GB2333913A - Control system for a plurality of fluorescent lamps - Google Patents

Abstract

Closure of a master switch 8 enables a common inverter 6 to supply a plurality of fluorescent lamp circuits 2 via a screened cable 10. The inverter 6 initially operates at a higher frequency (eg. 46 KHZ) for a short period (up to 5 seconds, eg. 500 msec.), after which the frequency is reduced to a lower running frequency (eg. 23 KHz). Each lamp circuit 2 has a switch 20 to connect a fluorescent lamp 12 to inverter 6 via a DC blocking capacitor 22, the lamp 12 being connected across a capacitor 26 of an L-C circuit 24, 26 which is resonant at the higher frequency. A further capacitor 28 connected in series with lamp heaters 14, 16 is effectively in parallel with capacitor 26, the L-C circuit formed by inductor 24 and capacitors 26, 28 being resonant at the running frequency. If a lamp switch 20 is closed before the main switch 8 is closed and the heaters of the corresponding lamp are intact, the lamp will not strike until after the inverter has switched to the running frequency, the heaters being preheated while the inverter is operating at its higher frequency. However, if either of the lamp heaters 14, 16 is burnt out, the lamp will strike during the initial period when the inverter operates at the higher frequency, the absence of a delay in striking the lamp giving a visual indication of a faulty heater. A lamp with intact heaters can also be struck by closing its switch 20 even after the inverter has switched to its running frequency. The arrangement allows a standard wall switch to be used for the switch 20, because high voltages do not appear across the switch 20, and the running frequency voltage appearing across capacitor 26 and lamp connection terminals 18 when a lamp is faulty or has been removed can be kept within safe limits. A faulty lamp can be replaced without affecting continued operation of the other lamps.

Description

CONTROL SYSTEM FOR FLUORESCENT LAMPS This invention relates to a control system for fluorescent lamps, and more particularly to such systems for enabling selective operation of a plurality of fluorescent lamps.

It is established practice for areas to be lit to incorporate a plurality of fluorescent lamps all of which are simultaneously triggered on actuation of a common control switch.

More particularly, a single electronic ballast is provided, initially to supply a high frequency voltage to the lamps to strike the lamps, and thereafter to supply a lower frequency running voltage thereto.

Such a ballast typically includes an input filter, a full wave rectifier, a half or full wave inverter, one or more resonant circuits, and, in most cases, a power factor correction circuit, the ballast being connected to an AC mains input by way of a master switch.

For lamps up to 2.5 metres in length, a voltage of about 1 Kv RMS at 20 to 40 Khz is required to cause a lamp to strike, and this is normally achieved by using a series resonant circuit incorporating a capacitor and an inductor supplied by the inverter. The lamp is connected across the capacitor, the voltage generated across the capacitor being sufficient to cause initiation of the lamp circuit and therefore to turn the lamp on.

High striking frequencies are preferred because the size of the capacitor used in the resonant circuits can be reduced, thereby saving in space and cost.

Furthermore it is easier to obtain the higher voltages necessary to cause striking of the lamps, which is particularly important in cold start systems where the lamp heaters do not contribute to the starting procedure and are only used as a source of electrons during operation of the lamp.

Such cold start systems are often preferred, in that heaters can become open circuit and operation of the lamp in a conventional heated system would thereby be prevented. It is however acknowledged that lamps will strike more easily at lower voltages if the heaters are used in the start-up procedure.

A number of fluorescent lamps may be supplied from a common inverter via separate resonant circuits, one for each lamp and each supplying the high frequency, high voltage output to its associated lamp necessary for striking.

However, if any one of the lamps is faulty or is not connected, the voltage appearing at the open circuit lamp terminals will remain high during operation of the other lamps, and will thereby exceed the limits determined by safety directives.

It is therefore essential to reduce the terminal voltage to an acceptable level as soon as possible after striking of the lamps. This can be achieved by supplying the striking frequency for only a short period of time sufficient to actuate the lamps, and thereafter reducing the frequency so that the circuits are no longer resonant and the voltage is thereby reduced.

However, such an arrangement suffers from the disadvantage that, as a lamp will not strike at the reduced running frequency, it is not possible to replace and start a faulty lamp without first of all switching off the ballast or the inverter and thereafter switching it back on to achieve the higher frequency necessary to strike the replaced lamp.

Furthermore, as all the lamps must be switched on and off together, more lamps may be in operation than is necessary, particularly where areas requiring illumination are some distance apart. This results in the use of more current than may be necessary, and detracts from the improved efficiency that may be expected from the use of electronic ballasts.

