Drive arrangement for portable fluorescent tube light fittings

Technical field: The present invention relates to a drive arrangement for portable fluorescent tube light fittings comprising input terminals for connection to a supply voltage, a ballast connected in series Kith a fluorescent tube with two filaments and a capacitor.

Technical problem:

Conventional solariums are relatively heavy due to the fact that the associated drive arrangement contains heavy and cumbersome components. The degree of efficiency is low, furthermore, resulting in high heat emission, which in many cases is experienced as being inconvenient. It is for this reason that portable solariums have generally been of a smaller type for sunning a part of the body. Full—length solariums have until now been executed as stationary units because of the high weight and space requirement of the drive arrangement, The object of the present invention is to propose a drive arrangement by means of which a considerable reduction in the weight of a portable fluorescent tube light fitting and an increase in the efficiency are achieved, in such a way as to reduce the disadvantageous emission of heat.

Solution:

The aforementioned object is achieved by means of a drive arrangement which is characterized in that the drive arrangement comprises at least two controllable electronic rectifiers so arranged together in the form of a push—pull circuit as to permit to flow only a generated alternating voltage with a relatively low frequency exceeding the frequency of the a.c. mains supply for the purpose of actuating an oscillating circuit for driving the fluorescent tubes with a current passing through the rectifiers at a considerably higher frequency for. the purpose of starting and operating the fluorescent tube.

Brief description of the drawing:

The invention is explained below in more detail as an illustrative embodiment, with reference to the accompanying drawing in which there is illustrated an example of a wiring diagram for the drive arrangement.

Preferred embodiment: .

Shown in the Figure is a wiring diagram for the drive arrangement in accordance with the invention in the form of an example which is connected at its input end via two input terminals 1, 2 to an a.c. mains supply, for example 220 volt. The drive arrangement exhibits two pairs of output terminals, 3, 4, 5, 6, 7, 8, 9, 10. for each fluorescent tube 11, 12 which is connected to the drive arrangement. Illustrated in the example are two fluorescent tubes connected in parallel to the drive arrangement. Depending on the power supplied, it is possible for a large number of fluorescent tubes to be driven simultaneously, for example seven 100— att fluorescent tubes.

Included in the drive arrangement is a full-wave rectifier 13 in the form of a diode bridge with four diodes 14—17. The rectifier 13 is connected to the two input terminals 1, 2 of the drive arrangement for the rectification of the incoming a.c. voltage. Connected in parallel with the rectifier is a 'bleeder* resistance 18, so arranged as to discharge a filter capacitor 19 for the purpose of filtering the d.c. voltage emitted from the rectifier 13. A resistor 20 of low resistance connected in series with the capacitor 19 is so arranged as to permit reactive power and switching current to flow through the resistor. A capacitor 21 connected in parallel with the outlet from the rectifier 13 compensates for any unavoidable line inductances and is connected as close as possible to the terminals 22, 23 of the field effect transistors 24, 25 contained in the drive arrangement, which transistors are two in number in the example shown here.

A saw-tooth generator is constituted by a resistor 26, a capacitor 7 and a diac 28 for the purpose of generating a saw—tooth voltage, the frequency of which is dependent to a high degree on

the input voltage, The breakdown voltage for the diac element 28 should preferably be of the order of 30 V. The field effect transistors 24, 25 constitute voltage-controlled, rapid-switching rectifiers or electronic switches via which the power supply to the fluorescent tubes 11, 12 takes place. A transformer 26 is connected to the control electrodes 42, 43 of the field effect transistors24, 25 and exhibits three windings 29, 30, 31 around a common core 32 made of a ferritic material. A capacitor 33 isolates the d.c. components in such a way that only a.c. voltage is supplied to the fluorescent tubes 11, 12, Arranged in series with each of these is a ballast 34, 35 which, since the drive arrangement operates at a relatively high frequency, can be given considerably lower inductance than in conventional fluorescent tube light fittings, and thus exhibits considerably lower weight and volume. The space requirement for the drive arrangement is reduced in this particular example by a factor of from 5 to 10 times, for a given power output. Arranged in parallel with each of the fluorescent tubes is a capacitor 36, 37 which together with the ballasts 34, 35 forms a series resonance circuit for each fluorescent tube at a high frequency, in this particular case of the order of 25 kHz, relative to the mains frequency of 50 Hz, i.e. the frequency of the supply voltage to the drive arrangement. The frequency is determined by the specified quantities for the capacitors 36, 37 in the first place and the ballasts 34, 35. By selecting the aforementioned frequency range, rather than a considerably higher frequency, for example 150 kHz, problems of radio interference are avoided.

