GB2388722A

GB2388722A - Monitoring the operation of a fluorescent lamp

Info

Publication number: GB2388722A
Application number: GB0210974A
Authority: GB
Inventors: Richard Beesley
Original assignee: MACKWELL ELECTRONICS Ltd
Current assignee: MACKWELL ELECTRONICS Ltd
Priority date: 2002-05-14
Filing date: 2002-05-14
Publication date: 2003-11-19
Anticipated expiration: 2022-05-14
Also published as: GB0210974D0; GB2388722B

Abstract

The operating conditions of a fluorescent lamp 12, supplied from a DC supply 14 through a DC inverter 16 and transformer 18, are monitored by sensing the voltage phase difference across a reactive impedance, e.g. capacitor 20, in series with the lamp 12. Two taps 24, 26 are provided either side of the capacitor 20, which control transistors 30a, 30b of switching circuits 28. The switching circuits 28 change state at zero crossings of the tapped voltages, which are separated by a known time difference (tp1, tp2 fig.3) in normal operation. A microprocessor 42 can be programmed to discriminate between normal and abnormal operation when the expected time difference (tp1, tp2 fig.3) is not observed. Additionally, the microprocessor may make other measurements, such as the frequency of the supply voltage, to determine which of the lamp or power supply is faulty.

Description

/ Monitoring Apparatus The present invention relates to monitoring

apparatus and in particular, to apparatus for monitoring the operating condition of a fluorescent lamp.

The operating condition of a fluorescent lamp is particularly important when the lamp is being used for emergency lighting. Emergency luminaires which incorporate fluorescent lamps, commonly have a battery from which the lamp is supplied in the event of a failure of mains supply. The battery drives an AC inverter from which current is supplied to the lamp. Legislative requirements exist, which require the luminaires to be tested regularly. These tests will include a test to ensure that the lamp strikes when required and that the battery is able to maintain the lamp alight for a specified minimum period of time. It is therefore desirable to be able to monitor the operating condition of an emergency lamp in order to complete these tests. Other situations may exist in which it is useful to monitor the operating condition of a non-

emergency fluorescent lamp.

The invention provides monitoring apparatus operable to monitor the operating condition of a fluorescent lamp supplied through a reactive impedance in series with the lamp, the monitoring apparatus comprising sensor means operable to sense the voltage phase difference across the reactive impedance and to determine whether or not the sensed phase difference is indicative of normal operation.

The sensor means may be operable to tap the voltage at either side of the reactive impedance. The sensor means is preferably operable to detect zero crossings of the tap voltages. The sensor means may be operable to measure the time separation of zero crossings at either side of the impedance, and to derive phase information therefrom.

Preferably, the tap voltages are applied to voltage dependent switching circuits, the state of which is indicative of the contemporaneous phase angle of

the tap voltage. The switching circuits are preferably high impedance and may include switching transistors. The state of the switching circuits is preferably detected by discriminator means which responds to the states to discriminate I between normal and abnormal operation. The discriminator means may measure the time difference between state changes of the switching circuits.

The discriminator means may be a microprocessor. i Preferably, the sensor means is further operable to measure frequency of at least one of the tap voltages. The sensor means is preferably operable to use frequency information to determine whether or not the power supply is operating normally. The frequency is preferably measured from a voltage I tapped at the common terminal of the reactive impedance and the power I supply. The invention also provides a method of monitoring the operating condition of a fluorescent lamp supplied through a reactive impedance in series with the lamp, in which the voltage phase difference across the reactive impedance is sensed to determine whether or not the sensed phase difference is indicative of normal operation. I Preferably, the voltage is tapped at either side of the reactive impedance.

Preferably zero crossings of the tap voltages are detected. The time separation i of zero crossings at either side of the impedance may be measured to derive phase information.

Preferably, the tap voltages are applied to voltage dependent switching circuits, the state of which is indicative of the contemporaneous phase angle of the tap voltage. The switching circuits are preferably high impedance and may include switching transistors. The state of the switching circuits is preferably detected to discriminate between normal and abnormal operation.

Discrimination may be by measuring the time difference between state changes of the switching circuits.

