IE62034B1 - "An Emergency Light Circuit" - Google Patents

Description

kn. Emergency Light Circuit T The invention relates to an emergency light circuit for controlling emergency lighting for operation when a mains • AC source fails .

Generally, emergency lighting is provided for safety 5 reasons, for example, to illuminate escape routes, firealarm points, and fire-fighting equipment. The lighting may be either maintained, in which case the lighting generally operates from the mains AC source in normal conditions and via an emergency light circuit in emergency conditions, or non-maintained in which case the lighting operates only under emergency conditions from the emergency light circuit.

Because of the situations under which emergency lighting is required to operate, reliability of the circuits is very important. Many presently available emergency light circuits (such as described in U.K. Patent Specification No. 2,185,646 (Hubbell)) do not provide protection features to prevent damage to the circuit in the event of battery voltage dropping to a dangerously low level. Such features are described in U.K. Patent Specification No. 1,414,052 (General Electric), however, the circuit arrangements to achieve this are rather complex and require a relatively large number of components.

A further problem in presently available light circuits is that emergency lighting is only operated on the conditions of complete mains failure. This is clearly • unsatisfactory.

• The invention is directed towards providing an emergency light circuit having improved reliability. A second object is to provide an emergency light circuit which - 2 operates under partial failure of a mains AC source. A further object is to provide an emergency light circuit which is relatively simple and inexpensive to produce.

According to the invention, there is provided an emergency light circuit comprising :a ballast transformer feeding a rectifier; a re-chargeable battery bank fed by the rectifier; a high-frequency inverter driving the emergency light and being connected to the output terminals of the battery; an electronic switch means for switching of the inverter and being connected to a mains AC source monitoring circuit and to a battery monitoring circuit so that the inverter is disabled if the mains AC source is at a normal voltage level, the inverter is enabled if the mains AC source is below a preset voltage level while the battery voltage level is above a preset level, and the inverter is disabled if the battery voltage level falls below the preset level while the mains AC source voltage is below the preset level, and wherein:the mains AC source monitoring circuit is operatively connected to the electronic switch means, and comprises voltage level detecting means connected to a diverting switch connected for diversion of an energising signal for the electronic switch means to earth during normal AC source voltage levels, and to open on dropping of the AC source voltage below a pre-set level, and the battery monitoring circuit is operatively connected to the switch means to provide an energising signal for it, and comprises a thyristor, the anode of which is connected to the battery positive output and the cathode of which is connected to the battery negative output and to the electronic switch means, the thyristor being adapted to form an open circuit on dropping of voltage across the device.

In one embodiment, the cathode of the thyristor is connected to the battery negative output via a voltage regulator.

In another embodiment, the voltage level detecting means comprises a voltage regulator and a charge storage device connected in series across the rectifier output, the charge storage device being connected to the diverting switch for maintaining it closed when in the charged state, the voltage regulator being adapted to open, on dropping of the rectifier output voltage below a pre-set level causing the charged storage device to discharge in turn causing the diverting switch to open.

The invention will be more clearly understood from the following description of some preferred embodiments thereof, given by way of example only with reference to the accompanying drawings in which :Fig. 1 is a block diagram illustrating an emergency light circuit of the invention; and Fig. 2 is a circuit diagram of the circuit.

Referring to the drawings, there is illustrated an emergency light circuit of the invention, indicated generally by the reference numeral 1. The circuit 1 is for operation of a fluorescent lamp 2 under conditions of failure of a mains 220V AC source 3. The circuit comprises a ballast transformer 4 for stepping down AC voltage from 220V to approximately 7.5V AC, the output of which is connected to a full-wave rectifier 5, the output of which feeds a positive DC rail 6 and a negative DC rail 7. The DC rails 6 and 7 supply power to the circuit 1 and are connected for charging a re-chargeable battery bank 8 comprising three 1.2V NiCd batteries via a charge resistor R3 . The circuit 1 also includes a high-frequency inverter 9 for conversion of DC power to a high-frequency AC supply for the fluorescent lamp 2 under emergency conditions. Operation of the inverter 9 is controlled by an electronic switch 10, which is in turn controlled by a battery monitoring circuit 11 and a mains monitoring circuit 12. The battery monitoring circuit 11 includes an output line 14 for delivering an energising signal to the electronic switch 10, which signal may be diverted to earth via a switch 13 controlled by the mains monitoring circuit 12.

In more detail, and referring specifically to Fig. 2, the full-wave rectifier 5 comprises diodes Dl, D2, D3 and D4, and the circuit 1 also includes a light emitting diode LED connected in series with a resistor R2 across a resistor R3 in the positive supply rail 6 for indicating satisfactory charge operation. The high-frequency inverter 9 is of the type having a pair of push-pull transistors T3 and T4 connected to first and second primary windings having 8 and 5 turns respectively. The secondary winding has 430 turns and is connected for delivery of power to the fluorescent lamp 2 via a capacitor C3. The electronic switch 10 comprises a transistor T2 connected to the second primary winding for control of the bases of the push-pull transistors T3 and T4. The emitter of the transistor T2 is connected to the inverter 9 via a resistor R8 and the collector of the transistor T2 is connected to the positive rail 6. The battery monitoring circuit 11 comprises a voltage sensitive device, namely, a thyristor or silicon controlled rectifier SCR, the anode of which is connected to the positive rail 6 via a resistor R6. The cathode of the SCR is connected to the negative rail 7 via a zener diode Z2 having a 2.7V break-down voltage. The cathode of the SCR is also connected to the base of the transistor T2 via a resistor R7. Thus, the base of the transistor T2 is effectively connected to the positive rail 6 via the SCR and the resistor R6. The gate of the SCR is connected to the line lead of the AC source 3 via a forward biased diode DS, a capacitor Cl and a resistor Rl.

