Note: Descriptions are shown in the official language in which they were submitted.
~ 33~ 410D-5434
The present invention relates to operating circuits for
gaseous discharge lamps, and more particularly concerns
direct current operating circuits for high pressure sodium
vapor discharge lamps.
The present invention concerns an improvement in the
A circuits disclosed in U.S. Patent No.
dated ~ ~
all assigned to the same assignee as the present invention.
It is a general object of the invention to provide
an improved DC operating circuit for pulsed operation of
loads.
It is a particular object of the invention to provide
an improved DC operating circuit for applying DC pulses
to gaseous discharge lamps, especially of high pressure
sodium vapor type, to produce improved color properties
of the lamp light output.
It is another object of the invention to provide a
circuit of the above type which is simple in construction,
efficient and reliable in operation, and relatively low ;~
in cost.
It is still another object of the invention provide
a circuit of the abo~e type for controlling lamp wattage
in respect to changes of lamp voltage.
Other objects and advantages will become apparent
from the following description and the appended claims.
With the above o~iects in view, the present invention
in one of its aspects relates to a lamp operating circuit
comprising a DC power source, a charging capacitor, a first
inductor in series with the charging capacitor connected
across the DC power source and forming therewith a charging
circuit for charging the capacitor, a second inductor
connected across the charging capacitor and forming there-
-- 1 --
ll~.t~33~3
-~^ 41-OD-5434
with a discharging circuit for discharging the capacitor,
a sontrolled switch in series with one of the inductors,
the latter inductor having a substantially smaller in-
ductance than the other inductor, control means coupled to
the controlled switch for repetitively operating the same
at predetermined intervals, a gaseous discharge lamp con-
nected in swries in one of the charging and discharging
circuits, and unidirectional circuit means connected
across the series connected one inductor and controlled
switch for limiting the voltage across the controlled switch.
Specifically, the unidirectional circuit means com-
prises a diode and an induction coil connected in series.
In one embodiment, the controlled switch and the
smaller inductor are in the charging circuit, whereas in
another embodiment these components are in the discharging
circuit.
In a typical embodiment of the invention, the lamp
is of high present sodium vapor type, and the controlled
switch is a silicon controlled rectifier.
A related type of circuit is disclosed in Canadian
application Serial No. 287,991 - Owen et al, dated October
3, 1976, and assigned to the same assignee as the present
nventlon .
The operating circuit of the invention may be used
for applying DC pulses of predetermined duty cycle and
repetition rate on the lamp for improving the color and
other properties thereof. A method and apparatus for
pulsed operation of high pressure sodium vapor lamps for
improving the color rendition of such lamps are dis-
closed in Canadian application Serial No. 275,374 - Osteen,
dated April 1, 1977. and assigned to the same assignee as
the present invention.
-- 2 --
lllS339 41-OD-5434
The invention will be better understood from the
following description taken in conjunction with the
accompanying drawings, in which:
FIGURE 1 is a circuit diagram of a DC pulse operating
circuit in accordance with an embodiment of the invention;
FIGURE 2 is a graphical representation of the voltage
and current waveforms relating to the operation of the
circuit shown in FIGURE l;
FIGURE 3 is a graph showing the relationship of lamp
volts and watts characterizing the circuit of the invention;
and
FIGURE 4 is a circuit diagram of another embodiment
of the invention.
Referring now to the drawings, and particularly to
FIGURE 1, there is shown a circuit diagram of a typical -~
embodiment of the invention comprising terminals 1 of a
source of alternating current, and induction coil Ll
connected at one side to one of the source terminals and
at the other side to an input terminal of full wave
bridge rectifier 2, which comprises dioded Dl, D2, D3
and D4 arranged in conventional manner as shown, the
other input terminal of bridge rectifier 2 being con-
nected to the other source terminal 1. Auxiliary in- -
duction coil L2 is inductively coupled to main induction
coil Ll, such as by arrangement of the two coils on a
common magnetic core on opposite sides of a magnetic
shunt. Such an arrangement of inductively coupled coils
is shown, for example, in the U.S. patent to Willis
3,873,910 dated March 25, 1975 and assigned to the same
assignee as the present invention. Auxiliary induction
coil L2 is connected at opposite sides respectively to
the input terminals of another full wave bridge rectifier
~ ,5339 41-OD-5434
3 constituted by diodes D5 and D6 co-acting with diodes
D2 and D4 to provide full wave rectification of the
current from auxiliary coil L2. Capacitor 5 connected
between auxiliary coil L2 and the input terminal of
bridge rectifier 3 is selected such that in conjunction
with the leakage reactance existing between induction
coils Ll and L2, it serves to provide the necessary phase
shift and power factor. If induction coil L2 and cap-
acitor 5 are selected so that the portion of the magnetic
core associated with coil L2 is saturated, a higher degree
of lamp wattage regulation is achieved for a wide range of
input voltage.
