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US3714641A - Ionization fire alarm - Google Patents

Ionization fire alarm Download PDF

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Publication number
US3714641A
US3714641A US00019242A US3714641DA US3714641A US 3714641 A US3714641 A US 3714641A US 00019242 A US00019242 A US 00019242A US 3714641D A US3714641D A US 3714641DA US 3714641 A US3714641 A US 3714641A
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United States
Prior art keywords
ionization
field
ionization chamber
effect transistor
fire alarm
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Expired - Lifetime
Application number
US00019242A
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English (en)
Inventor
A Scheidweiler
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Cerberus AG
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Cerberus AG
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Publication date
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    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B17/00—Fire alarms; Alarms responsive to explosion
    • G08B17/10—Actuation by presence of smoke or gases, e.g. automatic alarm devices for analysing flowing fluid materials by the use of optical means
    • G08B17/11—Actuation by presence of smoke or gases, e.g. automatic alarm devices for analysing flowing fluid materials by the use of optical means using an ionisation chamber for detecting smoke or gas

Definitions

  • the voltage value of the voltage source and the resistance value of a resistor element are selected such that the voltage drop at the ionization chamber without the influence of combustion aerosols is smaller than the threshold voltage of the field-effect transistor so that the fieldeffect transistor is therefore non-conductive or blocked, and additionally these values are simultaneously chosen such that the field strength, in the chamber regions of the ionization chamber in which at least 85 percent of the ionization current flows/is smaller than 5 V/cm.
  • the present invention relates to an improved ionization fire alarm comprising at least one ionization chamber connected by a resistance element in series with a direct-current voltage source, and further includes an electric circuit equipped with at least one transistor possessing high input resistance, the control electrode of which is coupled with the junction point of the ionization chamber and the resistance element.
  • fire alarms of this type contain an ionization chamber accessible to the surrounding air.
  • This ionization chamber possesses a radioactive radiation source for producing the required ionization.
  • this ionization chamber also commonly known as a measuring chamber, is connected in series with a resistance element and electrically coupled to a voltage supply delivered from a central signal station.
  • a second ionization chamber which is hermetically sealed or at least sealed to such an extent that combustion aerosols can only enter with great difficulty.
  • the second ionization chamber is designed such that it only weakly responds to combustion aerosols.
  • a voltage change takes place at the junction point of the ionization chamber with the resistance element due to a change in current in the ionization chamber.
  • This voltage change id delivered to an electric circuit which produces an appropriate signal, for instance a current change at the central signalstation, when the smoke density in the measuring chamber exceeds a predetermined threshold.
  • the operational reliability of an electronic device depends to a large extent upon the type andnumber of the employed components.
  • ionization fire alarms one is particularly concerned with the amplification of an electrical signal which originates from an extremely high ohm source with an internal resistance of to 10 ohms. This means that the electric circuit must possess a correspondingly high input resistance. Furthermore, the circuit must be able to differentiate between the presence and non-presence of an electrical magnitude, this is to say, it must contain a thresholdforming element.
  • the measuring chamber is located directly parallel to the control electrode-cathode path and as the threshold valve there is used the ignition voltage of this tube path, which in the conventional ignition voltage range for gas discharge tubes is about 100 volts.
  • this circuit works very reliably, it requires the use of a special tube with very narrow tolerances, and additionally, because of the required high ignition voltages, is limited to the operation of ionization chambers with relatively high chamber voltages.
  • a different problem is the smoke sensitivity of the ionization fire alarm. It has been found that for this purpose the electric field strength in that portion of the measuring chamber in which the major portion of the ionization current flows, approximately percent, is decisive.
  • a further known ionization fire alarm attains optimum smoke sensitivity in that, the field strength in the major portion of the measuring chamber is below 5 V/cm. However, it is necessary to work in the region of relatively low measuring chamber voltages, and specifically below about 20 volts. Yet, from the foregoing it has become apparent that an optimum sensitive ionization fire alarm cannot be realized when using gas discharge tubes as the amplifier element since the reduction of the ignition voltage of a gas discharge below 20 volts is not possible for physical reasons.
  • a known circuit incorporates, for instance, at the input a capacitance diode which is located in an oscillator circuit.
  • this required a large number of components and renders the circuitry subject to breakdown.
  • Field-effect transistors are differentiated by two types, the enhancement type and the depletion type. Whereas the first type does not conduct current with zero control gate voltage, that is to say, provides a very high electrical resistance and only upon application of a voltage of the same polarity as the operating voltage of the transistor begins to conduct, in the second type a maximum current already flows with zero control gate voltage. In order to cut off the current there is required, just as in the case of an electron tube, an auxiliary voltage of opposite polarity in the operating voltage.
  • a known ionization fire alarm uses a field-effect transistor of the depletion type which without the presence of smoke particles in the measuring chamber conducts considerable current. This current, upon entry of combustion aerosols into the measuring chamber, only changes so slightly that additional components, for instance controlled rectifier s, are required as the threshold detector. This increases the susceptibility to breakdown.
  • Another known ionization fire alarm indeed utilizes a field-effect transistor of the enhancement type which in its rest state or condition does not conduct current, and only upon exceeding a certain smoke density concentration in the measuring chamber switches intoits conductive state.
  • known field-effect transistors possess a threshold voltage between 2 and volts. In the event that the transistor should be non-conductive or blocked in its rest condition, then either the chamber voltage must be smaller than 5 volts or by using additional components there must be applied a pre-biasing to the transistor.
  • the first proposal cannot be carried out since it has been found that the ionization current of ionization chambers with currents in the order of magnitude of l0 amperes is no longer stable with chamber voltages below 5 volts.
  • a further object of this invention is the provision of an ionization fire alarm having a larger ratio or relationship of alarm current to rest current.
  • the inventive ionization fire alarm is generally manifested by the features that there is employed a transistor with high input resistance, such transistor being a field-effect transistor of the enhancement type having a threshold voltage above 7 volts.
  • the voltage of the voltage source and the resistance of the resistor element are selected such that the voltage drop at the ionization chamber, without the influence of combustion aerosols, is smaller than the threshold voltage of the field-effect transistor so that the field-effect transistor is therefore non-conductive or blocked, and additionally are simultaneously so chosen that the field strength, in the chamber regions of the ionization chamber in which at least 85 percent of the ionization current flows, is smaller than 5 V/cm.
