Title:
DISCHARGE-LAMP OPERATING DEVICE USING THYRISTOR OSCILLATING CIRCUIT
United States Patent 3753037


Abstract:
A discharge-lamp operating device is provided which comprises a power source connected in series with a discharge lamp and arc discharge stabilizing means of the linear inductor type, and a starting device including a capacitor connected in parallel with the discharge lamp and a series circuit including a voltage responsive switching element or a symmetrical switch element such as a bidirectional diode thyristor and a saturable non-linear inductor connected in parallel with the discharge lamp. Each filament of the discharge lamp is preferably interposed between the capacitor and the series circuit for improved preheating of the filaments. In addition, a bias coil which is magnetically coupled with the saturable nonlinear inductor can be connected in series with the capacitor to control oscillating frequency and voltage of the starting device. Further, there can be provided a second bias coil in series with the linear inductor so as to obtain desired effects according to the connections of the first and/or second bias coils which are magnetically coupled with the saturable nonlinear inductor.



Inventors:
Kaneda, Isao (Osaka, JA)
Takeuchi, Kiyokazu (Osaka, JA)
Application Number:
05/202766
Publication Date:
08/14/1973
Filing Date:
11/29/1971
Assignee:
NEW NIPPON ELECTRIC CO LTD,JA
Primary Class:
Other Classes:
315/106, 315/243, 315/244, 315/DIG.5
International Classes:
H05B41/04; H05B41/14; (IPC1-7): H05B37/00; H05B41/23
Field of Search:
315/99,100,105,107,106,DIG
View Patent Images:
US Patent References:



Other References:

transistors-Kiver McGraw-Hill, New York, 1962, TK 7872 T73K5; Title page and pp. 334-336.
Primary Examiner:
Rolinec, Rudolph V.
Assistant Examiner:
Nussbaum, Marvin
Parent Case Data:


CROSS-REFERENCES TO RELATED APPLICATION

This application is a continuation-in-part of our copending application Ser. No. 14,325 filed Feb. 26, 1970, now U.S. Pat. No. 3,665,243, issued May 23, 1972, entitled "Discharge-Lamp Operating Device Using Thryistor Oscillating Circuit."
Claims:
What is claimed is

1. In a discharge lamp operating device for a discharge lamp characterized by a virtual tube voltage: a power source generating a voltage having a maximum instantaneous value; said discharge lamp being connected to said power source; arc discharge stabilizing means including a linear inductor; lamp starting means including a capacitor connected in parallel with said discharge lamp, a saturable nonlinear inductor, and a voltage responsive switching element having a break-over voltage within a range of from above the virtual tube voltage of said discharge lamp to below the maximum instantaneous voltage of said power source, said non-linear inductor and said switching element forming a series circuit connected in parallel with said discharge lamp; said power source, said linear inductor and said capacitor forming a first closed circuit in oscillation condition; said capacitor, said non-linear inductor and said switching element forming a second closed circuit selectively in or out of oscillation condition; said non-linear inductor including a core which is a dielectric so as to enable the inducing of a back swing voltage across said nonlinear inductor when said switching element turns off, said nonlinear inductor being adapted, due to the inductance thereof in the unsaturated state, to temporarily impede the conduction of said switching element so that an output voltage across said capacitor is boosted sufficiently to discharge said lamp.

2. A device as claimed in claim 1, wherein said discharge lamp includes filaments at each end thereof, said filaments being interposed within said first closed circuit whereby said filaments are preheated by current flowing through said first closed circuit.

3. A device as claimed in claim 1, wherein said discharge lamp includes filaments at each end thereof, said filaments being interposed within said second closed circuit whereby said filaments are preheated by current flowing through said second closed circuit.

4. A device as claimed in claim 1, wherein said starting means further includes a bias coil connected in series with said capacitor, said bias coil being magnetically coupled with said non-linear inductor so as to control the frequency and voltage of the output of said capacitor.

5. A device as claimed in claim 4, wherein said bias coil is connected to increase the inductance of said nonlinear inductor in the unsaturated state.

6. A device as claimed in claim 4, wherein said bias coil is connected to decrease the inductance of said nonlinear inductor in the unsaturated state.

7. A device as claimed in claim 4, wherein the starting device further includes a second bias coil in series with the linear inductor in the first closed circuit, said bias coil being magnetically coupled with said nonlinear inductor to enable decreasing the sectional area of said core.

8. A device as claimed in claim 1, wherein the starting device further includes a first bias coil in said first closed circuit and a second bias coil in said second closed circuit.

9. A device as claimed in claim 1, wherein said discharge lamp includes a pair of filaments at each end thereof, said capacitor consisting of a first capacitor element connected on the power source side of and across said discharge lamp and a second capacitor element connected across said discharge lamp on the side of the latter remote from said power source to interpose said filaments between said first capacitor element and said second capacitor element for preheating said filaments by the currents flowing both into said first closed circuit and said second closed circuit.

10. A device as claimed in claim 2, wherein said nonlinear inductor includes reverse windings connected in series with said capacitor to provide for a minus bias.

