Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 35-EL-1496
AN ARC LA~P; LIGHTING UNIT ~ITE MEANS TO PREVENT
PROLONGE~ APPLICATION OF STA~TING POTENTIALS
The present invention deals with a lighting
unit energized from a 110 volt 60 hertz source
in which the principal source of light is an arc
lamp supplemented by a standby filamentary lamp,
and which includes a power supply unit providing
high frequency energy for starting the arc lamp
while using dc for normal lamp operation.
The present invention deals with a lighting
unit in which the principal source of light is a
high pressure discharge lamp having up to six times
the efficient of an incandescent lamp. High
pressure metal vapor lamps have been available for
some time in high power units. Recently, as
disclosed in U.S. Patent Number ~,161,672
issued July 17, 1979 to Cap and Lake, entitled
"High Pressure Metal Vapor Discharge Lamps of
Improved Efficiency" and assigned to the present
assignee, smaller, low wattage metal halide lamps
with efficiencies approaching those of the larger
size have been inverted. Such lamps are an
energy efficient replacement for the incandescent
lamp. When part of a lightiny unit suitable for
.~
3~S5
35-EL-1496
-- 2 --
home use, provisions must be made for standby
illumination and assuming two light sources,
means must be provided for supplying the diverse
electrical requirements of the two light sources.
The power supply of the present lighting unit
employs a hi`gh frequency power supply in which a
ferrite tra~sformer controlled ~or non-saturated
operation, a transistor switch, and a trigger
oscillator are the principal components. Such
lQ power supplies have been termed static inverters
in de~erence to the fact that "dc" quantities
are converted to ac through static or non-moving
parts.
In the starting sequence, such a power will
initially produce high frequency energy at a hi~h
voltage for starting the arc lamp, after which the
power supply reverts to a dc mode in which continuous
dc is supplied. In such supplies r failure of the
arc lamp may cause the supply to remain in the
starting mode for an indefinite period. Continued
operation in the starting mode is undesirable,
resulting in the production of unnecessary
electromagnetic interference and wasted power.
Accordingly, it is an object to the invention to
provide a lighting unit combining a main arc lamp with
~3~55
_3 35-EL~1496
a standby filamentary lamp having an improved operatin~
network providing energization at an above audible
frequency during starting and dc energization during
warm-up and normal lamp operation.
It is another object of the invention to provide
an improved lighting unit combining a main arc lamp
with a standby filamentary lamp having ar. improved
operating network providing high frequency starting
energization and direct current running energization
for the arc lamp, wherein means are provided far dis-
continuing the starting energization for the arc lamp
if the arc lamp does not start within a reasonable
period.
These and other objects of the invention are achieved
in a novel lighting unit utilizing an energy efficien~
metal vapor arc lamp as the main source of light, supple-
mented by a standby filamentary light source, the filament
thereof serving as a resistive ballast for the arc lamp.
The lighting unit also includes a dc power supply and an
operating network for converting 120V 60 hertz energy
into the forms needed for operating the main and standby
lamp.
The operating network comprises an electrical trans-
former, a monostable, normally noncon2ucti~e, solid state
switch, a trigger oscillator ~or causing intermittent con~
duction of the solid state switch, and latching means,
typically a positive temperature coefficient resistor for
la~ching the trigger oscillator in a non-oscillatory
condition when intermittent operation is excessively long.
~ith the production of trigger pulses prevented, the solid
state switch remains off and starting potentials are dis-
continued.
In the practical circuit, the main arc lamp is
connected in a series path between a node and one output
t~rminal of the dc supply. The filamentary resistance is
connected betwe~n the other output terminal of the dc
~3~5S
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supply and the node for normal energization and resistive
ballasting of the main lampO The electrical transformer
has a primary winding connected between the other supply
output between the terminal and the node~ and a second
winding connec~ed ~etween the node and one terminal of the
main lamp for applying lamp starting potentials.
The monostable, normally non-conductive, solid state
switch comprises a transistor connected between the node
and the one supply output terminal. Intermittent operation
of the switch develops an alternating potential in the
primary winding and a transformed alternating potential in
the second winding for starting or restarting the main
lamp. A pulsating current in the filamentary resistance
is also produced for standby illumination during starting.
The trigger oscillator is responsive to the electrical
state of the main lamp for causing intermittent switch con-
duction for starting or restarting the main lamp, and COM-
prises a second transistor having base, emitter and
collector electrodes.
~0 The thermal latch comprises a positive temperature
coefficient thermistor, thermally coupled to a member ex-
periencing a greater temperature rise during starting or
restarting than during normal energizationr typically the
switching transisto- The thermistor is electrically con-
nected in the current path between the collector of the
second trigger oscillator tra~sistor and the node. ~hen
the thermistor is at a low temperature corresponding to
normal operationr its resistance is low and trigger oscilla--
tions are permitted. Whan at an elevated temperature
corresponding to abnormal operation, its resistance is high
and trigger oscillations are stopped by biasing th~ second
transistor into saturation. The saturation current level is
set to provide sufficient self-heating in the ther~istor to
keep it at a high resistance adequate to prev~nt further
oscillations until the lighting unit is de-enexgized and
the ther~istor is allowed to cool.
5S
- -5-- 35-EL-1496
~ he trigger oscillator is a relaxation oscillator
whose base and emitter electrodes are used to sense the
voltage and the current in the arc lamp for starting or
restarting. Using saturation induced by a collector con-
nected thermistor as the mean~ of turning off the triggeroscillator when the starting process is too long, does
so without interference with the starting sensors.
In accordance with another facet of the invention,
one terminal of the positive temperature coefficient
resistor is electrically connected to the node to which
the collector of the switching transistor is also connected.
