GB2205206A

GB2205206A - Filament driver for dimmable fluorescent lamp

Info

Publication number: GB2205206A
Application number: GB08809997A
Authority: GB
Inventors: Gregory Yates
Original assignee: Macquarie University; HPM Industries Pty Ltd
Current assignee: Macquarie University; HPM Industries Pty Ltd
Priority date: 1987-05-26
Filing date: 1988-04-27
Publication date: 1988-11-30
Anticipated expiration: 2008-04-27
Also published as: GB2205206B; GB8809997D0

Abstract

A filament driver circuit 33, interposed between a fluorescent lamp 30 and a phase controlled dimmer 32, comprises a transformer 38 having a primary winding 39 connected to the dimmer 32 and secondaries 40 connected to the lamp filaments, and a high pass filter which passes to the filaments only harmonic components of the output voltage from dimmer 32. The filter may be an active device, or may be formed by the primary 39 and a capacitor 41. A bleed device 43 discharges capacitor 41 before each firing cycle of the dimmer 32 and may be formed by a switched device or a resistor such as an incandescent lamp. Circuit components may be arranged so that secondaries 40 have an output that rises from 3 volts with the dimmer 32 at a maximum light setting, to 4.5 volts with dimmer 32 at a minimum light setting. The transformer 38 may have a further secondary (44), (Fig. 4) auto wound with primary 39 to provide an increased lamp starting voltage, a capacitor (45) acting effectively to open circuit secondary (44) when the lamp is operating. <IMAGE>

Description

FILAMENT DRIVER FOR FLUORESCENT LAMPS This invention relates to a filament driver for fluorescent lamps and, in particular, to a filament driver which obviates the need for three-wire connections between dimmer control devices and fluorescent lamps.

Fluorescent lamps that are energised directly from the main supply without any intervening dimmer control may be regarded as falling within two general categories. The first category uses a glow switch starter to cause starting current to flow through the cathode filaments prior to ionisation of the gas within the lamp and the second uses a filament transformer to provide substantially constant current to the cathode filaments under both starting and operating conditions. However, when the light emission from a fluorescent lamp is required to be controlled, a filament transformer must always be used in conjunction with a dimmer control device to provide a suffiently high level of current to the cathode filaments to ensure that electron emission is sustained (when low level illumination is required) when the voltage to the lamp is reduced by the dimmer control device.

In order that the filament current may be sustained at the required level, the primary winding of the filament transformer must be permanently connected to the mains supply voltage and, thus, be powered independently of the output voltage from the dimmer control device. This then necessitates a three-wire connection between the lamp switch/dimmer device and the lamp circuit, the connection comprising a switched active wire, a dimmed active wire and a neutral wire.

Three-wire connections in new installations are feasible but expensive in terms of additional wire cost. However, considerable problems are encountered in existing installations when dimmer control is required. In some cases the cost and/or inconvenience of replacing a two-wire connection with a three-wire connection makes the operation impossible, impracticable or, at best, inconvenient.

The present invention is directed to a filament driver which permits the use of a two-wire connection between a fluorescent lamp circuit and an associated dimmer control device and which thereby facilitates the installation of dimmer control using existing wiring.

Broadly defined, the filament driver in accordance with the present invention comprises a transformer having a primary winding that is connectable in circuit with the output of a phase controlled dimmer control device and at least one secondary winding that is connectable to the cathode filaments of a fluorescent lamp, and a high pass filter that is arranged to pass to the cathode filaments only harmonic components of the output voltage from the dimmer control device.

The high pass filter is preferabaly located at the primary side of the transformer and it most preferably comprises the primary winding of the transformer and a capacitor in series with that winding. Also, a bleed device is preferably incorporated in the filament driver for bleeding from the capacitor, before the commencement of each firing cycle of the dimmer control device, any residual charge that exists on the capacitor.

However, the high pass filter may alternatively be located at the secondary side of the transformer and, in such case, the filter preferably comprises the combination of a capacitor in series with the or each secondary winding.

In operation of the preferred form of the filament driver, as the phase control of the dimmer device is advanced to reduce the rms value of the voltage to the fluorescent lamp, the effective voltage level of the harmonic components that are passed by the high pass filter increases, this providing a high transformer primary voltage and a proportionately high secondary voltage. Conversely, as the phase control of the dimmer device is retarded to increase the rms value of the voltage to the fluorescent lamp, the effective voltage level of the harmonic components decreases to reduce the secondary voltage. This form of regulation is, in fact, what is required to produce a relatively high filament current when the applied ionising voltage is low and a lower filament current when the ionising voltage is high and ionisation is self sustaining.By appropriately choosing the values of the circuit components the output of the secondary winding of the transformer may be controlled to remain at a desired level over the range of illumination levels of the lamp. The circuit components are preferably selected to provide a (secondary) filament voltage that rises from about 3 volts rms with the dimmer control set for maximum lighting levels to about 4.5 volts rms with the dimmer control set to provide for minimum light emission from the lamp.

