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(11) | EP 0 544 956 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Fire detector circuitry |
| (57) Circuitry is described for decreasing the susceptibility of a fire detector to unwanted
alarms in response to variations in the supply voltage powering said fire detector.
The fire detector contains a fire sensor (RC and SC), a regulator (TR1, ZD1), and
an alarm comparator circuit (A2, R8, P1, R9). Switching means (TR3) are connected
to the comparator circuit and switching control means (ZD1, R11) are connected so
as to respond to a drop, below a predetermined level, in the supply voltage. The switching
control means (ZD1, R11) are connected to the switching means (TR3) so as to cause
a difference between the alarm threshold voltage input and the sensor voltage input
to increase in response to a drop in the supply voltage. |
[0001] This invention relates to improved circuit means for ensuring that a fire detector that uses a comparator, or an operational amplifier arranged as a comparator, for the purpose of alarm threshold detection, does not produce an alarm in response to supply voltage variations, including variations produced when the supply is first applied to the detector.
[0002] Detectors that are intended for use in fire detection and alarm systems which protect industrial and commercial premises are normally powered from a direct current supply derived from the mains electrical supply or from a central standby battery in the event of an interruption of the mains electrical supply. Such detectors may be subjected to wide variations in supply voltage. These variations may be caused by events such as an alarm signal from another detector connected to the same circuit, interruption of the voltage supply to the circuit of detectors to affect the resetting of detectors following a fire alarm, induced voltage transients from nearby electrical equipment, or by rapid variations in the mains or standby battery supply. Any unwanted alarm produced in response to such variations in the voltage supply to the detectors will normally result in the unnecessary evacuation of the protected premises and the automatic attendance of the fire brigade. It is therefore essential that detectors intended for such applications do not produce unwanted alarms in response to wide variations in supply voltage.
[0003] Fire detectors primarily intended for use in industrial and commercial premises have two or more operational states, including normal and alarm states. Such detectors commonly use the threshold voltage of a FET (field effect transistor) or the zener voltage of a zener diode to establish an alarm threshold voltage, such that an excursion of a voltage related to the fire parameter being sensed beyond the threshold voltage, produces an alarm signal. The accuracy to which the absolute value of an alarm threshold voltage can be established is normally determined by the manufacturing tolerance and temperature coefficient of the FET threshold voltage or zener voltage. These tolerances in devices which are available at low cost and in high volume are sufficiently wide, particularly at the low current consumption commonly required of the fire detector circuits, as to require the incorporation of means for compensating for such tolerances in order to achieve a high degree of consistency of sensitivity between detectors of the same type. A significant advantage of using a FET or zener diode is that the alarm threshold voltage is established as an absolute voltage which must be exceeded to initiate an alarm signal. Provided the sensor output voltage that is being compared with the alarm threshold voltage does not exceed the alarm threshold voltage when the detector is subjected to supply voltage variations, the detector will not produce an alarm signal in response to supply variations. Such a detector characteristic can be readily realized using a FET or zener diode by those experienced in the art.
[0004] Recent developments in semiconductor technology have brought about the availability of low cost operation amplifiers and comparators which feature very low quiescent supply and input bias currents which make them ideally suitable for interfacing with fire sensors, and potentially for use in alarm threshold detection circuits. Moreover, these devices can, with advantages to manufacturers of fire detectors, be used in circuit configurations which do not require the incorporation of significant means to compensate for the tolerances of electrical components such as FETs and zener diodes.
[0005] Such circuits have been introduced in battery powered fire detectors primarily intended for use in domestic dwellings. Because such detectors are powered by an integral battery they are not subject to wide variations in supply voltage in normal operation and therefore do not necessarily require circuitry to prevent false alarms caused by such variations. The Motorola integrated circuits MC14466 and MC14468 are examples of such a circuit.
[0006] This invention provides a simple, low cost and novel solution to the above-mentioned problem.
[0007] More particularly, the invention provides circuitry for decreasing the susceptibility of a fire detector to unwanted alarms in response to variations in the supply voltage powering said fire detector, said fire detector comprising a fire sensor, a voltage regulator, and an alarm comparator circuit having an alarm threshold voltage input and a sensor voltage input, a voltage difference normally being maintained between said inputs to prevent the generation of a false alarm characterised in that switching means and switching control means are provided, said switching control means being responsive to a drop, below a predetermined level, in the supply voltage, so as to cause said difference to increase.
