Background Art To turn. on a preheated internal-discharge fluorescent lamp it is necessary to supply an a. c. current at a predetermined frequency to a resonating circuit applied to the lamp, such that the voltage at the terminals of the lamp builds up gradually and a current begins to flow. The energy issued by the ions of the low-pressure gas within the tube causes fluorescence, either of the atoms of vapour within the tube or of phosphor coating the inner surface of the tube.

Fig. 1 illustrates how such an oscillating current is produced by a conventional starting circuit reactor. The power supply, which is rectified and filtered upstream of the illustrated circuit, provides a d. c. voltage of 310 V. The capacitor 10 is charged through the resistor 20 until a point that the diac 30 conducts, discharging all the energy stored in the capacitor 10 onto the base of the transistor T2, turning the transistor T2 on. A current thus passes through inductors 40 and 50 (flowing from right to left in Fig. 1) and is grounded by transistor T2.

When the current passes through the inductor 50, a mutual inductance with inductors 60 and 70 is created, the three inductances being arranged so as to form a three coil transformer. As a result, by virtue of the suitable orientation of the primary inductor 50 and of the secondary inductors 60 and 70, transistor T2 is cut off and transistor T1 is activated in its saturation region.

When transistor Tl starts conducting, the current passing through the inductor 40 changes direction since the current now comes from the emitter of transistor T1 and flows towards the load (from left to right in Fig. 1). By driving the two transistors T1 and T2 in this way, i. e. by causing their alternating conduction, a square wave current is created

which causes resonance of the circuit formed by the exit inductance 50, capacitor 80 and the internal connections of the lamp 90, thereby provoking the ignition of the lamp 90.

The inductor 40 performs a current limiting function during running of the lamp 90.

A reactor as described above is able to turn on only a specific kind of lamp, having a predetermined power (for example 18 watts, 36 watts, etc.) ; it cannot be used for lamps having different power ratings or characteristics (for example when different gases are contained in the lamp).

Conventional reactors have the same life as that of the lamp. When the lamp no longer operates (for example when the low-pressure gases no longer react with the phosphor coating) but remains connected to the power supply, a high energy dissipation is generated due to the repeated turning-on of the cathode.

Disclosure of the Invention It is an object of the present invention to substantially overcome, or at least ameliorate, one or more disadvantages of existing arrangements.

Therefore, in one form, there is provided an electronic control circuit for one or more fluorescent lamps, said control circuit comprising: a power buffer responsive to an a. c. voltage supply to produce a first d. c. voltage; an oscillator producing a square-wave signal; a final power stage receiving said first d. c. voltage and said square-wave signal, and providing said square-wave signal to a switching circuit that switches said d. c. power supply into a resonating circuit that produces an amplified alternating output of a level sufficiently high to ignite said one or more fluorescent lamps; and a first protection circuit which is triggered to stop an ignition current when a voltage derived from said switching circuit and proportional to the duration of the ignition period of said one or more fluorescent lamps exceeds a predefined limit.

In another form there is provided a method of controlling the starting operation of one or more fluorescent lamps, said method comprising the steps of : producing a first d. c. voltage from an a. c. voltage supply; producing a square-wave signal; providing said first d. c. voltage and said square-wave signal to a switching circuit that switches said first d. c. voltage into a resonating circuit that produces an amplified alternating output of a level sufficiently high to ignite said one or more fluorescent lamps, said switching circuit supplying the full current demanded by said one or more fluorescent lamps; and triggering a first protection circuit to stop an ignition current if a voltage derived from said switching circuit and proportional to the duration of the ignition period of said one or more fluorescent lamps exceeds a predefined limit.

