Discharge Lamp Starting Apparatus

Abstract

A discharge lamp starting apparatus comprising a hot cathode start fluorescent discharge lamp having a cathode at each end thereof, a pulse generating circuit consisting of a pulse transformer connected to one of the cathodes of the discharge lamp and having a primary winding and a secondary winding, a first capacitor, a first bi-directional diode thyristor and a resistor, a circuit portion for increasing preheating current to the cathodes including a diode and a second capacitor which are connected in parallel with a circuit of the first bi-directional diode thyristor and the resistor, and a second bi-directional diode thyristor adapted to be turned off upon operation of the discharge tube and connected between the diode, the resistor and the second capacitor, on one hand, and the other cathode of the discharge tube, on the other hand.

Description

The present invention relates to a starting apparatus for starting a hot cathode start fluorescent discharge lamp. The discharge lamp starting apparatus of this invention makes rapid starting of a discharge lamp possible by means of a starting circuit whose principal components consist of semiconductor switching elements.

More specifically, the discharge lamp starting apparatus of this invention comprises a pulse transformer, a first semiconductor switching element, a pulse generating capacitor, a capacitor charging resistor, a diode for increasing preheating current to a fluorescent discharge lamp when starting the discharge lamp, a second capacitor, and a second semiconductor switching element for interrupting the supply of current to a starting circuit subsequent to the operation of the fluorescent discharge lamp.

It is an object of the present invention to provide a starting apparatus fully compatible with various types of hot cathode start fluorescent discharge lamps.

It is another object of the present invention to provide such starting apparatus in which the reliability of operation is increased by reducing the amount of electric strain applied to the semiconductor switching elements during starting of the discharge lamp.

It is a further object of the present invention to provide such starting apparatus which ensures an equal or longer life of a fluorescent discharge lamp as compared with the conventionally used glow starter or manual starter systems.

Various techniques by which semiconductor switching elements are employed as principal component parts for starting hot cathode start fluorescent discharge lamps have been proposed, for example, in U.S. Pat. Nos. 2,871,409, and 3,476,976 and 3,584,256 and their commercial commerical uses have also been attained recently. However, none of these prior art apparatus have successfully met all of the previously mentioned objects.

It is therefore a still further object of the present invention to provide a starting apparatus which accomplishes the previously mentioned objects and other objects.

The invention will be described further in detail with reference to preferred embodiments illustrated in the accompanying drawings, in which:

FIG. 1 is a basic electrical circuit diagram of an embodiment of a discharge lamp starting apparatus according to the present invention;

FIG. 2 is an electrical circuit diagram of another embodiment of the apparatus according to the present invention;

FIG. 3 is an electrical circuit diagram of a further embodiment of the invention;

FIG. 4 is an electrical characteristic diagram of the semiconductor switching element employed in the present invention;

FIG. 5 is a diagram showing the electrical characteristic of another semiconductor switching element employed in the present invention;

FIG. 6. is a diagram showing the voltage waveform applied during starting across the cathodes of the fluorescent discharge lamp shown in the circuit diagram of FIG. 1;

FIG. 7 is a diagram showing the waveform of current which flows through the point C in the circuit diagram of FIG. 1;

FIG. 8 is a diagram showing the voltage waveform applied during lamp operation across the cathodes of the fluorescent discharge lamp in the circuit diagram of FIG. 1;

FIG. 9 is a conventional fluorescent discharge lamp starting circuit diagram; and

FIG. 10 is a diagram showing the lamp voltage waveform during operation of the discharge lamp shown in FIG. 9.

In the drawings, like reference numerals refer to like parts.

The prior art devices and the effects attributable to the apparatus of this invention will be first explained in detail with reference to specific examples.

Referring now particularly to FIG. 9 of the drawings, there is illustrated one form of the previously mentioned prior art devices which has been designed giveing the greatest consideration to the starting characteristic and life of the fluorescent discharge lamp. The illustrated circuit is broadly divided into a fluorescent discharge lamp 1, a power supply section I and a starting circuit section II, and cathodes 2 and 2' provided at the ends of the discharge lamp 1 have their one ends connected to an AC source 3 and a ballast 4 and the other ends connected to the starting circuit section II consisting of a pulse transformer 5, a bi-directional diode thyristor 6, a diode 7, capacitors 8 and 9, and a resistor 10.

