Note: Descriptions are shown in the official language in which they were submitted.
S3'73
LD 7759
REFRACTORY HELICAL OVERWOUND ELECTRODE
FOR HIGH PRESSURE METAL VAPOR ~MP
This invention relates to a self-heating
electrode for high pressure metal vapor lamps. It may be
used in metal halide lamps wherein conventional alkaline
earth oxides cannot be used, but it is also suited to
carry emission materials when needed. It lends itself
particularly well to cathode designs suitable for
miniature metal halide lamps not using oxide emitters and
operating on d.c. with a discharge current of 1 ampere or
less.
B~CKGROUND''OF THE INVEN ION
Until recently, it has been the common view that
the efficacy of high intensity discharge lamps inevitably
goes into a rapid decline at lower wattage ratings starting
at about 250 watts, and metal halide lamps in sizes below
175 watts were considered impractical for general lighting.
However, in Canadian patent 1,111,483 issued October 27,
1981 to Daniel M. Cap and William H. Lake, titled "High
Pressure Metal Vapor Discharge Lamps of Improved Efficacy"
and assigned to the present assignee, design principles
are set forth which permit high efficacy to be
' achieved in previously unheard-of small sizes of
lamps. New miniature metal halide discharge lamps
are disclosed having envelope volumes less than 1
cubic centimeter, ratings going down to less than 10
watts, and which operate with discharge currents of
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LD 7759
-- 2 --
1 ampere or less. For good efficacy, a high ra-tio of
arc w~$~s whi~h produ~e light~ to el~ctro~æ ~tts which
do not, is necessary. In these new lamps, a high ratio
approaching those found in larger sizes is achieved by
increasing the mercury vapor pressure at the same time
as the discharge volume is decreased. However it is
necessary to attain the electrode temperature required
for adequate electron emission even with the reduced
energy input, and this is achieved primarily by reducing
the physical size of the electrodes, inleads and end
seals in order to reduce the heat loss from them. As
physical size is reduced, ~ire commensurate in fineness
must be used and this tends to make manu~acture more
d f~icult.
In lamps where~n the electrodes do not carry elec-
tron emission material in the conventional sense of
alkaline earth metal or oxides, the criterla for elec-
trode design and the permissible heat loss from the
electrode are much more stringent than in lamps con-
taining emission material. By ~ay of example, in a
mercur~ ~apor lamp containing barium o~iae as emission
material which has a wor]s function of 1.5 to 2 volts,
the electrode temperature should not exceed 1500 K~
By contrast, in lamps ~ithout emission material or re-
lying upon the presence o~ thorium or thorium iodide inthe fill for elec~rode activation with a work function
of 3O5 to 4.5 electron volts, a temperature of 2500
to 3000 K is nece~sary ~or adequate electron emission.
On the other hand at temperatures above 3300 K, tung-
sten vaporizes at such a rate that the small en~elopeso~ miniature lamps blacken rapidly and this imposes
another design constraint.
When a lamp is operated on alternating current,
each electrode sexves alternately as cathode and anode,
and the heating ~rom the cathode half cycle i5
.~
~3~
LD 7759
-- 3 --
supplemented by that from the anode half cycle, resulting
in an operating temperature which is less dependent
on the cathode function~ Thus, when an electrode
which was receiving adequate heat for i-ts cathode function
in an a.c. circuit at a given current is used as the
cathode in a d.c. circuit, it may no longer receive
adequate heat at the same current. It is economically
advantageous to operate miniature metal vapor lamps on
~ d.c. using transistors or solid state control devices
; 10 in the starting and ballasting circuit. Accordingly,
the cathode must be designed to properly manage the
energy balance in order to ensure that the cathode
hot spot rapidly reaches its desired operating temperature
during starting and does not exceed it during running.
If this is done, cathode damage and envelope blackening
will be minimized. These requirements, in addition to
those stemming from the small size and low operating
current, must all be met if a miniature metal halide
lamp is to operate satisfactorily on d.c.
