LAMPHOLDER AND LIGHTING UNIT COMPRISING A LAMPHOLDER

Abstract: A lampholder in accordance with the invention can suitably be used for a low-pressure discharge lamp (a) which operates at a high frequency and which comprises an elongated, tubular discharge vessel (b), said lamp being provided with a pair of electrodes (c, c') for maintaining an electric discharge in the discharge vessel. The lampholder (0) has contacts (1, 1') for connecting a high-frequency power supply (4), said contacts being electrically connected to a first (2) and a second terminal (2') for connecting the lamp. The second terminal is further removed from the contacts than the first terminal. A compensation conductor (6) extending between the first (2) and the second terminal (2') is connected to said first terminal (2). This leads to a reduction of conducted interference.

Description: Lampholder and lighting unit comprising a lampholder. The invention relates to a lampholder which can suitably be used for a low-pressure discharge lamp which operates at a high frequency and which comprises an elongated, tubular discharge vessel, said lamp being provided with a pair of electrodes for maintaining an electric discharge in the discharge vessel, said lampholder having contacts for connecting a high-frequency power supply, said contacts being electrically connected to a first and a second terminal for connecting the lamp, said second terminal being further removed from the contacts than the first terminal. The invention further relates to a lighting unit comprising a lampholder. A lamp for use in such a lampholder is known from GB 2066559. This Patent document provides a solution for counteracting interference caused by high-frequency magnetic fields. Another type of interference is caused by conduction via the mains. In this type of interference, hereinafter also referred to as conducted interference, high-frequency voltage variations of the discharge arc with respect to ground cause a high-frequency current to flow from the lamp vessel, via parasitic capacitances between the lamp and ground, via ground, via parasitic capacitances between ground and the mains, and via the power supply back to the lamp vessel. Particularly in low-pressure discharge lamps which operate at high frequencies and which have a relatively long discharge vessel, for example in excess of 50 cm, this type of interference plays an important role as the relatively large surface of the discharge vessel results in a large capacitance between the discharge vessel and ground. This type of interference does not only cause interference at the frequency at which the lamp operates but also at higher harmonics thereof since the voltage across the interference sources, in particular the discharge arc and electrodes, deviates substantially from the sine shape. It is an object of the invention to provide a measure by means of which conducted interference can be counteracted. To achieve this, the lampholder is characterized, in accordance with the invention, in that a compensation conductor extending between the first and the second terminal is connected to said first terminal. A parasitic current, which is in phase opposition to the parasitic current flowing via the parasitic capacitance between ground and the conductor to the second terminal, can flow via the parasitic capacitance between ground and the compensation conductor. As a result, the parasitic current flowing via the power supply to the mains is reduced. Although the compensation conductor preferably extends as far as or beyond the second terminal, favorable results are also achieved with a compensation conductor extending over a part of the length between the first and the second terminal. Said compensation conductor may be integral with the conductor connecting the first terminal to its contact, for example by bending the latter conductor so as to be U-shaped, with a first part extending from said contact to a location beyond the first terminal and a second part extending from said location to the first terminal. The second part and the region of the first part situated between the terminal and said location then form the compensation conductor. It is noted that a high-pressure discharge lamp comprising a first terminal which is connected to a conductor extending towards the second terminal is disclosed in GB 1531280. The conductor must counteract migration of filling constituents from the discharge vessel. In low-pressure discharge lamps, whose discharge vessel is at a relatively low temperature, this phenomenon occurs hardly, or perhaps not at all. Also the voltage drop across the electrodes of the lamp is responsible to a substantial degree for guided interference. In order to counteract also this type of interference, an attractive embodiment of the lampholder in accordance with the invention