HIGH PRESSURE DISCHARGE LAMP APPARATUS

Abstract: In a high pressure discharge lamp apparatus comprising a discharge tube (3) and a current limiting device (2) connected in series, an over-current protecting semiconduc­ tor switching device such as a triack (TC) is further connected in series to said discharge tube (3), the triack (TC) being controlled of its firing angle responding to an output of a current detection circuit (9) which detects a change of the lamp current through a change of the lamp voltage, and a pulse generator (4) is further connected in parallel to the discharge tube (3) and gives reignition pulse thereto at least during the zero-current period in each cycle of the lamp current.

Description: Title of the Invention High pressure discharge lamp apparatus Background of the invention 1. Field of the invention This invention relates to an improvement in a high pressure discharge larrp apparatus such as a high pressure mercury lamp apparatus, a high pressure sodium lamp apparatus, or a high pressure metal-halide lamp apparatus. The present invention especially concerns an improvement in a high pressure discharge lamp apparatus of the type cotrrising a high pressure discharge tube, a current limiting device, for instance a choke coil, and a pulse generator output terminals of which are connected in parallel to the electrodes of the discharge tube for lighting the tube with a lamp voltage nearly equal to a power source voltage. 2. Description of the Prior Art Generally, in a conventional high pressure discharge lamp. apparatus, a current limiting device such as a choke coil and a discharge tube are connected in series across a power source. In the abovementioned type of the high pressure discharge lamp apparatus, a voltage of the power source should be maintained over 1.5 times a voltage of the discharge tube for preventing an extinction of ignition in the tube. Therefore a voltage drop across the current limiting device is high, and a power loss at the current limiting device is rather large. Accordingly, a dimension and a weight of the current limiting device become undesirably large. For improvi the high pressure discharge lamp apparatus to eliminate the abovementioned defects, a pulse generator may be disposed in a manner to give pulses to the disdischarge tube. By such impression of pulses across the discharge tube, the lamp voltage can be selected so high as nearly equal to the power source voltage. The conventional discharge lamp apparatus having a pulse generator as mentioned has the folling problems: (1) In a case that the high pressure discharge lamp is ignited in every cycle of the power source voltage by impressing of pulse by the pulse generator, a lamp voltage can be selected so high as nearly equal to the power source voltage, and an impedance of the current limiting device can be selected small. But, when the discharge tube starting to ignite, a lamp current grows excessively large, and therefore the impedance of the discharge tube such as the choke coil becomes almost zero for a magnetic saturation due to the overcurrent. Therefore, for avoiding the magnetic saturation, the choke coil as the current limiting device should be designed so as to have relatively large sectional area of a core thereof, thereby making the current limiting device large in dimension and expensive in cost. (2) In order to solve the abovementioned problem, another conventional high pressure discharge lamp appa- ratus has been devised as shown in FIG. 1, whrerin a current limiting device 2 such as a choke coil, an auxiliary current limiting device 5 and a discharge tube 3 are connected in series across a power source 1. The auxiliary current limiting-device 5 of a relatively large size has more large capacity of current limitation than that of the current limiting device 2. And a semiconductor switching device 6 is connected in parallel to the auxiliary current limiting device 5. A pulse generator 4 is connected in parallel to the discharge tube3 for supplying reignition pulses to the discharge tube 3. In the abovementioned high pressure discharge lamp. apparatus, a duty time of the current limiting device 5 is controlled by firing angle controlling action of the semiconductor switching device 6 in such manner that at the stating the duty time of the switching device 6 is very small and at the steady lighting state the duty time is very large, so that effectively the auxiliary current limiting device 5 is substantially shot-circuited by the switching device 6 at a