However, this circuit is disadvantageous in that since the preheating of the cathodes of the fluorescent discharge lamp and the application of high pulse voltage across the cathodes of the lamp are effected by the same single switching element 6, the switching element 6 is subjected to an increased electrical stress and moreover since the preheating of the cathodes of the lamp and the application of high pulse voltage are effected simultaneously in the same phase, the cathode preheating current is limited so that the cathodes act as so-called "cold starting" with resultant rapid consumption of the oxide coated on the cathodes and hence a reduced life of the fluorescent discharge lamp.

Moreover, since the preheating of the cathodes and the application of high pulse voltage are effected simultaneously and in the same phase, if the external impedance (i.e., mainly the impedance of the ballast differs with different discharge lamps used, the value and phase of the high pulse voltage vary considerably under the influence of the preheating current and therefore it is difficult to ensure satisfactory starting for various fluorescent discharge lamps of different types under the same specification.

There is a further drawback in that since the series connected capacitors are connected in parallel with the fluorescent discharge lamp, during operation of the fluorescent discharge lamp the charging and discharging voltages of the capacitors are superposed on the lamp voltage of the fluorescent discharge lamp with the result that the lamp voltage e'.sub.2 rises abruptly as shown in FIG. 10 and hence the dispersion of the characteristics of a semiconductor switching element to be employed is restricted reducing productivity and thus making the apparatus expensive to manufacture.

The foregoing drawbacks are overcome by the discharge lamp starting apparatus of the present invention.

Referring now to FIG. 1 illustrating an embodiment of the starting apparatus according to the present invention, numeral 1 designates a hot cathode start fluorescent discharge lamp having cathodes 2 and 2' at the ends thereof, and block I designates a power supply section consisting of a commercial AC source 3 and a ballast 4. Block II designates a starting circuit section according to the present invention, in which numeral 5 designates a pulse transformer having a primary winding n.sub.1 and a secondary winding n.sub.2 with the turns ratio of the winding n.sub.1 to the winding n.sub.2 being about 1:20. Numeral 6 designates a first semiconductor switching element consisting of a bi-directional diode thyristor which constitutes a pulse voltage generating circuit with a first capacitor 7, the pulse transformer 5 and a resistor 8. Numeral 9 designates a diode whose purpose is to permit, during starting, the flow of a preheating current subjected to half-wave rectification through the circuit so as to magnetically saturate the magnetic circuit of the ballast 4 and to thereby supply a sufficient preheating current to the cathodes 2 and 2' of the fluorescent discharge lamp 1 (hereinafter referred to as a lamp). Numeral 10 designates a second capacitor which serves to asist the pulse voltage generating circuit composed of the components 5 through 8 to more efficiently generate high voltage and which also serves with the diode 9 to increase the preheating current to the cathodes 2 and 2' of the lamp 1 during starting thereof. The second capacitor has a larger capacity than that of the first capacitor 7. Numeral 11 designates a second semiconductor switching element consisting of a bi-directional diode thyristor having characteristics so that it is rendered conductive during starting of the lamp 1, while it is rendered non-conductive during operation of the lamp 1. As shown in FIG. 4, the electrical static characteristic of the bi-directional diode thyristor constituting each of the first and second switching elements is such that when the applied voltage reaches the breakover voltage V.sub.BO of the element, the state of the element is rapidly changed into conductive, whereas when the current through the element decreases below its holding current I.sub.H, the state of the element transfers to the nonconductive state. Accordingly, of the first and second thyristors used in the apparatus of this invention, the breakover voltage V.sub.BO of the first thyristor must be selected lower than the peak value of the power supply voltage and the breakover voltage V.sub.BO of the second thyristor must be selected lower than the maximum value of the power supply voltage, but higher than the peak value of the lamp voltage of the lamp 1 in operation.

While the thyristor having the characteristic as shown in FIG. 4 is generally known as a triggering thyristor, if the second thyristor of the two thyristors consists of one which is generally known as a power thyristor shown, for example, in FIG. 5, its operation will be the same.

In the starting apparatus having this circuit construction, the charging current for the pulse voltage generating first capacitor 7 during starting depends on the value, e.g., 3 to 5 K.OMEGA., of the resistor 8 whose impedance is much higher than that of the ballast 4. Therefore, even if the impedance of a ballast varies in accordance with different types of lamps, the switching characteristics of the thyristors will not be affected and thus the starting apparatus according to this invention is compatible with different kinds of lamps.