[0008] Preferably, said switching means is connected to cause an increase in said alarm threshold voltage input with respect to said sensor voltage input. Alternatively, or in addition, said switching means is connected to cause a decrease in said sensor voltage input with respect to said alarm threshold voltage input.
[0009] Preferably, said switching control means is connected to said regulator so that said predetermined level is a level at which said regulator ceases to provide a regulated voltage.
[0010] Preferably, the circuitry further includes an amplifier connected between said sensor and said comparator circuit. Filtering means may also be included to filter the output of the sensor.
[0011] The circuitry may be used in any suitable fire detector. Moreover, an alarm installation for premises may include a plurality of such fire detectors, the fire detectors being electrically connected together in parallel relationship and having a common power supply, and an alarm indicator actuable upon detection of fire by one or more of the fire detectors.
[0012] Embodiments of the invention will now be described with reference to some of the accompanying schematic Drawings, in which:
Figure 1 illustrates a circuit diagram of a prior art two-state smoke detector circuit which uses the threshold voltage of a p-channel FET to establish an alarm threshold voltage;
Figure 2 illustrates a circuit diagram of a prior art two-state smoke detector circuit which uses in combination the threshold voltage of a n-channel FET and the zener voltage of a zener diode to establish an alarm threshold voltage;
Figure 3 illustrates a circuit diagram of a prior art two-state smoke detector circuit which uses a potential divider across a stabilized supply to set the switching voltage of a voltage comparator to establish an alarm threshold voltage; and
Figures 4-8 illustrate circuit diagrams of various embodiments of the invention.
[0013] In order to be tolerant of changes in supply voltage most fire detectors incorporate some form of voltage regulator circuit (Figs. 1 to 8; components R1, TR1, ZD1, CD5). Such regulators are designed to produce a substantially constant output voltage for powering a fire sensor circuit. The regulated voltage is normally less than the minimum operational supply voltage specified for the detector. Figure 1 shows a fire sensor comprising two ionisation chambers, a sensing chamber (SC) open to the ingress of smoke, and a reference chamber (RC) closed to the ingress of smoke. The two chambers are connected in series to form a potential divider circuit. When smoke enters the sensing chamber the voltage developed across the sensing chamber increases and the voltage developed across the reference chamber decreases. When the voltage at the output of the ionisation chamber potential divider exceeds the source (S) to gate (G) threshold voltage of FET1, current flows between the source and drain (D) into the gate of SCR1 which in turn latches into an electrically conductive state. This results in a current flow through R2 and LED1 of a sufficiently high level for a control unit connected to the detector to recognise that the detector has detected a fire. Resistor R3 is a bias resistor for the gate of SCR1 and capacitor C1 provides improved immunity to false triggering of SCR1 in response to high voltage transients across SCR1. Capacitor C2 and resistor R6 control the rate of voltage rise across the ionisation chambers when the detector is first connected to a power supply such that the voltage developed across the smoke chamber does not exceed the threshold voltage of FET1 due to the high impedance ionisation chambers acting as a capacitive potential divider in response to a rapidly increasing voltage.
[0014] It will be noted from the above description that the smoke density at which the detector signals a fire alarm is a function of the threshold voltage of FET1. Commonly used low cost FETs have a threshold voltage manufacturing tolerance of several volts which has to be allowed for during calibration to ensure that the detector will produce a fire alarm at a specified smoke density. As the threshold voltage of any particular FET cannot be adjusted, it is usual to adjust the output of the sensor to suit the threshold voltage of the particular FET. This may be achieved by adjusting the voltage across the sensor circuit as illustrated in Fig. 1 where the potentiometer P1 in the potential divider circuit formed by R4, P1, and R5 is used to adjust the voltage across the ionisation chambers, or by shielding an ionising source, or by adjusting the geometry of one of the chambers. In practical detectors the current that flows through the ionisation chambers is in the order of a few picoamps which makes it difficult to monitor the sensor circuit output voltage without affecting the output voltage, furthermore the time constant of the sensor circuit is an inherent barrier to a fast change in sensor circuit output voltage in response to the means used to affect the change. Account should also be taken of the change in the sensor's output voltage sensitivity to smoke resulting from any electrical or mechanical change made to the sensor.
