EP2693415A1

EP2693415A1 - Line drive circuit for fire alarm system and corresponding fire alarm system

Info

Publication number: EP2693415A1
Application number: EP13172598.8A
Authority: EP
Inventors: Urs Kästli; Hong PANG; Shu Chun Hou
Original assignee: Siemens AG; Siemens Corp
Current assignee: Siemens Schweiz AG
Priority date: 2012-07-31
Filing date: 2013-06-19
Publication date: 2014-02-05
Also published as: RU2013135767A; CN103578218A; CN103578218B

Abstract

The present invention proposes a drive circuit and drive method for a fire alarm system. The drive circuit comprises: a current generation circuit, capable of selectively supplying either a first current I_N1 or a second current I_ALARM to the line, wherein the first current I_N1 is used as a line monitoring current in a normal operating state, and the second current I_ALARM is greater than the first current I_N1; and a detection circuit, for controlling the current generation circuit to supply the second current I_ALARM to the line when detecting that a line voltage V_Line on the line is lower than a threshold V_th, wherein the threshold V_th is lower than a normal line voltage but higher than a maximum alarm voltage indicated on the line when the fire detector gives an alarm.

Description

Technical field

The present invention relates to the field of fire alarms in general, in particular to a line drive circuit in a fire alarm system and a drive method.

Background art

An automatic fire alarm system, or fire alarm system for short, is a type of automatic fire control installation which people set up inside buildings or other areas so that they may discover and report a fire early, and take prompt and effective measures such as controlling and extinguishing the fire. Fire alarm systems are installed in most buildings nowadays, such as office blocks, hotels and large shopping centres.
Fig. 1A shows the construction of a typical fire alarm system schematically. As shown in Fig. 1A, a fire alarm system 100 typically comprises a central control system 110, and multiple fire detectors 120 and/or multiple manual alarms 130, etc. connected to the central control system 110 by lines "Line". The fire detectors 120 and manual alarms 130 all transmit alarm signals to the central control system 110 via the lines "Line". In response to these alarm signals, the central control system 110 automatically takes corresponding measures for extinguishing the fire and for communication. Thus the fire alarm system shown in Fig. 1A enables the alarm to be raised and the fire extinguished automatically, while also enabling safe dispersal and guidance, system process display, fire control file management, and so on, so as to form a complete fire-fighting control system.
An alternating current power supply 111 or storage battery power supply 112 can be used for the central control system 110 in Fig. 1A. The storage battery 112 of the central control system 110 is generally required to be able to support the system for 72 hours of fire monitoring. In some countries, the storage battery 112 is also required to be able to support an alarm state for approximately half an hour. In order to meet these requirements, the central control system 110 must be as power-saving as possible.
In Fig. 1A, the fire detector 120 is an instrument for detecting a fire. Since the onset of fire is accompanied by the generation of smoke, high temperatures and light, etc., this smoke, heat and light can be converted to electrical alarm signals by a smoke probe 120 ("S") or temperature probe 120 ("H"), so that the central control system automatically activates a fire-extinguishing device to promptly extinguish the fire. The central control system 110 drives each fire detector 120 or manual alarm 130 via the lines "Line", so that they obtain enough electrical energy to operate normally. At the same time, the central control system 110 also monitors the lines periodically to detect whether an open circuit or short circuit fault has occurred, to guarantee the normal operation of the fire detectors 120 and manual alarms 130.
Fig. 1B shows how the fire detectors 120 for example are specifically connected to the central control system 110. In the example shown in Fig. 1B, multiple fire detectors 120 are connected in parallel between lines Line+ and Line-, while a terminating element (EOL: End of Line) is connected to the far end of the line "Line". The terminating element EOL may be a resistive element, for detecting whether the line has a fault. Specifically, when an open circuit occurs in a line "Line", the line resistance detected from the central control system 110 side will be higher than the resistance of the EOL; when a short circuit occurs in a line "Line", the line resistance detected from the central control system 110 side will be lower than the resistance of the EOL. Thus the EOL can assist in detecting whether the line is normal.
The central control system 110 also monitors the line voltage V_Line so as to monitor whether an alarm has arisen on a line. The fire detectors 120 will send an alarm signal along the line "Line" when they detect a fire, for example by pulling down the line voltage V_Line. Figs. 2A and 2B show schematically how two types of fire detector 120 currently in common use indicate an alarm. The fire detector 120-1 shown in Fig. 2A closes a switch K1 when a sensing module 220 thereof detects a fire; the switch K1 is connected in series with a Zener diode (with a reverse breakdown voltage of about 5.6 V, for example) between Line+ and Line- of the line. Thus the closing of the switch K1 causes the line voltage to be pulled down from its normal value, such as 24 V, to the reverse breakdown voltage of the Zener diode, which is 5.6 V. In this way, the central control system 110 can discover a fire by detecting the drop in line voltage.
Fig. 2B shows another way in which a fire detector 120-2 gives an alarm. As shown in Fig. 2B, when a sensing module 220 thereof detects a fire, a switch K2 is closed; the switch K2 is connected in series with a small resistor (e.g. 450 ohms) between Line+ and Line- of the line. Thus when the switch K2 is closed, the current on the line, e.g. 30 mA, will flow into the small resistor and thereby cause the line voltage to be pulled down from 24 V to about 12 V. At this point, it is still necessary to maintain a large current on the line to flow through the small resistor, so that the voltage of about 12 V can be maintained for a period of time, to ensure that the central control system 110 can detect this alarm. Otherwise, if the current on the line were to drop, the voltage across the resistor would fall, and the fire detector 120-2 would mistakenly trigger the reset mechanism (RESET), thereby missing this alarm.
In existing central control systems, separate drive circuits are generally designed for the different fire detectors shown in Figs. 2A and 2B, and a jumper wire or software recognition is used to determine in advance whether the connected fire detector is of the type shown in Fig. 2A or the type shown in Fig. 2B. Another existing solution is to always supply a relatively high current on the wire, to avoid the mistaken resetting described above. However, considering the requirement for the central control system to be power-saving, this method is not ideal.