In another form there is provided an electronic control circuit for one or more fluorescent lamps, said control circuit comprising: a power buffer responsive to an a. c. voltage supply to produce a first d. c. voltage; an oscillator producing a square-wave signal ; and a final power stage receiving said first d. c. voltage and said square-wave signal, and providing said square-wave signal to a switching circuit that switches said d. c. power supply into a resonating circuit that produces an amplified alternating output of a level sufficiently high to ignite said one or more fluorescent lamps, said switching circuit comprising a push-pull transistor circuit in which only one said transistor has a gate shunt diode arrangement, the non-gate shunted transistor allowing said switching circuit to supply the full current demanded by said one or more fluorescent lamps.

In another form there is provided a method of controlling the starting operation of one or more fluorescent lamps, said method comprising the steps of : producing a first d. c. voltage from an a. c. voltage supply; producing a square-wave signal; and providing said first d. c. voltage and said square-wave signal to a switching circuit that switches said first d. c. voltage into a resonating circuit that produces an amplified alternating output of a level sufficiently high to ignite said one or more fluorescent lamps, said switching circuit supplying the full current demanded by said one or more fluorescent

lamps, and wherein said switching circuit comprises a push-pull transistor circuit in which only one said transistor has a shunt diode.

In another form there is provided an electronic control circuit having the capability of starting and running a fluorescent lamp having a power rating of between i 8- 56 watts.

In another form there is provided an electronic control circuit having the capability of starting and running two or more fluorescent lamps of different power ratings in the range 18-56 watts.

In another form there is provided an electronic control circuit for one or more fluorescent lamps, said control circuit comprising: a power buffer responsive to an a. c. voltage supply to produce a first d. c. voltage; an oscillator producing a square-wave signal; a final power stage receiving said first d. c. voltage and said square-wave signal, and providing said square-wave signal to a switching circuit that switches said d. c. power supply into a resonating circuit that produces an amplified alternating output of a level sufficiently high to ignite said one or more fluorescent lamps; a first protection circuit which is triggered to stop an ignition current when a voltage derived from said switching circuit and proportional to the duration of the ignition period of said one or more fluorescent lamps exceeds a predefined limit; and a second protection circuit which is triggered to stop said oscillator producing said square-wave signal if an input current to said resonating circuit is not flowing.

Brief Description of the Drawings In the drawings: Fig. 1 shows a conventional reactor starting circuit connected to a fluorescent lamp ; Fig. 2 is a schematic block diagram of a control circuit for a fluorescent lamp ; Fig. 3 shows a detailed circuit diagram of the circuit shown in Fig. 2.

Fig. 4 is a schematic block diagram of another control circuit for fluorescent lamps;

Fig. 5A and Fig. 5B show a detailed circuit diagram of a control circuit according to the block diagram of Fig. 4; Fig. 6 (a) and Fig. 6 (b) show a further control circuit for fluorescent lamps; and Fig. 7 (a) and Fig. 7 (b) show a further control circuit for fluorescent lamps.

Detailed Description Including Best Mode First Arrangement In a first arrangement, a fluorescent lamp reactor circuit comprises a series of main blocks as illustrated in Fig. 2.

A low-pass filter 100 is formed by a double-phase coil and a capacitor, and serves to filter the a. c. power supply, which can vary from a minimum of 220 V to a maximum of 280 V. It should be appreciated that by adding a suitable power-factor pre-regulator upstream of the illustrated circuit, it is also possible to operate the circuit at a mains voltage of between 110 V and 125 V.

The power buffer 110 is formed by a diode bridge between the ends of which an selectrolytiepolarisedcapacitorislocated. The low-voltage buffer 120 is connected in parallel to the low-pass filter 100 and comprises a polyester capacitor, an electrolytic capacitor and a diode bridge. The stabiliser circuit 130 exploits the amplification of a BJT (Bipolar Junction Transistor) to provide a suitable input voltage to the downstream components. The oscillator 140 is implemented by an integrated CMOS circuit, and the driver stage 150 comprises high-frequency transformers and components adapted to the control and protection of the apparatus. The final high-frequency power stage 160 uses a pair of N-channel MOSFET transistors. The circuit also includes protective circuitry 170.

