U.S. patent number 3,603,949 [Application Number 04/738,046] was granted by the patent office on 1971-09-07 for fire alarm installation.
This patent grant is currently assigned to Cerberus AG. Invention is credited to Beat Walthard.
United States Patent |
3,603,949 |
Walthard |
September 7, 1971 |
FIRE ALARM INSTALLATION
Abstract
A fire alarm installation is disclosed which provides positive
detection of the response or nonresponse of an individual fire
alarm during the normal operational condition thereof as well as
during a checking operation. Each of the individual fire alarms are
provided with circuits which exhibit a high impedance for supply
voltages of a first polarity and a low impedance for supply
voltages of a second polarity during normal operational conditions.
During alarm conditions, however, the impedance-polarity
relationships reverse and such is detected at a remote central
station.
Inventors: |
Walthard; Beat (Stafa,
CH) |
Assignee: |
Cerberus AG (Mannedorf,
CH)
|
Family
ID: |
4347556 |
Appl.
No.: |
04/738,046 |
Filed: |
June 18, 1968 |
Foreign Application Priority Data
Current U.S.
Class: |
340/509; 340/517;
340/579; 340/511; 340/629 |
Current CPC
Class: |
G08B
29/145 (20130101); G08B 17/06 (20130101) |
Current International
Class: |
G08B
17/06 (20060101); G08B 29/00 (20060101); G08B
29/14 (20060101); G08b 029/00 () |
Field of
Search: |
;340/214,409 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Habecker; Thomas B.
Claims
What is claimed is:
1. A fire alarm installation comprising:
a. a central station;
b. a plurality of fire alarms;
c. a pair of common conductors for connecting said plurality of
fire alarms in parallel to one another and to said central
station;
d. each of said fire alarms comprising a detector element sensitive
to fire phenomena and electric circuit means controlled by its
associated detector element;
e. each of said circuit means possessing two stable conditions,
defining a normal condition when the detector element assumes a
normal operating state in a condition of readiness for detecting a
fire and an alarm condition when said detector element assumes a
response condition when it has been subjected to fire phenomena
f. said circuit means possessing two parallel current paths for the
respective flow of currents of different polarity;
g. each of said circuit means including switching means for
connecting one of said parallel current paths to said two common
conductors when the circuit means is in said normal condition and
for connecting the other of said current paths across said two
conductors when the circuit means is in said alarm condition;
h. said central station embodying means for generating a supply
voltage of continuously and periodically alternating polarity which
is delivered to said two conductors;
i. said central station further including means for generating a
test signal applied to said two conductors; said test signal
controlling said electric circuit means in like manner as said
detector elements; and
j. said central station further comprising measuring means for
separately detecting the current in said two conductors during the
phases of different polarity of said supply voltage.
2. A fire alarm installation as defined in claim 1, wherein each of
said detector elements comprises a measuring ionization chamber
accessible to the surrounding atmosphere, at least one resistance
element connected in series with said measuring ionization chamber,
the current flowing through said measuring ionization chamber
changing when the atmosphere contains combustion aerosols or smoke,
each of said circuit means being responsive to the thus produced
voltage changes appearing across its associated measuring
ionization chamber.
3. A fire alarm installation as defined in claim 1, wherein said
parallel current paths of each of said circuit means comprises two
circuit branches, each circuit branch being connected across said
supply conductors via respective diode means of opposite polarity
and via controllable semiconductor means, said controllable
semiconductor means being connected in said circuit means such that
one of said semiconductor means is conductive and the other
semiconductor means is blocked during said normal condition of said
electric circuit means, whereas said one semiconductor means is
blocked and said other semiconductor means is conductive during
said alarm condition of said electric circuit means.
4. A fire alarm installation as defined in claim 3, wherein each of
said detector elements comprises a measuring ionization chamber
accessible to the surrounding atmosphere connected in series with a
reference ionization chamber by a common electrode, said one
controllable semiconductor means being a field-effect transistor
having gate, source and drain electrodes, the gate electrode of
which is electrically connected with said common electrode of both
said ionization chambers, said other semiconductor means being a
further transistor, said circuit means possessing voltage divider
means for coupling both said ionization chambers with said further
transistor such that said field-effect transistor is positively fed
back by means of said further transistor as well as by said
ionization chambers.
5. A fire alarm installation as defined in claim 4, wherein a Zener
diode is connected in series with said source electrode of said
field-effect transistor, said test signal generating means at said
central station including control means for reducing the amplitude
of said supply voltage during one polarity for a short period of
time.
6. A fire alarm installation as defined in claim 1, wherein said
parallel current paths of each of said circuit means comprise diode
means of opposite polarity, at least one of said current paths
possessing a bistable element containing at least two transistors
and wherein the other current path possesses at least one further
transistor, said transistors being coupled together to provide a
switching arrangement controlling either both transistors of said
bistable element to be conductive or said further transistor in
said other current path to be conductive, said bistable element
being connected with and controlled by its associated detector
element, said further transistor being connected with and
controlled by said bistable element.
7. A fire alarm installation as defined in claim 6, wherein each of
said detector elements comprises a measuring ionization chamber
accessible to the surrounding atmosphere connected in series with a
reference ionization chamber by a common electrode, one of said two
transistors of said bistable element being a field-effect
transistor having gate, source and drain electrodes, the gate
electrode of which is connected to said common electrode of both
ionization chambers, said further transistor having a base
electrode, the second transistor of said bistable element
possessing a collector electrode connected with a second electrode
of one of the ionization chambers and with said base electrode of
said further transistor in the other current path and via a
resistor with one of the supply conductors, said bistable element
being connected via one of said diode means with the other of said
supply conductors, one said electrode of said field-effect
transistor being connected via a Zener diode with said one supply
conductor, said further transistor in said other current branch
being connected via the other of said diode means with said other
supply conductor.
8. A fire alarm installation as defined in claim 1, wherein one of
said current paths comprises a first diode and a first bistable
element containing at least two transistors, said second current
path comprises a second diode connected electrically antiparallel
to said first diode and a second bistable element comprising at
least two further transistors, said second bistable element being
connected with and controlled by said first bistable element such
that only one of said bistable elements is conductive and the other
one is blocked.
