WO2008059308A1

WO2008059308A1 - Electronic circuit means for increasing the ability of fluorescent lamps to be dimmed using standard dimmers

Info

Publication number: WO2008059308A1
Application number: PCT/IB2006/003251
Authority: WO
Inventors: Daniel Alfonso Corte; Riccardo Arthur De Wet
Original assignee: Daniel Alfonso Corte; Riccardo Arthur De Wet
Priority date: 2006-11-17
Filing date: 2006-11-17
Publication date: 2008-05-22

Abstract

Electronic circuit means is provided for increasing the ability of fluorescent lamps to be dimmed using standard dimmers, such as phase-cut dimmers. The electronic circuit means includes at least one resistor connected between input terminals of a fluorescent lamp circuit to thereby improve the power factor of the fluorescent lamp circuit. A sensing circuit operable to sense when the phase-cut dimmer is dimming beyond a predefined threshold and to then introduce a lower value resistance between the output terminals of a bridge rectifier of a fluorescent lamp circuit is also provided. The invention includes an optional second sensing circuit operable to sense when the phase-cut dimmer is dimming beyond a predefined threshold and to then introduce a higher value reactance in series with a fluorescent lamp of the fluorescent lamp circuit.

Description

ELECTRONIC CIRCUIT MEANS FOR INCREASING THE ABILITY OF FLUORESCENT LAMPS TO BE DIMMED USING STANDARD DIMMERS

FIELD OF THE INVENTION

This invention relates to electronic circuit means for increasing the ability of fluorescent lamps to be dimmed using standard dimmers, such as phase-cut dimmers.

BACKGROUND TO THE INVENTION

Every electrical load has an impedance (Z). The impedance of a load is the sum of its resistance (R) and its reactance (X). A purely resistive load, such as a heating element or an incandescent lamp, has only resistance and no reactance whereas a purely reactive load, such as a capacitor or an inductor, has only reactance and no resistance. A load with a complex impedance is a load that has both resistance and reactance. The alternating current (AC) mains power supplied by electricity utility companies is normally a 50Hz, 230V RMS sinusoidal wave. One cycle of a sinusoidal wave is 360 degrees. When AC power is supplied to a complex impedance load, the reactive part of the load induces a phase shift between the current wave and voltage wave of the supplied power. The amount of phase shift that occurs is a measure of the power factor of that load.

Power factor is defined as the cosine of the phase angle between the current and voltage waves, which is a dimensionless number between 1 and 0. When the power factor is 1, the current wave and voltage waves are perfectly in step and all the energy supplied is consumed by the load. The power factor of purely resistive loads is equal to 1. When the power factor of a load is less than one, the current and voltage waves are out of step and only a portion of the energy supplied is consumed by the load, the rest being cyclically absorbed and reflected back at the frequency of the AC supply. A purely inductive or capacitive load results in a phase shift of positive or negative 90 degrees and a power factor of 0. Purely inductive or capacitive loads therefore on average consume no power, but merely cyclically absorb and reflect power. The closer the power factor is to 0, the less real power is available in the circuit to do work.

Circuits for dimming incandescent lamps are known. By varying the average power supplied to an incandescent lamp it is possible to vary the intensity of the light output. Variable autotransformers, which vary the amplitude of the mains power by using variable winding ratio transformers, have been used for dimming incandescent lamps but are bulky and expensive. Thyristor dimmers were introduced to overcome this difficulty. For lower powered applications, such as home lighting, a form of thyristor known as a triac is used. A triac can conduct in both directions and is triggered by a Resistive - Capacitive (RC) time constant circuit. By varying the value of the resistor in the RC circuit, the point at which the triac is fired is varied, thereby varying the output of the triac and implementing a dimming function. Figures 1A and 1B show the output voltage of a triac dimmer at two different dimming levels. The method of dimming using triac thyristors is known as phase-cut dimming.

