Electronic power supply module for a self-switching short-arc xenon lamp

Abstract

Electronic power supply module for a short-arc xenon lamp, comprising a first power supply (1), delivering a first DC voltage, referred to as the "operating voltage". According to the invention, the electronic module comprises a voltage booster (12) delivering a second DC voltage, referred to as the "ignition voltage", which is added to the operating voltage, the sum of the two voltages defining a supply voltage, a low-frequency filter (3) to filter the supply voltage and a current switch (2) to cancel the ignition voltage above a predefined lamp current threshold.

Description

Title of the invention: Electronic power supply module for a self-switching short-arc xenon lamp

[0001] The invention relates to an electronic power supply module for a short arc xenon lamp, comprising a first power supply, delivering a first DC voltage, called the "operating voltage".

[0002] The present invention relates more particularly to the field of cinematographic film projectors, using two voltages.

[0003] Regardless of their power, all short-arc xenon lamps are characterized, by manufacturers and in the field of cinematographic film projectors, by two voltages: - the supply voltage - the operating voltage

[0004] These two voltages increase with the lamp's power. Regarding the supply voltage, its value is typically between 65 and 75 volts for a 500 or 1000 watt lamp. For the operating voltage, which is around 20 volts, the maximum current specified by the manufacturer must be taken into account, as the voltage is automatically determined from this.

[0005] The conversion between the two voltages in early rectifiers was based on the voltage drops of the transformers used when under load and on the filter cells, especially the inductor, to obtain direct current with a maximum tolerated ripple percentage of 5%. It should be noted that these rectifiers are several decades old. They have drawbacks in terms of weight and size, operating at only 50 or 60 Hz. At this frequency, the transformers and output filter inductors are responsible for the weight of the modules.

[0006] More recently, over the last twenty years or so, converters—or switching power supplies—have appeared. The best way to reduce the size of a magnetic circuit, particularly for a transformer, is to increase its operating frequency. We have moved from 50 / 60 Hz to high-frequency currents, in the 20 to 100 kHz range, allowing for a significant reduction in the size and weight of the aforementioned components. Today, we find very high-performance products: 12 volts, adjustable, miniature (equivalent to 1.8 dm³) and capable of delivering up to 100 amps continuous, which was unimaginable just a few years ago.

[0007] With power supplies delivering a practically constant voltage, such as the switching power supplies used for the operating voltage, the problem of switching between the two voltages has arisen in new terms.

[0008] One of the aims of the invention is to provide a solution to this problem.

[0009] To this end, the invention relates to an electronic power supply module for a short arc xenon lamp, comprising a first power supply, delivering a first DC voltage, called "operating voltage", characterized in that it comprises a voltage booster, delivering a second DC voltage, called "ignition voltage", which is added to the operating voltage, the sum of the two voltages defining a supply voltage, a low-frequency filter for filtering the supply voltage and an ammeter switch, for canceling the ignition voltage, above a predefined lamp current threshold.

[0010] At the time of the lamp's ignition, the current switch allows the supply voltage to return to the operating voltage.

[0011] Preferably, the current-sensing switch is mounted between the voltage booster and the low-frequency filter. This arrangement prevents, in particular, the propagation of an electrical ignition flash to the current-sensing switch. Preferably, the current-sensing switch also includes a relay, controlled in the open position, to chronologically add the ignition voltage to the operating voltage and then cancel it, according to the predefined lamp current threshold.

[0012] In one embodiment the voltage booster comprises a transformer and a rectifier bridge, the two alternating input poles of which are connected to the secondary of the transformer while the negative pole is connected to a positive pole of the first supply and the positive pole, to a negative pole of the current switch.

[0013] In another embodiment, the first power supply and the voltage booster both comprise a switching power supply.

[0014] Other advantages of the invention are described in the exposition below, illustrated by the following drawings:

[0015] [Fig-1] is a schematic view of an electronic power supply module of a short arc xenon lamp, according to a first embodiment of the invention.

[0016] [Fig.2] is a schematic view of a switch used in the first mode of realization of the invention.

[0017] [Fig.3] is a schematic view of a test module, allowing adjustment of a lamp current threshold.

[0018] [Fig.4] is a schematic view of an electronic power supply module of a short arc xenon lamp, according to a second embodiment of the invention.

[0019] With reference to [Fig. 1], an electronic power supply module for a short-arc xenon lamp comprises a first power supply 1, delivering a first DC voltage, referred to as the "operating voltage." A voltage booster 12 delivers a second DC voltage, referred to as the "ignition voltage," which is added to the operating voltage, the sum of the two voltages defining a supply voltage. A low-frequency filter 3 filters the supply voltage, and a current switch 2 cancels the ignition voltage above a predefined current threshold. The current switch 2 is mounted between the voltage booster and the low-frequency filter 3 and includes a relay 6, which is normally open, to sequentially add the ignition voltage to the operating voltage and then cancel it, depending on the predefined current threshold.

[0020] In this embodiment, the voltage booster 12 comprises a transformer Tl and a rectifier bridge Dl, the two AC input poles of which are connected to the secondary of the transformer Tl. The negative pole is connected to the positive pole of the first power supply 1 and the positive pole to the current switch 2.

[0021] The additional continuous voltage comes from the transformer TL. As an example, a 30 volt secondary transformer (50 VA) delivers an ignition voltage of 30 x 1.414 = 42 volts.