The oscillator 140 and the driver stage 150 can each be substituted by corresponding programmable logic units. The protection circuit 170 can also be replaced by a programmable logic unit programmed so that the circuit achieves the same results and is still capable of being connected to any kind of lamp associated with the reactor.

A more detailed description of the circuit of Fig. 2 will now be given with reference to Fig. 3.

The mains voltage is supplied through the fusible resistor Rl and is fed to the low-pass filter 100, comprising the capacitor Cl and two coils 200,202 which are push- pull wound on a toroidal core. The mains voltage, rectified by the diode bridge PD1 and filtered by the electrolytic capacitor C2, is applied between the drain of the transistor MS1 and the source of the transistor MS2, with a value of about 320 V.

The oscillator 140 and the driver 150 are powered by voltages lower than that required by the final stage 160. Instead of using resistors or transformers, which create heating problems in traditional ballasts, the required low-voltage power supply is obtained from the mains supply using the diode bridge PD2 and the capacitor C10 connected between the mains and the low-voltage stages. The capacitor C10, in particular, provides the necessary voltage drop, furnishing a suitable impedance (about 4,700 Q) at a frequency of 50 Hz without an undue increase in temperature. The output voltage provided by the capacitor C10 and diode bridge PD2 is limited to 15 V by the zener diode DZ3. This voltage is further reduced to a value of about 12 V by the stabiliser circuit 130 which makes use of the bipolar junction transistor TR3. This reduced voltage feeds the oscillator circuit 140 and the stabiliser circuit 130. The oscillator 140 comprises a base oscillator, whose operational frequency of 65 ldIz is fixed by the resistor R8 and the capacitor C7, and by the two pairs of inverters, AB and CD, that are used by bridge- connected followers. The square wave (with a duty-cycle of 50%) produced by the base oscillator is applied to the transistor TR1 and, in a push-pull way, to the transistor TR2.

The two transistors TR1 and TR2 in turn transfer the signal to the final power stage 160 through two high-frequency transformers Tl and T2.

The two final N-channel MOSFET transistors MS1 and MS2 are brought into conduction alternately, forming a switching circuit which creates a square wave at the input of the coil LI having an amplitude of 150 V and an initial current of 0.3 A, as required for the preheating of the lamp 90. The capacitor C5, connected in parallel with the lamp 90, begins to resonate with the exit coil Ll, thus increasing the voltage to about 500 V. This leads, after about 0.7 seconds, to the ignition of the lamp 90.

Inside the final stage 160, the conduction of the MOSFET MS2 is limited by the zener diode DZ1, since it is necessary to limit the current returning to the negative end of

the diode bridge PD1. The MOSFET MS1, in contrast, does not have a diode across its gate-source junction. This arrangement allows the reactor to turn on lamps of different power and/or kind without any modification to the circuit. Since the conduction of the transistor MS1 is not limited, its output adjusts as a function of the load features of the lamp 90. Substantially, transistor MS1 works as a potentiometer and since it outputs the exact current needed by the lamp, it avoids undue dissipation and unwanted power wastage. The usual lighting period of a lamp is about 0.7 seconds, as mentioned above.

After this period, the circuit stops operating.

It should be appreciated that in the event the lamp 90 is broken or has finished its normal life, the circuit will continue to apply an ignition current to the end terminals of the lamp. This may provoke accidents caused by any direct contact with the electrodes, and is in addition a waste of energy. The protection circuit 170 is connected to the final stage 160 in order to overcome this problem. The protection circuit 170 measures the ignition time with a current which is proportional to the ignition current. This proportional current is obtained from the final stage 160 by means of the transformer T3, and is rectified by means of the diode D3. This current charges the capacitor C6 such that the voltage measured across the capacitor C6 is substantially proportional to the time passed since the reactor was connected to the mains supply. When this voltage exceeds a predetermined value (calculated as a function of the correct ignition time), the thyristor SCR1 starts conducting and cuts off the oscillator power supply.