Content of the invention

In view of the above shortcomings in the prior art, the embodiments of the present invention propose a drive circuit in a fire alarm system, the drive circuit being used to drive a line connected to one or more fire detectors. Using such a drive circuit, fire detectors of the two types shown in Figs. 2A and 2B can be supported in a self-adaptive manner, while also achieving the objective of saving power. The drive circuit is used to supply electrical energy to a line connected to one or more fire detectors, and comprises: a current generation circuit, capable of selectively supplying either a first current I_N1 or a second current I_ALARM to the line, wherein the first current I_N1 is used as a line monitoring current in a normal operating state, and the second current I_ALARM is greater than the first current I_N1; a detection circuit, for detecting whether a line voltage on the line is lower than a threshold, and when the line voltage is lower than the threshold V_th, controlling the current generation circuit to supply the second current I_ALARM to the line "Line", wherein the threshold V_th is lower than a normal line voltage but higher than a maximum alarm voltage V_Alarm indicated on the line when the fire detector gives an alarm. Preferably, the size of the second current I_ALARM enables the maximum alarm voltage V_Alarm to be maintained for a predetermined time period, so that the fire alarm system detects the alarm voltage V_Alarm. Even more preferably, when a remote end of the line "Line" is terminated by at least one End of Line element (EOL), the first current I_N1 > (maximum number of fire detectors which can be connected to the line x quiescent current of each fire detector + minimum forward current of the End of Line element EOL).
The drive circuit can supply a small monitoring current I_N1 in a monitoring stage and a larger alarm current I_ALARM in an alarm stage. Thus it enables the fire alarm system to detect alarm signals from fire detectors of different types, while also being able to reduce the energy consumption of the fire alarm system to a certain extent. Since such a drive circuit can save power more effectively, the storage battery of the fire alarm system can correspondingly maintain operation for a longer time.
In one embodiment of the present invention, the detection circuit comprises: a threshold judgment circuit, connected to the line "Line" and used for judging, in a hardware manner, whether the line voltage V_Line is lower than the threshold V_th; and a control circuit, for controlling the current generation circuit to be switched from supplying the first current I_N1 to supplying the second current I_ALARM when the line voltage V_Line is lower than the threshold V_th. Since the drive circuit uses a hardware detection circuit to accomplish threshold judgment and feedback control, the drive circuit can detect an alarm signal in a very short time, and supply a larger alarm current I_ALARM to maintain an alarm signal from a fire detector such as that shown in Fig. 2B.
In another embodiment of the present invention, the threshold judgment circuit comprises: a series branch, the two ends of which are coupled to the line voltage V_Line, the series branch comprising at least one Zener diode and a first resistor, wherein the sum of reverse breakdown voltages of the at least one Zener diode is equal to the threshold V_th; and the control circuit uses a voltage at a connection point between the first resistor and the Zener diode as a threshold judgment result, for controlling the current generation circuit. Preferably, the control circuit comprises a switch circuit for converting the threshold judgment result to a switching value. A Zener diode is used as a threshold judgment element in this embodiment, being distinguished by a simple structure and a fast response.
In one embodiment of the present invention, the current generation circuit is preferably a current source. Even more preferably, the current source comprises: a transistor, connected to form a common-emitter circuit, and used for supplying a current to the line "Line"; a variable resistance circuit, connected to the emitter of the transistor, and having a resistance capable of being switched between at least a first resistance value and a second resistance value; and a voltage control circuit, for stabilizing the voltage across the variable resistance circuit at a predetermined voltage; wherein the variable resistance circuit switches the resistance value thereof in response to a control signal from the detection circuit. Preferably, the variable resistance circuit comprises: at least two resistance branches connected in parallel, wherein a controlled switch is provided on at least one of the resistance branches, the controlled switch closing in response to a control signal from the detection circuit.
In another embodiment of the present invention, the current source comprises: a transistor, connected to form a common-emitter circuit, for drawing a current from the line "Line", a second resistor being connected to the emitter thereof; a voltage control circuit, for stabilizing the voltage across the second resistor at either a first voltage value or a second voltage value, wherein the voltage control circuit switches the first voltage value to the second voltage value in response to a control signal from the detection circuit.
In another embodiment of the present invention, a fire alarm system is further proposed. This fire alarm system comprises: one or more fire detectors, for detecting whether a fire has arisen in a neighborhood thereof; the drive circuit as described above, for supplying electrical energy to a line connected to one or more fire detectors; an End of Line element (EOL), connected at a remote end of the line, and used to terminate the line. Preferably, the End of Line element (EOL) is a transient voltage suppressor (TVS). Preferably, the fire detector comprises at least one detector which requires an additional current to maintain an alarm voltage thereof when giving an alarm. When such a fire alarm system is used, there is no need to distinguish between fire detectors of different types, while the objective of saving power can also be achieved.
In another embodiment of the present invention, a line drive method for a fire alarm system is further proposed, wherein the line is connected to one or more fire detectors to supply electrical energy thereto, the method comprising: in a normal operating state, supplying a first current I_N1 to the line, wherein the first current I_N1 is used as a line monitoring current in the normal operating state; detecting a line voltage V_Line on the line; judging whether the line voltage V_Line is lower than a threshold V_th, wherein the threshold V_th is lower than a normal line voltage but higher than a maximum alarm voltage V_Alarm indicated on the line when the fire detector gives an alarm; if it is judged that the line voltage V_Line is lower than the threshold V_th, switching the current supplied to the line from the first current I_N1 to the second current I_ALARM, the second current I_ALARM being greater than the first current I_N1. Preferably, the size of the second current I_ALARM is enough to enable the maximum alarm voltage V_Alarm to be maintained for a predetermined time period, so that the fire alarm system detects the alarm.

Description of the accompanying drawings

The object, characteristics, features and advantages of the present invention will be made more obvious by the following detailed description, which makes reference to the accompanying drawings.