steady lighting state of the lamp. Therefore a lamp current may be maintained for the same degree between at a steady lighting state and at a starting trangent state. Generally, in the high pressure discharge tube an ignition of the discharge tube extincts every period when the lamp current is zero. Accordingly, if a series insertion of a thyristor to the high pressure discharge lamp of conventional type for attempting to limit the lamp current by changing its firing angle, the tube does not work due to zero current period in every cycle. Therefore, in the lamp apparatus of FIG. 1, lamp current is always supplied from the power source 1 through the auxiliary current limiting device 5, which is connected in parallel to the thyristor or the like current limiting device 6, thereby to allow the current to flow without zero period made by the thyristor 6. However the problem of the conventional lamp apparatus is that though the current limiting device 2 can be designed relatively small, a large auxiliary current limiting. device 5 has been necessary. Accordingly the lamp apparatus can not be made in small size. Summary of the Invention The pourpose of the invention is to provide a high pressure discahrge lamp apparatus having a pulse generator to impress pulses across the discharge tube for allowing selecting a lamp voltage very close to a power source voltage, and having no auxiliary current limiting device like the device 5 of the conventional high pressure discharge lamp apparatus shown in FIG. 1. Brief Explanation of Drawing FIG. 1 is a block diagram of a conventional high pressure discharge lamp apparatus. FIG. 2 is a block diagram of a fundamental apparatus embodying the present invention. FIG. 3 is a block diagram of the actual apparatus embodying the present invention. FIG. 4 is a graph showing characteristic curves of a high pressure discharge tube at a starting transient state. FIG. 5 is a detailed circuit diagram of the apparatus of the present invention shown in FIG. 3. FIG. 6 is a detailed circuit diagram of a pulse generator shown in FIG. 5. FIG. 7A and FIG. 7B are wave-form chart showing wave-forms at various parts of the circuit shown in FIG. 5. FIG. 8 is a wave-form chart showing wave-forms at various parts of the circuit shown in FIG. 6. FIG. 9 is a graph showing performance curves of a high pressure discharge lamp apparatus of present invention at a starting trangent state. Description of Preferred Embodiments The high pressure discharge lamp apparatus of the present invention comprises a discharge tube and a current limiting device connected in series, an over-current protecting device is further connected in series to said discharge tube, over-current protecting device being controlled of its firing angle responding to an output of a current detection circuit, and a pulse generator is further connected in parallel to the discharge tube. A fundamental apparatus embodying the present invention is shown in FIG. 2. In FIG. 2, a current limiting device 2 and an over-current protecting device 11 are connected in series with a discharge tube 3 across a power source 1. A pulse generator 4 is connected in parallel to the discharge tube 3 for supplying reignition pulses to the discharge tube 3. And a lamp current detecting circuit 9 for detecting a lamp current and giving control signal to the over current protecting device 11 is connected in parallel to the discharge tube 3. The pulse generator 4 is for preventing extinction of the tube and allowing a selection of a lamp voltage high enough close to a voltage of the power source 1, by supplying the reignition pulses to the discharge tube 3, during a period from near the zero cross point of the voltage of power source 1 to a stated point for from the zero-cross point in every cycle of the voltage of power source 1. By means of the reigni tion pulses, extinction of the ignition in the discharge tube 3 may be effectively avoided even in the zero current period in each cycle of the lamp current. FIG. 3 shows an apparatus embodying the present invention which uses a semiconductor switching device 11 as the over-current protector and uses a lamp current detecting circuit 9, which in actual circuit is a voltage detection circuit which outputs a signal to indicate a voltage of the discharge tube 3 and has a specified relation with the lamp current, is shown in FIG. 3 In an embodiment of the present invention shown in FIG. 3, a current limiting device 2 (of passive element device such as a choke coil) and the semiconductor