GB2337644A

GB2337644A - Starting and warming up of fluorescent lamps

Info

Publication number: GB2337644A
Application number: GB9810969A
Authority: GB
Inventors: David V Pardington; Richard Faulkner
Original assignee: MACKWELL ELECTRONICS Ltd
Current assignee: MACKWELL ELECTRONICS Ltd
Priority date: 1998-05-22
Filing date: 1998-05-22
Publication date: 1999-11-24
Anticipated expiration: 2018-05-22
Also published as: GB9810969D0; GB2337644B

Abstract

In a method of striking a fluorescent lamp 12, a relatively high lamp current is passed through the lamp for an initial period after striking, to provide a warming up period while the lamp is illuminated, and then the lamp current is reduced. A relatively high cathode filament heating current is also maintained for a shorter initial period, to provide a cathode warming period while the lamp is illuminated, and the filament current is then reduced. The lamp current during warming up may be greater than after by a factor of at least two, and the filament heating current may also be greater during warming up than after by a factor of at least two. The lamp may be supplied by a high frequency inverter 10 which is operated at 90KHz for a 60 second lamp warmup period, after which a timer 44 opens a switch 42 to disconnect an additional inductor 40 from the primary winding 30 of a transformer 32 so that the inverter then operates at 30KHz resulting in a reduction in the lamp current. The cathode filaments 16 receive current from a further transformer 54 supplied via a switch 52 from a tapped winding 50 on transformer 32. The switch 52 is initially at a higher voltage tap 56a on winding and is switched by timer 44 to a lower voltage tap 56b after a 0.5 second cathode warmup period. When the lamp current is reduced on the opening of switch 42, the resulting reduced loading on the transformer 32 gives rise to a slight increase in the cathode heating current. The arrangement has advantages when used in a battery powered emergency light, since striking of the lamp does not need to be delayed while the cathodes warm up, and the lamp 12 may be an amalgam type which would normally require a long period to approach full light output.

Description

1 2337644 1 Fluorescent Lamps The present invention relates to fluorescent

lamps and in particular, but not exclusively, fluorescent lamps for emergency lighting applications.

It is a common requirement for public buildings such as shops, hotels and offices, to provide an emergency lighting system which illuminates automatically to provide lighting in the event of a power failure, allowing safe evacuation of the building. Emergency lighting installations of this nature are battery powered. Fluorescent lamps are attractive for this application, because of their low current requirements, thus increasing the period of time during which lighting is available from a particular battery capacity. However, the operating characteristics of fluorescent lamps can present difficulties in achieving a practical implementation which meets safety requirements. In particular, it is clearly not acceptable for evacuation to be delayed while emergency lighting is brought to its operating condition, if that requires an extended period of significantly reduced light output, as is common with fluorescent lamps.

The present invention provides a method of striking a fluorescent lamp, in which a relatively high lamp current is provided for an initial period after striking, to provide a warming up period while the lamp is illuminated, the lamp current being reduced after the warming up period.

Preferably the lamp current is reduced by a factor of at least two after the warming up period.

Preferably the lamp current is changed by changing the frequency of the lamp current. The lamp current may be provided by an oscillator circuit. The oscillation frequency is preferably changed by switching at least one oscillator componentinto or out of circuit. Preferably the circuit contains an inductor switchable in and out of circuit.

2 The lamp may be an amalgam lamp. The lamp current frequency is preferably at least 9OkHz during a warming up period, and may be approximately 30kHz thereafter. Preferably the warming up period is at least one minute.

The fluorescent lamp may form part of a battery operated emergency lighting installation.

The invention also provides apparatus for striking a fluorescent lamp, comprising lamp current supply means operable to provide a relatively high lamp current for an initial period after striking the lamp, to provide a warming up period while the lamp is Wuminated, and the supply means being operable thereafter to reduce the lamp current.

Preferably the lamp current is changed by changing the frequency of the lamp current. The lamp current may be provided by an oscillator circuit. The oscillation frequency is preferably changed by switching at least one oscillator component into or out of circuit. Preferably the circuit contains an inductor switchable in and out of circuit.

The lamp may be an amalgam lamp. The lamp current frequency is preferably at least 90kHz during a warming up period, and may be approximately 30kHz thereafter. Preferably the warming up period is at least one minute.

In another aspect, the invention provides a method of striking a fluorescent lamp, in which relatively high cathode current is maintained for an 3 initial period after striking, to provide a cathode-warming period while the lamp is illuminated, the cathode current being reduced after the cathode-warming period.

Preferably the cathode current is greater during the cathode-warming period than after, by a factor of at least two.

The cathode current may be changed by changing the cathode supply voltage. The cathode current is preferably provided from a transformer, the transformer ratio being changeable to change the cathode supply voltage. Preferably switch means are provided to change the number of transformer turns tapped to provide the cathode current.