SUMMARY OF THE INVENTION
The general object of the invention is to provide
a new self-heating electrode design of general utility
operable with or without alkaline earth emission material
and adaptable to a wide range of operating currents.
: 25 A more specific object is to provide a cathode particularly
suitable for miniature metal halide arc tubes operating
on d.c. discharge currents of 1 ampere or less. Our
design (1) permits the electrode surface area to be
maximized, (2) allows the electrode heat conduction loss
into the seal area to be minimized and (3) achieves the
foregoing together with structural rigidity and ease of
manufacture.
According to our invention, an electrode for a
high pressure metal vapor lamp comprises a hollow helix
of refractory metal wire which is provided with
an open-wound overwind of refractory metal wire
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LD 7759
on the turns of the helix. The open-wound overwind
provides quasi-point-contact spacers between adjacent turns
of the helix so -that conductive heat flow is compelled
to follow a long helical path. The arrangement permits
an increase in total electrode surface area as compared
with the surface area of the helix without the overwind,
and at the same time the arrangement assures a low axial
heat conduction loss into the seal area.
In a preferred embodiment suitable for a
miniature metal halide lamp, the open-wound overwind on
the helix is wound to provide an optimum number of quasi-
point-contact spacers between adjacent turns of the helix
in order to give structural rigidity without appreciable
increase in axial heat flow~ The electrode comprises a
helix as described which is attached to a refractory metal
inlead and projects distally from the inlead. The helix
can be spudded onto a shank serving as an inlead and can
be terminated in a solid cap by melting back a few turns
at the distal end of the helix. The wire of the helix can
be torsionally preloaded in order to provide a built-in
force that biases the turns of the helix together against
the spacers for extra rigidity.
DESCRIPTION OF DRAWINGS
In the drawings:
FIG. 1 illustrates, to the scale shown above the
figure, a miniature discharge lamp for d.c. operation
- provided with a cathode embodying the invention.
FIG. 2 is an enlarged view of a cathode
embodying the invention.
FIG. 3 is an enlarged view of the primary over-
wind wire open-wound on the primary mandrel.
FIG. ~ is a partly sectioned side view of cathode
embodying the invention, enlarged to a greater extent than
that of FIG. 2.
FIG. 5 is an end view of the cathode of FIG. 4.
DETAILED DESCRIPTION
An optimized electrode design must be capable of
serving through the various modes encountered in lamp
~, J
,5~'J~
LD 7759
-- 5 --
operation such as ~reakdown, glow-to-arc transition,
normal operation ~whtch may be a.c. cr d.c.), -and ~ot
restart after a temporary power loss. A given design
will have a specific structure and surface condition
including emission mix, a specific shape including sur-
face curvature whicIl may influence the electric field,
a specific mass distribution, and a particular ther-
mal balance ~etween heat generation and heat loss in
the structure. Our ~nventton provides an electrode de-
sign which ofers a ~ide choice of independent param-
eters which may be varied to achieve the desired opti-
mization. Among these parameters are the refractory
metal (e.g. tungsten) chosen for the structure, the
emission material such as a coating if used, and parti-
lS cularly the physical dimensions inherent in the struc-
ture as will appear hereafter such as mandrel diameter,
overwind diameter, over~ind pitch on the mandrel, tight-
ness of the overwind on the mandrel, shank or inlead
diameter and insertion length, overall length of the
electrode and tip or end cap size. A11 of these may
be varied to achieve t~e desired optimization.
When an electrode operates as a cathode on d.c.,
the forces that drive the point of arc attachment toward
the tip are muc~ less than when the same electrode op-
erates alternately as cathode and anode on a.c. This isbecause when an electrode operates as anode, electrons
are collected at the point of least separation from the
opposite electrode, namely at the tip, and the tip is
hea~ed up thereby. Thus on a.c. operation the tip tem-
perature is built up on successi~e anode half-cycles.