[0015] Although the circuits of the type illustrated in Fig. 1 do present calibration problems, such circuits are not usually susceptible to producing unwanted alarms in response to supply voltage excursions outside the operating voltage range specified for fire detection. This feature is mainly attributable to the use of a fixed voltage, the threshold voltage of FET1, as the fire alarm threshold voltage. Any increase in the voltage supplied to the detector beyond the capability of the voltage regulator circuit can be limited by protective circuitry (not shown) within the detector or by circuitry in the control unit. Any reduction in the voltage supplied to the detector that results in a decrease of the voltage applied to the sensor circuit normally decreases the output of the sensor relative to the threshold voltage of FET1.
[0016] Fig. 2 illustrates prior art circuitry based on an n-channel FET and features circuit means for adjusting the fire alarm threshold voltage. This circuit will not be described in detail but those experienced in the art will recognise that the fire alarm threshold voltage is primarily a function of the threshold voltage of FET2, the zener voltage of the zener diode ZD2, the VBE of transistor TR2, and the setting of the potentiometer P2. Because of the voltage tolerances associated with these components, the detector may be calibrated by applying a voltage to R7 that is known to produce a change in sensor output voltage equal to that produced by smoke of the density specified for the detector to alarm. The potentiometer is then slowly adjusted to a position at which an alarm is signalled. If the potentiometer is adjusted too quickly, it is likely to be incorrectly set. This problem is usually compounded by the need to use a potentiometer that produces a wide voltage range in order to accommodate the tolerance of the FET2 threshold voltage. However, the circuit shown in Fig. 2 has the advantage that it is generally not susceptible to producing an unwanted alarm in response to wide variations in the supply voltage to the detector.
[0017] Fig. 3 illustrates a prior art circuit in which a high input impedance unity gain amplifier (A1) is used to buffer the output of the sensor circuit. A further amplifier A2 and potential divider R8, P3, R9, arranged as a comparator, is used to set the alarm threshold level. As the voltage between the buffered output and the voltage on the potentiometer wiper can be measured directly the potentiometer can be quickly adjusted to give the required voltage difference for correct calibration. The circuit also has the advantage that the sensor output voltage can be quickly and accurately checked for deviation from the design value by measuring the buffered output voltage with respect to the 0 volt supply. These features of the circuit are advantageous to those engaged in the manufacture of detectors. Unfortunately the circuitry is susceptible to producing unwanted alarms in response to voltage variations.
[0018] For example, if the voltage supply to the detector is rapidly reduced to a value less than the regulation voltage of the regulator, the voltage on the wiper of the potentiometer may fall more quickly than the sensor's buffered output voltage. The buffered output voltage reflects the time constant of the ionisation chambers and that of R10 and C3. If the potentiometer wiper voltage falls below that of the buffered output, an unwanted alarm may be produced, particularly if the supply voltage is now suddenly increased. This disadvantage of the circuit may be ameliorated by replacing R9 with a zener diode such that the fixed zener voltage provides the major contribution to the voltage on the potentiometer wiper. However this approach has the disadvantage that the potentiometer range must be increased to allow for the voltage tolerance of the zener diode, with a consequent loss of adjustment resolution. Moreover, if the difference between the buffered output voltage and the wiper voltage is small, either by design, chamber contamination, or because of a normal ambient level of smoke, the zener diode may not provide a sufficient fixed voltage contribution to prevent an unwanted alarm.