9. A fire alarm installation as defined in claim 8, wherein each of
said detector elements comprises a measuring ionization chamber
accessible to the surrounding atmosphere connected in series with a
reference ionization chamber by a common electrode, a field-effect
transistor having a gate, drain and source electrode and
controlling said first bistable element, said gate electrode of
said field-effect transistor being connected to said common
electrode of said two ionization chambers, a still further
transistor providing an alarm threshold means connecting said
field-effect transistor with said first bistable element, said
field-effect transistor and said first bistable element being
blocked when said circuit means is in said normal condition, said
second bistable element being further connected via a diode with a
capacitor and with a voltage divider for connecting said second
bistable element with the series connection of both of said
ionization chambers, said further capacitor during the normal
condition of said circuit means being charged and in said alarm
condition discharges due to blocking of said second bistable
element and thereby stores said alarm condition by displacing the
potential at said gate electrode of said field-effect
transistor.
10. A fire alarm installation as defined in claim 1, wherein said
central station comprises control means for increasing as well as
for decreasing the supply voltage during the phases of one polarity
for short periods of time, said electric circuit means including
circuitry for maintaining said electric circuit means in a
self-holding state during the presence of said alarm condition.
11. A fire alarm installation as defined in claim 1, further
including a conductor terminal element connected in parallel to the
last fire alarm of said plurality of fire alarms, said conductor
terminal element comprising an asymmetrical astable multivibrator
having a higher pulse repetition frequency with respect to the
repetition frequency of said supply voltage.
12. An alarm installation comprising a central station, a plurality
of alarms, at least one pair of conductors for electrically
connecting said plurality of alarms in a group in parallel with
said central station, said central station including supply voltage
means for said group of parallely connected alarms, a conductor
terminal element means connected in parallel to the last alarm of
said plurality of alarms for monitoring possible interruptions at
the conductors of said plurality of alarms, said conductor terminal
element means including means for applying to said conductors a
signal having an alternating voltage component for periods during
which said central station is delivering a supply voltage of a
certain polarity to said conductors and different from the signals
drawn by said plurality of alarms.
13. An alarm installation comprising a central station, a plurality
of alarms, at least one pair of conductors for electrically
connecting said plurality of alarms in a group in parallel with
said central station, said central station including supply voltage
means for said group of parallely connected alarms, a conductor
terminal element means connected in parallel to the last alarm of
said plurality of alarms for monitoring possible interruptions at
the conductors of said plurality of alarms, said conductor terminal
element means including means for applying to said conductors a
signal different from the signal delivered by said supply voltage
means to said plurality of fire alarms and different from the
signals drawn by said plurality of alarms, said supply voltage
means delivering signals of alternating polarity, said terminal
element means applying to said conductors a signal of a different
frequency than the signal frequency of said supply voltage
means.
14. An alarm installation comprising a central station, a plurality
of alarms, at least one pair of conductors for electrically
connecting said plurality of alarms in a group in parallel with
said central station, said central station including supply voltage
means for said group of parallely connected alarms, a conductor
terminal element means connected in parallel to the last alarm of
said plurality of alarms for monitoring possible interruptions at
the conductors of said plurality of alarms, said conductor terminal
element means including means for applying to said conductors a
signal different from the signal delivered by said supply voltage
means to said plurality of fire alarms and different from the
signals drawn by said plurality of alarms, said supply voltage
means delivering a signal of alternating polarity, said means of
said conductor terminal element means for applying said signal to
said conductors comprises an asymmetrical astable multivibrator
having a higher pulse repetition frequency than the repetition
frequency of said supply voltage means.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an improved fire alarm
installation of the type incorporating a plurality of
two-conductor, parallely connected fire alarms operably coupled
with a central signal station.
Fire alarm installations of the type wherein fire alarms are
arranged together in groups and are parallely connected and coupled
via only two conductors with a central station have proven to be
particularly advantageous because of their low installation cost.
Known installations of this type are supplied with direct current.
Moreover, the individual fire alarms, during their normal state or
condition, exhibit a high impedance and, during their alarm state
or condition, exhibit a low impedance, whereby the current flowing
through the groups of fire alarms during response of one or a
number of fire alarms is directly or indirectly utilized for
triggering an alarm at the central station. In this instance, the
criteria for triggering of the alarm at the central station is
designated "current" in contrast to "no current."
It is necessary to periodically check fire alarm installations at
regular intervals to determine their operational reliability so as
to insure reliable response in the case of a fire. In modern fire
alarm installations, in which up to several hundred fire alarms are
connected to each central station, it is only possible to carry out
one remote triggering of a fire alarm by delivering
alarm-simulating conditions from the central station. The
difficulty with this system resides in the inability to positively
determine that all fire alarms of the group have actually responded
during the checking operation. In a known fire alarm installation,
in which, for the purpose of carrying out a checking operation,
electrical signals periodically cause all of the fire alarms to
simultaneously respond, a separate conductor is lead from each fire
alarm back to the central station. Signals appear on each conductor
during the checking operation regarding the response condition of
the relevant fire alarm. However, this installation is technically
far too complicated and costly for use in larger installations.
According to another known two-conductor fire alarm installation,
all fire alarms are likewise simultaneously activated or alarmed by
the central station. The total current flowing through the group of
fire alarms is then measured by means of an ampere meter and the
measurement result is compared with a reference value.
Consequently, this system is also suitable only for installations
which have relatively few fire alarms per group. The total current
varies because of the tolerances associated with the response
currents of the individual fire alarms, a consequence of the quite
considerable fluctuations or variations of the electrical
characteristics of different components which occur in practice.
This variation is of such a degree that with a larger number of
fire alarms per group, even when using extremely accurate measuring
devices, a positive detection of the threshold value is not
possible.
SUMMARY OF THE INVENTION
Accordingly, it is a primary object of the present invention to
provide an improved fire alarm installation of the aforementioned
type wherein the response of an individual fire alarm can be just
as positively detected during the normal operational condition
thereof as the nonresponse of an individual fire alarm during the
checking operation.
