U.S. patent number 3,611,021 [Application Number 05/025,684] was granted by the patent office on 1971-10-05 for control circuit for providing regulated current to lamp load.
This patent grant is currently assigned to North Electric Company. Invention is credited to Kenneth A. Wallace.
United States Patent |
3,611,021 |
Wallace |
October 5, 1971 |
CONTROL CIRCUIT FOR PROVIDING REGULATED CURRENT TO LAMP LOAD
Abstract
Control circuit for gaseous discharge lamps including a variable
frequency inverter for driving a high-reactance transformer having
a first capacitor in the transformer secondary tuned to a harmonic
of the supply voltage to provide ignition voltage for the lamps,
and a second capacitor in near series resonance with the
fundamental frequency of the supply voltage to provide series
impedance at the fundamental frequency for stable operation after
ignition, and lamp current sensing means for providing a feedback
signal to a variable reference comparator circuit which adjusts the
frequency output of the inverter to provide regulated lamp current
for changes in input voltage and lamp voltage.
Inventors: |
Wallace; Kenneth A. (Columbus,
OH) |
Assignee: |
North Electric Company (Galion,
OH)
|
Family
ID: |
21827487 |
Appl.
No.: |
05/025,684 |
Filed: |
April 6, 1970 |
Current U.S.
Class: |
315/239; 315/307;
315/DIG.5; 331/113A |
Current CPC
Class: |
H02M
7/53806 (20130101); H05B 41/392 (20130101); Y10S
315/05 (20130101) |
Current International
Class: |
H02M
7/538 (20060101); H05B 41/392 (20060101); H05B
41/39 (20060101); H03k 003/281 (); H05b
041/14 () |
Field of
Search: |
;315/DIG.5,DIG.2,307,239
;331/113A |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lake; Roy
Assistant Examiner: Demeo; Palmer C.
Claims
While what is described is regarded to be a preferred embodiment of
the invention, it will be apparent that variations, rearrangements,
modifications and changes may be made therein without departing
from the scope of the present invention as defined by the appended
claims.
1. A control circuit for providing regulated current to a gaseous
lamp comprising an input circuit over which direct current power is
supplied, a variable frequency inverter circuit connected to said
input means including a control input for adjusting the frequency
of the output signals from said inverter circuit, a high-reactance
transformer having a primary and a secondary winding, means
connecting said primary winding to the output of said variable
frequency inverter, a further winding means on said transformer
connected to energize the filaments of said lamp, a shunt capacitor
connected in shunt of said secondary winding for providing harmonic
resonance during start, a series capacitor connected in series with
said secondary winding and said gaseous lamp to provide fundamental
resonance for lamp energization subsequent to start, and lamp
current regulating means for providing a control signal to said
control input to adjust the frequency output of said variable
frequency inverter circuit in a current regulating mode.
2. A circuit as set forth in claim 1 in which said lamp current
regulating means includes a sensing circuit for providing a signal
representative of the value of the lamp current, a reference
circuit for providing a preset reference signal level, and means
for providing a control signal to said variable frequency inverter
circuit of a magnitude related to the differential of the sensed
signal relative to said preset reference signal level.
3. A circuit as set forth in claim 2 in which said reference
circuit includes means for adjusting said preset reference to
different values.
4. A circuit as set forth in claim 1 in which said variable
frequency inverter circuit includes an oscillator circuit and a
pair of switching transistors driven by said oscillator circuit,
and in which said control signal is fed to said control input to
vary the output frequency of said inverter circuit to maintain a
constant output current and thereby a constant light intensity from
said lamp.
5. A control circuit as set forth in claim 4 in which said
oscillator circuit is a saturable core oscillator.
6. A control circuit as set forth in claim 1 in which said series
capacitor and said secondary winding of said transformer have a
value which establishes the operating frequency of the variable
frequency inverter circuit to be above the starting frequency of
the inverter circuit.