[0019] Fire detector circuits such as that in the Motorola chip MC14468 do include circuitry in the form of "power on reset" circuitry which prevents a false alarm occurring when a battery is first connected or changed. A power on reset circuit is usually the first circuit to become active as the supply voltage to the detector increases upon first application of the supply voltage to the detector. On becoming active, a power on reset circuit provides signals, reset signals, to other circuits to cause these circuits to power up into a predetermined state when the supply voltage is sufficiently high to activate them. In the case of the MC14468 its power on reset circuit acts through other circuits to inhibit current flow through the potential divider that establishes the alarm threshold voltage such that the alarm threshold voltage substantially equals and follows the supply voltage applied to the detector. As the sensor output voltage cannot exceed the supply voltage the action of the power on reset circuit prevents a false alarm occurring in response to the first application of the supply voltage. After a predetermined time or upon receipt of a signal from another circuit, the power on reset circuit switches off the reset signals, and the detector circuit becomes fully operational. Thereafter, the power on reset circuit is non functional unless the supply to the detector and the power on reset circuit is first reduced to a level at which the detector becomes inoperative and then increased, as upon first application of the supply voltage. The power on reset circuit is not responsive to a drop in supply voltage to a level at which the detector would still be capable of initiating an alarm and which would also reduce the alarm threshold voltage. The Motorola circuit uses other techniques such as strobing and alarm confirmation logic to inhibit an alarm and provide time for the supply voltage to stabilize. As the Motorola circuit is intended to be powered by a battery in close proximity to the circuit it is unlikely to be subject to the range of voltage variations to which a remotely powered detector could be subject. Thus, the Motorola circuit is adequate for its intended purpose.
[0020] Embodiments of the invention will now be described. These embodiments each provide a simple, lower cost and novel solution to the problem of using an operational amplifier or comparator to establish an adjustable alarm threshold voltage in a fire detector circuit such that the detector is not susceptible to producing false alarms as a consequence of supply voltage variations.
[0021] Fig. 4 illustrates an embodiment of the invention in the form of circuitry according to the prior art illustrated in Fig. 3 but modified by the incorporation of transistor TR3 in series with the potential divider (R8, P3 and R9) such that the state of conduction of TR3 is controlled by the presence or absence of current through ZD1. The incorporation of resistor R11 in the circuitry improves the switching action of TR3 in response to changes in the conducting state of ZD1 in the event that the zener diode continues to pass a small leakage current when supplied with a voltage less than its zener voltage. T3 may be replaced with a FET or similar device.
[0022] In response to the first application of a supply voltage to the circuit (Fig. 4), no current will flow through the potential divider circuit (R8, P3, and R9) until the supply voltage exceeds the zener voltage of ZD1 plus the VBE voltage of transistor TR3. Until this voltage is attained the voltage on the wiper of P3 will follow the output voltage of the regulator (TR1 and ZD1) and will at all times be greater than the buffered output voltage of the sensor, thus no unwanted alarm will be induced. When the supply voltage to the detector exceeds the zener voltage of ZD1 plus the VBE of transistor TR3, transistor TR3 will switch to a conducting state. At this time the output voltage of the regulator circuit will be substantially constant, the voltage on the wiper of P3 will switch to the alarm threshold voltage and the detector will become fully operational. The potentiometer P3 can, if necessary, be adjusted to give the required alarm threshold voltage without any adjustment to the sensor circuitry or sensor geometry.
[0023] In the event that the detector is subject to short term voltage variations or transients such that the detector is subject to a voltage less than the zener voltage of ZD1 plus the VBE of TR3, then TR3 will switch off and the voltage on the wiper of P3 will switch to the voltage level then present at the output of the voltage regulator (TR1 and ZD1). Thus, the alarm threshold voltage is increased in response to a significant short term negative variation or transient and no unwanted alarm will be produced. When the supply voltage returns to a level greater than the zener voltage of ZD1 plus the VBE of TR3 then the voltage on the wiper of P3 will decrease to its normal level as already described.
[0025] In response to the first application of a supply voltage to the circuit (Fig. 5) the output voltage of the regulator will increase, current will flow through R14 into the base of transistor TR4 to cause TR4 to turn on and hold the sensor voltage input to the comparator low, inhibiting the possibility of a false alarm. When the supply voltage to the detector exceeds the zener voltage of ZD1 plus the VBE voltage of transistor TR3, transistor TR3 will switch to a conducting state and transistor TR4 will switch to a non conducting state and thus allow the sensor input voltage to the comparator to rise to its normal level. As the regulator is providing a regulated voltage to the potential divider, the alarm threshold voltage input to the comparator will be greater than the sensor input voltage and a false alarm will not be initiated.
[0026] In the event that the detector is subject to short term variations or transients, such that the detector is subject to a voltage less than the zener voltage of ZD1 plus the VBE of TR3, then TR3 will switch off and TR4 will switch on. TR4 switching on will pull the sensor voltage input to the comparator low, increasing the difference in voltage between the inputs to the comparator and thus preventing the initiation of a false alarm. When the supply voltage to the detector again exceeds the zener voltage of ZD1 plus the VBE of TR3 the circuit will revert to normal operation as already described in relation to the first application of a supply voltage.