switching device 11 as the over-current protector and a discharge tube 3 are connected in series across a power source 1. A pulse generator 4 for impressing reignition signal on the discharge tube 3 is connected in parallel to the latter. The lamp current detecting circuit 9 is connected in parallel to the discharge tube 3. And a firing angle controlling circuit 10 is disposed so as to receiving an out-put signal of the lamp current detecting circuit 9 and supply a firing angle controlling signal to the switching device 11. A firing angle of the switching device 11 is controlled based on a relation between the lamp current and the lamp voltage which is detected in the lamp current detecting circuit 9. And the switching device 11 controls the firing angle, hence effective value of the current of the discharge tube 3 from the power source 1, and thereby protects an over-current at the starting transient state. The pulse generator 4 supplies reignition pulses in every cycles to the discharge tube 3 at least during the zero current period of the lamp current in order for avoiding an extinction of the discharge tube 3 due to zero current period both in starting transient state and steady lighting state. Generally, in a high pressure discharge tube connected in series with choke coil or some impedance, for a starting transient state of the tube, a lamp voltage VLa and a lamp current ILa of the high pressure discharge tube show the curves as shown in FIG. 4. In FIG. 4, ZLa' which is relatively small at incipient period of starting of lighting in the discharge tube wherein a vapour pressure in the tube is relatively small. Then as time lapses the impedance ZLa increases. For that reason, the lamp current ILa, that is a current through the choke coil, decreases at the time lapses, and simultaneously the lamp voltage VLa increases as shown in FIG. 4. Accordingly, the amount of the lamp current ILa can be known by detect ing the lamp voltage VLa based on abovementioned. relation between the current ILa and the voltage VLa Consequently, the lamp current ILa can be controlled by detecting the lamp voltage VLa primarily. The operation of the over-current protection in accordance with the high pressure discharge lamp apparatus of present invention is elucidated hereinafter referring to employing FIG. 5, FIG. 7A and FIG. 7B. FIG. 5 is a circuit diagram of the apparatus of the present invention. FIG. 7A and FIG. 7B are wave-form chart of various parts of the circuit shown in FIG. 5. In the chart of FIG. 5, a voltage across the discharge tube 3 is impressed to the lamp current detecting circuit 9 which detects a lamp current by detecting a lamp voltage. That is, in present invention, the lamp current is easily detected by a simple circuit as shown in FIG. 5. In the lamp current detecting circuit 9, the lamp voltage having a certain relation with the lamp current as shown in FIG. 4 is stepped down by a transformer Tl, and rectified by a diode bridge circuit DB2, and further smoothed by a circuit of resistor 1 and a capacitor C1. A voltage of the- capacitor C1 is divided by a circuit of resistors R2 and R3, and - then impressed to the base of a transistor m in a-firing angle controllzE circuit 10. At immediately after a starting of lighting of the discharge tube 3, lamp voltage VLa is low as shown in FIG. 4. Therefore a voltage impressed to the base of the transistor Ql is also in a low level, so that the transistor Ql maintains off state. On the other hand, a power source voltage having the wave-form V5 shown in FIG. 7A is given to the firing angle controlling circuit 10, wherein the power source voltage V5 is led through a resistor R4, DB and R5, and then is level clipped by a regulate diode Dl, whose voltage wave form Vl is shown in FIG. 7A. The voltage wave-form V1 is supplied to an anode of PUT (programmable unijunction transistor) Q2 through a resistor R6. At the immediately after a starting of lighting of the discharge tube 3, the transistor Ql maintains off state as abovementioned, and hence the voltage of waveform Vl is supplied to a capacitor C2 for charging only through the resistor R6. As a result of the charging, a voltage wave-form V2 of the capacitor C2 is formed as shown in FIG. 7A. As shown in FIG. 5, the voltage of wave-form V1 is divided by resistors R8 and Rg, and the divided voltage is impressed to a gate of the PUT Q2. Hereupon, the voltage V2 of the capacitor C2 grows up and comes above the voltage impressed to the gate of the PUT Q2' hence the PUT Q2 turns on. Consequently