GB2303978A - Control of electromagnetic emissioins in battery backed apparatus - Google Patents

Abstract

In a battery backed electrically powered apparatus, a non-switched mains supply input 10 is connected through a filter 32 to a battery charger 34 to charge the backup battery 14, the filter 32 preventing electrical noise from the apparatus being conducted into the mains supply at input 10. An operational part of the apparatus, particularly an emergency light 8, can be powered from a switched mains input 12 during normal use. A switching circuit 20, 22, 24, 26, 28 is operable to connect the battery 14 to power the light 8 in the event of mains failure, and in this state the switching circuit breaks all electrical connection to the switched mains input 12. This arrangement allows for a total absence of noise filtration between the apparatus and the switched mains input 12. The lamp 8 may be an incandescent lamp. Alternatively, the lamp 8 may be a fluorescent lamp operated from the mains input 12 via a ballast 18 and operated from the battery 14 via a high frequency inverter 30. The switching circuit is in the form of a relay 20 with changeover contacts 22, 24, 26, 28 which not only completely disconnect the lamp 8 from the mains input 12 on mains failure, but also disconnect the ballast 18 from input 12 and from inverter 30 in this state, thereby avoiding the possibility that an electronic ballast 18 is damaged by becoming connected to the output of inverter 30.

Description

CONTROL OF ELECTROMAGNETIC EMISSIONS The present invention relates to control of electromagnetic emissions from electrical and electronic devices.

Forthcoming legislation will require products to incorporate more stringent controls on their electromagnetic emissions. One particular form of electromagnetic emission is conducted emission where emissions in the form of electrical noise are sent from a piece of equipment into a mains supply, where they might interfere with other apparatus connected to that supply. Such emission must be controlled even where the mains supply fails, this being a serious issue for apparatus which is provided with a back-up power supply.

Conventionally, conducted emissions have been reduced by providing a filter on the mains supply lines of a piece of apparatus. Such a filter may include some or all of a capacitor connected between live and neutral supply lines (X-capacitor); a capacitor connected between the live and/or the neutral supply and earth (Ycapacitor); and a common-mode inductor connected in series with the supply. If necessary, a differential mode inductor is also connected in series with the supply.

This arrangement works well in its function of reducing emissions.

Some electrical apparatus, such as emergency lights, have a backup battery to ensure that they continue to function in the event of mains failure. Typically, such apparatus will be provided with a permanent mains supply to maintain the battery in a charged state, and a switched mains supply by means of which the apparatus may be controlled in normal use. In such apparatus, it is often the case that the risk of emissions arises from only the part of the apparatus that is concerned with battery backup (such as charging circuits. and inverters).

However, it has often been the case that a filter has been required for both mains supplies since it is conventional to perform only minimal isolation between the supplies to reduce circuit complexity.

It is an aim of the present invention to provide a power supply for a battery backed electrical apparatus in which the requirement for duplication of filtration circuits is minimised or avoided.

According to the invention there is provided a battery backed electrically powered apparatus comprising a first mains input connected through a filter circuit to maintain a battery in a charged condition, a second mains input connected to power an operational part of the apparatus in normal use, and a switching circuit operable to connect the battery to power the operational part in the event of failure of the first mains input, in which, on failure of the first mains input, the switching means breaks all electrical connection to the second mains input.

Thus, it will be seen that the second mains input is isolated from the components likely to cause emissions.

In a typical embodiment, in which the operational part is a lamp, the emissions from it may be substantially non-existent, so allowing for a total absence of filtration for the second mains input.

An embodiment of the invention will now be described in detail, by way of example, with reference to the accompanying drawings in which: Figure 1 is a block circuit diagram of an emergency light incorporating a circuit embodying the invention; and Figures 2 and 3 are diagrams of the operational components of the light of Figure 1 respectively in normal operation and in the event of failure of the permanent mains supply.

With reference to the figure, an emergency light has a lamp 8, a permanent mains supply 10 and a switched mains supply 12. Its operation can be summarised in that during normal conditions, the lamp 8 is supplied with power under the control of the switched supply such that it can be turned on and off at will, while the unswitched supply 10 provides power to keep a battery 14 charged.

