GB2382238A

GB2382238A - Emergency lighting battery charging circuit

Info

Publication number: GB2382238A
Application number: GB0120755A
Authority: GB
Inventors: N Levesley
Original assignee: MACKWELL ELECTRONICS Ltd
Current assignee: MACKWELL ELECTRONICS Ltd
Priority date: 2001-08-25
Filing date: 2001-08-25
Publication date: 2003-05-21
Anticipated expiration: 2021-08-25
Also published as: GB0120755D0; GB2382238B

Abstract

A charging circuit 20 charges a battery 14. A variable voltage power supply 22 supplies voltage at 32. A current limit resistor 24 is connected between the supply 22 and the battery 14. A feedback arrangement 26 senses the charge condition of the battery 14 by current through an opto-isolator 36 which provides a feedback signal 28 to control the voltage output at 32. The effect is to adjust the voltage of the power supply 22 to maintain a constant voltage difference across the current limit resistor 24, thus maintaining a constant current to the battery 14.

Description

Emergency Lighting The present invention relates to emergency lighting and in particular to emergency lighting which is powered, during an emergency, from a rechargeable battery.

Emergency lighting is provided in many situations, particularly in public buildings and the like. Each emergency light fitting will include a lamp bulb and a circuit to provide power to the lamp in the event that mains failure disables conventional lighting. This provides illumination for emergency evacuation, for instance. This power will be drawn from a rechargeable battery which must therefore be maintained in a charged condition during normal (non-emergency) conditions. Previous proposals have provided for the rechargeable battery to be charged from the mains supply, when available.

Various conflicting design requirements are imposed on a designer of this type of emergency lighting. For example, space may be severely limited, which can lead to problems of heat dissipation in charging circuits, in turn leading to shortened battery life. However, minimum battery life is prescribed in regulations relating to emergency lighting for public buildings. Difficulties can be experienced in adequately meeting all these different requirements.

The present invention provides an emergency light battery charging circuit having a variable voltage power supply, current limit means connected between the power supply and the battery being charged and operable to supply a current in dependence on the voltage difference across the current limit means, and a feedback means operable to sense the charge condition of the battery being charged and to adjust the power supply voltage in accordance with the charge condition to maintain a constant voltage difference across the voltage current means and thus to maintain a constant current to the battery being charged.

The current limit means is preferably resistive. The current limit means

may be a current limiting resistor.

The sensed charged condition is preferably used to provide a feedback signal to the power supply, the power supply being operable to change the power supply voltage in dependence on the feedback signal. Preferably the feedback means senses the battery voltage. The feedback means may be connected between the battery and the output of the power supply, the feedback means including an opto-coupler through which the current in the feedback means flows, and which provides the feedback signal in response thereto. Preferably the feedback means includes, in series, the opto-isolator and a voltage-limiting device. The voltage-limiting device may be a zener diode. Preferably the feedback means is connected between the battery and the output of the power supply and consists of circuit components across which the voltage is fixed when current is flowing.

Preferably the power supply is a switched mode power supply.

The invention also provides an emergency light incorporating a charging circuit as set out above.

The invention also provides an emergency light power module having a rechargeable battery and a charging circuit as set out above.

The battery is preferably a Nicad battery.

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 highly schematic diagram of an emergency light of the type in which the present invention may be implemented; and Fig. 2 is a circuit diagram of an emergency light battery charging circuit

in accordance with the present invention.

Fig. 1 illustrates in simple diagrammatic form, an emergency light module 10 which has a lamp bulb 12, a battery 14 and a control circuit 16. The light 10 is connected to mains power at 18.

The battery 14 is rechargeable and in this example is a Nicad (Nickel- Cadmium) battery. During normal (non-emergency) conditions, in which the mains supply 18 is available, the control circuit 16 connects the battery 14 to the mains supply 18 to allow the battery 14 to be charged, or maintained fully charged. In the event that the mains supply 18 fails, so that ordinary electric lights will cease to function, the control circuit 16, shown schematically as a switch, changes state to connect the battery 14 to the lamp bulb 12 as indicated schematically by the alternative switch position indicated by broken lines in Fig.

1. Current for the lamp bulb 12 can then be drawn from the battery 14, until exhausted, thus providing emergency lighting until the battery is exhausted.

