Note: Descriptions are shown in the official language in which they were submitted.
LOW ENERGY STARTING AID FOR ~IGX
INTENSIT~ DISCHA~GE LAMPS
Proud et al, "Method and Apparatus F~r-5~arting ~ig~
Intensity Discharge Lamps", assignee's Canadian Application
386,752 filed concurrently with the present application and
assigned to the same assignPe as the present application,
contains claims to portions of the subject matter herein
disclosed.
This invention relates to starting of high intensity
discharge lamps and, more.particularly, to new and improved
apparatus for efficiently coupling high voltage, short
duration pulses to high intensity. discharge lamps.
High intensity discharge lamps, such as high pres-
sure sodium lamps, commonly include nobel yases at pressures
below 100 Torr. Lamps containing noble gases at pres~ures
below 100 Torr can be started and operated by utilizing
an igniter in conjunction with a lamp ballast~ The lamp
; ballast converts the ac line voltage to the proper ampli-
tude and impedance leveI for lamp operation. The ig~iter
provides pulses which assist in initiating discharge. The
igniter is a rela~ively large and heavy circuit and is
typically bullt into or located near the lamp ballast.
It has been found that the inclusion in high pressure
sodium lamps of xenon as the noble gas at pressures well in
excess of 100 Torr is beneficial to lamp performance. How-
ever, the pulse energy requirements for starting of the dis-
charge lamp increase as the pressure of the xenon included
within ~he lamp increases and the conventional igniter des-
cribed above does not, by itself, produce reliable starting~Various approaches to starting discharge lamps containing
high:pressure~ xenon have been taken. A high voltage pulse
is typically coupled to ~he dischar~e tube by a conductor
. known as a starting aid,
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D-22,650 -2-
as shown in U.SO Patent i~O. 4,179,640 issued Decemher 18,
1979 to Larson et al. The starting aids shown in the
prior art ~ave had the form of a wire wrappea around the
discharge tube in a spiral con~iguration or a wire har-
ness surrounding the discharge tube.
Starting aid configurations which more e~ficientlycouple the starting pulse to the discharge lamp are
, , desirable for several reasons. When starting pulse energy
re~uirements' are reduced by efficient coupling, the physi-
cal size and cost of the starting pulse generator circuit
can be reduced. Phyfiical size of the .starting circuit is
of particular importance when it.is dasixed to include
the starting circuit within ~he outer jacket of the lamp.
Alternativelv, more efficient coupliny of the starting
pulse facilitates starting of discharye lamps having
higher starting ~ulse energy requirements.
Accordingly, th~ present inventi.on~provides a li~ht .. .
source compri.si.ng: a hi.gh pressure:,discharge lamp
including a discharge tube having electrodes sealed
therein at opposite ends for receiving ac power and
enclosing a fill material. which emits li,ght during
dischal-ge; pulse generating means operative to ',
provide at ~n output thereof.a high voltage, ~hort
duration pulse of predetermined energy' and an
elongated:collductor coupled to said output of said -
pulse generating means and disposed in close proximity
.~
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D- 22650 -3-
to an outer surface of said discharge tube in a
configuration path of minimum length, free of
circumferential portions, between said electrodes when
said conductor is energized by said pulse genera-tin~
means.
Some embodiments of the invention will now be
described, by way of example, with reference to the
following drawings in which:
--FIG. 1 is a schematic diagram of a li~ht source
according to the present invention;
FIG. 2 is a simplified schemati.c diagram oi a spiral
line pulse generator;
FIG. 3 is a partial cross-sectional view of the
spiral line pulse generator shown in FIG. 2;
:, FIG. 4 is a graphic representation of t-he volta~Je.
out.put of the spiral line pulse generator of FIG. 2;
:: FIG. 5 is a schematic diagram of a light source -
which provides automatic starting; ~.
