US3590321A

US3590321A - Fire alarm device

Info

Publication number: US3590321A
Application number: US749166A
Authority: US
Inventors: Andreas Scheidweiler; Max Kuhn
Original assignee: Cerberus AG
Current assignee: Cerberus AG
Priority date: 1967-08-09
Filing date: 1968-07-31
Publication date: 1971-06-29
Anticipated expiration: 1988-06-29
Also published as: GB1192298A; FR1575250A; CH460594A; NL6811153A; NL160101C; NL160101B; DE1766669B1; SE333696B; DE6603864U

Abstract

Fire alarm devices wherein a detector element such as an ionization chamber includes a field-effect transistor for amplifying an electrical signal produced by the detector and a cover to protect the gate electrode of the field-effect transistor including switch means operable on removal of the cover to short circuit and the gate electrode to another electrode of the transistor and to remove the short circuit on proper replacement of the cover.

Description

United States Patent Andreas Scheidweiler;

Max Kuhn, both of Stafa. Switzerland [21] Appl. No. 749,166

[22] Filed July 31, 1968 [45] Patented June 29, 1971 [72] Inventors [73] Assignee Cerberus AG Mannedorl', Switzerland [32] Priority Aug. 9, 1967 [3 3] Switzerland [54] FIRE ALARM DEVICE 9 Claims, 5 Drawing Figs.

sz1u.s.c1 317/16,

200/6162, 317/33, 317/52 51 1111.c1...........1.... 1102115/12 so FieldofSearch 335/186,

202; l74/l7.4; 317/9, 235/2l.1,16, 33, 52; ZOO/50.1, 61.62, 61.71

[56] References Cited UNITED STATES PATENTS 2.756.299 7/1956 Sunko et al.. .1 ZOO/61.62 3,222,473 12/1965 Delhase..... ZOO/61.62 3,413.497 12/1968 Atalla 317/33 X Primary Examiner-James D. Trammell Attorney-Werner W. Kleeman ABSTRACT: Fire alarm devices wherein a detector element such as an ionization chamber includes a field-effect transistor for amplifying an electrical signal produced by the detector and a cover to protect the gate electrode of the field-effect transistor including switch means operable on removal of the cover to short circuit and the gate electrode to another electrode of the transistor and to remove the short circuit on proper replacement of the cover.

FIRE ALARM DEVICE I BACKGROUND or THE INVENTION This invention relates to fire alarm devices of the type containing at least one detector element sensitive to a phenomenon resulting from combustion, a field-effect transistor for amplifying an electrical signal produced by the detector element, connections for an electrical circuit for giving an alarm and a cover which protects the gate electrode of the field-effect transistor and the circuit components connected with it again external contact.

The desire for tire alarm devices which can be operated with a low operating voltage has led to the increasing transistorization of the alarm devices, leading to less technical skill for installation, readier compliance with safety specifications and lower power consumption. It is true that for a long time the main difficulty was to replace the cold-cathode gasdischarge tubes or equivalent devices and circuits which had to be employed whenever a very high impedance amplifier element was needed to amplify the electrical signals appearing across the detector element. This is particularly the case in ionization fire alarm devices employing one or more ionization chambers, in which the amplifier element must possess an input resistance of the order of magnitude of at least ohms. An element suitable for this purpose could be found only in the field-effect transistor, the inclusion of which in mass-produced fire alarm devices has, however, led to certain difficulties. The breakdown voltage between the gate electrode and one of the other two electrodes (source and drain electrode) of the field-effect transistor amounts to only some 50 v.; its input capacitance may become charged to several hundred volts by a touch from the naked hand on the gate electrode. Since this charge cannot flow away, a breakdown occurs which in contrast to conventional transistors with diode characteristics, effects irreparable damage. It is, of course, true that in operation a field-effect transistor may be covered by a suitable cover, possibly the casing of the alarm device, to protect against any direct contact with the gate electrode or with any parts of the alarm device connected with the gate electrode. This covering must however be removed for purposes of cleaning or repair, so that it has been found by experience that the risk of damage to the field-effect transistor is very high.

SUMMARY OF THE INVENTION Accordingly, it is a primary object of the invention to provide an improved protective device for fire alarm devices of the kind described which will protect the field-effect transistor from deleterious charging during maintenance operations.

