WO2006043144A1

WO2006043144A1 - Mains control of load power output

Info

Publication number: WO2006043144A1
Application number: PCT/IB2005/003081
Authority: WO
Inventors: Daniel Alfonso Corte; Riccardo Arthur De Wet
Original assignee: Daniel Alfonso Corte; Riccardo Arthur De Wet
Priority date: 2004-10-18
Filing date: 2005-10-17
Publication date: 2006-04-27
Also published as: ZA200706289B

Abstract

An electronic circuit is provided for controlling the power output of a load connected to the circuit by manipulation of the amplitude of the mains voltage. The load may be fluorescent tube. A sensing circuit coupled to the mains voltage detects changes in the mains voltage amplitude and manipulates an oscillator that drives switching means, thereby changing the frequency of the wave passing through the load. The invention also extends to a dimmable fluorescent lamp including the electronic circuit described.

Description

MAINS CONTROL OF LOAD POWER OUTPUT

FIELD OF THE INVENTION

This invention relates to mains control of load power output. More specifically, it relates to electronic circuit means for controlling the power output of loads, such as fluorescent lamps, by manipulation of the mains voltage. The invention also extends to an integrated, dimmable fluorescent lamp.

BACKGROUND TO THE INVENTION

A fluorescent lamp comprises a tube coated on its inner surfaces with a fluorescent or phosphorescent substance and filled with an ionisable gas at a low pressure. A pair of electrodes, which may have filaments, are provided at opposite ends of the tube.

When a high potential difference, typically in the order of about 2 kV, is provided across the electrodes, the high impedance between the electrodes is overcome and a surge of current, called the ignition surge, flows through the tube. This surge sets in motion a discharge process in which the ionisable gas becomes a plasma, and electrons within the ionised plasma migrate from one electrode to the other. In some fluorescent lamps, the filaments are also preheated to increase the ease with which the gas discharge process is initiated. Once the lamp has been so ignited, the impedance between the electrodes drops dramatically and the discharge process can be maintained by a relatively low voltage potential, typically a few hundred Volts, between the electrodes.

The gas discharge process causes an emission of ultraviolet rays that impinge on the fluorescent or phosphorescent substance, exciting it and causing it to emit light in the visible spectrum due to the photon/emission effect.

The high voltage required for the ignition surge is produced by a ballast circuit. In basic designs the ballast circuit comprises an inductor which initially short circuits the tube. Once sufficient current flows through the inductor, a switch is opened resulting in a dramatic rise in voltage between the electrodes that provides the ignition surge. The switch may be opened by an electromechanical bimetallic switch, or glow lamp.

Electronic ballast circuits also exist. These ballast circuits include an inductor and capacitor that resonate at a predetermined frequency to produce the high voltage required for the ignition surge. Importantly, in all of these cases, substantially full mains voltage is required at the lamp for the ballast circuit to be able to generate the high voltage required for the ignition surge.

With incandescent bulbs, controlling the power output of the bulbs is usually achieved by changing the duty cycle of the mains voltage supplied to the bulb. This approach, however, has not been successful with conventional fluorescent lamps mainly due to the problem of keeping the lamp reliably ignited below substantially full mains voltage supply at the lamp. Various attempts have been made to provide dimmable fluorescent lamps, for example by providing controllable switching means that can adjust the light intensity by changing the frequency of the AC voltage supplied to the fluorescent tube. However, these lamps require an additional port for controlling the switching frequency, making them generally unsuitable for direct retrofitting into existing lighting systems without extensive rewiring.

OBJECT OF THE INVENTION

It is an object of the invention to provide an integrated dimmable fluorescent lamp that, at least partially, alleviates some of the abovementioned problems. It is a further object of the invention to provide electronic circuit means by which the power output of loads such as, for example, fluorescent lamps, may be controlled by manipulation of the mains voltage.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided an electronic circuit for controlling the power supplied to a load connected to said electronic circuit by manipulation of the mains voltage supplied to the electronic circuit, comprising an oscillator operable to generate at least one variable-frequency oscillation signal; switching means powered by the mains voltage and controlled by the at least one oscillation signal for generation of an output power wave for connection to the load; and control means coupled to the oscillator for controlling the frequency of the oscillation signal; wherein the control means includes a sensing circuit coupled to the mains voltage supply and calibrated to manipulate the oscillation frequency between upper and lower frequency limits when the mains voltage amplitude is adjusted within a predetermined range.