11. A device as claimed in claim 3, wherein the ferrite core of said nonlinear inductor is adapted to have its temperature elevated above the Curie point.

12. A device as claimed in claim 1 wherein the nonlinear inductor has an inductance at saturation which is other then zero.

13. A device as claimed in claim 1 wherein the switching element is a bidirectional symmetrical switching type thyristor.

14. A device as claimed in claim 1 wherein the nonlinear inductor has a relatively low inductance at saturation and a relatively high inductance when in unsaturation condition.

15. A device as claimed in claim 1 wherein the nonlinear inductor has a value adapted to operate to retain said switching element turned off during a part of the time said capacitor is charged to an extent tending to turn the switching element on.

Description:
FIELD OF THE INVENTION

This invention relates to a discharge lamp lighting apparatus and more particularly to a novel and improved solid state starting device for reliably starting low-pressure discharge lamps, in which device an oscillatory high voltage produced by a nonlinear, bidirectional diode thyristor oscillator is applied between the electrodes of the discharge lamp.

BACKGROUND OF THE INVENTION

In order to start a discharge lamp, a high voltage than is usually available from conventional power lines is required to establish an arc discharge between the electrodes of the discharge lamp. Generally, this higher voltage is generated by a linear inductor or transformer. However, it is difficult to satisfy, by such means, all of the following conditions which are necessary to provide a satisfactory starting device: (1) to obtain rapid and stable ignition with high reliability in a broad operating temperature range; (2 ) to suppress discoloration of the discharge lamp; (3 ) to prevent shortening the life of the starting device; and (4 ) to provide a compact and economical starting device.

DESCRIPTION OF PRIOR ART

Solid state starting devices using pulse generators have been proposed as improvements of conventional starting devices. One such improvement is shown in U. S. Pat. No. 3,476,976 which issued to Hiroshi Morita et al.; however, sufficient voltage amplfication is not obtained in the pulse generator of this apparatus in which a pulse transformer is further required for the starting of discharge lamps. As a consequence, this starting device assembled with a low impedance load has the disadvantage that pulse energy is decreased in proportion to the winding ratio of the pulse transformer in which a large winding ratio must be used for voltage step-up purposes.

SUMMARY OF THE INVENTION

It is an object of the invention to eliminate the above-noted and other drawbacks inherent in the prior art and to provide a novel and improved discharge lamp operating device.

To achieve the above and other objects, there is provided, in accordance with the invention, a novel circuit including a starting device in which a discharge lamp is connected to the output terminal of a nonlinear, bidirectional diode thyristor oscillator. The bidirectional diode thyristor oscillator includes a first closed circuit which comprises a power source, a linear inductor or choke coil means for stabilizing the arc discharge of the lamp and a capacitor connected in parallel with the discharge lamp, and a second closed circuit which comprises the capacitor mentioned above, and a thyristor of the bidirectional symmetrical switching type and a saturable non-linear inductor connected in series with each other. The series circuit section consisting of the bidirectional diode thyristor and the nonlinear inductor is also connected in parallel with the discharge lamp.

Advantageously, the nonlinear inductor includes a coil wound about a ferrite core of dielectric material and has a relatively low inductance in its voltage-saturated state and a relatively high inductance in its insaturated state. For boosting the oscillating output voltage generated in the second closed circuit, these different inductance values and a back swing voltage induced across the nonlinear inductor are utilized together with the switching functions of the thyristor. The discharge lamp is ignited by applying the boosted oscillating voltage.

The above starting device is further improved by the combination therewith of a discharge lamp having a pair of filaments, in which the second closed circuit includes such filaments. In other words, the second closed circuit comprises a capacitor connected across the terminals of the discharge lamp in the power supplying side, a series circuit of a bidirectional diode thyristor and a saturable nonlinear inductor connected across the terminals of the discharge lamp in the "non-power" side (e.g., on the side remote from the power source), and lamp filaments connected respectively between the capacitor and the series circuit. Therefore, the filaments of the discharge lamp can be heated by the oscillating current in the second closed circuit, and the filament heating can be improved for the lamp starting operation. Further, the above-mentioned capacitor can be divided into two parts, of which one part is connected across the discharge lamp on the power supplying side and the other part is connected across the discharge lamp on the side remote from the power source to preheat the filaments by currents flowing through the first and second closed circuits.

The above-mentioned circuit can advantageously be provided with at least one bias coil for the nonlinear inductor. It has been found that when bias means such as a bias coil is added within the second closed circuit in series with the capacitor, oscillating conditions such as amplitude and frequency of the output voltage produced in the second closed circuit can be controlled. That is, the output voltage applied to the discharge lamp for its ignition is elevated by a plus or positive bias coil and is suppressed by a minus or negative bias coil. Hereinafter, the terms of "plus" or "positive" and "minus" or "negative" are used as meaning the effective function with regard to the output voltage. The bias coil of the above actually acts to change the level of the residual magnetic flux density (Br) of the nonlinear inductor, and to vary the amplitude between the maximum saturated magnetic flux density (Bms) and the residual magnetic flux density (Br) of the nonlinear inductor core.