Another object of this invention is to provide a switch means operable on removal of a cover over the field-effect transistor to short circuit the gate electrode to another electrode of the transistor.

Yet a further object of the invention is the use of either a resiliently biased switch member which is movable by a guide element on displacement of the cover or a reed switch and a permanent magnet with a ferromagnetic member interposable therebetween on proper positioning of the cover to prevent flux from the magnet from actuating the switch or wherein the member of ferromagnetic material forms on additional per manent magnet which neutralizes the field of the first permanent magnet.

Other and further objects will be particularly pointed out or will be obvious from the following detailed description.

According to the invention there is provided a fire alarm device including a detector element responsive to a phenomenon resulting from combustion to generate an electrical signal, a field-effect transistor arranged to amplify the signal and to supply an amplified signal to an alarm circuit and cover means normally preventing external contact with the to short circuit said gate electrode to another electrode of said transistor and operable in response to the correct placement of said cover means to remove the short circuit.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be further described with reference to the accompanying drawings in which:

FIG. 1 shows a cross section through a fire alarm device incorporating one embodiment of the invention;

FIG. 2a shows in cross section a modification of that part of the device of FIG. 1 lying to the left of the line A-A;

FIG. 2b shows a plan view of the modified embodiment shown in FIG. 2a, partly in cross section taken along line 8-8;

FIG. 3 shows in cross section another modification or the apparatus of FIG. 1; and

FIG. 4 shows a circuit diagram of a fire alarm device incorporating the inventive concepts.

Like reference characters refer to like parts throughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS S The fire alarm devices to which the illustrations relate are known as ionization fire alarms in which the detector element sensitive to a phenomenon resulting from a fire consists of an ionization chamber including a radioactive substance. As shown in FIG. 4 the measurement ionization chamber 10 is connected in series with a reference ionization chamber 11 and connected by way of supply leads or conductors l and 3 to a central station (not shown). The common connecting point or junction 50 (FIG. 4) of the two

ionization chambers

10 and 11 is taken to the gate electrode 14 of a field-effect transistor 12 which amplifies the alterations of voltage appearing at this point of the circuit and delivers at its source electrode 51 a signal which is applied by way of an alarm terminal 2 to a known alarm circuit (not shown). In FIG. 4 the source electrode 51 of transistor 12 is connected by way of a resistor 9 to supply lead 3. In accordance with the invention there is connected between the drain electrode 13 and the gate electrode 14 a short-circuiting switch 15 which is open during normal operation, that is with the cover of the alarm device closed. The short-circuiting switch may equally well be arranged between the gate electrode and the source electrode, as shown in broken lines at 15'.

FIG. 1 shows the greatly simplified construction of an ionization fire alarm device in accordance with FIG. 4. Into a plate 16 of insulating ceramic material there is introduced an electrode 17 common to a measuring ionization chamber 10 and a reference ionization chamber 11. Two

radioactive sources

18 and 19 ionize the air molecules present in the chambers 10 and II respectively. The reference ionization chamber 11 is provided with an electrically conductive housing 20 which rests upon an insulating base 30 and is effectively airtight; the external air has practically free entry into the measuring chamber 10, which is bounded by a housing 21 provided with perforations 22. The housing 20 of the reference ionization chamber 11 also forms the second electrode of that chamber, and the housing 21 of the measuring ionization chamber 10 forms the second electrode thereof. The housing 21 of the measuring ionization chamber 10 is push or screw mounted on to a baseplate 23 and may be removed for cleaning or repair purposes; the casing or housing of the measuring chamber 10 is connected by way of a contact member 29 with supply lead 3 and is usually grounded. During operation the measuring chamber housing 21 also serves as a cover which protects the individual parts of the alarm device, and especially those elements which are in connection with the gate electrode 14 of the field-effect transistor from direct contact.