F~G. 6 is an elevational view of a light source
according to the present invention wherein the starting :
circuit .is included withln ~he outer jacket;
FI:G. 7 is a schematic diagram of another light source
which pro~id~ aUtOLatiC starting;
:
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73
D-2~,650 -4-
FIC.. ~ is a graphic representation of voltage wave-
forms which occur in the light source of FIG. 7;
FIG. 9 is an elevational view, partly in cross-
section, of high intensity discharge lamps illustrating
starting aid configurations according to the prior a~t; and
.FIG. 10 is an elevational view, partly in cross-
section, of a high intensity discharge lamp illustrating
a low energy starting aid configuration.
. .
A hiyh intensity light source is shown in FIG. 1
and includes a;high pressure discharge lamp 10, a spiral
line pulse generator 12, a switch 14, and an elon~ated
conductor 7Ø The discharge lamp 10 is a hiyh pressure
sodium lamp and includes a discharge tube ~2, -typically
made of alumina or other transparent ceramic material,
having lectrodes 24 sealed therein at opposite ends.
The di.scharge tube 22 encloses a fill material, typica].~y
including sodium or a sodium amalgam and a noble gas or ..
mi~tures of noble gases, which emits light during dis- .
charge. The electrode.s 24 receive ac power from a lamp
ballast at a voItage and current suitable for operation
of the discharge lam.p 10. An output 26 of the spiral
l.ine pulse generator 12 is coupled to one end of the con-
ductor 20, typically a fine wire, wh.ich is located i.n
close proxi.mity to an outer surface of the discharge
tube 22. The confiyuration of:the conductor 20 is of
importan:ce in efficient starting of the light source of
FIG. 1 and is described in greater detail hereinafter.
The spiral line pulse generator 12 receives electrical
energ~ from a source of voltage V0 which can be the
ac input to the di.scharge lamp 10. The switch 14 is
coupled to the spiral line pulse generator 12. In a
manner which is full~ described hereinafter, the spiral
l.ine pulse generator l2, after closure of the switch 1~,
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D-22,650 -5-
pro~ides ~t its output a hiyh voltage, short duration
pulse which initiates discharge in the discharge l~np 10.
The spiral line pulse yenerator 12 is shown in sim-
plified form in FIG. 2 for ease of understanding. A pair
of conductors 30 a.nd 32 in the form of elongated sheets
of conductive material are rolled together to form a
multiple turn spiral configuration. FIG. 3 is a partial
cross secti'onal view of the spiral line pulse generator
12 illus~rating the layered construction of the device.
A four layered arrangement of alkernating conductors and
insulators, including the conductors 30 and 32 and a pair
of insulators 34 and 36, is rolled~onto a form 38 in a
multiple turn spiral configuration. The form 38 provide~
mechanical rigidity. The conductors 30 and 32 are separ~
ated by dielectric material at every point in the sp.iral
configuration~ '
The operation of the spiral line pulse generator 12
can be described with reference to FIG. 2, which schemat-
i.cally shows the conductors 30 and 32. The conductor 30
runs from polnt 40 to point 42 while the conductor 32
runs from point 44 to point 46. In the present example,
the switch 14 is coupled between the conduc'cors 30 and 3Z
at or near the points 40 and ~4. A voltage V0 is applied
- between the conductors 30 and 32. Prior to the closing
of the switch 14, the conductor 30 has a uniform poten-
t.ial between the points 40 and 42 and the conductor 32
has a uniform potential between the psints 44 and 45 and
the voltage difference between the innermost and the
outermost turns of the spiral configu.ration is at most
V0. This can be seell b~ summing the electric field vec-
tors at time t=0 as shown in FIG~ 2. When the switch 14
is rapidly closed, a field reversing wave propagates ..