Since the ignition time and the ignition current (about 300 mA) must be the same for any kind of lamp, the protection circuit 170 is able to detect breakdowns related to any kind of load. If for any reason the ignition phase is prolonged, the charge accumulated on capacitor C6 causes the voltage at the ends of capacitor C6 to exceed 0.8 V. The thyristor SCR1 then begins conducting and, as it is connected to the base of transistor TR3, sends the latter to ground, thus preventing the stabiliser circuit 130 from powering the oscillator 140. The control square-wave on the final components is thus eliminated and the output current is stopped.

When the protection circuit 170 is activated, the diode bridge PD2 is closed on the diode DZ2 and on the resistor R13, resulting in a current of only 77 mA. Further, the

capacitor C9 is protected by the zener diode DZ3, which prevents an excessive voltage drop at the ends of capacitor C9.

In order to substitute the broken lamp with another having different characteristics, it is necessary to turn off the circuit for a few seconds, to permit capacitor C6 to discharge to ground. On the other hand, a lamp, identical to the one broken or exhausted, can be connected without adopting any particular change-over arrangement.

Second AYraragenaefit A second arrangement of a control circuit for fluorescent tubes is described with reference to Fig. 4, Fig. 5A and Fig. 5B.

Fig. 4 shows a conceptual block diagram illustrating the functioning of the control circuit.

The line voltage is supplied to a low-pass filter 400. The output of the low-pass filter 400 is rectified and further filtered by the the circuit block 402. The output of the rectifier/filter 402 provides an input to the final power stage 414.

The line voltage is also supplied as an input to a low-voltage buffer 408. The output of the low voltage buffer 408 passes through a stabiliser circuit 410 to power the circuit block 412, which is an oscillator and a driver for the final power stage 414. The oscillator/driver 412 provides a square-wave signal to the final power stage 414, which is connected to the lamp 90.

A preheating circuit 404 provides an initial current for preheating the lamp 90.

The input to the preheating circuit 404 is the output of the rectifier/filter 402.

The circuit of Fig. 4 also contains two protection circuits, 406 and 416. The protection circuit 416 measures a current proportional to the ignition time of the lamp 90.

If the lamp 90 has not lit up within a specified time, the protection circuit 416 acts via the stabiliser circuit 410 to cut off the supply voltage to the oscillator/driver 412. The protection circuit 416 thus, when activated, acts to switch off the electronic ballast.

The protection circuit 406 verifies the presence of a lamp in the circuit. A current flowing between the final power stage 414 and the lamp 90 is monitored. If no lamp has been inserted the current will not flow and the protection circuit 406 acts on the stabiliser circuit 410 to cut off the supply voltage to the oscillator/driver 412, thereby switching off the electronic ballast. The protection circuit 406 thus prevents injuries that may occur if a user were to touch a live lamp-fitting with no lamp connected.

A detailed circuit diagram corresponding to the block diagram of Fig. 4 is shown in Figs. SA and 5B. Fig. 5B is a continuation of Fig. SA, and the outputs 620a, 622a, 624a and 626a of Fig. SA correspond to the inputs 620b, 622b, 624b and 626b of Fig. 5B.

The line voltage is supplied through the fusable resistor 500 and the NTC resistor 504 to a low-pass filter consisting of the capacitor 510, the capacitor 550 and the two coils 512 wound in counter-phase on a u-shaped ferrite core.

The output voltage of the low-pass filter is rectified by the diode bridge 552 and is further filtered by the capacitive-inductive set consisting of the capacitor 554, the inductor 556, the capacitor 558 and the resistor 560. The components 552,554,556,558 and 560 correspond to the rectifier/filter circuit 402 of Fig. 4.

The voltage across the resistor 560 (that is, the output of the rectifier/filter 402) is applied to the collector of transistor 608 and the emitter of transistor 610 with an amplitude of 320 volts. The transistors 608 and 610 form part of the final power stage 414 of Fig. 4.

