WO1998012901A1

WO1998012901A1 - Fluorescent lamp illumination level control

Info

Publication number: WO1998012901A1
Application number: PCT/AU1997/000606
Authority: WO
Inventors: Koichi Hayashi
Original assignee: H.P.M. Industries Pty. Limited
Priority date: 1996-09-18
Filing date: 1997-09-16
Publication date: 1998-03-26
Also published as: AUPO244796A0

Abstract

An illumination level controller for a fluorescent lamp. The controller comprises a dimmer control circuit (12) for generating a phase-angle controlled supply voltage which is connected across a circuit (11) containing a fluorescent lamp (16) and a series inductor (17). A solid state switching device (19) is connected in circuit with the lamp (16) and the switching device is arranged to be gated into conduction to direct heating current through the lamp cathodes (18). A gating circuit (20) is provided to generate gating signals to be applied to the switching device (19) for a time interval during each half-cycle of the supply voltage and to vary the gating time interval inversely with change in the conduction angle of the supply voltage.

Description

FLUORESCENT LAMP ILLUMINATION LEVEL CONTROL

FIELD OF THE INVENTION

This invention relates to a method of and apparatus for controlling the illumination level of fluorescent lamps. The invention has particular application to controlled dimming of fluorescent lamps and is hereinafter described in that context. However, whereas lamp dimming normally provides for selective variation of the illumination levels, the present invention has more general application, to adjustable setting of illumination to fixed levels. BACKGROUND OF THE INVENTION

Fluorescent lamp dimming is effected by gating a solid state switching device (usually a triac) into conduction at a selected phase angle and, hence, over a selected conduction angle during each half-cycle of a mains supply voltage. Also, m order to ensure that illumination is established and sustained during reduced conduction angles, filament heating current is fed through the lamp cathodes during lamp operation as well as during start-up. Various methods are employed to achieve this, for example by use of a filament transformer when a three-wire (active, dimmed active and neutral) connection may be made to the lamp from a switched dimmer control circuit or by use of a so-called filament driver when a two-wire (dimmed active and neutral) connection is to be made to the lamp.

However, it has been found m practice that, whilst controlled dimming may be employed m relation to a 38 mm (1.5 inch) diameter filament tube, it cannot be employed in relation to a 25 mm (1.0 inch) diameter tube, even when filament heating current is maintained following initial lomsation of discharge gas within the tube. SUMMARY OF THE INVENTION

The present invention seeks to avoid this problem by providing a fluorescent lamp illumination level controller which comprises: means for connecting a phase-angle controlled supply voltage across a circuit containing a fluorescent lamp and a series inductor, means for directing heating current through a circuit that includes the lamp cathodes and the inductor for a time interval during each half-cycle of the supply voltage, and means for varying the time interval inversely with change m the conduction angle of the supply voltage.

The invention also provides an illumination level controller as above defined when connected in circuit w th a fluorescent lamp and a series inductor .

Expressed in an alternative wav, the invention provides et method of controlling the illumination level of a fluorescent lamp wherein a phase-angle controlled supply voltage is connected across a circuit containing the fluorescent lamp and a series inductor, wherein heating current is directed through a circuit that includes the lamp cathodes and the inductor for a time interval during each half-cycle of the supply voltage and wherein the time interval of heating current flow through the lamp cathode is varied inversely with change m the conduction angle of the supply voltage. PREFERRED FEATURES OF THE INVENTION

The means for directing the heating current through the lamp cathodes and for varying the time interval of current flow during each half-cycle preferably includes a solid state switching device and a gating circuit arranged to gate the switching device into conduction for the required time interval during each half-cycle. In this preferred arrangement, the illumination level controller functions to maintain the lamp cathodes at a required temperature level when the supply voltage level is not sufficient to sustain gas discharge heating of the cathodes at a sufficiently high level. The cathode heating current is caused to flow for the time interval of each half-cycle of the supply during which the switching device is turned on, and as the conduction angle and hence level of the supply voltage increases, so the time interval during which the flow of heating current occurs decreases, and vice versa. Thus, the controller functions to maintain a balance between maximum illumination, when the supply voltage is at a maximum level (i.e., has a maximum conduction angle) and the duration of current flow to the cathode is minimised, and minimum illumination when the supply voltage is at a minimum level (i.e., has a minimum conduction angle) and the duration of current flow to the cathodes is maximised.