The invention will be more fully understood from the following description of various lighting circuits which are shown in the accompanying drawings wherein: Figure 1 shows a circuit diagram of a Prior art lighting circuit that incorporates a fluorescent lamp, a filament driver for the lamp, a dimmer control device and a three-wire connection between the control device and the filament driver; Figure 2 shows a simple dimmer control circuit for use in the lighting circuit of Figure 1 and for use also in the circuits of Figures 3 to 5; Figure 3 shows a circuit diagram of a lighting circuit that incorporates a filament driver in accordance with a first embodiment of the present invention, the filament driver being appropriate to a filament lamp that can be started with an ionising voltage equal to or less than the mains supply voltage;; Figure 4 shows a circuit diagram of a lighting circuit that incorporates a filament driver in accordance with a second embodiment of the invention, the filament driver in this case being appropriate to a fluorescent lamp that requires a starting voltage greater than that which is obtainable from the mains supply; and Figure 5 shows a circuit diagram that is similar to that illustrated in Figure 4 but adapted for driving two parallel connected fluorescent lamps.

As shown in Figure 1, in the prior art lamp circuits a fluorescent lamp 10 is connected across a single phase ac mains supply and in circuit with a switch 11, a dimmer control device 12, a filament transformer 13 and a ballast 14. A resistive load 15 is located across the output of the dimmer control device 12 to provide for latching of a triac 16 (Figure 2) in the control device 12, and a power factor correcting capacitor 17 may be connected across the main supply to correct for the predominantly inductive load imposed on the mains by the lamp circuit. The capacitor 17 would normally be provided only in large commercial or industrial lighting circuits and not in domestic circuits.

When the lamp 10 has been started and the gas within the lamp is ionised, current flows through the lamp circuit by way of the so-called "dimmed active" line 18, this current being limited in the usual way by the ballast 14. However, in order that the gas within the lamp may be ionised at switch-on and in order that ionisation may be sustained at low illumination levels, the lamp cathode filaments are heated by current which must be maintained at a minimum level under all operating conditions. The filament current is derived from the two secondary windings of the filament transformer 13 and, in order that the secondary current will be maintained at the required level when the output from the dimmer control device 12 is lowered, the primary winding of the transformer 13 is connected by a so-called "switched active" line 19 to the mains supply by way of the switch 11. Thus, it is necessary in the prior art circuit to run three wires to the lighting circuit 20; the dimmed active wire 18, the switched active wire 19 and a neutral wire 21.

The dimmer control device 12, which is employed in the arrangement shown in Figure 1, is illustrated in Figure 2 and such device may also be used in the circuits which are to be described, with reference to Figures 3 to 5, in the context of the present invention. Although various degrees of sophistication may be built into the circuit, it fundamentally comprises the triac 16, a gating circuit and an rf filter. The gating circuit comprises a potentiometer 22, a capacitor 23 and a diac 24, and, in having a variable RC time constant, the gating circuit may be used to vary the phase angle at which the triac 16 is gated into a conductive state during each half-cycle.

The rf filter comprises a low pass filter having an inductor 25 and a capacitor 26 for filtering out high order harmonics that would cause interference in the rf band.

The present invention, which is to be hereinafter described in the context of Figures 3 to 5, obviates the need for the three-wire connection that is shown in Figure 1.

As shown in Figure 3, a fluorescent lamp 30 is powered from a single phase ac mains supply and the lamp is connected in circuit with a switch 31, a dimmer control device 32 of the type shown in Figure 2, a filament driver 33, a ballast 34 and, if required, a power factor correcting capacitor 35. Two wires 36 and 37 are provided in this case to connect the lamp circuit components 30, 33 and 34 to the main supply by way of the switch 31 and dimmer control device 32. This is in contrast with the prior art which, as above mentioned, requires a three-wire connection to the lamp circuit.

The filament driver 33 comprises a transformer 38 having a primary winding 39 and two secondary windings 40 which are connected to the cathode filaments of the lamp 30. A capacitor 41 is connected in series with the primary winding 39 of the transformer and the capacitor/primary winding together form a high pass filter which passes harmonics of the output wave form from the dimmer control device 32. Thus, whereas the full output from the dimmer control device 32 is applied (by way of connection 42) to the cathodes of the lamp 30 and is used to control the illumination level of the lamp, only the higher order harmonics of the output from the dimmer control device are passed to the transformer primary winding.

Therefore, the voltage which is induced in each of the secondary windings of the transformer is proportional to the rms voltage of the harmonic components of the output from the dimmer control device, and the turns ratio of the windings on the transformer 38 are chosen so as to provide a required output voltage at the second windings over a range of outputs from the dimmer control device 32.The output voltage from the secondary windings is chosen such that the required current level will be driven through the cathode filaments under all operating conditions, and it has been found that optimum conditions will exist if the circuit components are selected to provide a transformer secondary voltage that rises from about 3 volts rms with the dimmer control set for maximum lighting levels to about 4.5 volts rms with the dimmer control set to provide for minimum light emission from the lamp.

A bleed device 43 is connected across the transformer primary winding 39 and the capacitor 41 for bleeding from the capacitor any residual charge that exists on the capacitor at the instant before each firing cycle of the dimmer control device 32.