7. A control circuit as set forth in claim 6 in which said control
signal to said variable frequency inverter circuit increases the
inverter output frequency to reduce lamp current responsive to
detection of an increase in lamp current by said lamp current
regulating means.
8. A control circuit as set forth in claim 1 in which said series
capacitor and said secondary winding of said transformer has a
value which establishes the operating frequency to occur below the
starting frequency of the variable frequency inverter circuit.
9. A control circuit as set forth in claim 8 in which said control
signal to said variable frequency inverter circuit decreases the
inverter output frequency to decrease the lamp current in response
to the detection of an increase in lamp current by said lamp
current regulating means.
10. A control circuit as set forth in claim 1 in which said shunt
capacitor is connected across only a part of said secondary
winding.
11. A control circuit as set forth in claim 1 in which said primary
and secondary transformer windings are connected in an
autotransformer configuration with the primary voltage in series
with the secondary voltage, and said shunt capacitor is connected
across the secondary winding and said series capacitor is connected
in series with the parallel connected secondary winding and shunt
capacitor.
12. A control circuit as set forth in claim 1 in which said
transformer and frequency inverter circuit have components which
provide a lamp current at the starting frequency which is slightly
higher than the maximum desired lamp current for the minimum input
voltage over said input circuit and the maximum drop across said
lamp.
13. A control circuit as set forth in claim 1 in which the signal
output of said adjustable frequency inverter current comprises an
AC square wave having a fundamental frequency component plus one or
more harmonics.
14. A control circuit as set forth in claim 1 in which said lamp
current regulating means comprises a current transformer having a
primary winding connected in series with said lamp, and a center
tapped secondary winding, a rectifier circuit connected to the
output of said secondary winding, and a resistor connected to the
output of said rectifier circuit to develop a DC signal
representative of the current in said lamp circuit.
15. A control circuit as set forth in claim 1 in which said
frequency circuit operates at a first frequency for ignition of
said lamp and a second frequency for operation of said lamp, and
wherein a momentary interruption of lamp power during operation of
said lamp and a resulting loss of lamp current causes said lamp
current regulating means to provide a control input signal to
return the variable frequency inverter circuit from said second
operating frequency to said first starting frequency for reignition
of said lamp.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a control circuit for starting and
operating gaseous discharge lamps which includes circuit means for
adjusting lamp current to provide automatic current regulation for
changes in input voltage and lamp voltage.
2. Description of Prior Art
The electrical characteristics of fluorescent lamps are such that a
high starting voltage is required for ignition, and a ballast (or
series impedance) is required for stable operation thereafter. The
light intensity output from an energized fluorescent lamp is
proportional to the RMS current through the lamp.
In certain prior art arrangements high-reactance ballast
transformers are used to provide the voltages required for starting
and operating one or more fluorescent lamps. By connecting a
shunting capacitor in parallel circuit relationship with the
high-reactance transformer secondary, a circuit is provided which
can be made to resonate with the fundamental or harmonic of the AC
input voltage, and develop a high starting voltage to ignite the
lamp. In addition a capacitor may be placed in series circuit
relationship with the lamp to provide a net capacitive reactance in
the lamp circuit during the period subsequent to lamp ignition.
While the above system is effective in starting and operating
fluorescent lamps, it does not, by itself, provide an adjustable,
regulated lamp current. In certain applications, as for example in
photographic or electrostatic copying machines, a regulated lamp
current is required to maintain constant light intensity. While
various attempts have been made to incorporate regulation in the
high-reactance transformer by saturation of the magnetic core, such
attempts have been generally inefficient and the arrangement in
general has been difficult to adjust.
SUMMARY OF THE INVENTION
It is the purpose of the present invention to provide a control
circuit for gaseous discharge devices, such as fluorescent lamps,
which has simple and efficient means for adjusting and regulating
lamp current. In a preferred embodiment of such arrangement, a
direct current input voltage is connected to a variable frequency
inverter which is operative to provide an AC voltage waveform
containing a fundamental frequency component, plus one or more
harmonics, to the primary of a tuned transformer. A first
capacitor, which is connected across the secondary of the
high-reactance transformer, has a value such that the capacitor
resonates with the leakage reactance of the transformer primary at
some selected harmonic which is present in the inverter output
waveform. The harmonic resonant voltage across the transformer
secondary, when added to the transformer fundamental voltage, is
made sufficient to ignite the lamp which is connected to the
transformer secondary.