By positioning the ballast 35 for every other fluorescent tube 12 on the opposite side of the fluorescent tube in relation to the position of the ballast 34 for the preceding fluorescent tube 11, every other fluorescent tube will be supplied in opposition, which provides a more even voltage distribution and thus less radiation of electromagnetic interference signals, that is to say less radio interference.

The drive arrangement in accordance with the invention functions in the following manner: when an alternating voltage is supplied to the input terminals 1, 2 for the drive arrangement.

the capacitor 27 is charged via the resistor 26, When the voltage across the capacitor 27 reaches the breakdown voltage of the diac component 28, this is fired and emits a short pulse to the control electrode 43 of the transistor 25. On each pulse a pulse—shaped 'current flows via the transistor 25, the winding 29 of the trans¬ former 26, the ballasts 34, 35, the capacitors 36, 37, the heater windings 38, 39 of the fluorescent tubes and the capacitor 33. The saw—tooth generator emits trigger pulses at about 600 Hz, in conjunction with which periodically decreasing voltages are induced via the winding 29 of the transformer 26 also in the other two windings 30, 31. Shortly after the firing of one of the transistors 25, and at the earliest one half—period later, and if the necessary voltage has been reached at the control electrode 42 of the other transistor 24, this transistor will also be fired, in conjunction with which the two transistors 24, 25 will be caused to work in push-pull fashion and to commute on passing through zero and to switch at a frequency exceeding the operating frequency. During this phase the pulses which pass through the heater windings 38, 39 of the fluorescent tubes 11, 12 will result in attempts at firing, and once these heater windings, that is to say the glow wires, have become hot and the necessary voltage has been reached, for example a voltage of the order of 1000 to 2000 V, firing will take place, and once the gas inside the fluorescent tubes, which are of the fluorescent discharge type, has been ignited causing the fluorescent tubes to glow, the voltage will drop to about 250 V. Even before this point is reached, the series resonance circuit formed by the capacitors 36, 37 and the ballasts 34, 35 for each fluorescent tube will thus have resulted in the desired high frequency of about 25 kHz having been reached. The oscillation of the series resonance circuits is maintained as a consequence of the feedback over the windings 29 and 31 in the transformer 26. Once this natural oscil¬ lation has started, the saw-tooth generator is stopped via a diode 40, since the capacitor 27 is discharged via the diode 40 and the transistor 25. It is thus not possible for a voltage which might be capable of firing the diac element 28 to occur across the capacitor 27.

By the precisely specified dimensioning of the constituent component parts a drive arrangement has been achieved which is particularly well suited to use in portable fluorescent tube light fittings for solariums. Because a suitable frequency of oscillation has in this way been selected for the voltage across the fluorescent tubes during operation, the possibility has been provided for achieving a radical reduction in the weight and the volume of the constituent ballasts, which, for relatively high outputs of, for example, 7 x 100 Watts, would otherwise require to be of consider— able weight and volume, without introducing problems of radio interference. The heat losses are also reduced in relation to conventional solariums due to the greater efficiency^ so that an unchanged level of sunburn is achieved without the uncomfortable feeling of the body heating up. The invention is not restricted to the illustrative embodiment described above and illustrated in the drawing, but may be varied within the scope of the following Patent Claims _* For example, other types of electronic rectifiers or switches may be selected. These can also be connected in parallel up to a relatively large number in order to permit the fluorescent tubes to be driven with greater effect. It must also be stated that temperature measurements performed on the fitting have revealed that the temperature was about 8° lower at the centre of the tube and about 30° lower at the ends compared with conventional fittings which are not driven electronically. The service life of the fluorescent tubes should accordingly increase in relation to the earlier types. The fitting can also be used as a light table for drawing work, etc., or for the occasional lighting of a construct¬ ion site or similar..