The bleed device 43 may comprise a resistor, but it preferably comprises a dynamic low resistance bleed device which is switched into a conductive state when required and which thereby avoids the heating losses associated with a resistive load.

When a mixed circuit consisting of a fluorescent lamp and an incandescent lamp is connected to the same dimmer control device, the incandescent lamp may be simply connected in the circuit in lieu of the bleed device 43.

The circuit which is shown in Figure 4 is similar to that which has been described above with reference to Figure 3 and like reference numerals have been employed to identify like parts. However, the circuit of Figure 4 is suitable for use with fluorescent lamps, such as those that are charged with Krypton gas, and require a high starting voltage to promote ionisation of the gas. In order to produce the high starting voltage, the transformer 38 is wound as an auto-transformer to provide a further secondary winding 44 which is capacitively coupled to the lamp 30 by a capacitor 45 and connecting wire 46.

With the arrangement shown, the total starting voltage across the lamp 30 will be the sum of the voltages being the sum of the voltages across the transformer windings 39 and 44, and the capacitor 45 will function effectively to open-circuit the secondary winding 44 following ionisation of the gas within the lamp.

The circuit arrangement which is shown in Figure 5 is similar to that which is illustrated in Figure 4, but adapted to accommodate simultaneous dimmer control of two parallel fluorescent lamps 30.

All of the circuit configurations shown in Figures 3 to 5 have a single inventive feature in common, namely, the inclusion of a high pass filter which functions to pass the harmonic components of the dimmer control output voltage to the primary of the transformer. This results in secondary voltages that increase slightly with a reduction in the rms voltage output from the dimmer control device, and such result is quite different from that which would be derived if the full (unfiltered) output from the dimmer control device were to be applied to the primary winding of the transformer.

It will of course be understood that the high pass filter may take various configurations and it need not be limited to the combination of a capacitor in series with the primary winding of the transformer. Other filter configurations, including active filters, may be adopted as an alternative to the configuration shown in the drawings.

Other variations may also be made in respect of the invention as above described without departing from the scope of the appendant claims.

Claims

CLAIMS:

1. A filament driver for location between a fluorescent lamp and a phase controlled dimmer control device, the filament driver comprising a transformer having a primary winding that is connectable in circuit with the output of the dimmer control device and at least one secondary winding that is connectable to the cathode filaments of the fluorescent lamp, and a high pass filter that is arranged to pass to the cathode filaments only harmonic components of the output voltage from the dimmer control device.

2. The filament driver as claimed in claim 1 wherein the high pass filter is located at the primary side of the transformer.

3. The filament driver as claimed in claim 2 wherein the high pass filter comprises the primary winding of the transformer and a capacitor in series with the primary winding.

4. The filament driver as claimed in claim 3 wherein a current bleed device is provided for bleeding from the capacitor, before the commencement of each firing cycle of the dimmer control device, any residual charge that exists on the capacitor.

5. The filament driver as claimed in claim 4 wherein the bleed device comprises a bleed resistor which is connected across the high pass filter.

6. The filament driver as claimed in any one of claims 2 to 5 wherein the transformer has two secondary windings one of which being connectable to a respective one of the cathode filaments of the fluorescent lamp.

7. A lighting circuit comprising a fluorescent lamp, a phase controlled dimmer control device and a ballast connectable in series across a single phase ac voltage supply, and a filament driver located in circuit between the dimmer control device and the lamp; the filament driver comprising a transformer having a primary winding that is connected in circuit with the output of the dimmer control device and at least one secondary winding that is connectable to the cathode filaments of the fluorescent lamp, and the filament driver further comprising a high pass filter that is arranged to pass to the cathode filaments only harmonic components of the output voltage from the dimmer control device.

8. The lighting circuit as claimed in claim 7 wherein the high pass filter is located at the primary side of the transformer.

9. The lighting circuit as claimed in claim 8 wherein the high pass filter comprises the primary winding of the transformer and a capacitor in series with the primary winding.

10. The lighting circuit as claimed in claim 9 wherein a current bleed device is provided for bleeding from the capacitor, before the commencement of each firing cycle of the dimmer control device, any residual charge that exists on the capacitor.

11. The lighting circuit as claimed in claim 10 wherein the bleed device comprises a bleed resistor which is connected across the high pass filter.

12. The lighting circuit as claimed in any one of claims 8 to 11 wherein the transformer has two secondary windings one of which being connectable to a respective one of the cathode filaments of the fluorescent lamp.

13. The lighting circuit as claimed in any one of claims 7 to 12 wherein the transformer has a further secondary winding that is auto-wound with the primary winding, wherein the output voltage is applied to one cathode of the lamp and wherein the output voltage of the further secondary winding is capacitively coupled to the other cathode of the lamp such that, during starting, the potential across the two cathodes is approximately equal to the sum of the voltages applied to the respective cathodes.

14. The lighting circuit substantially as hereinbefore described with reference to Figures 3, 4 or 5 of the accompanying drawings.