U.S. patent number 3,886,405 [Application Number 05/328,654] was granted by the patent office on 1975-05-27 for device for operating discharge lamps.
This patent grant is currently assigned to Mamiya Koki Kabushiki Kaisha. Invention is credited to Mitsuo Kubo.
United States Patent |
3,886,405 |
Kubo |
May 27, 1975 |
Device for operating discharge lamps
Abstract
The device disclosed herein fundamentally comprises current
limiting means for limiting a current in a discharge lamp to below
its rating until a predetermined point is reached in a half cycle
of the alternating current and control signal generating means for
applying a control signal to the current limiting means at the
predetermined current control changeover point to suspend or
depress the function of the current limiting means, so that the
supply voltage viewed from the discharge lamp is equivalently
increased and accordingly the device itself can be made smaller in
size and lighter in weight. The device further comprises means for
changing the current control point according to variation in the
characteristics of its circuit elements or the lapse of time from
the starting time of the discharge lamp in order to stably operate
a discharge lamp such as a mercury arc lamp which is variable in
vapor pressure and equivalent impedance. In addition, the device
further comprises time locking means for actuating the control
signal generating means after a given time, or keeping the function
of the control signal generating means in suspended state for the
given time after closing of a line switch, thereby to stably
operate a discharge lamp which requires starting pulse or has an
irregular discharge in the starting period.
Inventors: |
Kubo; Mitsuo (Osaka,
JA) |
Assignee: |
Mamiya Koki Kabushiki Kaisha
(Tokyo, JA)
|
Family
ID: |
27280245 |
Appl.
No.: |
05/328,654 |
Filed: |
February 1, 1973 |
Foreign Application Priority Data
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Feb 7, 1972 [JA] |
|
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47-13416 |
Feb 7, 1972 [JA] |
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47-13417 |
Mar 1, 1972 [JA] |
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47-20562 |
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Current U.S.
Class: |
315/246;
315/DIG.5; 315/224; 315/151; 315/283 |
Current CPC
Class: |
H05B
41/392 (20130101); Y10S 315/05 (20130101) |
Current International
Class: |
H05B
41/392 (20060101); H05B 41/39 (20060101); H05b
041/16 () |
Field of
Search: |
;315/DIG.5,149,151,156,224,227,240,246,283,362 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rolinec; R. V.
Assistant Examiner: Dahl; Lawrence J.
Attorney, Agent or Firm: Wenderoth, Lind & Ponack
Claims
I claim:
1. A device for operating a discharge lamp comprising: an
alternating current power source; a discharge lamp having a rated
current value; a low impedance element, said low impedance element
being a coil with a relatively few number of turns and relatively
small in size; current limiting means comprised of high impedance
means and bi-directional means coupled in parallel with said high
impedance means for limiting electric current flowing in said
discharge lamp to a value below the rated value thereof during a
first portion in each half cycle of the alternating current of said
power source, said current limiting means being inoperative during
the remaining portion of each half cycle and being switched from
operative to inoperative states by a control signal, said low
impedance element and said current limiting means being connected
between said alternating current power source and said discharge
lamp; and control signal generating means connected to said current
limiting means for applying a control signal to said current
limiting means at a predetermined current control changeover point
during each half cycle, said predetermined current control
changeover point occurring at substantially 90.degree. with respect
to current cross-over of each half cycle, whereby said current
limiting means become inoperative after the control signal is
applied to said current limiting means during each half cycle and
the electric current flowing in said discharge lamp increases so
that an effective current has said rated value for the half
cycle.
2. A device as claimed in claim 1 which further comprises time
locking means coupled to said control signal generating means for
actuating said control signal generating means after a given time
and suspending the function of said control signal generating means
for the given time until said lamp has achieved stable operation
after closing of a line switch.
3. A device as claimed in claim 1 in which said current limiting
means comprises a high impedance element and a bi-directional
controlled rectifier, said high impedance element being shunted by
said bi-directional controlled rectifier at said current control
point, and said control signal generating means comprises a
bi-directional switching element, a resistor and a capacitor, said
resistor and capacitor being coupled in series, said switching
element coupled to the junction of said resistor and capacitor,
said resistor and capacitor being selected so that a voltage of
said capacitor reaches the switching voltage of said bi-directional
switching element at the current control point, thereby applying a
control signal to said bi-directional controlled rectifier.
4. A device as claimed in claim 1 in which said current limiting
means comprises: a step-up transformer with a primary winding
serving as a low impedance element and a secondary winding serving
as a high impedance element, said secondary winding acting as a
current limiting element in series with said low impedance element;
and a bi-directional controlled rectifier rendered conductive by a
control signal from said control signal generating means at said
predetermined current control point to apply a supply voltage to
said primary winding of said step-up transformer, whereby a voltage
higher than the supply voltage is induced through electromagnetic
induction of said step-up transformer to suspend the function of
said current limiting element and thereby to increase said current
flowing in said discharge lamp.
5. A device as claimed in claim 1 in which said current limiting
means comprises a bi-directional controlled rectifier having a
commutating circuit which is substantially composed of an inductor
and a capacitor; and a bi-directional switching element for
controlling the bi-directional controlled rectifier, to interrupt a
current in said discharge lamp for the purpose of changing the
current into a high frequency current until said current control
point is reached, said control signal generating means providing a
control signal which is applied to a control electrode of said
bi-directional controlled rectifier to render the latter conductive
for that portion of alternating current cycle between said
predetermined current control point and the end of the respective
half cycle.
6. A device as claimed in claim 1 which further comprises means for
changing said current control timing point according to variation
in the characteristics of said circuit and current limiting means
and due to variation in the characteristic of said discharge lamp
in the starting period thereof and operatively coupled thereto.
7. A device as claimed in claim 3 in which said low impedance
element is connected in parallel to said high impedance element
through said bi-directional controlled rectifier.
8. A device as claimed in claim 3 in which said high impedance
element and said low impedance element are wound on one core.
9. A device as claimed in claim 6 in which said means for changing
the current control point comprises: light emitting means for
detecting variations in the characteristics of said circuit; and
photosensitive means combined in said control signal generating
means and controlling the latter according to light emitted by the
light emitting means.
10. A device as claimed in claim 1 which further comprises means
operatively coupled for changing said current control point with
the lapse of operating time after closing of a line switch.
11. A device as claimed in claim 10 in which said means for
changing said current control point is an indirectly heated
thermistor cooperating with said control signal generating
means.
12. A device as claimed in claim 2 which further comprises means
operatively connected for changing said current control point
according to variation in the characteristics of said circuit after
releasing the lock of said time locking means.
13. A device as claimed in claim 12 in which said means for
changing said current control point comprises light emitting means,
a resistor, and photosensitive means, said light emitting means
detecting variation in the characteristic of said circuit, said
resistor obtaining a suitable light output of the light emitting
means said photosensitive means being combined in said control
signal generating means and controlling the latter means according
to the light emitted by the light emitting means.
14. A device as claimed in claim 12 which further comprises means
operatively connected for changing said current control point with
the lapse of operating time after releasing the lock of said time
locking means.
15. A device as claimed in claim 14 in which said means for
changing the current control point is an indirectly heater
thermistor cooperating with said control signal generating means.
Description
BACKGROUND OF THE INVENTION
This invention relates to improvement of a device for operating a
discharge lamp (hereinafter referred to as "a discharge lamp
operating device" when applicable).
Heretofore, a large impedance element has been employed, as a
ballast, for operating a discharge lamp. This impedence element is
usually a coil wound on an iron core, and it is relatively heavy in
weight, relatively large in size and high in cost.
For overcoming those problems created by the impedance element, a
method of converting the voltage or current of an a.c. power source
applied to the discharge lamp into a high frequency voltage or
current thereby to equivalently increase the frequency of the power
source, has been considered. However, this method involves problems
such as a great power loss due to the iron core and a high cost in
manufacturing a device for operating a discharge lamp.