Preferably, the frequency of at least one of the tap voltages is measured.

Frequency information may be used to determine whether or not the power supply is operating normally. The frequency is preferably measured from a l voltage tapped at the common terminal of the reactive impedance and the power supply.

Examples 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 a circuit for supplying a fluorescent lamp I from a DC supply; I Fig. 2 shows the voltages arising in the circuit of Fig. 1, during normal operation; and Fig. 3 illustrates the output of switching circuits to which the tapped voltages of Fig. 2 are applied.

Fig. l shows a circuit 10 for supplying a fluorescent lamp 12 from a DC supply 14, such as a battery. The battery output is applied to a high frequency ' DC inverter 16, the output of which is applied to the primary winding 18a of i the transformer 1X. The lamp 12 draws current from the secondary winding 18b of the transformer 18, through a current limiting capacitor 20.

The circuit features described in the previous paragraph are conventional when driving a fluorescent lamp 12 from a battery 14. This arrangement is common in an emergency luminaire in which arrangements will be provided for sensing mains failure and, in response, providing power for the lamp 12 from the battery 14, as has been described. In some luminaires, the lamp 12 is dedicated only for emergency use. In others, arrangements are provided for mains supply to the lamp l2 until mains failure occurs. A luminairc in which the present invention is embodied may therefore have many other features

which are outside the scope of the present invention. are conventional in themselves, and are thus not described in this specification.

The circuit 10 also includes monitoring apparatus indicated generally at 22 to monitor the operating condition of the lamp 12. As has been described, the lamp 12 is supplied through a series capacitor 20 which therefore has a reactive impedance. During normal operation of the lamp 12, an arc discharge occurs within the lamp 12, resulting in the lamp 12 presenting a negative resistance characteristic to the secondary winding 18b. This negative resistance gives rise to a voltage phase difference across the reactive impedance of the capacitor 20.

The voltage across the capacitor 20 is tapped at 24 and 26 to provide, respectively, the voltage Vs at the supply side of the capacitor 20, and the voltage VLat the lamp side of the capacitor 20. Thus, Vs is taken at the terminal common to the capacitor 20 and the secondary coil 18b. Voltage V, is taken at common terminal of the capacitor 20 and the lamp 12.

The voltages Vs. V, tapped from the capacitor 20 are used by the monitoring apparatus 2 2 to sense the voltage phase difference across the capacitor 20 and thus to determine whether or not the sensed phase difference is indicative of normal operation. This occurs in the following manner.

Each tap at 24, 26 is applied to a respective switching circuit 28, based around a respective switching transistor 30a, 30b. The bases of the transistors 30a, 30b are connected to the common terminal of reverse biased diodes 34a, 34b and series resistors 36a, 36b.

Thus, the diode 34a and resistor 36a are in series between the tap Vs and the lamp terminal 12b and are thus, effectively, in parallel with the secondary winding 18b. Thus, the voltage applied to the base of the transistor 30a is in phase with the voltage across the secondary transformer 18b.

The diode 34b and resistor 36b are in series between the tap V, and the terminal 12b and are thus, in effect, in parallel with the lamp 12. Thus, the voltage applied to the base of the transistor 30b is in phase with the voltage across the lamp 12.

Fig. 2 represents a timing diagram of the voltages at Vs and V,, and thus of the voltages at the bases 38a, 381. The voltage 40a at Vs can be seen to lag the voltage 40b at Vl in Fig. 2, which represents normal operation. The phase difference to be expected during normal operation will be known to the designer of the circuit 10. The phase difference in typical emergency luminaires is commonly about 50 degrees.

Thus, as the voltages Vs and VL oscillate during normal operation, the transistors 30a, 30b will be switched on and off but not at the same time, by virtue of the phase difference between V5 40a and V, 40b. The switching times of the transistors 30a, 30b can therefore be used to discriminate between normal and abnormal operation, as follows.