Some ballasts include detector circuits which, for example, detect the heater currents in individual lamps, and, if the heater current ceases either because the lamp is faulty or the lamp is removed, switch off the ballast. Clearly such an arrangement overcomes the problems of high voltage at the faulty lamp but means that all lamps on the ballast output are extinguished.

It would be desirable to be able to provide a control system for supplying a plurality of fluorescent lamps and which enabled individual lamps to be removed without affecting operation of the othre lamps, the voltage at the terminals of a removed lamp being within predetermined safety limits.

According to the present invention there is provided, for a plurality of fluorescent lamps each including a pair of opposed heating elements one at each terminal end of the associated lamp, a control system comprising a common source of electrical supply connected across the terminals of said lamps by way of a primary switch and whereby, on actuation of said switch, a voltage at a first frequency is supplied to each lamp for a predetermined period, after which the frequency of said voltage is reduced to a second, lower value, and, between said source of supply and each lamp, a secondary switch and a resonant circuit, said circuit being arranged to resonate at said first frequency and to be non-resonant at said second, lower frequency associated with normal running of the lamps, characterised by, for each lamp, electrical means connected between the heating elements of the lamp and arranged to adapt the associated circuit whereby resonance occurs at said second, lower frequency associated with normal running of the lamp.

It will thus be appreciated that, with such a system, and in the event that the heating elements of a lamp are electrically intact, current will flow through the elements and, after said predetermined period, the associated circuit will resonate at said second, lower frequency to enable striking thereof.

If one or both of the heating elements are opencircuit, the electrical means between the elements is out of circuit and the circuit resonates at the first frequency to enable striking thereof.

In all events, the lamps are actuated independently of one another, and, on removal of a lamp, or on failure of a lamp, the voltage across the terminals thereof is kept to a safe value after said predetermined period as the associated circuit is not resonant at the running frequency.

In a preferred embodiment of the invention, the common source of electrical supply comprises AC mains electricity fed to a single inverter by way of said primary switch, although a DC supply may be used.

Conveniently, for each lamp, a capacitor is provided between the secondary switch and the resonant circuit to effect DC isolation between the inverter and the resonant circuit.

Preferably the resonant circuit comprises an inductor and a capacitor, while it is further preferred that the electrical means for each lamp comprises a further capacitor.

By way of example only, an embodiment of the invention will now be described in greater detail with reference to the accompanying drawing which shows a control system according to the invention.

Referring to the drawing, the illustrated system is arranged to control the operation of a plurality of fluorescent lamp circuits 2 only one of which is shown in detail.

A normal AC mains supply shown at 4 supplies a squarewave inverter 6 by way of a master switch 8, the inverter 6 being designed to operate for a predetermined short initial period, typically 500 milliseconds, at a high frequency, typically 46 Khz associated with striking of the associated lamps, after which the frequency is reduced to a lower value, typically 23 Khz, associated with running of said lamps.

The output of the inverter 6 is supplied, via a screened cable schematically indicated at 10, to all of the lamp circuits 2, the total load not exceeding the rating of the inverter 6.

Each of the circuits 2 includes a fluorescent lamp indicated generally at 12 and having a pair of heaters 14, 16 one at each end of the lamp 12, and a pair of opposed terminals 18. Each circuit 2 further includes a secondary switch 20, a capacitor 22 and a resonant circuit comprising an inductor 24 and a capacitor 26 selected to resonate at the high frequency supplied by the inverter 6 during said initial period. The capacitor 22 provides DC isolation between the inverter 6 and the resonant circuit comprising the inductor 24 and the capacitor 26.

A further capacitor 28 is connected across the heaters 14,16 of each lamp 12 whereby it is effectively in parallel with capacitor 26, the value of the capacitor 28 being selected such that the circuit comprising the capacitors 26, 28 and the inductor 24 resonates at the lower, lamp runing frequency of the output from the inverter 6.

The described control system operates in either of two modes depending upon which of the switches 8 and 20 are closed first.

If switch 8 is closed and, for a given lamp circuit 2, the switch 20 remains open, the inverter 6 provides an initial output at the high, striking frequency for said short, predetermined period, the output frequency then reducing to the lower running frequency.

On closing of the switch 20, and in the event that the heaters 14,16 are active whereby the capacitor 28 is in circuit and the circuit resonates at the running frequency, said circuit resonates to produce a voltage across the lamp terminals 18 sufficient to strike the lamp 12. A proportion of the current flowing in the capacitor 28 also flows through the heaters 14,16 to increase their temperature and to assist in striking of the lamp 12. The lamp 12 continues to run until either of the switches 8 or 20 are opened.