the electric charge in the capacitor C2 discharges through the PUT Q2. As a result, a trigger pulse V3 as shown in FIG. 7A is produced across both ends of a resistor R10. A thyristor Th is triggered by the pulses V3 impressed through a resistor Rll. When the thyristor Th turns on, both out-put ends of the diode brindge circuit DBl are short-circuited by the thyristor Th, and hence the voltage Vl becomes zero, and then triggering action of the PUT is stopped. As abovementioned, the PUT Q2turns on at a firing angle corresponding to the time constant resistor R6 and the capacitor C2, and therefore, the thyristor Th turns on by receiving the triggering pulses V3 impressed to the gate thereof through the resistor R11. As a rectified source voltage is supplied to the thyristor Th through the diode bridge circuit DB1, the thyristor Th maintains "on" state until a forward current thereof becomes lower than a holding current of the thyristor Th at a timing near an end of every positive half cycle of the power source voltage Vs. When the thyristor Th is in "on" state and the power source voltage Vs is in the positivehalf cycle, a gate current flows in the triack TC as a switching device 11 through the path of: power source 1 - current limiting device 2 - discharge tube 3 - gate of the triack TC - diode D2 - thyristor Th - diode bridge circuit DB1 - power source 1. And when the power source voltage Vs is in the negative half cycle, the gate current of the triack TC flows through the path of: power source 1 - diode bridge circuit DB1 - thyristor Th - diode D3 - gate of the triack TC - discharge tube 3 - current limiting device 2 - power source 1. In both cases of the positive half cycle and negative half cycle, the triack TC is "on" during the while the gate currents exist. Accordingly, a main load current ILa of the triack TC is appropriately controlled by the firing angle controlling circuit 10 which determines a firing angle pl of the triack TC as shown in FIG. 7A. Although the main load current ILa has a zero current period in each cycle, by means of the reignition pulses VP are supplied to the discharge tube 3 at least during the zero current period in each cycle of the main load current 1La After lapse of some time period, the lamp voltage increases, and also the base potential of the transistor Q1 rises, so that the transistor Ql turns on. When the transistor Ql turns on, a series circuit of the resistor R7 and the internal resistance (collector-emitter resist ance) of the on-state transistor Ql 1 is connected in parallel to the resistor R6, and therefore a charging time of the capacitor C2 is shortened. Accordingly a time till the PUT Q2 turns on is shortened, and therefore the time phase of trigger pulses V3 across the resistor R10 is led, hence the firing angle of the triack TC becomes very small as shown by #2 (82 < < #1) in FIG. 7B. As a result, main load current of a wide firing angle flows through the triack TC as shown in FIG. 7B. As abovementioned, an apparatus of present invention has a pulse generator 4 and a switching device as an over-current protecting device, and the lamp current at the starting transient state is suppressed by over-current protecting device nearly equal to the lamp current at the steady lighting state, preventing the extinction by the reignition pulses from the pulse generator 4. Therefore, the current limiting device 2 can be designed small enough and further a loss therein can be satisfactorily diminished. The aforementioned pulse generator 4 is elucidated in detail in reference to FIG. 6 and FIG. 8. FIG. 6 is a detailed circuit 'diagram of the pulse generator 4, and FIG. 8 is a wave-form chart of various parts of the circuit shown in FIG. 6. When the power source voltage V8 is applied, the pulse generator 4 issues ignition pulses to the discharge tube 3, and the discharge tube 3 is ignited. Operation of the pulse generator 4 after starting of ignition is elucidated hereafter separating in two parts. First part is for the positive half cycle period of the power source voltage V8 and the second part is for the negative half cycle period of the power source voltage Vs. In the positive half cycle period: In the positive half cycle of the power source voltage Vs, the voltage V5 is rectified and level clipped by a power source rectifying circuit 12a consisting of a diode Dada' a resistor R12a, and a regulate diode D5a An output voltage of the power source rectifying circuit 12a, that is a voltage of a point C in FIG. 6 is shown by a wave