Further, while the pulse generating circuit generates a pulse voltage of a high frequency (on the order 5 k hertz) during starting, the amount of current flow through the first thyristor 6 constituting part of the pulse generating circuit is very small due to the provision of the resistor 8. Therefore, the first thyristor 6 is subject to only a small electric stress and hence its operating reliability is increased.

Moreover, when the phase of the power supply voltage is in the forward direction of the diode 9, only the second thyristor 11 is turned on preheating the cathodes 2 and 2' of the lamp 1, whereas when the phase of the power supply voltage is in the reverse direction of the diode 9, the second thyristor 11 is turned off and only the first thyristor 6 is turned on applying a high voltage pulse across the cathodes 2 and 2' of the lamp 1. In other words, for each cycle of the power supply voltage, each of the thyristors has a quiescent period corresponding to one-half cycle of the power supply voltage, so that the thyristors are subject to only a small electric stress and the thyristors can easily recover form the effect of this stress with resultant improvement in their operating reliability.

Furthermore, since the preheating current amplifier circuit consisting of the parallel connection of the diode 9 and the second capacitor 10 is connected, through the primary winding n.sub.1 of the pulse transformer 5, in parallel with the pulse generating circuit consisting of the pulse transformer 5, the first thyristor 6 and so on, the magnetic circuit of the ballast 4 is fully saturated during starting supplying a sufficient preheating current to the cathodes 2 and 2' of the lamp 1 and thus the occurrence of so-called "cold starting" phenomenon is prevented, thereby ensuring a longer life of the lamp 1.

Still furthermore, the capacitors are both connected to the lamp 1 through the second thyristor 11 and thus the capacitors will not be charged owing to the turning off of the thyristor 11 during the operation of the lamp 1. Consequently, there is no occurrence of a phenomenon in which, as was the case with the previously explained prior art apparatus shown in FIG. 9, the charging and discharging currents are supplied to the capacitors even during the operation of the lamp 1 thereby increasing the peak value of the lamp voltage and imposing a restriction on the lower limit to the breakover voltage V.sub.BO of thyristors employed. The apparatus according to the present invention can be applied to various lamps having different lamp voltages.

The operation of the apparatus according to the present invention will be explained hereunder with reference to the drawings.

In the embodiment shown in FIG. 1, prior to the operation of the lamp 1, a very high impedance exists across the cathodes 2 and 2' of the lamp 1 and the power supply waveform is applied as such to the starting circuit section shown in the form of a block II. Assuming now that there is a positive potential at a point a and a negative potential at a point b, i.e., for the phase of the applied waveform during a time period t.sub.0 to t.sub.1 (FIG. 6), the voltage applied to the capacitor 10 is zero and practically the whole power supply voltage is applied to the second thyristor 11. In this case, since the breakover voltage V.sub.BO of the second thyristor 11 is preselected lower than the maximum value of the power supply voltage as previously mentioned, the second thyristor 11 is rapidly rendered conductive at the phase of the applied waveform appearing at a time t.sub.2 in FIG. 6, thereby supplying a charging current to the second capacitor 10. As the charging of the second capacitor 10 proceeds with resultant increase in the charging potential for the second capacitor 10, the amount of charging current flowing through the second thyristor 11 gradually decreases and eventually at a time t.sub.3 when the charging current becomes less than the holding current I.sub.H, the second thyristor 11 is changed from the conductive state to the nonconductive state. Then, the second thyristor 11 is negatively biased by the charging potential for the second capacitor 10 and it is thus maintained nonconductive. On the other hand, the first capacitor 7 having a capacity smaller than that of the second capacitor 10 receives its charging current from the second capacitor 10 through the resistor 8 so that the whole potential on the second capacitor 10 is ultimately applied to the first capacitor 7 and thus the voltage applied to the first capacitor 7 is also applied to the first thyristor 6 through the primary winding n.sub.1 of the pulse transformer 5. Since the breakover voltage V.sub.BO of the first thyristor 6 is preset lower than the charging potential for the second capacitor 10, the state of the first thyristor 6 becomes rapidly conductive and thus the charge on the first capacitor 7 is discharged through the primary winding n.sub.1 of the pulse transformer 5. Consequently, a voltage equal to the breakover voltage V.sub.BO of the first thyristor 6 is applied to the primary winding n.sub.1 of the pulse transformer 5, thereby inducing a high pulse voltage n.sub.2 /n.sub.1 x V.sub.BO in the secondary winding n.sub.2. This induced pulse voltage is higher than the maximum value of the power supply voltage and it is thus applied across the cathodes 2 and 2' of the lamp 1 by firing the second thyristor 11. In this case, since the second thyristor 11 has been reverse-biased by the second capacitor 10, the firing of the second thyristor 11 does not place it in a conductive state and it immediately returns to the nonconductive state. On the other hand, upon the discharging of the charge on the first capacitor 7, its charging voltage is reversed to charge the first capacitor 7 in the reverse direction. Consequently, the first thyristor 6 is biased in the reverse direction and it is immediately placed in a nonconductive state. Thereafter, the charge stored in the second capacitor 10 is again supplied to the capacitor 7 through the resistor 8 so that in the same manner as previously mentioned the generating of pulse voltage is continued until the charging potential for the second capacitor 10 drops below the breakover voltage of the first thyristor 6 (at a time t.sub.4 in FIG. 7).

