is described therein as having a first terminal
connected to a first bimetal, a second terminal connected to a
second bimetal, and a third terminal connected to a rigid
ccnductive member, the bimetals being electrically connected
together at one end which makes a normally closed contact with
the rigid member in the quiescent state of the device; means
connecting the second and third terminals of the starter device
across the output of the ballast circuit; and means for
connecting the first and third terminals of the starter device
across the terminals of a discharge lamp. Upon initial
energization of the ballast, short circuit current through the
second and third terminals of the starter device is operative
to flex the second bimetal for separating the bimetals from the
rigid member to provide an open circuit thereat and a switching
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transient across the lamp. Upon occurrence of this open
circuit, the lamp is adapted to start. Lamp current flowing
through both bimetals maintains the switch open and the lamp
lit.
The transient voltage résulting from the opening of the
shorting switch is equal to L di/dt. L is the inductance of
the reactor; i is the current flowing at the instant the switch
opens; and t is the time for the switch to open. If the
switching transient voltage is sufficiently high, lamp ignition
into the glow state results.
The open circuit steady state voltage (OCV~ applied across
the lamp immediately after the switching transient is the
summation of the line voltage and the voltage across the
capacitor of the ballast circuit at the inst~nt the switch
opens, or, OCY = Vc ~ V input. Thus, the open circuit
voltage is the input AC voltage displaced from a zero reference
by the DC capacitor voltage. Thus, the peak value of the open
circuit voltage is the peak of the input voltage in the
direction of th~ ca~acitor voltage plus the capacitor voltage
itself. Because the capacitor voltage wave shape resembles a
sine wave with it~ top flattened, there is a good probability
that the voltage across the capacitor is somewhat near its peak
after the switch opens. When the capacitor voltage is
sufficiently high, the open circuit voltage is large enough to
initiate the transition from the glow to the arc state.
Simultaneously, the energy stored in the ballast capacitor (l/2
CE2) is discharged through the lamp. If there is
insufficient energy the arc extinguishes. Therefore,
sufficient energy must be presented to the discharge to ensure
30 arc sustainability.
In the aforementioned United Sta-te~ Datent 4,329,621,
the three terminal bimetal starter switch is sealed in
a controlled gas atmosphere to control the amplitude of the
switching transient voltage. The controlled gas atmosphere may
be an argon gas fill. The control of the amplitude of the
transient voltage is necessary to ensure that excessively large
. voltages are not generated which might otherwise cause a
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component failure. In the aforementioned Unite~
States Patent 4,329,621, the circuit described therein with the
use of a gas atmosphere enclosing the switch did indeed reduce
the maximum transient voltage. However, it was also found that
5 the gas atmosphere switch provided a path in parallel with the
discharge lamp through which the capacitor would a~ least
partially discharge. Thus, a significant portion of the
capacitor energy, intended to be transmitted to the lamp for
glow-to-arc transition, was being dissipated in the gas
switch. This resulted in numerous arc extinguishments and
overall less reliable starting.
SUMMARY OF THE INYENTION
Accordingly, it is an object of the present invention to
provide a new and improved starter device and an improved
circuit ~or starting a high intensity discharge lamp.
A further object of the present invention is to provide an
improved means for starting high intensity discharge lamps with
improved starting reliability.
Still another object of the present invention is to provide
~ an improved starter device for high intensity discharge lamps
by substantially eliminating the problem mentioned hereinbefore
relating to the parallel capacitor discharge path by
substantially removing the parallel capacitor discharge path.
This enables substantially all of the capacitor energy to be
transferred to the discharge, facilitating the glow-to-arc
transition, thus enhancing the starting reliability.
A further object of the present invention is to provide a
new and improved starter device for high intensity discharge
lamps and having means for controlling the amplitude of the
switching transient voltage so as to alleviate component
failure due to excessively high voltages in the circuit.
To accomplish the foregoing and other objects of this
invention, the circuit shorting switch is enclosed in a vacuum
which, fo~ all practical purposes, removes the parallel
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capacitor discharge path. This enables almost all of the
capacitor energy to be transferred to the discharge,
facilitating the glow-to-arc transition and thus providing more
reliable starting. However, by providing the vacuum there now
remains the problem of controlling the maximum switching
transient voltage if the lamp does not light. To-overcome this
problem, in accordance with the present invention there is
provided a spark gap disposed in parallel with the discharge
lamp. The arc over voltage of the spark gap is selected to be
greater than the breakdown voltage of the discharge lamp to
ensure reliable lamp starting. Moreover, the arc over voltage
of the spark gap is also selected to be less than a value that
could cause damage to the starting circuit..