In order that the field-effect transistor 12 shall also remain protected against excessive charging even with the cover removed, a switch 15 is provided, as already explained with reference to FIG. 4 which, by a device described below in more detail, is automatically closed upon removal of the measuring chamber housing 21, thus short circuiting the gate elec trode 14 of the field-effect transistor 12 to one of the other electrodes, as shown for example, to the drain electrode 13. In the embodiment of H6. 1 the switch 15 consists of a movable spring contact which, if no external 'force is applied, is resiliently biased into engagement with a fixed contact 24. Fixedly connected to the measuringchamber housing 21 is a disc 25 of insulating material with a protruding cylindrical flange 26. A guide member 27, aligned with flange or ring 26, restrains pin 28 which, when the chamber housing .21 is correctly positioned, moves spring contact 15 from its rest position and away from the fixed contact 24. The short circuit between gate electrode 14 and drain electrode 13 by way of reference-chamber housing 20, spring contact 15, fixed contact 24 and electrode 17 is thus broken and the field-effect transistor is made operative. On the other hand, on removal of the measuring-chamber housing 21, the pin 28 is urged upward (relative 'to the showing in the drawing) and gate electrode 14 of the field-effect transistor 12 is connected to the drain electrode 13.

FIG. 2a which represents a modification of that part of the fire alarm device of FIG. 1 which lies to the left of line A-A shows another arrangement of switch 15 and its actuating means. FlG. 2b is a partial sectional plan view of the device of FIG. 2a, taken along the line B-B. Switch 15 which in its rest position short circuits the gate and drain electrodes of the field-effect transistor, employs here a reed switch 31 having its contacts enclosed in a protective gas atmosphere. Since, in contradistinction to the spring contact of FIG. 1, the contacts of reed switch 31 are normally open, a permanent magnet 32 mounted on a base 33 is provided, which closes the contacts of reed switch 31 when the housing 21 of the measuring chamber is removed. With the cover 21 correctly placed there is interposed between the magnet 32 and the switch 31 a member of ferromagnetic material which prevents theflux from the permanent magnet from actuating the switch. Thus, more specifically, there is fixedly secured to the disc 25 of the measuring chamber a ring 34, which in this case consists of a high permeability ferromagnetic material and which, when the measuring chamber housing 21 is in position for operation, is positioned between the permanent magnet 32 and the reed switch 31 to screen the flux of the magnet from the switch, thus preventing'the magnet from actuating the reed switch. One contact of reed switch 31 is connected by way of a lead 35 with electrode 17 and the other by way of a lead 36 with the housing 20 of the reference chamber. A cover 37 is provided to protect the components of the alarm device against the entry of external air, dust, etc.

Finally, FIG. 3 shows a modification of the alarm device of FIG. 20, using a reed switch, in which ring 32 fitted to disc 25 now consists of a further permanent magnet, which when the measuring-chamber housing is correctly positioned for operation is positioned so as to neutralize the first permanent magnet and to prevent actuation of the reed switch 31 so that the contacts of reed switch 31 remain open. The remaining references correspond with those of FIG. 1.

The invention has been particularly described only with reference to ionization fire alarm devices, but it will readily be appreciated that the inclusion of a short circuiting switch and a device actuated by the cover which determines the setting of the switch can advantageously be utilized in any kind of fire alarm device in which a field-effect transistor is employed as an amplifying device. Thus in FIG. 4 the detector element 10 might be replaced by a temperature-dependent resistor or a photocell, and the reference element 11 by a resistor or by some other suitable bridging element, as will be apparentto those skilled in the art.

Having now described in detail the instant inventive concepts,

' What we claim is:

1 In a fire alarm device including a detector element responsive to a phenomenon resulting from combustion to generate an electrical signal, a field-effect transistor having gate, source and drain electrodes, arranged to amplify the signal and to supply an amplified signal to an alarm circuit, and cover means normally preventing external contact with the gate electrode of said field-effect transistor, the improve ment which comprises switch means operable in response to the displacement of said cover means to short circuit said gate electrode to another electrode of said transistor, to thereby protect said transistor against damage from static charges, and said switch means being operable in response to the correct placement of said cover means to remove the short circuit.

' 2. In a fire alarm device as defined in claim 1, wherein said switch means includes a movable contact resiliently biased into engagement with a fixed contact, and actuating means operated on correct positioning of said cover means to force said movable contact away from said fixed contact.

3. in a fire alarm device as defined in claim 2, wherein said actuating means comprises a pin freely movable in a guide, a member secured to said cover means which engages said pin when said cover means to correctly positioned, said pin engaging said movable switch contact to force it away from said fixed contact.