The module 10 can be incorporated into the housing of an emergency light fitting, which may also provide a conventional light, whose output is to be replaced by the output of the lamp 12, in the event of mains failure.

Various regulations exist to govern the length of time for which the battery 14 must be capable of maintaining the lamp bulb alight, once the mains supply has failed. Furthermore, these requirements must be met even after the light 10 has experienced non-emergency conditions for a prolonged period of time.

Problems therefore arise in meeting these requirements when using Nicad batteries, which are vulnerable to prolonged exposure to heat or to inappropriate charging regimes.

Fig. 2 illustrates a charging circuit 20 in accordance with the present invention, for use in charging a battery 14 in an emergency light of the type

illustrated in Fig. 1. The charging circuit 20 has a variable voltage power supply 22 which receives mains supply at 18 and provides a supply voltage V. A current limit resistor 24 is connected between the power supply 22 and the battery 14 being charged. The effect of the resistor 24 is to supply a current to the battery which is set by the voltage difference across the resistor 24 as will be described in more detail below.

The circuit 20 also has a feedback arrangement indicated generally at 26.

This senses the charge condition of the battery 14 as will be described, and adjusts the voltage of the power supply 22 to maintain a constant voltage difference across the current limit resistor 24, thus maintaining a constant current to the battery 14.

In more detail, the power supply 22 is a switch-mode power supply (SMPS), preferably an SMPS provided in a single integrated circuit package. Many suitable circuits and components will be known to the skilled reader. For example, a SMPS can be built around a TINYSWITCH (TM) device (reference no.

TNY254) manufactured by Power Integrations Inc. This device is based around a switching circuit of constant frequency but which can be enabled and disabled to create AC at the primary coil of a transformer, from which an AC output can be drawn by means of a secondary coil. The AC can be rectified and smoothed to provide a DC output. Variation in the regime of enabling and disabling the switching element allows the DC voltage to be varied.

Variation is controlled by a feedback signal at 28, provided by the feedback circuit 26. Before describing the feedback circuit in more detail, it is first appropriate to note that the current resistor 24 is connected in series with the battery 14 and a rectifier diode 30, between the output 32 of the power

supply and ground at 34. Accordingly, the following equation applies :

IR = V.-V,, (Equation 1) .,....... --~~4. h~. -R R

Where IR is the current through resistor 24; Vo is the voltage at the power supply side of the resistor 24; V is the voltage across the battery 14: and V is the value of the resistor 24.

The feedback circuit 26 can now be described in more detail.

An opto-isolator 36, such as a TLP181GB device manufactured by Toshiba, incorporates a transistor 38 having its collector connected to provide the feedback signal 28 to the power supply 22, and its emitter connected to ground. The base of the transistor 38 is exposed, within the opto-isolator package 36, to the output of a light-emitting diode (LED) 40 connected at its anode to the junction of the diode 30 and resistor 24, and having its cathode connected in series with a zener diode 42 to the common terminal of the resistor 24 and battery 14.

The arrangement of the LED 40 and zener 42 provide the following equation:

Vs = VB+ Vz + VL (Equation 2)

Where V, is the breakdown voltage of the zener diode 42 and VL is the forward bias voltage of the LED 40.

Thus, Vo is held by the diode 42 and LED 40 at a voltage which is fixed relative to the battery voltage, while the components 40,42 are conducting.

The charging current flowing to the battery 14 will be the sum of the currents flowing through the current-limit resistor 24 and the zener diode 42.

However, appropriate choice of components allows the current through the zener diode 42 to be negligibly small in comparison with the current through the resistor 24. In particular, the paver supply 22 is very sensitive to the feedback signal at 28, and the opto-isolator has current gain, thus further reducing zener diode current.

Consequently, the charging current li can be considered equal to the resistor current I and thus

1, = V,-VB (Equation 3) COB P,

It can be seen by combining equations 2 and 3 that :

Ie = Vz + Vl (Equation 4) R R Thus, while the power supply output voltage V is forced to vary with battery voltage, rising as battery voltage rises and falling as battery voltage falls, the charging current is fixed, being dependent only on the fixed values of V, V, and R. The battery 14 is therefore provided with constant charging current regardless of its state of charge.

