US 3797008 A
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United States Patent Yuasa Mar. 12, 1974 FIRE DETECTING SYSTEM 3,056,120 9/1962 Mobarry 340/409 ux 4 1751 Inventor: Hayami Tokyo, Japan 312E132; 12/133 2121???? 343/183 73 Assignee; in Company, Limited, Tokyo, 3,643,247 2/1972 Ratcliffe 340/256 X Ja an p FOREIGN PATENTS OR APPLICATIONS  Flled: 1972 1,002,291 8/1965 Great Britain 340/253  Appl. No.: 219,723
Primary Examiner-John W. Caldwell  Foreign Application Priority Data Assistant Examiner-William M. Wannisky Feb. 4, 1971 Japan 46-4032 52 us. c1 340/227, 340/253 R, 340/409,  ABSTRACT  I t Cl 307/285 f iZQZ A fire detecting system having a voltage source, a pair  i i C 248 B of conductors extending from the source with a pluralle 3 13 23? R 3O7/285 ity of fire detectors connected across the conductors, 8 317/33 means at the end of said conductors placing a constant load on the voltage source and means adjoining the voltage source for detecting loss of said load resulting  References cued from a break in one of said conductors.
UNITED STATES PATENTS 3,500,394 3/1970 Egesdal 340/409 1 Claim, 4 Drawing Figures 1 1 Y VOLTAGE T 6 /7 "DETECTOR x 0 2 l 1 1 'f 6 1 7 1 B 2 2 5 1 i VOLTAGE 5:; 9 9 DETECTOR 7 7 K P /0 8 2 4'! 3/ 3. v 7 DISCON- 4 2 NECT/ON MONITOR FIRE DETECTING SYSTEM This invention relates to a fire detecting system and more particularly to a fire detecting system provided with improved means for disconnecting detectors.
A conventional fire detecting system comprises a single central unit and a plurality of fire detectors connected in parallel between a pair of conductors extending from the central unit. Each detector includes a normally open switch which is closed by heat or smoke to short-circuit both conductors, and the central unit includes a power supply for supplying an operation voltage between both conductors and an alarm device for sensing increase of current flowing through the conductors due to closure of the normally open switch in any of the detectors to produce an alarm.
When either of the conductors is disconnected or broken off at some position by an accident in such fire detecting system, the detectors disposed remote from this disconnected point become ineffective and such disconnection of the conductor is difficult to locate unless special testing means are used. As a way oflocating disconnection of the conductor at the central unit, a method of providing a bridge circuit in the central unit and measuring a change of resistance between the output terminals and a method of providing a special signal source at the ends of the conductors and receiving the signal at the central unit have been proposed. However, such methods have required very complicated and costly circuit arrangements.
Therefore, an object of this invention is to propose a fire detecting system provided with simple and inexpensive disconnection detecting means whereby any disconnecting of the conductors is automatically located and information is provided at the central unit.
The above object can be accomplished in accordance with this invention by inserting a Zener diode between the ends of both conductors to maintain the voltage between the conductors at the Zener voltage and providing means for detecting an increase of said voltage and producing an alarm at the central unit.
Other features and operation of the system of this invention will be described in more detail hereinunder with reference to the accompanying drawings.
In the drawings:
' FIG. 1 is a block diagram representing an embodiment of the system of this invention,
FIG. 2 is a circuit diagram representing an example of the voltage detector shown in block form in FIG.
FIG. 3 is a circuit diagram representing an example of the disconnection monitor 13 shown in block form in FIG. 1, and
FIG. 4 is a block diagram representing another embodiment of a system of this invention.
Throughout the drawings, like reference numerals are used to denote like components.
Referring first to FIG. 1, the system of this invention includes a central unit 1 and a plurality of fire detectors 2. Each of the detectors 2 is symbolized as a normally open switch since it has generally a switching element which is closed in response to detection of heat or smoke to short-circuit a pair of conductors 5 and 6 which extend from a pair of terminals 3 and 4 of the central unit 1. Between the other ends of the conductors 3 and 4, inserted is a Zener diode 7 according to this invention.
The central unit 1 includes a power supply 8 and a resistor 9 connected in series with the terminal 3, a voltage detector 10 which senses a voltage rise across the resistor 9 to energize a relay element included therein, and a normally open switch which is to be actuated by the relay element of the voltage detector 10 and an alarm generator 12 such as bell or buzzer connected in series with the relay contacts across the power supply 8. The central unit 1 further includes a disconnection monitor 13 in accordance with the invention, and which is connected in series with a diode 14 between the conductors 5 and 6.
An example of circuit configuration of the voltage detector 10 in FIG. 1 is shown in FIG. 2. An electromagnetic winding 15 of the relay element which actuates the normally open switch 11 is connected in series with a silicon controlled rectifier (hereinafter referred to as SCR) 16 between one end of the resistor 9 and the conductor 6, and a series connection of a resistor 17 and a Zener diode 18 is connected in parallel to the series connection of the winding 15 and the SCR 16. The connection point of the resistor 17 and the zener diode 18 is connected to the emitter electrode of a transistor 19 having a collector electrode connected to the control electrode of the SCR 16 and also connected through a resistor 20 to the conductor 6. The base electrode of the transistor 19 is directly connected to the other end of the resistor 9. Thus, a bridge circuit is formed which includes the resistor 17, the Zener diode 18, the resistor 9 and the Zener diode 7. The Zener voltage of the Zener diode 18 is previously selected to be lower than that of the Zener diode 7.