Furthermore, the fundamental circuit for operating a discharge lamp
is, in general, formed by a discharge lamp and an impedance element
which are connected in series to an a.c. power source. In this
circuit, it is necessary to decrease a supply voltage for the
discharge lamp approximately to a lamp voltage, in order to operate
the discharge lamp with a lower impedance value of the impedance
element.
However, with such a supply voltage, the stability of the lamp
operation for the variation of the supply voltage may be decreased,
and it is therefore not suitable for practical use. Accordingly,
usually a high impedance element and a high supply voltage are
used.
These problems have been solved by the present invention with the
result that the impedance element can be made smaller in size and
lighter in weight, as will be seen in the detailed description of
the specification.
For convenience of description, the method for solving the problem
will be briefly described.
Although the conventional supply voltage is used in the present
invention, the current flow in the discharge lamp is limited by
current limiting means until a predetermined instant (which will be
described in detail later) in a half cycle of the alternating
current thereby making its output lower than its rated value and
thereafter the function of the current limiting means is suspended
or depressed thereby to increase its output, so that the rated
output is obtained throughout each half cycle of the alternating
current. As a result, the effective supply voltage viewed from the
discharge lamp is lowered and the impedance element can be
therefore made smaller in size.
Since the lamp current is limited to a low value by the current
limiting means until a predetermined instant in each half cycle of
the alternating current, this invention has the following
advantages. This lamp current, thus limited to the low value,
maintains a low current flow in the lamp, and therefore the peak
voltage of the lamp is low, whereby dropout of the lamp can be
prevented. Furthermore, since the current flowing in the lamp is
thus slight, the ignition of the lamp can be readily accomplished
at the predetermined time at which the function of the current
limiting means is suspended or depressed. In addition, since the
small current flowing until the predetermined time in each half
cycle heats the cathode of the discharge lamp, the temperature of
the lamp is maintained at a temperature suitable for the cathode
oxide, and the service life of the cathode is therefore
extended.
More specifically, for instance, in one example of the device for
operating a discharge lamp in which a low impedance coil is
employed as the impedance element while a high impedance element
and a bi-directional controlled rectifier are employed as the
current control means, electric current flows through the high
impedance element connected in series with the circuit formed by
the power source, the discharge lamp and the low impedance element
until the current control point is reached in order to limit the
current to a value lower than its rated value, and at the
predetermined time the state of the bi-directional controlled
rectifier becomes conductive thereby bypassing the high impedance
element so that a large current flows the circuit.
However, when the discharge lamp such as a mercury discharge lamp
which has an equivalent impedance changing for the starting period
is operated by this method, another problem is encountered. In this
case, the equivalent impedance of the lamp in the starting period
is lower as the lamp has a low vapor pressure for the starting
period. When the lamp operation is in a stationary state, the
equivalent impedance is larger as it has a high vapor pressure.
If the current control point is chosen to occur when the discharge
lamp produces its rated output during the starting period, an
over-current will flow in the discharge lamp whereby the phenomena
as to the equivalent impedance as described above is observed,
which can damage the discharge lamp or the circuit elements.
Furthermore, if the discharge lamp requires starting pulses or has
an irregular discharge in the starting period, the operation of the
control signal generating means or the current limiting means is
affected by the starting pulses or the irregular discharge. As a
result, satisfactory operation and stability of the discharge lamp
will not be achieved.
SUMMARY OF THE INVENTION
Accordingly, a first object of this invention is to provide a
device for operating a discharge lamp which comprises means for
equivalently lowering a voltage to be supplied to the discharge
lamp and which is therefore small in size, light in weight, simple
in handling, and high in reliability.
A second object of the invention is to provide an improved device
for operating a discharge lamp which can stably operate a discharge
lamp which has different characteristic between the starting time
and the operating time, or which will require high pulsive voltage
at the starting time, or which will have an irregular
discharge.
A third object of the invention is to provide an improved device
for operating a discharge lamp which functions to maintain the
discharge lamp output constant despite variation in the
characteristics of the discharge lamp, variations in the conditions
of its circuit elements, and variations in its environmental
conditions such as supply voltage.
A fourth object of the invention is to provide a device for
operating a discharge lamp in which the discharge lamp is
exchangeable with other discharge lamps and which has
characteristics that are constant overtime.
The foregoing objects and other objects of the invention will
become more apparent from the following detailed description and
the appended claims when read in conjunction with the accompanying
drawings, in which like parts are designated by like numerals.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a circuit diagram illustrating one example of the device
for operating a discharge lamp according to the invention;
FIGS. 2(a)-2(c) showing the waveforms of a supply voltage, a lamp
current and a lamp voltage in the circuit shown in FIG. 1;
FIG. 3 is a graph showing the relationship between impedance of an
impedance element and phase angle of a current control timing
instant;
FIGS. 4, 5, 6 and 7 are circuit diagrams of other examples of the
device for operating a discharge lamp according to the
invention;
FIG. 8 is a diagram showing the waveform of a lamp current in the
circuit shown in FIG. 7;
FIG. 9 shows other combinations of discharge lamps and impedance
elements;
FIGS. 10(a) and 10(b) show circuits which can be connected in place
of the discharge lamp and impedance element in FIGS. 1 and 7;
FIGS. 11(a), 11(b), 11(c) and 11(d) are block diagrams illustrating
various detecting systems employed in the invention;
FIGS. 12(a), 12(b), 12(c), 12(d), 13 and 14 are circuit diagrams
illustrating devices for operating discharge lamps which are based
on a system of detecting the voltage, light output, or temperature
of the discharge lamp;
FIG. 15 is a circuit diagram illustrating a device for operating a
discharge lamp in which a system of detecting the voltage, magnetic
flux, or temperature of an impedance element is employed;
FIG. 16 is a circuit diagram illustrating a device for operating a
discharge lamp in which a system of detecting a current flowing in
the discharge lamp is employed;
FIG. 17 is a circuit diagram illustrating a device for operating a
discharge lamp in which a system of detecting the lapse of time is
employed;
FIGS. 18(a), 18(b), 18(c) and 19 illustrate other examples of the
control signal generating means;
FIG. 20 is a circuit diagram illustrating a typical example of the
device for operating a discharge lamp in which time locking means
is provided for stably operating a discharge lamp which requires
starting pulses or has an irregular discharge in the starting
period;
FIGS. 21(a), 21(b) and 21(c) are schematic diagrams illustrating
time locking means employed in a device for operating a discharge
lamp according to the invention;
FIG. 22 is a circuit diagram illustrating a device for operating a
discharge lamp which is provided with time locking means and
detecting means adapted to detect a voltage of the discharge lamp;
and
FIG. 23 is a circuit diagram illustrating another example of the
control signal generating means which has a time locking means
formed by transistors, resistors and capacitors and serves to have
a constant light output characteristic.
DETAILED DESCRIPTION OF THE INVENTION
Broadly speaking, the invention comprises three means: interrupting
a voltage or a current, and transformer means for controlling an
electric current flow in a discharge lamp.
With reference to FIG. 1, there is shown one example of a device
for operating a discharge lamp according to the invention in which
the impedance means is employed. The circuit for this device
comprises: an a.c. power source 1; a line switch 2; a discharge
lamp 3; a coil 4 employed as an impedance element having a
relatively low impedance; current limiting means 5 constituted by a
coil 6 employed as an impedance element having relatively high
impedance and a bi-directional controlled rectifier 7; and control
signal generating means 8 constituted by bi-directional switching
means 81, a resistor 83 and a capacitor 82. The current limiting
means 5 operates to limit current flowing through the discharge
lamp 3 and the coil 4.
The operation of the circuit (FIG. 1) is as follows. When the line
switch 2 is closed the discharge lamp 3 is operated through the
coil 4 and the coil 6 which are connected in series. This current
can be limited because the coil 6 is a high impedance device.