Connected across the thus described DC supply circuit
to the common output terminals of bridge rectifiers 2 and
3 is a lamp pusling circuit including gaseous dis-
charge lamp 11, particularly of high pressure sodium
vapor type, as described above.
By virtue of the described DC supply circuit, the
direct current supplied to lamp 11 by main induction coil
Ll via bridge rectifier 2 is substantially out of phase
with the direct current supplied to the lamp by auxiliary
coil L2 and capacitor S via bridge rectifier 3. As a
result, the average current through the lamp and the
voltage across the lamp is substantially increased over
the average magnitude of current and voltage which would
be applied in the absence o~ auxiliary coil L2 and its
associated rectifier circuit, and therefore the tendency
of the lamp to drop out because of de-ionization at
current zero is largely prevented, and at the same time
a sufficiently high re-ignition voltage is thereby pro-
vided to maintain operation of the lamp. In the operation
of the circuit, main induction coil Ll also serves as a
111.~33~
410D-5434
current limiting reactance to limit current flowing through
the lamp after it starts and thereby provides a ballasting
function.
A DC supply circuit of the above described type is
disclosed in Canadian application Serial No. 279,737 -
Neal, dated June 2, 1977, and assigned to the same assignee
as the present invention. -
In the embodiment of ,he present invention illustrated ~ ~
in FIGURE 1, filter capacitor 15 connected across the DC ~ -
10 supply circuit provides a filtered DC voltage supply
for the pulse generating circuit described hereinafter and
increases the average voltage supplied thereto. The type
of pulse generating circuit shown in FIGURE l for pulsed
operation of the lamp is disclosed in the above-mentioned
A Canadian Neal application Serial No. ~ ~ q,~l~ ~
As disclosed in the latter Neal application, a charging
circuit comprising serially connected inductor L4, diode
12 and capacitor 4 is connected across filter capacitor
15, and a discharging circuit comprising serially
connected inductor 13, lamp 11 and controlled unidirectional
thyristor switch, such as silicon controlled rectifier
(SCR) 13, is connected across capacitor 4.
The inductance of inductor L4 is substantially higher
than that of inductor L3, and in a typical circuit for
practicing the invention the L4 inductance would be about
10 times that of L3. However, the ratio may be in the
range of about 2:1 to about 50:1 or higher while the
obtaining satisfactory results. In general, the L4
inductance should be sufficiently high to ensure proper
discharging of capacitor 4 through the discharge circuit
and to provide for sufficient reversal of the capacitor
_ 5 _
~ 3~ 410D-5434
4 through the discharge circuit and to provide for sufficient
reversal of the capacitor chrage to commutate the SCR as
described below.
The operation of SCR switch 13 is controlled by a timing
and triggering circuit A including an RC timing circuit
comprising, in the illustrated embodiment, capacitor 6
and resistors 7 and 8 connected across the SCR. A voltage
breakdown device 9 constituted by a diac in the circuit
shown is connected at one side to the junction of
capacitor 6 and resistor 8 and at the other side to the
control electrode (gate) 13a of SCR switch 13. Zener diode
10 is connected across capacitor 6 and resistor 8 of the
timing circuit.
A starting aid circuit (not shown) may be in-
corporated in the described circuit for initially applying
sufficiently high voltage pulses to lamp 11 for strating.
Such a starting circuit and the above-described control
circuit A are disclosed in the aforementioned Morais
application, and the disclosures thereo~ in the latter
application may accordingly be referred to.
Briefly, the operation of the RC timing circuit shown
in FIGURE 1 is such that capacitor 6 is charged at a rate
determined by the combination of resistors 7, 8 and
capacitor 6. When the potential on capacitor 6 reaches
the breakdown voltage of diac 9, capacitor 6 discharges
through the loop including SCR control electrode 3a and
turns on SCR 3. While a diac is shown as the voltage
breakdown device 9, other breakdown devices such as
silicon bilateral switch (SBS), a Shockley diode, a glow
tube, or a series combination of certain of these devices,
could be employed.