Further features of the invention provide for the predetermined range to be selected to be below the range of normal fluctuation in mains voltage; and for the range not to be more than 10% of the nominal mains voltage.

Still further features of the invention provide for the control means to vary the oscillation frequency between about 30 kHz and about 100 kHz; and for the sensing circuit to be coupled to the oscillator by an opto-coupler.

The mains voltage may be an alternating current (AC) voltage. Then, the circuit may include a rectifier for rectifying the mains voltage to produce a direct current (DC) voltage for powering the switching means.

Preferably, the load is a fluorescent tube and an inductive element is provided in series with the fluorescent tube. The inductive element may be the primary winding of a high frequency transformer, the secondary winding of which is coupled to a power input port of the oscillator to form a feedback loop.

Further features of the invention provide for the sensing circuit to include a comparator that compares a clamped reference voltage input with a follower input derived from the mains voltage, and then supplies a signal proportional to the difference of the inputs to the opto-coupler.

The sensing circuit may include a capacitor connected between the clamped voltage input and ground, so that when the circuit is powered up the clamped voltage input is held below its final level until the capacitor has charged up. The invention also extends to a method of controlling the power output of a load by manipulation of the amplitude of the mains voltage, comprising the steps of: generating at least one variable-frequency oscillation signal with an oscillator; driving switching means with the at least one oscillation signal; sensing the amplitude of the mains voltage; and manipulating the oscillation frequency between upper and lower frequency limits when the mains voltage amplitude is adjusted within a predetermined range.

Further features of the invention provide for the predetermined range to be selected to be below the range of normal fluctuation in mains voltage and for the range to not more than 10 % of the nominal mains voltage.

Still further features of the invention provide for the oscillation frequency to be manipulated between about 30 kHz and about 100 kHz; and for the the load to be a fluorescent tube.

The method may include the step of generating a feedback voltage at the fluorescent tube and supplying the feedback voltage to a power input port of the oscillator

The invention yet further extends to an integrated, dimmable fluorescent lamp comprising: a housing having two electrodes for connection to a mains voltage supply; a fluorescent tube; an oscillator operable to generate at least one variable-frequency oscillation signal; switching means powered by the mains voltage supply and driven by the at least one oscillation signal for generation of a wave for connection to the fluorescent tube; and control means coupled to the oscillator for controlling the frequency of the oscillation signal; wherein the control means includes a sensing circuit coupled to the mains voltage supply and calibrated to manipulate the oscillation frequency between upper and lower frequency limits when the mains voltage amplitude is adjusted within a predetermined range.

Further features of the invention provide for the predetermined range to be selected to be below the range of normal fluctuation in mains voltage and to not be more than 10 % of the nominal mains voltage.

Preferably, the control means varies the oscillation frequency between about 30 kHz and about 100 kHz; and the sensing circuit is coupled to the oscillator by an opto-coupler.

The mains voltage may be an alternating current (AC) voltage. Then the fluorescent lamp may house a rectifier for rectifying the mains voltage to produce a direct current (DC) voltage that powers the switching means.

A further feature of the invention provides for a primary winding of a high frequency transformer to be coupled in series with the fluorescent tube, the secondary winding of which is coupled to the power input port of the oscillator to form a feedback loop.

The sensing circuit may include a comparator that compares a clamped voltage input with a follower input derived from the mains voltage, and supplies the difference to the opto-coupler. It may also include a capacitor connected between the clamped voltage input and ground, so that when the circuit is powered up the clamped voltage input is held below its final level until the capacitor has charged up.