The line voltage is also stepped down in order to drive the oscillator 412. This function is performed by the capacitor 508 and the resistor 506, together with diodes 514 and 516. The voltage provided to the downstream circuitry is limited by the Zener diode 520 which is connected to the base of the stabiliser transistor 526. The voltage is filtered by the capacitor 524.

The voltage across the capacitor 524 is further stabilised by the circuit consisting of the resistor 536, the Zener diode 538 and the transistor 534. The components 534,536 and 538 form part of the stabiliser circuit 410 of Fig. 4.

The output voltage of the stabiliser circuit 410 serves to energise the downstream oscillator circuit, which also functions as a driver for the final power stage 414.

The oscillator is formed by the base oscillator 576, which has an operational frequency of 44 kHz set by resistor 574 and capacitor 572. The oscillator provides a square wave to the primary winding of the transformer 606 via the two inverter pairs 586, 588, 590,592, connected in a bridge circuit, the diodes 578,580,582,584 and the capacitor 604. The square wave is provided to the transformer 606 with a 50% duty cycle.

The voltage provided by the secondary windings of the transformer 606 is applied to transistor 608 and, inverted by 180°, to transistor 610. Transistors 608 and 610 work in an alternating conductance mode to generate a square wave with an amplitude of about 150 V and an initial current of 0.001 A, as required for the preheating of the lamp. The square wave, via connection 626a and 626b is applied to the input of the coil 648 which is part of a resonant circuit connected to the lamp 90.

Also seen in Fig. 5B is timer circuitry corresponding to the preheating circuit 404 of Fig. 4. Transistors 632 and 636 provide a current of about 0.100 A via the relay 662 in order to preheat the lamp 90. The preheating circuit has a delay time of between 0.4 seconds and 0.7 seconds, depending on the value chosen for capacitor 644. For a time period determined by capacitor 644, the relay 662 connects the additional capacitor 658 to the capacitor 656 that is connected in parallel with the lamp 90. While the capacitor 658 is connected, a preheating current flows, maintaining the voltage at the ends of the lamp to about 250 V.

At the end of the time period determined by capacitor 644, the relay 662 isolates capacitor 658 from capacitor 656 and as a result the inductor 648 enters into resonance, increasing the voltage across the lamp 90 to about 1,000 V and causing the lamp 90 to light up.

The final power stage formed by transformers 608 and 610 supplies a square wave to inductor 648. The capacitor 650, which is connected in series with the inductor 648 converts this square wave to a triangular wave, thereby reducing the time of maximum conductance of the transistors 608 and 610 and, consequently, the temperature generated.

Furthermore, the voltage at the lamp 90 is increased without need of a further power increase from the start, with the lamp 90 getting into the'SOFT LINGT'ignition mode.

In the absence of protection circuits, current would still flow to the lamp fitting even when the lamp is broken or has reached the end of its life cycle. Besides causing unnecessary power consumption, injury could be caused if a person made direct contact with the live electrode.

In a first protection circuit, shown as 416 in Fig. 4, a charge is accumulated in the capacitor 594 in proportion to the ignition time. If ignition occurs normally, the protection circuit is not activated. However, if the lamp 90 does not ignite within a specified time, the charge in capacitor 594 passes a threshold and the protection circuitry is activated.

The capacitor 594 is charged in accordance with the potential difference across resistor 612, via diode 602, resistor 600 and the parallel combination of resistor 596 and the capacitor 594.

During normal operation, the voltage across the terminals of capacitor 594 remains below a preset critical value (set at 0.8 V) and the thyristor 540 is not activated. If for some reason the ignition phase lasts too long, the accumulation of charge in capacitor 594 takes the voltage across its terminals beyond 0.8 V and the thyristor 540 starts conducting. When thyristor 540 starts conducting, it takes the base of the transistor 534 to ground. The effect of this is to cut off the supply power to the oscillator 576, thus eliminating the controlling square wave ordinarily supplied to the inductor 648. In effect, if the ignition time exceed a preset value, the thyristor 540 acts to switch the ballast off.