Also, in operation of the illumination level controller, when the switching device is gated off in each half-cycle of the supply voltage, the switching device functions to interrupt current flow to the lamp cathodes and, so, to induce the generation of back e f in the inductor that is located m series with the fluorescent lamp.

Because the switching device must be gated on and off within the duration of each half-cycle of the supply, it may comprise an ac switching device or a dc switching device in circuit with a rectifier. The switching device preferably comprises a field effect transistor, but any other solid state switching device (or arrangement of devices) which can be made to function as a switch during each half-cycle may be employed.

The illumination level controller may be employed m conjunction with or be integrated with a more-or-less conventional dimming control circuit, and, in such circumstances, the phase-angle controlled supply voltage will be derived directly as the dimmed active voltage output of the dimming control circuit. A preferred embodiment of a fluorescent lamp illumination level controller which is suitable for use in conjunction with a dimming control circuit is now described by way of example with reference to the accompanying drawings .

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings :

Figure 1 shows a schematic representation of the illumination level controller in association with a dimming control circuit and a fluorescent lamp circuit,

Figure 2 shows a complete circuit diagram of the illumination level controller, again m association with a dimming control circuit and a fluorescent lamp circuit,

Figure 3 shows graphical representations of voltage waveforms in various parts of the controller circuit, and

Figure 4 shows comparative voltage waveforms under (A) maximum and (B) minimum levels of lamp illumination.

DETAILED DESCRIPTION OF PREFERRED MODE OF PERFORMING THE

INVENTION As illustrated in Figure 1, the illumination level controller 10 is connected in circuit with a fluorescent lamp circuit 11 and a dimming control circuit 12. The dimming control circuit 12 is connected to a single phase mains supply voltage 13 and it comprises a conventional triac circuit 14 for which the conduction angle may be varied by a gating circuit 15. The output from the dimming control circuit 12 comprises a phase-angle controlled supply voltage V_: which is connected across the series circuit 11 containing a fluorescent lamp 16 and an inductor (i.e., a ballast) 17.

The controller 10 may be considered as having three circuit portions, the first of which provides for connection of the controlled supply voltage V] across the lamp circuit

11, the second of which includes a solid state switching device 19 for directing current through cathodes 18 of the fluorescent lamp 16 and the third one 20 of which provides for controlled gating of the switching device 19.

As shown, the switching device 19 comprises a field effect transistor which is connected across two terminals of a rectifier bridge circuit 21. The rectifier bridge 21 is itself connected m circuit with the lamp cathodes 18 and, when gated into conduction, the transistor 19 provides for heating current flow through the lamp cathodes.

The control circuit 20 is provided to effect on-off gating of the transistor 19 during each half-cycle of the supply voltage Vi, and the control circuit is arranged also to vary the gating time interval as an inverse function of the level (i.e., the r. .s. value) of the supply voltage V, as determined by the conduction angle of the triac 14. This is described in more detail later m this specification.

In operation of the controller as described thus far, when the controller 10 and the dimming circuit 12 are first connected to the mains supply 13 the transistor 19 is initially turned ON and the voltage V_t across the transistor will be almost zero. Therefore, the lamp 16 will not be turned ON but heating current will flow through the two cathodes 18 to establish voltage drop

due to the resistance of the filaments that form the cathodes.

After expiration of a predetermined time interval of two or three seconds, when the temperature of the cathodes has been raised to a required level, the control circuit 20 functions to turn the transistor 19 OFF, causing the current to drop almost instantly to zero. This m turn induces generation of a back e f in the inductor 17 at a level sufficient to push the voltage V₂ up to approximately 1 kV, causing the lamp to turn ON.

Thereafter, the control circuit 20 functions to turn the transistor 19 ON and OFF during the period of each half- cycle of the supply voltage V and the ON period of the transistor is controlled as an inverse function of the conduction angle of the supply voltage V, . That is, with a long conduction angle and a resultant "high" level supply voltage, the ON period of the transistor is controlled to be "short". Conversely, with a short conduction angle and a resultant "low" level supply voltage, the ON period of the transistor is controlled to be "long". With a high level supply voltage, the lamp 16 is energised for maximum illumination and the current flow through the lamp under that condition is sufficient to maintain the cathodes 18 at the required operating temperature level. When the supply voltage level drops, the illumination level of the lamp drops and cathode heating is maintained at a required level by current flow through the transistor/cathode circuit for an increasing time interval during each half-cycle, so that lamp illumination is sustained.