along the transmission line formed by the conductors 30
and 32. When the wave reaches the poin-ts 42 and 46, at,
time t=T, the potential d.ifference between the points 42
and 46 is nV0, where n is the number o;~ turn.s in the
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D-G2, 650 -6-
spiral. conEiguratiorl, due to the ahsence of cancelling
stati.c field vectors. As is well known, the propagatin~
wavt- undexgoes an in-phase re.,,lection at -the points 42
and 46 when these points axe -terminated in a hiyh impe-
dance or are open circuited as shown in FIG. 2. ~'his
results in an additio~al incr~ase in the potential differ-
ence between the innermost ancl outermost cond~ctors wilh '
a'maximurn occurring at time t-2~ at which time the ~ield
vectors are aligned as shown in FIG. ~ .The out.put vol-
tage waveform of t.he spiral line pulse generator ].2 i.5
shown in FIG. 4. The output taken. between point 42 or 46
and point 40 reaches:a maY.imum voltage of 2nV0 ~t t=2T
after the closure of the switch 1~. The operation of the
spiral line pulse generator is described in further
detail in U.S. Pa-tent No. 3,289,015 and in Fitch et al,
Novel Principle of Transient High Voltage Generation,
Proc. IEEr Vol. 111l No. 4, Ap~il 1964~
The operati.on and properties of ~ne spiral line
pulse generator 12 can be expressed in terms of the fol-
lowing parameters: :
VG Charginy voltage
Vm Peak pulse voltage
n Number of turns
V(t) Transient voltage wavefo~m
I Transit time in spiral line
D Diameter of spiral
v ~elocity of propagation i.n spiral
W Width ~f line composing spiral
d Thickness of dielectric
c Velocity of EM waves in vacuum
CO Static capacitance of li,ne
C Effective output capacitance ,.
z Impedance of line composing sp.i.ral
k Kelative dl.electric constant
~O Die].ectr.ic~ constant in vacuum
Permeability of vacuum
'
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v-22,65~ ~7~
;L Inductance of ~ast switch
~ Thickness oE build-up
E Energ~ available in spiral line
Relationships descriptive of the output pulse are yiven
by:
(1) Vm 2nVo
(2)- V(t) = (nk/l)VO 0 < t ~ 2
~3) ` V(t) = 2n(1-t22~)VO 2c ~ t ~ 4
(~ n~/v, v = ck 1/
The capaci.tance of the spiral line and its effective out-
p~lt c~pacitance are given by.
(5) CO = ~nk~ODW/d
(~) C = CO/(2n)2
The stored energ~ is:
~7) E = C V 2/2
The characteristic impedance of the s-trip line composing
the spiral is:
(8) z (~/f ) 1/
In optimizing performance of the spiral line pulse ~
generator 12, it is important to utilize low loss dielec- ~ -
;` tric materials and conductors in order that the propaya-
ting wava maintain a ~ast risetime compared to the
transit time ~ of electromagnetic waves-between the
innexmost turn and the outermost turn of the spiral line
pulse generatox. It is additionally important to main-
tain a lar~e ratio of diame-ter to ~Jinding buildup (D/~
and to provide for a vexy low inductance switch to
insure that the voltaye between the conductors is switch-
ed in a time interval which is much shorter than ~
The maximum permlssible value of inductance for the
switch 14 is determlned from the approximation known in
the ~rt that closure risetime i5 approximatel~ equal to
.
L/Zo. Therefore, the followiny inequality must be met:
L<<~Zo. For a t~pical design, L, ~he inductance oE the
.
~ 73
D-22,650 -8-
swi~ch, i.s on the order of one nanohenry or less.
As discussed hereina.ter, it is pre~erable to in-
clude the spiral line pulse generator 12 within an outer
jacket of the light source. In this situation, the spira~
line pulse generator 12 must meet certain additional
requirements. It is important that the spiral line pulse
generator 12 have a compact physical size. Furthermore,
when the spiral line pulse generator 12 is included
' within the outer jacket o~ the l'ight source, it must be
capable of withstanding the considerable heat generated
by the discharge lamp. In a t~pical application, the
spiral li~e pulse g~neratox 12 must be capable of Qpera-
tion at 200C.