A second capacitor connected in series with the lamp is selected to
be near resonance with the leakage reactance of the transformer at
the fundamental inverter frequency. Once the lamp is ignited the
high harmonic voltage across the first capacitor is swamped out by
the large fundamental current flowing through the second capacitor,
the lamp and the secondary winding of the transformer. The
equivalent series impedance at the fundamental inverter frequency
provides the necessary ballast for stable operation.
Lamp current control is accomplished by taking advantage of the
lamp current versus frequency characteristic of the tuned
transformer configuration consisting of the transformer and the
first and second capacitor. Lamp current is sensed and the sensing
signal is compared to a preset reference level. If the lamp current
attempts to exceed the reference level, an error signal is applied
to a control input of the variable frequency inverter circuit, and
the output frequency of the inverter is changed in a direction to
reduce the lamp current. With decrease of the lamp current below
the reference point, the output of the inverter frequency changes
in a direction to increase the lamp current.
BRIEF DESCRIPTION OF THE DRAWINGS
With reference to the drawings,
FIG. 1 is a showing of one embodiment of the novel lamp current
control circuit of the invention;
FIG. 2 is a graph of the harmonic starting voltage versus frequency
characteristics of a tuned transformer configuration in such a
circuit where the lamp is not ignited;
FIGS. 3A and 3B are graphs of the lamp current versus frequency
characteristics of a tuned transformer configuration in such a
circuit;
FIG. 4 is an illustration of a reference and comparator circuit
designed to produce the curve of FIG. 3B above; and
FIGS. 5-7 are illustrations of different tuned transformer
configurations for use in the novel circuit arrangement.
DETAILED DESCRIPTION
With reference to FIG. 1, there is shown thereat a preferred
embodiment of the invention. As there shown, a variable frequency
inverter 3 has a pair of inputs 1, 2 connected to any applicable
source of direct current input voltage. Variable frequency inverter
3 includes a saturable core oscillator 10 which drives a pair of
switching transistors 4, 8 in a manner to be described, to supply a
square wave output over conductors 13, 14 to a center tapped
primary winding 16 wound with the indicated polarity on core 17 of
a tuned transformer 15. The waveform output from inverter 10 will
contain a fundamental frequency component plus one or more
harmonics.
Transformer 15 includes a first, second and third secondary
windings 19, 20, 21 respectively wound on core 17 with the
polarities indicated by the dots adjacent the respective windings.
The secondary windings 19, 20 of transformer 15 are connected in
series with the filaments 30, 31 respectively of a gaseous device,
such as illustrated fluorescent lamp 27. Secondary winding 21 is
connected in series with capacitor 26, lamp 27, and the primary
winding 35 on a current transformer 36 in lamp sensing circuit
34.
A second capacitor 25 is connected across the secondary 21 of the
high-reactance transformer 15. During the "start up" condition
capacitor 25 is made to resonate with the leakage reactance of
transformer 15 at some selected harmonic present in the inverter
output waveform. The harmonic resonant voltage across secondary
winding 21, when added to the transformer fundamental voltage, is
made sufficient to ignite the lamp (see FIG. 2). The output
frequency of inverter 3 during this "start up" condition is denoted
the "starting" frequency.
Capacitor 26 is selected to be near resonance with the leakage
reactance of the winding of transformer 15 at the fundamental
inverter frequency. Once the lamp 27 is ignited, the voltage across
capacitor 25 is swamped out by a large fundamental current flowing
through secondary winding 21, capacitor 26, lamp 27, and the
primary winding 35 of current transformer 36. The equivalent series
impedance of these components at the fundamental inverter frequency
provides the necessary ballast for stable operation.