However, as the capacitor 82 is gradually charged through the
resistor 83, the voltage of the capacitor 82 increases.
In this connection, the resistance of the resistor 83 has been
predetermined so that the voltage of the capacitor reaches the
switching voltage of the switching element 81 at a predetermined
time when the alternating current is at a predetermined phase in
each half cycle. Accordingly, at this instant, the capacitor is
discharged through the switching element 81 and the control
electrode 7c and second electrode 7a of the bi-directional
controlled rectifier 7 thereby producing a control signal for the
bi-directional rectifier 7.
By means of this control signal, the first electrode 7b and the
second electrode 7a of the rectifier 7 are substantially shorted,
whereby the function of the current limiting is suspended.
Accordingly, the current flowing in the discharge lamp 3
(hereinafter referred to as a lamp current when applicable)
increases, that is, the output of the discharge lamp 3 increases.
Thereafter, the lamp current decreases with the a.c. voltage, and
when the lamp current through the bi-directional controlled
rectifier 7 has reached about zero, the bi-directional rectifier 7
is turned off. The half cycle is thus completed.
The operation of the circuit in the next half cycle of the
alternating current is the same as that described above except that
the polarity is revered. Thus, the discharge lamp 3 is operated at
its rated output throughout a complete cycle of the alternating
current. Since the lamp current is limited to a low value by the
current limiting means until a predetermined instant in each half
cycle of the alternating current, this invention has the following
advantages. This lamp current, thus limited to the low value,
maintains a low current flow in the lamp, and therefore the peak
voltage of the lamp is low, whereby dropout of the lamp can be
prevented. Furthermore, since the current flowing in the lamp is
thus slight, the ignition of the lamp can be readily accomplished
at the predetermined time at which the function of the current
limiting means is suspended or depressed. In addition, since the
small current flowing until the predetermined time in each half
cycle heats the cathode of the discharge lamp, the temperature of
the lamp is maintained at a temperature suitable for the cathode
oxide, and the service life of the cathode is therefore
extended.
In addition, if the predetermined, current control time described
above occurs at an earlier instant during each half cycle of the
alternating current without lowering the stabilization of the
discharge lamp, the output of the discharge lamp will be increased
more than the rated value thereof. In contrast, if the current
control timing made to occur at a later instant, the output of the
discharge lamp will be decreased. Thus the light output of the
discharge lamp can be adjusted as required.
In addition, if the current control point is changed according to
the variation of the voltage supply, the variation with time of the
circuit elements, and the variation of environmental conditions,
the output of the discharge lamp may be maintained constant, that
is, it is possible to give a constant power characteristic to the
discharge lamp.
The waveforms obtained by plotting the voltage supply, the voltage
of the discharge lamp (hereinafter referred to as a lamp voltage
when applicable) and the lamp current respectively against time (t)
are shown in FIG. 2, in which the current control time is
represented by reference symbols T.sub.1 and T.sub.2.
In the circuit described above, since the inductance of the coil 4
is lower than that of a conventional ballast, the number of turns
of the coil 4 is relatively small and the iron core of the coil 4
is also small. Therefore, the coil 4 is very small and is
accordingly light in weight.
In addition, the inductance of the coil 6 is relatively high, but
the current capacity thereof can be small. Therefore, although the
number of turns of the coil 6 is great, a wire small in diameter
and an a small iron core may be used for constructing the coil 6.
Accordingly, the coil 6 can be also made small in size and light in
weight. It is apparent that the other components of the device are
small. Especially, the resistor and the capacitor can be small in
current capacity since they are to be used in the control
circuit.
In FIG. 1, the input terminal C of the resistor 83 may be connected
to the power source side of the discharge lamp 3 or to the coil 4,
so that the above-described control signal generating means is not
affected by the operating conditions of the discharge lamp 3.
Furthermore, it is possible to provide a load equivalent to the
discharge lamp 3, for instance, a transformer in place of the
discharge lamp 3 in FIG. 1 in such a manner that the primary
winding of the transformer is connected between the power source 1
and the coil 4 and the secondary winding thereof is connected to
the discharge lamp 3. Furthermore, each of the coils in FIG. 1 may
be a complex circuit consisting of a coil and a capacitor. These
modifications may be applied to other examples, described
hereinafter, of the dvices for operating discharge lamps according
to the invention.
In addition, in FIG. 1, the input terminal A of the bi-directional
controlled rectifier 7 may be connected to an intermediate point B
of the coil 6. In the circuit thus modified, even when the
bi-directional controlled rectifier 7 becomes conductive, the
effect of the coil 6 continues so that the coil 6 takes a part of
the action of the coil 4. Similarly, the input terminal A of the
coil 6 may be connected to an intermediate point D of the coil 4,
so that the coil 4 takes a part of the action of the coil 6.
In the above-described example, the current flowing through the
discharge lamp is limited, by the high impedance element 6, to a
value smaller than the rated current until the predetermined time
in each half cycle of the alternating current at which the current
control point is reached, thereby maintaining a supply of ions in
the discharge lamp. Accordingly, the current of the discharge lamp
can be readily increased without application of a high voltage such
as a pulse voltage, that is, without changing the supply voltage at
the predetermined instant, thereby to stably operate the discharge
lamp.
The predetermined current control point, that is, an instant (in
a.c. phase angle) when the alternation of the impedance elements is
caused by the control signal generating means in the circuit should
be determined by taking into consideration the following
description.
In the case where the current control point is advanced to zero in
phase angle in a half cycle of the alternating current, no current
limitation is caused. This is the same as in the conventional
ballast, in order to stably operate the discharge lamp at its rated
output, the impedance element must be high in impedance.
In contrast, if the current control point is delayed from the zero,
an effective supply voltage viewed from the discharge lamp is
reduced as was described before. Therefore, in order to obtain the
rated output of the discharge lamp, the impedance element must have
a low impedance. The relationship between the impedance of the
impedance element and the phase angle is shown in FIG. 3.
In general, the waveform of an alternating current in an
approximately half cycle after the current control point has
occurred is approximately equal to a sine waveform, and its
equivalent frequency increases with the delay of the current
control function. For instance, if the phase angle at the instant
current control occurs is 90.degree., the equivalent frequency is
approximately twice the frequency of the power source. The phase
angle at the instant current control occurs, in general, between
50.degree. and 100.degree., for the purpose of achieving the
objects of this invention. Accordingly, if a coil is used as the
impedance element as shown in FIG. 1, the inductance of the coil is
reduced by the effect of such an equivalent frequency as described
above. In conclusion, with the delay of the current control point
the coil can be made small in size, that is, the discharge lamp
operating device can be made smaller in size and lighter in weight,
but the power factor of the device becomes lower. Therefore, in
determining the instant current control the effect of this power
factor should be taken into consideration.
FIG. 4 is a second example of the device for operating a discharge
lamp according to the invention, in which impedance is utilized as
the current limiting means.
In the circuit shown in FIG. 4 the coil 6 is connected in parallel
through the controlled rectifier 7 to the coil 4. In this circuit,
only the coil 6 operates until the occurrence of the current
control operation in the A.C. cycle, and then the state of the
controlled rectifier 7 becomes conductive, as a result of which
coils 4 and 6 are electrically directly connected in parallel to
each other. The current limiting function of the current limiting
means 5 is reduced by this operation. More specifically, since the
inductance of the coil 4 is much lower than that of the coil 6,
most of the current of the discharge lamp 3 will flow through the
coil 4.
In this example, it should be noted that a part of the current of
the discharge lamp 3 flows in the coil 6. Therefore, the current
capacity of the coil 4 may be smaller than that of the coil 4 of
FIG. 1, that is, the diameter of the wire of the coil 4 of FIG. 4
may be smaller than that of the coil 4 of FIG. 1. Accordingly, the
former coil 4 (FIG. 4) is smaller than the latter coil 4 (FIG. 1).