Further objects of the subject invention are:
a. To provide a fire alarm installation having high operational
reliability;
b. To provide a fire alarm installation in which all major
components of the system can be easily checked for operability;
c. To provide a fire alarm installation having low power
consumption; and,
d. To provide a fire alarm installation of relatively low cost.
These and other objects of the subject invention as will become
apparent are implemented in that, generally speaking, the invention
is characterized by the features that the individual fire alarms
are provided with circuits which, for the normal operational
condition, provide a high impedance for supply voltages of a first
polarity and provide a low impedance for supply voltages of a
second polarity. In the case of an alarm, these impedances reverse.
Furthermore, means are provided at the central station which
deliver a voltage of alternating polarity to the supply conductor
and, by carrying out impedance measurements, determine the
following three operating conditions of the group of fire
alarms:
a. No fire alarm is alarmed (high impedance during the first
polarity of the supply voltage);
b. At least one fire alarm is alarmed and at least one fire alarm
is not alarmed (low impedance during both polarities of the supply
voltage); and,
c. All fire alarms are alarmed (high impedance during the second
polarity of the supply voltage).
According to a preferred embodiment of the invention, the circuits
for the individual fire alarms, which, as stated above, provide a
high impedance for supply voltages of the first polarity and a low
impedance for supply voltages of the second polarity during the
normal operational condition and, in the case of an alarm reverse
these impedances, consist of at least two parallely arranged
current paths which, on the one hand, can be connected via a diode
of different polarity with one supply conductor and, on the other
hand, can be connected via a reversing or selector switch to the
other supply conductor, whereby the reversing switch changes its
position during response of a fire alarm.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood, and objects other than
those set forth above will become apparent, when consideration is
given to the following detailed description thereof. Such
description makes reference to the annexed drawings wherein:
FIG. 1 is a block diagram of the invention fire alarm
installation;
FIGS. 2a to 2e are diagrams which serve to explain the mode of
operation of the inventive fire alarm installation;
FIG. 3 is a first embodiment of a fire alarm suitable for use in
conjunction with the inventive fire alarm installation;
FIG. 4 is a second embodiment of the inventive fire alarm;
FIG. 5 is a third embodiment of a fire alarm designed in accordance
with the invention and employing individual optical indicator
means;
FIG. 6 is a variant of the portion of the fire alarm of FIG. 5
appearing at the left of the line A--A thereof;
FIG. 7 is an embodiment of a conductor terminal element of FIG.
1;
FIGS. 8a to 8c are diagrams serving to explain the mode of
operation of the conductor terminal element shown in FIG. 7;
and,
FIG. 9 is a block diagram of a central signal station.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Describing now the drawings, it will be understood that the fire
alarm installation depicted by way of example in FIG. 1 consists of
a number n of two-poled fire alarms M.sub.1, M.sub.2.... M.sub.n
which are arranged parallel to one another and connected via two
supply conductors 1 and 2 with a central signal station SZ. The
individual fire alarms M.sub.1, M.sub.2 ..... M.sub.n are of
identical construction and essentially consist of a circuit which,
in the normal operational condition, exhibits high internal
resistance for supply voltages U of a first polarity and a low
internal resistance for supply voltages of a second polarity and
which, in the event of an alarm, reverses this resistance
relationship. A circuit which fulfills these conditions is
illustrated in a simplified form in the block of the fire alarm
designated M.sub.2. The circuit comprises two diodes 3 and 4, each
of which is provided with a respective series connected resistor 5
and 6. The diodes 3 and 4 are oppositely poled, whereby the anode
of diode 3 and the cathode of diode 4 collectively form one pole of
the fire alarm and are connected to the supply conductor 1. A
switch 7, depending upon its position is connecting the resistor 5
or 6, respectively, with the other pole of the fire alarm or the
supply conductor 2. The position of switch 7 is determined by a
nonillustrated apparatus which, as a function of the measured
concentration of smoke, of changes in the ambient temperature, or
of different criteria indicative of a fire, triggers a switching
operation upon exceeding a threshold value. Apparatus which are
contemplated for this purpose, among others, are those equipped
with ionization chambers, thermoelements, optical smoke-measuring
devices, or flame alarm devices.
During the normal operational condition, switch 7 is located in the
indicated full-line position. During positive supply voltages U
(supply voltages of the second polarity), diode 3 is conductive and
the current flowing through the fire alarm is essentially
determined by the resistor 5. During negative supply voltages
(supply voltage of the first polarity), diode 3 blocks and
practically no current flows through the fire alarm. Thus, viewed
from the central station SZ, the fire alarm M.sub.2 comprises a
relatively low impedance determined by the resistor 5 for positive
supply voltages U, and, on the other hand, for negative supply
voltages -U, the fire alarm M.sub.2 comprises a high impedance. In
the case of an alarm, the exact opposite behavior of the fire alarm
is determined, wherein the switch 7 assumes the phantom line
position of FIG. 1. In other words, for positive supply voltages U,
the alarmed or activated fire alarm M.sub.2 exhibits a high
impedance and for negative supply voltages, fire alarm M.sub.2
exhibits a relatively low impedance.
This specific behavior of an individual fire alarm with regard to
positive and negative supply voltages, respectively, during the
normal operation and during a alarm, can be utilized in accordance
with the teachings of the invention with a number of parallely
connected fire alarms M.sub.1, M.sub.2, ...... M.sub.n which are
coupled via only two common conductors to a central station, to
determine the following conditions:
1. During normal operation:
All fire alarms are in a state of rest;
2a. During normal operation:
One or more fire alarms have responded (alarm);
2b. During a checking operation:
One or more alarms have not responded (disturbance);
3. During a checking operation:
All fire alarms have responded (installation intact).
In order to determine these conditions, a pulse-shaped voltage u,
as shown in FIG. 2a, is delivered to the supply conductors 1 and 2.
