GB2151115A

GB2151115A - Control circuit for a fluorescent tube

Info

Publication number: GB2151115A
Application number: GB08331053A
Authority: GB
Inventors: Richard Sidney Marshall
Original assignee: CONCORD CONTROLS Ltd
Current assignee: CONCORD CONTROLS Ltd
Priority date: 1983-11-21
Filing date: 1983-11-21
Publication date: 1985-07-10
Also published as: GB8331053D0

Abstract

The invention relates to a control circuit connected to an electronic ballast of a fluorescent tube. The control circuit comprises a gating circuit 44 having a plurality of stable states and a detector 54 for applying to the gating circuit 44 a triggering signal indicative of the presence of mains supply voltage at the fluorescent tube. The gating circuit changes state in response to brief interruption of the mains supply to the tube and the operation of the ballast is enabled or inhibited in dependence upon the state of the gating circuit 44. When fitted to some tubes in a room, the control circuit allows only some of the tubes to be switched on selectively without altering the wiring installation. When first the wall switch is operated only tubes without the control circuit are switched, whereas a brief interruption of the mains supply by the common wall switch will also switch on the tubes provided with the control circuit. Sets of tubes may be responsive to different numbers of mains interruptions. <IMAGE>

Description

SPECIFICATION Control circuit for a fluorescent tube The present invention relates to a ci rcuit for controlling fluorescent tubes forthe purpose of providing energy saving.

In a conventional fluorescent lightfitting, the mains supply is applied byway of a wall switch tothefitting which includes a ballast, the fluorescent tube, and a starter switch. Thefluorescent tube has two heater filaments at its opposite ends and these are connected across the starter switch in such a mannerthatwhen the switch is closed current flows through the ballast and the heaterfilaments whereas when the starter switch is open the current no longer passes through the heaterfilament and the mains voltage is applied across the fluorescent tube.

In operation, it is arranged that when the wall switch is first operated the starter switch is closed so that the mains current flows through the heater filaments and these are pre-heated as a result. After a time, the starterswitch opensautomaticallyandthe rapid interruption of the current through the ballast causes a surge in voltage which fires the tube. The starter is now by-passed by the current through the fluorescent tube and serves no further purpose.

The ballast in addition to causing a surge of current to fire the tube serves to limitthe currentthrough the tube after it has fired. When the mains voltage is sufficiently high,the ballast may consistofan inductance or choke but when the mains voltage is low the ballast also also acts to step up the mains voltage and may consist of a transformer.

More recently, it has been proposed to design electronic circuits to serve the different functions fulfilled by the ballast as this may be less expensive than a choke or a transformer. The designs have been perfected to such an extentthatthe electronic ballast may be formed as an integrated circuit. The present invention is concerned with a modification which may be incorporated in such an electronic ballast for a fluorescent tube to enable energy saving.

There are several light fittings available which have more than onefluorescenttube and in such fittings each tube is provided with its own ballast. However, the tubes are all connected in parallel so that all the tubes are either switched on or all are switched off.

This can often mean unnecessary energy consumption. A similar energy wastage occurs when several single lightfittings are installed in the same room and are connected in parallel for operation bythe same wall switch.

It is of course possible to rewire a building so that each tube is operated by its own light switch but the costs involved in modifying an existing installation are such that they would not be warranted by the energy savings possible.

The present invention seeks to enable fluorescent light tubes operated by the same light switch to be energised selectively without requiring extensive modification ofthewiring installation.

In accordance with the presentinvention,there is provided a control circuit connected to an electronic ballast circuit of a fluorescent tu be, which control circuit comprises a gating circuit having a plurality of stable states and means for applying to the gating circuit a triggering signal indicative of the presence of mains supplyvoltage at the fluorescenttube,the gating circuit being operative to change state in response to brief interruption of the mainssupplyto the tube and the operation ofthe ballast being enabled or inhibited in dependence upon the state ofthe gating circuit.

In its simplest form, the control circuit has onlytwo states and is initially in the state in which it inhibits the operation of the ballast and is triggered into its second state by brieftoggling of the main switch. The expression "toggling" is intended to mean cycling of the switch (switching off and then on again) within a relatively short time.

The modified electronic ballast may readily be incorporation in an installation and then acts to prevent operation ofthe tube if the mains light switch is merely switched on and left on. If only some ofthe tubes in a fitting or in a room are provided with the modified ballast of the invention those tubes will not operate when the wall switch isfirstoperated and only the remaining fluorescent tubes will fire. Should such illumination prove inadequate then by making the conscious effort oftoggling the mains switch, the control circuit will be switched into its second stable state in which it will enable operation of the electronic ballast and thereby permit operation ofthe associated fluorescent tube.

Using control circuits having only two stable states only two levels of illumination are possible but clearly the control circuit may have several states thereby requiring the mains light switch to be toggled more than once before the associated tube can be allowed to fire.

When the control circuit has several stable states it is preferable to arrange forthe circuitto be programmable by means of micro-switches incorporated within the control circuit so thatthe number of times that the mains switch must be toggled before the tube is energised may be selected by the user. For example, the multi-state control circuit may comprise a shift register or a presettable counter.

