COLD-CATHODE LAMP, IN PARTICULAR, A TABLE OR DISPLAY WINDOW LAMP

Abstract: The table or display window lamp (1) has a supporting structure (2, 3), and a head (4) connected in articulated manner to the supporting structure. The head (4) has a first portion (10a) housing a cold-cathode bulb (12), and a second portion (10b) housing a supply circuit (11) for supplying the cold-cathode bulb; and the first and second portions are aligned on opposite sides of a portion (13) connecting the head (4) in articulated manner to the supporting structure. The supply circuit (11) has an input rectifier (26) having a first and a second output node (26a, 26b); a first and a second bias circuit (28, 29) connected to each other in parallel between the first and second output node of the input rectifier; a first and a second control transistor (30, 31) connected to respective bias circuits (28, 29) and to respective terminals of two primary windings (41, 42) arranged in series with each other, of a transformer (16), the secondary winding of which is connected to the cold-cathode bulb (12). The control transistors are low-saturation-resistance MOS types.

Description: COLD-CATHODE LAMP, IN PARTICULAR, A TABLE OR DISPLAY WINDOW LAMP TECHNICAL FIELD The present invention relates to a cold-cathode lamp, and in particular to a table or display window lam . BACKGROUND ART As is known, cold-cathode lamps are currently used for rear lighting portable personal computers, and have sophisticated, high-cost supply and control electronics. In particular, currently used drive circuits comprise a transformer with three primary windings, one of which is used to feedback control the push-pull cycle of drive transistors forming part of the supply circuit. To operate properly, the transformer windings must be positioned accurately and therefore wound by hand; and the push-pull transistors are defined by expensive low- drop bipolar transistors . The drive circuit as a whole is therefore extremely expensive to produce, particularly on a small scale, and so increases the total cost of cold- cathode lamps as to make them unfeasible for use as table or display window lamps. A demand therefore exists for straightforward, low- cost cold-cathode lamps for use as table lamps or for lighting display windows. DISCLOSURE OF INVENTION It is an object of the present invention to provide a cold-cathode lamp designed to meet the above requirements . According to the present invention, there is provided a table or display window lamp comprising a supporting structure and an articulated head connected in articulated manner to said supporting structure; characterized in that said articulated head comprises a first portion housing a cold-cathode bulb; and a second portion housing a supply circuit for supplying said cold- cathode bulb . BRIEF DESCRIPTION OF DRAWINGS A non-limiting embodiment of the invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a side view of the lamp according to the present invention in different operating positions; Figure 2 shows a larger-scale cross section of part of the head of the Figure 1 lamp; Figure 3 shows an electric diagram of the cold- cathode lamp supply circuit . BEST MODE FOR CARRYING OUT THE INVENTION As shown in Figure 1, the lamp 1 comprises a base 2; and a telescopic upright 3 shown in both the withdrawn and extended position, and which rotates about a vertical axis, as shown by the dash line (position 1') in Figure 1. Telescopic upright 3 supports a head 4 articulated to rotate both about a hinge pin 18, as shown by the dash line (positions 1") , and about telescopic upright 3 itself, as shown by arrow 7 in Figure 1. Lamp 1 is turned on by a switch 8 - shown by the dash line in Figure 1 - located on base 2, and comprises a transformer (not shown) , either preferably located inside base 2 or external, and which is supplied with mains current by a cable 9, and generates at the output a low-voltage, e.g. 12 V, alternating current, which is supplied to head 4 housing a supply circuit (Figure 2) for supplying the cold-cathode bulb. As shown in Figure 2, head 4 comprises a casing 10 defining a first portion 10a housing the cold-cathode bulb 12, and a second portion 10b housing the supply circuit 11. More specifically, second portion 10b is substantially parallelepiped-shaped, is closed on all sides except for the one facing first portion 10a, and is shorter and deeper than, and forms an extension of, first portion 10a. Close to first portion 10a, second portion 10b comprises a projecting portion 13 extending crosswise with respect to head 4 and connected in articulated manner to telescopic upright 3 to enable rotation of head 4 in the direction of arrow 7 and about pin 18. Supply circuit 11 is carried on a printed circuit board 14 having a hole 15 beneath a transformer 16 to sufficiently dissipate the heat generated during operation. In a manner not shown, printed circuit board 14 has conducting tracks on both faces to ensure the required insulation distances within a small space . Supply circuit 11 receives the low-voltage current, supplied by the transformer (not shown) housed inside base 2, via an electric connector 20, and supplies the high-voltage power for operating bulb 12 via electric wires 21 shown only partly in Figure 2. With reference to Figure 3, supply circuit 11 comprises two input terminals 25 supplied with low- voltage current by connector 20 and connected to a diode- type rectifying bridge 26. Rectifying bridge 26 comprises a first and a second output terminal 26a, 26b, between which are connected in parallel a filtering capacitor 27, a first voltage divider 28, and a second voltage divider 29. First voltage divider 28 comprises a first resistor 28a and a second resistor 28b, and defines a first intermediate node 28c; and second voltage divider 29 comprises a first resistor 29a and a second resistor 29b, and defines a second intermediate node 29c. Transformer 16 is a two-primary type. More