GB2155258A - Control circuit for a fluorescent tube - Google Patents

Abstract

A control circuit for insertion into the starter socket 24 of a fluorescent light fitting permits the light to be switched off or dimmed while the mains light switch 20 remains on. When switches 32,34 are closed by a control circuit 36, a short is placed across the socket 24 so as to prevent the flow of discharge current through the gas in the 30 tube 14. An alternative embodiment uses the same switch (38, Figure 3) for starting and stopping. The control circuit may have timing means to turn off the light after a predetermined time, or may be responsive to the ambient light level, or may be remotely controlled by signals transmitted over the mains wiring or by radio, infrared or ultrasonic waves. To prevent the tube 14 firing again on removal of the short, the short circuit current may be caused to decay gradually or may be switched off at such a time relative to the current flow through choke 22 that the induced voltage will not fire the tube. Circuit 36 may be arranged to close switch 32 during starting only after switch 20 has been toggled a preset number of times. The shorting means may be activated cyclically to interrupt current through the tube during part of each mains cycle, thereby resuming the light output from tube 14 rather than turning it off completely. <IMAGE>

Description

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

In a conventional fluorescent light fitting, the mains supply is applied by way of a wall switch to the fitting which includes a choke, the fluorescent tube and a starter switch. The fluorescent tube has two heater filaments at its opposite ends and these are connected across the starter switch in such a manner that when the switch is closed current flows through the choke and the heater filaments whereas when the starter switch is open the current no longer passes through the heater filament 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 starter switch opens automatically and the rapid interruption of the current through the choke 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.

It has already been proposed in copending Patent Application No. 8325596 to incorporate circuitry into the starter in order to provide energy saving. In particular, the above application enables only selected ones of the tubes in a room to fire when the lights are first turned on, it being possible to vary the level of illumination by toggling the light switch.

The present invention is similarly directed at taking advantage of the availability of the starter socket in fluorescent light fittings to permit simple installation of circuitry to provide energy savings.

:According to the present invention, there is provided a control circuit for insertion into the starter socket of a fluorescent light fitting, comprising shorting means which when activated effectively short circuit the terminals of the socket leading to the heater filaments so as to prevent the flow of discharge current through the gas in the tube, and means for activating the shorting means during the course of discharge through the gas in the tube to enable the level of illumination to be reduced.

The shorting means may constitute the starter switch or may comprise a further switching device connected in parallel with the starter switch.

The means for activating the shorting means may comprise timing means operative to switch off the tube entirely after a predetermined length of time has elapsed. Devices seving such a purpose are already available and are usually integrated into the light switch but the present invention enables easier installation and better control in as much as the different fittings in a room may have different time constants.

Instead of or in addition to means for activating the shorting means after a predetermined time interval, means may be provided for causing such activation in response to an increase in the level of ambient illumination. Such devices are also currently available and used to avoid unnecessary current consumption. Once switched off by the increase in mabient light, the tube may remain off either until a decrease in the ambient light level or until the main light switch is operated.

When the short circuit is applied across the terminals of the starter socket, current flows through the heater filaments instead of flowing through the tube. Care must therefore be taken when stopping the short circuit current that the tube does not then fire again spontaneously. This may be achieved in a variety of ways such as by gradually causing the short circuit current to decay or by switching off the current suddenly at a time suitably symchronised with the current flow through the choke to ensure that the induced voltage does not suffice to refire the tube.

It is further possible using a control circuit of the invention to implement systems of remote signalling such as used in large buildings to provide general commands to all the fittings in a specified area. Such remote signalling may employ signals superimposed on the mains supply or any other form of remote control such as by radio, infrared or ultrasonic waves.

Thus the present invention when combined with that in copending Application No.8325596 enables full control of the light fittings without the need to operate the light switch. The tubes can not only be prevented from firing until certain conditions are fulfilled but having fired the tubes may be remotely switched off using the same access points to the circuitry of the fittings, namely the terminals of the socket provided for the starter switch.

It is possible additionally to activate the shorting means cyclically to interrupt a proportion of the current flowing through the tube in each cycle, thereby permitting the light output to be reduced.

This may be desirable in certain installation where the control of the level of illumination is necessary to achieve different lighting effects but this technique would not be recommended from the point of view of energy saving since current flows through the heater filaments when the discharge current is interrupted. This however is desirable from the point of view of refiring the tube since the longer the interruption during each cycle, the higher will be the temperature of the filaments.

The invention will now ber described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a conventional fluorescent light fitting, Figure 2 is a circuit diagram of a modified fitting incorporating a control circuit of the invention but employing all of the original installation including the starter, and Figure 3 is a circuit diagram of a modified fitting incorporating a control circuit of the invention, in which the conventional starter is replaced by a switch forming part of the control circuit.

In the interest of clarity, throughout the drawings like parts have been allocated like reference numerals and will in each case only be described once.