It has been determined that the eneryy content,
rather than the amplitude or pulse width, of the spiral
line pulse generator output pulse is the most .important
~actor in ef~ective starting of.high pressure discharge
lamps. The discharge lamp can be started by output
'` puls~s of less than ten kilovolts in amplitude b~ i.n-
cxeasing the energy content of the puls~. Since output
pulses of maximum amplitude and minimum duration are not
nècessarily re~uired, the spiral line pulse generatGr
design re~uirements and the switch speed requirements
described hereinabove can be rel.axed.
In one example of a spi.ral line pulse generator,
the conductors were aluminum foil having a thickness of
0.0007" and a width of 0.5" and the insulators were
polyimide film dielectric having a thic,kness of 0.00048"
and a width of 1". The two: conductors, separated by the
; 30 two insulators, were wound on a cyl.indrical form having
a diameter of 0.7". ~pproximatel~ 130 turns were to
provide a capacitance of approximately 0.5 microEarad.
The insulators were wider than the conductors to prevent
arcing between turns at the edye~ oE the conductors.
T~pically the voltage, yround, and output conne-,,tions are
made by means of tabs which are spot welded to the
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D-22,650 -9--
conductors du~in~3 the winding o~ the spiral line pulse
gener~tor. When 200 volts is applied to this spiral line
puls~ g~nerator, an OUtpllt pulse of approxirnately 3500
volts and 30 nanoseconds is provided.
The low inductance swi~ch 14, whiCh is shown in
FIG. 2 connected be~ween the conductors 30 and 32 on the
innermos~ turn of ~he spiral line pulse generator 1~l can
alternatively be connected bet~7een the conductors 3~ and
32 on the outexmost turn at or near the points 42 and 46
or between the conductors 30 and 32 at the midpoint of
the conductors 30 and 32. While the output voltage can
be takerl between an~ two points on the spiral line pulse
gene~ator 12, the maximum voltage multiplication facto-
is obtained when the OlltpUt is taken between the inner~
most turn and the outermost turn.
A light source configuration providing automatic
operation is illustrated in schematic form in FIG. 5.
A discharge lamp 50 corresponds exactly to the discharge
! lamp lO shown in FIG. 1 and described hereinabove. A
spiral l:ine pulse yenerator 52 shown symbolically in
FIG. 5 corresponds to the spiral line pulse generator 12
shown in FIGS. 1, 2, and 3 and described hereinabove.
AC power is coupled to electrodes 54 at opposite ends of
the discharge lamp 50 and is coupled through a current
~5 limiting resistor 56 to on~ end of one conductor of the
spir~l line pulse generator S2. The output of the spiral
line pulse generator 52 is coupled to one end of a con-
ductor 58 located in close proximity to an outer surface
of the discharge lamp 50 but not coupled to the electrode~
54. Alternatively, the output o~ the spiral line pulse
generator can be coupled to the electrodes 54 of the dis-
charge lamp 50 in~which case the ac power is coupled
through a filter circuit to block the high voltage pulse
from the source of power. A self-heated thermal switch
60 includes a bimetallic switch 62 having a normally
closed coIltact 64 and a normally open contact 66 and
~:. ; ' ~ '' ' ' '''`'
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~-22,650 -10-
further includes a heater element 68. The normall~ open
contact 66 of the bimetallic switch 62 is coupled to ~he
one conductor of the spiral line pulse generator 52.
The no~mally closed contact 64 of the bimetallic switch
62 is coupled through the heater element 68 and through
a normally closed disabling swi-tch 70 to the ac input.
A common contact 72 of the bimetallic switch 62 and the
other conductor of the spiral line pulse generator 5
are coupled to ground. The disabling switch 70 is a
bimetallic switch which is located in proximity to the
discharge lamp 50 ancl. senses the t~mperature o~ th~ dis-
charge lamp 50. A starting circuit. 7~, comprisin~ the
spiral line pulse gene.rator 52, the resistor 56, the
thermal switch 60, and the disabling switch 70, has an
output 78, which is the output of the spiral ~ine pulse
generator 52, coupled to the conductor 58.