The aforementioned bias coil, of which the winding ratio to the winding coil of the nonlinear inductor is above about 0.001, is magnetically connected with the nonlinear inductor in additive or bucking relation for increasing or decreasing the inductance of the second closed circuit in oscillation, thereby controlling the oscillating frequency and voltage in the second closed circuit. Use of the minus bias coil advantageously reduces discoloration of the discharge lamp. When a further bias coil is added within the first closed circuit in series with the linear inductor for magnetizing the nonlinear inductor, abnormal operation of the second closed circuit, which might be caused by spike voltages during lamp operation, can be prevented by the input current which results from maintaining a high impedance in the second closed circuit. This has economic merit for minimizing the cross-sectional area of the core used for the nonlinear inductor.

The nonlinear inductor, one of the main components employed in accordance with this invention, differs from a conventional pulse transformer as to the nonlinear characteristic relating to the voltage applied thereto and the inductance values thereof, which characteristic provides a low inductance value during the magnetic flux saturated state (therefore it is called a voltage-saturated inductor) and a high inductance value during the unsaturated state. We have found that when the nonlinear inductor is a coil on a core of ferrite material, which is a dielectric and has high initial permeability, the continuous oscillating voltage in the second circuit can be significantly elevated and such boosted voltage is quite enough to ignite a discharge lamp.

Boosting of the oscillating voltage is achieved by reason of the fact that the voltage rising characteristic of the back swing voltage which is induced, when the bidirectional diode thyristor turns off, between the terminals of the nonlinear inductor in the unsaturated state due to the distributed parallel elements of its equivalent circuit (namely, a relatively large resistance and capacitance), is substantially consistent with the voltage rising characteristic of the output voltage appearing between the terminals of the capacitor, and further because the nonconductive state of the bidirectional diode thyristor is maintained by impedance obstruction due to the non-linear inductor's being in the unsaturated state, while the capacitor voltage increases directly up to several times the source voltage. The capacitor subsequently acts as a noise preventing capacitor during lamp operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a circuit for boosting oscillating voltage in accordance with the invention;

FIGS. 1(B) and 1(C) are charts which illustrate waveforms of currents and voltages in the circuit of FIG. 1(A) relating to the discharge lamp lighting apparatus of the invention;

FIGS. 2(A), 2(B) and 2(C) respectively illustrate equivalent circuit diagrams for the explanation of the phenomena involved in each process for the boosting of voltage in FIG. 1A;

FIG. 3 is a schematic diagram of a circuit using an A.C. source instead of the D.C. source of FIG. 1A;

FIGS. 4(A), 4(B), 4(C) and 4(D) respectively illustrate circuit diagrams of four embodiments of the invention;

FIGS. 5(A) and 5(B) respectively illustrate two kinds of waveforms of the lamp voltage;

FIGS. 6(A), 6(B) and 6(C) respectively illustrate waveforms of the output voltage and input current for three of the embodiments in FIGS. 4(A)-(D); and

FIG. 7 illustrates the waveform of the output voltage in case abnormal operation of the discharge lamp occurs.

BRIEF DESCRIPTION OF THE INVENTION

The voltage boosting function of this invention is next explained with reference to FIGS. 1 to 3.

A D.C. operating circuit and waveforms for the explanation thereof are shown in FIGS. 1(A)-(C). In FIG. 1(A) is shown a circuit for boosting oscillation voltage in a manner applicable to the present invention. FIG. 1(B) shows waveform variation from the initial stage of starting the circuit to the stable state with respect to input current i1, the output of capacitor voltage ec and a source voltage E. FIG. 1(C) is an enlarged view of the part of the waveforms in the stable state, in which the relationship of input current i1 and output current ic and the relationship of source voltage E, output voltage ec and inductor voltage el including back swing voltage e b are shown in different scales.

In FIG. 1(A), the illustrated circuit comprises a first closed circuit in oscillating condition including a D.C. source 1 having voltage E, a linear inductor 3 having inductance L and loss resistance R, and a capacitor 4 having capacitance C; and a second closed circuit in oscillating condition including the capacitor 4 shared in common with the first closed circuit, a bidirectional diode thyristor 6 having an operational break-over voltage Vbo, and a saturable nonlinear inductor 5 having inductance ls while in its saturated state, inductance lu while in its unsaturated state and loss resistance r. The nonlinear inductor 5 has a core of dielectric and saturable magnetic material and a coil, the inductance of which is abruptly decreased to about 1/1000 of its original inductance by the application of a voltage higher than its saturation voltage and proportional to the frequency of the applied voltage.

When the D.C. source is connected to the above-mentioned circuit, variations of input current i1 flowing in the first closed circuit and of output voltage ec across the capacitor 4 in relation to the passing of time t occur from the instant of starting of the operation of the circuit and these are shown in the waveforms of FIG. 1 (B), in which the input current i1 is gradually increased during the passing of time from the beginning of the starting process and reaches a constant value at the end of the starting process, and the output voltage ec at high frequency rises voltage ec rises above E.