The illumination level controller may be implemented in a form that provides for maximum lamp illumination when the supply voltage output V, from the dimming control circuit 12 has a conduction angle of approximately 150°. Under this condition the OFF period of the transistor 19 is long, in the order of 9 mS during each half-cycle of a 50 Hz supply, and cathode heating is effected primarily as a consequence of "high" energy level gas discharge within the lamp.

Minimum lamp illumination may be provided when the supply voltage output V, from the dimming control circuit 12 has a conduction angle of approximately 90°. Under this condition the ON period of the transistor 19 is controlled to be long, in the order of 9 mS during each half-cycle of the supply. Under this condition, almost all of the power that is generated during the period of current conduction by the transistor 19 is dissipated in the form of heat m the cathode filaments 18 and a "small" amount of energy is expended in gas discharge within the lamp. The illumination level of the lamp under this condition may be reduced to something in the order of 5°₀ of the maximum illumination level but the lamp is prevented from extinguishing by the periodic generation of back emf within the inductor during each half-cycle of the supply, as a consequence of the transistor 19 being turned OFF during the duration of each half-cycle.

The illumination level controller 10 as shown in Figure 1 may be implemented in various circuit configurations, one of which is shown by way of example m Figure 2.

The circuit as illustrated in Figure 2 incorporates the same elements as those shown m Figure 1, principally the dimming control circuit 12, the fluorescent lamp 16 and inductor 17, the field effect transistor 19 and the gating control circuit 20. The functions of the various circuit components are largely self evident and, where appropriate, are referred to as follows m sub-circuit groupings.

The gating control circuit 20 includes a dc power supply 25 for a monostable IC oscillator 26. The output from the power supply s stabilised by a zener diode 27 and is connected to pin 8 of the oscillator 26. At initial switch-on, the output V₇ of the oscillator 26 is initially held low for the predetermined time interval of two to three seconds. This time interval is determined by the value of the capacitor connected to pin 5 of the oscillator 26. Thereafter, the oscillator 26 is triggered once during each half-cycle of the supply voltage via pin 2 by voltage V_L going low at or near the zero crossing point of the supply voltage Vi, and the output V derived from pin 3 of the oscillator goes high for a period which is determined by and increases with decreasing level of the voltage V applied to the pins 6, 7 of the oscillator 26. The voltage Vr, is derived as a dc voltage level which varies inversely as a function of the level of the supply voltage V, and, thus, the output V₇ from the oscillator remains high for a time interval which is proportional to the conduction angle (i.e., r.m.s. voltage level) of the supply voltage Vi .

The output V from the oscillator is coupled to the gate of the transistor 19 by way of an opto-coupler 28 and, thus, when voltage V goes high, voltage V< applied to the gate of the transistor 19 goes low.

The relationship of the waveforms of the various voltages V V , V_b, V₇ and V< is shown in Figure 3 and the following summary of the operation of the illumination level controller is provided in the context of the waveforms shown in Figure 3.

After expiration of the initial predetermined time interval following switch-on, the oscillator 26 is triggered by voltage V once during each hall-cycle of the supply voltage Vi, and the output voltage V, of the oscillator goes high. As a result, the gating voltage V applied to the transistor 19 goes low and the transistor is turned off. Current flow through the transistor is terminated and the generation of back emf is, as a result, induced m the inductor 17, resulting m a periodic spike m the voltage V- across the transistor 19. The voltage V_ lifts the supply voltage Vi that appears across the lamp 16 during a portion of each half-cycle of the supply voltage.

Following an interval of time as determined by the dc level of the voltage v., the output voltage V₇ of the oscillator goes low. The transistor gate voltage V^ consequently goes high and turns the transistor 19 ON until such time as the output voltage V₇ from the oscillator again goes high.

When the transistor 19 is turned ON, the voltage V_? goes to zero, and current is caused to flow through the lamp cathodes 18, by way of the transistor 19, for the interval during which the voltage V^ is high and the voltage V₇ is low.