If either of the heaters 14,16 burns out or otherwise goes open circuit during operation of the lamp 12, the lamp will continue to operate with the current flow therethrough limited by the value of the inductor 24. Capacitor 26 remains in circuit and ensures that the lamp voltage is reasonably sinusoidal and that distortion is kept within acceptable limits - ie. the crest factor is maintained below 1.7.

If the lamp 12 is removed with switch 20 closed, the voltage appearing across capacitor 26, which is applied to the lamp terminals 18, would not exceed the safe value because the circuit is not resonant at the applied running frequency.

If a lamp 12 with intact heaters 14,16 is subsequently reconnected, the circuit would once again resonate, and the lamp 12 would strike. The operation of the other lamp circuits 2 would not be affected.

If switch 20 of the lamp circuit 2 is closed first, and switch 8 is closed thereafter, the lamp circuit 2 is initially supplied by the inverter 6 at the high striking frequency, and then at the lower running frequency. If the heaters 14,16 are intact, the circuit containing the capacitors 26,28 and the inductor 24 will not resonate until the frequency drops to the lower value. Thus the striking of the lamp 12 will be delayed by the length of the predetermined time period typically 500 milliseconds - during which time the heaters 14,16 will be warmed up, the warmed heaters thence assisting in the subsequent striking of the lamp 12.

If either of the heaters 14,16 are open circuit, and the capacitor 28 is consequently out of circuit, the circuit will immediately resonate on application of the high frequency voltage from the inverter 6, and the lamp 12 will strike during the initial predetermined perido.

In this mode of operation, in which switch 20 is kept closed and the master switch 8 is used to actuate the system, it will be appreciated that an indication is given to the user whether or not the lamp 12 incorporates a faulty heater 14,16. More particularly, if there is a delay in striking of the lamp 12, the heaters are intact, while, if the lamp 12 strikes immediately, one or both of the heaters 14,16 are faulty.

In the latter instance, the lamp 12 can be changed without losing illumination from the other lamp circuits.

The predetermined period after which the frequency of the output from the inverter 6 falls to the lower value may be other than 500 milliseconds, and may be any period up to 5 seconds.

Because the lamp circuits 2 are switched after the inverter 6 and before the resonant circuit including the inductor 24 and the capacitor 26, the high voltages produced across the terminals 18 of the lamp 12 do not appear across the switch 20. The RMS voltage across the switch 20 is at all times less than the normal mains voltage, and consequently switch 20 can be a standard wall switch as used for general lighting purposes.

Thus the control system of the invention enables selective and independent switching of a series of fluorescent lamps supplied from a common inverter or electronic ballast, and replacement of any such lamp without affecting continued operation of the others.

Additionally, 5the voltage across the terminals of a faulty or removed lamp can readily be kept within predetermined safety limits.

The lamps can be switched via a normal wall switch without the need for interaction with the inverter or ballast to modify the frequency of the output therefrom, and without the need to use additional switching circuits. The heaters of the lamps are used to improve the striking efficiency of the lamps, but the lamps can continue to operate even if a heater burns out.

As illustrated, the basic resonant circuits comprising the inductor 24 and capacitor 26 are separate from the inverter 6 and are fitted externally of the inverter adjacent the associated lamps, conveniently in a housing 30.

Alternatively, the basic resonant circuits may be incorporated with the inverter to constitute an electronic ballast.

Claims (6)

1. A control system for a plurality of fluorescent lamps each including a pair of opposed heating elements one at each terminal end of the associated lamp, the control system comprising a common source of electrical supply connected across the terminals of said lamps by way of a primary switch and whereby, on actuation of said switch, a voltage at a first frequency is supplied to each lamp for a predetermined period, after which the frequency of said voltage is reduced to a second, lower value, and, between said source of supply and each lamp, a secondary switch and a resonant circuit, said circuit being arranged to resonate at said first frequency and to be non-resonant at said second, lower frequency associated with normal running of the lamps, characterised by, for each lamp, electrical means connected between the heating elements of the lamp and arranged to adapt the associated circuit whereby resonance occurs at said second, lower frequency associated with normal running of the lamp.

2. A control system as claimed in claim 1 in which the common source of electrical supply comprises AC mains electricity fed to a single inverter by way of said primary switch.

3. A control system as claimed in claim 1 or claim 2 in which for each lamp, a capacitor is provided between the secondary switch and the resonant circuit to effect DC isolation between the inverter and the resonant circuit.

4. A control system as claimed in any one of claims 1 to 3 in which the resonant circuit comprises an inductor and a capacitor.

5. A control system as claimed in claim 4 in which the electrical means for each lamp comprises a further capacitor.

6. A control ssytem substantially as described with reference to and as illustrated by the accompanying drawings.