This permits the mechanically convenient mounting of the
positive temperature coefficient resistor to the metal
collector tab of the switching transistor for efficient
thermal coupling.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel and distinctive features of the invention
are set forth in the claims appended to the presen~ -
application. The invention itself, however, together with
further objects and advantages thereof may best be under-
stood by reference to the following description and
accompanying drawings in which:
Figure 1 is an illustration of a novel lighting unit
suitable for connection to a standard lamp socket using
~5 an arc lamp as the principal light source, and a standbY
light ~ource and a compact power supplv unit nroviding
starting and operating potentials for the light sources;
Figure 2 is an electrical circuit diagram o the
lighting unit,
Figure 3 is an illustration cf a ferrite transformer
which forms a portion of the power supply unlt; and
Figure 4 is an illustration oE the switching power
transistor which forms a portion of the power supply unit
with an attached positive temperature coefficient thermist-
or designed to prevent prolonged application of starting
potentials after arc lamp failure.
~3~)~5
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DESCRIPTION OF PREFERRED EMBODIMENT
Referring now to Figure 1, a lighting unit for
operating from a conventional low fxequency (50-60 Hz)
alternating current power source is shown. The lighting
unit comprises a lamp assembly which produces light, and
a power supply unit which supplies electrical power to
the lamp assembly, with certain elements of the lighting
unit having dual light production and ballasting functions.
The lamp assembly includes a glass enclosure 9 which con-
tains a high efficiency arc lamp 11 and filamentary re-
sistance elements 12 and 13 for both ballastlng a-nd
supplemental light production. The power supply unit
includes a rigid case 10 attached to the glass enclosure
9 and a screw-in plug 14. The plug 14 provides both
electrical connection and mechanical attachment of the
lighting unit to a conventional ac lamp outlet. The power
supply unit develops the required energization for the arc
_ lamp during starting and operating conditions, provides
immunity to certain line transients, and supplies power
for supplemental incandescent illumination as needed
during starting of the lamp.
The lighting unit may be switched on, restarted, or
turned off with the same immediate productioll of light as
ar. incandescent lamp. During the half minute periods that
it may take for the arc lamp to reach full brightness af~er
a cold start or the longer periods required for a hot re-
start, supplemental incandescent illumination is provided.
The disposition of the elements of the lamp assembly
are best seen in Figure 1. ~he arc lamp 11, the 60 watt
filamentary resistance 12, and the 40 watt filamentary
resistance 13 are all installed insidethe single
large glass envelope 9. The elements 11 through 13
are supported on leads sealed into the base of the lamp
assembly. The gas filling the envelope 9 ls an inert
gas suitable for a conventional incandescent lamp.
The arc lamp 11 is shown with the positive electrode
~3~55
-7- 35-EL-1496
or anode down (near to the base) and the negative
electrode or cathode up (remote from the base). The
two electrodes are in turn sealed into the ends of a
small quartz vessel whose outer contour is cylindrical
except for a small central region of larger cross
section, of less than 1/2" in diameter. The interior
of the arc lamp, which is not specifically illustrated,
contains a spherical or elliptical central chamber
filled with an ionizable mixture: argon, an ionizable
starting gas, mercury, which is vaporized when hot, and a
vaporizable metal salt such as sodiu~ and scandium
iodides. When operating, an arc is formed between the
electrodes which creates illumination throughout the
chamber. Small, low power lamps of the type just
described a e referred to as metal halide or metal
vapor lamps. A suitable lamp is more fully described
in the earlier cited U. S. Patent No. 4,161,672.
Light production is shared between the arc lamp 11
and the filamentary resistance 12, while the latter and
the filamentary resistance 13 provide resistive ballast-
ing for the arc lamp. In dimmed operation, the current
levels and therefore the brightness of the arc lamp is
reduced by the imposition of resistance 13 in the current
path. In normal "final run" operation, the filamentary
resistance 12 tand 13 if dimmed) conducts the current
lowing in the arc lamp but primary light gereration
occurs in the arc lamp. During normal final run operation
the power supplied to the arc lamp is dc having some low
frequency (Sa-60 Hz) ripple which is unobjectlonable from
the point of view of electromagnetic interference (EMI).
In starting or restarting and warm-up of the main
arc lamp, the filamentary resistance (12 primarily)
produces supplemental illwmination. In starting or re-
starting, the power supplied to the arc lamp and filaments
36 has substantial high ~requency content, which may be
ob]ectionable from the point of view of electromagn~tic
~3~5S
-8- 35-EL~1496
intererence. In accordance with the invention, means
are provided for preventing prolonged starting r such as
might occur when arc lamp failure precludes ignition.
The arc lamp exhibits several distinct states
in conventional use and each active state requires
distinct energization. From a practical viewpoint, the
arc lamp has three essentially active states denominated
Phases I - III and an inactive state.
In Phase I, "ignition" occurs. The duxation of
ignition should be no longer than a second or two and is
often much shorter. It is the time required for a suitably
hign voltage to cause "electrical breakdown" of the gas
contained in the arc lamp to initiate a falling
maximum lamp voltage. This latter condition is also
referred to as the establishment of a "glow discharge".
Phase II - the glow to arc transition extends from
one-tenth of a second to perhaps two seconds and is
characterized by a more sustained ioniza-tion level and
a lower maximum voltage. As Phase II begins, the discharge
is typically unstable, swinging between a maximum and a
minimum value, with the voltage of the discharge falling
continuously toward a lower maximum with a recurring
minimum near 15 volts. As the average level of gas con-
duction increases, the maximum lamp voltage falls, the
consumed power increases, and the temperatures inside
the lamp also increase. As the maximum arc voltage falls
thro~gh values near 200-400 volts, a more substantial
energy (typically 2-4 watts) is required by a metal vapor
lamp.
Phase III begins with the establishment of the "arc"
which occurs when a portion of the cathode has reached
thermionic emission temperatures. At the marked transition
from Phase II to Phase III, the voltage of the discharge
loses its unstable quality and holds to an initial value
of about 15 volts. In Phase III, a sustained low lamp
impedance is exhihited, and current limiting is required
3~5~i
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to prevent e~cessive heating. At the beginning of
Phase III, the lamp dissipation is set to be between
lO and 15 watts and significant light pxoduction
starts.
The warm-up period, which is the initial portion
of Phase III, normally lasts from 30 - 45 seconds.