Preferably the lamp is an amalgam lamp. Preferably the cathodewarming period is less than one second, preferably approximately 0.5 seconds.

The fluorescent lamp preferably forms part of a battery operated emergency lighting installation.

The invention also provides apparatus for striking a fluorescent lamp, comprising cathode current supply means operable to provide a relatively high cathode current for an initial period after striking, to provide a cathodewarming period while the lamp is illuminated, the supply means being operable thereafter to reduce the cathode current.

Preferably the cathode current is greater during the cathode-warming period than, after, by a factor of at least two.

The cathode current may be changed by changing the cathode supply voltage. The cathode current supply means preferably comprises a transformer, the transformer ratio being changeable to change the cathode supply voltage. Preferably switch means are provided to change the number of transformer turns tapped to provide the cathode current.

4 Preferably the lamp is an amalgam lamp. Preferably the cathodewarming period is less than one second, preferably approximately 0.5 seconds.

It is within the scope of the present invention to provide any or all of the features of the first aspect of the invention in any combination with any or all of the features of the second aspect of the invention.

An embodiment of the present invention will now be described in more detail, by way of example only, and with reference to the accompanying drawings, in which:

Fig. 1 is a circuit diagram of an emergency lighting installation embodying the present invention; and Fig. 2 is a highly schematic timing diagram for operation of the installation of Fig. 1.

Fig. 1 illustrates a circuit 10 for striking and thereafter supplying a fluorescent lamp 12. The circuit 10 supplies lamp current at 14 and cathode current at 16. Lamp current 14 is relatively high during an initial period after striking, to provide a warming up period while the lamp 12 is illuminated, and the lamp current is then reduced after the warming up period, in a manner to be described. The cathode current 16 is relatively high for an initial period after striking, to provide a cathode warming period while the lamp is Wuminated, and the cathode current is then reduced, in a manner to be described.

In more detail, the circuit 10 is based around a pair of npn transistors 18,20. The transistors 18,20 have their emitters connected to each other at 22 and to the negative rail of a DC supply 26. The transistor collectors are connected acr oss a parallel LC circuit comprising a capacitor 28 and one winding 30 of a transformer 32. The centre of the winding 30 is connected to the positive rail 34 of the DC supply 26. The winding 30 is coupled with a winding 3 6 connected across the bases of the. transistors 18,2 0.

This arrangement forms a self-oscillating sine wave circuit powered by the supply 26. When used for emergency lighting, the supply 26 will monitor the presence of normal AC supply 38 and wW switch to operate the oscillator circuit when the supply 38 is disrupted, indicating an emergency.

In addition to the components described above, the oscillator also has a further inductor 40 and a switch 42 by which the inductor 40 can be switched into and out of circuit, in parallel with the capacitor 28 and winding 30. This has the effect of changing the oscillation frequency. In the preferred arrangement, the frequency is about 3 times higher when the inductor 40 is in circuit, than when the inductor 40 is out of circuit (about 30kHz and 9OkHz, respectively).

The condition of the switch 42 is controlled by a timer circuit 44, the operation of which will be described in more detail below. The timer 44 is initiated by the DC supply 26 when the AC supply is disrupted.

The winding 30 forms a primary winding of the transformer 32. A secondary winding 46 is connected through frequency turning capacitors 48 to provide lamp current at 14. A further secondary winding 50 is connected through a further switch 52 to a second transformer arrangement 54 to provide cathode heating current at 16. The switch 52 selects between two tapping positions 5 6a, 5 6b on the winding 5 0 so that the voltage applied to the primary winding 58 of the transformer 54 can be selected, thereby changing the voltage applied to the cathodes through the secondary windings 60.

The state of the switch 52 is set by the timer 44.

Operation of the circuit can now be described simultaneously with a 6 description of Fig. 2, in which time is shown on the x-axis. The y-axis is used to indicate reductions in magnitude of parameters, but is otherwise schematic and has an arbitrary scale.

In this description, it is assumed that T = 0 represents the time at which the supply 26 senses a disruption in the AC supply 38 and thus begins to apply power to the circuit 10 and initiate the timer 44.

When operation of the timer 44 is initiated by the supply 26 supplying power, the timer sets switch 42 to include the inductor 40 in the oscillator circuit, and sets the switch 52 to supply the greater of the two possible voltages to the cathodes (by tapping at 56a). In consequence, at T = 0, the frequency of the lamp current 14 is the higher of the two available frequencies. Lamp current, the current flowing through the gas of the fluorescent lamp, generally increases with frequency (in a roughly linear manner) and thus, the initial lamp current as the lamp strikes to begin emitting light, is at a relatively high level. This has the effect of boosting the operation of the lamp so that any reduction in light output arising from the lamp being cold is compensated, at least to some degree, by the increased frequency and current.

