GB2269062A - Emergency lighting; Battery charging - Google Patents

Abstract

The emergency lighting system has a fluorescent lamp connected to output terminals 36, 38 of an inverter 30 which is powered from a battery via a DC-DC converter 12, 14, and in order to reduce the warm-up time before the lamp becomes fully effective, the DC-DC converter initially boosts the input power to the inverter for a predetermined time to a level greater than that normally required to operate the lamp at normal brightness. The power boost is achieved by controlling the ON period duration of a switching transistor 12 by means including an R-C circuit 39, 40, 42, 40 in which a transistor 54 effectively gradually removes a capacitor 42 from the circuit so that the operating frequency of transistor 12 changes from 18 KHz to 36 KHz over an initial two minute period. The ON period duration of transistor 12 is also controlled to maintain light output as the battery voltage decreases. A battery charger has a switching converter in which an FET (76), (Fig 2), is turned on for a period inversely proportional to the main supply voltage so that the charging rate is independent of mains voltage fluctuations. An inductor (84) in series with the battery (72) sets the desired charge current. <IMAGE>

Description

LAMP CONTROL APPARATUS AND METHOD OF CONTROL This invention relates to a lamp control apparatus and method of control, and especially to a lamp circuit for an emergency lamp fitting.

Emergency light fittings wherein battery power is provided for use in the event of mains power supply failure are known and are often provided in industrial or commercial premises, particularly for the illumination of escape routes. If mains power supply should be interrupted, for example due to a fire or other similar problems, safe routes out of the premises need to be adequately indicated for people therein.

Because such emergency lamp fittings need to operate independently of mains power supply they are normally powered by their own internal battery packs, with the fitting being provided with means operative to actuate the battery pack in the event of interruption of the mains power supply. Such battery packs may be rechargeable - drawing charging power from the mains supply when it is available.

Known emergency lamp circuits suffer from several disadvantages, namely that it may take a noticeable period of time (possibly up to 10-20 seconds) for the emergency lighting to become fully effective, and that when the battery packs are reaching the end of their life the effectiveness of the emergency lighting may be reduced.

The present invention aims to reduce some or all of the above problems.

Accordingly, in a first aspect the present invention provides a control circuit for an emergency lamp including power supply means for supplying electrical power from a battery to a lamp, the power supply means including first power control means operable to supply power to the lamp for a predetermined time period at a rate greater than the minimum required by the lamp.

Preferably the predetermined time period is commencable when the first power control means are activated so that power is initially suppliable to the lamp at a rate greater than the minimum required.

In this way a "boost" start period is provided and the emergency lamp may have a minimum start up time between initial operation and peak effectiveness, and it may be assured that the lighting reaches adequate luminescence within the shortest possible time.

The term lamp includes all forms of lighting e.g., inter alia, fluorescent lamps and other lamps having a warm-up characteristic.

Preferably, the first power control means includes a switching transistor usable to control the power supplied to the lamp, and the on-time of the transistor is adjustable to adjust the rate of power supply. In a practical embodiment, the switching transistor is controlled by one or more timing capacitors and the timing capacitance is reducible at the end of said predetermined time period so as to reduce the on-time of the transistor.

Advantageously, the circuit includes a magnetic core connectable between the battery and the first power control means wherein the magnetic core is usable to store energy from the power supply means and to subsequently transfer the stored energy to the lamp. Thus, the power supply means, which may include a converter section, can be considered to be that of a - "discontinuous boost converter". That is to say that all the energy stored in the magnetic element is transferred to the load before the magnetic element is allowed to store energy once more.

The main switching transistor can be turned on for a fixed period of time which may be determined by the battery terminal voltage. During this time energy is being stored in the magnetic core, with the amount of energy being determined by the on-time of the main switching transistor whose on-time may be inversely proportional to battery voltage (see below). Thus constant throughput power to the load and hence constant lamp brightness may be maintained.

At start up it may be desirable to double the power going into a lamp inverter part of the circuit. In the circuit this may be achieved by doubling the on-time of the main switching transistor, thereby doubling the amount of energy stored in the magnetic element. This double power at start up "boost" helps the lamp e.g. a fluorescent lamp, to strike and warms the lamp up quicker thus increasing lamp brightness. This is very desirable because in an emergency this would be the time when maximum illumination would be required for safe passage from a building etc.