The invention will now be described furtherm by way of example, with reference to the accompanying drawings, in which; Figure lisa block diagram of a fluorescent light installation fitted with an energy saving ballast, and Figure 2 shows a more detailed block diagram ofthe control circuit in Figure 1.

In Figure 1, apartfrom the control circuit 10 and the electronic ballast 22 connected in place of a conventional ballast,the remaining componentsallconstitute part of a conventional installation. The fluores cent light tube 14 has heaters 16 at each end and the power supply to the tube is derived by way of an optional programmed timer switch 18 and a wall switch 20 connected in series with one and other and by way ofthe ballast 22. The circuit illustrated also comprises two power factor correction capacitors 24 and 26 butthese areoptional and are not present in all installations. The heaters 16 are further connected to a socket which receives the starter switch 28.

In the absence of the control circuit 10, when power is supplied across the tube 14 by closing ofthe two series connected switches 18 and 20, the starter switch initially closes so that current flows through the ballast 22 and the two heaterfilaments 16. Afterthe heaters 16 have been pre-heated, the starter switch 12 opens and the full mains voltage is applied across the gas in the fluorescent tube 14 which now fires. The voltage surge caused by the ballast 22 when the current is initially interrupted assists in thefiring ofthe tube.

The function ofthe control circuit 10 is to ensure that when the switches 18 and 20 are closed forthe first time, the ballast 22 does not operate. If, the mains supply is briefly interrupted and reconnected, however, then the ballast 22 is connected through the control circuit 10to operate in the manner previously de- scribed in orderto fire the tube.

The circuit 10 (see Figure 2) includes a full wave rectifier30 connected through lines 11 and 15 across the mains supply. The output signal ofthe control circuit 10 which is fed to the electronic ballast 22 over output line 19 is derived from a gating circuit 44, which signal is applied to the ballast 22 through an amplifier 40. The powerforthe gating circuit 44 as well as the remaining components is derived from a power supply circuit 52 connected to the rectifier 30 by way of a capacitor 38 which acts to isolate any DC component that may be present, for example when a power factor correction capacitor is used in the lightfitting. The triggering pulsesforthe gating circuit are produced by a detector54which serves to detect each time power is present.

A first trigger impulse is applied from the detector 54 by way of a reset circuit 50 which acts to ensure that upon the first actuation the gating circuit 44 is always in its first stable state in which the ballast 22 is prevented from operating.

Three furthertimers 56, 46 and 48 are associated with the gating circuit 44. The timer 56 ensures that any brief interruptions of the power supply, as may be caused by voltage transients or by contact bounce, are disregarded by the gating circuit 44. This timer 56 has a time period ofthe order of 100 ms and pulses of shorter durations are disregarded. The second timer 46 produces triggering pulses if the period of disconnection ofthe mains supply is between 1 OOms and 1 second and this represents the toggling action which is intended to alter the state of the gating circuit in orderto operate the ballast 22.Thetimer48 is provided for switching offthe gating circuit 44 if the toggle cycle exceeds 6 seconds and enables the illumination do be dimmed by a long interruption of the mains supply.

There is a tendencyfor room illumination to be left on at a maximum level even ifthe ambient light has improved during the course ofthe day. In orderto reduce such energy wastage, it is possible to use the optional switch 18 (Figure 1) which may be a central switch common to several light fittings, to interrupt the powersupplyfor long enough to cause itto come on again atthe lower illumination level. This may, for example, be performed automatically at certain times ofthe dayor in dependence upon the level of ambient light IFthe higher level is in fact essential, then the wall switch must be brieflytoggled to change the ight intensity by firing the tubes fitted with the control circuit of the invention.

If the ballast 22 is formed as an integrated circuit then it is possible to incorporate the entire control circuit on the same chip orin the same package. Thus the advantage of energy saving may be achieved without any significant effect on the cost of manufac ture of the electronic ballast.

Claims

1. A control circuit connected to an electronic ballast circuit of a fluorescent tube, which control circuit comprises a gating circuit having a plurality of stable states and means for applying to the gating circuit a triggering signal indicative ofthe presence of mains supply voltage at the fluorescent tube, the gating circuit being operative to change state in response to brief interruption ofthe mains supply to the tube and the operation ofthe ballast being enabied or inhibited in dependence upon the state ofthe gating circuit.

2. A combined ballast and control circuit as claimed in claim 1, wherein the gating circuit has only two states and is initially in the state in which it inhibits the operation of the ballast and is triggered into its second state by brieftoggling of the main switch.

3. A combined ballast and control circuit as claimed in claim 1, wherein the gating circuit has several states and requires the mains light switch to be toggled more than once before the associated tube can be permitted to fire.

4. A combined ballast and control circuit as claimed in claim 3, wherein the control circuit is programmable by means of micro-switches so that the number oftimes that the mains switch must be toggled before the tube is energised may be manually selected.

5. A combined ballast and control circuit as claimed in claim 4, wherein the gating circuit is constituted by a shift register or a presettable counter.

6. Acombined ballast and control circuit as claimed in any preceding claim, wherein means are provided to prevent triggering of the gating circuit by pulses having an excessively short duration, whereby to preventtriggering by noise or contact bounce.

7. A combined ballast and control circuit substantially as herein described with reference to and as illustrated in the accompanying drawings.