is characterized in that a metal plate is connected to each of the terminals. The voltage drop across the electrodes causes a current which flows, via the parasitic capacitance between ground and the lamp vessel, via parasitic capacitances of the mains and ground, and via the power supply, to the electrodes. By means of the metal plates connected to the terminals, an additional capacitance is created between the electrodes and ground. This causes a substantial part of the current which otherwise would flow via the mains to flow via said additional capacitance. In a favorable embodiment of the lampholder in accordance with the invention, the metal plates also serve as a reflector. A low-pressure discharge lamp may be in one piece with the lampholder or may be detachably provided therein. An attractive embodiment of the lampholder in accordance with the invention is characterized in that the lampholder is provided with further contacts, which are accessible from without, and which are connected to the contacts for connecting the high- frequency power supply, said further contacts and the contacts being provided at opposing ends and being constructed so that they cooperate with each other. By virtue thereof, the lampholders can be connected to each other, with the further contacts of the lampholder forming the high-frequency power supply for the contacts of a neighboring lampholder. Preferably, the lampholder in accordance with the invention forms part of a lighting unit in accordance with the invention which further comprises a high-frequency power supply which is provided with a transformer having a primary and a secondary winding, with the contacts of the lampholder being connected to the secondary winding of the transformer. The transformer electrically separates the DC/AC-converter and the lampholder, which results in a further reduction of the electromagnetic interference. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 and Fig. 2 successively show a first and a second embodiment of the lampholder in accordance with the invention. These Figures also schematically show a power supply. Figs. 3A through 3C show measuring results of conducted interference in dBμV as a function of the frequency in MHz. Fig. 4 shows a third embodiment of the lampholder in accordance with the invention. Fig. 1 shows a first embodiment of a lampholder 0 which is suitable for a low-pressure discharge lamp a, which operates at a high frequency and which comprises an elongated, tubular discharge vessel b, said lamp being provided with a pair of electrodes c, c' which are accommodated in the discharge vessel. In this case, a low-pressure mercury discharge lamp a having a discharge vessel b with a length of 77 cm and an outside diameter of 3.2 mm is accommodated in the lampholder 0. The lampholder 0 has contacts 1, 1' for connecting a high-frequency power supply 4, and a first terminal 2 and a second terminal 2' to which lamp contacts d, d' are connected. The second terminal 2' is further removed from the contacts 1, 1' than the first terminal 2. Copper conductors 3, 3' having a diameter of 0.8 mm extend from the contacts 1 , 1 ' to the terminals 2, 2'. The lampholder 0 forms part of a lighting unit in accordance with the invention, which further comprises a high-frequency power supply 4 which is provided with a transformer 5 having a primary winding 5a and a secondary winding 5b. In this case, the high-frequency power supply 4 comprises an AC/DC-converter (not shown) for converting a mains voltage supplied to mains-connection terminals Kl, K2 to a direct current, and a DC/AC-converter (not shown) for converting said direct current to a high-frequency alternating current of, in this case, 28 kHz, at an output (not shown) to which the primary winding 5a of the transformer 5 is connected. The contacts 1, 1' of the lampholder are connected to the secondary winding 5b. In the embodiment shown, the primary winding 5a has 179 turns. The secondary winding 5b has 1998 turns which are distributed among 6 sections, which are separated from each other by insulating partitions and which each comprise 333 turns. The lampholder is characterized in that a compensation conductor 6 is connected to the first terminal 2 and extends between said first terminal 2 and the second terminal 2'. In this case, the compensation conductor 6 does not form part of the conductor 3 extending from the first terminal 2 to its contact 1. The compensation conductor 6 extends throughout the length of the discharge vessel (77 cm) and, just like the other conductors 3, 3', has a diameter of 0.8 mm. A second embodiment of a lampholder is shown in Fig. 2. Parts corresponding to parts shown in Fig. 1 are indicated by a reference numeral which is increased by 10. In this embodiment, each one of the terminals 12, 12' is connected to a metal plate 17, 