As is apparent, the pulse-shaped voltage u possesses positive
components 11 (positive pulses) and negative components 12
(negative pulses) with respect to the zero line 10. Instead of
utilizing a rectangular shaped voltage, it is basically possible to
also choose a different voltage shape exhibiting both positive and
negative amplitudes, such as a trapezoidal-shaped voltage,
sinusoidal oscillations, and so forth. If all of the fire alarms
M.sub.1, M.sub.2, .... M.sub.n assume a normal operating condition
or state, then a low impedance is exhibited for the positive pulses
11, the low impedance being practically determined by the parallel
connection of all resistors 5 in FIG. 1, and a very large or high
impedance is exhibited for negative pulses 12. In FIG. 2b, this
normal operating condition is illustrated whereby, instead of the
total impedance of the group of fire alarms, the total fire alarm
current is plotted which is reciprocal to the impedance. As the
criterium for the "normal operating condition or state" there is
only determined at the central station whether the negative current
pulses 14 are practically equal to zero; the positive current
pulses 13, on the other hand, not being evaluated for the time
being.
Now, if one or more fire alarms respond, then, during the negative
voltage pulses, a current flows through the group of fire alarms
which differs from zero and which has a magnitude determined by the
number of fire alarms which have responded and the resistors 6
which during these phases are located in parallel. This situation
is represented in FIG. 2c, whereby during phase A, for instance,
one fire alarm has responded, and during phase B, a second fire
alarm has responded. As the criterium for the alarm condition of at
least one fire alarm, there can thus be employed at the central
station the noticeable deviation of the current i from the null or
zero value 10 for negative pulses 14'.
In modern fire alarm installations, it has become conventional, and
in certain countries even required by law, that the operational
reliability of the fire alarm be checked at regular time intervals.
For this purpose, alarm-simulating conditions are periodically
delivered to the fire alarms in order to be able to thereafter
check at the central station whether or not all fire alarms have
responded. The delivery or transmission of the alarm-simulating
conditions can take place in a standard manner known to the art, a
particularly advantageous technique being described hereinafter in
greater detail.
The purpose of checking with regard to functional or operational
reliability is to determine whether all fire alarms have responded
or whether one or a number of fire alarms have not responded. To
this end, the behavior of the group of fire alarms with respect to
the positive voltage pulses is observed at the central station. If
during the checking operation all of the fire alarms have
responded, then during the positive voltage pulses 11 (FIG. 2a)
practically no current 13" (FIG. 2d) flows through the group of
fire alarms, whereas there can be observed a clear deviation of the
positive current pulses 13'" (FIG. 2e) in the case of one or a
number of defective fire alarms. Analogous to FIG. 2c, it will be
observed from FIG. 2e that during the phase A, a fire alarm has not
responded, whereas during the phase B, a second defective fire
alarm appears.
A deviation of the negative current pulse amplitudes from zero is,
therefore, used for determining a fire alarm which has responded or
been alarmed during a normal operation, a deviation of the positive
current pulse amplitudes from null or zero being used for
determining a defective fire alarm during a checking operation. For
the time being, the behavior of the group of fire alarms to
positive voltage pulses during a normal operating condition and to
negative voltage pulses during a checking operation has not been
evaluated. It would therefore also be basically possible to employ
a series of pulses which, in the normal case, only exhibit negative
amplitudes and, in the case of testing or checking, exhibit only
positive amplitudes. Thus, further functions can be carried out by
the pulse portions of the supply voltage which are not employed for
evaluation, such as will be explained hereinafter in conjunction
with the example of the individual indicated responding or
defective fire alarm.
FIG. 3 illustrates a first embodiment of a fire alarm which
possesses the specific behavior or characteristics described in
conjunction with the fire alarm M.sub.2 of FIG. 1. Two ionization
chambers 20 and 21 are connected in series with one another. Their
point of connection or junction, generally formed by a common
electrode in practice, is coupled to the control or gate electrode
of a field-effect transistor 22. Generally, ionization chamber 20
normally consists of a compartment which is closed at all sides and
serves as a reference ionization chamber, whereas ionization
chamber 21, functioning as a measurement chamber, is accessible to
the surrounding air. The second electrode of the reference
ionization chamber 20 which is not connected with the gate
electrode of the field-effect transistor 22 is connected via a
junction or switching point 23 and a diode 3 with the supply
conductor 1. The source electrode of field-effect transistor 22 is
coupled via a Zener diode 24 to the junction 23, whereas the drain
electrode of field-effect transistor 22 is connected via a voltage
divider comprising resistors 25 and 26, and the diode 4 with the
supply conductor 1. The base of a transistor 27 is connected to the
top of voltage divider 25,26. The emitter of transistor 27 is
connected via a resistor 28 with the drain electrode of
field-effect transistor 22 and is also connected via a capacitor 29
with the diode 4, as shown. The emitter of transistor 27 is further
coupled via a conductor 31 to the supply conductor 2, and via a
capacitor 32 to junction 23. The collector of the transistor 27 is
connected via a resistor 30 with diode 4 and, via a voltage divider
comprising resistors 33 and 34, with the junction 23. The tap of
the voltage divider 33,34 is coupled with the second electrode of
the measurement ionization chamber 21.
Now, in order to explain the mode of operation of the exemplary
fire alarm illustrated in FIG. 3, it should be initially assumed
that the potential of the supply conductor 1 is positive with
respect to that of the supply conductor 2 (positive voltage pulse
11, FIG. 2a). As will be demonstrated shortly, the capacitor 29 is
charged to the negative voltage 12 (FIG. 2a).
Since the diode 3 conducts during positive voltage pulses and
therefore charges capacitor 32, the sum of the negative and
positive supply voltage U (FIG. 2a) appears across the voltage
divider 30, 33, 34. Capacitor 29 is designed or dimensioned to be
so large that its voltage only slightly drops during the negative
pulses, and thus can be considered to be constant. A potential is
applied to the gate electrode of field-effect transistor 22 which,
on the one hand, is determined by the relationship of the resistors
30, 33 and 34 and, on the other hand, by the relationship of the
internal resistance of the ionization chambers 20 and 21. The gate
supply voltage of the field-effect transistor 22 is chosen in such
a manner and is accommodated to the Zener voltage of Zener diode 24
such that field-effect transistor 22 conducts. Thus, a current 13
(FIG. 2b) flows during the positive pulses from the supply
conductor 1 via the diode 3, Zener diode 24, field-effect
transistor 22 and resistor 28, to the supply conductor 2. Resistors
25 and 26 are dimensioned such that during the normal condition of
operation, transistor 27 blocks.