[0027] Figure 7 illustrates a further embodiment of the invention in which the embodiments described in Figs. 4 and 5 are combined to further increase the voltage difference between the inputs to the comparator in response to supply voltage variation to a level less than the zener voltage of ZD1 plus the VBE of transistor TR3.
[0028] In response to the first application of a supply voltage to the circuit (Fig. 7) any current through the potential divider (R8, P3, R9) will also flow through the optional resistor R14 into the base of TR4 to cause TR4 to become conducting and thus hold the inverting input to amplifier A2 low and therefore incapable of initiating an unwanted alarm, when the supply voltage exceeds the zener voltage of ZD1 plus the VBE voltage of transistor TR3, transistor TR3 will switch to a conducting state and transistor TR4 will switch to a non conducting state. At this time the output voltage of the regulator will be substantially constant, the voltage on the wiper of potentiometer P3 will switch to the alarm threshold voltage, the voltage at the inverting input to amplifier A2 will switch to the voltage level of the sensor output, and the detector will become fully operational.
[0029] In the event that the detector is subject to short term voltage variations or transients such that the detector is subject to a voltage less than the zener voltage of ZD1 plus VBE of TR3, then TR3 will switch off causing the voltage on the wiper of P1 to increase and TR4 to switch on. When TR4 turns on the voltage at the inverting input to amplifier A2 will be pulled low. Thus, the combination of the voltage on the wiper of P1 increasing the voltage at the inverting input of A2 decreasing, increases the voltage different between the inputs of amplifier A2 so as to prevent the initiation of an unwanted alarm. Because the inverting input of A2 is pulled low the amplifier A2 will remain incapable of initiating an unwanted alarm whilst A2 is operationally functional whatever the voltage output of the regulator during supply voltage variations and transients. When the supply voltage again exceeds the zener voltage of ZD1 plus the VBE of TR3 the voltages at P1 and the inverting input of A2 will switch to the normal levels as in the manner already described for the circuit response upon the initial application of the supply voltage.
[0030] Figure 8 illustrates an embodiment of the invention applied to a detector incorporating means C4 and R12 for smoothing the sensor output voltage.
1. Circuitry for decreasing the susceptibility of a fire detector to unwanted alarms
in response to variations in the supply voltage powering said fire detector, said
fire detector comprising a fire sensor (RC and SC), a voltage regulator (TR1, ZD1,
C5), and an alarm comparator circuit (A2, R8, P1, R9) having an alarm threshold voltage
input and a sensor voltage input, a voltage difference normally being maintained between
said inputs to prevent the generation of a false alarm characterised in that switching
means (TR3, TR4) and switching control means (ZD1, R11) are provided, said switching
control means (ZD1, R11) being responsive to a drop, below a predetermined level,
in the supply voltage, so as to cause said difference to increase.
2. Circuitry according to Claim 1, wherein said switching means (TR3, TR4) is connected
to cause an increase in said alarm threshold voltage input with respect to said sensor
voltage input.
3. Circuitry according to Claim 1 or 2, wherein said switching means (TR3, TR4) is connected
to cause a decrease in said sensor voltage input with respect to said alarm threshold
voltage input.
4. Circuitry according to any of Claims 1 to 3, wherein switching control means (ZD1,
R11) is connected to said regulator (ZD1, R11, C5) so that said predetermined level
is a level at which said regulator ceases to provide a regulated voltage.
5. Circuitry according to any of Claims 1 to 4, further including an amplifier (A1) connected
between said sensor (RC, SC) and said comparator circuit (A2, R8, P1, R9).
6. Circuitry according to any of Claims 1 to 5, further including filtering means (C4,
R12) to filter the output of said sensor (RC, SC).
7. A fire detector containing circuitry according to any of the preceding Claims.
8. An alarm installation for premises comprising a plurality of fire detectors according
to Claim 7, the fire detectors being electrically connected together in parallel relationship
and having a common power supply, and an alarm indicator actuable upon detection of
fire by one or more of the fire detectors.