GB2318932A

GB2318932A - Fluorescent lamp failure warning device

Info

Publication number: GB2318932A
Application number: GB9714763A
Authority: GB
Inventors: Robert Joseph Desbois
Original assignee: LAB CRAFT Ltd
Current assignee: LAB CRAFT Ltd
Priority date: 1996-11-01
Filing date: 1997-07-15
Publication date: 1998-05-06
Anticipated expiration: 2017-07-15
Also published as: GB2318932B; GB9622768D0; GB9714763D0

Abstract

A fluorescent lamp 1, which may be in a vehicle, has spaced cathodes 2, each comprising a heating element. A power supply 6, drive circuit 5 and controller 3 supply operating alternating current to the cathodes 2, and heating current to the heating elements. A monitoring current, which may be the heating current itself, is passed through each of the heating elements, and is measured by sensors 4 at each terminal of the lamp. A warning signal 10 and/or a delayed shut down signal 11 is produced in response to detection of an imbalance between the sensed monitoring currents. The sensors 4 may be LED photodetectors, coils and ferrite cores, or Hall devices.

Description

FLUORESCENT LAMP FAILURE WARNING SYSTEM Field of the Invention This invention relates to a fluorescent lighting device having a lamp failure warning system.

Background to the Invention Fluorescent lamps typically comprise a cathode at each end of the tube, with a mains voltage alternating current supply being connected across the cathodes to strike the arc. The cathodes are formed as heater elements separately supplied on start-up with a heater current to pre-heat the cathodes, thereby facilitating striking of the arc.

The heater elements are generally disconnected automatically by means of a glow switch in the control circuit once the arc has been struck.

In fluorescent lamps used on low-voltage direct current supplies, for example those found in motor vehicles, the glow switch-based control circuit is not used. An inverter transformer provides the higher voltage alternating current needed to operate the lamp, while the heater current is supplied continuously while the lamp is operated, and can be controlled electronically to ensure efficient functioning of the tube, for example by increasing heater current when the arc current is reduced, and decreasing the heater current when the arc current increases. The usual cause of failure of such lamps is failure of a heating element. Although the lamp will usually continue to operate for a certain period after an element has failed, the arc will eventually fail and cannot then be re-ignited.

In some applications, such as public service vehicles, the failure of lamps, especially those illuminating exits from the vehicle, would place the vehicle in breach of safety regulations, requiring its removal from service. This would be costly for the vehicle operator. Unfortunately, it has not been possible to obtain warning of the imminent failure of a lamp, so the only way an operator can avoid the problem is to replace all lamps routinely before they reach the end of their working life. This increases costs, since the fullest possible life is not achieved, and there is still the possibility of premature failure occurring within the expected life of the lamp.

It is therefore desirable to produce a fluorescent lighting device which can provide an advance warning of imminent failure of the tube, and preferably maintain operation of the lamp long enough to permit its replacement, for example without the need to remove the vehicle in which the lamp is installed from normal service.

Summary of the Invention According to the invention, there is provided a fluorescent lighting device having a fluorescent lamp of the type having spaced cathodes, each comprising a heating element, the device also comprising means for supplying an operating alternating current to the cathodes, means for selectively supplying a heating current to the heating elements, means for supplying a monitoring current through the heating elements, means for sensing the monitoring current at each terminal of the lamp and means for providing a warning signal in response to detection of an imbalance between the sensed currents.

The heating current and the monitoring current may be the same or different For example, in the case of the inverter type of lamp, the heating current is usually applied continuously, and can therefore serve as the monitoring current Where the heating current is not applied continuously during operation of the lamp, a separate monitoring current will need to be applied, and means will therefore need to be provided within the lamp to generate such a current. It will be appreciated that such a monitoring current can be at a relatively low level, so as not to affect the normal operation of the lamp.

The heating current may be an alternating current and may be supplied from a mains supply. It is common for the heating current to a fluorescent tube to be provided from a mains supply, the circuit being provided with a suitable electronic ballast. Such circuits continue to operate the fluorescent tube after the cathode has failed. Provided that the electronic ballast is operating at a suffidendy high frequency, the tube failure device of the invention may be used to monitor the cathodes and indicate failure. The device may be adapted to switch off the supply to the ballast.