Such temperature build~up increases emission at the tip
and facilitates transfer of the hot spot thereto on the
ca-thode half-cycle. On d.c. operation, there is no such
force driving the hot spot towards the tip. However
our electrode design provides another driving force
LD 7759
-- 6 --
making it particularly suitable for d.c. operation.
The driving force arises through the resistance loss
in the long spiral path which the current must follow
coupled with the thermal insulation between electrode
tip and the heat sink at the seal. With our design,
the transfer characteristics in moving the arc terminus
to the tip rapidly and without damage to the electrode
are much superior to those of conventional electrodes
using a winding around the shank with the shank tip
protruding through it.
While our invention is useful in any size of
lamp including high curren-t lamps, it is particularly
valuable for miniature metal halide lamps such as those
described in the previously-mentioned patent of
Cap and Lake. An example of a miniature metal halide
lamp is illustrated in FIG. 1 comprising a small
arc tube 1 whose size may be judged from the centimeter
scale shown above. By way of specific example, in a
35 watt lamp such as illustrated, the internal diameter
of the arc chamber is from 6 to 7 millimeters. The
envelope is made of quartz or fused silica and comprises
a central bulb por-tion 2 which may be formed by -the
expansion of quartz tubing, and neck portions 3,3' formed
by collapsing or vacuum sealing -the tubing upon molyb-
denum foil portions 4,4' of electrode inlead assemblies.Leads 5,5' welded to the foils project externally of the
necks while electrode shanks 6,6' welded to the opposite
sides of the foils extend through the necks into the
bulb portion. The cathode shank 6 may be tungsten, or
alternatively molybdenum which reduces tendency to
back-arcing.
A suitable filling for the enevelope comprises
' argon or other inert gas at a pressure of several tens
of torr to serve as starting gas~ and a charge com-
prising mercury and one or more metal halides. A pre-
:,
65~73
L~ 7759-- 7 --
ferred filling comprises MaI, ScI3 and ThI4. The charge
ma~ be introduced into ~he a-~c ch~amber through one o~
the necks before sealing in the second electrode; in such
case the arc chamber portion is chilled during the heat
S sealing of the neck to prevent vaporization of the charge.
Alternatively, the charge may be introduced through an
exhaust tube extending from the side of the ~ulb which
is then eliminated by tipping of. The arc tube is
usually mounted within an outer pro~ective envelope or
jacket ~not shown~ having a base to whose contact ter-
minals the inleads 5~S' of the arc tube are connected.
In a direct current lamp, the anode is simply an
electron collector and a conductor such as lead 6' pro-
jecting into the envelope will suffice providing it has
sufficient heat~dissipating capacity. The anode is
made of refractory metal, suitably tungsten, and its
tip may be eroded slowly during operation. In order
to reduce such erosion and stabilize operation, an en-
larged head or ball 7 may be provided on the tip of
lead 6'. Such a ball is readily formed by directing a
plasma -torch on the upper end of the wire while it is
held upright. By way of example of dimensions, for the
illustrated lamp the anode is tungsten and the shank
may be 9 mil and the ball 7, 20 mil in dlameter.
Z5 The invention relates particularly to the cathode
structure 10 formed upon or attached to the end or in-
lead 6. As best seen in FIG~ 2 or in FIGS. 4 and 5, the
cathode proper comprises a hollow helix 11 which may be
described as consi~ting of a coiled primary mandrel 12
around whose coiled turns i5 wound a smaller primary
wire forming an overwind 13. sath primary mandrel and
primary wire are retained in the comple.ed cathode.
The wires are of tungsten or of other refractory me-tal
suitable for electrodes. As shown ln FI~. 3, the over-
3, wind wire 13' is wound around mandrel 12', such that
io53~73
LD 7759
when the composite of 12' and 13' is tightly coiled a-
round a secondary ~andrel to pr~duce t~e helica} struc~
ture 11, the turns of the over~ind or primary wire inier-
digitate, producing a spacing of one primary wire di~
ameter 13 between tl~e turns 12 of the primary mandrel
forming t~e main ~elix 11.