Zener diode 10 connected to the junction of resistors
- 6 -
~13~.33'~
410D-5434
7 and 8 of the RC timing circuit stabilizes the timing of
the triggering operation by establishing a fi~ed clamping
voltage toward which capacitor 6 is charged.
In accordance with the present invention, a feedback
branch comprising series-connected diode 14 and inductor
L5 is connected across the described discharging circuit
comprising serially connected inductor L3, lamp 11 and
SCR 13. The provision of this feedback branch serves to
limit the peak voltage across SCR 13 and thereby avoids
undesirable firing of the SCR and affords desirable
control of the lamp watts-lamp volts relationship as
explained more fully below.
In the operation of the described circuit, capacitor
4, which serves as an energy metering device in the circuit,
is initially charged by current flowing from filter cap-
acitor 15 through inductor L4 and diode 12. The charge
on capacitor 4 reaches a positive voltage substantially
higher than the supply voltage. When SCR 13 is triggered
on by operation of the RC timing circuit, capacitor 4
discharges through inductor L3, lamp 11 and SCR 13, and
subsequently this energy (minus the amount dissipated in
the lamp) is returned to capacitor 4 both with the polarity
of the voltage reversed, such that the upper electrode
of capacitor 4 goes to a negative potential. This
voltage reversal causes the SCR cathode voltage to be
more positive than its anode voltage, and a result com-
mutation and turn-off of the SCR switch occurs. This
negative potential is prevented from reversing again by
SCR 13. Capacitor 4 is then again charged by supply
current flowing through inductor L4 and diode 12, and
through inductor L5 and diode 14 as hereinafter described,
to a voltage higher than the supply voltage, and diode
111~33~'~
410D-5434
12 serves to prevent the re-charged energy on capacitor 4
from returning to the supply source. The circuit remains
quiescent until the next pulse is provided by operation
of the RC timing circuit. The latter circuit is adjusted
to trigger SCR 13 to produce pulses of desired repetition
rate for pulsing lamp 11 in the manner intended.
On subsequent cycles, the positive voltage drop across
SCR 13 increases to even higher levels, until an equilibrum
potential is reached as a function of the total resistive
losses in the circuit. This equilibrium potential can
assume values greater than twice the supply voltage. In an
illustrative case, with a supply voltage of about 180
volts, the equilibrium voltage across SCR 13 typically
reaches about 450 volts during operation. Such high
voltages, when imposed across lamp 11 during conduction
of SCR 13, serve to ensure re-ionization and continued
operation of the lamp, especially when the pulse re-
petition rate is relatively low.
The provision of the above-described feedback branch
in accordance with the invention serves to limit the peak
voltage across SCR 13 during the lamp starting interval
when the lamp voltage is low, and thereby avoids un-
desirable firing of the SCR during this period due to
excessive anode voltage, especially if it is of low
voltage capability.
Such inadvertent firing not only may cause degradation
of the SCR but also may result in higher peak currents
through lamp 11, causing higher lamp wattage and con-
sequent shorter lamp life. The feedback branch functions
(see FIGURE 1) to transfer energy from the cathode side of
SCR 13 to the power source side of the discharge circuit
combination of inductor L3 and SCR 13. As a result, the
-- 8 --
lii~i;~39 410D-5434
voltage across the SCR prior to switching on of the latter
is limited to an acceptable maximum level.
FIGURE 2 graphically shows the SCR voltage and current
pulse waveforms achieved after equilibrium is reached in
the operation of the described circuit. The initial
positive SCR voltage drop shown (anode positive with respect
to cathode) prevails before the SCR is gated on. When
the SCR switch is turned on at point A, as determined by
the RC timing circuit, the voltage across the switch
immediately drops to zero, as indicated at point B. The
voltage remains zero while the current flows through the
SCR switch. During this period, as seen in the SCR
current waveform, the current rises to a peak value and
then drops to zero to operation of the LC circuit com- .
prising inductor L4 and capacitor 4. The reversal of
current is prevented by the SCR, and due to the large
negative voltage (with respect to ground) on capacitor 4
as described previously, the SCR is reverse biased to
achieve commutation. As the SCR ceases to conduct, as
indicated at point C, the voltage drop across the SCR is
reversed, i.e., assumes a negative polarity with its
cathode voltage more positive than its anode voltage, as
indicated at point D. Capacitor 4 then charges through
inductor L4 and diode 12, as well as through feedback
branch L5 - diode 14. The RC timing circuit starts the
timing interval when the SCR voltage goes positive (point
F) so that the interval from F to A is determined by the
timing circuit.