The magnitude of the reactance of a capacitor or an inductor is a function of the frequency of the AC power supplied to it. However, the frequency of the AC power is only a single value when the AC voltage supply is a pure sinusoidal wave. In the case of phase-cut dimming, the resultant waveform is no longer sinusoidal in shape, but includes harmonics that tend to affect the reactance of a load. Whereas incandescent lamps are purely resistive loads, fluorescent lamps include a substantial capacitive reactance. They therefore induce a phase shift between the incoming AC voltage and the incoming AC current that reduces the available real power in the circuit. Phase-cut dimmers are generally not suitable for dimming fluorescent lamps because the harmonics on the resultant waveform increase the reactance of the fluorescent lamps which leads to a greater phase shift and resultant loss of available real power. This manifests itself in mild to severe flickering of 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 lamp. However, these lamps require expensive dedicated circuitry, as well as an additional port for controlling the switching frequency. It would be an advantage if fluorescent lamps, especially low-powered compact fluorescent lamps (CFLs), could be dimmed using standard triac phase-cut dimmers.

OBJECT OF THE INVENTION

It is an object of the invention to provide electronic circuit means that will, at least partially, increase the ability of fluorescent lamps to be dimmed using standard phase-cut dimmers. SUMMARY OF THE INVENTION

In accordance with the invention there is provided electronic circuit means for increasing the ability of a fluorescent lamp to be dimmed using a standard dimmer, comprising at least one resistor connected between input terminals of a fluorescent lamp circuit to thereby improve the power factor of the fluorescent lamp circuit.

Further features of the invention provide for the standard dimmer to be a triac phase-cut dimmer.

Still further features of the invention provide for the electronic circuit means to include a sensing circuit operable to lower the value of the resistance between the input terminals of the fluorescent lamp circuit when dimming of the phase-cut dimmer is detected.

Yet further features of the invention provide for the sensing circuit to be connected between the output of a bridge rectifier and a fluorescent lamp circuit; for the sensing circuit to include a first resistor and a second resistor connected in series between the output terminals of the bridge rectifier, and for the second resistor to be short circuited by a flip-flop arrangement of transistors when the voltage at the output of the bridge rectifier drops below a predefined threshold.

Further features of the invention provide for the first resistor to have a lower resistance than the second resistor.

Still further features of the invention provide for the electronic circuit means to include a second sensing circuit operable to sense when the phase-cut dimmer is dimming beyond a predefined threshold and to then introduce a higher value reactance in series with the fluorescent lamp of the fluorescent lamp circuit.

Yet further features of the invention provide for the second sensing circuit to include a ballast coil connectable in parallel with an existing ballast coil of the fluorescent lamp circuit by means of an electromagnetic or electronic switch, and for the switch to be actuated when the phase-cut dimmer is dimmed beyond a predefined threshold so as to disconnect the ballast coil from its parallel connection with the existing ballast coil.

The invention extends to a method of increasing the ability of a fluorescent lamp to be dimmed using a standard dimmer, comprising connecting at least one resistor between input terminals of a fluorescent lamp circuit to thereby improve the power factor of the fluorescent lamp circuit.

Still further features of the invention provide for the method to include sensing when the phase-cut dimmer is dimming beyond a predefined threshold and then lowering the value of the resistance between the input terminals of the fluorescent lamp circuit.

Yet further features of the invention provide for the method to include sensing when the phase-cut dimmer is dimming beyond a predefined threshold and then increasing the reactance of a ballast coil arrangement provided in series with the fluorescent lamp of the fluorescent lamp circuit. BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the invention will now become apparent from the description of some examples of the invention with reference to the following drawings. In the drawings,

Figure 1A is a graph showing the voltage output waveform of a standard triac phase-cut dimmer at a moderate dimming level;

Figure 1 B is a graph showing the voltage output waveform of a standard triac phase-cut dimmer at a high dimming level;

Figure 2 is a circuit diagram of electronic circuit means for improving the power factor of a fluorescent lamp circuit; Figure 3A is a graph showing the voltage waveform at node (20) in Figure 2 when no dimming is applied; Figure 3B is a graph showing the voltage waveform at node (20) in Figure 2 when moderate dimming is applied; Figure 3C is a graph showing the voltage waveform at node (20) in Figure 2 when high dimming is applied;

Figure 4 is a circuit diagram of a further embodiment of electronic circuit means for improving the power factor of a fluorescent lamp circuit; and Figure 5 is a circuit diagram of an alternative embodiment of the circuit of

Figure 4.

