Electric lamp and discharge devices: systems – Surge generator or inductance in the supply circuit
Reexamination Certificate
1999-01-20
2001-01-30
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Surge generator or inductance in the supply circuit
C315S276000, C315S219000
Reexamination Certificate
active
06181081
ABSTRACT:
The invention relates to a starting device for a discharge lamp in accordance with the preamble of Patent Claim
1
or
12
, and to a method for starting a discharge lamp.
I. TECHNICAL FIELD
The invention relates in particular to a starting device for a high-pressure discharge lamp, for example a low-wattage halogen metal vapour high-pressure discharge lamp for a motor vehicle headlamp. The high-pressure discharge lamp has a discharge vessel which is sealed in a gastight fashion. Projecting into the discharge space are two gas discharge electrodes which are connected in an electrically conducting fashion to external supply leads. During operation of the discharge lamp, a light-emitting discharge arc is formed between its gas discharge electrodes. To operate the lamp, an operating unit is required which supplies the discharge lamp with electric power and limits the discharging current via the discharge arc. The operating unit also comprises a starting device for the discharge lamp which initiates the gas discharge. In order to start the gas discharge in a high-pressure discharge lamp, a starting voltage of a few kilovolts is required for a cold lamp, while to restart the same lamp when hot—that is to say to start it in the still hot state—a starting voltage of more than 20 kV can be required. After starting of the gas discharge lamp has been performed, the operating voltage of the high-pressure discharge lamp drops, that is to say the voltage drop over the discharge path which is required to maintain the discharge arc, to only approximately 80 V to 100 V. The starting device can be constructed, for example, as a pulse starting unit which applies unipolar high-voltage pulses to one of the two gas discharge electrodes of the high-pressure discharge lamp during the starting phase.
II. PRIOR ART
A starting device corresponding to the preamble of Patent Claim
1
is disclosed in the PCT application with the international publication number WO 97/04624. This starting device is a pulse starting device for a high-pressure discharge lamp. The pulse starting device has a starting transformer with a primary winding and a secondary winding, a starting capacitor, a resistor element via which the starting capacitor is charged, and an automatic switch. One terminal of the secondary winding is connected to one of the gas discharge electrodes of the high-pressure discharge lamp, while its other terminal is connected to the voltage input of the starting device. The primary winding of the starting transformer and the switching path of the automatic switch are arranged in such a way that the discharging current of the starting capacitor flows through them.
Since the starting voltage required to restart the high-pressure discharge lamp when hot is substantially higher than the voltage present at the voltage input of the starting device, the starting transformer must have an appropriately high transformation ratio. As a consequence of the high transformation ratio of the starting transformer, the known and commercially available starting devices have a high space requirement because of the large volume of the starting transformer. Consequently, in the case of low-wattage halogen metal vapour high-pressure discharge lamps which are provided for use in motor vehicle headlamps, it is not possible to accommodate the starting device for these lamps in the lamp cap.
III. DESCRIPTION OF THE INVENTION
It is the object of the invention to provide an improved starting device for a discharge lamp, and an improved method for starting a discharge lamp. In particular, the starting device is intended to have as compact a design as possible, so that it can still be accommodated in the lamp cap even in the case of the low-wattage halogen metal vapour high-pressure discharge lamps which are used in motor vehicle headlamps and are of very small design.
This object is achieved according to the invention by means of the characterizing features of Patent Claim
1
or
13
. Particularly advantageous embodiments of the invention are described in the subclaims.
According to the invention, the starting device according to the invention has a transformer which has a parallel circuit, comprising at least two primary windings, and at least one secondary winding, the parallel circuit of the primary windings being inductively coupled to the at least one secondary winding. For a given transformation ratio of the transformer, this measure permits the number of turns per unit length on the secondary side of the transformer to be substantially reduced without the inductive coupling between the primary and secondary sides being impaired by the low number of turns per unit length of the primary windings. The induced voltage available on the secondary side does not change if the transmission ratio is preserved.
The primary windings have advantageously respectively at most two turns. As a result, there is also a reduction in the number of turns per unit length of the at least one secondary winding in accordance with the desired transformation ratio. It has emerged that, for example, a transformer with two primary windings, which are connected in parallel and have two turns in each case, has just as good inductive coupling between the primary and secondary sides as a transformer with only one primary winding which has four turns. However, for a prescribed transformation ratio of the transformer, only half as many turns are required for the case of the two primary windings which are connected in parallel and respectively have two turns on the secondary side as for the case of the one primary winding with four turns. In order to improve still further the inductive coupling between the primary and secondary sides of the transformer, the primary windings connected in parallel can advantageously consist in each case of a copper strip.
The reduction of the number of turns per unit length on the secondary side permits a compact design of the transformer and of the entire starting device, with the result that all the subassemblies of the starting device, including the transformer, can be accommodated in the lamp cap. The expensive electric connections, provided with insulation which is proof against high voltage, between the lamp holder and the starting unit are thereby eliminated. The high-voltage pulses for starting the gas discharge are then generated inside the lamp cap and are therefore no longer accessible from outside.
It has proved advantageous to use a transformer with a ferrite core and a coil former which has at least one chamber for the transformer windings. To avoid electric breakdowns and to prevent eddy currents in the ferrite body, the ferrite core advantageously consists of a high-resistance material, with the result that the ferrite core has an electric resistance of more than 1 M&OHgr;. It is advantageous to use E cores or cylindrical cores, such as cylinder cores, tubular cores or threaded cores, for example, as ferrite body.
The starting device according to the invention comprises a capacitor, a resistor element, an automatic switch and a transformer with at least two primary windings connected in parallel and at least one secondary winding. The components of the starting device are arranged and interconnected in such a way that the capacitor discharges abruptly for starting the gas discharge in the lamp, the discharging current of the capacitor flowing via the parallel circuit of the primary windings and via the switching path of the automatic switch, with the result that the voltage pulses induced in the at least one secondary winding are applied to one of the gas discharge electrodes of the lamp.
In a first preferred exemplary embodiment of the invention, the starting device is constructed as an asymmetrical pulse starting device which applies unipolar starting voltage pulses to only one of the lamp electrodes. In this starting device, the transformer has only one secondary winding, which is connected to a terminal of the starting voltage output for the discharge lamp.
In the second, particularly preferred exemplary embod
Becker Juergen
Behr Gerhard
Helbig Peter
Hirschmann Guenther
Wittig Christian
Lee Wilson
Meyer William E.
Patent-Treuhand-Gesellschaft Fuer Elektrische Gluehlampen MBH
Wong Don
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