The effect of the components 40,42 in setting the voltage at Vo will affect the operation of the transformer within the SMPS 20, causing the amount of power lost in the windings to vary. Thus, the output of the power supply 20 will be of constant current but variable power.

A smoothing capacitor 44 may be provided, parallel with the battery 14 and resistor 24, In a typical emergency lighting situation, Nicad cells would be used with

a nominal value of 1.2V per cell, the emergency lighting incorporating from 2 to 6 cells to provide between 2.4V and 7.2V for driving the lamp bulb 12. After powering the lamp bulb 12 for 3 hours or more, the actual voltage of the cells may have dropped to 0.9V per cell. When fully charged, the cell voltage may rise to I. SV per cell. Charging at 210mA will charge the Nicads in about 24 hours. Maintaining substantially constant charging current is advantageous in various ways, as follows.

The longevity of Nicad cells can be improved by charging at a constant charging current. In the circuit of Fig. 2, the charge condition of the battery 14 is sensed by sensing the battery voltage, and using this to create a feedback signal by means of the opto-isolator 36 to control the power supply 22 to vary the output voltage of the power supply 22 (as set down in equation 2), the result being that the power supply voltage is varied in accordance with the charge condition and substantially constant charge current is maintained at all times. This is achieved with a simple current limit arrangement, which may be as simple as a single resistor.

Maintaining constant charging current in this way ensures that the resistor 24 is under the same amount of stress regardless of battery charge condition and in particular, heat dissipation requirements are constant, allowing for greater ease in design of the rest of the light 10.

Furthermore, equation 2 holds even in the event of failure of the battery 14, resulting in a short-circuit through the battery. Even in that extreme situation, the current through the resistor 24 will be held at the constant value icy so that even in short-circuit conditions, heat dissipation from the resistor 24 does not increase. Again, this facilitates design of the light 10, particularly in contrast with previous arrangements which have required multiple resistors solely to be able to cope with heat dissipation during short-circuit conditions, giving rise to space problems, increased heat dissipation and an increased component count.

In a typical example, the power supply may have an input voltage in the range of 110Vac to 240Vac, and may provide an output voltage of approximately IOV when charting 5 cells in series.

Many variations and modifications can be made to the apparatus described above, without departing from the scope of the present invention. In particular, different component values, current and voltage values could be chosen. The circuit described above in relation to Fig. 2 can be built as part of an emergency light, or as a module for incorporation into an existing light.

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 herein before referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

CLAIMS 1. An emergency light battery charging circuit having a variable voltage power supply, current limit means connected between the power supply and the battery being charged and operable to supply a current in dependence on the voltage difference across the current limit means, and a feedback means operable to sense the charge condition of the battery being charged and to adjust the power supply voltage in accordance with the charge condition to maintain a constant voltage difference across the voltage current means and thus to maintain a constant current to the battery being charged.
2. A circuit according to claim 1, wherein the current limit means is resistive.
3. A circuit according to claim 2, wherein the current limit means is a current limiting resistor.
4. A circuit according to any preceding claim, wherein the sensed charged condition is used to provide a feedback signal to the power supply, the power supply being operable to change the power supply voltage in dependence on the feedback signal.
5. A circuit according to claim 4, wherein the feedback means senses the battery voltage.
6. A circuit according to claim 4 or 5, wherein the feedback means is connected between the battery and the output of the power supply, the feedback means including an opto-coupler through which the current in the feedback means flows, and which provides the feedback signal in response thereto.
7. A circuit according to claim 6, wherein the feedback means includes, in series, the opto-isolator and a voltage-limiting device.

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8. A circuit according to claim 7, wherein the voltage-limiting device is a zener diode.
9. A circuit according to claim 4 or 5, wherein the feedback means is connected between the battery and the output of the power supply and consists of circuit components across which the voltage is fixed when current is flowing.
10. A circuit according to any preceding claim, wherein the power supply is a switched mode power supply.
11. An emergency light incorporating a charging circuit as defined in any preceding claim.
12. An emergency light power module having a rechargeable battery and a charging circuit as defined in any of claims I to 10.
13. A module according to claim 12, wherein the battery is a Nicad battery.
14. An emergency light battery charging circuit substantially as described above, with reference to Fig. 2 of the accompanying drawings.
15. 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.