Similarly, the cathode current, which heats the cathodes to allow the lamp current to pass, is initially relatively high. This provides a heating effect for the cathodes and therefore compensates for the fact that the lamp has been struck without the cathodes having been pre-heated. Cathode pre-heating (before striking) is normally considered a requirement for proper operation of at least some types of fluorescent lamp.

The boosted cathode current is required only for a relatively short period, preferably 0.5 seconds. At T = 0.5s, the timer 44 changes the position of the switch 52 to drop the cathode supply voltage to a lower level. Cathode voltage remains at a reduced level thereafter.

Warming up the lamp by means of the increased lamp current has been 7 found to take longer than this and consequently, the frequency of the oscillator remains at the high level for a longer period which, in this example, is 1 minute. At T = 60s, the timer 44 switches the switch 42 to take the inductor 40 out of circuit, to drop the oscillator frequency by a factor of about 3 and thus reduce the lamp current by about the same factor. (In practice, the nonlinearity is such that a frequency factor of 3 is likely to change the current by a factor of about 2.5). Lamp current remains at the lower value thereafter.

This description has assumed that lamp current and cathode current are completely independent. However, it has been found that the excessive load placed on the transformer 32 during the boosted lamp current phase (the first minute) depresses the secondary voltage of the transformer, which in turn reduces the voltage applied to the cathodes. When the lamp current returns to normal (T = 60s) this load is reduced and the secondary voltage of the transformer will rise to a final steady value (below the initial value). In one example, for a 42W amalgam lamp, cathode current has been arranged to drop from an initial value of about 70OrnA, to a reduced value of about 220m.A and then to stabilise at about 2 70niA at T = 60s. Other values will occur in other lamps.

It has been found by experiment that by operating the fluorescent lamp in the manner described above, adequate lamp output can be provided immediately the lamp is struck, and striking the lamp does not need to be delayed for preparation such as cathode warming. Thus, the striking arrangement is particularly appropriate for use with emergency lighting installations, where lighting is required immediately an emergency situation is detected.

Naturally, the power requirement of the circuit is initially greater than normal, resulting in a reduced time for which the supply 26 can drive the lamp 12. However, this is not considered to represent a difficulty in practice. A typical emergency lighting arrangement would have a 4 AH battery supply and be expected to provide approximately 1 A for three hours (thus leaving 1 AH of capacity to cover for degradation of the battery, or other factors). In this context, it can be readily understood that the modified operation during the first one minute makes an insignificant difference to the overall time for which the lamp can be driven and indeed, it is likely that normal tolerances in battery condition or charge state will have a much greater effect on operation than the modifications according to the invention.

It is envisaged that the method and apparatus described above can be used with or modified for use with various types of fluorescent lamp. However, the arrangements are considered particularly appropriate for use with lamps of the type called "amalgam" lamps, such as those manufactured by Philips Lighting and others. Amalgam lamps are a type of low pressure mercury gas discharge lamp. Lamp current from a heated cathode causes mercury within the lamp to emit ultraviolet radiation which is then converted to visible light by a fluorescent coating on the inside surface of the tube. An amalgam lamp uses a mercury amalgam to provide enhanced control of the mercury pressure in the lamp and ensures a near constant mercury pressure over a wide temperature range, by releasing and reabsorbing mercury at various temperatures. This has a number of operating advantages, but has the disadvantage that the warm-up (or running-up) time for the lamp can be considerable. For instance, after one second, an amalgam lamp may be producing only 10% of its final output. Even after one minute, this amount may only be 65%. It may take five minutes for output to rise to 95% of the final value. These figures are based on manufacturers recommendations for correct operation, including preheating the cathode before striking the lamp.

These characteristics suggest that amalgam lamps are inappropriate for emergency lighting applications, where good light output is required immediately. However, the present invention allows an amalgam lamp to be driven in a manner which meets safety requirements and therefore enables advantages of amalgam lamps to be realised without compromising emergency lighting requirements.

9 It will be apparent that various modifications can be made to the apparatus described above without departing from the scope of the present invention. In particular, other drive circuits could be used, other oscillators could be used and other arrangements could be used for changing the lamp current and cathode current.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

1

Claims

1. A method of striking a fluorescent lamp, in which a relatively high lamp current is provided for an initial period after striking, to provide a warming up period while the lamp is illuminated, the lamp current being reduced after the warming up period.

2. A method according to claim 1, wherein the lamp current is reduced by a factor of at least two after the warming up period.

3. A method according to claim 1 or 2, wherein the lamp current is changed by changing the frequency of the lamp current.