Preferably, the frequency of the electronic ballast is greater than 50 Hertz.

The detection of an imbalance between the sensed currents will indicate a tube failure condition, in which one of the heating elements has broken. The warning signal is suitably an indicator light, which can either be arranged to switch on when a failure condition is detected, or to be on when the lamp is working satisfactorily and to switch off when a failure condition is detected. An audible warning may also be initiated by the warning signal.

The warning signal may be used to operate a timer which causes the power supply to the tube (arc current) to switch off after a predetermined time period from detection of the failure condition, to prevent the risk of further damage to the tube.

The means for sensing the current may comprise a light-emitting diode in series with the heating current supply to or from the terminal of the lamp, and a photodetector associated with each lightemitting diode, the photodetectors being arranged such that a change in any one of them causes a switching action which produces the warning signal.

In another embodiment of the invention, the means for sensing the current comprises a sensing coil wound on to a core. The monitoring current induces a voltage in the sensing winding which voltage may be applied to a comparator to cause a switching action which produces a warning signal.

Preferably, the monitor current supply wire is passed through the said core.

Preferably the core is ferrite. The core may be in the shape of a toroid.

The device of the invention signals imminent failure of a fluorescent tube, permitting it to be readily identified and replaced before complete failure occurs.

Brief Description of the Drawings In the drawings, which illustrate exemplary embodiments of the invention: Figure 1 is a block diagram of a fluorescent lamp according to a first embodiment; Figure 2 is a circuit diagram of the tube failure warning device according to a second embodiment; Figure 3 is a block diagram of the power supply for the tube shown in Figure 2; and Figure 4 is a circuit diagram of the tube failure warning device according to a third embodiment of the invention.

Detailed Description of the Illustrated Embodiment Referring first to Figure 1, the lamp comprises a fluorescent tube 1 having heating elements/cathodes 2 at each end thereof connected to a combined transformer and control circuit 3 via current sensors 4 in each connection.

The transformer and control drcuit 3 is connected to a driver circuit 5 comprising an oscillator and a switch or switched to induce an alternating current at low voltage in the combined transformer control circuit 3. The transformer is suitably a step up transformer.

The driver circuit 5 is connected to a power supply 6, for example that of a motor vehicle supplying a low voltage direct current. Drive circuit 5 comprises an oscillator and at least one switching device which induce an alternating current at low voltage in the step up transformer and control drcuit 3.

Tappings are provided on the transformer to produce the low voltage heater current that passes through the current sensors 4, and the much higher voltage required to provide the arc current to the cathodes 2. Optionally, an arc delay circuit 7 can delay application of the arc current until the heating elements have been running for a predetermined period, to provide longer fluorescent tube life when the fluorescent tube is frequently switched on and off, and to assist in startup, for example under cold conditions.

The current sensors 4 may be any device capable of providing an output in response to passage of an electric current therethrough, for example a Hall effect device, an optsolator device. The outputs from the current sensors are passed to a 4-input AND gate 8 which gives an output signal when one of the inputs to it is low, as a result of the failure of one of the heating elements.

The output signal passes to a driver circuit and latch 9 which in turn operates an imminent tube failure warning device 10, for example a light-emitting diode and/or buzzer, and a timer device 11 which switches off the driver circuit 5 after a predetermined delay.

Figures 2 and 3 illustrate a preferred implementation of the device, in which the inverter is represented at 30 in Figure 3, with heater windings 31 (effectively tappings from the transformer), to give four supply terminals A, B, C and D, B and C being the higher voltage arc supply terminals, while the pairs A and B, and C and D, are the low voltage heater current terminals. These terminals are indicated separately in Figure 2 for the sake of clarity.

The tube is represented, in broken form for convenience, at 20 in Figure 2.

