EP0028986A1 - Feeding device for a discharge lamp with preheating circuit - Google Patents

Abstract

1. Device for providing power to a discharge lamp and particularly to a fluorescent tube, comprising a source of AC electrical current (31) which feeds a transformer delivering, on a first secondary winding (33) a high voltage for maintaining an arc in the tube and, on second secondary windings (34, 35), voltages for pre-heating the electrodes, and switching means comprising a thermistor having a positive temperature coefficient, which shortcircuit the first winding (33) until a delay has expired from the application of voltage to the device, characterized in that said thermistor is located in series with a voltage threshold element (38).

Description

The subject of the present invention is a device for supplying a discharge lamp and in particular a fluorescent tube, comprising a source of alternating electric current which supplies a transformer providing, on a first secondary, a high arc maintenance voltage in the tube and, on a second secondary, preheating voltages of the electrodes, and switching means comprising a thermistor, which short-circuit the first secondary until a time has elapsed from the power up of the device.

Such a device (patent FR-A-2 266 421) reduces the blackening of the tube caused by successive ignitions of the latter, because, thanks to preheating, each electrode has emitted a sufficient electron cloud before the voltage Arc striking is applied. The blackening, due to a progressive tearing off of the active part of the cathode and to the deposition of the particles torn off on the walls, is thus reduced.

In return, this device has drawbacks. As the thermistor remains permanently energized, it constitutes a non-negligible source of heat and reduces the yield. In the event of an interruption in the power source and immediate re-energization, the resistor does not have time to cool down and the fluorescent tube reboots without preheating.

The present invention aims to provide a device avoiding the above drawbacks, in particular in that it does not affect the yield and that it practically does not dissipate heat in normal operation.

For this purpose, the invention proposes a device of the above-defined type in which the thermistor, constituted by a resistance with a positive temperature coefficient (called PTC) is placed in series with a voltage threshold element. This element can in particular be constituted by a discharge lamp, a set of threshold diodes (Zener diodes in particular) or any other member which does not start until a voltage higher than a determined threshold is applied to it.

Dans.un advantageous embodiment, the two secondary are placed in series and associated with a common primary.

It should be noted that the operation obtained is different from that of a system comprising a preheating circuit in which a threshold element is provided on part or all of the secondary of the supply transformer. If, apparently, this solution is advantageous, because the threshold element is in service only during preheating, in practice it faces serious drawbacks. The threshold voltage of the element must lie within a very narrow range, since it is limited above by the arc voltage without preheating and, below, by the arc voltage with preheating. The dispersion of characteristics of commercially available threshold elements requires individual verification of the elements. In addition, the ignition voltage with preheating increases with the aging of the fluorescent lamp: as soon as it exceeds the threshold voltage of the circuit element, the element dissipates significant power and heats up. Finally, the decommissioning of the lamp by wear generally leads to the destruction of the threshold element.

In the case of the invention, the threshold voltage of the element is chosen so as to be slightly higher than that corresponding to the operating voltage of the fluorescent lamp. In the event of parallel mounting on only a fraction of the secondary, this threshold voltage will obviously be chosen as a function of the voltage ratio corresponding to the number of turns in parallel.

As will be seen below, the series association of the threshold element and the resistance with positive temperature coefficient. Results in a complete change of operation compared to the use of a resistance alone or a threshold element alone.

In particular, since the resistor is only energized when its intervention is necessary, the power dissipated in this element can be very reduced.

An adjustable resistor will advantageously be fitted in series with the threshold element and the resistance with positive temperature coefficient: it will allow the use of commercially available elements, having dispersed characteristics, without risking malfunction. .

The device according to the invention lends itself perfectly to the operation of the fluorescent tube or tubes at several power levels. Devices are already known which make it possible to reduce the power applied to a fluorescent tube, the adjustment element of which consists of a self-induction coil placed in series between the secondary winding and the lamp. This coil is short-circuited to operate at nominal power. It is left in circuit to operate at reduced power, for example by 50%. According to a secondary characteristic of the invention, the device comprises a capacity placed in parallel on the primary winding of the transformer. The power reduction is carried out by modifying this capacity, for example by constituting it with a variable number of capacitors in parallel. This system has the advantage that a reduction in the power consumed by the tube is accompanied by an increase in the frequency of the self-oscillating circuit supplying the transformer. This increase in frequency reduces the impedance of the capacitors usually placed in series on the preheating secondaries. A higher preheating current then circulates continuously and guarantees that the ignition of the tube supplied under reduced power.