During the warm-up period, t~e lamp reaches full operating
temperature and the contained gases reach their high,
final operating pressures~ The voltage across the lamp
increases to a value of typically 87 volts with an
accompanying reduction in lamp conductance. When the `
final run condition occurs, the lamp absorbs the maximum
power (typically 32 watts) and the maximum light output
is produced.
The pre-ignition period is a variable period having
a nominal minimum value of zero at standard ambient con-
ditions and a maximum value between 45 seconds and 4minutes if there has been a failure of the arc and a hot
restart is required. If the lamp is de-energized in the
course of normal operation, the lamp will be at an
elevated tempera-ture and at a high gas pressure for a
short while. To restrike the arc when the lamp is hot,
the potential required may be in excess of an order of
magnitude more than for a cold start (e.g., 10-30 KV).
The thermal time constants of the lamp are such that the
time required fox cooling from a hot operating condition
to the point where a conventional (1-2KV) voltage will
restrike an arc may be from 45 seconds to 4 minutes.
Supplemental incandescent illumination is particu-
larly important during the longer warm-up and pre ignition
periods, but in tne interests of steady illumination,
supplemental incandescent illumination is provided tnrough
the shorter periods (ignition and the glow to arc
transition) as well.
Suita~le operating power for the arc lamp and the
standby light producing filament i5 provided by the power
supply illustrated in Figure 2.
3~5
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The lighting unit whose electrical circuit diagram
is illustrated in Figure 2, has as its principal compon-
ents the arc lamp 11, a dc power supply (14, 15, 16) for
converting the 120 volt 60 Hz to dc, an operating network
(17-37) for converting electrical energy supplied by the
dc power supply into the forms required for operation of
the lamp assembly and finally two filamentary resistances
(12 and 13) which perform a ballasting ~unction in the
operating network, and one (12) of which enters into the
production of standby light.
The dc power supply circuit of the generating network
is conventional. Energy i5 supplied from a 120 volt
60 hertz ac source via the plug 1~ and two input connections
to the ac input terminals of a full wave rectifier bridge
15. The positive output terminal of the bridge becomes
the positive output terminal of the dc supply and the
negative output te~minal o the bridge bridge becomes the
common or reference output terminal of the dc supply. The
filter capacitor 16 is connected across the output terminals
of the dc supply to reduce ac ripple. The output of the
dc power supply during normal run operation of the arc
lamp 11 is 1~5 volts at about 1/2 amperes current, pro-
ducing an output power of approximately 50 watts of which
32 watts is e~pended in the lamp. The power required o~
tne dc power supply by the lighting unit during a hot
restart is approximately 60 watts and the maximum
required during warmup of the arc lamp is approximately
75 watts.
The operating network, which derives its power from
tne dc supply, and in turn supplies energy to t~e lamp
assembly, comprises the elements 17-37 (optionally 12 and
13) connected together as follows: The filamentary re-
sistance 12 and 13, diode 17, arc lamp 11 and lamp current
sensing resistance 33 are serially connected in the order
recited ~etween the positive terminal and the common
terminal of the dc supply. A switch 18 shunts the
~3~DSS
~ 35-EL-1496
filamentary resistance 13 producing dimming of the
arc lamp when open and undimmed operation when closed.
The diode 17, which is poled for easy current flow from
the dc source to the arc lamp, has its anode coupled to
one terminal of the resistance 13 and its cathode coupled
to one terminal of the arc lamp 11. The arc lamp,
which has a required polarization, has its anode coupled
to the cathode of the diode 17 and its cathode coupled to
one terminal of the current sensing resistance 33.
Continuing with a description of the operating net-
work, a triggered monostable solid state switch is
provided, constituted of a power transistor 19, a step-up
transfonmer 20, and passive components 28, 29. The power
transistor has base, emitter and collector electrodes.
The step-up transformer 20 has a ferrite core for high
frequency operation (>20 Khz), a main primary winding 21,
a main second winding 22, a primary control winding 23
and a secondary contxol winding 24, all associated with
the core. The control windings, as will be described,
provide a transistor conduction control whose sense is
responsive to the magnetic state of the ferrite core and
produce monostable action, avoiding full core saturation.
The main primary winding 21 has its undotted terminal
coupled through the capacitor 25 to the positive source
terminal and its dotted terminal connected to the inter-
connection terminal or node 26 between filamentary resist-
ance 12 and 13. The main second winding of transformer 22
has i-ts undotted terminal connected to the node 26 and its
dotted termi~al connected through the capacitor 27 to
the anode of the arc lamp ll. The collector of the power
transistor is connected to the node 26. The emitter of
the power ~:ransistor l9 is coupled to the unmar~ed ~
terminal of the primary control windiny 23. The marked
terminal of the primary control winding 23 is connected
to the cathode of the arc lamp ll. The base of transist-
or 19 is coupled to the cathode of a clamping
~ ~3~5~i
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diode 28, whose anode is coupled through resistance 29
to the common dc terminalO The secondary control winding
~4 has its unmarked terminal coupled to the base of
transistor l9 and its marked terminal connected to the
emitter. The base of transistor 19 is the point for
application of a trigger pulse for initiating each
conduction cycle.
The operating network is completed by the transistor
30 which, with its associated components, forms a
triggering oscillator for recurrently turning on the solid
state switching transistor 19. The trigger oscillator is
turned on and off and also shifted in frequency in response
to electrical conditions attributable to the electrical
state of the arc lamp. The trigger oscillator
is also responsive to the temperature of the switching
transistor, thus preventing prolonged triggering in the
event of arc lamp failure. The transistor 30 has its
emitter coupled to the emitter of transistor 19, its base
coupled through the capacitor 31 to the base of translstor
19, and its collector serially connected through the
resistance 32 and positive temperature coefficient re-
sistance 37 to the node 26. A voltage sensing voltage
divider is provid~d consisting or resistance 34 connected
between the anode of diode 17 and the base of transistor
30 and resistance 35 connected between the base of trans-
istor 30 and the common source terminal. During warm-up
and final run operation, both dc states of the lighting
unit, the diode 17 is forward biased, and the divider
output voltage, at the base of transistor 30, is a direct
measure of the lamp volt~ge. During the high frequency
states of the lighting unit, the diode 17 is reversely
biased when power is delivered to the lamp, so that the
voltage on the voltage divider reflects the loading effect
of the arc lamp upon the transformer circuit and is an
3~ indirect measure of the lamp voltage. The connection of
the emitter of translstor 30 to the non-referenced
3~S~
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terminal of the resistor 33 in series with the arc
lamp 11, makes the trigger oscillator responsive to
lamp current in the rorm of the voltage proportional
to the lamp current developed in resistance 33. The
trigger oscillator is connected to respond in the
manner noted above to the difference in sensed voltages.