Advantageously the converter comprises means to cause operation thereof at a predetermined frequency, and the circuit may further include a delay capacitor connected to a booster transistor operatively connected by a second capacitor to a timing circuit so that switching off the second capacitor has the effect of doubling the operational frequency of the converter.

After this initial boost period the lamp power may be reduced to normal running power and stabilised over the remaining battery discharge time by means of its control circuit.

The lamp output transformer may be optimised by impedance matching it to the expected group of fluorescent lamps, and this may greatly reduce the number of transformers otherwise required. The characteristic impedance of the groups of lamps may be determined by their struck voltage/current components. Therefore for maximum power transfer this gives the inverter transformer output impedance required.

In a second aspect the present invention provides a control circuit including power supply means for supplying electrical power from a battery to a lamp, the power supply means including second power control means operable to control the rate of power supply, wherein the duty cycle of the second power control means is adjustable to compensate for variations in the terminal voltage of the battery.

Thus as the battery approaches its end point i.e., becomes close to discharged, the circuit compensates for the drop in battery terminal voltage by means of its control circuit, thereby extending the end point of the battery and maintaining constant lamp brightness.

The second power control means may include or be included in the first power control means of the first aspect of the present invention.

Preferably the circuit includes a switching transistor usable to control the power supplied to the lamp, and the on time of the transistor is adjustable to adjust the rate of power supply.

In a practical embodiment the on-time of the switching transistor is controlled by at least one timing capacitor and the time taken for the timing capacitor(s) to charge is adjustable according to the terminal voltage of the battery, so that the on-time of the transistor is adjustable to compensate for variations in said terminal voltage.

In a third aspect, the present invention provides a battery charging circuit for supplying power from a power supply to a rechargeable battery, including third power control means for controlling the rate of power supply, wherein the duty cycle of the third power control means is adjustable to compensate for variations in the voltage of the power supply.

Thus the battery charger may maintain the same charge current into the battery irrespective of mains input voltage range or fluctuations therein.

To accomplish this the battery charger is preferably arranged to include a transformer for storing energy from the power supply wherein the duty cycle of the third power control means is adjustable to maintain a substantially constant rate of energy storage in the transformer. The amount of energy stored may be held constant by allowing the supply current to flow for a time which is inversely proportional to the value of the supply voltage. In this way the time-current product remains constant.

Preferably the circuit includes current control means to supply a substantially constant current from the transformer to the battery.

In a preferred embodiment this is achieved by allowing a power transistor to conduct charge for a predetermined period, determined by the charging time of a capacitor which is fed via a resistor from a supply voltage. The result of this arrangement is that the charge received by the battery means is independent of the supply voltage and thus the battery charger is adaptable to any variation or fluctuation in input voltage range. This minimises power consumption, internal luminaire temperature and battery temperature. It also maximises battery life.

The constant current charger means that at high mains excursions (all nominal mains vary by +/- tolerance) the battery is still charged with the optimum amount, thereby minimising power dissipated in the circuit and minimising battery temperature and maximising battery life. i.e., The charger circuit only draws from the power supply the actual power that is required. Similarly the maintained circuit only applies the minimum power necessary to the lamp, whatever the mains. Therefore the temperature is minimised, again keeping battery life at a maximum.

The battery charger may further include a resistor which defines a predetermined threshold value such that a voltage preference circuit is established causing selective operation of the lamp via the battery when the mains voltage falls below a predetermined low threshold value. It may also restore mains voltage supply to the lamp when a second higher threshold is exceeded by the mains supply.

The invention may also provide an improved lamp circuit which can be easily adapted to a range of battery voltages, thereby allowing the flexibility of using many lamp sizes and powers.

To accomplish this the charge circuit is arranged to give a constant charge current irrespective of nominal battery voltage. Additionally, during emergency operation, the combined converter/inverter circuits demand a constant current from the battery, again irrespective of normal battery voltage.

The invention will be described further by way of example with reference to the accompanying drawings in which: Fig. 1 is a circuit diagram of a first embodiment of the invention including timing and inverting sections; Fig. 2 is a circuit diagram of a charger circuit according to-on aspect of the invention; and Fig. 3 is a circuit diagram of modified charger circuit.