  • Field-effect transistors of the enhancement type with threshold voltages above 7 volts were not previously used, since any advantage for their prior utilization was not recognized; in fact, in complete contrast thereto, low threshold voltages were intentionally preferred. It is, however, possible to manufacture transistors with higher threshold voltages in that the thickness or the material of the dielectric between the semiconductor body and the control electrode are appropriately changed. For instance, it is possible to produce a sufficiently thick silicon oxide layer for a silicon semiconductor and, thereafter to apply a control electrode in such a manner that the threshold voltage of the thus produced transistor amounts to 9 volts.
  • control path of such a transistor can be connected directly parallel to the measuring ionization chamber without necessitating a further auxiliary voltage. Consequently, all of the mentioned advantages can thus be obtained, namely, increased smoke sensitivity, increased operational reliability, less susceptibility to breakdown, simpler manufacturing operations and larger relationship of alarm current to rest current.
  • FIG. 1 is a circuit diagram of an ionization fire alarm using one transistor
  • FIG. 2 is a circuit diagram of an ionization fire alarm using two transistors.
  • FIG. 3 is an ionization fire alarm with an integrated switching circuit.
  • FIG. 1 the ionization chamber 1 which is open to the surrounding air is connected in series with a resistor or resistance 2.
  • Ionization chamber 1 and resistor 2 are supplied via conductors 3 and 4 from a non-illustrated central signal station with a direct-current voltage of 12 to 24 volts.
  • the spacing and the configuration of the electrodes in the ionization chamber 1 are chosen such that, there where the major portion of the ionization current flows the electric field strength is smaller than 5 V/cm.
  • junction point 20 of the ionization chamber 1 and the resistor 2 is coupled with the control electrode or gate of a field-effect transistor 5 of the enhancement type, the source electrode and drain electrode of which are coupled with the supply conductors 3 and 4 respectively.
  • the threshold voltage of transistor 5, that is, the-control voltage above which the field-effect transistor 5 becomes conductive, is about 9 volts.
  • the resistance 2 is dimensioned such that the voltage drop across the electrodes of the ionization chamber 1 under normal conditions, that is, when the surrounding atmosphere does not contain combustion aerosols, is about 6 volts to 8 volts, in other words below the threshold voltage of the field-effect transistor 5 so that such therefore is non-conductive or blocks and only delivers a very small rest current. If combustion aerosols enter the ionization chamber 1, then its resistance increases and therefore also the voltage drop across the ionization chamber and the control path of the field-effect transistor 5 arranged parallel to the ionization chamber, so that the field-effect transistor begins to conduct and an increased alarm current appears at the supply conductors 3 and 4 which is recorded or detected at the central signal station.
  • this circuit only contains a single semi-conductor element it is extremely operationally reliable. Since the field-effect transistor 5 itself forms the alarm path of the fire alarm through which the main current flows between the supply conductors in the case of an alarm, the resistance change of the field-effect transistor group response is directly employed for forming the alarm current. As a result, the relationship of the alarm current to the rest current can become especially large without further components being required for this purpose.
  • a second ionization chamber 6 is connected in series with the ionization chamber 1.
  • the ionization chamber 1 is accessible to the surrounding atmosphere, the other ionization chamber 6 is either completely or almost completely closed, so that changes in the surrounding atmosphere only affect the ionization chamber 6 with a time delay.
  • ionization chamber 6 can also be constructed such that it responds considerably weaker to changes in the surroundings than the ionization chamber 1. It is advantageous if the ionization chamber 6 is saturated.
  • both ionization chambers l and 6 are connected viasupply conductors 3 and 4 respectively, with a central signal station.
  • junction point 20 of both ionization chambers l and 6 is again coupled with the gate or control electrode of field-effect transistor 5.
  • a field-effect transistor 5 of the enhancement type with a threshold voltage of about 9 volts, and the voltage at the ionization chamber under normal circumstances is again less than 9 volts.
  • the source electrode of the field-effect transistor 5 is coupled with the base of a further transistor 7.
  • a resistor 9 is connected between the base and supply conductor 4.
  • the ionization chamber 1 and the ionization chamber 6 are again connected in series and the junction point 20 of both chambers I, 6 is coupled with the gate or control electrode of a field-effect transistor.
  • a resistor 10 which, however, is smaller by at least one order of magnitude, i.e., by a factor of 10, than the resistance of ionization chamber 1, so that the ionization current is not appreciably influenced by this resistor 10.
  • the electronic circuit indicated generally by reference numeral 12, once again will be seen to contain a field-effect transistor, a second transistor and a base resistor, but in this case these components are combined into an integrated circuit 12.
  • inventive fire alarms clearly indicate that through the use of a field-effect transistor of the enhancement type with a threshold voltage above 7 volts, it is possible to build an ionization fire alarm which possesses both optimum sensitivity due to low field strength in the measuring chamber as well as also increased functional reliability and less susceptibility to breakdown because of the reduced number of components, and thus can be manufactured cheaper. Additionally, it is possible in this manner, and without having to use additional components, to build a tire alarm with increased ratio or relationship of alarm current to rest current.
  • an optional resistance element can be connected in series with the ionization chamber 1.
  • this resistance element was designed in the form of the ohmic resistor 2, in FIG. 2 as the reference ionization chamber 6 which, as is known, exhibits a non-linear current-voltage characteristic, and in FIG. 3 as a combination of reference ionization chamber 6 and ohmic resistor 10.
  • the characteristic of the resistance element is without importance. Under the term resistance element there can be thus understood a resistor with optional dependency upon voltage drop and current.
  • An ionization fire alarm comprising an ionization chamber accessible to the surrounding air, said ionization chamber being provided with two electrodes, a source of direct-current voltage, a resistance element with two terminals, one electrode of said ionization chamber being connected to one terminal of said resistance element and the other electrode of said ionization chamber and the other terminal of said resistance element being connected to said direct-current voltage source, an electric circuit incorporating at least one field-effect transistor having gate, source, and drain electrodes, the gate electrode of said field-effect transistor being connected to the connection point of said one electrode of said ionization chamber and said one terminal of said resistance element, the source and drain electrodes being electrically connected to said voltage source, said field-effect transistor being of the enhancement type and having a threshold voltage above 7 volts, the control path of said transistor connected directly parallel to the measuring ionization chamber without the need for further auxiliary elements, said direct-current voltage source having a voltage-value and said resistance element having a resistance value such that the voltage drop across said two electrodes of said i
  • said at least one ionization chamber is connected in parallel between the gate electrode and one of said source or drain electrodes of the field-effect transistor.
  • said ionization chamber and said further resistance being connected between the gate electrode and one of the other electrodes of said field-effect transistor.

