Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Impedance or current regulator in the supply circuit
Reexamination Certificate
2002-06-05
2004-03-02
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Impedance or current regulator in the supply circuit
C315S309000
Reexamination Certificate
active
06700331
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the field of control circuits used to power fluorescent lamps and in particular to those circuits which permit the illumination of a fluorescent lamp to be dimmed in response to a reduction in input power.
2. Prior Art
In order for a fluorescent lamp to illuminate, the voltage across the filaments thereof must be greater than the minimum voltage specified for the lamp. Under typical conditions, it is necessary for a lamp supply circuit to include a current limiting element. The fluorescent lamp itself acts as a voltage limiting component. The voltage across the filaments of a fluorescent lamp is independent of the power supply voltage and is determined by the power of the fluorescent lamp itself. Therefore, to illuminate a fluorescent lamp with conventional alternating current power (i.e., 115 volts/60 Hz), a ballast or current limiting component is utilized.
To turn on or illuminate a fluorescent lamp, it is necessary to utilize a triggering or starting component. This is generally referred to as a starter circuit which initially heats the filaments of the fluorescent lamp. The lamp is illuminated when the voltage across the filaments exceeds the minimum necessary for a particular fluorescent lamp. The prior art exhibits several basic circuits for powering fluorescent lamps. One of the conventional designs for a fluorescent lamp power supply circuit is shown in FIG.
1
.
A fluorescent lamp
10
is a gas discharge tube, the inner surface of which is coated with a light-emitting substance, typically fluorescent or phosphorescent metallic salts (e.g., calcium tungstate, zinc sulphide or zinc silicate). The tube is filled with mercury vapor at extremely low pressure.
FIG. 1
illustrates an exemplary fluorescent lamp
10
. Excitation filaments F
1
and F
2
are placed at alternative ends of lamp
10
and are powered by an input alternating current voltage Vac. As can be seen in
FIG. 1
, input voltage Vac is applied at terminals
11
and
12
. In the circuit shown in
FIG. 1
, the current limiting component is formed of a high value iron inductance L which is connected between terminal
11
and supply terminal
13
of filament F
1
. The second supply terminal
14
of filament F
1
is connected to terminal
15
of filament F
2
by way of a starter circuit
16
which is typically a thermal switch
19
. Terminal
17
of filament F
2
is connected to input terminal
12
. A capacitor C interconnects power supply terminals
11
and
12
.
A starter circuit
16
in the form of a thermal switch is used to heat up filaments F
1
and F
2
of lamp
10
by connecting together or otherwise short circuiting terminals
14
and
15
until the filaments F
1
and F
2
are no longer cold. The starter circuit
16
opens as soon as filaments F
1
and F
2
have reached a predetermined temperature. This will result in an over voltage which triggers or otherwise turns the fluorescent lamp
10
on by means of power stored as a result of inductance L.
Under normal operations, the function of inductance L is to limit the current in lamp
10
in order that it does not exceed the value for which it is designed. The function of capacitor C is to compensate for the dephasing associated with the inductive assembly in order to improve the power factor and to make lamp
10
acceptable for a connection to a network.
The disadvantages of the prior art circuit illustrated in
FIG. 1
are inherent in its design. The design of the prior art illustrated in
FIG. 1
is a conventional power system. The system uses a high inductor (e.g., 1 Henry) which will result in a structure which is cumbersome, bulky and heavy. Furthermore, the inductive nature of the assembly requires a capacitor C of high value (e.g., 10 &mgr;F) which necessitates use of a heavy electrolytic capacitor. The primary disadvantage of the prior art circuit shown in
FIG. 1
is that it cannot be used to dim the light emitted from a fluorescent lamp.
Another control circuit disclosed by the prior art is illustrated in FIG.
2
.
FIG. 2
constitutes a conventional electronic circuit which employs active components to limit the current drawn by the fluorescent lamp. The circuit illustrated in
FIG. 2
employs a diode bridge D having a pair of input terminals
20
and
21
which are connected to terminals
22
and
23
, respectively, of input alternating current voltage Vac. By means of a high value electrolytic capacitor C, the output terminal
22
of bridge D provides a direct current power source to a switched-mode converter
23
which is used to supply fluorescent lamp
24
. Switched-mode converter
23
is a conventional circuit generally formed by a control circuit
25
which is associated with two MOS power transistors M
1
and M
2
which are connected in series between terminal
22
of bridge D and the ground, capacitor C being connected in parallel thereto. The terminal
30
of the switched-mode converter
23
is connected to a first terminal of a high frequency inductance L which is then connected in series with input terminal
19
of filament F
1
of lamp
24
. A capacitor C
2
of low value interconnects terminals
26
and
27
of filaments F
1
and F
2
and enhances the ability to trigger or otherwise start fluorescent lamp
10
.
Terminal
28
of filament F
2
is connected to ground through a capacitor C
3
. Another capacitor C
4
connects terminal
28
of filament F
2
to input terminal
29
of switched-mode capacitor C
5
. Capacitor C
4
and C
5
are used to filter the direct current component in fluorescent lamp
24
. Terminal
29
receives the direct current voltage provided by capacitor C
1
. Transistor M
1
is connected between terminals
29
and
30
and transistor M
2
is connected between terminal
30
and ground. Transistors M
1
and M
2
are controlled by circuit
14
which also includes a feedback input connected to terminal
30
and which is supplied from terminal
29
through resistor R. A capacitor C
5
interconnects terminals
29
and
30
and contributes to the generation of an auxiliary power supply necessary for the control of transistor M
1
.
The disadvantage of the prior art circuit illustrated in
FIG. 2
is that, as in the circuit shown in
FIG. 1
, it requires electrolytic capacitors of high value (e.g., more than 10 &mgr;F) to filter the rectified voltage output at terminal
22
. The use of electrolytic capacitors will result in the reduction of the life of the circuit. Another disadvantage of the circuit shown in
FIG. 2
is that harmonics from the supplied current will affect the power factor in the absence of a correction circuit. As with the prior art circuit illustrated in
FIG. 1
, that shown in
FIG. 2
cannot be employed for dimming the illumination of a fluorescent lamp.
The present invention substantially resolves the inadequacies inherent in the devices disclosed by the prior art. Principally, the present invention control circuit allows the illumination from a fluorescent lamp to be responsive to the input power and, most importantly, to permit illumination to commence at an input voltage which is less than that specified for the selected fluorescent lamp. The objectives of the present invention are achieved through the use of a half-wave voltage doubler circuit. The alternating current signal is processed to alter the sinusoidal form of the input alternating current power and apply voltage to the fluorescent lamp filaments only once per cycle. The effect of the present invention is to double the voltage that drives the fluorescent lamp. This will cause the fluorescent lamp to illuminate at a higher level and will permit the fluorescent lamp to be dimmed when the alternating current voltage input to the present invention is reduced even to that which is less than the minimum generally required for operation of the fluorescent lamp. When the input alternating current voltage is reduced, the light emitted from the fluorescent lamp will be reduced or dimmed accordingly.
SUMMARY OF THE INVENTION
The present invention co
Lusa Lighting, Inc.
Painter Michael A.
Tran Chuc
Wong Don
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