Fig. 1A shows the structure of an existing fire alarm system schematically;
Fig. 1B shows how fire detectors are connected in the fire alarm system shown in Fig. 1A;
Figs. 2A and 2B show two types of fire detector which give an alarm in different ways;
Fig. 3 shows a line drive circuit in a fire alarm system according to one embodiment of the present invention by way of example;
Fig. 4 shows a line drive circuit in a fire alarm system according to another embodiment of the present invention by way of example;
Fig. 5 shows schematic diagrams of the waveforms of voltage and current on the line when an alarm arises on the line;
Fig. 6 shows a schematic diagram of a line drive circuit according to another embodiment of the present invention by way of example;
Fig. 7 shows a flow chart of a line drive method for a fire alarm system according to one embodiment of the present invention by way of example.

Particular embodiments

A fire alarm system needs to be compatible with the two types of fire detector shown in Figs. 2A and 2B, but these two types of fire detector have different alarm voltages (e.g. 5.6 V and 12 V, respectively); moreover, in the case of the fire detector shown in Fig. 2B, a relatively large current is also required to maintain the alarm state thereof, otherwise the fire detector will mistakenly reset automatically. In view of this, the inventors of the present invention put forward the concept of supplying different line drive currents in a line monitoring stage and an alarm stage in the following embodiments. Specifically, a lower current is used to monitor whether the line is normal in a line monitoring stage, and a higher current is used to maintain an effective alarm signal when an alarm is discovered, to avoid erroneous resetting of the fire detector. This concept can support both types of fire detector, with no need to identify the detector type in advance. Furthermore, this concept achieves the objective of saving power by having power supplied at a low current in the monitoring stage.
Fig. 3 shows a line drive circuit 300 for a fire alarm system according to one embodiment of the present invention, and a fire detector 120 and EOL connected to the drive circuit 300 by a line "Line". As Fig. 3 shows, the drive circuit 300 supplies a line current I_Line to the line "Line". The fire detector 120 is connected between Line+ and Line- of the line. The remote end of the line is terminated by an End of Line element (EOL), which is for example a resistive element or preferably a transient voltage suppressor (TVS). Due to the existence of internal resistance (Rcable) of the line (or cable), the line current I_Line flowing through the line "Line" forms a certain voltage drop V_R = I_Line*Rcable on the line before being delivered to the fire detector 120. As a result, the voltage across the fire detector 120 is Vd = V_Line - V_R, wherein V_Line is the line voltage observed at the drive circuit 300 side. The current flowing through the fire detector 120 is Id. The current flowing through the End of Line element EOL at the remote end is I_EOL.
In the example shown in Fig. 3, the drive circuit 300 comprises a current generation circuit 310 and a detection circuit 320. The current generation circuit 310 can selectively supply at least two line currents I_Line of different sizes to the line "Line", i.e. either a first current I_N1 or a second current I_ALARM. The first current I_N1 is used as a line monitoring current in a normal operating state, and the second current I_ALARM is used as an alarm current when an alarm is given, the second current I_ALARM being greater than the first current I_N1. The detection circuit 320 is connected to the line, and used to control the current generation circuit 310, via a control path C1, to supply the second current I_ALARM to the line "Line" when detecting that the line voltage V_Line on the line "Line" is lower than a threshold V_th (which indicates that an alarm signal has arisen on the line). The threshold V_th can be set to be lower than a normal voltage on the line, and higher than a maximum alarm voltage V_Alarm indicated on the line "Line" when the fire detector 120 gives an alarm. Supposing the fire detector connected to the line comprises the two detectors 120-1 and 120-2 shown in Figs. 2A and 2B, then V_th can be set to be greater than 12 V, for instance, which is the alarm voltage of the fire detector 120-2.