Figure 4 shows a comparison of the voltage waveforms V,, V_. and V^ under maximum and minimum levels of lamp illumination. As illustrated, when the conduction angle of the supply voltage V_: increases, the dc level of voltage V- decreases, causing the period of the transistor gating voltage V- to decrease. Consequently, the interval during which heating current is fed to the lamp cathode decreases and the period during which the lamp is held ON increases. Thus, maximum illumination is derived when the conduction angle of the supply voltage is at a maximum.

As also can be seen from Figure 4 of the drawings, the magnitude of the spike m the voltage waveform V_^ is significantly greater when the voltage supply V is controlled to provide a low level of lamp illumination.

Claims

THE CLAIMS

1. A fluorescent lamp illumination level controller which comprises means for connecting a phase-angle controlled supply voltage across a circuit containing a fluorescent lamp and a series inductor, means for directing heating current through a circuit that includes the lamp cathodes and the inductor for a time interval during each half-cycle of the supply voltage, and means for varying the time interval inversely with change m the conduction angle of the supply voltage.

2. A fluorescent lamp illumination level controller which comprises means for connecting a phase-angle controlled supply voltage across a circuit containing a fluorescent lamp and a series inductor, a solid state switching device arranged for connection in circuit with the lamp and arranged when gated into conduction to direct heating current through a circuit containing the lamp cathodes and the inductor during each half-cycle of the supply voltage, and a gating circuit arranged to gate the switching device into conduction for a time interval during each half-cycle of the supply voltage and to vary the time interval inversely with change in the conduction angle of the supply voltage.

3. The illumination level controller as claimed in claim 2 wherein the switching device comprises a dc switching device m circuit with a rectifier circuit. . The illumination level controller as claimed in claim 3 wherein the switching device comprises a field effect transistor. 5. The illumination level controller as claimed in any one of claims 2 to 4 wherein the gating circuit includes a pulse generator which is controlled to generate an output pulse following each zero crossing of the supply voltage, the output pulse having a duration which is proportional to the r.m.s. voltage level of the supply voltage, and wherein a signal which is generated as an inverted form of the output pulse is applied to the gate of the switching device.

6. The illumination level controller as claimed in claim 5 wherein an opto-coupler is provided for coupling the output pulse of the pulse generator to the gate of the switching device.

7. The illumination level controller as claimed in any one of claims 2 to 6 wherein the gating circuit is controlled to inhibit gating of the switching device for a predetermined time period following initial connection of the supply voltage across the circuit containing the fluorescent lamp and the series inductor.

8. The illumination level controller as claimed m any one of the preceding claims and including a dimming circuit which generates the phase-angle controlled supply voltage, the dimming circuit including an ac switching device and a further gating circuit for gating the ac switching device at a selected phase angle during each half- cycle of an alternating voltage supply. 9. An illumination level controller connected in circuit with a fluorescent lamp and comprising means for generating a phase-angle controlled alternating supply voltage and for connecting the supply voltage across a circuit containing the fluorescent lamp and a series inductor, a solid state switching device connected in circuit with the lamp and arranged when gated into conduction to direct heating current through a circuit containing the lamp cathodes and the inductor during each half-cycle of the voltage supply, and a gating circuit arranged to effect on-off gating of the switching device during each half-cycle of the supply voltage and to vary the gating time interval as an inverse function of the level of the supply voltage as determined by the conduction phase angle of the supply voltage. 10. The illumination level controller as claimed in claim 9 wherein the switching device comprises a dc switching device which is connected by way of a rectifier circuit with the means for generating the voltage supply and the lamp cathodes. 11. The illumination level controller as claimed in claim 10 wherein the gating circuit comprises a pulse generator which has inputs derived from the supply voltage, which is controlled to generate an output pulse following each zero crossing of the supply voltage and which is controlled such that the output pulse has a duration which is proportional to the level of the supply voltage.

12. A method of controlling the illumination level of a fluorescent lamp wherein a phase-angle controlled supply voltage s connected across a circuit containing the fluorescent lamp and a series inductor, wherein heating current is directed through a circuit that includes the lamp cathodes and the inductor for a time interval during each half-cycle of the supply voltage, and wherein the time interval of heating current flow through the lamp cathode is varied inversely with change m the conduction angle of the supply voltage.

13. A fluorescent lamp illumination level controller substantially as shown in the accompanying drawings and substantially as hereinbefore described with reference thereto.

14. A method of controlling the illumination level of a fluorescent lamp, substantially as hereinbefore described with reference to the accompanying drawings.