- Referring firstly to Fig. 1, a battery (not shown) is connected to the circuit at terminal 10 and to a mains switching transistor 12 (Q3) via a magnetic core 14 having two windings 16,18. In this condition the transistor 12 is held in an "off" condition as no base current can flow. The transistor's collector consequently rises to the battery voltage level biasing a diode 20 in a forward direction. An output capacitor 22 is hence charged up to the battery's voltage minus the voltage drop due to diode 20.

This voltage is applied to an inverter section referred to generally by the reference 30 and including two transistors 32,34. The voltage applied is however too low to allow the invertor 30 to start up and the circuit is dormant and a fluorescent lamp (not shown) connected between terminals 36,38 is not energised.

Alternatively, when terminals 24 and 26 are connected to the battery a base current flows into the switching transistor 12 allowing a current to build up in a linear fashion in the first winding 16. This commences the charge period of a switching cycle during which energy is being stored in the magnetic core 14.

After a period of time determined by the product of R multiplied by C, being the Resistance and the Capacitance respectively of resistors 39,40 and time capacitors 42,44, the voltage on the base of a timing transistor 42,44, is turned on (0.65 volts nominal). At that time the transistor 46 is turned on which clamps the base voltage of the switching transistor 12 to zero volts thus turning the main switching transistor off again.

The voltage on the first winding 16 is therefore reversed and the voltage at the collector of the switching transistor rises to a high voltage. As this voltage collapses, the voltage on the second winding 18 reverses direction and a further diode 48 conducts thereby discharging the timing capacitors 42,44. This causes the timing transistor 46 to turn off allowing base current to flow into the mains switching transistor 12 via an inductor 50. This turns the switching transistor 12 "on" once more, which cycle is repeated in a self-oscillating manner at a frequency of approximately 18kHz.

In this embodiment the timing circuit 60 is set such that after approximately two minutes, the charge on boost capacitor 52 reduces to a point where a boost transistor 54 begins to turn off. This effectively removes one of the two timing capacitors 42 from the timing circuit 60 and halves the capacitance thereof.

This removal happens gradually over a period of time and is not visibly detectable. The effect of halving the timing capacitance is to double the operating frequency of the converter to approximately 36kHz. Thereby, the main switching transistor 12 is turned "on" for only half its former period, i.e. half of that the aforementioned 2 minute boost period.

The energy thus stored in the first winding 16 is halved and therefore the energy passed to the inverter section 30 is also halved resulting in the fluorescent lamp attached to the terminals 36,38 running at a normal brightness.

This circuit further has the capacity to maintain lamp brightness even when the battery is operating at a reduced voltage. In this mode the time taken for the second timing capacitor 44 to reach a base threshold of the timing transistor 46 is increased. This has the effect of increasing the "on" time of the main switching transistor 12 thereby increasing energy stored in the first winding 16. A closed loop control is formed compensating for reduced battery voltage across the lamp terminals 36,38.

Turning now to Fig. 2, a charger circuit 70 according to a further aspect of the invention delivers charge into a battery 7by storing energy in a transformer 74 and transferring its energy to the battery in its second phase of operation. The amount of energy stored is held constant by allowing a supply current to flow for a time which is inversely proportional to the value of the supply voltage. In this way, the time ~ current product will remain constant, thereby determining the amount of energy stored in the transformer 74.

This operating characteristic is achieved by allowing a power transistor 76 to conduct for a period determined by the charging time of a capacitor 78. This capacitor is fed via a resistor 80 from a supply equal to the peak value of supply voltage and the conduction time of the power transistor 76 is again inversely proportional to the value of the supply voltage.

As the peak current value that flows in the power transistor 76 after a given conduction time, assuming a fixed value of transformer inductance, is proportional to the value of the supply voltage, both these effects cancel each other.

Therefore, the peak current in the power transistor 76, and therefore the energy stored in the transformer, remains the same. The result of this arrangement is that the charge received by the battery is independent of the actual supply voltage.

The batteries are fed with charge current from a secondary winding 82 on the transformer 74 which produces sufficient voltage to make battery voltage changes insignificant in comparison therewith. The same charge current is therefore maintained. Preferably a wattless dropper in a form of an inductor 84 is used to set the charge current to the desired value.