specifically, transformer 16 comprises a first and a second primary winding 41, 42 arranged in series between a first and a second input terminal 43, 44 and defining a common node 45. Transformer 16 also comprises a secondary winding 46 having a first and a second output terminal 47, 48, and the second output terminal 48 of which is connected to the second output terminal 26b of rectifying bridge 26. First intermediate node 28c of first voltage divider 28 is connected to the gate terminal of an N-channel MOS transistor 30 having the drain terminal connected to first input terminal 43 of transformer 16, and the source terminal connected to output terminal 26b of rectifying bridge 26. Second intermediate 29c of second voltage divider 29 is connected to the gate terminal of an N- channel MOS transistor 31 having the drain terminal connected to second input terminal 44 of transformer 16, and the source terminal connected to output terminal 26a of rectifying bridge 26. MOS transistors 30, 31 have a low saturation resistance R ds,on, so as to have practically no drop between the source and drain terminals, when turned on, and may be defined, for example, by Harris components RFD 3055. A first diode 33 has the anode connected to first intermediate node 28c of first voltage divider 28, and the cathode connected to second terminal 44 of transformer 16; and a second diode 34 has the anode connected to second intermediate node 29c, and the cathode connected to first terminal 43 of transformer 16. A capacitor 35 extends between the first and second input terminal 43, 44 of transformer 16, and forms a resonant circuit together with primary windings 41, 42 of transformer 16. The output terminals 47, 48 of transformer 16 are connected to bulb 12; a ballast capacitor 50 is located between first output terminal 47 of transformer 16 and bulb 12; and an inductor 51 is interposed between first output terminal 26a of rectifying bridge 26 and node 45, and acts as a current/voltage converter to isolate the direct current from rectifying bridge 26 with respect to the pulsating current supplied to transformer 16. Supply circuit 11 in Figure 3 operates as follows. Between output terminals 26a and 26b, rectifying bridge 26 supplies a low direct voltage (typically 15 V) which is supplied to identical voltage dividers 28, 29. When supply circuit 11 is turned on, an increasing voltage is therefore supplied between output terminals 26a and 26b, and an accordingly low voltage is established at intermediate nodes 28c, 29c. At this step, though voltage dividers 28, 29 and transistors 30, 31 are identical, minor differences in the actual components cause one of the two transistors 30, 31 to be turned on first, thus preventing the other from being turned on. For example, assuming transistor 31 is turned on first, this connects respective input terminal 44 to a potential close to that of output terminal 26b, so that first diode 33 connected to output terminal 26b is turned on and biased directly. Consequently, intermediate node 28c is forced to the potential of output terminal 26b of rectifying bridge 26 plus the drop across first diode 33, thus preventing transistor 30 from being turned on. The other diode (34 in this case) , on the other hand, connects first output terminal 26a of rectifying bridge 26 to the other terminal (in this case, first input terminal 43) of transformer 16, so that, between input terminals ,43, 44 of transformer 16, a voltage is supplied to activate the resonant circuit defined by primary windings 41, 42 and by capacitor 35, and so generating an alternating voltage. More specifically, as the positive half-wave voltage falls, diode 34 forces second intermediate node 29c to follow the potential of first input terminal 43, thus eventually turning off transistor 31; and, during the negative half-wave, diode 33 is disabled, so that transistor 30 can be turned on. As the positive and negative half-waves succeed one another, transistors 30 and 31 are therefore turned on and off alternately to regulate the frequency of the alternating voltage at input terminals 43, 44 of transformer 16. The alternating voltage so generated is multiplied by transformer 16 to the amplitude required to power bulb 12 (e.g. 500V) . Supply circuit 11 in Figure 3 therefore supplies the power required by bulb 12 using only two primary windings 41, 42 and low-cost components, such as transistors 30, 31. The advantages of the cold-cathode lamp according to the present invention will be clear from the foregoing description. In particular, by arranging supply circuit 11 in portion 10b of head 4 of the lamp, by using a printed circuit board 14 with a hole beneath transformer 16, and by virtue of the electronics employed, supply circuit 11 is compact and cheap enough for lamp 1 to be used as a table or] display window lamp. Moreover, the articulating structure described enables the lamp to be oriented freely as required by the user. Using a two- as opposed to a three-primary transformer provides for reducing not only the cost, as indicated above, but also consumption of the lamp; and using low-resistance MOS transistors provides for reducing the power dissipated by operation of the drive circuit, and hence the amount of heat generated. • Being located inside second portion 10b of head 4, supply circuit 11 is safely insulated from the outside, thus ensuring lamp 1 is electrically safe. Clearly, changes may be made to the lamp as described herein without, however, departing from the scope of the present invention. In particular, by appropriately designing electric connector 20, head 4 may be made fully replaceable to prevent user access to the high-voltage components of supply circuit 11 when changing bulb 12. If the lamp is used for lighting a large display window area, the head may be designed to comprise a further housing for a second bulb controlled by the same supply circuit by simply providing an additional ballast capacitor.