The invention will be better understood on reading the following description of a device without supply choke for at least one fluorescent tube constituting a particular embodiment of the invention, given by way of nonlimiting example , and variants.

• - la figure 1 est un schéma de principe du dispositif ;
• - la figure 2 montre l'allure de la variation de résistance en fonction de la température pour une thermistance du genre couramment dénommé PTC.

The description refers to the accompanying drawings, in which:

• - Figure 1 is a block diagram of the device;
• - Figure 2 shows the shape of the resistance variation as a function of temperature for a thermistor of the type commonly referred to as PTC.

The device shown in FIG. 1 comprises an alternating current source 31 constituted by a converter which supplies the primary 32 common to two transformers. This primary winding 32 belongs to a self-oscillating output circuit of the converter 31, as well as an adjustable capacitor 37. The first transformer comprises a secondary 33 whose terminals are each connected to a corresponding electrode of the fluorescent tube 30 to be supplied. The second transformer includes two additional secondary 34 and 35, placed in series with the secondary 33 and on the same core. Each of the secondary 34 and 35 feeds a corresponding electrode of the tube 30.

Switching means intended to limit the duration of the application of the preheating current connect the terminals of the secondary 33. These means comprise a thermistor 36 with positive temperature coefficient, constituted by a semiconductor wafer whose resistance increases suddenly when its temperature exceeds a predetermined value. This thermistor, or PTC, is placed in series with a threshold element 38, which will advantageously be cons sparkled by a spark gap.

The characteristics of the thermistor 36 and of the threshold element 38 must be chosen appropriately, as a function of those of the tube or tubes 30 supplied.

The thermistor will present a resistance variation curve R as a function of the temperature T of the kind shown in FIG. 2. The resistance R increases very rapidly, when its temperature increases from a value T _m to a slightly higher value T _M. T _m will be chosen so that it is much higher than the maximum ambient temperature that can be encountered during operation.

The thermistor is chosen so that its resistance at ambient temperature is low enough so that the voltage across the secondary 33 is reduced to a small fraction of the voltage with open secondary. The thermistor must also be provided so that a time of the order of 0.3 to 1 second elapses from the instant of application of the voltage until the instant when the temperature of the thermistor exceeds T . As for the value R of the resistance after switching of the thermistor, it must be sufficiently high to cause the arc of the spark gap to blow (in the event of lighting of the lamp 30) or to leave only one low residual current, not exceeding a few milli-amperes (in the case where the fluorescent tube does not light up, for example due to its previous destruction).

As for the spark gap 38, its ignition voltage is chosen to be lower than the ignition voltage of the fluorescent tube 30, when the cathodes thereof are cold. Its maintenance voltage is on its side chosen high enough for the arc in the spark gap to be blown when the thermistor switches, when passing from temperature T to temperature T _M. It must also be able to withstand the passage of the discharge current during the time that elapses from the application of the voltage to the switching of the thermistor.

To avoid having to use elements of precise characteristics, an adjustable impedance 39 is advantageously placed in series with the elements 36 and 38 and allows adaptation to the characteristics of these elements.

Instead of mounting the switching means between the terminals of the secondary 33, these means can be placed so that they only short-circuit a fraction of the secondary 33. In the corresponding arrangement, the adjustable impedance 39, generally constituted by a resistance, is replaced by an impedance placed in 39a.

Instead of a spark gap, many other components can be used as threshold element 38, such as in particular a discharge lamp, a set of two Zener diodes mounted head to tail, a varistor, etc.

The device can obviously be supplemented by additional components in addition to those which have just been described; in particular, capacitors 40 may be placed in series on the preheating circuits, so as to reduce the preheating current once the lamp is on. Another capacitor can be placed in parallel on the windings to ensure a suppressor function.