DETAILED DESCRIPTION OF THE DRAWINGS

The invention comprises an electronic circuit means and method for increasing the ability of a fluorescent lamp to be dimmed using a standard dimmer, by improving the power factor of a fluorescent lamp circuit. In its simplest embodiment, the electronic circuit means comprises a single resistor connected between the input terminals of a fluorescent lamp circuit. This resistor causes the input impedance of the fluorescent lamp circuit to be more resistive, improving the power factor of the circuit and therefore increasing the amount of real power available to the circuit to do work. Due to its simplicity, this embodiment of the invention has not been illustrated.

Figure 2 illustrates a more advanced embodiment of the invention that includes means for adjusting the input resistance of the fluorescent lamp circuit depending on the amount of dimming applied by the standard dimmer. In Figure 2, a sensing circuit (10) is shown that increases the ability of a fluorescent lamp circuit (12) to be dimmed with a standard dimmer, in this case a phase-cut dimmer (14).

The output (16) of the phase-cut dimmer (14) is connected to a bridge rectifier (18), and the sensing circuit (10) is connected between the output of the bridge rectifier (18) and the fluorescent lamp circuit (12). Figure 3A shows the waveform of the output of the bridge rectifier (18), between an upper rail (20) and a lower rail (26), when there is no dimming. As the dimmer (14) starts to apply dimming, the waveform between the upper rail (20) and lower rail (26) starts to take the shape shown in Figure 3B and eventually takes the shape shown in Figure 3C when the dimmer applies high dimming levels.

The sensing circuit (10) includes resistors R3 and R4 that form a voltage divider. The voltage waveform between resistors R3 and R4, at node (22), therefore has an identical shape to the waveform at the upper rail (20) but with smaller absolute values. All voltages are now described with reference to the lower rail (26). Diode D1 is a Zener diode. When the voltage at node (22) is lower than the breakthrough voltage of diode D1, no current can flow through diode D1 and transistor Q1 is therefore switched off. Little current therefore flows through resistor R2 and the voltage at the base of transistor Q2, at node (24), is high. Transistor Q2 is therefore switched on and the current that flows between the upper rail (20) and the lower rail (26) flows only through resistor R1. Because resistors R3 and R4 have a very high cumulative value they can be regarded as an open circuit for the purposes of this analysis and the resistance "seen" between the upper rail (20) and the lower rail (26), is therefore the value of resistor R1.

When the voltage at node (22) exceeds the breakthrough voltage of diode D1 , which will occur at a pre-selected point on the upward voltage curve of the upper rail (20), diode D1 will break through and start to conduct current, causing transistor Q1 to switch on. This will cause the voltage to drop at node (24), the base of transistor Q2, thereby switching off transistor Q2. Current between the upper rail (20) and the lower rail (26) now flows through both resistors R1 and R2, and the resistance "seen" between the upper rail (20) and the lower rail (26) is therefore the sum of the values of resistors R1 and R2, which is, of course, higher than the value of resistor R1 alone.

Transistors Q1 and Q2 therefore function as a flip-flop. When the one is on, the other one is off. When the voltage between the upper and lower rails is high, the resistance seen between the rails is high, and when the voltage between them is low, the resistance between them is low. The sensing circuit (10) therefore provides a variable resistance circuit that adjusts the input resistance of the fluorescent lamp circuit (12) depending on the magnitude of the voltage between the upper and lower rails (20, 26).

As can be seen from Figures 3A to 3C, when no dimming is applied the magnitude of the voltage between the upper and lower rails (20, 26) is high for longer than when more dimming is applied. Consequently, as the phase-cut dimmer (14) applies more dimming, the input resistance of the fluorescent lamp circuit (12) is lower on average. At high dimming levels, as shown in Figure 3C, the voltage between the upper and lower rails (20, 26) will not exceed the threshold required to switch on transistor Q1 and the input resistance will remain equal to lower value of R1. A low input resistance improves the power factor of the fluorescent lamp circuit (12) and helps counteract the phase-shift effect that occurs as a result of the distorted waveform supplied to the fluorescent lamp circuit (12). By placing the circuit (10) between an existing bridge rectifier (18) and an existing fluorescent lamp circuit (12), a standard triac phase-cut dimmer (14) can be used to dim a fluorescent lamp. The invention finds particular application in the dimming of compact fluorescent lamps (CFLs) which have a power output lower than about 25 Watts. The embodiment of Figure 2 has the advantage over a single permanently connected resistor that less power is dissipated in the resistors, since the lower valued resistance is only introduced when dimming is detected. Because less power is dissipated, lower power-rated resistors can be used.