A microprocessor 42 is provided for sensing the switch state of the transistors 30a, Job. The transistors 30a, 30b each have a collector 44a, 44b connected to a respective input 46a, 46b of the microprocessor 42. The emitter 48a, 48b of the transistors 30a, 30b are connected to terminal 12b which serves as the ground for the circuit 10. Consequently, the inputs 46a, 46b will have two states according to the switching state of the transistors 30a, Job. This is illustrated in Fig. 3 which is a timing diagram of the state of the inputs 46a, 46b. The time axis is consistent with that of Fig. 2. The two drawings are connected by broken lines 50 which indicate that at each zero crossing of the voltages Vs. V,, there is a change in state of the corresponding switching circuit 28, switching the corresponding input 46a, 46b to the logic low level 52, or to the logic high level 54.

In Figs. 2 and 3, the change of state of the switching circuits 28 is shown to occur precisely at the zero crossing point of the voltages Vs'V, but in

( practice, bias conditions on the transistors 30a, 30b may result in the change of state occurring slightly to one side of the zero crossing point, but nevertheless associated with it, as will be apparent to the skilled reader.

The input 46a from Vs is shown in a solid line in Fig. 3. A chain dotted line 58 indicates the state of the input 46b, derived from voltage Vi.

A tine difference tpl can clearly be seen to arise between the trailing edges of the pulse trains at the inputs 4Ga, 46b, with the input 46b changing state before the input 46a. A further time difference tp2 arises between the leading edges. The time separations are characteristic of the phase difference between Vs and V,, at a particular frequency. One of the time differences tpl or tp2 is measured by the microprocessor 42, which is programmed by conventional techniques to detect the trailing or leading edge of a pulse at input 46b, and to begin timing until a trailing or leading edge of a pulse is detected at input 46a. Since the time difference to be expected from normal operation is known, the microprocessor 42 can therefore be programmed to discriminate between normal and abnormal operation. An output 60 is used for the result of this decision, for instance to drive an annunciator lamp.

In brief summary, the design of the circuit 10 determines the frequency

of operation of the lamp 12 and the voltage phase difference to be expected across the capacitor 20 during normal operation. This allows the expected value of tpl or tp2 to be calculated and thus for the microprocessor 42 to be appropriately programmed to watch for variations of tpl or tp2 away from the expected value by more than an acceptable tolerance level.

In the event that the expected value of tpl or tp2 is not observed by the microprocessor 42, a determination can be made that the circuit 10 is not operating correctly. In a preferred arrangement, additional information can be derived to assist in identifying the type of fault which has occurred when abnormal operation is detected. This is derived by using the microprocessor 42 to make an additional measurement, of the frequency of the voltage Vs. This

( can readily be determined by the microprocessor 42 by observation of the pulses at input 4(ia.

In this modified arrangement, a full sequence of monitoring by the microprocessor 42 will begin by a measurement of the frequency of Vs. If the I measured frequency is the expected frequency for normal operation, within acceptable tolerances, the microprocessor 42 can conclude that the battery 14, inverter 16 and transformer 18 are all functioning satisfactorily to provide the designed power supply to the lamp 12.

If the frequency of Vs is found to differ from the expected frequency, the microprocessor 42 will conclude that a fault exists within the power supply. If Vs is not oscillating, that would suggest a fault in the battery 14. If there is oscillation at a frequency other than the design frequency, that would suggest a fault within the inverter 16.

If the frequency of Vs indicates that the power supply is operating correctly, the microprocessor 42 then begins to monitor the time difference tpl or tp2, as described above. If the expected value of tpl or tp2 is found, the microprocessor 42 determines that the lamp 12 is functioning correctly. If the expected value of tpl or tp2 is not found, the microprocessor 42 determines that the lamp is not functioning properly but moreover, because the power supply has previously been found to be functioning properly, the microprocessor 42 is able to report that the fault is with the lamp 12.

Thus, the microprocessor 42 is able to report 3 different states, namely normal operation, a faulty lamp or a power supply fault. These three states can be indicated by an annunciator lamp driven by the output 60, for example with on, off and flashing states to correspond with the three results. Alternatively, the results can be sent to a remote location, for example, for monitoring a test on an emergency luminaire.

It may be desirable to measure both time differences tpl and tp2. This

( allows asymmetry in the waveforms to be detected, which may allow diagnosis of faults likely to cause such asymmetry, such as a single cathode fault which causes partial rectification on the supply current.

