High-pressure discharge lamp

Abstract: The invention relates to a high-pressure discharge lamp (2) provided with an ignition circuit (10) which comprises a voltage-dependent capacitor (8). According to the invention, a voltage-dependent resistor (9) is connected in series with the capacitor. This realizes both a favourable moment of generation of a voltage pulse and a limitation of the level of the generated voltage pulse in a simple and reliable way.

Description: The invention relates to a high-pressure discharge lamp comprising a discharge vessel, an outer bulb enclosing said discharge vessel with intervening space and having a lamp cap, and an ignition circuit within said lamp cap and/or said outer bulb comprising a voltage-dependent capacitor. A lamp of the type mentioned in the opening paragraph is known from German Patent DE-C-3330266. A semiconductor breakdown element is included in the ignition circuit in the known lamp, which is suitable for being operated in series with a stabilizing ballast from an AC voltage supply source. The semiconductor breakdown element is connected in series with the voltage-dependent capacitor. A favourable characteristic is that the moment at which the voltage-dependent capacitor strongly changes its capacitance can be favourably chosen in relation to the AC voltage applied as the supply source through a suitable choice of the breakdown level. Very high voltage peaks of up to 3.5 kV can be realized in this way. A further favourable characteristic is that the suitable choice of the breakdown level can also achieve that the ignition circuit is switched off in the operational state of the lamp. In the known lamp, the generated voltage pulses are applied across the lamp, and thus directly on the lamp cap. This renders the use of the known lamp subject to major restrictions with regard to safety requirements, e.g. in relation to overloading of the ballast. The invention has for its object to provide a method by which the voltage pulses generated in the ignition circuit are limited, while at the same time the favourable characteristics are retained. This object is achieved in a high-pressure discharge lamp of the type described in the opening paragraph in that a voltage-dependent resistor is connected in series with the capacitor. An advantage of the lamp according to the invention is that the level of the voltage pulse is limited by the resistance character of the voltage-dependent resistor included in the ignition circuit, while the adjustment possibility for the voltage pulse generation moment is retained. Also, the favourable characteristic that the ignition circuit is effectively switched off in the operational state of the lamp is retained. An additional advantage is that a voltage-dependent resistor is generally more heat-resistant than a semiconductor breakdown element. A further improvement is possible in that the ignition circuit is also provided with a fuse. This ensures that even under unfavourable conditions, such as, for instance, short-circuit in the capacitor, overloading of the stabilizing ballast through extremely high currents is prevented by melting of the fuse. The ignition circuit may be accommodated in the outer bulb or in the lamp cap. It is also possible to arrange a part of the ignition circuit in the lamp cap and another part in the outer bulb. If the capacitor is arranged in the outer bulb, it is advisable to mount the capacitor in a gas-filled gas-tight glass capsule in order to counteract adverse results of a strong heating of the voltage-dependent capacitor during lamp operation. The gas pressure ensures that dissociation and/or evaporation of components of which the voltage-dependent capacitor is built up, is counteracted. A gas of such a composition should be chosen that no reactions with components of the capacitor take place under the prevailing conditions. Besides rare gases, for instance, nitrogen, oxygen and sulphur-hexafluoride are suitable gases. Combinations of suitable gases are also possible. Although it is conceivable to fill the outer bulb itself with a suitable gas instead of using a separate capsule, in practice this is a less attractive solution. It is true that this provides an equivalent protection of the voltage-dependent capacitor, but the thermal losses owing to convection and conduction in the gas present in the outer bulb will increase. The said thermal losses generally lead to a reduction of the luminous