The other operation of the circuit is the same as in the circuit of
FIG. 1. A transient voltage applied to the bi-directional
controlled rectifier 7 is moderated by the coil 4, as a result of
which the bi-directional controlled rectifier is prevented from
erroneously becoming cocnductive by the transient voltage.
Accordingly, the discharge lamp operating device is improved in
reliability, and the operation of the discharge lamp is also
improved in stability.
In this example (FIG. 4) also, the input terminal F of the coil 4
may be connected to the intermediate point B of the coil 6, or the
input terminal E of the coil 6 may be connected to the intermediate
point D of the coil 4.
The circuit described above was actually embodied with the
following data:
Discharge lamp . . . a 400-watt mercury-arc lamp,
Supply voltage . . . 200 V,
Current control timing instant (in phase angle) . . . approximately
90.degree.
Coil 4
Inductance . . . 10 mH
Current capacity . . . 4 A
Size . . . 7 .times. 6 .times. 4 cm.sup.3
Weight . . . 640 g
Coil 6
Inductance . . . 800 mH
Current capacity . . . 0.4 A
Size . . . 7 .times. 5.5 .times. 4 cm.sup.3
Weight . . . 630 g
From a comparison of the above data with the following data of an
impedance element, namely, a coil of the conventional ballast used
with a similar discharge lamp, it is clear that the coil of the
conventional ballast is about 4 times as large and about 3.5 times
as heavy as the device of the invention. Therefore, it can be
understood how small and light the device of the invention is.
Conventional Ballast
Inductance . . . 95 mH
Size . . . 15 .times. 11 .times. 9 cm.sup.3
Weight . . . 4,400 g
Furthermore, in conventional constant power type ballasts, for
instance, a conventional ballast for 400 watts is approximately
11,500 g in weight. This weight can be reduced to about one-ninth
thereof and the effect of a device with such a conventional ballast
can be increased if the present invention is applied thereto,
because according to the invention the constant power
characteristic of the discharge lamp as was described above can be
obtained and since the number of the component parts to be added
thereto is small, the weight and volume of the device will hardly
increase.
Another example of a device for operating a discharge lamp
according to the invention, shown in FIG. 5, is the same as the
second example shown in FIG. 4 except that the coils 4 and 6 are
wound on the same iron core.
In this example (FIG. 5), since these coils 4 and 6 are wound on
the same iron core, the directions of the currents flowing in the
coils 4 and 6 are opposite to each other, and therefore the iron
core will be little saturated. Accordingly, the sectional area of
the iron core may be smaller than otherwise required. Consequently,
the coil assembly 4 and 6 and accordingly the device can be made
smaller in size and lighter in weight than those shown in FIG.
4.
With reference now to FIG. 6, there is shown a fourth example of a
device for operating a discharge lamp according to the invention;
reducing the supply voltage for the discharge lamp, thereby
limiting a current flow in the discharge lamp.
In this example, the voltage applied to the discharge lamp 3 is
lowered to below the rated value of the lamp by means of a
transformer and a bi-directional controlled rectifier 7 until the
current control point, previously described, in a half period of
the alternating current, and thereafter the bi-directional
controlled rectifier 7 becomes conductive to operate the discharge
lamp at its rated output.
This example (FIG. 6) can be effectively applied especially to the
case where, for instance, a 200-V rating discharge lamp with a
step-up transformer is used with a 100-V conventional power
source.
The circuit shown in FIG. 6 comprises the control signal generating
means 8, the line switch 2, the bi-directional controlled rectifier
7, the discharge lamp 3, the impedance element 4 and the a.c. power
source 1 all of which were described previously, and further
comprises windings 18a and 18b of transformer 18.
The operation of the circuit shown in FIG. 6 is as follows. In each
half cycle of the alternating current appearing at the upper
terminal of the power source 1, the a.c. voltage is divided by the
primary winding 18b and the bi-directional controlled rectifier 7.
Since the impedance of the bi-directional controlled rectifier 7 is
much higher than that of the primary winding 18b, most of the a.c.
voltage is applied to the bi-directional controlled rectifier 7.
Therefore, the secondary winding 18a of the transformer is operated
as a series impedance of the discharge lamp. Accordingly, the
current 3 flows through the secondary winding 18a of the
transformer and the impedance element 4, thus operating the
discharge lamp 3.
In this case, the voltage applied to the discharge lamp 3 is below
its rated voltage. Therefore, the current flowing in the discharge
lamp 3 is limited by this low voltage supply and by the series
conduit of the secondary winding 18a and the impedance element 4.
At the current control point in the a.c. cycle, the bi-directional
controlled rectifier 7 becomes conductive due to a control signal
produced by the control signal generating means 8 as was described
previously, as a result of which almost all of the supply voltage
is now applied to the primary winding 18b, and at the same time, a
voltage higher than the supply voltage is induced between the
terminals G and H of the coil assembly 18a and 18b owing to
electromagnetic induction. The step up voltage thus induced is
applied to the discharge lamp 3, thereby increasing the output of
the discharge lamp. This condition will be maintained till the
termination of the a.c. half cycle.
The operation of the device in the next a.c. half cycle is the same
as in the case described above except that the polarity is
reversed. Thus, the discharge lamp 3 is operated at its rated
output throughout one cycle of the alternate current.
In this example, since the current flowing in the discharge lamp 3
is limited until the current control point of the a.c. cycle, the
primary winding 18b can be small on the basis of the equivalent
frequency described before, and the device can be therefore made
small in size and light in weight.
Depending upon the kinds of discharge lamps used, it is possible to
connect the impedance element in parallel to the bi-directional
rectifier 7 so that the voltage applied to the winding 18b can be
increased to a certain degree even before the current control point
occurs. In this case, since a voltage obtained by electromagnetic
induction is added to the voltage supply, it follows that the
voltage applied to the discharge lamp will increase.
Furthermore, if the magnetic coupling of the windings 18a and 18b
is loosened to form a leakage transformer, the winding 18a can
serve as the impedance element 4.
FIG. 7 shows a further example of a device for operating a
discharge lamp according to the invention by employing the method
of interrupting a voltage or a current.
The operation of this device is as follows. In each half cycle of
the alternating current, when the line switch 2 is closed, a
capacitor 15 is charged through a discharge lamp 3, a coil 4, and a
resistor 14. When the voltage of the capacitor 15 reaches the
switching voltage of a bi-directional switching element 51, the
capacitor 15 is discharged through the switching element 51, thus
producing a discharge current. This discharge current flows to the
control electrode of a bi-directional control rectifier 7 thereby
making the latter conductive.
At the same time, the the capacitor 13 which has been charged
through the discharge lamp 3 and the coils 4 and 12, is discharged
through the bi-directional controlled rectifier 7 and coil 2
thereby to oppositely charge the capacitor 13. Then the electric
charge in the capacitor 13 is transferred in a reverse direction
through the bi-directional controlled rectifier 7 again owing to a
commutating circuit consisting of the coil 12 and the capacitor 13.
In this case, the direction of the current flowing through the
bi-directional controlled rectifier 7 and caused by the transfer of
the electric charge is opposite to the direction of the discharge
lamp current through the bi-directional controlled rectifier 7.
Therefore, when the current through the bi-directional controlled
rectifier 7 has become approximately zero, bi-directional
controlled rectifier 7 is turned off. That is, the bi-directional
controlled rectifier 7 resumes its nonconductive state. Then, the
voltage of the capacitor 15 again rises.
Thus, the conductive state and nonconductive state of the
controlled rectifier 7 are repeated, as a result of which the
current flowing in the discharge lamp is interrupted as shown in
the first half of each half cycle of the alternate current in FIG.
8.
At the current control point in the a.c. cycle described above, the
control signal generating means 8 operates in the same manner as
described above, thereby to feed a control signal through a
resistor 11 and a small bi-directional controlled rectifier 821
having small capacity to the control electrode of the
bi-directional controlled rectifier 7, as a result of which the
bi-directional controlled rectifier 7 becomes conductive. This
conductive state of the circuit is maintained until the current of
the bi-directional controlled rectifier 7 becomes approximately
zero. Therefore, during this period, the function of the current
limiting means 5a is suspended, that is, the current limiting means
5a operate as conductive means only.