The mains monitoring circuit comprises a voltage regulator, namely, a zener diode ZI having a break-down voltage of 4.7V, the cathode of which is connected to the positive rail 6, and the anode of which is connected to the negative rail 7 via a charge storage device namely, a capacitor C2. The capacitor C2 is connected in parallel to ground with a resistor R4. The diverting switch 13 is formed by a transistor TI, the base of which is connected to the junction between the zener diode ZI and the capacitor C2 via a resistor R5. The collector of the transistor TI is connected to the base of the transistor T2, and the emitter is connected to the negative rail 7.

In operation, when the mains AC source 3 is at its correct level, the rectifier 5 will provide a 10V DC output to the rails 6 and 7. Further, the output of the ballast transformer 4 fires the SCR of the battery monitoring circuit 11. Thus, the DC voltage appears across the resistor R6, the SCR and the zener diode Z2, resulting in an energising signal being delivered on the line 14 to the base of the transistor T2. However, the DC voltage also appears across the zener diode ZI and the capacitor C2 of the mains monitoring circuit 12. The voltage appearing between these two components maintains a signal to the base of the transistor TI to keep the transistor closed, thus diverting the energising signal on the line 14 directly to the negative rail 7 (ground). Thus, the energising signal on the line 14 does not reach the base of the transistor T2, which remains off, thus maintaining the push-pull transistors T3 and T4 in the off condition. Accordingly, the inverter 9 does not operate and the discharge lamp 2 remains off.

If the voltage level of the mains AC source 3 drops to a level of approximately 70% of the normal level, the voltage across the zener diode Zl of the mains monitoring circuit 12 drops below the break down voltage of 4.7V, thereby shorting the capacitor C2 to earth (negative rail 7). There is then no base current to the transistor Tl which switches off, thus opening the path to ground for the energising signal on the line 14. Accordingly, the transistor T2 switches on and drives the bases of the transistors T3 and T4 of inverter 9 resulting in highfrequency output to the lamp 2 which then operates. If the lamp 2 remains in emergency operation for a period approaching three hours, the voltage level of the battery 8 drops towards the recommended minimum level for the battery and the minimum level at which the current can operate the fluorescent tube. When this happens, the anode voltage across the SCR drops to a cut-off level so that the SCR opens, thus disconnecting the base of the transistor T2 from the positive rail 6. Accordingly, the transistor T2 switches off, thus disabling the push-pull transistors T3 and T4 to switch off the inverter 9.

It will be noted that control of the inverter 9 is achieved through control of the switch 10 (transistor T2) in a simple manner with relatively few components. No additional connections are required to the inverter 9 other than that of the transistor T2. It will also be appreciated that the mains monitoring circuit 12, in a simple manner and reliable manner provides for operation of the fluorescent lamp on dropping of the mains voltage y below a pre-set level. Thus, it is not necessary that the mains AC source disappear completely before the fluorescent lamp 2 is operated.

The invention is not limited to the embodiments illustrated. For example, it is envisaged that a pair of capacitors may be connected in parallel at the output of the inverter 9 to provide a choice of brightness and duration of emergency lighting for the fluorescent lamp 2 . It is also envisaged that the input of the ballast transformer 6 may be connected by an RC circuit to an additional input line lead terminal to provide a choice of mains input voltages, for example, between 110V and 220V.

The invention is not limited to the embodiments hereinbefore described, but may be varied in construction and detail.

Claims (4)

1. An emergency light comprising :a ballast transformer feeding a rectifier; a re-chargeable battery bank fed by the rectifier; a high-frequency inverter driving the emergency light and being connected to the output terminals of the battery; an electronic switch means for switching of the inverter and being connected to a mains AC source monitoring circuit and to a battery monitoring circuit so that the inverter is disabled if the mains AC source is at a normal voltacfe level, the inverter is enabled if the mains AC source is below a preset voltage level while the battery voltage level is above a preset level, and the inverter is disabled if the battery voltage level falls below the preset level while the mains AC source voltage is below the preset level, and wherein :the mains AC source monitorincj circuit is operatively connected to the electronic switch means, and comprises voltage level detecting means connected to a diverting switch connected for diversion of an energising signal for the electronic switch means to earth during normal AC source voltage levels, and to open on dropping of the AC source voltage levels, and to open on dropping of the AC source voltage below a pre-set level, and the battery monitoring circuit is operatively connected to the switch means to provide an energising signal for it, and comprises a thyristor, the anode of which is connected to the battery positive output and the cathode of which is connected to the battery negative output and to the electronic switch means, the thyristor being adapted to form an open circuit on dropping of voltage across the device.

2. An emergency light circuit as claimed in claim 1, wherein the cathode of the thyristor is connected to the battery negative output a voltage regulator.

3. An emergency light circuit as claimed in claims 1 or 2, wherein the voltage level detecting means comprises a voltage regulator and a charge storage device connected in series across the rectifier output, the charge storage device being connected to the diverting switch for maintaining it closed when in the charged state, the voltage regulator being adapted to open, on dropping of the rectifier output voltage below a pre-set level causing the charged storage device to discharge in turn causing the diverting switch to open.

4. An emergency light circuit substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.