Lamp Current Regulation
In accordance with a novel concept of the invention, current to the
lamp 27 is automatically regulated by utilization of the lamp
current versus frequency characteristic of the tuned transformer
configuration consisting of transformer 15, capacitor 25, and
capacitor 26. If the current flow through lamp 27 attempts to
exceed a reference level preset in reference and comparator circuit
45, the circuit 45 generates and feeds an error signal over
conductor 60 to the saturable core oscillator 10 in the variable
frequency inverter 3, and the inverter output frequency is changed
in a direction to reduce lamp current.
More specifically, with capacitor 25 tuned with the leakage
reactance of transformer 15 to be resonant at the third harmonic of
the starting frequency, the high harmonic voltage across capacitor
25 is sufficient to ignite the lamp 27, and thereafter current at
the fundamental square wave frequency begins to flow through
capacitor 26, lamp 27, current transformer primary winding 35 of
transformer 36 and transformer secondary winding 21. The lamp
current through the primary winding 35 of current transformer 36 is
transformed to the center tapped secondary 37 for rectification by
diodes 38 and 40 and filtering by capacitor 41. The filtered DC
voltage developed across resistor 42 is proportional to the lamp
current through lamp 27, and is fed over conductor 43 to the base
of comparison transistor 46 in the reference and comparator circuit
45.
FIG. 1 shows a reference and comparator circuit 45 for a device
wherein the starting frequency, (the fundamental inverter
frequency) is above the peak of the lamp current versus frequency
curve, as shown in FIG. 3A. If the device were designed so that the
starting frequency were below the peak of the curve, as shown in
FIG. 3B, a reference and comparator circuit 45', such as shown in
FIG. 4, would be used. The following description is of the
reference and comparator circuit shown in Fig. 1.
Comparison transistor 46 is connected to compare such signal with a
preset reference voltage the value of which is determined by the
setting on potentiometer 50, and to such end has an emitter
connected through the adjustable arm 49 of potentiometer 50 to
negative conductor 2. The collector of transistor 46 is connected
to the base of the control transistor 47 to thereby vary the value
of the control signal fed over conductor 60 to oscillator 10. More
specifically, the emitter of transistor 47 is connected to a stable
voltage point established at the junction of Zener diode 51 and
resistance 52 which are series connected across the DC input
conductors 1, 2. The variable current output from the collector of
transistor 47 (as determined by the output of transistor 46) is fed
over conductor 60 to the input for oscillator 10. The collector of
transistor 47 is also connected through resistor 48 and resistor 50
to negative potential on conductor 2.
In operation, the starting frequency of the oscillator 10 is
determined by the voltage of the reference Zener diode 51 minus the
voltage drop across resistor 48. During the startup condition,
transistor 47 is off. After startup, with variation of the lamp
current above the preselected value, the input signal on conductor
60 will be adjusted to vary the frequency output of inverter 3 in a
related manner. More specifically, if the voltage across resistor
42 attempts to exceed the reference level established over
adjustable resistor 50 at the emitter of transistor 46 by more than
the emitter-base drop of transistor 46, collector current will
begin to flow in transistor 46, and transistor 47 will be turned on
to cause an increased voltage to appear on conductor 60 and the
input for the saturating core oscillator 10. Consequently the
frequency output of inverter 10 will increase, and in a system
having the characteristics of Fig. 3A, lamp current will decrease
to hold the lamp current constant at the value determined by the
setting on potentiometer 50. Correspondingly, as the lamp current
decreases, and the error signal provided across resistor 42
decreases, the conductivity of transistors 46 and 47 decreases to
reduce the value of the control signal over conductor 60 to
oscillator 10 to decrease the output frequency of inverter 3.
Reduction of the frequency output of inverter 3 will result in the
increase of lamp current, whereby current to the lamp tends to
remain constant despite normal variations in DC input voltage and
lamp voltage drop.