These and other features of the invention will become apparent from the following description of an embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, with reference to the drawings in which:

Figure 1 shows a prior art dimming and ballast circuit;

Figure 2 shows a block diagram of a circuit for controlling the power output of fluorescent tubes including a dimming and ballast circuit in accordance with the invention and a voltage controller;

Figure 3 shows a circuit diagram of the dimming and ballast circuit of Figure 2; and

Figure 4 shows an integrated, dimmable fluorescent lamp in accordance with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the drawings, like reference numerals refer to like components.

Figure 1 is a circuit diagram of a prior art circuit (100) for a dimmable fluorescent lamp. High voltage DC is supplied to a first node (102) which is connected through a resistor (104) to the power supply port (106) of an oscillator (108), which may be an oscillator manufactured by International Rectifier and sold under the reference IR2153(S). The oscillator (108) is operative to generate switching signals at ports (110, 112) that drive a pair of transistors (114, 116). An output is taken between the transistors (114, 116) at node (118), and passed through a capacitor (120) that blocks the DC voltage at node (118). The output is then supplied to a reactive load, such as a fluorescent light tube (122). The fixed frequency of the wave supplied to the load (122) can be varied by means of a resistor (127) between control ports (126, 128) of the oscillator (108).

A capacitor (130) is connected between further ports (132, 134) of the oscillator (108), and a diode (136) is provided between the power supply port (106) and port (132). The capacitor (130) and diode (136) provide a boot-strap supply drive to the upper transistor (114). They are required for the proper functioning of the oscillator (108) and, as they are recommended by the datasheet of the oscillator (108) and well understood in the art, are not further expanded upon.

A reservoir/timing capacitor (138) is provided between the power supply port (106) and ground, and another timing capacitor (140) is provided between one of the control ports (128) and ground. These capacitors are also required for the proper functioning of the oscillator (108) and are recommended by the datasheet and well understood in the art.

Figure 2 is a conceptual overview of the invention. It shows a block diagram of a system (10) for controlling the power output of loads. The system (10) includes a dimming and ballast circuit (12) and a generic voltage controller (14). The voltage controller (14) is connected to the power grid via Live and Neutral wires (15), and is able to control the voltage of the mains (16) AC power coupled to the dimming and ballast circuit (12). The voltage controller (14) may, for example, be a variable voltage controller, such as a variac or the like, but is not described further as it does not form part of the invention. The dimming and ballast circuit (12) includes a high voltage AC switching arrangement (18) that supplies high voltage, high frequency AC power to a fluorescent tube (20). A sensing circuit (22), coupled to the mains voltage (16), is able to sense when the mains voltage (16) drops below a predetermined voltage level and to then generate a control signal at its output (24). This control signal is input into the high voltage AC switching arrangement (18) to control the frequency of the high voltage AC power supplied to the fluorescent tube (20).

Current sensing means (26) is provided in the current flow path through the fluorescent tube (20) that provides feedback to stabilise the power supply of the high frequency AC switching arrangement generator (18), as will be explained in more detail below.

Generally, a voltage fluctuation of about +/-10% can be expected on the power grid. In countries where the nominal power grid voltage is 230 V AC, this means the actual voltage on the power grid can fluctuates between about 207 V AC and

253 V AC. In countries where the nominal voltage is 110 V AC, the actual voltage is typically between 99 and 121 V AC. For simplicity, the invention will be described with reference to a nominal mains power voltage of 230 V AC, but it will be understood that the invention can be applied equally where the nominal mains voltage is any other value, such as 110 V AC (with integration of a AC voltage doubler).

In a 230 V AC power grid, the predetermined voltage level below which the sensing circuit (22) is able to sense is selected at 200 V AC, so that the sensing circuit (22) does not sense voltage fluctuations that are inherent in the power grid. The sensing circuit (22) only generates an output when the mains voltage supply (16) drops below 200 V. It will also be appreciated that, when the voltage controller (14) of Figure 2 lowers the voltage below the range of the 10% nominal allowed fluctuation of the grid, the AC mains supply (16) at its output does not have these voltage fluctuations.