Corresponding to the activation of the protection circuit 416, it is observed that the low voltage buffer formed by the resistor 506, the capacitor 508, the transistor 526 and the Zener diode 520 reduces the total consumption of the ballast unit to only 0.018 A.

The second protection circuit, shown as 406 in Fig. 4, monitors for the presence of a lamp 90, and is formed by transistor 528, diodes 566,570, resistor 568, capacitor 564 as well as the encapsulated toroidal transformer 646.

During the start phase of normal operation, the current flowing through the primary winding of the transformer 646 generates a voltage of 0.7 V in the secondary winding of transformer 646, which, is applied to the base of the transistor 528 after rectification and filtering by the diodes 566 and 570, the resistor 568 and the capacitor 564. This keeps the voltage at the collector of transistor 528 sufficiently low to prevent the thyristor 542 from conducting.

However, if the start phase does not succeed in establishing a current flow in the primary winding of the transformer 646, for example because no lamp is connected, the voltage of the secondary winding of transformer 646 will be equal to 0.3 V. In consequence, transistor 528 is shut off and the voltage at the gate of the thyristor 542 reaches a value of 0.8 V. As a consequence the ballast is immediately blocked.

The second protection circuit 406 avoids the danger of direct contact with a live lamp fitting where no lamp is connected. When the protection circuit 406 is activated, the voltage at the output terminals is 0 V.

In order to replace a failed lamp or to insert one having different characteristics, the circuit power must be switched off for some seconds in order to allow the capacitors 548 and 594 to discharge to ground.

Third Arraztge7ßte7tt A third arrangement of the control circuit for fluorescent tubes is shown in Fig. 6.

The form of the circuit shown in Fig. 6 is described by the functional block diagram of Fig. 4. However, the circuit implementation of each of the functional blocks differs from the implementation shown in Fig. 5A and 5B. One of the main differences is the use of a pulse width modulation (PWM) integrated circuit 709 to provide the oscillator functionality.

The line voltage is supplied to a low-pass filter via a fusible resistor 701. The low-pass filter is formed by two coils wound in counter-phase on a u-shaped ferrite core 703, and a capacitor 702. The output of the low-pass filter is rectified by the diode bridge 704. The output voltage of the diode bridge 704 is applied to the source of the transistor 705 and the drain of the transistor 706. The transistors 705 and 706 are preferably

insulated-gate bipolar transistors (IGBT). The transistors 705,706 form part of a final power stage that drives the lamp 90 via the inductor 712.

The oscillator circuitry operates at a lower supply voltage than the final power stage. Accordingly, it is necessary to step down the line voltage. In the circuit of Fig. 6, the line voltage is stepped down via the resistor 708 and the transformer 707 to provide a lower voltage which is applied to the integrated circuit 709 via the transistor 723.

The pulse width modulation (PWM) integrated circuit 709 is used to generate a square wave signal which is fed to the power stage transistors 705,706 via the half-bridge driver integrated circuit 710.

The integrated circuit 709 is preferably an SG3525A chip supplied by STMicroelectronics of Geneva, Switzerland. The integrated circuit 709 generates rectangular pulses having a duty cycle which is a function of a comparison voltage applied to the inputs of an internal comparator.

The square wave is output at pin 11 of the integrated circuit 709. The rectangular wave form is then applied to the gate of transistor 706.

The square wave signal is also output at pin 14 of the integrated circuit 709 but with a 180° phase shift compared with the signal at pin 11. The square wave signal output at pin 14 is applied to the integrated circuit 710, which in turn applies the square wave signal to the gate of transistor 705.