During the negative voltage pulses 12 (FIG. 2a), the diode 3
blocks. Capacitor 32 slightly discharges via the field-effect
transistor 22 and transistor 27 continues to remain blocked. Only a
relatively small current 14 (FIG. 2b) flows between the supply
conductor 1 via diode 4, resistors 26,25 and 28, conductor 31, and
the supply conductor 2. During this phase, capacitor 29 is charged
to the negative supply voltage.
Now, if smoke enters into the measurement ionization chamber 21,
then its internal resistance increases. The voltage across
reference ionization chamber 20 and thus between the gate electrode
and source electrode of field-effect transistor 22 drops until it
reaches a value which causes field-effect transistor 22 to block.
As a result, the voltage at the base of the transistor 27 is also
reduced and its emitter-base voltage is increased, whereby
transistor 27 begins to conduct. Thus, for negative voltage pulses,
a current 14' (FIG. 2c) flows between the supply conductor 1, via
diode 4 and collector resistor 30, and the supply conductor 2,
which enables the detection of an alarm at the central station. A
large voltage drop appears across resistor 30 which, via both of
the voltage dividers, resistors 33, 34 and ionization chambers 20,
21, additionally reduces the voltage between the gate electrode and
the source electrode of field-effect transistor 22, whereby an
increased smoke effect is simulated in the measurement ionization
chamber. Accordingly, the circuit is positively fedback and shifts
from the rest condition in self-holding fashion into the alarm
condition. The alarm condition is maintained even after
disappearance of the alarm-triggering conditions.
The remote triggering of the fire alarm or the attainment of
alarm-simulating condition from the central station necessary for
checking operational reliability can be undertaken in a number of
different ways. A simple possibility resides in reducing, for a
short period of time, the positive pulse amplitudes of the supply
voltage. The reduction of voltage, because of the
voltage-stabilizing properties of the Zener diode 24, acts
completely upon the source electrode of the field-effect transistor
22, whereas the potential at the gate electrode is only partially
reduced or lowered due to the voltage dividers 33,34 and 20,21.
Field-effect transistor 22 now blocks and simulates the presence of
smoke. During the positive voltage pulses 13" (FIG. 2d) there is,
with the fire alarm intact, an interruption of current flow between
the supply conductor 1 and the supply conductor 2 on the one hand,
through the blocked field-effect transistor 22 and, on the other
hand, through the diode 4. With defective fire alarms such as those
having a short circuited field-effect transistor 22, there is
maintained, on the other hand, the current flow 13'" (FIG. 2e) from
the supply conductor 1 to the supply conductor 2, which can be used
at the central station for triggering a disturbance alarm.
By increasing the amplitudes of the positive voltage pulses for a
short period of time, it is possible to reset a responsive or
alarmed fire alarm back into the rest condition state.
The fire alarm circuit of FIG. 3 although being particularly simple
in construction, has the drawback that different components therein
are not supervised or monitored. For example, an interruption in
diode 4 results in capacitor 29 discharging via the resistors
33,34, whereby, the potential at the gate electrode of field-effect
transistor 22 shifts towards positive values and causes such to
block. Although the fire alarm is therefore apparently alarmed, an
alarm function is nevertheless suppressed because of the
interruption at the diode 4. During the checking operation,
field-effect transistor 22 is nonetheless blocked in spite of this
defect so that, during the positive voltage pulses, there is no
current flow. During cutout of the diode 4, it is neither possible
to determine the defect for itself nor an eventually occurring
alarm.
The circuit arrangement shown in FIG. 4 overcomes these
disadvantages, yet requires a greater expenditure because it uses a
further transistor. The gate electrode of field-effect transistor
22 is again connected to the common electrode of both ionization
chambers 20 and 21. Zener diode 24 in series with the source
electrode of the field-effect transistor 22 is now directly coupled
with the supply conductor 2. The drain electrode of field-effect
transistor 22 is coupled via the voltage divider comprising
resistors 25 and 26, to the junction or switch point 40, at which
there is also connected the second electrode of the reference
ionization chamber 20 and the emitter electrode of transistor 27.
The collector electrode of transistor 27 is coupled at the junction
or switching point 41 with the second electrode of the measurement
ionization chamber 21 and is connected via a resistor 46 with the
supply conductor 2 as well as via a resistor 42 with the base
electrode of a transistor 43. The base electrode of transistor 43
is connected via a resistor 44 and the collector electrode via a
resistor 45 to the cathode of the diode 3, whereas the emitter
electrode of transistor 43 is directly connected with the supply
conductor 2. Once again, the capacitor 29 is situated between the
anode of diode 4 and the supply conductor 2. Whereas the
field-effect transistor 22 in the circuit arrangement of FIG. 3 was
conductive in the rest condition, now it blocks during the normal
operating condition. Capacitor 29 is again charged to the negative
operating voltage. The voltage divider relationship: reference
ionization chamber 20--series connection of the measurement
ionization chamber 21, resistor 46 and the value of the Zener
voltage of the Zener diode 24 are accommodated to one another such
that the gate-source voltage of field-effect transistor 22 is below
the response voltage thereof.
During the positive voltage pulses a current flows via the diode 3
and the series connection of resistors 44, 42 and 46. A voltage
appears between the base and the emitter electrodes of transistor
43 which causes such to conduct. A current which is essentially
dependent upon the resistor 45 flows via the diode 3, resistor 45
and transistor 43 from the supply conductor 1 to the supply
conductor 2. On the other hand, the diode 3 blocks during the
negative voltage pulses. Since transistor 27 is likewise not
conductive due to the field-effect transistor 22 which still
remains blocked, during this phase only a negligibly small charging
current flows in the capacitor 29, the current through the
ionization chamber being of the order of magnitude of
10.sup.-.sup.10 amperes.