The system (10) therefore enables the brightness of the fluorescent tube (20) to be adjusted by manipulation of the mains supply (16) within a narrow range. No additional control wires or ports are necessary.

Figure 3 shows the circuit diagram of the dimming and ballast circuit (12) of Figure 2. In the circuit (12), the nominal 230 V AC mains voltage supply (16) is fed through a full wave bridge rectifier (28) to produce a high voltage DC with an average amplitude of about 320 V at node (30). Capacitor C2 at the output of the rectifier minimises the AC ripple to produce a relatively constant DC voltage at node (30).

The output of the rectifier (28) at node (30) is fed to a high voltage AC switching arrangement (18) that includes a pair of transistors (Q2, Q3), which may be

MOSFET transistors sold under reference IRF830. The output of the high voltage

AC switching arrangement (18) is a high frequency AC wave and is taken between the transistors (Q2, Q3) at node (32). Capacitor C6 blocks the DC voltage at node (32), while resistor R11 connected in parallel with capacitor C6 allows a very small DC current to flow through filaments (42) of the fluorescent tube (20). This small DC current helps to keep the filaments (42) warm and avoid flickering, especially at low dimming settings and when the ambient temperature around the fluorescent tube (20) is low. The output at node (34) is then connected to a first terminal (36) of the fluorescent tube (20).

The transistors (Q2, Q3) are driven through their respective gates (38.1, 38.2) by control signals produced by an oscillator (40), which may be an integrated circuit oscillator manufactured by International Rectifier and sold under the reference IR 2153. The oscillator (40) switches the transistors (Q2, Q3) at high frequency, typically about 30 to 100 kHz, so that a high voltage, high frequency AC wave is produced at the first terminal (36) of the fluorescent tube (20).

The fluorescent tube (20) has filaments (42) provided at both ends thereof, and second and third terminals (44, 46) of the fluorescent tube (20), each being one end of each of the filaments (42), are connected together through capacitor C7. Capacitor C7 is chosen so as to regulate the ignition set-point of the fluorescent tube (20), which is the frequency at which the fluorescent tube (20) is ignited as will be explained below.

The return path to ground passes from a fourth terminal (48) of the fluorescent tube (20) and through the primary winding (50) of a high frequency transformer (52). A secondary winding (54) of the transformer (52) is connected to the power supply port (56) of the oscillator (40) through diode D3 at its one end, and its other end is connected through resistor R12 to ground. The output of the secondary winding (54) provides the current sensing means (26) illustrated in Figure 2, and thereby forms a feedback loop. The winding ratio of primary (50) to secondary (54) windings is chosen to be about 10:1 and the resistance of R12 is rated to give sufficient current flow through diode D3 and to the power supply port of the oscillator (40). This feedback loop ensures that the voltage at the power supply port (56) of the oscillator (40) is always at or above the minimum required operating voltage, by cyclically recharging capacitor C3 when the AC current generated in the secondary winding (54) is positive and able to flow through diode D3. By ensuring that the oscillator (40) always has enough power, the instantaneous operating output frequency of the oscillator (40) is stabilised within the entire desired frequency range.

Diode D2 and capacitor C5 are provided in accordance with the specification of the oscillator (40). These elements provide a boot-strap supply drive to the upper switching transistor Q2 and, because they are required for the internal operation of the oscillator (40), will not be described in further detail herein. As the frequency of the wave that passes through the fluorescent tube (20) increases, the reactance of the primary winding (50) of the transformer (52) increases, resulting in a drop in current flowing through the fluorescent tube (20) and consequent dimming. The brightness of the fluorescent tube (20) can therefore be changed by controlling the frequency at which the transistors (Q2, Q3) alternately switch.

The oscillator (40) allows the switching frequency to be controlled by changing the resistance between control ports (58, 60). An opto-coupler (62) having a light dependant resistor (64) and a light-emitting diode (66) can be used to vary the resistance between the control ports (58, 60). As the brightness of the LED (66) changes, the resistance of the light-dependant resistor (64) inversely changes. The LED (66), in turn, is controlled by the sensing circuit (22), which is described in more detail below.

