Alarm system

Abstract

 A fire alarm system comprises a warning device 18 and a control panel 10 for providing an alarm signal to trigger the sounder device 18 when a fire is detected. The control panel 10 comprises a power supply 12 which incorporates a rechargeable battery 13, the normal output voltage of the power supply 12 being greater than the voltage at which the output of the sounder device 18 is at or above a predetermined acceptable level. Thus, in the event that the sounder device 18 is triggered before or during a power failure, the output of the sounder device 18 will remain at or above the predetermined acceptable level for a considerable length of time, until the battery becomes exhausted.

Description

[0001] This invention relates to an alarm system.

[0002] Alarm systems are known which comprise audible and/or visual warning devices which are activated in the event that an alarm condition is detected. It is well known to provide alarm systems with back-up batteries, which enable the system remain functional when there is a loss of mains power.

[0003] In fire alarms systems for hotels and other residences, there is a requirement that the back-up battery must be able to provide sufficient power after 72 hours that audible warning devices in each room can output a volume of at least 75dB, when monitored from a point located at the head of the bed in that room. Thus, if a fault occurs with the supply to the alarm system, the back-up battery should have sufficient power for the alarm system to remain operational until the fault is detected and rectified.

[0004] Alarm systems consume a considerable amount of power when in the stand-by mode and thus it is difficult for back-up batteries to meet the above-mentioned objective. The problem can be solved by providing a large back-up battery, or by limiting the number of alarm detectors connected to the system, so that less current is consumed in the stand-by mode.

[0005] It will be appreciated that both of the above-mentioned solutions to the problem are unsatisfactory.

[0006] Another problem with alarm systems is that there is often several kilometres of cable between the audible warning devices and the control panel. The cable exhibits electrical resistance, with the result that there can be a considerable voltage drop along the cable, particularly when a high current is being drawn during an alarm condition. Accordingly, there is a risk that the alarm voltage at the audible warning devices will be considerably than their proper operating voltage, with the result that the output noise level of the audible warning devices will be considerably less than is required.

[0007] We have now devised an alarm system which alleviates the above-mentioned problems.

[0008] In accordance with this invention there is provided an alarm system comprising a warning device and power supply means for providing power to the warning device, wherein the normal output voltage of said power supply means substantially exceeds a set voltage at which an output of the warning device is at a predetermined acceptable level.

[0009] The output of the warning device at said level is arranged so that it is sufficient to alert persons that an alarm condition has been detected. Thus, if the voltage applied to the warning device is less than the output of the power supply, say because either the output of a standby battery of the power supply has fallen or because there is drop in voltage between along a supply cable, the output level of the warning device will remain at said acceptable level as long as the output voltage of the power supply substantially exceeds the set voltage.

[0010] Preferably the warning device is connected to the power supply means via a voltage regulator means, which provides the warning device with the set voltage when the output of the power supply means is greater than the set voltage. Thus, the output level of the warning device does not fall as the output voltage of the power supply means falls towards the set voltage.

[0011] In one embodiment, the voltage regulator means outputs an alternating voltage such as a square wave, the regulator means being arranged to stabilise the average value of the alternating voltage, in order to provide the warning device with the set voltage. Thus, as the peak value of the alternating voltage falls with the output of the power supply means, the voltage regulator means will vary the mark-to-space ratio of the regulator voltage, thereby stabilising its average value.

[0012] In an alternative embodiment, the voltage regulator means is arranged to output a d.c. voltage having a value which remains at the set voltage of the warning device, as long as the output voltage of the power supply means remains at or above the set voltage.

[0013] Preferably the warning device comprises an audible warning device.

[0014] Preferably the power supply means comprises a rechargeable battery.

[0015] Embodiments of this invention will now be described by way of examples only and with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of an embodiment of a fire alarm system in accordance with this invention; and

Figure 2 is a schematic diagram of an alternative embodiment of fire alarm system in accordance with this invention.

[0016] Referring to Figure 1 of the drawings, there is shown a fire alarm system comprising a control panel 10 having a power supply 12 provided with terminals 11 for connecting to the mains supply. A rechargeable battery 13 is connected to the power supply 12, and is arranged to provide power in the event of an interruption in the mains supply.