In the example shown in Fig. 3, the detection circuit 320 can specifically comprise two parts, one being a threshold judgment circuit 321 and the other being a control circuit 325. The threshold judgment circuit 321 is coupled to the line, and used for judging whether the line voltage V_Line is lower than the threshold V_th (e.g. 12 V). The control circuit 325 switches the current generation circuit 310 from supplying the first current I_N1 to supplying the second current I_ALARM when the line voltage V_Line drops below the threshold V_th. Thus the small current I_N1 is used for monitoring in a normal state, and the large current I_ALARM is used to maintain an alarm signal in an alarm state. In this way, not only can fire detectors of different types be supported, but power can also be saved in the central control system.
Fig. 4 shows a drive circuit 400 according to another embodiment of the present invention. In the example shown in Fig. 4, the current generation circuit 410 is preferably two current sources connected in parallel, for instance, a controlled switch K being provided on an output path of one of these current sources. In the normal state, the controlled switch K is open, and a first current source alone supplies a monitoring current I_N1 to the line "Line". When a detection circuit 420 detects that V_Line has dropped below V_th, i.e. an alarm (ALARM) has arisen, the detection circuit 420 controls the switch K to close, so that both current source I_N1 and current source I_AL supply a current I_ALARM = I_N1 + I_AL to the line "Line".
In the example shown in Fig. 4, the detection circuit 420 comprises a series branch used for threshold judgment, connected between Line+ and Line- of the line. The series branch comprises a Zener diode D1 and a resistor R3. The output of the detection circuit 420 is the voltage Vt at the connection point between the Zener diode D1 and the resistor R3. The reverse breakdown voltage of the Zener diode is the threshold voltage V_th, for example 15 V (greater than the maximum alarm voltage, 12 V, of the fire detector). Of course, in the example shown in Fig. 4, the Zener diode D1 could also be replaced by two or more Zener diodes, in which case the sum of the reverse breakdown voltages of these Zener diodes would be the threshold voltage V_th. In the example shown in Fig. 4, Vt is HIGH during normal operation (V_Line = 24 V), because the Zener diode D1 is in the reverse breakdown state; when an alarm arises (V_Line < 15 V), the Zener diode D1 is in the high resistance state, so Vt is LOW. In the example shown in Fig. 4, the control circuit is simplified to using the voltage Vt directly to control the closing or opening of the switch K via a control path C1. Fig. 4 shows a simple manner of implementation. According to actual requirements, those skilled in the art could also use a comparator to realize the threshold judgment circuit 321 in Fig. 3, while the control circuit 325 could control the switch K after suitably transforming the output of, for instance, the comparator. All these variations are obvious to those skilled in the art.
Fig. 5 shows waveforms of current and voltage on the line in the example shown in Fig. 4. As Fig. 5 shows, in the normal operating state (NORM), the drive circuit 400 outputs a monitoring current I_N1 to the line "Line", while the line voltage V_Line is kept at, for instance, the normal voltage V_N = 24 V. When one or more of the fire detectors gives an alarm, the line voltage V_Line is pulled down below V_th (e.g. V_th = 15 V), for instance to V_AL = 12 V or 5.6 V. At this point, the drive circuit 400 outputs I_ALARM = I_N1 + I_AL, so that the alarm voltage V_AL is maintained for a period of time Ta. The fire detector then resets automatically and enters the next cycle of normal operation. In the example shown in Fig. 5, I_N1 is, for instance, about 6 mA, I_AL is about 24 mA, and I_ALARM = I_N1 + I_AL = about 30 mA. However, in practical applications the sizes of I_N1 and I_AL can be suitably chosen according to requirements.
In the examples shown in Figs. 3 and 4, it is preferable that I_N1 must be greater than (sum of quiescent currents flowing through all fire detectors in the monitoring stage + minimum forward current flowing through the End of Line element EOL). If we suppose that the quiescent current of each detector is Im, where m = 0, 1,..., M, wherein M is the maximum number of detectors that can be connected to the line, then I_N1 can satisfy the following formula: $I_{N 1} \geq (M x Max (Im) + Min (I_{EOL}))$