In operation, when ac power, typically provided by
a lamp ball.ast, is appl.i.ed to the light souxce of FIG. 5,
the spiral line pulse generator 52 is charged throug~l the
resistor 56. At the same ti.me, current flows through the
switch 70, the heater.68 and the bimetallic switch 62,
thus increasing the temperature.of the heater element 68.
The heater element 68 is in close proximity to the
bimetallic switch 62 and cau~es heating of the bimetallic
switch 62. When the heater elemen.t 68 reaches a prede-
te.rmined temperature, the bimetallic switch 62 sw.itches
from normally closed contact 64 to normally open contac~
~6. The closure of normally open contact 66 provides a
short circuit across the conductors of the spiral line
pulse generator 52, thus prodùcing at the output of the
spiral line pulse generator 52 a high voltage, short
duration pulse which in~tiates discharge in the dis-
charge lamp 50~ The heat produced by the discharge in
the lamp 50 causes the clisabling swi~tch 70 to open,
thereby disabling the thermcll switch 60.
If, for any raason, t~ie first spiral line pulse
``.. ' :
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~-22,650 -11-
generator 52,output pulse did not initiate discharge in
the discharge lamp 50, the'switch 70 remain~ in the
closed position and the bimetallic switch 62 cools since
the heater element 68 is no longer energized. When the
bimetallic switch 62 cools to a predetermined temperature,
it switches back to the normall~ closed contact 64 and
current again flows through the heater element 68. The
temperature of the heater element 68 and,the bimetallic
switch 62 again rises and auses switching'of the bi- -
metallic switch 62 to the normally open contact 56 and a
second high voltage, short duration pulse is generated
by the spiral line pulse generator 52. This process
eont.inues automatically until a ~ischarge is initiated
in the discharge lamp 50. At that time -the increase in
temperature of the discharge lamp 50 causes the switch
70 to open and the thermal switch 60 to be disabled. As
discussed hereinabove, the bimetallic switch 62 must
provide a low inductanee short ci.rcuit across the spiral
line pulse generator 52 ~or optimum per~ormance of'the
spiral line pulse generator 52. The configuration of
; - FIG. 5 provides automatic generation of starting pulses
until a discharge is initiated in the discharge lamp 50.
; A ph~sical embodiment of the light source ~hown in
sehematic form in FIG. 5 is illu,.trated in,FIG. 6. r~he
~5 discharge lamp 50 is enclosed,by a light transmitting
outer jacket 80. Power i.s received by a lamp base 82
and conducted through a lamp stem 84 by conductors 86
and 88 to the electrodes of the discharge lamp 50. The
conductors 86 and 88 are sufficientl~ rigid to provide
mechanical support for the discharge lamp 50. The start-
ing circuit 76 is located in the base region of the outer
jacket 80 surrounding the lamp stem 84. This location
of the starting ,circuit 76 is chosen to minimize block~
age of light emitted h~ the dischargc lamp 50. The
starting circui' 76 includes the spiral line pulse
ge}lerator 52, the resistor 56,, the therrnal switch 60 a71d
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.