Claims: CLAIMS 1) A table or display window lamp (1) comprising a supporting structure (2, 3) and a head (4) connected in articulated manner to said supporting structure; characterized in that said head (4) comprises a first portion (10a) housing a cold-cathode bulb (12) ; and a second portion (10b) housing a supply circuit (11) for supplying said cold-cathode bulb. 2) A lamp as claimed in Claim 1, characterized in that said supply circuit (11) comprises a transformer (16) having a first and a second primary winding (41, 42) in series with each other, and a secondary winding (46) connected to the supply cables (21) of said cold-cathode bulb (12) . 3) A lamp as claimed in Claim 2, characterized in that said supply circuit (11) comprises an input rectifier (26) having a first and a second output node (26a, 26b) ; a first and a second bias circuit (28, 29) connected to each other in parallel between said first and said second output node of said input rectifier; and a first and a second control transistor (30, 31) connected to respective bias circuits (28, 29) and to said first and said second primary winding (41, 42); said control transistors being low-saturation-resistance MOS types . 4) A lamp as claimed in Claim 3, characterized in that said bias circuits (28, 29) each comprise a voltage divider having an intermediate node (28c, 29c) ; in that each said control transistor (30, 31) has a gate terminal connected to a respective intermediate node (28c, 29c) of a respective said voltage divider; in that said first control transistor (30) has a first terminal connected to said first primary winding (41) , and a second terminal connected to said second output node (26b) of said input rectifier (26) ; and in that said second control transistor (31) has a first terminal connected to said second primary winding (42) , and a second terminal connected to said second output node of said input rectifier. 5) A lamp as claimed in any one of Claims 2 to 4, characterized in that said supply circuit (11) is connected to a printed circuit board (14) having an opening (15) at said transformer (16) . 6) A lamp as claimed in Claim 5, characterized in that said printed circuit board (14) carries an electric connector (20) facing an articulation portion (13) connected to said supporting structure (2, 3); and in that said transformer (16) is housed in said second portion (10b) in a remote position with respect to said first portion (10a) . 7) A lamp as claimed in Claim 5 or 6, characterized in that said second portion (10b) is in the form of a closed parallelepiped, and surrounds said printed circuit board (14) on all sides. 8) A lamp as claimed in any one of the foregoing Claims, characterized in that said supporting structure comprises a base (2) ; and an upright (3) connected in articulated manner to said base to enable rotation of said upright, with respect to said base (2) , about an axis perpendicular to said upright. 9) A lamp as claimed in Claim 8, characterized in that said upright (3) is connected in articulated manner to said head (4) to enable rotation of said head about an axis perpendicular to said upright (3) , and rotation of said head about an axis coaxial with said upright. 10) A lamp as claimed in Claim 8 or 9, characterized in that said upright (3) is telescopic.

Inventors: GISMONDI ERNESTO, GISMONDI, ERNESTO
Applicants: ARTEMIDE SPA, GISMONDI ERNESTO, ARTEMIDE S.P.A., GISMONDI, ERNESTO
Application year: 2000
Main classification: H05B41
IPC codes: F21V21, F21V23, H05B41