Since the a.c. voltage or current is thus interrupted, forming a
high frequency until the current control point therein is reached,
even if the current flowing through the discharge lamp becomes an
over-current because of the small inductance of the coil 4, it will
be prevented because the bi-directional controlled rectifier 7
becomes immediately nonconductive. Then, at the current control
point is reached, this current limitation is suspended to increase
the output of the discharge lamp. Thus, the discharge lamp can be
operated at its rated output for a full cycle of the alternating
current.
This control signal generating means 8 (in FIG. 7) is applicable to
a device for operating a discharge lamp in which another type of
a.c. interrupting circuit with a bi-directional controlled
rectifier is employed.
In the example shown in FIG. 7, if a series circuit consisting of a
resistor and a capacitor is connected in parallel with the
bi-directional controlled rectifier, the bi-directional controlled
rectifier may be operated under suitable conditions. Furthermore,
if a reactor is connected in series with the bi-directional
controlled rectifier 7, stable operation of the bi-directional
controlled rectifier 7 may be achieved.
In all of the examples described above, a bi-directional switching
element, controlled rectifiers connected in parallel with and in
opposition to each other, a saturable reactor or the like can be
used in place of the bi-directional controlled rectifier 7 or 821,
if these elements are equivalent in performance to the rectifier 7
or 821.
In these examples described above, instead of one series circuit
consisting of the discharge lamp 3 and the impedance element 4, a
parallel circuit consisting of a plurality of circuits in which
discharge lamps 3a, 3b, 3c, etc. and impedance elements 4a, 4b, 4c,
etc. are connected in series respectively, can be connected as
shown in FIG. 9.
In addition, in the circuits shown in FIGS. 1 and 7, the series
circuit of the discharge lamp 3 and the impedance element 4 can be
replaced by a circuit comprising a transformer 19, a discharge lamp
3 and an impedance element 4 as shown in FIG. 10(a) or by a circuit
comprising a leakage transformer 20 and a discharge lamp 3 as shown
in FIG. 10(b).
Furthermore, the discharge lamp 3 shown in FIG. 4 or 5 can be
replaced by a circuit shown in FIGS. 9, 10(a) or 10(b).
As is apparent from the above descriptions, in operating the
discharge lamp by alternating current according to the invention,
the output of the discharge lamp is limited to less than its rated
value by the current limiting means until the current control point
of a half cycle of the alternating current, and therefore the
function of the current limiting means is suspended or depressed to
increase the output of the discharge lamp and the rated output
thereof can be thus obtained for each cycle of the alternating
current. Accordingly, the effect whereby the effective supply
voltage (viewed from the discharge lamp) is lowered to shorten the
period of time during which the discharge lamp produces its output,
is utilized so that the value of the impedance element can be
reduced while the rated output is produced from the discharge
lamp.
Unlike the case where the supply voltage is merely reduced, the
supply voltage for operating the discharge lamp again is the same
as that in the case where the conventional ballast is employed.
Therefore, the device for operating a discharge lamp according to
the invention is lighter in size, smaller in size, stable against
the supply voltage variation, inexpensive and highly stable.
However, in this invention, if the predetermined current control
timing instant in each of the half cycles is selected so that the
rated current is obtained from the beginning of operation of the
lamp, discharge lamps such as mercury are lamps whose impedance
become low during the starting period will suffer from the
following disadvantage. That is, in this case, the lamp current
becomes several times as large as the rated current. As a result,
the discharge lamp may be damaged, or the circuit elements may be
burned, that is, the service life of the discharge lamp may be
rendered short. This disadvantage is overcome by this
invention.
Described below are several examples of the device for operating a
discharge lamp in which the current control point is changed
according to variations in the characteristics of a discharge lamp,
variations in the physical quantities of circuit elements, or the
lapse of time after closing of a line switch whereby any type of
discharge lamp can be stably operated.
For providing, the device as described above, there are considered
several systems such as illustrated in FIG. 11(a), (b), (c) and
(d). FIG. 11(a) is a block diagram illustrating a first system in
which variations of the voltage, light output and temperature of a
discharge lamp are detected, and the variations thus detected are
converted into electrical resistances which will be applied to
control signal generating means. FIG. 11(b) is a block diagram
showing a second system in which variations of the voltage,
magnetic flux, and temperature of an impedance element are
detected. FIG. 11(c) is a block diagram showing a third system in
which a current flowing through a discharge lamp is detected. FIG.
11(d) shows a fourth system in which the time after closing of a
line switch is detected.
A circuit shown in FIG. 12(a) is constructed in accordance with the
first system (FIG. 11(a)) and comprises an a.c. power source 1, a
line switch 2, a discharge lamp 3, an impedance element 4 having
relatively low value and current limiting means 5 comprising a
bi-directional controlled rectifier 7 and an impedance element 6
having a relatively high impedance.
The circuit further comprises control signal generating means 8
which includes a bi-directional switching element 81, a capacitor
82, photosensitive means 83 such as a photosensitive resistance,
photo cell, or a photo diode, and resistors 84 and 85, and
detecting means 9 which has a resistor 92 and light emitting means
91 such as an incandescent lamp, a glow lamp, an electroluminescent
lamp, or a photo emitting diode.
The operation of this circuit is as follows: when the line switch 2
is closed, a current flowing in the discharge lamp 3 is limited by
the impedance element 6, that is, this current is smaller than its
rated value. On the other hand, the capacitor 82 in the control
signal generating means 8 is gradually charged through the resistor
85, thus increasing its voltage.
In the starting period, the equivalent impedance of the discharge
lamp 3 is low and the voltage across the discharge lamp is also
low. Therefore, the light output of the light emitting means 91 in
the time locking means 9 is small, and the resistance of the
photosensitive means 83 is therefore very high. Consequently, this
circuit 83 is negligible for the charging of the capacitor 82.
When the voltage of the capacitor 82 reaches the switching voltage
of the switching element 81, it will quickly become conductive.
Therefore, the capacitor 82 is discharged through the element 81
thereby forming a trigger control signal for the bi-directional
controlled rectifier 7. As a result, the bi-directional rectifier 7
is substantially shorted, that is, the impedance element 6 is
shorted thereby to suspend the function of the current limiting
means 5. Therefore, only the low impedance element 4 is left as an
impedance element in the circuit, as a result of which the current
flowing in the discharge lamp 3 is increased to operate the
discharge lamp 3.
In this connection, the current control point instant in a half of
the alternate current, at which the current limiting function of
the impedance element 6 is suspended, is determined by the
resistance of the photosensitive means 83 in the starting period.
For instance, the current control point is determined so that the
current flowing in the discharge lamp in the starting period is of
the order of 150 to 200 percent of the rated current.
Then, when the current flowing through the bi-directional
controlled rectifier 7 becomes about zero, the bi-directional
controlled rectifier 7 is changed to its nonconductive state, and
the circuit is restored to its initial conditions.
The operation of the circuit in the next half cycle is the same as
in the case described above except that the polarity is
reversed.
Soon, the vapor pressure, equivalent impedance, and voltage of the
discharge lamp increase. At this time, if the current control point
were kept unchanged, the lamp current would decrease. However, upon
an increase of the voltage of the discharge lamp, the light output
of the light emitting means 91 in the detecting means 9 increases,
and therefore the resistance of the photosensitive resistance
element decreases. As a result, the voltage of the capacitor 82
increases more quickly. Therefore, the instant at which the
bi-directional controlled rectifier 7 is changed to its conductive
state occurs earlier, that is, the current control point of the
a.c. cycle when the current limiting function caused by the
impedance element 6 is suspended, occurs earlier, thereby
preventing the decrease, described above, of the lamp current.