For analyzing each cycle of the oscillation, the waveforms in the stable state of FIG. 1(B) are shown in FIG. 1(C) on enlarged scales with respect to the time axis. Voltage e1 including a back swing voltage e b appears across the nonlinear inductor 5. It has a rising curve similar to that of the voltage ec, with a partially different part for keeping the nonconductive state of the thyristor 6. FIG. 1(C) also shows the relationship of the input current i1 and current ic in the capacitor 4. From these figures, it is understood that ec can be elevated to several times the value of E by using this circuit arrangement.

For explanation of such boosting of the voltage, reference is next made to the equivalent circuits shown in FIGS. 2(A)-(C). For the first step of the starting operation, the equivalent circuits of FIGS. 2(A) and (B) are considered. The capacitor 4 will be charged until ec equals E. If the break-over voltage Vbo of the bidirectional diode thyristor 6 is less than E, the bidirectional diode thyristor 6 turns on. The impedance of inductance lu which is proportional to the frequency of the applied voltage can not impede the conduction of the bidirectional diode thyristor 6, because a D.C. source is being used. The current ic flows in the second closed circuit which is a high Q circuit and the amplitude of the current is increased significantly, as shown in FIG. 1(C), due to the voltage-saturated inductance ls of the nonlinear inductor 5. The polarity of ec, which is equal to -E at the initial starting of the circuit, is reversed at about a half-cycle of ic if the loss resistance rs is negligible.

The current ic will be significantly amplified, particularly in case the Q of the second closed circuit is very high, since ic is compressed by the small inductance ls in the saturated state of the nonlinear inductor 5 in contrast to the current i1 flowing through the large inductance L of the linear inductor 3. At the same time, the current i1 flows through the closed circuit consisting of elements 1-3-5-6-1, the value of current is negligible in comparison with that of ic.

However, the inductance ls is selected to be other than zero at saturation and, because of this, the oscillating condition of the second closed circuit is essentially maintained in order to assure the nonconductive state of the thyristor 6. Then, currents ic plus i1 flowing through the thyristor 6 inevitably approach zero due to oscillation and the amplitude of ic and the thyristor turns off.

Next, the equivalent circuit of FIG. 2(C) is considered. Therein a distributed capacitance cu and a distributed resistance ru are shown as equivalent elements of the nonlinear inductor 5 in the unsaturated state, after a turning off of the thyristor 6. The conduction of the bidirectional diode thyristor 6 is impeded for a while, since the equivalent circuit generates a back swing and return swing oscillation and induces a back swing voltage eb (as shown by the dashed line in FIG. 1(C)), due to the stored magnetic energy resulting from i1 plus ic which opposes ec and therefore the developing of ec. After that, ec is further impeded for a while by the impedance of inductance lu, after which ec is further raised with a steep ascending slope by the first closed circuit from -E at the initial starting due to i1 at the initial condition of each charging cycle.

Thus, ec can be elevated above Vbo and i1 is held constant due to inductance L in the linear inductor 3.

It is a most remarkable feature of this circuit that the thyristor keeps its nonconducting state for a while in spite of ec being higher than Vbo by reason of the fact that the difference between ec of the capacitor 4 and el of the nonlinear inductor 5 is maintained below or equal to Vbo of the bidirectional diode thyristor 6 by the unsaturated state of the nonlinear inductor 5.

The rising rate of ec has a steeper ascent than i1 during each initial charging of the capacitor 4 and reaches the stable condition shown in FIG. 1(C). When ec meets with el including a part of eb, ec is increased until the relationship of ec = e1 + Vbo is established. By selection of suitable components, impedance of the nonlinear inductor 5 can be improved significantly. In this circuit, the maximum ec is determined mainly by a peculiar characteristic of the nonlinear inudctor 5. The frequency of the oscillating circuit is also determined mainly by the same. Therefore, the most important component of the circuit is the non-linear inductor 5 which is characterized by the use of ferrite material. This oscillation circuit is not limited to the use of a D.C. source.

FIG. 3 shows a circuit for a commercial A.C. source, in which an output voltage ec is modulated at the frequency of the source as shown in FIG. 6(A). This is because ec is proportional to each of the initial oscillation conditions of an input current i1.

As described above, the waveform of ec becomes higher than the source voltage e with respect to voltage as well as the frequency thereof. For example, the ratio of e c to e of the power source reaches about 10, and the frequency of oscillation reaches about 25 kHz. This boosted voltage ec is utilized in the discharge lamp lighting apparatus of the invention.

In FIGS. 4(A)-(D), there are shown several types of circuits of the present invention. It is to be understood that the choke coil means for stabilizing arc discharge in these circuits can be changed to another type of stabilizing means such as a leakage transformer.

In FIG. 4(A), a discharge lamp 12 having a pair of filaments 13 and 14, each of which has a pair of terminals, is connected to an A.C. power source 10 through a choke coil 15 of linear inductor type thereby forming a conventional arc discharge circuit including the power source 10, the choke coil 15, and the discharge lamp 12 through the terminals of the filaments 13 and 14. For improved starting of the discharge lamp 12, a starting device of this invention is associated with the above circuit.