Figure 4 shows a further embodiment of the invention that, in addition to improving the power factor as described with reference to Figure 2, also alters the output reactance of the fluorescent lamp circuit (12). In Figure 4, the dimmer

(14), bridge rectifier (18), sensing circuit (10) and fluorescent lamp circuit (12) are as described in Figure 2, except that the output stage (40) of the fluorescent lamp circuit (12) has been shown in detail. The output stage (40) includes the fluorescent tube (50), capacitor C4 which regulates the ignition set-point of the fluorescent tube (50), capacitor C5 which blocks DC current from flowing through the fluorescent tube (50) and inductor B1. Inductor B1 is called a ballast coil, and is necessary to limit the current flowing through the fluorescent tube (50) as well as to provide the high voltage necessary to fire the fluorescent tube (50) when it is switched on. The use of ballast coils in conjunction with fluorescent tubes is well known.

The value of the reactance of a ballast coil is chosen so as to provide the correct degree of current limiting at normal operation of a fluorescent lamp. However, when the power to a fluorescent lamp is reduced, the ballast coil is no longer of the right value and flickering of the lamp often results. It would be advantageous to be able to increase the reactance of the ballast coil when power is reduced to the fluorescent lamp circuit. Having a ballast coil of higher reactance while reducing the power input to the lamp improves the quality of the high frequency AC waveform supplied to the lamp and helps keep the crest factor within acceptable limits.

The embodiment of Figure 4 includes a second sensing circuit (100) that is operable to switch a second ballast coil (B2) out of parallel connection with an existing ballast coil (B1) when operation of the dimmer (14) is detected. The sensing circuit includes an electromagnetic switch, in this embodiment a relay (102). The relay (102) includes an electromagnet (104) which, when energised, actuates a switch (106) to switch both ballast coils (B1 , B2) into parallel connection. The electromagnet (104) is energised by a capacitor (C7). Capacitor C7 is cyclically charged by a diode pump power supply comprising diodes D3 and D4 and capacitor C6. The operation of such a diode pump power supply is well known and will not be described in detail. When there is no dimming, the diode pump power supply is able to charge capacitor C7 to its full value, which keeps the electromagnet (104) energised and the switch (106) actuated. In this condition, the ballast coils (B1 , B2) are connected in parallel and the overall reactance of the parallel combination is lower.

When the dimmer (14) starts to reduce input power the diode pump power supply can no longer maintain full charge on capacitor C7. The electromagnet (104) is then de-energised and the switch (106) released. In this condition, only ballast coil B1 is now connected to the fluorescent tube (50). The overall reactance is therefore higher than when B1 and B2 are connected in parallel.

Figure 5 shows an alternative embodiment of Figure 4. Instead of connecting ballast coils B1 and B2 in parallel when the electromagnet (104) is energised, this circuit disconnects them from their parallel connection when the electromagnet

(104) is energised. The electromagnet is therefore not energised until dimming is detected, and this arrangement therefore reduces power losses in the relay (102) when the dimmer (14) is not dimming.

The embodiment of Figure 5 includes a second sensing circuit (200) that has a voltage divider formed by resistors R8 and R9 and an PNP-type transistor Q3 that only switches on when the voltage at its base, at node (202), drops below the voltage at its emitter at node (204). Diode DZ1 is a 24 V Zener diode that maintains a constant 24 volts across capacitor C6. When the dimmer (14) starts to dim, and the average voltage level at the upper rail (20) starts to drop, the voltage at node (202) will fall below 24 V and transistor Q3 will switch on. This will energise the electromagnet (104) which will actuate the switch (106) and disconnect the ballast coils (B 1 , B2) from their parallel connection.