form C of FIG. 8 A voltage of the wave-form C is supplied to an oscilating circuit 14a and to an oscillation control circuit 13a. In the oscillation control circuit 13a, the voltage of the wave-form C is impressed to an integration circuit consisting of a resistor R17a and a capacitor C4a, then an integrated voltage of the capacitor C4a at a point D in FIG. 6 is impressed to the base of a transistor Q5a. A voltage wave-form D at the point D is shown in FIG. 8. A comparator consisting of two transistors Q5a' Q6a and three resistors R18a, Alga, R20a is formed in the oscillation control circuit 13a. A voltage of the point C is divided by resistors R21a and R22a and impressed to the base of the transistor Q6 in the comparator. DC component voltage of the collector of the transistor Q6a as an output of the comparator is cut off by a capacitor C5a, and the AC component is impressed on the base of a transistor Q in the oscillating circuit 14a through a diode D6a In the oscillating circuit 14a, an oscillation frequency determined by a time constant of a resistor R 13'a and a capacitor C 3a is maintained during the off state of the- transistor 03a. (A resistor R14a and a resistor R15a are adequately selected in a manner that a PUT Q4a maintains the oscillation.) When both ends of the capacitor C3a is short-circuited, an anode potential of the PUT Q4a changes to zero, and the oscillation stops. That is, in a duty time of a wave-form E, the oscillation is stopped as shown in FIG. 8. The wave-form E is for a voltage of an out-put of the oscillation control circuit 13a at a point E in FIG. 6. Pulse train is issued for the period of 63 from the oscillating circuit of the PUT Q4a and impressed on a switching transistor Q7, through a pulse transformer T2a thereby giving the input signal of the wave-form F shown in FIG. 8 for operation of the transistor Q7. In the negative half cycle period: In a negative half cycle of the power source voltage V5, the voltage Vs is rectified and level clipped. by the power source rectifying circuit 12b. A negative cycle circuitry comprises a power source rectifying circuit 12b, an oscillating circuit 14b, and an oscillation control circuit 13b; and these are same with the positive cycle circuitry comprising the power source rectifying circuit 12a, the oscillating circuit 14a, and the oscillation control circuit 13a except a connection to the power sources thereof which are opposite each other. Accordingly, detail explanation about the negative cycle circuit construction is omitted here. In the negative half cycle of the power source voltage Vs, the voltage wave-form Vs is transduced to a wave-form G of a out-put voltage of the power source rectifying circuit 12b as shown in FIG. 8. The oscillating circuit 14b issues pulses H-shown in FIG. 8. Then the pulses H impressed to a switching transistor Q,, As a result, the transistor Q7 is "on" in positive half cycle of the power source Us; and the transistor Q8 is "on" in negative half cycle of the power source V5. The power source voltage VS is allwave-rectified by a diode bridge circuit DB3 and is smoothed by a smooth ing capacitor 6 C capacitor , then impressed on the transistors Q7 and Q8 through a transformer T3. A pulse wave-form I shown in FIG. 8 having both polarity of pulses (reignition pulses is issued from secondary winding of the transformer T3, and is given to the discharge tube 3 through a capacitor C7. Then the reignition pulses start an ignition and maintain a lighting. The capacitor C7 cuts off a low frequency voltage component from the discharge tube 3 and prevents a magnetic saturation in the transformer T3. A period that the reignition pulse issued from the pulse generator- 4 can be controlled by a integration time of the oscillation control circuit 13a by determining the time constant of the resistor and' and the capacitor C4a, or the period can be controlled R17a by a reference voltage of the comparator in the oscillation circuit 13a by determining the voltage by the ratio of the resistors Rzla and R22a An oscillation frequency at the oscillating circuit 14a can be controlled by, for instance, selecting the values of the resistor R13a and the capacitor C3a A reignition pulse impressing angle 93 should be selected as 63 > 01 with respect to the firing angle 61 or the zero-current period in each cycle of the lamp current. Performance curves of the high pressure discharge lamp apparatus of present invention at a starting trangent state are shown in FIG. 9. In FIG. 9, characteristic of the lamp