Now, if smoke enters into the measurement ionization chamber, then
the voltage between the gate electrode and the source electrode of
field-effect transistor 22 increases, whereby such begins to
conduct. A current flows in the base of the transistor 27 which
therefore likewise conducts.
A voltage forms across the resistor 46, whereby the junction point
41 becomes more negative. The voltage drop at the resistor 46 is
transmitted via the measurement ionization chamber 21 to the gate
electrode of field-effect transistor 22, so that such becomes more
conductive. The field-effect transistor 22 and the transistor 27
therefore conjointly form a complementary switching stage with
positive feedback. The fire alarm which has responded remains
self-holding. At the fire alarm which has responded, current flows
during the negative voltage pulses on the one hand via diode 4,
resistors 26 and 25, field-effect transistor 22 and Zener diode 24,
and, on the other hand, via diode 4, transistor 27 and resistor 46
between the supply conductor 1 and the supply conductor 2 and
serves for evaluation of the alarm at the central signal
station.
During the checking of the fire alarms, they are again caused to
respond from the location of the central station by means of
alarm-simulating signals, for example, by increasing the negative
voltage amplitudes for a short period of time. When the fire alarms
are intact or operable, the field-effect transistor begins to
conduct and maintains this condition or state because of the
positive feedback and the charge stored in the capacitor 29 also
during the positive voltage pulses. The negative voltage drop
occurring across resistor 46 is transmitted via the voltage divider
42,44 to the base of transistor 43, so that transistor 43 is also
blocked during the positive voltage pulses. Consequently, during
checking of the fire alarms, a current flows during the positive
voltage pulses from the supply conductor 1 to the supply conductor
2, if the fire alarm had not responded, especially also if the
diode 4 is defective. In this case, capacitor 29 finally discharges
and the negative voltage drop across resistor 46 responsible for
blocking transistor 43 for positive voltage pulses disappears after
several periods. On the other hand, what have not been supervised
or checked are the transistor 43 and the diode 3 itself, yet the
disadvantages associated therewith are much less than those present
with the circuit arrangement of FIG. 3, since even during
interruption or failure of the elements, the sounding of an alarm
is possible. In order to reset the fire alarm which has responded,
it is sufficient to suppress the negative voltage pulses.
A circuit arrangement in which all of the essential components of
the fire alarm are checked with respect to their operational
reliability is illustrated in FIG. 5. The fire alarm circuit shown
by way of example in such Figure is additionally designed
particularly for low power consumption, contains protective devices
for the most sensitive circuit components, and is additionally
equipped with an optical indicating device for fire alarms which
have responded or which are defective.
Once again, the gate electrode of field-effect transistor 22 is
connected with the common electrode of the measurement ionization
chamber 21 and the reference ionization chamber 20. A resistor 34
is situated in parallel to the series connection of both ionization
chambers 20 and 21. Resistor 34, together with the resistor 33,
forms a voltage divider. The drain electrode of field-effect
transistor 22 is coupled with the second electrode of the reference
ionization chamber 20 and is connected via a resistor 60 with the
anode of the diode 4. The source electrode of field-effect
transistor 22 is connected to the emitter electrode of a transistor
58, the collector electrode of which is connected via a resistor 59
to a conductor 57. Conductor 57 forms the second output of the fire
alarm and provides the connection with the supply conductor 2. The
base electrode of transistor 58 is connected at the tap of a
voltage divider formed by resistors 54 and 55, the voltage divider
being directly connected between both supply conductors 1 and 2. A
capacitor 56 is connected in parallel with a resistor 55. The
capacitor 29 once again is located between the anode of diode 4 and
the supply conductor 2, and the conductor 57, respectively. The
second electrode of the measurement ionization chamber 21 is
connected via a diode 50 with conductor 57, whereas the second
electrode of the reference ionization chamber 20 is coupled by
means of a Zener diode 51 to conductor 57.
Transistors 64 and 68 collectively form a first complementary
switching stage, whereby the voltage appearing across the resistor
59 is applied to the base electrode of transistor 68. The emitter
electrode of transistor 68 is directly connected with conductor 57.
The collector electrode of transistor 68 is coupled via the voltage
divider, resistor 62 and resistor 63, to the anode of diode 4. The
base electrode of transistor 64 is connected at the tap of the
voltage divider, the emitter electrode of which is directly
connected with the anode of diode 4. The collector electrode of
transistor 64 is lead back via the resistor 61 to the base
electrode of transistor 68, and is further connected via a resistor
66 and glow lamp 67 with supply conductor 1 and finally via a
voltage divider comprising resistor 65 and resistor 70, to the
cathode of diode 3. At the tap of the voltage divider 67,70, the
base electrode of transistor 69 is coupled, the emitter electrode
of which is coupled with conductor 57. The collector electrode of
transistor 69 leads via a voltage divider, comprising resistor 71
and resistor 72, to the cathode of diode 3. The base electrode of
transistor 73 is situated at the tap of the voltage divider 71,72,
and its emitter electrode is directly connected with the cathode of
diode 3, whereas the collector electrode, on the other hand, is
connected via a resistor 74 to the base electrode of transistor 69
and, on the other hand, via a diode 63 to the resistor 33 of the
voltage divider 33,34. Transistors 69 and 73 collectively form a
second bistable switching stage which is controlled by the first
switching stage (transistors 64, 68) in a master-slave technique. A
capacitor 52 is connected between the cathode of diode 53 and the
supply conductor 57.
In order to explain the operation of the circuit arrangement of
FIG. 5, it is initially assumed that capacitor 29 has been charged
to the negative supply voltage and capacitor 52 to the positive
supply voltage. A voltage appears across the series connection of
both ionization chambers 20 and 21 which is determined by the
voltage divider relationship of the resistors 33,34 and 60. The
working or operating point of the gate electrode of field-effect
transistor 22 is, analogous to the circuit of FIG. 4, chosen such
that field-effect transistor 22 and therefore also transistor 58
block in the rest condition. The Zener voltage of Zener diode 51 is
greater than the voltage appearing during normal operational
condition across the series connection of both ionization chambers,
so that the Zener diode also blocks. Only upon the appearance of
possible overvoltages at the supply conductor 1 does the Zener
diode 51 begins to conduct and produce a corresponding voltage drop
across the resistor 60. Diode 50 insures that the second electrode
of the measurement ionization chamber 21 starting from zero
potential can only assume negative values.