Our helical electrode with overr~ind meets the pre-
viously stated criteria ~or good design, namely maxi-
mized surface area for more rapid glow-to-arc transi-
tion~ and controlled heat conduction loss into th~ seal.The surface area o~ the helix is large by comparison
with that o~ a shank type electrode, even one with an
overwind. The long helical path ~hich conduction heat
from the electrode tip must follo~ greatly reduces the
loss of heat ~y comparison with that in a shank type
electrode. The overwind by providing points of support
between turns of the helix assures structural rigidity
which is particularly difficult to achieve in a small
electrode.
The density of overwind turns 13' o~ the mandrel
wire 12l, that is the pitch ratio, is determined by con-
siderations o~ electrode sur~ace, thermal conductivity
and structural rigidity with compromises or txade-off
between these. It is desirable for stability to approach
at least 3 evenly distributed spacers or xest points per
turn of the primary mandrel (meaning that the angular
interval between rest points cannot ~e much less than
120); less than 3 reduces rigidity and onl~ 2 rest
points per turn corresponding to 180 between rest
points) is of course inadequate. A density or pitch
ratio of 1-1/2 turns of overwind ~ire 13' per turn of
primary mandrel 12 generates 3 rest points per turn in
the electrode structure. Elowever the distribution is
uncertain, and unless the rest points are evenly spaced
circumferentially when the overwirld turns interdigi-
tate, the rigidity may be lessened. For this reason
3~3
LD 7759
_ g _
a minimum of 3 over~nd turns 13 ? per turn o~ the primary
mandre~l 1i2' is preferred as illustrated in the drawings~
This generates 6 rest points which assures rigidity even
under the worst condltion of contiguous pairing of rest
points wh~ch would e~feckively reduce the 6 to 3.
The separation and resulting thermal insulation between
primary mandrel turns ~hich the overwind assures is even
more important later in the li~e of the cathode when sinter-
ing, in the absence of spacers, woula tend to increase the
thermal contact between mandrel turns. With the structure
provided by our invention, sintering merely makes the helix
into a mechanically stronger structure without significant
change in heat flo~ characteristics~ This ls a great ad-
vantage over other structures such as loop electrodes which
tend to change shape during operation of the lamp, especial-
1~ when the loops are made or ~ine wire as they must in
miniature lamps.
The helix 11 is attached to inlead 6 in a mannex
to project distally into the envelope. A convenient way
to do so is to spud the helix onto the end o~ the wire
inlead or shank 6. For such an attachment, the bore
(diameter of the axial cavity) of the helix is made
slightly less than that of the inlead; this causes the
helix to expand slightly over the extent 15 as it is
screwed onto the inlead and assures a tigh-t grip. -With
our helix which caxries an overwind, the overwind pre-
vents direct contact betwean wire 12 of the helix and
the shank 6 and thus assures low-thermal conduction into
~ the shank. Another convenient manner of attachment is
; 30 by welding which may be used where greater thermal con-
duction into the shank is desira~le. In ~IG. ~, the
helix is shorter and ~ewer turns are spudded onto the
shank or inlead than in FIG. 2; our electrode config-
uration facilitates such variations in design to achieve
the desired heat balance. The portion 15 of the elec-
trode which grips the shank 6 may be embedded in the
.5;3'73
LD 775g
-- 10 --
silica as shown in FIG. li embedding makes it easier to
center the electrode in the bulb at the sealing step in
lamp manu~acture.
- In FIGS. 1 and 2, the electrode 10 is ter~inated
in a solid cap 14. Such an end provides a place where
a hot spot may ~evelop where the arc attaches during
normal operation and reduces the rate of erosion o~ the
electrode. An end cap may be formed simply by heating
the end O F the electrode~ suitably by a plasma torch,
and melting back the last few ~urns of he helix. A1-
ternatively, a small mass of suitable refractory metal
may be welded or sintered to the distal end of the helix
. portion 11.