voltage is designated by the curve VLa , the lamp current by ILa t and firing angle at a condition that the starting lamp current is controlled during a period tc from starting of lighting of the discharge tube by e. The transformer T1 can be replaced by a suitable isolator for example, photo-isolator. The pulse generator 4 of FIG. 6 can be constituted by a digital IC, thereby a similar function is obtainable. device is high, and a power loss at the current limiting device is rather large. Accordingly, a dimension and a weight of the current limiting device become undesirably large. For improving the high pressure discharge lamp apparatus to eliminate the abovementioned defects, a pulse generator may be provided in a manner to give pulses to the discharge tube. By such impression of pulses across the discharge tube, the lamp voltage can be selected so high as nearly equal to the power source voltage. The conventional discharge lamp apparatus having a pulse generator as above-mentioned still has the following problems: 1, In a case that the high pressure discharge lamp is ignited in every cycle of the power source voltage by impressing of pulse by the pulse generator, a lamp voltage can be selected so high as nearly equal to the power source voltage, and an impedance of the current limiting device can be selected small. But, when the discharge tube starts to ignite, a lamp current grows excessively large, and therefore the impedance of the discharge tube such as the choke coil becomes almost zero for a magnetic saturation due to the overcurrent. Therefore, for avoiding the magnetic saturation, the choke coil as the current limiting device should be designed so as to have relatively

Claims: What we claim is 1. A high pressure discharge lamp apparatus camprising: a discharge tube, a current limiting device and an over-current protecting device which are connected in series with said discharge tube across a power source, a pulse generator connected in parallel to said discharge tube to supply pulses to said discharge tube, and a lamp current detecting circuit to detect a lamp current and give control signal to said overcurrent protecting device. 2. A high pressure discharge lamp apparatus of claim 1, wherein said over-current protecting device is a semiconductor switching device. 3 A high pressure discharge lamp apparatus of claim 1, wherein said lamp current detecting circuit includes a means for detecting lamp voltage. 4. A high pressure discharge lamp apparatus of claim 2, wherein a zero-current period in each cycle of said semiconductor switching device is controlled by a firing angle controlling circuit. 5. A high pressure discharge lamp apparatus of claim 4, wherein said pulse generator issues pulses for making a reignition of said discharge tube during at least said zero-current period in each cycle of said semiconductor switching device. High pressure discharge lamp apparatus New claims 1. A discharge lamp apparatus comprising: a discharge tube (3), a current limiting device (2) and an over-current protecting device (11), a power source (1), and a lamp current detecting circuit (9) to detect a lamp current and to give control signal to said over-current protecting device (11), characterized in, that said discharge lamp apparatus is AC-operated, that said current limiting device (2) and said overcurrent protecting device (11) are connected in series with said discharge tube (3) across said power source (1), that a pulse generator (4) is connected in parallel to said discharge tube (3) to supply pulses to said discharge tube (3), and that said lamp current detecting circuit (9) detects the lamp current through detection of a voltage across a pair of electrodes of the lamp. 2. A high pressure discharge lamp apparatus of claim 1, characterized in that said ove-current protecting device (11) is a semiconductor switching device. 3. A high pressure discharge lamp apparatus of claim 2, characterized in that a zero-current period in each cycle of said semiconductor switching device (11) is controlled by a firing angle oontrolling circuit (10). 4. A high pressure discharge lamp apparatus of claim 3, characterized in that said pulse generator (4) issues pulses for making a reignition of said discharge tube (3) during at least said zero-current period in each cycle of said semiconductor switching device (11).

Inventors: HORII, SHIGERU, KOYAMA, KAZUTAKA, YOSHIKAWA, NOBUHISA
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1981
Main classification: H05B41
IPC codes: H05B41