A sample of the rectified DC voltage at port (30) is obtained by means of a resistive voltage divider comprising resistors R1 and R2. The voltage between resistors R1 and R2 at node (68) is a small sample of the voltage at node (30). This voltage is fed through resistor R3 to the base (70) of transistor Q1 , which may be a PNP transistor sold under reference BC327-40. A reference voltage is obtained by connecting the rectified DC voltage at port (30) through Zener diode DZ1 and resistor R4 to ground. The voltage between resistor R4 and Zener diode DZ1 , at node (74), is a clamped DC voltage which is then fed into the emitter (76) of transistor Q1. Capacitor C1 ensures that the voltage across the Zener diode DZ1 is zero when the circuit (12) is initially powered up, so that the circuit is powered up with maximum initial illumination, as will be explained in more detail below.

As long as the voltage at the base (70) of transistor Q1 stays above the voltage at the emitter (76), transistor Q1 will stay switched off and no current will flow through the LED (66). However, if the voltage at the base (70) of the transistor drops below the voltage at the emitter (76), the transistor will switch on and current will start to flow through the LED (66).

The sensing circuit (22) is therefore calibrated so that at the point where the mains voltage (16) drops below the predetermined voltage of 200V, the voltage at the base (70) of transistor Q1 starts to drop below the voltage at the emitter

(76), the transistor Q1 switches on and current starts to flow through the LED. In this embodiment, the sensing circuit is calibrated so that the LED will be fully turned on when the mains voltage (16) drops to about 190V. With the LED fully turned on, the resistance of the light dependant resistor (66) will be at a minimum and the resistance between the control ports (58, 60) of the oscillator (40) will be at a minimum. This, in turn, will result in a maximum output frequency for the oscillator (40) and, consequently, maximum dimming of the fluorescent tube (20).

The brightness of the fluorescent tube (20) can thus be controlled by varying the mains voltage (16) between 200 and 190 V AC. It has found that, even with the mains voltage (16) at the minimum voltage of 190 V AC, the circuit (12) still has sufficient energy to fire the fluorescent tube (20).

Firing of the fluorescent tube (20) and pre-heating of the filaments (42) is provided for by the ballast and dimmer circuit (12). The oscillator (40) includes a feature whereby the oscillation frequency starts at about 100kHz when power is first supplied to the power supply port (56) of the oscillator (40). As the voltage at the power supply port (56) increases, the output frequency proportionately decreases, so that when, eventually, the supply has reached its maximum voltage, the output frequency equals the final operational frequency which is determined by the resistance between the control ports (58, 60).

Resistor R6 and capacitor C3 are chosen to form an RC-timer circuit having a time constant of about 1.5 seconds. Capacitor C7 is chosen so that it resonates, together with the inductance of the primary winding (50) of the high frequency transformer (52), at a frequency of about 60 kHz.

When power is first supplied to the circuit (12), capacitor C3 is discharged and the voltage at the power supply port (56) of the oscillator (40) is zero. As soon as the minimum starting voltage has been reached at the power supply port (56), the oscillator (40) starts to oscillate at about 100 kHz. As capacitor C3 is charged the voltage at the power supply port (56) rises and the output frequency of the oscillator starts a downward sweep.

The circuit (12) also provides for pre-heating of the filaments (42). Prior to the ignition of the fluorescent tube (20), a small high frequency current starts flowing through the filaments (42) and through the primary winding (50) of the high frequency transformer (52) to the ground return path. This current is enough to warm the filaments to facilitate the ionisation process prior to the ignition of the tube. Pre-heating of the fluorescent tube (20) increases the life expectancy of the filaments (42) and the overall performance of the tube (20).