The starting device of this invention in FIG. 4(A) consists essentially of a capacitor 16 and a series circuit including a saturable nonlinear inductor 17 and a bidirectional diode thyristor 18. The capacitor 16 and the series circuit are each connected across the discharge lamp 12 to the same terminals of the filaments 13 and 14. This enables preheating the filaments 13 and 14 by the input current i1 at the frequency of source 10 during the starting period. Compared with the circuit of FIG. 3, the circuit of FIG. 4(A) is similarly constituted of the first closed circuit to which filaments 13 and 14 are added, and the second closed circuit. Therefore, it will be understood by reference to the explanation given hereinbefore that the output voltage ec and input current i1 have substantially the same waveforms as are shown in FIG. 6(A).

The nonlinear inductor 17 of this invention comprises a coil or winding and a magnetic core having a square hysteresis loop and a high initial permeability. The magnetic core is of a material such as a ferrite which provides a suitable dielectric constant and has good qualities at high frequencies. Immediately after the switching element turns off, the coil of the nonlinear inductor 17 induces the back swing voltage eb between the terminals thereof due to the magnetic energy stored in the saturated state. The voltage varying characteristic across the nonlinear inductor 17 is predetermined by selection of the constants of inductance lu in the unsaturated state, distributed capacitance Cu and distributed resistance ru as illustrated in the principal circuit of FIG. 2(C). Thus, it is necessary to oppose the characteristic of the voltage eb across the nonlinear inductor 17 with that of the voltage ec across the capacitor 16.

It is, however, preferred to interpose the filaments 13 and 14 within the second closed circuit, as shown in the embodiments of FIGS. 4(B), (C) and (D) hereinafter described, in order to effect preheating of the filaments 13 and 14 by the high frequency current flowing in the second closed circuit.

In any of these embodiments, since the capacitor 16 is connected across the discharge lamp 12, the output voltage, which is produced across the capacitor 16, can be applied across the discharge lamp 12 for ignition thereof. This output voltage has a continuous oscillating waveform the frequency and amplitude of which are determined by the choice of elements mentioned above, and the further possible addition of a bias coil as described hereinafter.

As to the break-over voltage Vbo of the bidirectional diode thyristor 18, it has been found that there is no occurrence of a spike voltage (or peaked voltage) in the initial part of the tube voltage of the discharge lamp 12. Possible waveforms of the tube voltage are shown in FIGS. 5(A) and (B). Generally, a spike voltage is developed when the lamp is operated under a lower ambient temperature or with the use of large capacitance capacitor, as shown in FIG. 5(A). In this case, the lower limit of the break-over voltage Vbo in the conventional solid state starting device must be higher than the spike voltage (or peaked tube voltage) in order to prevent improper operation. Thus, if the lamp is to be installed in a lower ambient temperature, operation may sometimes become impossible in the conventional type of device. However, such trouble is avoided by the invention by relating an impedance function to the spike voltage due to the presence of the non-linear inductor 17 in unsaturated state which presents a relatively large inductance lu. This is because the frequency of the spike voltage is in the order of several kHz. Stated otherwise, current caused by the spike voltage is suppressed below the holding current level of the thyristor 18 by the nonlinear inductor 17 thereby keeping the nonconductive state of the thyristor and preventing undesired operation. As a result, a thyristor having a break-over voltage Vbo which is lower than the spike voltage can be used in circuits of this invention.

In such a case, however, abnormal spike voltages as shown in FIG. 5(B) sometimes appear in the tube voltage. To impede the application of the spike voltage to the bidirectional diode thyristor 18 is to improve reliability of the circuit used in such a lower temperature, or to broaden the range of bidirectional diode thyristors available for use in the starting circuits of the invention. In other words, any bidirectional diode thyristor having a lower limit above the virtual tube voltage, which means the value of the r.m.s. voltage appearing across the lamp, and an upper limit below the maximum instantaneous source voltage can be used in a circuit of this invention, and selection of the Vbo is not affected substantially by the operating condition or the use of the large capacitance capacitor which cause a higher spike voltage.

When the discharge lamp 12 is lighted, the bidirectional diode thyristor 18 holds its nonconducting state, even if the spike voltage exceeds the break-over voltage Vbo thereof, and the choke coil 15, which is the linear inductor, acts as a stabilizing ballast. Further, the capacitor 16 acts as a radio-noise preventing capacitor.

In the above-mentioned embodiment, however, it happens sometimes that an excess output voltage ec causes sputtering of the lamp electrode coating materials, or that preheating by the input current i1 is insufficient for some types of lamps. The following embodiments are improvements to avoid such difficulties.

The circuit of FIG. 4(B) comprises: a first closed circuit including power source 10, choke coil 15 and capacitor 16; and a second closed circuit including the capacitor 16, saturable nonlinear inductor 17, thyristor 18, and filaments 13 and 14 of the discharge lamp 12,which are interposed in the second closed circuit. The capacitor 16 is connected across the terminals of the filaments 13 and 14 on the power supply side. The series circuit including nonlinear inductor 17 and the thyristor 18 is connected across the other terminals of the filaments 13 and 14 remote from the power supply side. after the thyristor turns off, the coil of the nonlinear inductor 17 induces the back-swing voltage eb between the terminals thereof due to the magnetic energy stored in the saturated state. The voltage varying characteristic across the nonlinear inductor 17 is predetermined by selection of the constants of inductance lu, distributed capacitance Cu and distributed resistance ru in the unsaturated state as illustrated in the principal circuit of FIG. 2(C). Thus, it is necessary to oppose the characteristic of the voltage e1 across the nonlinear inductor 17 with that of the voltage ec across the capacitor 16.