[0022] The rectifier bridge Dl receives both the DC operating voltage from the first switching power supply 1, via the low-frequency filter 3 and the AC voltage, coming from the secondary of the transformer TL. At the output of the low-frequency filter 3, the desired DC supply voltage is obtained, the sum of the operating voltage and the ignition voltage.

[0023] During lamp ignition, the ignition voltage is canceled when the predefined current threshold is reached and the rectifier bridge Dl exhibits zero resistance at its poles connected to the secondary of the transformer TL

[0024] With reference to [Fig. 2], the current-measuring switch 2 includes a current-measuring system using a Hall effect sensor 4, located in the positive branch at the output of the rectifier bridge DL. The Hall effect sensor 4's output voltage per ampere increment is very small: approximately 12 millivolts per ampere, for a nominal supply voltage of 5 volts. The positive terminal of the comparator 5 is connected to the output of the Hall effect sensor 4, which is 2.5 volts under no-load conditions, and the second input, to a voltage adjustable by a multi-turn potentiometer, corresponding to the lamp's current threshold, set at 3 amperes. At the output of the Hall effect sensor 4, a comparator 5 allows direct connection to the relay 6, which has a coil resistance of 600 ohms and a supply voltage of 24 VDC.

[0025] When activated in the open position, relay 6 disconnects transformer Tl, thereby canceling the ignition voltage. The response of comparator 5, between the current measurement and the output voltage of the Hall effect sensor 4, is less than one microsecond.

[0026] The current threshold setting uses a test setup, [Fig.3], which includes an autotransformer 7, a rectifier bridge 8, a low-frequency filter 9, an ammeter 10 and a load resistor 11. Increasing the voltage through the autotransformer 7 and reading the current delivered on the ammeter 10 allows the multi-turn potentiometer to set the current threshold to 3 amperes, the chosen threshold, to actuate the relay 6.

[0027] It goes without saying that the stability of all voltages is controlled by regulators 13 and 14, of 5 volts and 24 volts respectively, to prevent any drift with the 220 VAC input. An independent power supply connected to the mains provides a continuous 40 VDC. Relay 6 switches off the 30-volt transformer, which allows the switching power supply to step down from 65-70 volts to the operating voltage of approximately 25 volts, enabling the lamp to function.

[0028] In a manner known per se, the electrical ignition of the lamp takes place in the part attached to the projection apparatus, called the lantern, initiated by an electrical flash via a high-voltage transformer, igniter, or starter. It is the positive electrode, which is larger than the negative electrode, that provides the illumination, but which requires a few seconds to reach its temperature and color, and to increase the current drawn from the rectifier. The operating voltage of the switching power supplies must be adjusted by the associated potentiometer, for a lamp current of approximately 25-30 amps for a 450-500 watt xenon lamp, and 35-40 amps for a 900-1000 watt xenon lamp.

[0029] The different phases of ignition of a short-arc xenon lamp, before current stabilization, must depend on the electrical configuration of a generator. We present some observations made on the variations in the lamp's electrical current intensity over time. We can state that the positive electrode preheats; it is this electrode that provides the full brightness, at half maximum current, around 20 amperes for a 1000-watt lamp. After a few seconds, there is a sudden current surge, which necessitates a reduction in the operating voltage. Calibrating the variable 12-volt power supply allows ignition close to a chosen minimum current before final adjustment to the defined maximum current.

[0030] The second embodiment illustrated in [Fig. 4] differs from the previous one in that the first power supply 1 and the voltage booster 12 comprise one and The other is a switched-mode power supply, for example a 48-volt / 1-amp switched-mode power supply.

[0031] The inductor of the low-frequency filter 3, used for stabilizing the current of the lamp, being as close as possible to the output of the rectifier, the two specific cables connected to the lantern can be several meters long depending on the organization of the cinema projection booth, lengths which are compatible with the attenuation of the electrical initiation flash.

[0032] By way of example, a prototype, tested with two lamps of 450 watts and 1000 watts, was built with the components listed below: a 230 / 3V 50VA transformer a 50 A rectifier bridge two 12V and 12V adjustable switching power supplies a LEM CAS 50 A series current transducer an LM 311 comparator a 24V 600 ohm relay four 2200 microfarad capacitors a 3 mm / 40 millihenrys wire inductor.

Claims

Demands

1. Electronic power supply module for a short arc xenon lamp, comprising a first power supply (1), delivering a first DC voltage, called "operating voltage", characterized in that it comprises a voltage booster (12) delivering, a second DC voltage, called "ignition voltage", which is added to the operating voltage, the sum of the two voltages defining a supply voltage, a low-frequency filter (3) for filtering the supply voltage and an ammeter switch (2), for canceling the ignition voltage, above a predefined lamp current threshold.

2. Electronic power supply module for a short arc xenon lamp according to claim 1, characterized in that the current switch (2) is mounted between the voltage booster (12) and the low-frequency filter (3).

3. Electronic power supply module for a short arc xenon lamp according to claim 1, characterized in that the current switch (2) includes a relay (6), controlled to open to chronologically add the ignition voltage to the operating voltage and then cancel it, depending on the predefined lamp current threshold.

4. Electronic power supply module for a short arc xenon lamp according to claim 1, characterized in that the voltage booster (12) comprises a transformer (Tl) and a rectifier bridge (Dl), the two alternating input poles of which are connected to the secondary of the transformer (Tl) while the negative pole is connected to a positive pole of the first power supply (1) and the positive pole, to a negative pole of the current switch (2).

5. Electronic power supply module for a short arc xenon lamp according to claim 1, characterized in that the first power supply (1) and the voltage booster (12) both comprise a switching power supply.

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