However, should the mains supply fail (causing interruption of the unswitched supply 10), the lamp 8 automatically will be supplied with power from the battery 14. Hitherto, both of the unswitched supply 10 and the switched supply 12 would have been provided with a filter to reduce conducted emissions.

The lamp 8 in this embodiment is a fluorescent tube, but the invention is equally applicable to any other type of lamp, such as conventional incandescent, halogen, compact fluorescent, etc. To drive the lamp 8 there is provided a starter 16 and ballast circuit 18 of conventional configuration. There is also provided a high-frequency inverter 30 which serves as a power converter to allow the lamp 8 to be powered by the battery 14.

Power from the permanent mains supply 10 is fed to a filter circuit 32, so isolating the mains supply from any electrical noise that it might otherwise receive from the various circuits within the emergency light. Power from the filter circuit 32 is fed to a charging circuit 34 which maintains the battery 14 in a charged condition.

The negative battery terminal is connected to earth at 36.

There is provided a control circuit 38. A first stage of the control circuit 38 is connected between the positive terminal of the battery 14 and the inverter 30.

A second stage of the control circuit 38 is connected in series with the relay coil 20 across the output of the filter circuit 32.

The emergency light comprises switching stage having a relay comprising a coil 20 and four two-way contacts 22, 24, 26, 28, each connecting a respective common terminal 22c. .28c to normally open terminal 22a. .28a and normally closed terminal 22b..28b under selective control of the coil 20.

For as long as the permanent supply is operational, the relay coil 20 is energised. The control circuit 38 senses a voltage proportional to mains from the charger through sensing lines 40. The control circuit 38 ensures that no power is fed from the battery 14 to the inverter 30. Should this voltage fall, this is detected by the control circuit 38, which operates to complete the circuit between the battery 14 and the inverter 30. It will be appreciated that the relay changes state in accordance with the presence or absence of the permanent mains supply 10.

While the permanent mains supply 10 is live, the relay is in the condition opposite to that shown in Figure 1. Functionally, the circuit operates as shown in Figure 2. It will be seen that the lamp 8 (together with is associated ballast 18 and starter 16) is powered by the switched mains supply 12, the effective circuit diagram being shown in Figure 2. Thus, the lamp 8 can be switched on and off under the control of the switched supply 12. The battery 14 is maintained in a charged state by the charger 34. Since the high frequency inverter 30 is not operating and the inverter 30 and the charging circuit 34 have absolutely no connection with the second supply, filtration for the second supply is unnecessary.

In the event that the switched mains supply fails, the relay drops out, and adopts the configuration shown in Figure 1. The operational components of this circuit are shown in Figure 3. In this configuration the control circuit 38 supplies power to the high-frequency inverter 30 from the battery 14. The lamp 8 is powered by a circuit arrangement as shown in Figure 3. It will be seen that the lamp 8 is run scold. That is to say, no current is passed through the heaters of the lamp 8, so as to reduce power consumption.

It will be clear from Figure 3 that the ballast 18 of the lamp 8 is completely disconnected in this condition of the circuit. This has been found to give rise to a second significant advantage of this invention as applied to emergency lighting. In cases where the ballast 18 is an electronic ballast, as opposed to a conventional ballast choke, the ballast circuit can be damaged if it becomes connected to the output of the high-frequency inverter 30. This possibility is avoided in the arrangement of the present embodiment.

One output of the high-frequency inverter 30 is connected to earth at 40 through the normally-closed relay terminal 26b. Thus, the inverter 30 is tied to earth at both its input and its output, this arrangement having been found to cause rapid decay of any current circulating in the circuit when the light changes to battery operation. Additionally, the earthing arrangement facilitates starting of the lamp 8.

Claims (3)

CLAIMS:

1. A battery backed electrically powered apparatus comprising a first mains input connected through a filter circuit to maintain a battery in a charged condition, a second mains input connected to power an operational part of the apparatus in normal use, and a switching circuit operable to connect the battery to power the operational part in the event of failure of the first mains input, in which, on failure of the first mains input, the switching means breaks all electrical connection to the second mains input.

2. Apparatus according to claim 1 in which the operational part is a lamp, there being a total absence of filtration for the second mains input.

3. Battery backed electrically powered apparatus, substantially as described herein with reference to the accompanying drawings.