It is apparent that adjustment of potentiometer 50 of different
values will vary the operating frequency range of the circuit. If
the current is too high, the setting on potentiometer 50 is lowered
so that the reference circuit will increase the voltage to the
oscillator circuit 3 to control same to operate at a higher
frequency and thereby reduce the current. Raising of the setting on
potentiometer 50 effects a decrease of the voltage to the
oscillator 3 and a decrease in the oscillator frequency to increase
the current.
If the lamp were to extinguish, the current at input 43 to
transistor 46 would go zero and transistor 46 will turn off to in
turn effect turnoff of transistor 47. The voltage on output path 60
will go to minimum value, and at minimum voltage the frequency of
the oscillator drops back to f.sub.o , the harmonic frequency drops
to nf.sub.o and the lamp will refire.
The position of the peak of the lamp current versus frequency curve
(FIG. 3A) on the frequency axis is determined by the value of the
leakage reactance 21 and capacitor 26. Thus by changing the value
of capacitor 26 it is possible to shift the peak of the curve along
the frequency axes.
If the circuit components (i.e., capacitor 26 and reactance 21) are
selected so that the starting frequency is below the peak of the
lamp current versus frequency curve (FIG. 3B), a reference and
comparator circuit 45', such as shown in FIG. 4, would be used.
With reference thereto, components similar to those shown in FIG. 1
are identified by a corresponding number. In such arrangement, the
voltage on line 60 equals the voltage established by Zener diode
51' less the voltage across resistor 101. As the voltage across
resistor 101 goes up, the voltage on line 60 goes down, and vice
versa. The voltage drop across resistor 101 is dependent on the
amount of current going through resistor 101, and the value of
current through resistor 101 is dependent on the conductivity of
transistor 100, which conductivity in turn is dependent on the
conductivity of transistor 46'.
Current on line 43 to the base of comparison transistor 46', and
the related base voltage when compared to the present reference
voltage determined by the setting on potentiometer 50', will
determine the conductivity of transistor 46'. An increase in
current, and a corresponding increase in voltage on line 43 will
cause transistor 46' to become more conductive, causing transistor
100 to become more conductive, and an increased current flow
through resistor 101. With the greater voltage drop across resistor
101 as the result of the increased current flow, there is a
decreasing voltage on line 60 to the oscillator, causing a decrease
in frequency and, as seen in Fig. 3B, a corresponding decrease in
lamp current.
By the same analogy, a decrease in current on line 43 will cause an
increased voltage on line 60 delivered to the oscillator causing an
increased frequency and a corresponding increase in lamp
current.
Adjustment of potentiometer 50 to a lower setting will lower the
current range, and adjustment of potentiometer 50 to higher setting
will raise the current range for the unit in an obvious manner. If
the lamp current and the resulting current on line 43 were to be
reduced to zero, then the voltage on line 60 would be at maximum
and the oscillator frequency would be increased to f.sub.o and the
resulting harmonic frequency nf.sub.o would cause the lamp to
fire.
With specific reference now to the variable frequency inverter 3 as
shown in Fig. 1, it will be recalled that switching transistors 4,
8 are alternately switched on by the output signals from the
saturating core oscillator 10 to provide a square wave AC voltage
output to transformer 15 for energizing the lamp load. As shown in
Fig. 1, oscillator 10 basically comprises a pair of switching
transistors 61, 62, the collector outputs of which are connected to
opposite ends of the primary winding 63 which is wound on a
saturable core 69 of transformer 70. The center tap 64 of primary
winding 63 is connected to negative input conductor 2. Feedback
windings 66, 68 wound on saturable core 69, with the indicated
polarities, are series connected through resistances 65, 67 to
bases of transistors 61, 62 respectively and through their
respective emitters to diode 74, and also through resistor 73 to
negative conductor 2. The emitters of transistors 61, 62 are
connected common to one another and to the input conductor 60 over
which the control signals are received from the reference and
comparator circuit 45.