In Figure 1,the mains supply includes a neutral line 10 connected directly to one of the heater filaments 12 of the tube 14, while the line line 16 is connected to the second heaterfilament 18 by way of a wall mounted light swith 20 and a choke 22. The heater filaments have their other ends connected to a starter socket 24 which receives a removable starter 26.

The conventional starter 26 is a gas discharge device which is conductive when first the mains switch 20 is closed. This current flow heats a bimetallic contact within the starter and causes a current two flow through the two filaments 12,18.

After a short time sufficient to heat the filaments 12,18, the cooling of the bimetallic strip causes it to interrupt the circuit of the filaments and the full mains voltage is applied across the tube 14. The interruption also causes an induced voltage developed by the choke 22 to be applied across the tube 14 and the result voltage rise coupled with the increased filament temperature is sufficient to cause the tube 14to fire. The impedance of the tube 14 then drops considerably and shorts out the starter 26, the voltage across the tube after firing being below the strike voltage of the starter 26.

The circuit of Figure 2 differs from that of Figure only in that the starter 26, has been removed from its socket 24 and a control circuit generally designated 30 has been interposed. The control circuit 30 comprise a first switching 32 connected in series with the starter 26 and a second switching element 34 connected in parallel with the starter 26. Both of the switching elements 32,34 are controlled by means of a control unit 36 which permits the tube 14 to be switched on and off.

To duplicate the function of the circuit of Figure 1, the control unit would switch the element 32 on permanently and the element 34 off permanently.

The starter then functions in the conventional manner.

If it is required to implement the invention set forth in Patent Application No. 93255596, then the control unit 36 may comprise means for sensing the presence of mains voltage, and a counter for closing the element 32 only after the mains switch 20 has been toggled a preset number of times.

The present invention however is concerned with the switching off of the tube after it has fired. To achieve this objective, the control circuit close both of the elements 32,34 for a predetermined length of time sufficient to permit the gas in the tube 14 to cool. The closing of both switching elements 32,34 causes current again to flow through the heater filaments and though the discharge in the tube has been discontinued it is important to ensure that it does not recommence when the short circuit across the tube is removed. This may be achieved by gradually reducing the current in the short circuit path or by opening the short circuit path at a suitable time when the induced voltage from the choke 22 would not reignite the tube 14.

The control unit 36 may comprise timing circuits to switch off the tube automatically after a certain length of time or it may include an ambient light sensorto indicate when artificial illumination is no longer necessary. As a further possibility, the control unit may respond to an external stimulus received either as wave energy or down the mains lines.

It should be mentioned that the switching element 34 may alternatively be connected across the series combination of the starter 26 and the element 32 whereupon only that element need be rendered conductive to switch off the tube 14.

Because the switching elements 32,34 are connected across the terminals of the socket 24 for the starter 26, they may themselves be controlled by the control unit 36 to fulfill the function of the starter as described above. Such an embodiment is shown in Figure 3 in which a single electronic switching element 38 is controlled by the control unit 40 to achieve the functions of starting and switching off the tube discharge as required.

Claims (10)

1. A control circuit for insertion into the starter socket of a fluorescent light fitting, comprising shorting means which when activated effectively short circuit the terminals of the socket leading to the heater filaments so as to prevent the flow of discharge current through the gas in the tube, and means for activating the shorting means during the course of discharge through the gas in the tube to enable the level of illumination to be reduced.

2. A control circuit as claimed in claini 1, wherein the shorting means constitute the starter switch.

3. A control circuit as claimed in claim 1, wherein the shorting means comprise a further switching device connected in parallel with the starter switch.

4. A control circuit as claimed in any preceding claim, wherein the means for activating the shorting means comprise timing means operative to switch off the tube entirely after a predetermined length of time has elapsed.

5. A control circuit as claimed in any preceding claim, in which the means for activating the shorting means include a light sensor for causing such activation in response to an increase in the level of ambient illumination.

6. A control circuit as claimed in any preceding claim, in which the means for activating the shorting means includes means responsive to an externally applied stimulus.

7. A control circuit as claimed in claim 4,5 or 6, in which in order to prevent the tube from refiring when the short circuit is removed, means are provided to cause a gradual decay of said short circuit current.

8. A control circuit as claimed in claim 4,5 or 6, in which in order to prevent the tube from refiring when the short circuit is removed, the current in the short circuit path is timed in relation to the current flow through the choke of the light fitting in such a manner that the voltage across the tube is insufficlient to cause firing.

9. A control circuit as claimed in any preceding claim, comprising means for activating the shorting means cyclically to interrupt a proportion of the current flowing through the tube in each cycle, thereby permitting the light output to be reduced.

10. A control circuit constructed substanbtially as herein described with reference to and as illustrated in Figure 2 or Figure 3 of the accompanying drawings.