GB2316818A

GB2316818A - Emergency luminaire power supply with reduced HF emissions

Info

Publication number: GB2316818A
Application number: GB9718117A
Authority: GB
Inventors: Mark James Charles Dale
Original assignee: JSB Electrical PLC
Current assignee: JSB Electrical PLC
Priority date: 1996-08-28
Filing date: 1997-08-27
Publication date: 1998-03-04
Also published as: GB9617946D0; GB9718117D0

Abstract

A fluorescent lamp 10, which is normally operated from a mains supply L, N, E via a ballast choke 24 in the live or neutral line, is connected to a battery powered HF inverter 28 by change over switches 20, 22 in the event of mains failure, and the switches 20, 22 are so arranged that the lamp 10 remains connected to the choke 24 even when operating from the inverter 28. Capacitive coupling between the lamp 10 and a metal casing 18 of the luminaire results in HF leakage current I L . Leaving the choke 24 connected to one end of the lamp 10, and disconnecting the other end of the lamp from the mains supply, ensures that the only circuit loop through which the HF leakage current I L can flow includes the choke 24 which presents a high impedance to the HF current, whereby contamination of the mains supply by HF signals is reduced. Connection of a capacitor C between the casing 18 and one side of the inverter 28 further reduces any such HF contamination.

Description

POWER SUPPLY CIRCUIT AND AN EMERGENCY LUMINAIRE The present invention relates to a power supply circuit, and in particular to a power supply circuit in which apparatus is supplied by power from a mains supply or from a battery in the event of failure of the mains supply, and also to an emergency luminaire incorporating such a power supply circuit.

Power supplies of the above described type are often used in emergency lighting apparatus whereby a light is illuminated (or continues to be in illuminated) in the event of failure of a mains supply. In many situations, such lights will comprise a fluorescent tube. This poses particular problems.

A fluorescent tube cannot be operated directly from a battery. An inverter must be provided to convert the d.c. output of the battery to a high voltage a.c. supply suitable for driving the tube.

The presence of the inverter gives rise to highfrequency signals, must be taken into account when the power supply circuit is designed. Regulations prohibit contamination of a mains supply (even if that supply is temporarily inactive) by signals originating from apparatus connected to it.

Previous attempts to provide a power supply circuit which complies with requirements for minimising high frequency emissions have resulted in addition of costly filter components or a multiplication of the number of switch contacts which must be provided within the power supply. This can lead to increased cost and also introduces further components where failure could occur.

It is thus an aim of the present invention to provide a power supply of the general type described above but which can provide control of emission with a minimum of components.

According to a first aspect of the invention, there is provided a power supply circuit for supplying power to a load, the power supply circuit being operable in a normal condition to supply power from a mains input and in a failure condition to supply power from a reserved power supply during failure of the mains input, in which power supply there is an inductor connected between the load and the mains input during operation of the power supply in its normal and in its failure conditions. By breaking with the convention formerly adapted for emission control devices, and not disconnecting substantially all components from the load during the failure condition, the inductor serves as a low pass filter which provides a high impedance barrier to the high frequency emissions of the inverter.

Where the power supply is intended for use with a fluorescent lamp, the inductor can conveniently be constituted by the lamp choke. This arrangement helps to minimise the number of components of the power supply circuit.

In a second of its aspects, the invention provides an emergency luminaire comprising a fluorescent lamp, a battery, an inverter, a choke and change-over switches, all contained within an earthed metal case, in which the choke is connected between the lamp and the live or the neutral mains supply line, and the change-over switches are operative to selectively connect the lamp to the mains supply or to the inverter, without disconnecting the lamp from the choke.

Preferably, there is further provided a capacitor which is connected between an output of the inverter and the case.

Embodiments of the invention will now be described in detail, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a conventional power supply circuit for an emergency luminaire; Figures 2 and 3 show two further separate diagrams of an emergency luminaire being first and second embodiments of the invention; and Figure 4 shows a modification to the circuit of Figure 3.

With reference first to Figure 1, a conventional emergency luminaire comprises a fluorescent lamp 10 having first and second electrodes 12,14, each electrode having a pair of connection terminals. A mains supply 16 has a live, an earth, and a neutral supply line L,E, N.

The luminaire also comprises a double pole, two-way change-over relay. The switch elements 20,22 only are shown in the circuit diagram for the sake of clarity. A ballast choke 24 and a starter unit 26 are provided to drive the lamp 10. An inverter 28 powered by a battery 30 provide an alternative source of power for the lamp.

The live supply line L is connected through the choke 24 to a normally open terminal of the first switch element 20. The normally closed terminal of the first switch element 20 is connected to a first output of the inverter 28. The common terminal is connected to a first terminal of the first electrode 12 of the lamp 10.

The second terminal of the first electrode 12 is connected to the common terminal of the second switch element 22. The normally closed terminal of the second switch element 22 is unconnected, while the normally open terminal is connected to a first connection of the starter 26.