wherein Max(Im) represents the maximum value of quiescent current of all the detectors, and Min(I_EOL) represents the minimum value of forward current flowing through EOL. Preferably, I_N1 is approximately 6 mA or less.
In the examples shown in Figs. 3 and 4, it is preferable that I_ALARM must supply a large current, so that the alarm voltage of the fire detector shown in Fig. 2B can be kept at, for instance, 12 V. For example, in the case of the structure shown in Fig. 2B, the alarm voltage can be kept above 12 V if I_ALARM is approximately 30 mA, and so this is sufficient to avoid automatic resetting of the detector. Of course, those skilled in the art can choose the size of I_ALARM appropriately according to actual requirements.
Fig. 6 shows by way of example a specific circuit structure of the block diagram shown in Fig. 3. As Fig. 6 shows, the drive circuit 600 comprises a current generation circuit 610 and detection circuit 620.
The current generation circuit 610 is preferably a current source with a variable output current. The current source comprises a transistor V101, which operates in an amplifying state and is connected to form a common-emitter circuit, a variable resistance circuit 612 connected in series to the emitter of the transistor V101, and a voltage control circuit 613 for controlling the voltage across the variable resistance circuit 612.
As Fig. 6 shows, the transistor V101 in the current source is, for instance, a PNP-type transistor, the collector C thereof being connected to the line "Line" as an output electrode. The emitter E of the transistor V101 is also connected to Vcc via the variable resistance circuit 612. Although Fig. 6 shows a scheme in which a PNP-type transistor is used to realize the current source, those skilled in the art can appreciate that the current source could also be realized using another circuit used to set up a current source, such as an NPN-type transistor, MOSFET or op-amp.
In Fig. 6, a stable reference voltage Vref is obtained at point R once Vcc has been divided by the series resistors R118, R114 and R119. A Zener diode 614 is used to stabilize the voltage across the series resistors R118 and R114. The voltage control circuit 613 acquires a sample voltage from the emitter E of the transistor, and compares this sample voltage with the reference voltage Vref; the comparison result is preferably used to control the base B of the transistor V101 after proportional integration (PI), and hence adjust the size of V_CE. In this way, a substantially constant voltage across the variable resistance circuit 612 can be achieved under the control of the voltage control circuit 613. In other words, the size of the current outputted by the transistor V101 is determined by the resistance of the variable resistance circuit 612 connected in series with the emitter E of V101.
In the example shown in Fig. 6, the variable resistance circuit 612 comprises two resistance branches R108 and R111, connected in parallel with each other. A controlled switching element K is further provided in the resistance branch in which R111 is located, the controlled end of K being connected to the output end of the detection circuit 620. In the example shown in Fig. 6, the controlled switching element K is preferably a MOSFET V102, the gate G thereof being the controlled end. When the gate voltage is HIGH, the MOSFET V102 cuts off so that only resistor R108 is in operation, and the size of the current outputted by the transistor V101 is I_N1 = V_R108/R108. When the gate voltage is LOW, the MOSFET V102 conducts, so that resistor R111 and resistor R108 are connected in parallel and the size of the current outputted by the transistor V101 is I_ALARM = V_R108/(R108//R111).