Thereafter, the lamp current is gradually and stably reduced to the
rated value.
As the light output of the discharge lamp 3 reaches its rated
value, the resistance of the photosensitive means 83 becomes very
low. However, if each concerned constant is determined so that the
current control timing point is such that the rated output is
obtained by the resistor 84 connected in series with the
photosensitive means 83, the lamp current will stably become its
rated value, that is, the discharge lamp 3 will be operated at its
rated conditions.
Thus, according to the invention, in the device for operating a
discharge lamp in which a current flow in the discharge lamp is
limited until a current control timing point is reached and the low
impedance element is employed as a stabilizing impedance element to
make the device smaller in size and lighter in weight, even if the
discharge lamp is such that the equivalent impedance and lamp
voltage thereof in the starting period is different from those at
the time when the rated light output is emitted from the discharge
lamp, it can be stably operated by changing the current control
timing point through detection of the conditions of the lamp in the
starting period.
In the circuit shown in FIG. 12(a), a terminal A, on the power
source side, of the control signal generating means 8 may be
connected to a point B where the discharge lamp 3 and the light
emitting means 91 are connected together, or to a point C between
the small impedance element 4 and the bi-directional controlled
rectifier 7. In these cases, the supply voltage for the control
signal generating means 8 corresponds to the variation in the
characteristics of the discharge lamp, and therefore the starting
operation of the discharge lamp 3 can be made stabler.
This can be applied to other examples which will be briefly
described with reference to FIGS. 12(b), 12(c) and 12(d). In these
examples, means for improving the starting characteristic of the
discharge lamp are the same as those in FIG. 12(a).
A device for operating a discharge lamp shown in FIG. 12(b) is also
based on the system of switching the impedance elements. The
impedance elements 4 and 6 are in series with the discharge lamp 3
until the current control point is reached, that is, the current is
limited by the high impedance element 6, and at the time the
current control point is reached the bi-directional controlled
rectifier 7 is changed to its conductive state to increase the
current flow in the discharge lamp.
A circuit shown in FIG. 12(c) employs a system for interrupting a
current flowing in a discharge lamp. In this circuit, a current
interrupting circuit 5a consisting of a bi-directional controlled
rectifier 7, capacitors 15 and 13, a resistor 14, a bi-directional
switching element 51, and a coil 12 operates to interrupt the
current flowing in the discharge lamp 3 so as to change it into a
high frequency current thereby to limit the current until the time
a current control point is reached in the half period of alternate
current. At the current control point the controlled rectifier 7 is
changed to its state, and this condition of the bi-directional
rectifier 7 is maintained until the end of the half cycle.
A circuit shown in FIG. 12(d) employs the system for reducing a
supply voltage. In this circuit, as the bi-directional controlled
rectifier 7 is maintained non-conductive until the current control
point, if the voltage supply is lower than the rated voltage of a
discharge lamp 3, no primary current will flow in a transformer 16,
that is, the voltage supply will not be increased. Accordingly, a
low voltage is applied to an impedance element 4 and the discharge
lamp 3, thus limiting the current flow in the discharge lamp 3.
At the time the current control point is reached, the
bi-directional controlled rectifier 7 is changed to its conductive
state, as a result of which the primary current flows in the
transformer 16, the supply voltage increases, and the current flow
in the discharge lamp 3 increases.
In the circuits described above, the discharge lamp may be replaced
by an equivalent load such as the primary side of a transformer
with its secondary side being connected to the discharge lamp.
The circuit shown in FIG. 12(a) can be modified as illustrated in
FIG. 13. In this modified circuit, the phase of a voltage across a
capacitor 86 lags behind that of a voltage supply owing to the
capacitor 86 and resistor 85, while a capacitor 82 is charged
through a resistor 87. Therefore, the operation of a bi-directional
switching element 81 is stable even in the case where the current
control timing instant is delayed (in phase) in the starting
period.
FIG. 14 illustrates a modification of the circuit shown in FIG. 13.
In this modification, instability in the starting operation of the
discharge lamp, which may be caused by the variation of a voltage
supply, is suppressed by means of Zenor diodes 88 which are
connected in series and back-to-back.
FIG. 15 illustrates one example of a device for operating a
discharge lamp in which a voltage across an impedance element is
detected according to the invention. In this example, in the
starting period, a lamp current is large, and the light output of a
light emitting means is therefore great, that is, the resistance of
photosensitive means 83 is low. Therefore, the resistance of a
circuit formed by the photosensitive means 83 and a resistor 84 can
be determined substantially by the resistance of the resistor 84.
Accordingly, a part of the current flowing through a resistor 85 to
a capacitor 82 will flow to the circuit consisting of the
photosensitive means 83 and the resistor 84.
As a result, the instance at which the voltage of the capacitor 82
reaches the switching voltage of a bi-directional switching element
81 will be delayed, that is, the switching instant of the switching
element 81 or the current control point described above will be
delayed (in phase) by a half cycle of the alternating current.
As the voltage of the discharge lamp 3 increases, the voltage of
the impedance element 4 decreases. Accordingly, the resistance of
the photosensitive means 83 increases, that is, the part of the
current described above decreases. Therefore, the voltage of the
capacitor 82 will reach the switching voltage of the switching
element 81 earlier.
Soon, the light output of the discharge lamp 3 reaches its rated
value, and the discharge lamp 3 is operated at its rated output at
the current control point. This example is provided according to
the system described with reference to FIG. 11(b).
Shown in FIG. 16 is a circuit provided in accordance with the
system describe with reference to FIG. 11(c). In this circuit, a
current flowing through a discharge lamp 3 is converted into a
voltage across a resistor 93. The operation of this circuit is the
same as that of the circuit of FIG. 15.
FIG. 17 shows one example of a device for operating a discharge
lamp in which the current control point is changed as a function of
the time passing after the starting time of the discharge lamp.
This example is provided on the basis of the system described with
reference to FIG. 11(d). However, it has an additional effect,
which will become more apparent later, in that the starting of the
function of control signal generating means is delayed.
The circuit comprises a resistor 94 for heating a thermistor 83a
whose resistance is very high at room temperature, and a variable
resistor 95 which is used to control a current flowing through the
resistor 94 thereby to adjust a temperature rise thereof.
In this circuit, when a line switch 2 is closed, the discharge lamp
3 starts to discharge, and at the same time, a current flows in the
heater 94 thereby to heat the latter. Since the resistance of the
thermistor 83a is very high at room temperature, a current for
charging the capacitor 82 flows through resistors 84 and 85.
Therefore, the current control timing instant will occur at a
relatively late point in a half cycle of the alternate current.
With the lapse of time, the temperature of the thermistor 83a is
raised by the heating operation of the resistor 94, that is, the
resistance of the thermistor is decreased. Therefore, the charging
speed of the capacitor 82 gradually increases. Therefore, the
current control point is advanced in each half cycle of the
alternating current, and soon the discharge lamp 3 will be operated
at its rated output. At this time, the resistance of the thermistor
83a is low and the charging speed of the capacitor 82 is therefore
determined substantially by the resistance of the resistor 85.
Therefore, the values of the capacitor 82 and the resistor 85
should be determined so that the current control point lies in the
range such that the discharge lamp is operated at its rated
output.
In this connection, it is possible to use other elements whose
characteristics are changed with time, in place of the indirectly
heated thermistor described above.
FIGS. 18(a), 18(b) and 18(c) show other examples of the control
signal generating means.
In the examples illustrated in FIGS. 12(a) through 17, the
resistance 83 or 83a is changed with the variations of the voltage,
current, output and temperature of the discharge lamp, and with the
variations of the voltage, magnetic flux and temperature of the
impedance element, until the discharge lamp is operated at its
rated output from the starting time.