In operation, a voltage which may be somewhat higher than the Zener voltage of the Zener diode 7 is applied from the power supply 8 through the conductors 5 and 6 to all detectors 2, but the voltage applied practically to the detectors 2 is limited to and maintained at the Zener voltage of the Zener diode 7. Therefore, the Zener voltage of the Zener diode 7 is previously selected to be optimum for the detectors 2 to exhibit maximum sensitivity. In such a normal moni toring condition wherein none of the detectors 2 is actuated, the base voltage of the transistor 19 is substantially zero with respect to the emitter voltage and the transistor 19 is in a cut-off condition. Consequently, the SCR 16 which is controlled by the collector voltage of the transistor 19 is also in a cut-off conditon and no current flows through the electromagnetic winding 15.
When one of the detectors 2 is actuated by a fire, the conductors 5 and 6 are substantially short-circuited through a low impedance and, accordingly, the voltage across the resistor 9 is suddenly raised to about the source voltage. Thus, the transistor 19 is driven into conduction and the SCR 16 is also driven into conduction. Thus, the electromagnetic winding 15 is energized to close the switch 11. This results in actuation of the alarm 12.
An example of circuit configuration of the disconnection monitor 13 in FIG. 1 is shown in FIG. 3. A Zener diode 21 and a resistor 22 are connected in series between the diode 14 and the conductor 6. A SCR 23 has a control electrode connected to the connection point of the Zener diode 21 and the resistor 22. The SCR 23 is connected in series in a closed circuit with a known alarm circuit 24 and a power supply 25, and the cathode electrode of the SCR 23 is connected to the conductor 6. The Zener voltage of the Zener diode 21 is previously selected to be higher than that of the Zener diode 7 but lower than the source voltage. Therefore, in the normal monitoring condition, the Zener diode 7 is in an ON-condition but the Zener diode 21 is in an OFF-condition, and no current flows in the disconnection monitor 13.
When either the conductor or 6 is broken or cut, however, the Zener diode 7 is removed from the circuit and, therefore, the voltage between the conductors 5 and 6 is suddenly raised to about the source voltage. Consequently, the Zener diode 21 conducts to raise the anode voltage thereof, and the SCR 23 is thus driven into conduction. Thus, the alarm circuit 24 is actuated to give an alarm.
Referring then to FIG. 4, this embodiment of fire detecting system includes a single central unit 1 and a plurality of branches (two branches in the drawing) of the fire detectors 2. Such configuration is often used in practice for dividing the fire detectors into plural groups to facilitate location of a fire. The central unit 1 includes a single power supply 8 but plural pairs of output terminals 3-1, 4-1, 3-2, 4-2, from which respective pairs of conductors 5 and 6 extend. Between each pair of conductors 5 and 6 there are a plurality of fire detectors 2 and a terminating Zener diode 7 as in the case of FIG. 1. The branches of the detectors are provided with their respective voltage detectors 10-1, 10-2, but with a common disconnection monitor 13. Instead of the switch 11 of FIG. 2, a parallel connection of switches 11-1, 11-2, which are respectively actuated by the voltage detectors 10-1, 10-2, are provided, so that the alarm 12 is actuated if any fire detector in any branch is actuated. Indication lamps may be provided respectively in conjunction with the switches 11-1, 11-2, or voltage detectors 10-1, 10-2, for indicating visually that branch which includes an actuated detector. Since the disconnection monitor 13 is connected in common with all branches through respective diodes 14-1, 14-2, the
monitor 13 can indicate a discorinection occurring in any branch but cannot determine the particular branch in which the disconnection has occurred. For this purpose, a disconnection monitor 13 may be provided in each of the branches or indicating lamps may be arranged so as to be excited in response to conduction of the diodes 14-1, 14-2,
As described in the above, .disconnection of distributed conductors of fire detecting system can be automatically determined at the central unit by simple and inexpensive additions, such as a Zener diode 7 and a disconnection monitor 13. Moreover, it is unnecessary to sufficiently stabilize the source voltage because of the inclusion of the Zener diode 7. This is especially advantageous when the fire detectors are those of ionization smoke detector types which require a wellstabilized dc. voltage source.
It should be noted that the above mentioned embodiments are only examples presented as an aid for explanation of this invention and various variations, modifications and changes can be made without departing from the scope and spirit of this invention. For example, various circuit configurations can be considered by those skilled in the art for the voltage detector 10 and disconnection monitor 13, other than the circuits in FIGS. 2 and 3.
What is claimed is:
l. A fire detecting system comprising a voltage source, a pair of conductors extending from both terminals of said voltage source, a plurality of fire detectors connected in parallel between said pair of conductors, each detector being provided with switch means for short-circuiting said pair of conductors upon actuation, a first alarm and means for sensing an increase in the current flowing through said conductors and actuating said first alarm, a first zener diode connected between the other ends of said pair of conductors, a disconnection monitor including a second zener diode connected between said pair of conductors, a second alarm, means interconnected with said second zener diode for sensing an increase in the current flowing through said second zener diode, and a connection between the last said means and said second alarm to actuate said second alarm, the zener voltage of said second zener diode being selected to be higher than the zener voltage of said first zener diode but lower than the terminal voltage of said voltage source.