The neutral supply line N is connected to a first terminal of the second electrode 14, the second terminal of which is connected to a second connection of the starter 26. A second output of the inverter 28 is also connected to the neutral supply line N. The earth line E is connected to the casing 18.

During such time as the mains supply is active, the relay is energised. Thus, the lamp 10 is connected, through the choke 24, across the mains supply 16. The starter 26 is also connected across the lamp 10. On failure of the mains supply, the relay drops out. This leaves the lamp 10 connected directly across the inverter 28, while the choke 24 is connected only to the mains supply, its other side being unconnected, and similarly the starter 26 is connected only to the second terminal 14 of the lamp 10.

It has been found in practice that the lamp 10 can become capacitively coupled to the case 18. This results in a current 1L which flows along the earth line E, and a return current 1R which flows in the neutral line N.

These currents 1L'1R are considered to be unacceptable high frequency emissions.

With reference now to Figure 2, there will be described a first embodiment of the invention This embodiment comprises the same components as discussed above, but connected in a different arrangement. In the following description, these components will be given reference numerals corresponding to those used above.

The live supply line L of the mains input 16 is connected through the choke 24 directly to a first terminal of the first electrode 12 of the lamp 10. The second terminal of the first electrode 12 is connected to the common terminal of the first switch element 22. The normally closed terminal of the first switch element 22 is connected to a first output of the inverter 28. The starter 26 is connected between the normally open terminal of the first switch element 22 and a second terminal of the second electrode 14. The first terminal of the second electrode 14 is connected to the common terminal of the first switch element 20. The normally open terminal of the first switch element 20 is connected to the neutral supply line N, while the normally closed terminal of the second switch element 20 is connected to the second output of the inverter 28.

It will be seen that, during normal operation, the lamp 10 is connected across the mains supply 16, exactly as is the case in the conventional arrangement described above. In the event of a mains supply failure, the lamp 10 becomes connected across the inverter 28. The starter 26 is taken out of circuit. The neutral supply line N becomes disconnected entirely. However, the live supply line L remains connected to the first electrode 12 through the ballast choke 24.

In this embodiment, the only circuit loop through which a high frequency current could run is described by the capacitive coupling between the lamp 10 and the casing 18, a current IL in the earth line E, and a return current 1R in the live supply line L. However, this return current 1R must pass through the choke 24. Since the current is generated at a high frequency by the inverter 28, the choke 24 presents to it a high impedance. Thus, the emission current which can flow in this embodiment is substantially less than that which can flow in the conventional arrangement described above.

However, this achieved using no additional components.

Figure 3 shows a circuit arrangement which is virtually identical to that of Figure 2. The difference is that the live and neutral supply lines are effectively reversed. In this arrangement, the return current flows in the neutral supply line N, in which the ballast choke is connected.

A modification to the circuit of Figure 3 is shown in Figure 4. In this arrangement, a capacitor C is connected between the casing 18 and the normally closed terminal of the second switch element 24. The corresponding position of this capacitor in the circuit of Figure 2 is shown in dotted lines. This capacitor performs no function while the luminaire is operating under mains power. However, on failure of the mains supply, it provides a poor resistance path between the casing 18 and the inverter 28 for noise signals which are capacitively transmitted to the casing 18. This means that a substantial part of the energy which would be responsible for generating a high frequency emission is, instead, contained within the luminaire as a return current 1R1 This further reduces the size of signals emitted by the device, as compared with the arrangements of Figures 2 and 3.

Claims

1. A power supply circuit for supplying power to a load, the power supply circuit being operable in a normal condition to supply power from a mains input and in a failure condition to supply power from a reserved power supply during failure of the mains input, in which power supply there is an inductor connected between the load and the mains input during operation of the power supply in both its normal and in its failure conditions.

2. A power supply circuit according to claim 1 intended for use with a fluorescent lamp, in which the inductor is constituted by the lamp choke.

3. A power supply circuit substantially as herein described with reference to any one of Figures 2, 3 or 4 of the drawings.

4. An emergency luminaire comprising a fluorescent lamp, and a power supply circuit including a battery, an inverter, a choke and change-over switches; in which the choke is connected between the lamp and the live or the neutral mains supply line, and the change-over switches are operative to selectively connect the lamp to the mains supply or to the inverter, without disconnecting the lamp from the choke.

5. An emergency luminaire according to claim 4 in which the battery, the inverter, the chock and the change-over switches are all contained within a case.

6. An emergency luminaire according to claim 5 in which the case is metal and is earthed.

7. An emergency luminaire according to claim 6 in which there is further provided a capacitor which is connected between an output of the inverter and the case.

8. An emergency luminaire according to any one of claims 4 to 7 in which the change-over switches are constituted by first and second switch elements of a relay.

9. An emergency luminaire substantially as herein described with reference to any one of Figures 2, 3 or 4 of the drawings.