A small leakage current flows only during the normal operation
condition through field-effect transistor 22 and through transistor
58. A voltage of practically zero appears across the collector
resistor 59 and therefore also between the base and emitter
electrode of transistor 68, so that transistor 68 also blocks. The
voltage divider 62,63 is without current, whereby transistor 64
likewise blocks due to the absence of a base-emitter voltage.
During the positive pulse phases, diode 3 conducts and draws a
current through resistors 70, 65, 61 and 59. A voltage drop appears
between the base and the emitter electrode of transistor 69 so that
such begins to conduct and draws a collector current through the
voltage divider 71 and 72, which, in turn, causes transistor 73 to
conduct. Thus, for positive supply voltage pulses, a current flows
from the supply conductor 1 via diode 3 and essentially via
transistor 69 to the supply conductor 2. At the same time,
capacitor 52 is positively charged by means of diode 53. On the
other hand, during negative pulses, diode 3 blocks and since the
field-effect transistor 22 remains blocked, only a relatively small
current flows in the capacitor 9 as well as across the voltage
divider 54,55 to the supply conductor 2.
Now, if smoke enters the measurement chamber 21, then field-effect
transistor 22 begins to conduct in the manner already described
above. During the negative pulse phases or cycles, a current flows
between the supply conductor 1, via diode 4, resistor 60,
field-effect transistor 22, transistor 58, transistor 68 and the
supply conductor 2. Transistor 58 serves to form a response
threshold for field-effect transistor 22 and replaces the Zener
diode 24 of FIGS. 3 and 4. Its base electrode is prebiased by the
charge stored in capacitor 56, whereby the voltage represents an
average value of the negative and positive voltage pulses
alternately appearing across resistor 55. Because of the current
flowing from transistor 58 into the base electrode of transistor
68, transistor 68 begins to conduct, and because of the current
which now would flow via resistor 62 into the base electrode of
transistor 64, transistor 64 also begins to conduct, so that at the
fire alarm which has been alarmed, a larger or greater current
flows via both of these transistors between the supply conductor 1
and the supply conductor 2, such being used in the central signal
station for triggering an alarm. The first bistable switching stage
formed by both transistors 64 and 68 remains in a conductive state
or condition until it is reset.
The possibility of an individual optical indication of an alarmed
fire alarm will now be briefly considered. For this purpose, an
indicating glow lamp 67 is employed which is connected in such a
manner that it basically only tends to ignite during the positive
pulses of the supply voltage. With afire alarm which is not
alarmed, the voltage applied across glow lamp 67 during the
positive voltage phases, is determined by the voltage divider
resistor 59,61 and resistors 65,70 and is smaller than the response
voltage of glow lamp 67. On the other hand, when a fire alarm has
responded, the potential at the collector electrode of transistor
64 becomes more negative, thus the voltage across glow lamp 67 is
increased during the positive pulse phases. However, if glow lamp
67 should first illuminate after the alarm signal has been properly
received at the central station, whereby the illumination of the
glow lamp at the same time can be evaluated as indicative of the
reception of an alarm signal at the central station, or if the
optical indicator should be completely suppressed for example
during a subsequent checking operation to be described, then the
response voltage of the glow lamp 67 is chosen such that the glow
lamp, both when a fire alarm has responded or when transistor 64 is
conductive, still does not ignite. By increasing the amplitude of
the positive supply voltage pulses it is then possible to cause the
glow lamp to burn or ignite during the positive pulse phases; the
optical indicator therefore functions as a blink light.
The remote triggering of the fire alarm for the purpose of checking
can take place in a number of different ways, for example, by
increasing the positive voltage pulse amplitudes. One very
sophisticated technique is to increase the repetition frequency of
the positive pulses or to decrease their periods, whereby the
average value of the direct-current voltage across capacitor 56
increases, which once again brings about a increase of the
source-gate voltage of field-effect transistor 22, and such
responds. As a result, as described above, transistors 58,64 and 68
begin to conduct. The collector electrode of transistor 64 becomes
strongly negative, so that with a suitable voltage divider
relationship of the resistors 65 and 70 during the positive pulse
phases, the base electrode of transistor 69 becomes more negative
than its emitter electrode and transistor 69 remains blocked. As a
result, transistor 73 also remains blocked so that during the
positive impulse phases with the fire alarm intact, current can no
longer flow from the supply conductor 1 to the supply conductor 2.
On the other hand, the capacitor 52 does not receive any new charge
during the positive pulse phases, so that it discharges to the null
value. The potential of the gate electrode of field-effect
transistor 22 becomes more negative due to the discharging of
capacitor 52. This corresponds to the positive feedback already
described in connection with FIGS. 3 and 4, but now the
self-holding of an alarmed fire alarm is brought about in the first
instance by the bistable switching circuits 64,68 and 69,73,
respectively, which have switched into their other stable state or
position.
The circuit arrangement according to FIG. 5 manifests itself, among
other things, by the fact that it provides a high degree of
supervising or monitoring possibilities for the individual
components. A defect in any one of the structural components at the
actual region or part of the fire alarm (components above the
conductor 57) brings about a blocking of transistors 64 and 68,
whereby during the checking phase, transistors 69 and 73 further
conduct and therefore deliver a disturbance signal 13'" (FIG. 2e)
to the central station. An interruption at the actual monitoring
circuit (elements beneath the conductor 57) on the other hand,
causes a discharge of capacitor 52 and therefore a response of
field-effect transistor 22, which, during the normal operating
condition, indicates a (false) alarm. Naturally, it is conceivable
that defects can also simultaneously occur in both circuit portions
which, under these circumstances, will not be determined by the
remote checking operation. However, this type of situation is
highly improbable and can be taken into consideration without any
great drawback.