Although Fig. 6 shows a solution in which a variable emitter resistance is used to regulate the output current, those skilled in the art can appreciate that the current generation circuit 610 could also selectively supply two drive currents of different sizes in other ways. For example, the current generation circuit 610 could also supply a switchable output current by means of a voltage source and a variable resistance. Alternatively, the current generation circuit 610 could also be a current sink used to draw current from the line. Similarly, the current sink could, for instance, consist of a transistor connected to form a common-emitter circuit, the collector of the transistor being the output end, and the emitter being connected to ground via a second resistance. A voltage control circuit is used to stabilize the voltage across the second resistance at either a first voltage value or a second voltage value, wherein the voltage control circuit can switch the first voltage value to the second voltage value in response to a control signal from the detection circuit (620).
In the example shown in Fig. 6, the detection circuit 620 comprises a Zener diode V112 and a resistor R121 connected in series therewith; the Zener diode and resistor are connected in series between Line+ and Line-, and used as a threshold judgment circuit 621. The reverse breakdown voltage of the Zener diode V112 is equal to the threshold V_th, e.g. 15 V. Optionally, the Zener diode V112 may be replaced by multiple Zener diodes, the sum of reverse breakdown voltages of the multiple Zener diodes being equal to the threshold V_th. Additionally, a control circuit 625 regards a voltage Vp at a joining point P between the Zener diode V112 and resistor R121 as a threshold judgment result.
In Fig. 6, the control circuit 625 comprises a switch circuit for converting Vp to a switching value, the switch circuit comprising cascaded transistors V109 and V107. Both transistors V107 and V109 are connected to form a common-emitter circuit, wherein Vp is the input of V109, and the output of V109 serves as the input of V107. The output of V107 is used to control the controlled end of switch K in the variable resistance circuit 612. Thus when V_Line is lower than V_th, Vp changes from HIGH to ground (GND), causing cut-off of the transistor V109 which was originally conducting, so that V_{Alarm_Line} changes to HIGH. Driven by the HIGH V_{Alarm_Line}, V107 conducts, so that the collector C end of V107 changes to LOW, thereby causing the MOSFET V102 to conduct, i.e. connecting resistors R111 and R108 in parallel. In this way, a larger drive current can be supplied on the line in response to an alarm signal, to maintain the alarm voltage.
Fig. 7 shows by way of example a line drive method according to an embodiment of the present invention for the fire alarm system shown in Fig. 1B, for instance. The fire detectors connected into the line may comprise the detector(s) shown in Fig. 2A and/or Fig. 2B. As shown in Fig. 7, the line drive method begins with step S710. In step S710, a first current I_N1 is supplied to the line "Line" in a normal operating state, wherein the first current I_N1 is used as a line monitoring current in the normal operating state. In step S720, a line voltage V_Line on the line "Line" is detected. In step S730, it is judged whether the line voltage V_Line is lower than a threshold V_th, the threshold V_th being lower than a normal line voltage but higher than a maximum alarm voltage V_Alarm indicated on the line when a fire detector gives an alarm. For example, if V_Alarm is the alarm voltage of the detector shown in Fig. 2B, e.g. 12 V, then 15 V, for instance, can be chosen as V_th. If the judgment result is negative, then the method flow returns to step S720, to detect the line voltage V_Line again. If the judgment result is positive, this indicates that an alarm has arisen, and so the method flow proceeds to step S740. In step S740, the current supplied to the line "Line" is switched from the first current I_N1 to a second current I_ALARM. Here, I_ALARM > I_N1. Preferably, the size of the second current I_ALARM is sufficient to enable the maximum alarm voltage V_Alarm to be maintained for a predetermined time period, so that the fire alarm system detects the alarm.
Those skilled in the art should appreciate that a variety of changes and modifications could be made to the embodiments disclosed above without departing from the substance of the invention; all such changes and modifications should fall within the scope of protection of the present invention. For this reason, the scope of protection of the present invention should be defined by the attached claims.