Capacitor C7 and the primary winding (50) of the transformer (52) are connected in series through the filaments (42). At the resonant frequency of about 6OkHz, the reactances of the capacitor C7 and the inductive primary winding (50) are equal in magnitude but 180 degrees out of phase. At this frequency the impedance of the series combination reaches a minimum and a large current is produced. Although the overall voltage across the series combination is low, high voltages that are out-of-phase with one another are produced across the capacitor C7 and the primary winding (50) respectively. The high voltage across the capacitor C7 causes the impedance between the filaments (42) to be overcome, thereby igniting the fluorescent tube (20). Once the fluorescent tube (20) has ignited, current flows through the fluorescent tube (20) and through the primary winding (50). This increases the voltage across the secondary winding (52) of the transformer (52), thereby increasing the voltage at the power supply port (56) of the oscillator (40), and bringing the oscillator into its normal running condition.

To ensure that the fluorescent tube is powered up at full brightness setting, capacitor C1 is provided that initially short circuit the Zener diode DZl When power is first supplied to the circuit (12), capacitor C1 is discharged and the voltage at the emitter (76) of transistor Q1 is zero. Capacitor C1 and resistor R4 are chosen to have an RC time constant in excess of the time constant of capacitor C3 and resistor R6. In this embodiment, a time constant of about 5 seconds is chosen. Thus, even when the mains voltage (16) is at the minimum dimming setting of 190 V AC, the LED will remain switched off, and the fluorescent tube (20) consequently at its brightest setting, for the first few seconds after power is switched on. This gives the fluorescent tube (20) a chance to warm up before the dimming is affected.

As shown in Figure 4, the entire dimming and ballast circuit (12) is built onto a printed circuit board (202) and integrated into the base (204) of a compact fluorescent lamp (200) to provide a dimmable compact fluorescent lamp with an integrated ballast and dimming circuit (12) that operates by manipulation of the mains voltage (16) alone.

It is an advantage of the invention that each fluorescent lamp integrates the ballast and dimming circuitry required for that lamp, so that any number of such lamps can be connected into a lighting system and the problems commonly associated with dimming circuits are minimized.

Other embodiments of the invention may be devised that fall within the scope of the invention. For example, a plurality of fluorescent lamp tubes, each with their own feedback transformer, may be connected to a single driving ballast circuit that provides the switching means for each of the fluorescent lamp tubes. Alternatively, the single ballast circuit may provide a master ballast unit having a single oscillator, and a plurality of slave ballast units each with their own switching arrangement could be provided, all fired by the oscillator and energised by the rectifier of the master ballast unit.

It will be appreciated that the invention is not limited to applications in which the load is a fluorescent lamp. The load may, for example, be an electrical motor, in which case the circuit of the present invention will be operable to change the rotation speed of the motor. It will also be appreciated that the circuit may be adapted to function at any mains voltage and that the range of voltages in which the sensing circuit operates may be any suitable range. In the case of a stable AC mains supply, the sensing range need not be below the nominal mains AC voltage as in this case, but the sensing circuit may be configured to detect voltage fluctuations about the nominal voltage.

The invention therefore provides electronic circuit means by which the power output of loads, such as fluorescent lamps, can be controlled by manipulation of the mains voltage alone. Fluorescent lamps embodying the electronic circuitry of the invention can easily be retrofitted to existing lighting networks, and the only additional hardware required is a centrally located common mains voltage controller of suitable rating.

Claims

1. An electronic circuit for controlling the power supplied to a load connected to said electronic circuit by manipulation of the mains voltage supplied to the electronic circuit, comprising: an oscillator operable to generate at least one variable-frequency oscillation signal; switching means powered by the mains voltage and controlled by the at least one oscillation signal for generation of an output power wave for connection to the load; and control means coupled to the oscillator for controlling the frequency of the oscillation signal; wherein the control means includes a sensing circuit coupled to the mains voltage supply and calibrated to manipulate the oscillation frequency between upper and lower frequency limits when the mains voltage amplitude is adjusted within a predetermined range.

2. An electronic circuit as claimed in claim 1 in which the predetermined range is selected to be below the range of normal fluctuation in mains voltage.

3. An electronic circuit as claimed in claim 1 or claim 2 in which the control means varies the oscillation frequency between about 30 kHz and about

100 kHz.