The integrated circuit 710 is a half-bridge driver capable of providing the necessary current. The integrated circuit 710 is used to control the transistor 705. In order to make transistor 705 conducting, a positive pulse must be supplied having a voltage amplitude of 302 V (290 V + 12 V). In practice, for integrated circuit 705 to conduct the voltage between the gate and the source must be higher than that at the drain electrode of the transistor 705 relative to ground.

In order to maintain the correct value of the voltage at the input of inductor 712, the pulse width modulation capabilities of the integrated circuit 709 are used. Initially the

duty cycle of the square wave generated at pins 11 and 14 of the integrated circuit 709 is 50%, that is, each pulse lasts for half a period. In practice the duty cycle is dynamically modified depending on the load conditions of the output. The modification of the duty cycle is achieved by feedback of the voltage value at the inductor 712, which is returned to pin 1 of the integrated circuit 709 via the resistor 713, the diode 714 and the resistor 715.

The duty cycle of the square wave output by integrated circuit 709 is thus varied in order to keep the input voltage of the inductor 712 constant.

The control circuit of Fig. 6 contains two protection mechanisms, the circuit design of which is substantially the same as the protection mechanisms shown in Fig. 5A and Fig. 5B.

The first protection circuit (corresponding to circuit 416 of Fig. 4) monitors the length of the ignition period. If the lamp does not light up within a specified time, the charge accumulated in capacitor 719 exceeds a specified limit, the thyristor 721 becomes conductive and, as a consequence, transistor 723 ceases to conduct, thereby cutting off the power supply to the integrated circuit 709. In this way the control circuit is switched off if ignition does not occur.

The second protection circuit (corresponding to circuit 406 of Fig. 4) verifies the presence of a lamp 90. When the circuit is functioning normally, a supply current flows through inductor 712. This current is detected by means of the transformer 711. If the lamp 90 is removed or otherwise ceases to function, current no longer flows through the inductor 712. The protection circuit including transistor 722 and thyristors 720 and 721 then acts to cut off the supply voltage to the integrated circuit 709. In this way, the lamp fittings are no longer live when the lamp is not working or is not present, thereby reducing the danger of injury to users.

Fourth Arrangetet Fig. 7 shows a futher arrangement of a control circuit for fluorescent lamps. The circuit of Fig. 7 is substantially the same as the circuit shown in Fig. 6. The difference is that the half-bridge driver 730 is used instead of the half-bridge driver 710. Driver 730 and 710 have different pin configurations. For example pin 3 of driver 730 is'earth'whereas

'earth'is pin 4 of driver 710. Preferably, the drivers 730,710 are IR2111 or IR2108 chips supplied by International Rectifier Corporation of El Segundo, California, USA.

Advantages The arrangements described offer one or more of the following advantages: The control circuitry disclosed above can operate with lamps of different kinds (i. e. containing different gases) and different powers (from 18 watts to 56 watts) even at the same time. No modification to the circuit or manual intervention is required when changing to a different type of lamp.

When the apparatus is controlling different lamps, and one of these lamps fails or is exhausted, all the other lamps continue to work correctly.

The light emitted by the lamp is safe, clear and cold-coloured, thus avoiding any visual disturbance.

The temperature of the lamp is lower by 25% than that reached using conventional reactors, and in consequence the lamp can be affixed to a double ceiling, since the temperature is kept sufficiently low and the equipment is sufficiently light. The ceiling light fixture does not need internal wiring clamps.

The circuitry is free of vibration and does not generate background noise (as occurs in the prior art). In addition, it does not induce noise on radios, computers or any other apparatus connected to the mains supply.

The preferred embodiment of the invention ensures a low energy consumption with savings of up to 50% with respect to conventional reactors. No power-factor correctors are necessary, since the power factor is sufficiently high (about 0.96).

The life of the lamp is prolonged by 50% with respect to lamps using conventional reactors.

The foregoing describes only four circuit arrangements of the present invention, and modifications or changes can be made thereto without departing from the scope and spirit of the invention.