Due to the relatively long period of time taken to adjust to supply voltage variations, the charge remains fixed at a particular duty cycle across the sinusoidal variation of the income unsmoothed supply. Peak rectification of the supply by a diode 86 and a capacitor 88 ensure that the regulating circuit is fed from a source that changes with supply voltage yet remains constant during the short duration of a supply cycle.

This results in the charge drawing current proportional to the instantaneous value of the supply, and the average current taken by the charger remains sinusoidal so that the harmonic content of the supply amount is acceptable.

In Fig. 3 the charger circuit is modified to provide for self selection of mains failure changeover voltage threshold.

In this modification, a changeover and maintained control circuit 90 is altered such that capacitor 92 is charged to a negative value proportional to the mains supply voltage. The relative values of a resistor 77 in the inverter and further resistor 94 in the changeover control 90 determines at what specific voltage the mains switching transistor 12 switches and changes from battery to mains operation.

This switching operation is enhanced by a latch formed by a triac 96, which, once set in its conducting state by operation of the luminaire in the high range of 180-275 v AC, causes the changeover switch to operate at an elevated point appropriate to the nominal supply voltage in use. Operation of the luminaire from a mains supply voltage in the low range (99134 v AC) means the triac 96 is not set and the changeover switch remains at its default low operating points. The latch formed by the triac can be reset by disconnection of the batteries should it be desirable to clear the current changeover state.

The invention is not confined to foregoing details and variations being made thereto within the scope are with the invention.

Claims (15)

CLAIMS:

1. A control circuit for an emergency lamp, including power supply means for supplying electrical power from a battery to a lamp, the power supply means including first power control means operable to supply power to the lamp for a predetermined time period at a rate greater than the minimum required by the lamp.

2. A control circuit according to claim 1 wherein the predetermined time period is commencable when the first power control means are activated so that power is initially suppliable to the lamp at a rate greater than the minimum required.

3. A control circuit according to claim 1 or claim 2, wherein the first power control means include a switching transistor usable to control the power supplied to the lamp and the ontime of the transistor is adjustable to adjust the rate of power supply.

4. A control circuit according to claim 3 wherein the on-time of the switching transistor is controlled by one or more timing capacitors and timing capacitance is reducible at the end of said predetermined time period so as to reduce the on-time of the transistor.

5. A control circuit according to any one of the above claims including a magnetic core connectable between the battery and the first power control means wherein the magnetic core is usable to store energy from the power supply means and subsequently transfer the stored energy to the lamp.

6. A control circuit for an emergency lamp, including power supply means for supplying electrical power from a battery to a lamp, the power supply means including second power control means operable to control the rate of power supply; wherein the duty cycle of the second power control means is adjustable to compensate for variations in the terminal voltage of the battery.

7. A control circuit according to claim 6 wherein the second power control means includes a switching transistor usable to control the power supplied to the lamp and the on-time of the transistor is adjustable to adjust the rate of power supply.

8. A control circuit according to claim 7 wherein the on-time of the switching transistor is controlled by at least one timing capacitor and the time taken for the timing capacitor(s) to charge is adjustable according to the terminal voltage of the battery, so that the on-time of the transistor is adjustable to compensate for variations in said terminal voltage.

9. A control circuit substantially as any one embodiment herein described with reference to figure 1 of the accompanying drawings.

10. An emergency lamp fitting including a control circuit according to any one of the above claims operatively connected to a lamp.

11. A battery charging circuit for supplying power from a power supply to a rechargeable battery, including third power control means for controlling the rate of power supply, wherein the duty cycle of the third power control means is adjustable to compensate for variations in the voltage of the power supply.

12.- A battery charging circuit according to claim 11 including a a transformer for storing energy from the power supply wherein the duty cycle of the third power control means is adjustable to maintain a substantially constant rate of energy storage in the transformer.

13. A battery charging circuit according to claim 12 including current control means to supply a substantially constant current from the transformer to the battery.

14. A battery charging circuit substantially as herein described with reference to figures 2 and 3 of the accompanying drawings.

15. An emergency lamp fitting including a battery charging circuit according to any one of claims 11 to 14, a control circuit according to any one of claims 1 to 9 , and a lamp.