In pre-ignition, ignition and ylow to arc trans-
ition, the transformer 20, the transistor switch 19 and
the trigger oscillator (30, etc.) of the operating
network assume an active role in generating a high
frequency output. The change in electrical output to
dc occurring between the glow to arc transition and
warmup is in response to conditions in the main lamp.
More gradual changes in electrical output of the operat-
ing network occur between pre-ignition and ignition and
between ignition and the glow to arc transition, and
these changes are also in response to conditions in the
main lamp.
In pre-ignition, ignition and the glow to arc
~0 transition, the operating network produces short duration,
high voltage pulses for ignition of the arc
lamp, the voltage falling to a lower value in response
to lamp loading in the glow to arc transition. During
pre-ignition, the unidirectional high voltage pulses
have substantial ringing, and they occur at a rate of
50 KHz. In the glow to arc transition, the ringing is
reduced and the frequency shifts to 35 KHz. The downward
shift in frequency produces a shorter transistor con-
duction duty cycle, which increases the energy supplied
to the lamp in the glow to arc transition. The operating
network also supplies current to the filamentary resist-
ance 1~ in the form of a series of unidirectional pulses
ai the ~0-35 KHz rate.
The operating network produces the high frequency
electrical energization described above as a result of
high freauency switching of the monostable transistor
~3~755
-14- 35-E~-lq96
switch. Intermittent switching of the transistor switch
produces an alternating component in the main primary
winding 21 of the step-up transformer 20, a stepped up
alternating component in the transformer output and a
pulsating current in the filamentary resistance 12
which is primarily unidirectional.
Alternating current flow in the main primary windin~
takes place in the following manner. Assuming that the
transistor 19 has been turned on by a suitable trigger
signal coupled to its input junction, a displacement
current path is completed between the positive and common
terminals of the dc supply. That path comprises in order
the capacitor 25, the main primary winding 21, the NPN
switching transistor 19 (collector and emitter electrodes
respectively), the primary feedback winding 23 and the
current sensing resistance 33. The switching transistor
presents a low impedance when conducting, and the capacit-
or 25, the primary feedback winding 23 and the resistance
33 are also low impedances. As the current in the circuit
increases, the primary feedback winding 23, which is
inductively coupled to the secondary feedback winding 24,
produces regenerative feedback in the input circuit of the
transistor and turns it on more strongly. Accordingly,
when the transistor conducts, the current rapidly builds
up in the transformer primary winding, limited primarily
by the primary inductance. The current build-up continues,
however, until a prescribed flux level is reached in the
core of the power transformer. At that point, the feed-
back is inverted to become degenerative, turning off the
transistor 19 before rull core saturaiion is reached. The
The discontinuance of conduction through transistor 19
opens the prior path for current flow through the primary
winding and allows a portion of the energy stored in the
circuit to dissipate in the form of a reverse current
through the filamentary resist~nce 12. Thus, the current
flow, which was initially out of the dotted terminal of
3a355
-15- 35-EL-1496
the primary winding when transistor 19 was conducting,
reverses and the current now flows into the dotted
terminal.
The transformed version of the high frequency
alternating voltage appearing across the transformer
primary winding during pre-ignition, ignition, and the
glow to arc transition appears at the terminal of the
winding ~2 remote from winding 21. The output is coupled
from the winding 22 by means of the capacitor 27 to the
anode of the arc lamp 11. The output takes the form of
unidirectional pulses by virtue of the presence o~ the
diode 17 whose anode is coupled through filamentary
resistance 13 (or the closed switch 18) to the undotted
terminal of the secondary winding and whose cathode is
coupled to the anode of the arc lamp. The diode 17 is
poled to permit application of a stepped-up secondary
voltage to the arc lamp developed during the reverse
current flow in the transformer primary circuit and to
- suppress application of the secondary voltage developed
during forward current flow when the switching transistor
is conducting. With the indicated parameters, and
assuming substantial ringing, the available pre-ignition
potential is the 1600 volts peak to peak referred to
earlier. Pre-ignition is nominallyof zero dura~ion when
the lamp is cold and from 45 seconds to 4 minutes when
the lamp is hot.
The transformer 20 is essentially an auto~transformer
although in certain respects it may be regarded as a c~n-
ventional transformer with separate primary and-secondary
windings. The windings 21 and 22 are serially connected,
and wound in the same sen~e and the input is applied across
the primary winding 21. When transistor 19 is conductive,
the co~mon terminal (node 26) between the primary and
second windings is at re~erence potential and the voltage
developed in the second winding reflects the primary to
second turns ratio 500/140, with the diode 17 providing a
~3~5~
-16~ 35-EL-1496
short circuit and precluding the application of an
output voltage to the main lamp. When the ~ransistor
19 is nonconductive, stored energy developed across
winding 21 and referenced through capacitor 25 to the B+
terminal of the power supply is released, and the device
appears as an auto-transformer with the transformer ratio
being 640/140. Thus, during the critical period when the
transformex is delivering energy to the arc lamp, the
transformer is in an auto-~ransformer configuration.