D22,650 -12-
the swi*ch 70 connectecl as shown in FIG. 5. The outpu-t
78 o the starting circuit 76 is coupled to the conduc-tor
58 which is located in close proximity to an outer sur-
face of the discharge lamp 50. The location of the
startirlg circuit 7~ as shown in FIG. 6 is advantageous
because the generally cylindrical shape of the spiral
line pulse gene~ator 52 is compatible with the annular
space available in the lamp base. When very hiyh energy
levels are required to start the discharge lamp 50, the
lQ spiral line pulse generator 52 can become too large for
inclusion within the outer jacket 80. In this instance,
the starting circuit 76 can be located externa]. to the
outer jacket 80, for example, in the light fixture in
which the light source is mounted. The pulse energ~
requi.rements for startincJ of the discharge lamp 50 in-
crease as the pressure of the noble gas included w~thin
the lamp increases. For example, a lamp having a xenon
pressure of about 10 Torr re~uires a starting pulse of
approximately 2 to 5 millijoules while a lamp having a
xenon pressure of about 300 Torr requi.res a starting
pulse of approximatel~ 70 to 100 millijoules. The igni-
ter commonly used in high pressure soclium lamp ballasts :~
does not provide pulses of sufficient voltage to start
lamps cc,ntaining noble gases at pressures above about
lOQ Torr. Therefore, such lamps cannot be used in stand-
ard high pressure sodium lamp fi.xtures. In the config-
uration shown in FIG. 6, the starting circuit 76 is in-
cluded within the outer jacket 80 of the light source
and is tailored for effective starting of the discharc~e
lamp 50. Therefore~ the light source shown in FIG. 6
can be used with standard high pressure sodium lamp
ballasts. Furthermore, since the starting circuit is
self-contained within the light source, the configuratio
of FIG. 6 can be utilized with mercur~ lamp ballasts,
which do not contain an igniter.
,: .
.~ ~2,650 -13-
~ n alternative light source configuratiorl providing
automatic operation is illustrated in schematic forrn in
FIG. 7. The discharge lamp 50 and khe spiral line pulse
ger.erator 52 are connected as shown in FIG. 5 and de-
scribed hereinabove except that the thermal swil:ch 60 andthe disabling switch 70 of FIG. ~ are replaced by a spark
gap 90. The spark gap 90 is a two terminal device ~-hich
is connected directly across the conductors.of the spiral
line pulse generator 52. The spark gap 90 is normally
an open circuit but switches to a short circuit when a
voltage greater than a pred~termi~.ed value is applied to
the device. In FI~ 7, the predetermined firing voltage
of the spark gap 90 is selected to be slightly less than
the peak ac input voltage so that the spiral l~ne pulse
generator 52 achieves maxirnum output voltage. A starting
circuit 92, including the spiral line pulse generator 52.
the resistor 58, and the spark gap gO, has an ou~put 94
coupled to the conductor 58. The s-tarting circuit 9.2 can
replace the starting circui.t 76 shown in the light source
20, of FIG. 6.
In operation, an ac voltage, typically provided by a
- lamp ballast, is applied to the configuration of FIG. 7.
The voltage across the spiral line pulse generator 52,
illustrated in FIG. 8A, i.ncreases until the firing vol-
tagQ of the spark gap 90 is reached at time To~ Thespark gap 90 rapidly short,circuits the spiral line pulse
generator 52 and a high voltage, short duration pulse,
illustrated ;n FIG. 8B, i.s provided at the OlltpUt of the
spiral line pulse generator 52 at time To a.s described
hereinabove. By repetition of this process, a high vol-
tage pulse is produced by the spiral line pulse generator
on each half ~yc].e of the ac input voltage, as shown in
FIG. ~B, until starting of the discharge lamp 50. After
the discharge lamp 50 is started, the voltage supplied
by the lamp ballast to the light source is reduced and
the spark gap 90 does not fire.
D-22,650 -14-
The canfig~l~atior~ of FIG. 7 provides several advan-
tages. (1) Starting pulses are produced when maximum
potential exists across the discharge lamp 50, thus maxi-
mizing the probability of starting. (2) Starting pulses
S are produced at 120 Hz until starting occurs. (3~ The
starting circuit stops functioning automatically after
the discharge lamp 50 starts. ~4~ The nurnber of cixcuit
components is minimal.