In this embodiment, a larger oscillation current of higher frequency preheats the filaments 13 and 14, and the resistance of the filaments 13 and 14 in the second closed circuit controls the output voltage to decrease sputtering. For instance, since the output voltage of the circuit of FIG. 4(A) sometimes exceeds about five times that of the source voltage and since such voltage is too high for certain types of discharge lamps, a voltage control means is required. However, the use of the resistance of the filaments 13 and 14 is effective to decrease this problem.

To compensate the increased resistance of the filament due to accident or deterioration, such as the consumption of emitting materials on the filaments, the circuit of FIG. 4(B) can be modified by adding a bias coil 20 of the negative type as shown in FIG. 4(C). In this embodiment, the bias coil 20 has windings connected in bucking relation to the windings of the nonlinear inductor 17, and is connected in series with the capacitor 16. The winding ratio of the bias coil 20 to the windings of the non-linear inductor 17 is in the range of 0.01 to 0.001. By adding the negative-type bias coil 20, the inductance lu in the unsaturated state of the nonlinear inductor 17 is decreased so as to decrease the amplitude of and to increase the oscillating frequency of the output voltage for effective preheating.

After starting of the lamp, the inductance of the non-linear inductor is returned to its original state due to the inaction of the bias coil 20. Thus, the merit of bias coil 20 is to provide for the possible selection either of the inductance value of the normal lu or of a different lu'. The different value lu of the inductor 17 can be voluntarily varied to increase or decrease its value by selection of winding turns and connecting direction of the first bias coil 20.

A further modification is shown in FIG. 4(D) in which two bias coils 20 and 22 are added to the circuit of FIG. 4(B). A first bias coil 20 of the negative type is the same as that of the circuit of FIG. 4(C). The second bias coil 22, which is of the positive type, is added to the first closed circuit in series with the choke coil 15. The second bias coil 22 is connected magnetically with the nonlinear inductor 17 and is operated by the input current i1 of the first closed circuit to add to the bias of the nonlinear inductor 17 for preventing erroneous operation of the starting device due to a spike voltage during normal lighting operation.

The merit of the positive coil in the first and second closed circuits is that the cross-section of the core for the non-linear inductor 17 can be decreased for economic purposes. In case the positive coil is used in the first closed circuit, the upper limit of the winding ratio is not limited.

The following tables show some results of some applications of the embodiments of FIGS. 4(B) and (C), in which a bidirectional diode thyristor having a break-over voltage Vbo within a range above the virtual tube voltage and below the maximum instantaneous source voltage is used as the thyristor 18, and in which the core of the saturable nonlinear inductor 17 is a Mn-Zn ferrite material, which provides an equivalent circuit including inductance lu, distributed capacitance Cu and distributed resistance ru as shown in FIG. 2(C).

Table I shows the results in cases in which no bias and minus bias are used with 15-watt and 40-watt fluorescent lamps, where the necessary starting energy is less than the energy stored in the core of the nonlinear inductor 17. Table II shows the results when no bias and plus bias are used with a particular type of 30-watt fluorescent lamp where the necessary starting energy is above the energy stored in the core of the nonlinear inductor 17. It is shown that when the constants of Table II are selected, the plus bias is effectively used to excite the core for increasing the energy stored in the core up to the starting energy required for such type of fluorescent lamp.

It is noted that since the stored energy of the core is proportional to the sectional area of the core, the plus bias effect of Table II is also obtained by increasing the sectional area of the core. In other words, the use of a plus bias coil makes it possible to theoretically cut the sectional area of the core in half for obtaining the same stored energy. --------------------------------------------------------------------------- TABLE I

Type of Discharge 15 Watt T8 tube 40 Watt T12 tube Lamp 12 Fluorescent Lamp Fluorescent Lamp __________________________________________________________________________ Power Source 10 100V A.C., 60Hz 200V A.C., 60Hz Capacitor 16 0.033 μF 0.022 μF Nonlinear (Saturable) Inductor 17 Type of Core H5C EE19 H5B EI22 Sectional Area 0.23 cm.2 0.41 cm.2 Winding Turns 330 T 220 T Bidirectional Diode Thyristor 18 Type K2F2D K2F1D3D Vbo 120V 240V Bias Coil 20 none minus none minus bias bias Winding Turns 0 2 0 1 Output Voltage 370V 280V 520V 450V (zero to peak) Ignition Time 1.2 sec. 0.7 sec. 0.7 sec. 0.4 sec. (at 20°C) Allowed Ambient -10° to -10° to -15° to -10° to Temperature + 70°C. + 60°C. +75°C. + 70°C. __________________________________________________________________________ --------------------------------------------------------------------------- TABLE II