The current for standby illumination during pre-
ignition, ignition, and the glow to arc transition is
produced by high frequency switching of the transistor
switch. At the instant that the transistor switch becomes
conductive, a direct current path is completed between the
positive and common terminals of the dc supply. The dc
path includes the standby light producing filamentary re-
sistance 12, the transistor 19 (collector and emitter
electrodes, respectively), the primary feedback winding 23
and the current sensing resistance 33. The transistor 19
~0 presents a low impedance, when conducting, and the primary
feedback winding 23 and the resistance 33 are also low
impedances. At the start of pre-ignition, the resistance
of the filamentary resistance may also be low, and a large
initial current ensues. Self-heating is rapid, and the
resistance quickly reaches a relatively stable, larger
value near 200 ohms, which persists throughout the balance
of the starting procedure. The heat dissipation in the
filamentary resistance during pre-ignition is set primarily
by its own relatively large resistance, the duty cycle of
the transistor switch and the dc voltage available from
the dc power supply, and may be increased by adjustment
of these parameters.
In addition to the intermittent current supplied to
the filamentary resistance in the dc path just described~
3~ the return portion OL the alternating current flowing in
the primary winding 21 of the transformer also flows
~3~5~
-17- 35-EL-1496
through the filamentary resistance as discussed earlier.
During pre-igni~ion, with the secondary winding of the
transformer ~0 being substantially open-circuited, the
heating effect of the reverse current in the primary
circuit is negligible. During the glow to arc transition,
when the lamp draws the more substantial energy, the
alternating current adds significantly to the total
dissipation in the filament, in which pulsating dc
energization is reduced.
The operating network is responsive to the electrical
state of the arc discharge lamp to produce the outputs
previously characterized during pre-ignition, ignition and
the GAT period. The means by which this responsiveness
is accomplished includes the triggering oscillator (trans-
lS istor 30, etc.) lamp current sensing resistor 33 and the
vol~age sensing resistors 34, 35.
The trigger oscillator causes active operation of
the transistor switch 19 during pre-ignition, ignition and
the GAT eriod and controls the transistor duty cycle to
supply additional energy to the arc discharge lamp during
the GAT pericd. Since the transistor switch is monostable,
each trigger pulse supplied from the trigger oscillator
initiates a conduction sequence.
In the event that the arc lamp does not "start" (i.e.
reach warm-ùp) within a normal period and sensed conditions
still dictate continuing high frequency operation, then
after a prescribed additional period, the trigger oscillator
will discontinue oscillations and the power supply will
effectively ~urn itself off. Turn-off is achieved by a
thermal latch, responsive to the temperature of the trans
istor switch 19. When thermally latched, the po~er supply
can not be reactivated until it has been de-energized and
allowed to cool. The operation of the thermal latch, which
protects against a protracted starting sequence, will be
taken up at the end of the discussion of the trigger
oscillator.
~'3~S~
-18- 35-EL-1496
The trigger oscillator is activated at the time
the operatiny network is first energized, and remains
energized through the pre-ignition, ignition and glow
to arc transition. During pre-ignition, there is no
lamp current, while during ignition and the glow to arc
transitior., the lamp current increases to one-fifth of
an ampere peak in short pulses. The voltage developed
in the transformer primary winding at node 26 is high
(>300V) during pre-ignition, falls appreciably under the
loading affect of the lamp duriny ignition, and the glow
to arc transition, and consists of a series of pulses
initially with substantial ringing.
The foregoing current and voltage conditions
reflecting the lamp condition during pre-ignition,
ignition and glow to arc transition are sensed in the
operating network and combined differentially at the
input junction of the oscillator transistor, and used to
activate the trigger oscillator. Any lamp current flowing
in the lamp current sensing resistance, to which the
emitter electrode o the junction transistor 30 is
coupled via the low impedance feedback winding 23, pro-
duces a voltage in a sense tending to back-bias the input
junction. (The lamp current is zero at the start and
remains small during these lamp conditions.) The voltage
at the node 26 is applied across the voltage di~ider
34, 35, the output tap of which is coupled to the base
electrode of the transistor 30. The voltage appearing at
the node 26 is positive and a fraction (1/181th) of that
voltage is applied to the base electrode. Here, the
voltase is in a sense tending to forward bias the input
junction. During pre-ignition, the voltage at node 26
is a maximum and sufficient, assuming time has been
allowed for the capacitor 31 to charge up, to ~orward bias
the transistor 30 and initiate oscillation.
The trigger oscillator operates as a relaxation
oscillator 31 being recurrentl~ charyed through the
~3~SS
-19- 35-EL-1496
passive elements of the operating network and re-currently
discharged by the transistors 19, 30. The charging period
of capacitor 31 is determined primarlly by the value of
capacitor 31, the value of resistor 35 and, as will be
shown, the differential voltage applied to charge the
capacitor 31. The capacitor 31 has one terminal coupled
to the base of the transistor 30, the output tap on the
voltage divider 34, 35, and the other terminal coupled to
the base of the switching transisior 19. The other
capacitor terminal is led to ground through one path in-
cluding the reversely poled diode 28 serially connected
with resistance 29, and a second path including the
serially connected, low resistance, feedback windings
24, 23 to the unreferenced terminal of the lamp current
sensing resistance 33. The discharge of the capacitor 31
starts when transistor 30 begins to conduct, and is
completed after the transistor switch 19 is turned on by
transistor 30. With both transistors conducting, both
te~minals of capacitor 31 are coupled through a conduct-
ing junction to a common point, discharging the capacitor
31, and removing the forward bias on transistor 19, turning
it off. The turn-off action of the transformer 20 leaves
a residual inverse voltage on th~ capacitor at the end of
switch conduction.
As an examination of the circuit will show, when
sufficiently hi~h potentials are present at the node 26
and assuming a low lamp current, the oscillator will start
to conduct when the capacitor 31 reaches the value re-
quired to fo~ard bias the input junction of the transistor
30 (+0.6 volts) as indicated above. The voltage on the
capacitor is determined by the difference between the
voltage at the voltage divider output and the voltage due
to lamp current in resistor 33.
Once the transistor 30 conducts, current flows in
the primary feedback winding 23 and the strongly regenera-
tive reedbac~ action involving secondary feedback winding 24
;3-~55
-20- 35-EL-1496
and capacitor 31 produces a short duration trigger
pulse for turning on transistor switch 19.