4. A method according to any preceding claim, wherein the lamp current is provided by an oscillator circuit.

5. A method accurding to claim 4, wherein the oscillation frequency is changed by switching at least one oscillator component into or out of circuit.

6. A method according to claim 5, wherein the circuit contains an inductor switchable in and out of circuit.

7. A method according to any preceding claim, wherein the lamp is an amalgam lamp.

8. A method according to any preceding claim, wherein the lamp current frequency is at least 9OkHz during a warming up period.

9. A method according to any preceding claim, wherein the lamp current frequency is approdmately 30kHz after the warming up period.

10. A method according to any preceding claim, wherein the warming up period is at least one minute.

11 11. A method according to any preceding claim, wherein the fluorescent lamp forms part of a battery operated emergency lighting installation.

12. A method of striking a fluorescent lamp, substantially as described above with reference to the accompanying drawings.

13. Apparatus for striking a fluorescent lamp, comprising lamp current supply means operable to provide a relatively high lamp current for an initial period after striking the lamp, to provide a warming up period while the lamp is illuminated, and the supply means being operable thereafter to reduce the lamp current.

14. Apparatus according to claim 13, wherein the lamp current is reduced by a factor of at least two after the warming up period.

15. Apparatus according to claim 13 or 14, wherein the lamp current is changed by changing the frequency of the lamp current.

16. Apparatus according to claim 13, 14 or 15 wherein the lamp current is provided by an oscillator circuit.

17. Apparatus according to claim 16, wherein the oscillation frequency is changed by switching at least one oscillator component into or out of circuit.

18. Apparatus according to claim 17, wherein the circuit contains an inductor switchable in and out of circuit.

19. Apparatus according to claim 13 to 18, wherein the lamp is an amalgam lamp.

20. Apparatus according to any of claims 13 to 19, wherein the lamp current frequency is at least 90kHz during a warming up period.

p 12

21. Apparatus according to any of claims 13 to 20, wherein the lamp current frequency is approximately 30kHz after the warming up period.

22. Apparatus according to any of claims 13 to 21, wherein the warming up period is at least one minute.

23. Apparatus according to any of claims 13 to 22, wherein the fluorescent lamp forms part of a battery operated emergency lighting installation.

24. Apparatus for striking a fluorescent lamp, substantially as described above, with reference to the accompanying drawings.

25. A method of striking a fluorescent lamp, in which relatively high cathode current is maintained for an initial period after striking, to provide a cathodewarming period while the lamp is illuminated, the cathode current being reduced after the cathode-warming period.

26. A method according to claim 2 5, wherein the cathode current is greater during the cathode-warming period than after, by a factor of at least two.

271. A method according to claim 2 5 or 26, wherein the cathode current is changed by changing the cathode supply voltage.

28. A method according to claim 27, wherein the cathode current is provided from a transformer, the transformer ratio being changeable to change the cathode supply voltage.

29. A method according to claim 28, wherein switch means are provided to change the number of transformer turns tapped to provide the cathode current.

30. A method according to any of claims 25 to 29, wherein the lamp is an amalgam lamp.

1 q 1 13

31. A method according to any of claims 25 to 30, wherein the cathodewarming period is less than one second.

32. A method according to any of claims 2 5 to 3 1, wherein the cathodewarming period is approximately 0.5 seconds.

33. A method according to any of claims 2 5 to 3 2, wherein the fluorescent lamp forms part of a battery operated emergency lighting installation.

34. Apparatus for striking a fluorescent lamp, comprising cathode current supply means operable to provide a relatively high cathode current for an initial period after striking, to provide a cathode-warming period while the lamp is illuminated, the supply means being operable thereafter to reduce the cathode current.

35. Apparatus according to claim 34, wherein the cathode current is greater during the cathode-warming period than after, by a factor of at least two.

36. Apparatus according to claim 34 or 35, wherein the cathode current is changed by changing the cathode supply voltage.

37. Apparatus according to claim 36, wherein the cathode current supply means comprises a transformer, the transformer ratio being changeable to change the cathode supply voltage.

38. Apparatus according to claim 3 7, wherein switch means are provided to change the number of transformer turns tapped to provide the cathode current.

39. Apparatus according to any of claims 34 to 38, wherein the lamp is an amalgam lamp.

40. Apparatus according to any of claims 34 to 39, wherein the cathodewarming period is less than one second.

14

41. Apparatus according to any of claims 34 to 40, wherein the cathodewarming period is approdmately 0.5 seconds.

42. Apparatus according to any of claims 34 to 41, wherein the fluorescent lamp forms part of a battery operated emergency lighting installation.

43. A method according to any of claims 1 to 12 and any of claims 2 5 to 3 3.

44. Apparatus according to any of claims 13 to 24 and any of claims 34 to 42.

45. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.