Type of Discharge 30 Watt T10 Circular Lamp 12 Fluorescent Lamp __________________________________________________________________________ Power Source 10 100V A.C., 60Hz Capacitor 16 0.047 μF Nonlinear (Saturable) Inductor 17 Type of Core 3E2 EI19 Sectional Area 0.23 cm.2 Winding Turns 220 T Bidirectional Diode Thyristor 18 Type K2F2D Vbo 120V Bias Coil 20 none plus bias Winding Turns 0 1 Output Voltage 330V 380V (zero to peak) Ignition Time 1.2 sec. 0.7 sec. (at 20°C.) Allowed Ambient *only the -15° to Temperature normal + 75°C. __________________________________________________________________________ l6 *At high or low temperatures, starting energy is lacking.

Referring to the operation of the circuit of FIG. 4(B), when the commercial power source 10 is supplying power, the output voltage ec across the capacitor 16 follows the source voltage and reaches the break-over voltage Vbo of the bidirectional diode thyristor 18, as a result of which the thyristor 18 turns on and enters the conductive state. Since the saturable nonlinear inductor 17 saturates easily due to the relatively low impedance at the source frequency of 60Hz, electric charges of the capacitor 16 are discharged through the second closed circuit 16-13-17-18-14-16. The nonlinear inductor 17 stores magnetic energy in the core due to excitation by the current ic of discharge, while the output voltage ec varies in value and polarity during the increasing of resistance loss which takes place mainly in filaments 13 and 14. At the same time, a small current i1 flows in a third closed circuit 10-15-13-17-18-14-10. Accordingly, when the total of currents ic + i1 is decreased below a holding current IH of the thyristor 18 due to the oscillation condition, the thyristor 18 which is in the conductive state is changed to the nonconductive state once again.

Immediately thereafter, a back swing voltage eb is induced across the nonlinear inductor 17, since the magnetic energy stored therein is released from the parallel equivalent closed circuit as explained hereinbefore. During this time, the output voltage ec increases further due to the charging function of first closed circuit 10-15-16-10 at the initial condition of the current i1. The differential voltage between the output voltage ec and the back swing voltage eb or terminal voltage el of the inductor 17, which are in reversed polarity with respect to each other, is applied to the thyristor 18. However, the thyristor 18 maintains its nonconductive state for a while, because the applied differential voltage is at a value below the break-over voltage Vbo. It should be noted that both oscillations ec and el are predetermined to have similar voltage rise characteristics.

Further, there is another factor to maintain the non-conductive state as mentioned above. This factor is the saturation time of the nonlinear inductor 17 with respect to the high-frequency output voltage ec. Since the conduction of the thryristor 18 is impeded by the impedance function of the nonlinear inductor 17 in the unsaturated state, the value of the output voltage ec becomes higher than that of the break-over voltage Vbo of the thyristor. During such period, the capacitor 16 is continuously charged by the initial condition of the current i1.

Since both oscillations can not retain the same relation and the differential voltage ultimately exceeds the break-over voltage Vbo due to the saturation state of the nonlinear inductor 17, the bidirectional diode thyristor 18 turns on after a while. Then, the charges of the capacitor 16 are discharged in oscillating manner through the second closed circuit. Similar charging and discharging steps are repeated in this circuit so as to boost the output voltage ec to a value several times that of the source voltage, by reason of the fact that the capacitor 16 can store a substantial charge for a short time and that the impedance function of the nonlinear inductor 17 is substantially proportional to the applied frequency of the output voltage ec.

The boosted output voltage ec, which is of high frequency, is also proportional to the current i1 in the initial condition so far as the inductance of the linear inductor of the choke coil 15 is significantly larger than the saturated inductance ls of the non-linear inductor 17. Thus, the waveform of the output voltage ec is generally modulated by the current i1 of the commercial frequency as shown in FIG. 6(A).

FIG. 6(B) shows the waveforms of the output voltage ec and input current i1, together with a dotted line showing of the maximum value of the output voltage which is obtained in the circuit of FIG. 4(A) as shown in FIG. 6(A). As seen in the waveform of FIG. 6(B), the height and density of the oscillating lines are different from that of FIG. 6(A). This means that the voltage of the output is adjusted by the filaments 13 and 14 which are interposed in the second closed circuit. That is, compared with the waveforms of FIG. 6(A), the amplitude of the output voltage ec is decreased, and the frequency of the output voltage ec is increased. Therefore, the preheating current of this circuit is about twice that of the circuit of FIG. 4(A), and the voltage applied to the discharge lamp 12 is controlled to suppress discoloration of the lamp and to ignite the lamp rapidly.

In the above-mentioned circuit, it is also found that the function of limiting oscillation insures safety for the lighting apparatus. When the lamp is burnt up by consumption of emitting materials on the filaments, filament resistance in the second closed circuit is increased and decreases the peak value of current ic due to discharging of the capacitor 16, thereby decreasing the back swing voltage across the nonlinear inductor 17. As a result, the oscillation is limited as shown in FIG. 7 in which it is seen that the oscillating period is limited to within about one third of a radian of the source voltage. Accordingly, the virtual current flowing in the second closed circuit is decreased and during the remaining two-thirds of a radian is at the same value as that of the input current i1.