17'. The metal plates 17, 17' form a parabolic reflector having a depth of 10 mm and a maximum width of 8 mm. In accordance with the CISPR-15 standard for the average-value measurement, conducted interference in the frequency range from 10 kHz to 30 MHz on the mains-connection terminals Kl , K2 of the lighting units in accordance with the invention was measured (invl and inv2, respectively), as shown in Fig. 1 and Fig. 2, respectively. For comparison, also the conducted interference on the mains-connection terminals of a lighting unit (ref), not in accordance with the invention, was measured. The difference between said lighting unit (ref) and the lighting unit "invl " in accordance with Fig. 1 is that the former does not comprise a compensation conductor. For the rest, the lighting units "invl" and "ref are identical. The level (in dBμV) of the conducted interference, measured in the lighting units "invl ", "inv2" and "ref", as a function of the frequency (in MHz) is successively shown in Figs. 3A, 3B and 3C. These measurements show that the lighting unit comprising the lampholder which is not in accordance with the invention exceeds the CISPR- 15 standard for the average value in the range from 0.23 MHz to 0.72 MHz. The lighting unit "invl" meets the standard throughout the range from 10 kHz to 30 MHz. An even stronger reduction of the interference is achieved by means of the lighting unit "inv2". Fig. 4 shows a further embodiment of the lampholder in accordance with the invention. Parts shown in said Figure which correspond to parts shown in Fig. 1 bear a reference numeral which is increased by 20. In the case of the lampholder 20 shown, the discharge vessel b is accommodated in a transparent housing 28 which is provided at a first end 28a with contacts 21, 21 ' which are constructed as contact pins which are accessible from without. At a second, opposing end 28b, the lampholder 20 is provided with further contacts 29, 29', which are constructed as contact sockets which are accessible from without. The further contacts 29, 29' are connected to the contacts 21, 21 ' for connecting a high- frequency power supply, via the conductor 23' which extends to the second terminal 22' and via the compensation conductor 26. The contact pins 21 , 21 ' and the contact sockets 29, 29' of the lampholder are constructed so that they cooperate with each other. By virtue thereof, lampholders of this construction can readily be used to form a chain by connecting contact pins of lampholders in said chain to contact sockets of the neighboring lampholder.

Claims: CLAIMS: 1. A lampholder (0; 10, 20) which can suitably be used for a low-pressure discharge lamp (a) which operates at a high frequency and which comprises an elongated, tubular discharge vessel (b), said lamp being provided with a pair of electrodes (c, c') for maintaining an electric discharge in the discharge vessel, said lampholder (0; 10; 20) having contacts (1, 1'; 11, 11'; 21, 21 ') for connecting a high-frequency power supply (4; 14), said contacts being electrically connected to a first (2; 12; 22) and a second terminal (2'; 12'; 22') for connecting the lamp, said second terminal being further removed from the contacts than the first terminal, characterized in that a compensation conductor (6; 16; 26) extending between the first (2; 12; 22) and the second terminal (2'; 12'; 22') is connected to said first terminal (2; 12; 22). 2. A lampholder as claimed in Claim 1 , characterized in that a metal plate (17, 17') is connected to each of the terminals (12, 12'). 3. A lampholder as claimed in Claim 2, characterized in that the metal plates (17, 17') form a reflector. 4. A lampholder as claimed in any one of the preceding Claims, characterized in that the lampholder (20) is provided with further contacts (29, 29'), which are accessible from without, and which are connected to the contacts (21, 21 ') for connecting the high-frequency power supply, said further contacts (29, 29') and the contacts (21, 21') being provided at opposing ends (28a and 28b, respectively) and being constructed so that they cooperate with each other. 5. A lighting unit comprising a lampholder (0; 10), as claimed in any one of the preceding Claims, and a high-frequency power supply (4; 14) which is provided with a transformer (5; 15) having a primary and a secondary winding (5a, 5b; 15a, 15b), with the contacts (1, 1'; 11, 11') of the lampholder (0; 10) being connected to the secondary winding (5b; 15b) of the transformer (5; 15).

Inventors: BOUWMAN LAMBERTUS JOHANNES MAR, DE BIJL ADRIANUS MARTINUS JOHA, OP HET VELD JOHANNES HUBERTUS, BOUWMAN, LAMBERTUS, JOHANNES, MARIA, DE BIJL, ADRIANUS, MARTINUS, JOHANNES, OP HET VELD, JOHANNES, HUBERTUS, GERARDUS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1997
Main classification: H01J61
IPC codes: H01J61, H01R33, H05B41