The initial starting conditions and charging
interval for each oscillation of the relaxation oscillat-
or-are established by the operating ne~_work. The capacitor
31 is fully discharged when both transistors 19 and 30
become conductive. The capacitor 31 assumes a reverse
charge as a result of the feedback reversal in windings
23 and 24 attributable to maximum conduction by transistor
switch 19. When conduction ends, a conduction inhibiting
voltage o~ approxima~ely 4 or 5 volts is produced on
capacitor 31. The inverse voltage is limited by the
serially connected diode 28 and resistor 29, and repre-
sents the starting point for each charging interval of
the relaxation oscillator. While the transistor switch
19 is conducting, the virtual generators embodied by
the voltage divider 34, 3S and the current sensing
resistor 33 are inactive, precluding recharging of the
capacitor 31, and precluding the starting of the ne~t
oscillation cycle.
Assuming that the arc lamp current has begun to
flow and the voltage across the lamp has begun to increase,
t~e differential voltage used to charge capacitor 31 falls
on the average, increasing the period required to turn on
the transistor 19 and initiate the next trigger pulse.
This provides more time for the energy stored in the input
circuit o~ the operating network to be released to the
lamp. Earlier in the starting cvcle, the lamp cathode
current may be truncated by the next conduction interval,
and less stored energy is delivered to the arc lamp. The
circuit has been designed so that the nonconduction
period is maximum when the lamp voltage is in the glow
region (appro~imately 200-400 volts), to maximize
the output power at about 9 watts for metal vapor lamps.
The chargi~g time constant is about 5 microseconds
and provides some smoothing within each pulse, re~ucing
.
~3~55
-21- 35-EL-1496
the noise sensitivity, but negligible pulse to pulse
averaging. The principal function of the capacitor
31 is to serve as the integrating capaci~or in the RC
network used to time the off interval of the power
transistor.
During pre-ignition, ignition and the glow to
arc transition region, high frequency operation contin-
ues, with the trigger oscillator recurrently turning
on the transistor switch 19 while the transistor switch
turns itself off through feedback reversal in the trans-
former 20. The trigger oscillator transistor 30 is
turned off shortly after conduction by transistor switch
19 removes the conduction favoring charge on capacitor 31.
Transistor 30 remains quiescent through the balance of
lS switch conduction. Turn-on of the transistor switch is
achieved through the coupling of the base electrode of
transistor 30 through the capacitor 31 to the base of
transistor l9, the interconnection of the emitters of
transistor 19 and 30 together, and the shared connection
of the transistors 19 and 30 to the transformer feedback
windings ~3 and 24. ~hen transistor 30 becomes forward
biased, and starts to conduct, emitter current is develop-
ed in the primary feedback winding 23. This produces the
regeneration needed to create a trigger pulse on the
order of l/lOth ampere and having a sub-microsecond
duration at the secondary winding 24. ~he trigger current
23 flowing in the secondary winding 24 turns on the main
switching transistor 19, initiating monostable switching
action. Transistor 19 completes its con~uction cycle,
which is set by transformer design to be shorter than the
interval between trigger pulses, and turns off in response
to the reversal in feedback provided by the feedback
windings 23, 24. High frequency operation o~ the switch
continues so long as the trigger oscillator generates
trisger pulses.
~;3~55
-22- 35-EL-1496
Once the arc lamp has reached thermionic operation
corresponding to warm-up, the high frequency output pro-
duced by transistor switching is designed to stop and
the dc state commences. The trigger oscillator 31,
S which triggers the monostable transistor switch l9 into
active operation, remains reversely biased due to a new
set of current and voltage conditions in the operating
network and becomes inactive. The rectified high
frequency voltage at node 26, previously applied across
the voltage divider 34, 35 is replaced by a sustained dc
voltage with some ripple, represen~ing the lamp voltage.
The dc voltage continues in a sense favoring conduction,
but is lower by 1 or 2 orders of magnitude. The diode 17,
now forward biased, connects the voltage divider across
the lamp, and the voltage divider now senses 1/181th of
the new lamp voltage, initially 15 volts. Simultaneously,
a maximum initial lamp current of 6/lOthS of an ampere
occurs in resistor 33, developing a conduction inhibiting
voltage of approximately 1.2 volts. The differential
voltage produces a reverse bias on the input junction of
the transistor switch l9.
As warm-up continues into final run condition t the
lamp voltage rises and the lamp current falls. The lamp
condition sensors are set to keep the trigger oscillator
inactive through warm-up and final run. In final run, the
lamp reaches a current of 0.3 amperes and a voltage of 87
volts. Should the lamp voltage rise 10 volts above the
normal value (e.g., 97 volts) and the current fall ko
0.050 ampere, then the trigger oscillator will b* reactivat-
ed as a safeguard against transistor dropout.
If the arc lamp does not reach thermionic operationduring a predetermined period, the operating network is
designed to turn itself off and to so remain until it is
de-energized and allowed to cool. The turn-off mechanism
is automatically responsive to the temperature rise of the
power transistor l9. It relies on the circumstance that
~ ~3;~55i
~23- 35-EL 1496
higher temperatures occur in the power transistor -
which operates only during starting or restarting - when
the arc lamp is either not starting at all or starting
very slowly. Normal starts or restarts are of shorter
duration and because the periods involved are shorter
than the times required for the devices to reach thermal
equilibrium, lower temperatures are attained. In short,
the temperature rise of the switching transistor may be
regarded as a reliable symptom of arc lamp or circuit
malfunction. In accordance with this principle, the
system is designed to shut down when pre-assigned
temperatures are exceeded in the switching transistor.
Without such a provision an unattended lighting unit
would continue to operate in the starting mode for an
appreciable period - normally measured in days. The
technique herein described avoids the generation of
prolonged electromagnetic interference and wasted power.
The positive temperature coefficient resistor or
thermistor 37 is the sensor for the automatic turn off
mechanism ~ust described. As shown in Figure 2, the
positive temperature coefficient resistor 37 is electric-
ally connected in the collector current path of the trigger
transistor 30, being connected between the one kilohm
resistance 32 and the node 26. As shown in Figure 4, the
PTC resistor 37 is attached through an optional inter-
mediate metallic member 39 to the heat conducting
collector tab of the power transistor 19, the attachment
providing both mechanical support to the PTC resistor and
good ther~al contact between it and the power transistor.