The operation of the device which has just been described comprises three phases, from the energization of the cold tube.

After energizing, until the voltage across the threshold element 38 reaches the ignition threshold voltage, the cathodes of the tube are preheated to a voltage fixed by the transformation ratio between primary 32 and secondary 34 and 35. It should be noted that the thermistor 36 does not absorb any power. As a result, there is no delay in preheating, whereas, if it were mounted alone, there would be current flow in the switching means and risk of saturation of the transformer ferrite, due to the low resistance of the cold thermistor.

The second phase begins as soon as the ignition threshold voltage of the element 38 is exceeded. The switching means 38, 36 and 39 constitute a parallel circuit on the secondary 33, a circuit whose resistance remains low as long as the thermistor 36 has not reached the temperature T.

As soon as this temperature is reached, the resistance increases considerably, in a ratio which can reach or exceed 200: the short-circuit of the secondary 33 ceases and the ignition voltage appears at the terminals of the tube 30. The voltage at the terminals of the spark gap decreases, typically by about half, and the spark gap arc is blown out.

During the third phase, the operating voltage of the fluorescent lamp being much lower than the starting voltage (at least half), the transformer delivers only this voltage at most. The voltage across the threshold element 38 remains below the maintenance voltage, even when the thermistor 36, which is no longer energized, has returned to ambient temperature. The threshold element is advantageously provided so that the thermistor does not reach the temperature T _M. Thus, a second ignition can take place very quickly after a first extinction (5 to 10 seconds), which would be impossible if the thermistor was mounted alone.

As an example, we can give the characteristics of a 13 W fluorescent tube supply device having an operating current of 250 mA, an arcing voltage after preheating of 140 V and a voltage of 97 V operation. The device includes a thermistor, the resistance goes from 500 n at 25 ° C to 40 kn after switching and a spark gap with a starting voltage of 150 V and an operating voltage of 80 V. This spark gap could withstand a current of 2 A * for one second, superabundant for a tube operating current of 250 mA.

The device can obviously be modified to power several fluorescent lamps placed in series. In this case, the secondary 33 will generally be mounted so as to supply all the lamps in series. The terminal cathodes of the series connection will each be supplied by transformers 34 and 35. The intermediate cathodes may be grouped in series two by two, each pair being supplied by a separate secondary or on the contrary supplied by independent secondary.

The presence of the capacitor 37 makes it possible to operate the tubes at several power levels. By increasing the value of the capacitor 37, it is possible both to decrease the applied power and to increase the frequency. This increase in frequency causes a decrease in the impedance of the capacitors 40, therefore an increase in the preheating current which guarantees that the tube remains on despite the lower discharge voltage.

The device which has just been described is susceptible of numerous variants and numerous applications other than to a fluorescent tube. In particular, it can be used to power sodium vapor lamps under pressure and then solves the problem of re-lighting these lamps after extinction, re-ignition which, with conventional devices, is only carried out after cooling of the lamp.

Claims (5)

1. Device for supplying a discharge lamp and in particular a fluorescent tube, comprising a source of alternating electric current (31) which supplies a transformer providing, on a first secondary (33), a high maintenance voltage for the arc in the tube and, on a second secondary (34, 35), preheating voltages of the electrodes, and switching means comprising a thermistor, which short-circuit the first secondary (33) until a delay has elapsed from the power-up of the device, characterized in that the thermistor, consisting of a resistor with a positive temperature coefficient (36), is placed in series with a voltage threshold element (38). 2. Device according to claim 1, characterized in that the threshold element is constituted by a discharge lamp, a set of threshold diodes, a set of Zener diodes, or a spark gap. 3. Device according to claim 1 or 2, characterized in that the switching means comprise an adjustable resistor in series with the threshold element and the thermistor. 4. Device according to any one of the preceding claims, comprising capacities in series in the preheating circuits, characterized in that it comprises a variable capacity for adjusting the level of illumination in the primary circuit of the transformer. 5. Device according to any one of the preceding claims, characterized in that the secondary (33, 34, 35) are in series and associated with a common primary (32).

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