The heater elements 21 are each connected to their respective terminals via current limiting resistors 22 and 23 to the lightemitting diode (LED) element 24 of a respective opto-isolator device 25. It will be seen, therefore, that if current ceases to flow in any one terminal of the lamp 20, the respective LED 24 will cease to emit light Each opto-isolator device 25 also comprises a phototransistor 26, and the devices 25a, 25b, 25c and 25d are connected together such that the collectors and emitters of the phototransistors 26a-d are connected in series. While the lamp 20 is operating normally, each of the LED's 24a-d is emitting light, causing the phototransistor to conduct If one LED 24 ceases to emit light, the path through all the phototransistors 26 is broken, and so the voltage on the base of a first detector transistor 27 drops to zero and the transistor 27 is thereby switched off. The potential on the base of a second detector transistor 28 increases as a result of transistor 27 switching off, causing it to switch on, allowing current to flow through indicator lamp 29, illuminating it to give a warning.

In an alternative arrangement, the indicator lamp 29 is connected between the line voltage and the ground via the first transistor 27, the second transistor then being omitted. In this configuration, the indicator lamp 29 is normally on when the fluorescent lamp is operating normally, and is switched off when an imminent failure condition is signalled.

Referring now to Figure 4, there is shown a circuit for sensing the monitoring current at each cathode terminal of lamp 20 (see Figure 2). A, B, C, and D each represent a cathode terminal, B and C being the higher voltage arc supply terminals, while the pairs A and B, and C and D, are the low voltage heater current terminals. A monitoring means 40 comprises a ferrite core 42 in the shape of a toroid, and having coil 43 wound therearound. A monitor current carrying cathode leg 41 passes through the centre of the ferrite core 42. When a monitor current is present, voltage is induced in the coil 43, and hence the output from comparator 44 is low. If the monitor current is absent, i.e. the fluorescent lamp has failed, no voltage is induced in winding 43 and hence the output from comparator 44 is high turning on transistor 45 which operates an indicator, in this case LED 46, or a shut down circuit

Claims

Claims

1) A fluorescent lighting device having a fluorescent lamp of the type having spaced cathodes, each comprising a heating element, the device also comprising means for supplying an operating alternating current to the cathodes, means for selectively supplying a heating current to the heating elements, means for supplying a monitoring current through the heating elements, means for sensing the monitoring current at each terminal of the lamp and means for providing a warning signal in response to detection of an imbalance between the sensed currents.

2) A device according to Claim 1, wherein the heating current and the monitoring current are the same or different.

3) A device according to Claim 2, wherein the heating current is applied continuously during operation of the lamp, and serves as the monitoring current

4) A device according to Claim 2, wherein the heating current is not applied continuously during operation of the lamp, and a separate monitoring current is applied, the device further comprising means to generate the said monitoring current

5) A device according to Claim 4, wherein the heating current is an alternating current supplied from a mains supply.

6) A device according to Claim 5, wherein the circuit is provided with electronic ballast

7) A device according to Claim 6, wherein the warning signal produced in response to detection of an imbalance between the sensed currents causes supply of current to the ballast is switched off.

8) A device according to Claim 6 or 7, wherein the frequency of the electronic ballast is greater than 50 Hertz.

9) A device according to any preceding claim, wherein the warning signal is an indicator light 10) A device according to Claim9, wherein the indicator light can either be arranged to switch on when a failure condition is detected, or to be on when the lamp is working satisfactorily and to switch off when a failure condition is detected 11)A device according to any preceding claim, wherein the warning signal is audible.

12)A device according to any preceding claim, wherein the warning signal operates a timer which causes the power supply to the tube (arc current) to switch off after a predetermined time period from detection of the failure condition.

13) A device according to any preceding claim, wherein the means for sensing the current comprises a light-emitting diode in series with the heating current supply to or from the terminal of the lamp, and a photodetector associated with each light-emitting diode, the photodetectors being arranged such that a change in any one of them causes a switching action which produces the warning signal.

14) A device according to any of Claims 1 to 12, wherein the means for sensing the current comprises a sensing coil wound on to a core.

15) A device according to Claim 14, wherein the monitoring current induces a voltage in the sensing winding which voltage is applied to a means to cause a switching action which produces a warning signal.

16) A device according to Claim 15, wherein the said means is a comparator.

17) A device according to Claim 15 or 16, wherein the monitor current supply wire is passed through the said core.

18) A device according to any of Claims 14 to 17, wherein the core is ferrite.

19) A device according to any of Claims 14 to 18, wherein the core is in the shape of a toroid.

20) A fluorescent lighting device substantially as described with reference to, or as shown in, the drawings.