The electrical connection of one thermistor terminal to
the collector electrode of the power transistor is permitted
since both are connected to the node 26.
The positive temperature coefficient rssistor or
thermistor may be of a conventional type. When exposed
to a heat source, causing the device to exceed a threshold
temperature, the device e~hibits a very strong incrPase in
~3~35~
-24- 35-EL-1~96
resistance with tempeature. The rate of resistance
increase may be by a factor o 1~ for each 10C increase
in temperature. The conventional thermistor consists of
a suitably electroded semiconductor ceramic. The ceramic
S is manufactured by sintering a material whose principal
ingredient is barium ~itanate and whose exact composition
is adjusted by non-stoichiometric combination and/or the
addition of small quantities of other materials for
optimi2ation of its semi-conducting properties. The
thermistor frequently takes the form of a shall cylindric-
al member with electrodes on the ends. A suitable
thermistor for use in the present application has a value
of less than 20 ohms when below the knee of the resist-
ance/temperature curve which occurs at 70. Values in
excess of 10,000 ohms may be reached when the thermistor
is exposed to temperatures in excess of 110C.
In the practical circuit illustrated in Figure 2,
there is a "window" between the transistor temperatures
accompanying a normal start or restart and those implying
lamp circuit abnormality. At normal temperatures, the
thermistor has a value sufficiently low that its influence
on the circuit is negligible. On the other hand, at
elevated temperatures, the resistance value becomes
sufficiently high to preclude normal operation of the
trigger oscillator.
The operation of the thermal latch will now be
explained. During a normal start, the transistor switch 19
may operate for a few seconds before the arc lamp "starts",
i.e., begins ~hermionic emission, after which the power
transistor is turned off. For a start at room temperature,
the case of thQ lighting unit is typically 28C. During
the starting process, the temperature of the power transist-
or may increase slightly but it will remain well under the
knee or the thermistor resistance characteristic. As
normal operation or the arc lamp continues, the trigger
oscillator and the switchin~ transistor remain quiescent,
but the temperature in the cas~ housing the thermistor and
~3i~35~i
-25- 35-EL-1496
power supply unit will increase from toom temperature to
an equilibrium tempèrature of 70C. ~t this point, the
thermistor is still below the knee of its temperature
characteristic and has a value of less than 30 ohms.
A "hot restart" provides a set of higher tempera-
ture conditions still indicative of circuit normalcy.
In a hot restart, the user has turned off the lighting
unit, but without allowing it to cool has turned it back
on again. Under these conditions, the arc lamp is at
maximum temperature (much higher than the room temperature)
and pressure and will not respond to the available ignition
potentials until it has been allowed to cool. The cooling
o the arc lamp to the point where it will ignite,
normally takes from 45 seconds to several minutes. During
this period, the trigger oscillator and the switching
transistor continue to operate, producing normal ignition
potentials until the arc lamp has started (entered warm-up)~
At the beginning of th~ hot restart, the thermistor is at
the case temperature of approximately 70C. Assuming that
a single hot restart may require two minutes of operation,
the temperature at the tab of ~he switching transistor will
elevate as a result of self-heating from 5 to 10 degrees
above the case temperature. In the event that a second hot
restart is attempted, and assuming that no additional time
~5 is allowed for cooling, the tab temperature will increment
an additional 5 to 10 degrees. This final temperature,
assuming a second voluntary hot restart and normally lying
between 85 and 100, corresponds to the highest temperature
that one may expect assuming normal operation. This
temperature sets the lower boundary to the thermal "window"~
The upper limit to the thermal window is the temperature
that one would expect from a failed lamp. In the event of
a failed lamp, the tab temperature will normally continue
to elevate until thermal equilibrium is reached in the
vicinity of 125C.
. ~ _ L ~ 5
-26- 35-EL-1496
Under the indicated circumstances, the positive
temperature coefficient resistor should be selected to
cause a circuit response at tab temperatures between
90C and 123C, or typically 100C. A satisfactory
positive temperature coefficient resistor exhibits a
knee in its characteristic of 70C, a cool resistance
of approximately 10 ohms at 28C and 10,000 ohms in the
vicinity of 105C.
The circuit is designed to respond to the resistance
change of the thermistor, when it reaches approximately
3000 ohms, corresponding to a temperature near 100C.
When this resistance is reached, the current pulse from
the trigger transistor is reduced from its maximum value
to a value no longer capable of turning on the power
transistor. Since the power transistor cannot come on to
discharge the capacitor 31, relaxation oscillations cease.
Without the drain produced by the switching transistor 19,
the voltage on the electrolytic capacitor 16 rises to
about 170 volts. The dc circuit conditions force the
trigger transistor 30 into medium current (10 ma) saturation.
Base current is supplied by the 180K resistor 34 to
forward bias the input junction. The output junction loses
in reverse bias, with a major portion o~the B+ voltage in
the collector circuit being developed across the positive
temperature coefficient resistor 37, which now has a large
value. Under the indicated conditions, collector current
flowing through the thermistor will be about 10 milliamperes,
producing a dissipation of about a watt in the thermistor.
This dissipation is set to maintain the thermistor in a
high resistance state (greater than 3000 ohms) fxom self-
heating. The power unit will remain latched in a low
current latched condition so long as the power remains on.
Should the power be removed long enough for the thermistor
to cool down below the transition temperature (a period of
above 5 seconds) the circuit will again revert to a hot
restart condition. The present circuit reaches the latching
3~35S
-27- 35-EL-1496
state after about 10 minutes of hot restart operation,
and unlimited starting is precluded.
The thermistor must meet several requirements. The
thermisto~ must reach a sufficiently high resistance
state in response to an abnormal temperature rise in the
switching transistor (19) to halt oscillations by the
trigger oscillator, this result being accomplished by
forcing the oscillator transistor 30 into medium direct
current saturation, pre-cluding significant current gain.