If described with reference to FIG. 18(a), these variations are
considered the same as those obtained by varying the resistance of
a resistor R1 or the resistance of a resistor R4 connected parallel
to a capacitor C1. In the circuit of FIG. 18(a), even if the
resistance R1 or R4 is linearly varied, the variation of the time
at which the control signal is generated from the control signal
generating means is not linear, that is, this time is not varied
until a certain value of the resistance is reached, and thereafter
is abruptly varied. Therefore, the unstable operating conditions of
the device are sometimes observed till the rated light output of
the discharge lamp is obtained.
In the examples shown in FIGS. 18(a), 18(b) and 18(c), the
resistance of the resistor R1 or R4 is fixed, while the resistance
of a resistor R2 or R3 is varied by sensing variations in the
characteristic of the discharge lamp or circuit so as to
substantially linearly change the current control point described
above. This is a so-called pedestal control, and the current
control point can be stably changed in a wide range.
FIG. 18(a) illustrates an example in which alternating current
subjected to full-wave rectification is applied to a uni-junction
transistor (UJT), FIG. 18(b) illustrates an example which employs a
silicon controlled switch (SCS), and FIG. 18(c) illustrates an
example in which alternating current is applied to a bi-directional
switching element.
Shown in FIG. 19 is a further example of the control signal
generating means which controls the output of the discharge lamp
such that it is constant. In this example, a voltage proportional
to the supply voltage is added through a resistor R7 and a
capacitor C2 to the constant voltage obtained by a Zenor diode D,
and the voltage obtained by this addition is used as the base
voltage of a unijunction transistor UJT.
When the supply voltage increases, the peak voltage of the
uni-junction transistor UJT increases also. As a result, the
current control point occurs late thereby to suppress or prevent
the increase of the lamp current. In contrast, when the supply
voltage decreases, the peak point voltage of the unijunction
transistor UJT decreases also. As a result, the current control
point occurs early thereby to prevent the decrease of the lamp
current.
Thus, it is possible to make the light output of the discharge lamp
constant, regardless of the variation of the voltage supply, by
properly determining the values of the resistor R7 and the
capacitor C2.
Thus, by the additional provision of means which operates to change
the current control point in the a.c. waveform according to
variation in the characteristics of the discharge lamp in its
starting period, resulting variation of the physical quantities,
such as current and voltage, of the circuit elements, or the lapse
of time after the starting time, a device for operating a discharge
lamp which can stably operate any discharge lamp, can be
obtained.
Hereinafter, several examples of the device for operating a
discharge lamp according to the invention, in which the discharge
lamp requires starting pulses or has an irregular discharge will be
described.
Shown in FIG. 20 is one example of a device for operating a
discharge lamp according to the invention which comprises: an
alternating current power source 1; a line switch 2; a discharge
lamp 3; an impedance element 4 having a relatively low impedance;
current limiting means 5 constituted by an impedance element 6
having a relatively high impedance and a bi-directional controlled
rectifier 7; control signal generating means 8 comprising a
bi-directional switching element 81, a capacitor 82 and a resistor
83; time locking means 9 having a coil 91a and its contact means
92a.
The operation of this circuit is as follows. Upon closing of the
line switch 2, the discharge lamp 3 is operated, but its current is
limited by the high impedance element 6 to a value lower than its
rated current.
In general, discharge lamps are liable to have irregular discharge
characteristics in the starting period. For instance, some
discharge lamps will have an irregular discharge for a period of
several cycles of an alternating current applied thereto after
being switched on, and some discharge lamps, for several tens of
seconds. The cause for this irregular discharge cannot be simply
determined, because various factors such as lamp current, ambient
temperature and lapse of time, can be considered for the irregular
discharge.
Therefore, if the time locking means 9 is set so as to close its
contact 91 after the irregular discharge is over and the discharge
lamp has been stably operated, upon termination of the irregular
discharge a terminal a of the control signal generating means 8 is
connected to a terminal A of the power source 1.
During each half cycle of the alternating current, since the
terminal a is thus connected to the power source 1, the capacitor
82 of the control signal generating means 8 is gradually charged
through the resistor 83.
In this case, if the resistance of the resistor 83 is determined in
advance so that the voltage of the capacitor 82 reaches the
switching voltage of the bi-directional switching element 81 at a
current control point during a half cycle of the alternating
current, at the time the predetermined current control point is
reached the bi-directional switching element 81 becomes conductive
and the capacitor 82 is therefore discharged through the switching
element 81 thereby to produce a control signal for the
bi-directional controlled rectifier 7.
The bi-directional controlled rectifier 7 becomes substantially
conductive by the control signal thus produced, that is, the high
impedance element 6 is short-circuited by a circuit formed by the
bi-directional controlled rectifier 7 and the low impedance element
4. In this case, since the only impedance element left active in
the circuit is the low impedance element 4, the current flowing in
the discharge lamp 3 increases, that is, the output thereof
increases.
Thereafter, the lamp current decreases with the voltage of the
alternating current, and the half cycle of the alternating current
waveform terminates.
During the next half cycle, the operation of the circuit is carried
out in the same manner as described above except that the polarity
is reversed.
Accordingly, the discharge lamp 3 can be operated at its rated
output for a full cycle of the alternating current.
As is apparent from the above description, in order to prevent the
incorrect operation of various means in the circuit which are found
when a discharge lamp which requires pulses for starting discharge
after switched being on is operated on when the operation of
current limiting means such as a bi-directional controlled
rectifier is made unstable by the irregular discharge of the
discharge lamp, the control signal generating means is actuated by
the time locking means after a given time thereby to suspend or
depress the function of the current limiting means at the instant
the current control point is reached during every half cycle of the
a.c. input for a given time until the discharge lamp beings to
operate in a stable manner. As a result, the operation of the
discharge lamp can be carried out more stably.
In the circuit shown in FIG. 20, the contact means 92a of the time
locking means 9 may be provided at a point b or c of the control
signal generating means 8, or may be inserted in series with the
bi-directional controlled rectifier 7.
In addition, if the contact means of the time locking means 9 is
such that it will open by the operation of the time delay relay for
the time locking means, the contact may be connected between the
points b and c of the control signal generating means 8.
Instead of the time delay relay for the time locking means an
element such as a bimetal, transistor, bi-directional controlled
rectifier or a circuit comprising resistors, capacitors and
semiconductors which is equivalent in effect to the time delay
relay, may be employed.
The contact means 92a of the time locking means 9 may be connected
between the terminal a of the control signal generating means 8 and
a point B where the discharge lamp 3 and the impedance element 4
are connected, or may be connected to a point C between the
impedance element 4 and the bi-directional controlled rectifier 7.
In this case, the supply voltage for the control signal generating
means 8 responds to variations in the characteristic of the
discharge lamp. Therefore, it is possible to make the starting
operation of the discharge lamp stabler.
These modifications applied to the circuit may be given to the
following circuits (FIGS. 21(a), 21(b) and 21(c) ) which are
provided according to the invention for the same purpose as that
described above.
In these circuits, time locking means are the same as that shown in
FIG. 20.
The circuit shown in FIG. 21(a) is a modification of the circuit
illustrated in FIG. 20.
An electric current flowing in a discharge lamp 3 is limited by a
relatively high impedance element 6 which, together with a
relatively low impedance element 4 and the discharge lamp 3 forms a
series circuit until the current control point described above is
reached in each half cycle of the alternating current. At instant
the current control point is reached, a bi-directionl controlled
rectifier 7 becomes conductive in the same manner as described
above, and short-circuits the high impedance element 6. As a
result, the current flowing in the discharge lamp 3 increases.
The circuit shown in FIG. 21(b) is designed for reducing a supply
voltage for limitation of a current flow in a discharge lamps. In
this circuit, a supply voltage is kept lower than the rated voltage
of a discharge lamp 3 and therefore a bi-directional controlled
rectifier 7 is also kept nonconductive until a current control
point is reached. Accordingly, no primary current flows in a
transformer 16, that is, the transformer does not work as a booster
in this case. Therefore, a voltage applied to the discharge lamp 3
and a relatively low impedance element 4, is relatively low, thus
limiting the current flow in the discharge lamp 3. At the current
control point, the bi-directional controlled rectifier 7 is caused
to be conductive, and therefore the primary current flows in the
transformer 16 and the voltage supply is increased. That is, the
higher voltage is applied to the discharge lamp 3, which increases
the current flow in the discharge lamp 3.