It is desirable that in the circuit 10, the switching circuits 28 present a high impedance to the tapping points at the capacitor 20. High impedance can be designed into the circuit, for example by using high value resistors 36.

Furthermore, the presence of the transistors 30 buffers the voltage taps from the input capacitance of the microprocessor 42. The effect of high impedance is to minimise current drawn by the switching circuits 28, so that the voltages at Vs and VL are substantially unaffected by the measuring process.

Many variations and modifications can be made to the apparatus described above. In particular, various components and component values can be used. Many other circuit arrangements could be envisaged for performing the measurement functions described above and in particular, many other arrangements for dividing the functions between discrete hardware, microprocessor or other computing hardware and software could be devised.

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.

Claims

:! ( CLAIMS

1. Monitoring apparatus operable to monitor the operating condition of a fluorescent lamp supplied through a reactive impedance in series with the lamp, the monitoring apparatus comprising sensor means operable to sense the voltage phase difference across the reactive impedance and to determine whether or not the sensed phase difference is indicative of normal operation.

2. Apparatus according to claim 1, wherein the sensor means is operable to tap the voltage at either side of the reactive impedance.

3. Apparatus according to claim 2, wherein the sensor means is operable to detect zero crossings of the tap voltages.

4. Apparatus according to claim 3, wherein the sensor means is operable to measure the time separation of zero crossings at either side of the impedance' and to derive phase information therefrom.

5. Apparatus according to claim 2, 3 or 4, wherein the tap voltages are applied to voltage dependent switching circuits, the state of which is indicative of the contemporaneous phase angle of the tap voltage.

6. Apparatus according to claim 5, wherein the switching circuits are high impedance.

7. Apparatus according to claim 6, wherein the switching circuits include switching transistors.

8. Apparatus according to claim 5, 6 or 7, wherein the state of the switching circuits is detected by discriminator means which responds to the states to discriminate between normal and abnormal operation.

9. Apparatus according to claim 8, wherein the discriminator means measures the time difference between state changes of the switching circuits.

(

10. Apparatus according to claim 8 or claim 9, wherein the discriminator means is a microprocessor.

11. Apparatus according to any of claims 2 to 10, wherein the sensor means is further operable to measure frequency of at least one of the tap voltages.

12. Apparatus according to claim 11, wherein the sensor means is operable to use frequency information to determine whether or not the power supply is operating normally.

13. Apparatus according to claim 11 or 12, wherein the frequency is measured from a voltage tapped at the common terminal of the reactive impedance and the power supply.

14. Monitoring apparatus substantially as described above, with reference to the accompanying drawings.

15. A method of monitoring the operating condition of a fluorescent lamp supplied through a reactive impedance in series with the lamp, in which the voltage phase difference across the reactive impedance is sensed to determine whether or not the sensed phase difference is indicative of normal operation.

16. A method according to claim 15, wherein the voltage is tapped at either side of the reactive impedance.

17. A method according to claim 16, wherein zero crossings of the tap voltages are detected.

18. A method according to claim 17, wherein the time separation of zero crossings at either side of the impedance is measured to derive phase information.

19. A method according to claim 16, 17 or 18, wherein the tap voltages are applied to voltage dependent switching circuits, the state of which is indicative

( of the contemporaneous phase angle of the tap voltage.

20. A method according to claim 19, wherein the switching circuits are high impedance.

21. A method according to claim 20, wherein the switching circuits include switching transistors.

22. A method according to claim 19, 20 or 21, wherein the state of the switching circuits is detected to discriminate between normal and abnormal operation.

23. A method according to claim 22, wherein discrimination is by measuring the time difference between state changes of the switching circuits.

24. A method according to any of claims 16 to 23, wherein the frequency of at least one of the tap voltages is measured.

25. A method according to claim 24, wherein frequency information is used to determine whether or not the power supply is operating normally.

26. A method according to claim 24 or 2 5, wherein the frequency is measured from a voltage tapped at the common terminal of the reactive impedance and the power supply.

27. A method of monitoring, substantially as described above, with reference to the accompanying drawings.

28. 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.