As noted hereinabove, the configuration OL the con-
ductor 20 in FIG. 1 and the conductor 58 in FIGS~ 5-7 is
of importance in efficient starting of the light source
described herein. Conductors, such as the conductors 20
and 58, used for starting of discharge lamps are ~ommonl~
re~erred to as starting aids. By providing ef~icient
transfer of energy from the spiral line pulse generator
to the discharge lamp, the energy required in the output
pulse of the spiral line pulse generator can be reduced.
A reduction in energv requirements is beneficial in two
ways. For a given discharge lamp, the size of the spiral
line pulse generator can be reduced, thus resulting in
easier packaging of the spiral line pulse generato:r and
lower cost. Second, a given spiral line pulse generator
can be used to start discharge lamps with higher nobl2
gas pressures.
Various starting aid confiyurations are known in the
prior art. Referring now to FIG. gA, there is shown a
discharge lamp 100, corresponding to the discharge ~amp
10 shown in FIG. 1 and described hereinabove. The dis-
charge la~lp 100 includes a light transmitting discharge
tube 102 having electrodes loa sealed therein at opposite
ends. A starting aid 106, in the form of a fine wire, is
wrapped around the outer surface of the discharge tube
102 in a spiral configuration having several turns. The
starting aid 106 is coupled at its ends to a pulse gener-
ator. Upon application of a high ~701tage, short duration
pulse to the startiny aid 106l ~n ionization path lOg is
: .
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D-22,650 -15-
form~d in t~lO in~erior o~ ~he discharcJe lamp 100 be~een
the electrodes 104. The ionization path 10~ follows the
path of the starting aid 106 and therefore is spiral in
configuralion.
A similar configuration of a startiny aid according
to the prior art is shown in FIG. 9B. A discharge lamp
110, corresponding to the discharge lamp 10 shown in
FIG. 1 and described hereinabove, includes a discharge
tube 112 having electxodes 114 sealed therein at opposite
ends. ~ starting aid 116, in the form of a conductive
wire harness, is disposed around the outer surface of the
discharge tube 11?. The startin~ aid 116 includes a
number of circumferential portions~118 which surround the
discharge tube 112 and a number of interconnecting por-
tions 120 ~hich connect the circumferential portions 118,
thus forming a harness. When a high voltage, short dura-
tion pulse is applied to the starting aid 116, an ioniza-
tion path 122 is formed wi-thin the discharge tuke 112
between the electrodes 114. The ionization path 122
fo]lows the path of the conductor which forms the star-t-
ing aid 116~ Thus, the ionization path 122 includes
portions 124 which follow the circumferential portions
118 of the starting aid~116, and portions 126 which fol-
low the interconnecting portions 120 of the startlng aid
; 25 116.
It has been found that the use of a straight wire
starting aid results in superior starting of high inten-
sit~ discharge lamps. Referring now to FIG. 10, there is
shown a c~ischarge lamp~130~ corresponding to the dischar~e
lamp 10 shown in FIG. 1 and described hereinabove. The
discharge lamp 130 includes a transparent discharge tube
13~ having electrodes 134 and 136 sealed therein at oppo-
site ends. A starting aid 138, in the form of an elon-
gated conductor in a generally straight configuration, is
located in proximity to an outer surface of the discharge
tube 132 . The starting aid 138 is coupled to a generator
.