[0017] A plurality of sounders e.g. 16 are connected in parallel across a pair of wires 14, 15 that extend from the control panel 10. The sounders 16 each comprise a speaker 18 and a speaker driver circuit (not shown), which is connected to the wires 14, 15 via a voltage regulator 17. The speaker 18 is arranged to produce an output of 75dB, when a voltage of say 6 volts is applied to the driver circuit.

[0018] The alarm system also comprises a plurality of heat and/or smoke sensors and manual call points (not shown), which are connected to the control panel 10. The control panel 10 enters an alarm condition when one of the sensors or call points is activated, whereupon it applies an alarm signal to the wires 14, 15.

[0019] In one embodiment, the alarm signal comprises a d.c. voltage of a value which is normally much greater than 6 volts. When the alarm signal is applied across the wires 14, 15, the voltage regulator 17 produces a voltage on its output which is regulated at 6 volts. Thus, the speaker 18 generates an audible warning signal at 75dB.

[0020] In the event of a mains power failure, the battery 13 maintains the supply, so that the alarm remains operational. The control panel 10 and the detectors draw current from the battery in the standby mode of the alarm, and thus the battery 13 slowly becomes exhausted. The output voltage of the battery 13 fall as the battery becomes exhausted, which causes a corresponding reduction in the alarm voltage. Thus, in conventional alarm systems the output level of the speakers 18 will fall as the battery becomes exhausted.

[0021] However, the speakers 18 of an alarm system in accordance with this invention will continue to operate at 75 db, as long as the battery 13 can produce an alarm voltage of 6 volts or more. Typically the alarm voltage and battery voltage are equal to 24 volts. Thus, the battery voltage has to fall from 24 volts to 6 volts before the output level of the speaker starts to fall.

[0022] It takes a considerable amount of time for the battery voltage to fall from 24 volts to 6 volts and in this time the mains fault is likely to have been detected and rectified.

[0023] In an alternative embodiment, the regulator 17 produces a square wave on its output, having a peak-to-peak value which is dependent on the alarm voltage. The average voltage value of the square wave is adjusted by the regulator to 6 volts. Thus, when the battery voltage falls the peak-to-peak value of the regulator will fall by a corresponding amount. However, the regulator increases the mark-to-space ratio of the square wave, in order to keep the mean value of the square wave at 6 volts.

[0024] Referring to Figure 2 of the drawings, there is shown an alternative embodiment of alarm system and like parts are given like reference numerals. In this embodiment a supply of voltage is continuously applied to the wires 14, 15, the sounders 16 being activated by applying an alarm signal to a third wire 20. When the sounders 16 are activated current is drawn from the wires 14, 15.

[0025] If the mains power fails, the supply voltage on the wires 14, 15 will fall as the battery voltage falls, although the voltage of the alarm signal may not fall. The regulator 17 and speaker 18 operate in the same way as before, and thus the output of the speaker will remain at 75dB until the supply voltage on the wires 14, 15 falls below 6 volts.

[0026] In an alternative embodiment, the wires 14, 15 continuously carry a supply of voltage, say to power detectors which may be incorporated within the same housing as the sounders 16, and the sounders are activated by superimposing an a.c. alarm signal onto the supply voltage. The sounders 16 comprise a detector circuit which monitors the wires 14, 15 and activates the speakers in the event that the a.c. alarm signal is detected.

Claims

1. An alarm system comprising a warning device and power supply means for providing power to the warning device, wherein the normal output voltage of said power supply means substantially exceeds a set voltage at which an output of the warning device is at a predetermined acceptable level.

2. An alarm system as claimed in claim 1, in which the warning device is connected to the power supply means via a voltage regulator means, which is arranged to provide the warning device with the set voltage when the output of the power supply means is greater than the set voltage.

3. An alarm system as claimed in claim 2, in which the voltage regulator means outputs an alternating voltage, the regulator means being arranged to stabilise the average value of the alternating voltage, in order to provide the warning device with the set voltage.

4. An alarm system as claimed in claim 2, in which the voltage regulator means is arranged to output a d.c. voltage having a value which remains at the set voltage of the warning device, as long as the output voltage of the power supply means remains at or above the set voltage.

5. An alarm system as claimed in any preceding claim, in which the warning device comprises an audible warning device.

6. An alarm system as claimed in any preceding claim, in which the power supply means comprises a rechargeable battery.

Drawing