The circuits of Figure 4 and Figure 5 therefore provide means for increasing the ballast reactance in a fluorescent lamp circuit when dimming is detected. The circuits function by disconnecting one of a pair of ballast coils connected in parallel when dimming is detected. Increased ballast coil reactance improves the quality of the high frequency AC waveform supplied to the lamp and helps keep the crest factor within acceptable limits. It is important to keep the crest factor within limits to achieve the expected lifetime of the fluorescent lamp.

The invention therefore provides means for increasing the ability of fluorescent lamps to be dimmed using standard dimmers, such as phase-cut dimmers, by improving the power factor of a fluorescent lamp circuit and by optionally increasing the ballast reactance of the fluorescent lamp circuit when dimming is detected. Fluorescent lamps having ballast circuits that include the electronic circuit means of the invention can be retrofitted into existing lighting networks and dimmed using existing triac phase-cut dimmers which were previously intended only for dimming incandescent lamps. The electronic circuit means of the invention can be included into the ballast circuit built into the base of compact fluorescent lamps ("CFLs"), in which case no additional hardware other than the compact fluorescent lamp itself is required. The invention provides a simple and cost-effective solution to alleviate the problems in using triac phase-cut dimmers to dim fluorescent lamps.

It will be appreciated that Figures 2, 4 and 5 are merely illustrative of some of the embodiments of the invention, and it will be apparent to a person skilled in the art that other embodiments of the invention may be devised which nevertheless fall within the scope of the appended claims.

Claims

1. Electronic circuit means for increasing the ability of a fluorescent lamp to be dimmed using a standard dimmer, comprising at least one resistor connected between input terminals of a fluorescent lamp circuit to thereby improve the power factor of the fluorescent lamp circuit.

2. Electronic circuit means as claimed in claim 1 in which the standard dimmer is a triac phase-cut dimmer.

3. Electronic circuit means as claimed in claim 1 or claim 2 which includes a sensing circuit operable to lower the value of the resistance between the input terminals of the fluorescent lamp circuit when dimming of the phase-cut dimmer is detected.

4. Electronic circuit means as claimed in claim 3 in which the sensing circuit is connected between the output of a bridge rectifier and the fluorescent lamp circuit.

5. Electronic circuit means as claimed in claim 4 in which the sensing circuit includes a first resistor and a second resistor connected in series between output terminals of the bridge rectifier, and a flip-flop arrangement of transistors operable to short-circuit the second resistor when the voltage at the output of the bridge rectifier drops below a predefined threshold.

6. Electronic circuit means as claimed in claim 5 in which the first resistor has a lower resistance than the second resistor.

7. Electronic circuit means as claimed in any one of the preceding claims which includes a second sensing circuit operable to sense when the phase-cut dimmer is dimming beyond a predefined threshold and to then introduce a higher value reactance in series with the fluorescent lamp of the fluorescent lamp circuit.

8. Electronic circuit means as claimed in claim 7 in which the second sensing circuit includes a ballast coil connectable in parallel with an existing ballast coil of the fluorescent lamp circuit by means of electromagnetic or electronic switch, wherein the switch is actuated when the phase-cut dimmer is dimmed beyond a predefined threshold so as to disconnect the ballast coil from its parallel connection with the existing ballast coil.

9. A method of increasing the ability of a fluorescent lamp to be dimmed using a standard dimmer, comprising connecting at least one resistor between input terminals of a fluorescent lamp circuit to thereby improve the power factor of the fluorescent lamp circuit.

10. A method a claimed in claim 9 in which the standard dimmer is a triac phase-cut dimmer.

11. A method as claimed in claim 9 or claim 10 which includes sensing when the phase-cut dimmer is dimming beyond a predefined threshold and then lowering the value of the resistance between the input terminals of the fluorescent lamp circuit.

12. A method as claimed in any one of claims 9 to 11 which includes sensing when the phase-cut dimmer is dimming beyond a predefined threshold and then introducing a higher value reactance in series with the fluorescent lamp of the fluorescent lamp circuit.