A similar limiting of oscillation takes place above the Curie point of the ferrite core, at which oscillation is stopped. That is, when the abnormal temperature rising of the ferrite core used in the starting circuit employing high frequency current for filament preheating is predetermined to be the temperature above the Curie point, current capacities of the circuit elements can be decreased, and the circuit elements can be protected by stopping oscillation. For example, core material having a Curie point of 80°C to 150°C has been used to protect the circuit elements.

Next, the operation of the circuit of FIG. 4(C) is considered. This circuit is characterized by the adding of a bias coil 20 in series with the capacitor 16. The bias coil 20 is magnetically connected with and carried on the core of the non-linear inductor 17 in bucking relation therewith as a "minus bias." The use of magnetization as a plus bias is also used in this invention. For example, when the windings of the bias coil 20 is composed of two turns wound in reverse with respect to the 330 winding turns of the nonlinear inductor 17, a minus bias is achieved. In such case, charging of the capacitor 16 is similar to that of the circuit in FIG. 4(B), since inductance of the coil 20 can be neglected with respect to inductance lu of the nonlinear inductor 17 in the unsaturation state, except that the inductance lu is slightly decreased to lu' by the minus bias. This decreases the back swing voltage which is approximately proportional to the square root of the inductance value lu. On the other hand, inductance ls of the nonlinear inductor 17 in the saturation state is held at a constant value ls', since the maximum variation of flux density is adjusted by the bias coil 20. Therefore, continuous oscillation as shown in FIG. 6(C) is achieved without relating to the amplitude of the input current i1. Assuming the saturable nonlinear inductor 17 is perfectly saturated, the thyristor 18 can not keep its nonconductive state as against the large current i1 + ic, because of the non-oscillation condition in the second closed circuit, so that the oscillating period will be limited to a small phase angle of the input current.

Thus, the output voltage ec is decreased and preheating current is increased by high frequency. For instance, the use of such a bias coil 20 changes the oscillating frequency from 20kHz to 40kHz, and produces a suitable output voltage.

Increasing the frequency of the output voltage is advantageous for rapid preheating of the filaments 13 and 14, and for prevention of sputtering in various kinds of discharge lamps, due to the effective preheating and lower applied voltage.

After the lamp is lighted, the aggregated inductance lu' decreased by the minus bias recovers to the original value lu because of the lesser current through the bias coil 20. At such time, the capacitor 16 acts as a noise preventing capacitor. The inductance of the bias coil 20 is very small such as, for example, several μH. Therefore, a circuit having a minus bias is preferably used for the lighting apparatus employed at low ambient temperatures.

By adding the above-mentioned minus bias coil 20, certain undesirable results are prevented. For example, where the capacitance of the capacitor is increased for the purpose of obtaining sufficient preheating current, such capacitor produces a higher spike voltage against which a large inductance lu is required. However, such large inductance lu produces an excessive output voltage of a relatively lower oscillating frequency. Therefore, the capacitance of the capacitor would otherwise have to be further increased to provide for sufficient preheating current.

It is also found that for discharge lamps having a low spike voltage, the plus bias should be used. The use of the plus bias coil 20 in the circuit of FIG. 4(C) is also effective to compensate starting energy. As an example, Table II shows a case in which the non-bias circuit of FIG. 4(B) improves its range of allowed ambient temperatures by the adding of the plus bias of the coil 20 of FIG. 4(C).

The embodiment of FIG. 4(D) is characterized by adding a minus bias coil 20 connected in series with the capacitor 16 and a plus bias coil 22 connected in series with the choke coil 15. The plus bias coil 22 excites the nonlinear inductor 17 by operation of the input current i1, and thereby a large aggregate inductance lu' is obtained. While the inductance lu of the nonlinear inductor 17 is determined by the sectional area of the core, such plus bias is effective to allow a decrease in the core weight. Thus, a compact and economical starting device is provided.

When a plus bias coil in series with the capacitor 16 or in series of the linear inductor 15 is used in a case generating a relatively low spike voltage, oscillating energy can be increased. Also achieved is a savings in core material for the core of the nonlinear inductor 17.

When a minus bias coil in series with the capacitor 16 is used in a case generating a relatively high spike voltage and having large inductance for the nonlinear inductor 17, the condition of the output voltage is adjusted to suppress sputtering. Also achieved is the preheating of filaments efficiently by increasing the frequency of the output voltage and by eliminating cold cathode discharge of the lamp due to the decreased output voltage.

When a combination of the plus and minus bias coils is used with a suitably designed circuit, the circuit is operated by the minus bias during starting period and by the plus bias during normal operation. Accordingly, both of the above-mentioned advantages are achieved.

While the principles of this invention have been described above in connection wih specific embodiments and particular modifications thereof, it is to be clearly understood that this description is given only by way of example and not of limitation on the scope of the invention. Instead, the scope of the present invention should be determined by the following claims.