Next, the situation following the time t.sub.4 in FIG. 6, i.e., the case in which there is a positive potential at the point b and a negative potential at the point a will be explained hereunder.

At the phase of the power supply voltage during a time period t.sub.4 to t.sub.6, the instantaneous value of the power supply voltage has not attained the breakover voltage V.sub.BO of the second thyristor 11. However, since the charge stored during the time t.sub.1 to t.sub.4 is still resident in second capacitor 10, this residual charge is super-imposed on the instantaneous value of the power supply voltage and applied to the second thyristor 11. Consequently, the second thyristor 11 is changed into conduction at the time t.sub.4 where the phase of the power supply voltage is still much short of the phase of time t.sub.6 at which the instantaneous value of the power supply voltage attains the breakover voltage V.sub.BO of the second thyristor 11. Thus, the residual charge on the second capacitor 10 is supplied as a discharge current until the phase of time t.sub.4 - t.sub.5, thereby supplying a preheating current. After the phase of time t.sub.5, a closed circuit consisting of the second thyristor 11, the diode 9, the secondary winding n.sub.2 of the pulse transformer 5, ballast 4 and the power supply 3 is established through the cathodes 2 and 2' at the ends of the lamp 1. Consequently, the cathodes 2 and 2' of the lamp 1 are preheated by the magnetic saturation of the ballast 4 due to the rectifying action of the diode 9 and the current flow in this closed circuit continues until the current flow through the second thyristor 11 decreases below its holding current I.sub.H at a phase of time t.sub.8. The waveform of this current is shown in FIG. 7 and it flows through a point c in FIG. 1. The current which flows in this circuit is delayed due to the inductance component of the ballast 4 and thus the current continues to flow even after the time t.sub.4 to t.sub.7 where the polarity of the power supply voltage is in the same phase with the forward direction of the diode 9. Thus, the current flowing through the second thyristor 11 decreases below its holding current at the phase of time t.sub.8 as above explained.

While the construction and operation of the circuit shown in FIG. 1 has been explained, the circuits of another embodiments shown in FIGS. 2 and 3 will be explained hereunder.

The circuit of FIG. 2 differs from the circuit of FIG. 1 in that a resistor 12 is connected in parallel with the second capacitor 10. There are cases where the lamp is operated before the charge stored in the second capacitor 10 during starting has been completely discharged and in such instances the resistor 12 serves as a discharging resistor. If the lamp 1 is operated without the complete discharging of the charge stored in the second capacitor 10, the voltage applied to the second thyristor 11 during the lamp operation consists of the lamp voltage of the operated lamp 1 on which has been superposed the residual charge on the second capacitor 10. Consequently, the absolute value of this applied voltage tends to become so great that it may sometimes become higher than the breakover voltage V.sub.BO of the second thyristor 11. In such an instance, a so-called "re-ignition phenomenon" is repeated in which the once operated lamp 1 is again preheated and then put into operation. The provision of the resistor 12 permits the complete discharging of the remaining charge on the second capacitor 10.