The path for direct heat trans~er from the switching
transistor to the thermistor is of high thermal conduct-
ance, being designed to conduct sufficient heat to the
thermistor in the context of other cooling ef~ects to
bring the thermistor to the necessary high value within
the desired latch-out period.
Once at the elevated temperatures required to stop
oscillations, the high resistance value and the saturation
current level in the transistor 30 must provide sufficient
"self" dissipation in!the thermistor to sustain the
thermistor temperature at a value required to prevent
further oscillation, i.e., provide a thermal latch. The
saturation current is adjusted in part by the resistance
values in the voltage divider network (34, 35) and the
collector load resistances including the thermistor.
As~stated earlier, a current of ten milliamperes is a
practical value.
Thirdly, the thermistor is selected to be non-responsive
to sel~-heating at the current levels corre~ponding to
normal oscillations at maximum normal temperatures. The
trigger oscillator is a relaxation oscillator, which
permits the periods of conduction and non conduction to be
unequal. In the interests o~ energy conservation, the
trigger oscillator should dissipate a relatively small
amount of energy in relation to that dissipated in the
switching transistor which it controls. For this reason,
it is desirable that the conduction periods be very short
3 C~ 5 ~
-2~- 35-EL-1496
and the duty cycle low. The conduction need only persist
for a few hundred nanoseconds with current peaks of
100 milliamperes at a duty cycle of one-fiftieth for
effective triggering. Under these conditions, the
average current is 2 milliamperes. Assuming that the
thermistor is at the maximum design resistance, e.g.
10K, the self-heating effect of the thermistor is .040
watts and insufficient for self-latching, assuming
typical cooling within the case 10. On the other hand,
assuming 10 milliamperes current during saturation of
the transistor 30 and a 10K thermistor resistance, the
self-heating effect is approximately 1 watt and
su~icient for thermal latching.
The time constant for activation of the thermal
latch may be controlled by adding thermal insulation or
ventilating the thermistor; by choice of a larger or
smaller total thermal mass, including that of the
thermistor, the transistor and the mechanical support;
and by increasing or decreasing the thermal conductivity
between the power transistor and thermistor. The primary
heat source or thermal generator is the power transistor 27.
(A second potential heat source, not exploited in the present
embodiment is the current sensing resistor 33.) The power
transistor normally is on less than a minute, which is an
insufficient time for the transistor or the attached ther-
mistor to reach terminal temperatures. By adding to the
associated thermal masses it is possible to delay the sensed
temperature rise by one or more minutes. In addition, the
terminal temperatures reached by the power transistor and
attached thermistor can be made usefully high and well above
normal operating temperatures by controlling the surface
design and radiating area to xeduce convection, conduction,
and radiation heat loss. In short, it is practical to set
a threshold. Operation below this threshold may be char-
acterized as normal and allowed to continue and operationabove this threshold may be characterized as abnormal and
3~5~
-29- 35-EL-1496
fu~ther OperatiQn prevented.
Returning to the practicalities of thermal design;
several methods for mounting the thermistor to the tran-
sistor to establish practical electrical and thermal
S contact may be employed. One method applicable to the
disc-shaped thermistor employs conductive epoxy between
the thermistor electrode and the powex transistor tab or
between these elements and a third conductor (e.g., 38).
This provides both electrical connection between one side
of the thermistor and the power transistor collector and
a desirable good thermal path between them. The flying
lead o~ t~e thermistor is attached to the other ele~ctrode
for connection to the 1 kilohm resistor in the collector
circuit of the trigger oscillator. Insulation may be used
around the combination to speed up the latch-up process but
is normally not required. More heat sinking may be used to
slow the latch-up down. In a practical embodiment, the
metal conductor 38 is used, which also adds to the thermal
mass. It is a generally flat sheet of copper whose dimen-
sions are .375" x 1.00" x 0.010".
The thermistor employed should have a voltage ratingsuitable for the intended use and be of sufficiently low
electrical capacitance as not to interfere with the shut-
off process. Typical manufacturers' voltage ratings lie
between 120 and 170 volts. The capacitive reactance be-
tween the thermistor electrodes at the triggering fre-
quency should be small in relation to the latching resistance
of the thermistor. Preferably, the capacitance should be
no more than 500 picofarads. The proper capacitance value
may be achieved by selecting a thermistor with reduced
cross-sectional area and increased electrode spacing con-
sistent with maintaining adequate electrical power dissi-
pation to maintain the thermistor in a latched-up condition.
In a practical embodiment, the thermistor has a diameter of
approximately 1~4" and a thickness of 0.105".
The thermistor 37, as described, is ~oth electrically
355
-30- 35-EL-1496
and thermally-connected to the power transistor 19. The
thermal interconnection is desirable since it places the
thermistor in thermal contact with the circuit element
whose temperature experiences the greatest change in
temperature between normal and abnormal operation as
defined above.
The thermal contact of the thermistor to the collector
of the switching transistor is preferable to any other
circuit points because the temperature change there is
greatest between normal and prolonged staxting conditions,
and direct electrical connection may also be made. A
second choice is to the 2 ohm resistor 33, which also
becomes substantially hotter during prolonged starting than
during normal operation, but which has a substantially
narrower thermal window than the transistor collector tab.
While thermal contact between the thermistor and the
power transistor is essential, electrical contact is not.
Thus, the thermistor 37 could be intercharlged in the circuit
with the load resistance 32 of the trigger oscillator. ~~
The disadvantage of this interchange is that the thermal
contact between the thermistor and power transistor must
now be electrically isolated, as by the interposition of
a thin insulative barrier which would involve additional ex
pense. In practice, it is preferable to use the illustrated
simpler combined thermal and electrical contact. The il-
lustrated electrical contact is made at the node 26 where it
has no electrically adverse affect upon the operation of the
switching transistor 19. The position of the thermistor 37
in the collect~r circuit of the trigger oscillator transistor
30 is also advantageous in that in effecting saturation it
produces no impairment in the oscillator output function
except as it is desired, and no impairment of the input
response of the trigger oscillator to arc lamp voltage
and current which involve the base and emitter electrodes
3~ of the transistor 30.