The circuit shown in FIG. 21(c) employs the system of interrupting
an electric current. An electric current flowing in a discharge
lamp 3 is changed into a higher frequency current by means of a
current interruption circuit 5a which comprises a bi-directional
controlled circuit 7, a coil 12, capacitors 13 and 15, a resistor
14, and a bi-directional switching element 51. This operation is
continuously carried out until a control point is reached during
each half cycle of the alternating current applied to the circuit.
At the control point, a bi-directional controlled rectifier 821
relatively small in capacity is caused to be conductive. Since this
conductive condition of the rectifier 821 is maintained until the
half cycle of the alternating current is terminated, the
bi-directional controlled rectifier 7 is kept conductive for the
same period, thus suspending the operation of the current
interruption circuit 5a.
The principles described with reference to FIGS. 20, 21(a), 21(b)
and 21(c) can be applied to any of the circuits which will be
described later.
These principles can be applied to the discharge lamp whose
equivalent impedance in the starting period is roughly the same as
that in the rated operating time period.
However, with the discharge lamp which has low vapor pressures and
substantially shorted equivalent impedance in the starting period
and takes several minutes till it becomes stabilized by increasing
its vapor pressure to a certain value conditions, if the current
control timing instant is fixedly set to occur at such an instant
as the discharge lamp produces the rated output under the
conditions that the circuit for the discharge lamp operates after a
given time by means of the time delay means, there will be a danger
of an over-current flowing in the discharge lamp with the result of
damage of the latter.
This danger can be avoided as follows. The current control point is
set at a late instant in the half cycle of the alternating current
so that, after the starting switch has been closed and the
discharge has become stabilized, the lamp current is, for instance,
on the order of 150 to 200 percent of that obtained at the time the
discharge lamp is turned on at its rated value. In addition,
variation in the characteristics of the discharge lamp or resulting
variations in the characteristics of circuit elements are converted
into a variation such as electrical resistance which is applied to
control a signal generating means so that the current control
timing instant is automatically changed to occur at such an instant
as to obtain the rated light output of the discharge lamp, thereby
to stably light the discharge lamp.
For this purpose, the previously systems described with reference
to FIGS. 11(a) through 11(d) can be applied to the device for
operating a discharge lamp.
Devices for operating a discharge lamp in which the systems are
employed will be described hereinafter.
A circuit shown in FIG. 22 comprises a detecting means 221 having a
resistor 221a and light emitting means 221b used to detect the
voltage of a discharge lamp 3, control signal generating means 8
having photosensitive means 84 and a resistor 85, and several
elements such as the time locking means 9, the line switch 2, the
impedance element 4 and the current limiting means 5 described with
reference to FIG. 20.
The operation of the circuit is as follows. When the line switch 2
is closed, the lamp current, being limited by the impedance element
6, is smaller than the rated current of the discharge lamp. In this
case, if the current control point described above is set so that a
supply voltage is applied to the control signal generating means 8
through the time locking means 9 after the termination of the
irregular discharge of the discharge lamp from the control point
thus set the control signal generating means 8 starts its
operation. In this time, the discharge lamp 3 has a low vapor
pressure, equivalent impedance, and lamp voltage. Therefore, the
light output of the light emitting means 221b in the detecting
means 221 is small and the resistance of the photosensitive element
84 is very high.
Accordingly, in this case, the charge of the capacitor 82 is
determined only by the resistance of the resistor 85. On the basis
of this fact, the current control point can be set by the resistor
85 so that the lamp current in the period of starting the operation
of the discharge lamp is on the order of 150 to 200 percent of that
obtained when the discharge lamp produces its rated output.
Soon, the vapor pressure of the discharge lamp 3 increases while
the equivalent impedance thereof increases, as a result of which
the lamp voltage also increases. In this case, at the current
control point, the lamp current decreases, but the light output of
the light emitting means 221b of the detecting means 221 increase
with the lamp voltage, that is, the resistance of the
photosensitive means 84 decreases with the increases of the light
output, as a result of which the speed in charging the capacitor 82
will be changed faster. Consequently, the instant the
bi-directional controlled rectifier 7 is changed to its conductive
state, namely, the control point is reached earlier. Thus, the
decrease of the lamp current can be avoided.
As the discharge lamp 3 produces its rated light output, the
resistance of the photosensitive means 84 becomes low. In this
case, if the constants of the elements concerned are determined so
that the current control point is within a range such that the
rated light output of the discharge lamp is obtained by means of
the resistor 83, which will allow the lamp current to stably have
its rated value, the discharge lamp will be stably operated at its
rated conditions.
FIG. 23 shows another example of the control signal generating
means according to the invention, in which the time locking
described above is obtained by a circuit comprising capacitors,
resistors and transistors.
After application of the power source, the potential at a point c
will gradually lower. When this potential becomes lower than a
certain voltage determined by a Zenor diode Z2, a transistor
Tr.sub.1 will become conductive and the potential at a point d will
soon become equal to that at a point b, which will allow the
control signal generating means to generate a control signal.
In this system, a switching element having relatively small current
capacity may be used in place of the Zenor diode Z2 and the time
locking can be readily obtained by changing the resistance of a
resistor R3.
A circuit formed by resistor R5, R6, and R7, a diode D2, and a
capacitor C3 is stabler in control operation than a circuit formed
only by the resistor R7 and the capacitor C3.
A circuit formed by a current transformer CT, diodes D3, a
capacitor C4, a variable resistor R8, resistors R9 and R10, and a
transistor Tr.sub.3 is used to detect the lamp current and to
equivalently change the resistance of the resistor R6 depending on
a value obtained by the detection of the lamp current. For
instance, the current control point can be set by adjusting the
variable resistor R8 so that the lamp current is of the order of
150 to 200 percent of the rated lamp current during the starting
operation after the time delay.
Soon, the vapor pressure of the discharge lamp increases, the
equivalent resistance of the discharge lamp increases, the lamp
current decreases, and the base-emitter current of the transistor
Tr.sub.3 decreases. As a result, the equivalent resistance of the
resistor R6 increases, the current control point advances (in phase
angle) thereby to prevent the decrease of the lamp current. Thus,
the discharge lamp will soon produce its rated light output.
Thus, the occurrence of the control point can be adjusted mainly by
the resistor R6. The increase of the resistance of the resistor R6
will permit the discharge lamp to produce more light output than
its rated value. A Zenor diode Z1 can suppress the voltage
variation in the transistor circuit.
Lagging in phase in a detection circuit of such a current feedback
loop as described above can be readily compensated by capacitance
and resistance thereby to obtain the stable operation of the
discharge lamp.
Furthermore, in this example (FIG. 23), the peak voltage of the
uni-junction transistor Tr.sub.2 can be changed by a resistor R2
and a capacitor C1 for the variation of the voltage supply thereby
to control the occurrence of the current control point, and this
system provides a discharge lamp operating device having such a
constant power characteristic as to have a constant light output
during the period that the discharge lamp is operated at its rated
light output.
The locking time caused by the time locking means provided in the
discharge lamp operating device of the invention is roughly the
same as that of the conventional discharge lamp operating
device.
As is apparent from the above description, it is possible,
according to the invention, to provide a discharge lamp operating
device which is highly stable, highly reliable, smaller in size and
lighter in weight and which operates properly for a discharge lamp
requiring pulses or having the irregular discharge characteristics
during the starting period, a discharge lamp having characteristics
which vary with time, and a discharge lamp whose characteristics in
the starting period differs from that in its rated operating
period.
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