Claims

A drive circuit for a fire alarm system, the drive circuit being used to supply electrical energy to a line (Line) connected to one or more fire detectors (120), and to acquire a signal from the line, the drive circuit comprising:
a current generation circuit (310, 410, 610), capable of selectively supplying either a first current (I_N1) or a second current (I_ALARM) to the line (Line), wherein the current generation circuit supplies the first current (I_N1) for the purpose of line monitoring in a normal operating state, and the second current (I_ALARM) is greater than the first current (I_N1);

a detection circuit (320, 420, 620), for detecting whether a line voltage (V_Line) on the line (Line) is lower than a threshold (V_th), and when the line voltage (V_Line) is lower than the threshold (V_th), controlling the current generation circuit (310, 410, 610) to supply the second current (I_ALARM) to the line (Line), wherein the threshold (V_th) is lower than a normal line voltage but higher than a maximum alarm voltage (V_Alarm) indicated on the line when the fire detector (120) gives an alarm.
The drive circuit as claimed in claim 1, wherein the size of the second current (I_ALARM) enables the maximum alarm voltage (V_Alarm) to be maintained for a predetermined time period, so that the fire alarm system detects the alarm voltage (V_Alarm).
The drive circuit as claimed in claim 1, wherein when a remote end of the line (Line) is terminated by at least one End of Line element (EOL),
the first current (I_N1) > (maximum number of fire detectors which can be connected to the line x quiescent current of each fire detector + minimum forward current of the End of Line element (EOL)).
The drive circuit as claimed in any one of claims 1 - 3, wherein the detection circuit (320, 420, 620) comprises:
a threshold judgment circuit (321), connected to the line (Line) and used for judging, in a hardware manner, whether the line voltage (V_Line) is lower than the threshold (V_th);