., --- --- - - - : ` ~ ' '
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D--2 2 , 6 5 0 1 6--
of hi.gh voltage; short duration pulses and runs in a gen-
erally straight path between a region 140 proximate ~he
electrode 134 and a region 142 proximate the electrode 136A
The starting aid 138 can be mounted in proximity 1:o
the discharge tube 138 in any convenien-t manner which does
not appreciably block the li;ght ou-tput of the discharge
lamp 130. For example, insu].at.ing support brackets can
be located at opposi.te ends o~ the discharge larnp 130,
When the conductor which forms the starting ~id 138 is of
sufficient diameter to have mechanical rigidity, a single
insulating support bracket can be used. Alternatively,
the starting aid 138 can be affi~ed to thP oliter surface
of the discharge tube 132 b~r cement capable of withstand-
ing the heat generated by the discharge lamp 130,
When a hiyh voltage, short duration pulse,, such as
that generated by the spi,ral line pulse generator describ-
ed hereinabove, is applied to the start~ng aid 138, an
ionization path 144 is formed in the interior of the dis-
charge lamp 130 between ~he electrodes 134 and 136. The
ionization-path 144 follows the path of the starting aid
138 and thus runs in a yenerall~ straight path between
the electxodes 134 and 136. rrhe formatJon of the ioniza-
tion path 144 is dependent upon ths peak pulse voltage
applied to the startiny aid 138. Whether the degree of
ionization develops further to form an arc discharge
between the electrodes 13~ and 136 depends upon the ini-
tial conductivity of the ionization path 144. Conductiv-
ity :in turn depends on the degree of ionization and elec-
tron temperature and is directly related to the energy
,initia.~1~7 supplied b~ the st:arting pulse. Thus very
narrow high voltage pulses can, in some cases, produce
ionization but can fail to produce sufficient conductivit~
in the iani7ation path 144 to induce further deve'lopment
of a self-sustained discharge. In contrast to the ion.iza-
tion path 108 in FIG. 9A and t,he ioni2ation path 122 in .
E~IG. 9B, the ionization path 14~ in E~IG. ;0 is free O:e
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extraneous circum~eren~ial turns. As a result, -the
length o~ the .ionization path 144 is less than either of
the ionization paths 108 or 122, and less pulse eneryy
is required to establish conditions suitable or arc for-
mation or starting of the discharge lamp 130.
The reduction in requisite pulse energy h~s beenshown by experiment to be roughly a factor of two for the
starting aid 138, shown in FIG..lO, as compared with the
starting aids shown in FIGS. 9A and 9B. This is genera].ly
consistent with the reduction achieved ln the leng-th of
the i.onization path b~ utilizing a straight skarting ~id~
U5ing the prior art starting aid configuration illustrated
in FTG. 9B, i.t has been found that high pressure sodium
lamps contaîning 203 Torr xenon pressure require 35 kilo--
volt, 20 millijoules pulses, when the pulses are approxi-
mately 10 nanoseconds in width. A high pressure sodium
lamp containing 300 Torr xenon cannot be started ~ithin a --
reasonable voltage ranye using the starting aid shown in
FIG. 9B. When the starting aid 138, as shown in FIG. lO,
~0 is uti.lized, experiment has shown that a discharge tube
containing 200 Torr xenon can be started with a 25 kilo-
volt J 10 millijoules pulse of lO nanosecond pulse width.
The straight star-ting aid 138, shown in FIG. lO, enables
reliable starting o~ high pressure sodium discharge lamps
containing 300 Torr xenon with 33 kilovolt, 15 millijol.lles
pulses at a pulse width of lO nanoseconds.
It is to be understood that while the starting aid
138, shown in FIG. lO, has been described in connection
with a spiral line pulse generator, a starting aid having
3n a generally straight config~lration can be usea with any
pulse generator capable of generating the requisite high
voltage, short duration pulses. The starting aid 138 is
of particular i.mportance when it is desired to minimi~e
the si7.e of the pulse generator or when it is desi.red to
start discharge lamps having hi~h energy starting re~uire-
ments.
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Thus there is provided by the present invention a
light source in which a spiral line pulse generator pro-
vides starting pulses of sufficient ene~y to start a
discharge lamp containi~g high pressure noble gases. The
spiral line pulse generator reduces the mass and volume
associated with inductive starting circuits. In addition,
tha spiral line pulse generator has a physicai conigura~
tion which can advantageously be included within a dis~
charge lamp envelope.
While there has been shown and described what is at
present considered the preferred embodiments of the inven~
tion, it will ~e obvious to those skilled in the art that
various changes and modifications may be made therein
without departiny from the scope of the invention as
defined kv the appended claims.
.
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