In accordance with one embodiment of the invention, the
starter circuit comprises a ballast circuit having an input
connected to a source of AC line voltage and a power output and
a starter device having a first terminal connected to a
- conductor, a second terminal connected to a bimetal, and a
third terminal connected to a rigid-conductive member with the
conductor and bimetal being electrically connected together at
one end which makes a normally closed contact with the rigid
member in the quiescent state of the device. Means are
provided for both connecting the second and third terminals of
the starter device across the output of the ballast circuit and
for connecting the first and third terminals of the device
across the terminals of the discharge lamp. In another aspect
of the invention there is also provided a spark gap and means
connecting this spark gap across the first and third terminals
of the starter device thereby placing the spark gap across the
discharge lamp. The starter device has a sealed envelope
containing a vacuum to substantially reduce the ballast circuit
discharge therethrough. Upon initial energization of the
ballast, short circuit current through the second and third
terminals of the starter device is operative to flex the
bimetal for separating the bimetal from the rigid member to
provide an open circuit thereat and a switching transient
across the lamp. The arc over voltage of the spark gap is
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selected so that it is greater than the breakdown voltage of
the discharge lamp to ensure reliable lamp starting.
Furthermore, the arc over voltage of the spark gap is selected
so:that it has a value less than a value that would be expected
to cause damage to components of the circuit.
Emb~i~nts of the present invention ~ill now ~e
describe~, Dy way of example, with reference 'o ~e
~c~ .y~-~ drawi~gs in which:-
FIG. 1 is a circuit diagram of an improved starting andoperating circuit embodiment of the present invention;
FIG. 2 is a circuit diagram of an alternative embodiment of
the invention in which the spark gap and starter device are
directly disposed within the same envelope; and
FIG. 3. is a fragmentary Yiew of the starter device of
FIG. 2.
Referring to FIG. l, there is shown a circuit diagram
illustrating an embodiment of the present invention comprising
a ballast and associated starting system for igniting,
facilitating transition from glow-to-arc, and operating a high
intensity discharge lamp 20. The lamp 20 may be a low wattage
metal halide arc discharge lamp. The circuit shown in FIG. 1
includes a lead circuit ballast 13 in combination with a
shorting-starting switch lO. The input terminals ll and 12 of
the circuit are connected to an AC line source, e.g., t20
volts/60 hertz. The lead circuit ballast l3 is comprised of an
inductive reactor l4 such as a choke coil, and a capacitor 15
series connected in that order between the AC input terminal ll
and terminal 2 of the starter device lO. A discharge resistor
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16 is connected across capacitor 15. The AC input terminal 12
is connected to terminal 3 of the starter device 10, and
terminals 1 and 3 of the starter device 1~0 are connected across
the terminals of the discharge lamp 20.
The starter device 10 may ~e of the ty~e described
in said Uhite~ ~tates Patent 4,329,621. P~ illustrated
in FIG. 1, the starter device 10 may comprise two bimetals 6
and 7, and a rigid conductive member 9 disposed wiihin a
hermetically sealed envelope 4 which in accordance wi~h the
present invention has a vacuum atmosphere therein rather than
the controlled gas atmosPhere described in the aforementioned
UnitRd 5tates Pate~i 4,32~,621. ~breo~er, one o~ the
bimetals 6 may be replaced by an electrical conductor. Three
lead-in wires respectively connected both mechanically and
electrically to the bimetals 6 and 7 and rigid conductive
member 9 extend through a re-entrant stem sealed at the bGttom
of the lamp envelope 4, not shown in FIG. 1 but shown and
discussed in more detail with regard to the embodiment of FIG.
2. The terminal 1 as shown in FIG. 1 is connected to bimetal
6; the terminal 2 is connected to bimetal 7; and the terminal 3
is connected to the conductive member 9. The two bimetals 6
and 7 are electrically connected together at one end and
attached to a contact button 8, such as by welding. A mating
contact button 17 is welded to the end o~ the rigid conductive
member 9. In the quiescent state of the device, the bimetals 6
and 7 resiliently urge button 8 to make a normally closed
contact with the button 17 of the rigid conductive member 9.
Hence, at normal room temperature and in the absence of current
flowing through either of the bimetals 6 and 7, starter device
10 provides a normally closed switch across both the lamp 20
and the output of the ballast circuit, represented by the AC
input and lead circuit 13.
To essentially compensate for the use in accordance with
the present invention of a vacuum atmosphere within -the starter
device 10, there is provided, ~or controlling transient
voltages, the spark gap 24 and resistor 26. The spark gap 24
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has its own sealed envelope 25. The spark gap 24 and resistor
26 are connected in series9 and the series combination of spark
gap 24 and resistor 26 is connected in parallel across both the
discharge lamp 20 and terminals l and 3 of the starter switch
lO. The spark gap 24 is particularly selected in accordance
with the present invention to have an arc over vo~tage greater
than the break down voltage of the discharge lamp 20 with which
it is used. Moreover, the arc-over voltage of the spark gap 24
is also selected so that it is less $han a value that would be
expected to cause damaging transient voltages that could
destroy components of the system. The resistor 26 in series
with the spark gap 24 is selected at a value that will provide
sufficient current limiting so that the current through the
spark gap 24 is limited to a value which will ensure long spark
gap life.