a control circuit (325), for controlling the current generation circuit (310) to be switched from supplying the first current (I_N1) to supplying the second current (I_ALARM) when the line voltage (V_Line) is lower than the threshold (V_th).
The drive circuit as claimed in claim 4, wherein the threshold judgment circuit comprises:
a series branch, the two ends of which are coupled to the line voltage (V_Line), the series branch comprising at least one Zener diode (D1, V112) and a first resistor (R3, R121), wherein the sum of breakdown voltages of the at least one Zener diode (D1, V112) is equal to the threshold (V_th); and

the control circuit uses a voltage at a connection point between the first resistor (R3, R121) and the Zener diode (D1, V112) as a threshold judgment result, for controlling the current generation circuit (310, 410, 610).
The drive circuit as claimed in claim 5, wherein the control circuit (625) comprises a switch circuit for converting the threshold judgment result to a switching value.
The drive circuit as claimed in any one of claims 1 - 3, wherein the current generation circuit is a current source (410, 610).
The drive circuit as claimed in claim 7, wherein the current source (610) comprises:
a transistor (V101), connected to form a common-emitter circuit, and used for supplying a current to the line (Line);

a variable resistance circuit (612), connected to the emitter of the transistor (V101), and having a resistance capable of being switched between at least a first resistance value and a second resistance value;

a voltage control circuit (613), for stabilizing the voltage across the variable resistance circuit (612) at a predetermined voltage;
wherein the variable resistance circuit (612) switches the resistance value thereof in response to a control signal from the detection circuit (620).
The drive circuit as claimed in claim 8, wherein the variable resistance circuit (612) comprises:
at least two resistance branches (R108, R111) connected in parallel, wherein a controlled switch (K) is provided on at least one of the resistance branches, the controlled switch (K) closing in response to a control signal from the detection circuit.
The drive circuit as claimed in claim 7, wherein the current source comprises:
a transistor, connected to form a common-emitter circuit, for drawing a current from the line (Line), a second resistor being connected to the emitter thereof;

a voltage control circuit, for stabilizing the voltage across the second resistor at either a first voltage value or a second voltage value, wherein the voltage control circuit switches the first voltage value to the second voltage value in response to a control signal from the detection circuit (620).
A fire alarm system, comprising:
one or more fire detectors (120), for detecting whether a fire has arisen in a neighborhood thereof;

the drive circuit as claimed in any one of claims 1 - 10, for supplying electrical energy to a line (Line) connected to one or more fire detectors (120);

an End of Line element (EOL), connected at a remote end of the line (Line), and used to terminate the line (Line).
The fire alarm system as claimed in claim 11, wherein the fire detector (120) comprises at least one detector which requires a current to maintain an alarm voltage thereof when giving an alarm.
The fire alarm system as claimed in claim 12, wherein the End of Line element (EOL) is a transient voltage suppressor (TVS).
A line drive method for a fire alarm system, wherein the line is connected to one or more fire detectors (120) to supply electrical energy thereto, the method comprising:
in a normal operating state, supplying a first current (I_N1) to the line (Line), wherein the first current (I_N1) is used as a line monitoring current in the normal operating state;

detecting a line voltage (V_Line) on the line (Line);

judging whether the line voltage (V_Line) is lower than a threshold (V_th), wherein the threshold (V_th) is lower than a line voltage in the normal operating state but higher than a maximum alarm voltage (V_Alarm) indicated on the line when the fire detector (120) gives an alarm;

if it is judged that the line voltage (V_Line) is lower than the threshold (V_th), switching the current supplied to the line (Line) from the first current (I_N1) to the second current (I_ALARM), the second current (I_ALARM) being greater than the first current (I_N1).
The drive method as claimed in claim 14, wherein the size of the second current (I_ALARM) is enough to enable the maximum alarm voltage (V_Alarm) to be maintained for a predetermined time period, so that the fire alarm system detects the alarm.