4. In a fire alarm device including a detector element responsive to a phenomenon resulting from combustion to generate an electrical signal, a field-effect transistor having -gate, source and drain electrodes, arranged to amplify the signal and to supply an amplified signal to an alarm circuit, and cover means normally preventing external contact with the gate electrode of said field-effect transistor, the improvement which comprises switch means operable in response to the displacement of said cover means to short circuit said gate electrode to another electrode of said transistor and operable in response to the correct placement of said cover means to remove the short circuit, said switch means including a reed switch having resiliently biased contact members, a permanent magnet whose magnetic field extends between said permanent magnet and said reed switch in order to urge together said contact members against their resilient bias when the cover means is removed, and a member of ferromagnetic material which prevents the flux from the permanent magnet from actuating said reed switch when the cover means is correctly positioned.

5. In a fire alarm device as defined in claim 4, wherein said member of ferromagnetic material is of high permeability and screens the magnetic flux of the permanent magnet from said reed switch.

6. In a fire alarm device as defined in claim 4, wherein said member of ferromagnetic material forms a permanent magnet which neutralizes the field of the first said permanent magnet to prevent actuation of said reed switch.

7. In a fire alarm device including a detector element responsive to a phenomenon resulting from combustion to generate an electrical signal, a field-effect transistor having gate, source and drain electrodes, arranged to amplify the signal and to supply an amplified signal to an alarm circuit, and cover means normally preventing external contact with the gate electrode of said field-effect transistor, the improvement which comprises switch means operable in response to the displacement of said cover means to short circuit said gate electrode to another electrode of said transistor and operable in response to the correct placement of said cover means to remove the short circuit, said switch means being connected between the gate and the source electrodes of said field-effect transistor.

8. In a fire alarm device including a detector element responsive to a phenomenon resulting from combustion to generate an electrical signal, a field-effect transistor having gate, source and drain electrodes, arranged to amplify the signal and to supply an amplified signal to an alarm circuit, and cover means normally preventing external contact with the gate electrode of said field-effect transistor, the improvement which comprises switch means operable in response to the displacement of said cover means to short circuit said gate electrode to another electrode of said transistor and operable in response to the correct placement of said cover means to remove the short circuit, said switch means being connected between the gate and drain electrodes of the field-effect transistor.

9. In a fire alarm device including a detector element responsive to a phenomenon resulting from combustion to generate an electrical signal, a field-effect transistor having gate, source and drain electrodes, arranged to amplify the signal and to supply an amplified signal to analarm circuit,

Claims

1. In a fire alarm device including a detector element responsive to a phenomenon resulting from combustion to generate an electrical signal, a field-effect transistor having gate, source and drain electrodes, arranged to amplify the signal and to supply an amplified signal to an alarm circuit, and cover means normally preventing external contact with the gate electrode of said field-effect transistor, the improvement which comprises switch means operable in response to the displacement of said cover means to short circuit said gate electrode to another electrode of said transistor, to thereby protect said transistor against damage from static charges, and said switch means being operable in response to the correct placement of said cover means to remove the short circuit.

2. In a fire alarm device as defined in claim 1, wherein said switch means includes a movable contact resiliently biased into engagement with a fixed contact, and actuating means operated on correct positioning of said cover means to force said movable contact away from said fixed contact.

4. In a fire alarm device including a detector element responsive to a phenomenon resulting from combustion to generate an electrical signal, a field-effect transistor having gate, source and drain electrodes, arranged to amplify the signal and to supply an amplified signal to an alarm circuit, and cover means normally preventing external contact with the gate electrode of said field-effect transistor, the improvement which comprises switch means operable in response to the displacement of said cover means to short circuit said gate electrode to another electrode of said transistor and operable in response to the correct placement of said cover means to remove the short circuit, said switch means including a reed switch having resiliently biased contact members, a permanent magnet whose magnetic field extends between said permanent magnet and said reed switch in order to urge together said contact members against their resilient bias when the cover means is removed, and a member of ferromagnetic material which prevents the flux from the permanent magnet from actuating said reed switch when the cover means is correctly positioned.

9. In a fire alarm device including a detector element responsive to a phenomenon resulting from combustion to generate an electrical signal, a field-effect transistor having gate, source and drain electrodes, arranged to amplify the signal and to supply an amplified signal to an alarm circuit, and cover means normally preventing external contact with the gate electrode of said field-effect transistor, the improvement which comprises switch means operable in response to the displacement of said cover means to short circuit said gate electrode to another electrode of said transistor and operable in response to the correct placement of said cover means to remove the short circuit, said detector element being an ionization chamber.