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  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Business, Economics & Management (AREA)
  • Emergency Management (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Fire-Detection Mechanisms (AREA)
  • Other Investigation Or Analysis Of Materials By Electrical Means (AREA)
US00019242A 1969-03-27 1970-03-13 Ionization fire alarm Expired - Lifetime US3714641A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
CH462969A CH489070A (de) 1969-03-27 1969-03-27 Ionisationsfeuermelder

Publications (1)

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US3714641A true US3714641A (en) 1973-01-30

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US00019242A Expired - Lifetime US3714641A (en) 1969-03-27 1970-03-13 Ionization fire alarm

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US (1) US3714641A (de)
CH (1) CH489070A (de)
DE (1) DE2011329A1 (de)
GB (1) GB1253196A (de)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4023152A (en) * 1973-10-01 1977-05-10 Matsushita Electric Works, Ltd. Ionization type smoke sensing device
FR2441892A1 (fr) * 1978-11-20 1980-06-13 Anglo Amer Corp South Africa Detecteur et procede d'utilisation d'un detecteur
US4364031A (en) * 1979-12-14 1982-12-14 Cerberus Ag Ionization smoke detector with increased operational reliability
US20080252473A1 (en) * 2006-09-15 2008-10-16 Nano-Proprietary, Inc. Smoke Detector

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CH586941A5 (de) * 1975-07-25 1977-04-15 Cerberus Ag

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4023152A (en) * 1973-10-01 1977-05-10 Matsushita Electric Works, Ltd. Ionization type smoke sensing device
FR2441892A1 (fr) * 1978-11-20 1980-06-13 Anglo Amer Corp South Africa Detecteur et procede d'utilisation d'un detecteur
US4364031A (en) * 1979-12-14 1982-12-14 Cerberus Ag Ionization smoke detector with increased operational reliability
US20080252473A1 (en) * 2006-09-15 2008-10-16 Nano-Proprietary, Inc. Smoke Detector
US7821412B2 (en) 2006-09-15 2010-10-26 Applied Nanotech Holdings, Inc. Smoke detector

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Publication number Publication date
GB1253196A (de) 1971-11-10
DE2011329A1 (de) 1971-06-03
CH489070A (de) 1970-04-15

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