4. An electronic circuit as claimed in any one of the preceding claims in which the sensing circuit is coupled to the oscillator by an opto-coupler.

5. An electronic circuit as claimed in any one of the preceding claims in which the mains voltage is an alternating current (AC) voltage and the electronic circuit includes a rectifier for rectifying the mains voltage to produce a direct current (DC) voltage that powers the switching means.

6. An electronic circuit as claimed in any one of the preceding claims in which the load is a fluorescent tube and an inductive element is provided in series with the fluorescent tube.

7. An electronic circuit as claimed in claim 6 in which the inductive element is the primary winding of a high frequency transformer, the secondary winding of which is coupled to a power input port of the oscillator to form a feedback loop.

8. An electronic circuit as claimed in any one of claims 4 to 7 in which the sensing circuit includes a comparator that compares a clamped reference voltage input with a follower input derived from the mains voltage, and supplies a signal proportional to the difference of the inputs to the opto-coupler.

9. An electronic circuit as clamed in claim 8 in which the sensing circuit includes a capacitor connected between the clamped voltage input and ground, so that when the circuit is powered up the clamped voltage input is held below its final level until the capacitor has charged up.

10. A method of controlling the power output of a load by manipulation of the amplitude of the mains voltage, comprising the steps of: generating at least one variable-frequency oscillation signal with an oscillator; driving switching means with the at least one oscillation signal; sensing the amplitude of the mains voltage; and manipulating the oscillation frequency between upper and lower frequency limits when the mains voltage amplitude is adjusted within a predetermined range.

11. A method as claimed in claim 10 wherein the predetermined range is selected to be below the range of normal fluctuation in mains voltage.

12. A method as claimed in claim 10 or claim 11 in which the oscillation frequency is manipulated between about 30 kHz and about 100 kHz.

13. A method as claimed in any one of claims 10 to 12 in which the load is a fluorescent tube.

14. A method as claimed in claim 13 which includes the step of generating a feedback voltage at the fluorescent tube and supplying the feedback voltage to a power input port of the oscillator.

15. An integrated, dimmable fluorescent lamp comprising: a housing having two electrodes for connection to a mains voltage supply; a fluorescent tube; an oscillator operable to generate at least one variable-frequency oscillation signal; switching means powered by the mains voltage supply and driven by the at least one oscillation signal for generation of a wave for connection to the fluorescent tube; control means coupled to the oscillator for controlling the frequency of the oscillation signal; wherein the control means includes a sensing circuit coupled to the mains voltage supply and calibrated to manipulate the oscillation frequency between upper and lower frequency limits when the mains voltage amplitude is adjusted within a predetermined range.

16. A fluorescent lamp as claimed in claim 15 in which the predetermined range is selected to be below the range of normal fluctuation in mains voltage.

17. A fluorescent lamp as claimed in claim 15 or claim 16 in which the control means varies the oscillation frequency between about 30 kHz and about 100 kHz.

18. A fluorescent lamp as claimed in any one of claims 15 to 17 in which the sensing circuit is coupled to the oscillator by an opto-coupler.

19. A fluorescent lamp as claimed in any one claims 15 to 18 in which the mains voltage is an alternating current (AC) voltage and the fluorescent lamp houses a rectifier for rectifying the mains voltage to produce a direct current (DC) voltage that powers the switching means.

20. A fluorescent lamp as claimed in any one of claims 15 to 19 in which the primary winding of a high frequency transformer is coupled in series with the fluorescent tube, the secondary winding of which is coupled to the power input port of the oscillator to form a feedback loop.

21. A fluorescent lamp as claimed in any one of claims 18 to 20 in which the sensing circuit includes a comparator that compares a clamped voltage input with a follower input derived from the mains voltage, and supplies the difference to the opto-coupler.

22. A fluorescent lamp as claimed in claim 21 in which the sensing circuit includes a capacitor connected between the clamped voltage input and ground, so that when the circuit is powered up the clamped voltage input is held below its final level until the capacitor has charged up.