The described dual charging operation produces the
portion of the SCR voltage waveform from D to E as the SCR
anode potential is made positive by the resulting reversal
of charge on capacitor 4. At point E, capacitor 4 con-
13'~
- 410D-5434
tinues to be charged only through inductor L4 and diode 12,
producing a more gradual charging rate represented by the
waveform portion from E to A, and at point A the SCR anode
potential is at its maximum.
FIGURE 3 graphically illustrates the additional
benefit afforded by the described circuit in providing
for control of the lamp watts-lamp volts relationship,
whereby a flatter curve for this relationship me be
obtained. In the graph, Curve A in interrupted lines
shows the variation in lamp watts with lamp volts in a
circuit without the feedback arrangement of the present
invention as described above, while Curve B shows the
watts-volts relationship characterizing the circuit of
the present invention. In the circuit of Curve A, the
increase in lamp volts, which typically occurs over the
operating life of lamps such as here involved, is accom-
panied by a substantial increase in lamp watts, which
tends to shorten lamp life due to excessive heating. In
contrast, it is evident that Curve B is substantially
flatter than Curve A and that accordingly the lamp remains
relatively constant with increase in lamp volts, resulting
in longer life of the lamp and more nearly unifrom
illumination during its operating life.
FIGURE 4 shows a different embodiment of the invention
wherein the charging circuit connected to filter capacitor
15 and DC power source 20 (shown in simplified form)
comprises the series combination of SCR 13, smaller in- ~
ductor L3' and lamp 11, whereas the discharging circuit
connected across metering capacitor 4' comprises serially
~ 30 connected diode 16 and larger inductor L4'. Such a circuit
.
is shown in the U.S. Patent No. ~ ~b~ ~ ~
dated ~ In this embodiment, as in the FIGURE 1
-- 10 --
;331) 410D-5434
embodiment, the feedback branch comprising inductor L5' and
diode 14' is connected across the series combination of
inductor L3', lamp 11 and SCR 13 to produce similar results.
However, in this case, in reference to the description of
the waveform in FIGURE 2, at point D capacitor 4 dis-
charges through diode 16 and inductor L4', as well as
discharging through the feedback branch L5' - diode 14'.
This dual discharging operation produces the portion of
the SCR voltage waveform from D to E, and at point E,
capacitor 4 continues to discharge only through inductor
diode 16 and inductor L4', producing the waveform portion
E to A. This embodiment also provides the desirable lamp
watts-lamp volts relationship shown in FIGURE 3.
In a typical circuit such as shown in FIGURE 1, the
following components may have the values indicated:
Inductor L3 - 0.7 millihenries
Inductor L4 - 7 millihenries
Capacitor 4 - 3 microfarads
Capacitor 6 - .12 microfarad
Resistor 7 - 41K ohms
Resistor 8 - 7K ohms
Zener diode 10 - 62 volts
Diode 12 - lK volts
Diac 9 9 - 38 volts
Inductor L5 - .3 millihenries
Diode 14 - lK volts
A snubber circuit of conventional RC type (not shown),
if found necessary to desirable, may be connected across
diode 12, diode 14 or SCR 13 to reduce voltage spikes across
those components.
I,amp 11 may be arranged in various places in either the
charging or discharging circuit of either of the FIGURE 1 and
~ 33~ 410D-543~
FIGURE 4 embodiments. Such modifications will produce varied
but satisfactory results in accordance with the invention. -
While an SCR is disclosed as the unidirectional controlled
switch in the described circuit, it will be understood that
other equivalent switch devices may alternatively be em-
ployed in accordance with the invention. For example, a
triac or a transistor switch may be employed in combination
with a diode to provide unidireetional controlled switch
means" is intended to include all such equivalent switch
devices or arrangements.
The power supply may be any suitable source of DC
voltage, such as a battery or a rectified AC source different
from that described above. Preferably, the DC supply is
at least about 150 volts in order to aehieve the desired
improvement in color properties of lamp 1 (assuming the
lamp to be of 250-300 watt variety).
While a diode has been disclosed in series with the
larger induetor L4 (or L4'), this diode may be dispensed
with if induetor L4 has suffieiently high induetanee.
While the present invention has been described with
referenee to partieular embodiments thereof, it will be
understood that numerous modifieations may be made by
those skilled in the art with out aetually departing
from the scope of the invention. Therefore, the appended
elaims are intended to eover all sueh equivalent variations
as eome within the true spirit and seope of the invention.
- 12 -