A practical disadvantage of the circuit arrangement of FIG. 5
resides in the fact that the second electrode of the measurement
ionization chamber 21 is connected via the diode 50 with the supply
conductor 2 which is generally grounded. Experiments have shown
that it is possible to fulfill the desire of the designer to
directly connect the measurement ionization chamber 21 with the
supply conductor 2 in that the substrate electrode 80 of
field-effect transistor 22 is connected instead with the tap of the
voltage divider comprising resistors 33 and 34, rather than with
the source electrode. Such an arrangement is shown in FIG. 6 which
is a variation of the portion of the circuit arrangement to the
left of the section line A--A of FIG. 5.
Now, even if the fire alarms of FIG. 5 are supervised or monitored
to a high degree with regard to their operational reliability, with
the previously described measures it is still not possible to
detect an eventual break or interruption in the supply conductors.
To this end, there is utilized a conductor terminal element LE
(FIG. 1) which is connected after the last fire alarm M.sub.n and
parallel to such between both supply conductors 1 and 2 and whose
internal resistance previously was impliedly presupposed to be
infinitely large. If the internal resistance of the conductor
terminal element is chosen to be very large but still finite, then
during the normal operating condition a current will flow through
the groups of fire alarms during the phases of negative voltage
pulses, and this current will be greater than zero (10, FIG. 2c),
yet smaller than the response current 14' of an individual fire
alarm.
FIG. 7 depicts an embodiment of a conductor terminal element which
can be used to particular advantage in conjunction with the
inventive fire alarm installations. The function of this conductor
terminal element resides in indicating at the central station a
possibly occurring break or interruption at the conductor means of
the groups of fire alarms. The exemplary illustrated conductor
terminal element here shown consists of an asymmetrical astable
multivibrator which is connected via a diode 81 to the supply
conductor 1 as well as directly to the supply conductor 2. Both of
the transistors 82 and 83 are coupled with the supply conductor 2
through the agency of a respective collector resistor 86 and 89.
The base electrodes of both transistors 82 and 83 are coupled or
connected with the junction or tap of a respective voltage divider
circuit incorporating resistors 84,85 and 87,88, respectively. The
emitter electrode of transistors 82 and 83 are directly connected
with the anode of a diode 81. A capacitor 90 is arranged between
the base electrode of transistors 83 and the collector electrode of
transistor 82, and, between the base electrode of transistor 82 and
the collector electrode of transistor 83, a capacitor 91 is
placed.
The mode of operation of an astable multivibrator is well known to
the art. During those phases when the potential of the supply
conductor 1 is negative with respect to that of the supply
conductor 2, transistors 82 and 83 alternatingly conduct, whereby
with appropriate variable section of the collector resistors 86 and
80, respectively, a pulse-shaped current which varies between two
values flows between the supply conductor 1 and the supply
conductor 2. This behavior is illustrated in FIGS. 8a to 8c. FIG.
8a initially shows once again the pulse-shaped voltage u applied
between the supply conductors 1 and 2 and appearing at both inputs
of a conductor terminal element. It will be recognized that FIG. 8a
is identical to FIG. 2a. In FIG. 8b, there is shown the current
i.sub.LE flowing through the astable multivibrator, whereby the
pulses 92 of smaller amplitude signify the conduction of one
transistor and the pulses 93 of larger amplitude signify the
conduction of the other transistor. Finally, FIG. 8c shows, at the
left of the line S--S, the current i flowing through the group of
fire alarms during the normal operating condition and represents a
superimposition of FIGS. 2b and 8b, whereas at the right of the
line S--S there is indicated the current i received at the central
station when a fire alarm is alarmed or has responded, and which
results from superimposing the current shown in FIGS. 2c and 8b.
Accordingly, pulses continually appear at the central station, the
repetition frequency of which is considerably greater than that of
the supply voltage pulses and which therefore can be easily
separated by means of a frequency branching or selective network.
An absence of the pulses of the conductor terminal element
signifies a conductor break or interruption or a possible defect in
the conductor terminal element itself.
Finally, FIG. 9 illustrates a block diagram of a central station SZ
in which there is shown only the elements or components necessary
for carrying out the previously described functions. A function
transmitter 100 delivers the supply voltages with positive and
negative amplitudes according to FIG. 2a. Changes in the shape and
amplitude of the different phases of the supply voltages, remote
triggering of the fire alarm, triggering of the individual
indicators, and so forth, are delivered or fed into the function
transmitter 100 by means of a control device 101. The supply
conductor 1 is directly connected with the function transmitter 100
whereas the supply conductor 2 is connected via a measurement
resistor 102 with such function transmitter 100. Two threshold
value detectors 103 and 104 received the voltage drop appearing
across the resistor 102, whereby by switching in diodes 105 and 106
of different polarity, the threshold value detector 103 measures
the positive voltage drops and the threshold value detector 104
measures the negative voltage drops. The threshold value detector
103 is controlled by the control device 101 in such a manner that,
during the checking operation, a disturbance alarm is triggered
when the positive voltage drop across the resistor 102 exceeds a
certain threshold value. On the other hand, the threshold value
detector 104 is controlled by the control device 101 in such a
manner that, at its output 107, only those negative signals
appearing across the resistor 102 are transmitted which, during the
normal operation exceed a certain negative threshold value. These
signals are subsequently divided at the units 108 and 109 in
accordance with their repetition frequencies, whereby unit 108
measures the signals with high frequency derived or coming from the
conductor terminal element and, in the absence thereof, triggers a
disturbance signal, and unit 109 measures the signals with low
frequency and, during their occurrence, triggers a smoke or fire
alarm.
At this point, it is again mentioned that the circuit arrangements
illustrated in FIGS. 3 to 6 are only preferred, exemplary
embodiments of fire alarms which are particularly well suited for
use in a fire alarm installation of the inventive type shown in
FIG. 1. It is quite readily possible to use other circuits and, in
particular, individual components or groups of structural
components can be replaced by equivalent different components, such
as replacing transistors by tubes and the like.
It should now be apparent from the foregoing detailed description
that the objects set forth at the outset of the specification have
been successfully achieved.
* * * * *