In FIG. 4 the helix is not terminated by a solid
end cap and FIG. 5 merely shows the electrode in end
view. Some sintering together of the helix wire 12 and
overwind wire 13 ~ill occur in operation, particularly
at the distal end where the hot spot attaches in opera-
tionn In a metal halide lamp wherein thorium.iodide is
present, a bare electrode may be used as illustxated; in
.~ other metal ~apor lamps a coating of electron-emissive
material may be desirable and, in such case, the helical
structure and the overwind are useful to retain the coat-
ing~
~- 25 According to an optional feature of the in~ention
.. even greater structural rlgidit~ may be achieved by pre-
~ loading or over~inding the turns o~ the helix~ Greater
: rigidity may become relatively more important as lamp and
electrode are miniaturi~ed. When line or wire is coiled,
there i5 an inherent or equi~alent twist of 360 per
: loop put into it~ This is rea~ily seen when one pulls
a line sideways off a spool instead of unrolling it from
the spool; a 350 twist appears in the line for every
loop pulled ofE the spool. If a twist greater than 360
per loop i9 put into the line, it may be said to be over-
twisted or preloaded and the result is a built-in tor-
5~3
LD 7759
sional stress that, in the ca~e of a close-wound heli~
o~ resilient wire, biases the turns laterally together
and maintains them in tig!lt side-by-side contact. This
may be observed in preloaded springs, such as those fre-
quently used to close screen doors of houses; the springwill not stretch and the turns t~ill not open up at all
until a certain minimllm force is excee~ed, and t~ere-
alter the stretch is proportional to the excess of ~orce
over the minimum. This minimum corresponds to the built-
- 10 in torsional stress or pre-load that biases the turns
together.
An overt~ist or preloaded condition may be achieved
by putting a t~ist in the proper direction into t~e com-
posite 12',13' prior to or during coiling around the sec-
ondary mandrel. There are two coilings whic~ occur inthe electrode structure, that of overt~ind 13' around man-
drel 12' ~FIG. 3~ and that of the composite 12,13 ~FIG.
2~ around a mandrel (not shown but corresponding general-
ly to shank 6~. If both coilings are wound.in the sam~
di~ection (both left-hand or both right-hand), the ~inal
structure t~ill have the overwind 13 tight on its mandrel
wire 12. ~owever, i~ the coilings are wound in opposite
directions, the ~inal coiling will cause overwind wire
13 to loosen on its mandrel 12. T~is generates clear-
ance between the two exposing more o~ the electrode sur-
face and creaiing fissures for emission mix. ~he fissure
may also serve as an electrid field concentrator. This
. . ef~ect.can be utiIized over a wide range to vary the
physical characteristics of the electrode.
The ~ollowing is an example of a cathode in ac-
cordance ~ith the invention suitable for a 35 watt metal
halide lamp operating on d.c. current in the range of
200 to 500 milliamperes. The primary wire 13' is 5
mil tungsten and the primary mandrel 12' is 7 mil tung-
sten wire. The primary coiling is relatively open and
suitably provides an advance about equal to tt~ice mandrel
,~ j t~ '3
LD 775g
- 12 -
diame-ter, that is 14 mil per turn, the objective being
to hav~ a~pro~lmat~ly 3 tuxns of the o-~érwind around
the primary mandrel per turn of the primary mandrel
arouna the secQndary mandrel. Prior to coiling around
the secondary mandrel, the composite wire resulking
from the primary coil~ng is pretwisted approximately 1
turn per inch. If the primary coiling was conventional
right-hand coliling, then the pretwisting is done with a
rig~t-hand twist which has the ef~ect of loosening the
overwind slightly. Finally the composite is coiled with
a left-hand coiling around a 9 mil secondary mandrel
(not shown). In this example the coiling sequence re-
sults in a slight loosening of the overwlnd. The sec-
ondary mandrel is molybdenum and is dissolved out by
nitric and sulfuric acid which do not attack the tung-
sten wires, and the helix is then spudded onto an inlead
of more than 9 mil to assure a good grip.