The saturable core oscillator 10 is operative in a conventional
manner to provide square wave signals across secondary windings 71,
72 of transformer 70 through current limiting resistors 75, 76 to
the base circuits of switching transistors 4, 8 to effect alternate
switching of transistors 4, 8, and the provision thereby of a
square wave AC voltage at the primary winding of transformer 15
which waveforms have a frequency identical with that of the base
drive signals output from transformer 70 of oscillator 10. Feedback
diodes 5 and 11 connected between the collector of transistors 4, 8
and the negative conductor 2 permit flowback of reactive current to
the DC input.
With reference once more to Fig. 3A, a typical characteristic is
shown thereat for a circuit in which the components are selected so
that the resonant frequency of capacitor 26 and the leakage
reactance of transformer 15 falls somewhat below the starting
frequency. The values of the tuned transformer 15 and inverter 3
are selected so that the lamp current at the starting frequency is
slightly higher than the maximum desired lamp current for the worst
input case and worst lamp conditions, i.e., minimum DC input
voltage across conductors 1, 2 and maximum voltage drop across lamp
27.
It will be apparent that in the circuit shown in Fig. 3A, the
starting frequency for the lamp is at the lower end of the
operating frequency range and that the lamp current is at the
higher value at start. If lamp current tends to increase, the
regulating system will cause the inverter frequency to increase
(i.e. above the starting frequency) and the lamp current will be
reduced. As the lamp current drops, the inverter frequency is
decreased, and the lamp current is regulated to the desired
value.
It should be obvious that be selecting the components so that the
lamp current resonant peak after start falls above the starting
frequency as shown in FIG. 3B, the operating frequency range could
be made to occur below the starting frequency, and lamp current
would decrease as inverter frequency was made less than the
starting frequency. In either case the end result is the same, the
lamp current tends to remain constant at the reference level
despite normal variations in DC input voltage and lamp voltage
drop. The value of current is of course readily adjusted by
movement of potentiometer arm 49 to change the reference level.
Should the lamp become extinguished for any reason the inverter
frequency drops back to the starting frequency to reignite the
lamp.
The feedback control technique depicted in FIG. 1 can utilize a
number of different tuned transformer configurations as shown in
Figs. 5 through 7, all of which have essentially the same starting
and control characteristics. If the tuned transformer/lamp circuit
shown in Fig. 1 is replaced by the circuit shown in Fig. 5 the
operation is essentially the same as that described previously
except that harmonic resonance at "start" occurs between the
leakage reactance of transformer 15 and the series combination of
capacitors 25 and 26. Also the voltage available to ignite the lamp
at start is the voltage across the secondary 21 reduced by the
capacitance divider formed by capacitor 25 and 26.
The operation of the circuit shown in Fig. 6 is identical to that
of the corresponding parts shown in Fig. 1 except that the shunting
capacitor has been connected to a tap on the transformer secondary
21 instead of across the entire winding.
Fig. 7 shows a further alternate circuit to that shown in Fig. 1.
In this case an auto transformer connection is used which places
the primary voltage of winding 16 in series with the secondary
voltage; otherwise operation is essentially the same as described
previously.
Numerous advantages in the use of the foregoing arrangement include
the fact that no electrical or mechanical switch is required to
start the lamp while yet achieving wide current control with
relatively small frequency change. A nearly sinusoidal current is
provided by the series resonant circuit during stable operation and
by starting the lamp with harmonic resonance (rather than
fundamental resonance) the circulating energy and current supplied
by the source is greatly reduced, whereby less stringent
requirements are placed on the inverter which provides the voltage
for the lamp.
Representative values, which are not to be considered limiting,
could be as follows:
i.sub.L =0.5 -2 amps
f.sub.o =20 kHz.
nf.sub.o =60 kHz. (the third harmonic)
In the use of the arrangement of Fig. 3A, a typical operating range
might be 20-25 kHz. where f.sub.o is minimum. In the use of the
arrangement of Fig. 3B, a typical operating range might be 15-20
kHz. where f.sub.o is maximum.
* * * * *