The third embodiment shown in FIG. 3 differs from the embodiment of FIG. 1 in that the second capacitor 10 is connected to the center terminal of the pulse transformer 5. This change in the location of the second capacitor 10 gives rises to no inconvenience and thus the same operating principle and the effect as those of the embodiment shown in FIG. 1 can be expected. In the circuit of FIG. 1, the first capacitor 7 and the first thyristor 6 may change places without provoking any hindrance, and also the diode 9 can be connected inversely.

There are some differences in circuit construction among the embodiments so far described and illustrated in FIGS. 1 to 3, however, all of the embodiments operate such that a high voltage pulse is applied across the cathodes of a lamp, thereby supplying a sufficient preheating current to the respective cathodes to eventually bring the lamp into operation. The lamp voltage waveform of the lamp 1 which is applied to the starting circuit after the lamp has been operated, is shown in FIG. 8 at e.sub.2 and practically whole of this voltage is applied to the second thyristor 11. In FIG. 8, e.sub.1 represents the power supply voltage waveform. Once the lamp has been operated, the second thyristor 11 remains in its nonconductive state since its breakover voltage V.sub.BO is higher than the peak value of the lamp voltage during the lamp operation, and in this way a stable lamp operation can be ensured.

It should be appreciated that in the embodiments so far described, the second thyristor is connected in series with the first and second capacitors with the result that the charging and discharging voltages of the capacitors have no effect on the lamp voltage waveform during the lamp operation. Consequently, the maximum value of the lamp voltage waveform is made sufficiently lower than it has been in the previously known apparatus and the lower limits of the breakover voltage of the second thyristor can be settled to be sufficient low and also it can be chosen from a wide range of voltage.

It should also be appreciated that since the first thyristor is turned off by the second thyristor during the lamp operation, the lower limit to the breakover voltage of the first thyristor needs not be set higher than the lamp voltage during lamp operation as was the case in the previously known apparatus and it is possible to use a thyristor whose breakover voltage is on the order of several volts. Thus, in the apparatus according to the present invention, as compared with the previously known starting apparatus, the selection of the characteristics of the thyristors to be employed can be made very easily and those thyristors which ensure satisfactorily wide applications and desired effect of mass production may be employed.

Claims

What we claim is:

1. A starting apparatus for a hot cathode start fluorescent discharge lamp having a cathode at each end, said starting apparatus being connected in series with said cathodes and comprising:

a. a pulse generating circuit including a pulse transformer having a primary winding and a secondary winding coupled to one of the cathodes of said fluorescent discharge lamp, a first capacitor and a first bi-directional diode thyristor connected in series across the ends of the primary winding of said pulse transformer, and a resistor having one end thereof connected to the junction of said first capacitor and said first thyristor;

b. a diode having one end connected to one end of the primary winding of said pulse transformer;

c. a second capacitor having one end connected to the other end of said primary winding; and

d. a second bi-directional diode thyristor connected between the other ends of said resistor, diode and second capacitor and the other cathode of said fluorescent discharge lamp.

2. A discharge lamp starting apparatus according to claim 1 wherein said second capacitor is connected in parallel with a series circuit of said first capacitor and said resistor in said pulse generating circuit.

3. A discharge lamp starting apparatus according to claim 2, wherein a second resistor is connected in parallel with said second capacitor connected in parallel with the series circuit of said first capacitor and resistor in said pulse generating circuit.

4. A starting apparatus for a hot cathode start fluorescent discharge lamp having a cathode at each end, said starting apparatus being connected in series with said cathodes and comprising:

a. a pulse generating circuit including a pulse transformer having a winding including primary and secondary portions coupled to one of the cathodes of said fluorescent discharge lamp, a first capacitor having one end thereof connected to the junction between the primary and secondary portions of the winding of said pulse transformer, a first bi-directional diode thyristor having one end thereof connected to the other end of said primary winding, and a resistor having one end thereof connected to the other ends of said first capacitor and first bi-directional diode thyristor;

b. a diode and second capacitor connected in parallel, one end of said parallel combination being connected to the junction between the primary and secondary portions of the winding of said pulse transformer; and

c. a second bi-directional diode thyristor connected between the other ends of said resistor and parallel combination of said diode and said capacitor and the other cathode of said fluorescent discharge lamp.