efficacy of the lamp. It is possible to counteract excessive heating of the voltage-dependent capacitor by arranging this capacitor in the lamp cap, whether or not in a gas-filled capsule. It is also possible, if the voltage-dependent capacitor is arranged in a capsule in the outer bulb, to provide this capsule with an internal or external radiation reflecting layer. The discharge vessel of the lamp may be provided with an external auxiliary electrode. The invention will be explained in greater detail with reference to a drawing of an embodiment, in which Fig. 1 is an elevation of a lamp, and Fig. 2 is diagrammatic representation of a circuit formed by the lamp of Fig. 1 together with a stabilizing ballast. In Fig. 1, a lamp 2 according to the invention is provided with a discharge vessel 3, which is enclosed with intervening space by an outer bulb 30, which is provided with a lamp cap 31. The discharge vessel 3 is provided with lamp electrodes 4 and 5, between which a discharge takes place in the operational state of the lamp. Lamp electrode 4 is connected to a lamp contact C of lamp cap 31 via a rigid current conductor 40. Similarly, lamp electrode 5 is connected to a lamp contact D of lamp cap 31 via a rigid conductor 50. An ignition circuit (not drawn in the Figure) is also mounted in lamp cap 31. In Fig. 2, parts corresponding to those of Fig. 1 have corresponding reference numerals. A and B are connection terminals for connecting an AC voltage supply source. Terminal A is connected to lamp contact C via a stabilizing ballast 1. Terminal B is connected to lamp contact D. A circuit 10, built up of a fuse 7, a voltage-dependent capacitor 8, and a voltage-dependent resistor 9 is connected between the lamp contacts C and D. This circuit 10 forms the ignition circuit which, together with the stabilizing ballast 1, generates in known manner ignition voltage pulses between the lamp contacts C and D, and thus between the lamp electrodes 4 and 5. In a practical embodiment of a lamp according to the invention, the lamp was a high-pressure sodium discharge lamp with a power rating of 110 W, suitable for being operated at an AC voltage from a 220 V, 50 Hz source in combination with a type BHL125L stabilizing ballast, make Philips. The practical lamp was provided with an ignition circuit built up of a type NLB1250 voltage-dependent capacitor, make TDK, and a type 2322 592 59506 voltage-dependent resistor, make Philips. The discharge vessel of the lamp was also provided with an external auxiliary electrode. In the operational state of the lamp, the voltage between the lamp electrodes was approximately 115 V. The voltage-dependent resistor has a strongly increased conductance at an applied voltage of 135 V or more. Upon connection to the 220 V, 50 Hz supply voltage, voltage pulses with a level of 1000 V were realized. The choice of 135 V as the level of strongly increased conductance of the voltage-dependent resistor resulted in the generation of a voltage pulse taking place approximately 3 ms after each zero passage of the AC supply voltage. The generated voltage pulses ensured a quick ignition of the lamp. It was observed that the lamp had an ignition voltage of approximately 700 V. The choice of 135 V as the level of increased inductance of the voltage-dependent resistor also ensured that the ignition circuit was effectively switched off in the operational condition of the lamp.

Claims: A high-pressure discharge lamp comprising a discharge vessel, an outer bulb enclosing said discharge vessel with intervening space and having a lamp cap, and an ignition circuit within said lamp cap and/or said outer bulb comprising a voltage-dependent capacitor, characterized in that a voltage-dependent resistor is connected in series with the capacitor. A lamp as claimed in Claim 1, characterized in that the ignition circuit is also provided with a fuse. A lamp as claimed in Claim 1 or 2, characterized in that the voltage-dependent capacitor is mounted in the outer bulb in a gas-filled gas-tight glass capsule. A lamp as claimed in Claim 1 or 2, characterized in that the voltage-dependent capacitor is positioned in the lamp cap.

Inventors: BLEEKER HENDRIK MATTHEUS, DE MAN ROLF ERWIN, LUIJKS GERARDUS MARINUS JOSEPH, BLEEKER, HENDRIK MATTHEUS, DE MAN, ROLF ERWIN, LUIJKS, GERARDUS MARINUS JOSEPHUS FRANCISCUS
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1990
Main classification: H01J61
IPC codes: H05B41, H01J61