Referring now to the operation of the circuit of FIG. l,
upon initial energization of the circuit with AC input power,
ballast short circuit current is drawn through starter terminal
?, bimetal 7, rigid conductive member 9, and starter terminal
3. The resulting I2R in the bimetal 7 is sufficient to cause
the necessary heat to flex both of the bimetals 6 and 7 so as
to separate and open the contacts 8 and 17. When this open
circuit occurs at the starter lO, the current drawn from the
lamp ballast 13 rapidly decreases and the inductive output of
the ballast generates a high voltage pulse, thereby producing a
switching transient across the lamp 20 which provides
sufficient energy to initiate discharge in the lamp 20.
previously, when a controlled gas atrnosphere was used in the
starter lO, upon generation of the high voltage pulse, at least
a portion of the capacitor discharge went through the starter
device. However, by providing a vacuum atmosphere within the
starter device lO, substantially all of the discharge voltage
is coupled to the lamp 20. When the lamp 20 is ignited,
current is drawn through both of the bimetals 6 and i, and the
I2~ power is sufficient to maintain the bimetals 6 and 7
separated from the rigid member 9 and thereby keep the contacts
8 and l7 open.
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As mentioned previously, the use of vacuum within thestarter device lO solves the capacitor discharge problem at
open circuit. However, this would make the circuit susceptible
: to excessive switching transient voltage when the lamp does not
light. Accordingly, in accordance with the invention there is
provided the spark gap 24 and associated resistor-26 with the
aforementioned arc over voltage of the spark gap for proper
operation.
Alternatively, FIG. 2 illustrates a discharge lamp starting
and operating apparatus which includes a discharge lamp 28
having an outer bulbous envelope 30 with a conventional
screw-in base 32. The screw-in base 32 has a shell 34 and a
center contact 36 separated from the shell 34 by an insulating
material 38. A glass stem mount 40 extends inside the envelope
30 and includes a pair of support wires 42 and 44 sealed
therein. Moreover, the glass stem mount 40 is hermetically
sealed at one end to the envelope 30. The support wires 42 and
44 are electrically connected to the center contact 36 and the
shell 34 respectively.
Disposed within the glass outer envelope 30 is an arc tube
46 enclosing a pair of spaced electrodes 48 and 50 which are
electrically connected to a pair of lead wires 52 and 54. The
arc tube lead wire 52 is affixed to the support wire 42 while
the support wire 44 is electrically connected to a starter
device 56. In one specific embodiment, the arc tube 46 is
filled wi-th an inert gas at a predetermined pressure9 along
with quantities of mercury and one or more selected iodides and
metal.
Referring more specifically to the discharge lamp 28,
reference is made to the enlarged fragmentary view of FIG. 3.
Therein, the starter device 56 includes first and second
bimetals 58 and 60 and a rigid conductive member 62. The first
bimetal 58 is electrically and mechanically connected to the
lead wire 54. The second bimetal 60 is electrically and
mechanically connected to the support wire 44. Also3 -the rigid
conductive member 62 is electrically and mechanically connected
to the support wire 42. Moreover, the two bimetals 58 and 60
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are preferably in the form of flexible strips and are connected
together at one end and contact the rigid conductor member 62
to provide a normally ~closed switch. Moreover, a spark gap 64
and a resistor 66 are çonnected in series and shunted across
the lead wire 52 and 54.
As previously described, the envelope 30 is a vacuum sealed
container with the starter device 56 and the spark gap 64
sealed therein. However, it should be noted that the starter
device 56 as well as the spark gap 64 each may have a separate
sealed envelope with a given atmosphere therein. Moreover, the
spark gap 64 as well as the starter device 56 may include
separate sealed envelopes which are positioned outside the
envelope 30.
Additionally, the embodiment of FIG. 2 may include
circuitry substantially similar to the apparatus of FIG. l.
Herein, a ballast 68 which includes a series connected choke 70
and capacitor 72 shunted by a resistor 74 couples the support
wire 44 by way of the screw-in base 32 to a terminal 76 of an
AC potential source (not shown). Also, the other terminal 78
of the AC potential source is coupled to the center contact 36
and to the support wire 42.
As can readily be seen, the electrical connections are
essentially similar to the embodiment of FIG. l. In other
words, the ballast 68 is connected to the second bimetal 60,
the discharge lamp 46 is coupled across the first bimetal 58
and the rigid conductor member 56, and the series connected
spark gap 64 and resistor 66 are coupled across the lead wires
52 and 54 of the discharge lamp 46. Thus, energization causes
flexure of the bimetal 60 to cause the bimetals 58 and 60 to
disengage from the rigid conductive member 62 and application
of an energizing potential from the ballast 68 to the discharge
lamp 46 whereby starting thereof is effected.
While there has been shown and described what is at present
considered the preferred embodimemtns of the invention, it will
be obvious to those skilled in the art that various changes and
modifications may be made therein without departing from the
invention as defined by the appended claims.
