US3718919A - Ionization smoke detector - Google Patents

Abstract

An ionization smoke detector having an open and a closed chamber each including a pair of electrodes and a radioactive source. The chambers are connected in series across a power supply and the gate electrode of a junction type field effect transistor is connected to the junction of said chambers. The drain-source circuit of the field effect transistor includes means for limiting the current in that circuit to maintain the gate leakage current below a predetermined minimum and thereby avoid erroneous indications of the presence and absence of smoke.

Description

United States Patent [191 Sasaki et al.

IONIZATION SMOKE DETECTOR Inventors: Koju Sasaki, Tokyo; Akihiro Kobayashi, Fujisawa; Naoki Takahashi, Yokohama, all of Japan Nittan Company, Limited, Tokyo, Japan Filed: March 17, 1970 App]. No.: 20,353

Assignee:

Foreign Application Priority Data June 16, 1969 Japan ..44/46886 June 16, 1969 Japan ..44/46887 U.S. CI. ..340/237 S, 250/83.6 FT Int. Cl ..G08b 17/10 Field of Search ..340/237 S; 250/836 PT, 44,

References Cited UNITED STATES PATENTS 7/1969 Gelemter ..307/251X 10/1969 Wieczorek ..307/304X Feb. 27, 1973 3,500,368 3/1970 Abe ..340/237 S 3,530,450 9/1970 Walthard et al ....250/83.6 FT X 3,475,600 10/1969 Spence ..307/279 X FOREIGN PATENTS OR APPLICATIONS 1,088,976 10/1967 Great Britain ..250/43.5 D 950,183 2/1964 Great Britain ..307/304 Primary Examiner-John W. Caldwell Assistant ExaminerDaniel Myer Attorney-Eugene E. Geoffrey, Jr.

[57] ABSTRACT An ionization smoke detector having an open and a closed chamber each including a pair of electrodes and a radioactive source. The chambers are connected in series across a power supply and the gate electrode of a junction type field effect transistor is connected to the junction of said chambers. The drain5source circuit of the field effect transistor includes means for limiting the current in that circuit to maintain the gate leakage current below a predetermined minimum and thereby avoid erroneous indications of the presence and absence of smoke.

2 Claims, 4 Drawing Figures IONIZATION SMOKE DETECTOR BRIEF SUMMARY OF THE INVENTION This invention relates to ionization smoke detectors of the type utilizing a closed ionization chamber and an open ionization chamber, each of which has a radioactive source and a field effect transistor interconnected with the chambers for the detection of smoke in the open chamber. More specifically, the invention concems an improved smoke detector having improved stability and reliability which affords a more accurate and dependable indication of the presence of smoke.

In general, a field effect transistor (FET) particularly of the junction type has a characteristic which causes the gate leakage current Igs to decrease abruptly below a specific drain current Idl. Accordingly, the current Igs will increase sharply above a specific value Vsh of the source-drain voltage. Prior ionization smoke detectors using a FET presented difficulties since an increase in the drain current of the drain-source current during the operating procedure exceeded a critical value Idl or Vsh, the gate leakage current would change abruptly and produce an erroneous indication.

One object of the invention resides in the provision of an improved ionization smoke detector embodying means for maintaining the gate leakage current of a FET substantially uniform during the operating periods and thereby preventerroneous indications.

In accordance with the invention, a smoke detector utilizing a closed ionization chamber and .anopen ionization chamber each of which includes a pair of electrodes and a radio-active source are connected in series, a FET having the gateelectrode connected to the junctionof the ionization chambers, and a current limiting circuit interconnected with the source-drain path of the FET which circuit maintains the gate leakage current of the FET as low as possible during periods of operation.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS The above and other objects and advantages of the invention will become more apparent from the following description and accompanying drawing forming part of this application.

In the drawings:

FIG. 1 is a circuit diagram illustrating one embodiment of a smoke detector in accordance with the invention;

FIG. 2 is a graph illustrating the operation of the detector shown in FIG. 1;

FIG. 3 is a circuit diagram illustrating a modified embodiment of the invention; and

FIG. 4 is a graph illustrating the operation of the circuit shown in FIG.'3.

BRIEF DETAILED DESCRIPTION Referring now to FIGS. 1 and 2, the smoke detector includes. a closed ionization chamber 2 having a radioactive source 21 therein and an open ionization chamber 4 having a radioactive source 41. The chambers 2 and 4 also include pairs of electrodes 23,25 and 43,45, respectively. The electrode 23 of chamber 2 is connectedto a conductorG coupled to the positive side of a suitable power supply. Electrodes and 43 are connected together, and the electrode 45 is connected to conductor 8 which is coupled to the negative side of the power supply. The gate electrode 14 of FET 12 is connected at 10 to the conductor connecting the electrodes 25 and 43. The drain electrode 16 of the FET is connected directly to the conduct-or 6 while the source electrode 18 is connected through resistors 20 and 22 and a zener diode 24 to the conductor 8. The junction 26 of resistors 20 and 22 is connected through a zener diode 28 to the gate electrode 32 of a silicon controlled rectifier (SCR) 30. The gate electrode 32 of the SCR 30 is also connected through a resistor 34 to the conductor 8. The anode 36 and the cathode 38 of the SCR are connected respectively to the conductors 6 and 8.

In order to facilitate an understanding of the operation of the invention as described above, let it be assumed that the circuit of FIG. 1 does not include the zener diode 24. Under such conditions, if it is intended to produce a signal of sufficient magnitude to cause the SCR 30 to conduct, it is necessary that the values of resistors 20 and 22 be as small as possible. In so doing, the drain currentv or Id of the FET 12 during operation will increase and the gate leakage current Igs will also increase. If the drain currentld reaches the critical value Idl as shown in FIG. 2, the gate leakage current Igs will abruptly increase. Such a material change in the gate leakage current Igs affects the potential at the junction 10 between the two ionization chambers 2 and 4 with the result that the FET 12 cannot accurately detect: a change of impedance of the open ionization chamber 4 asa result of. the presence of smoke..Accordingly, erroneous operation will result and an alarm may be produced even though smoke is not present and r in the alternative it is possible that an alarm will not be produced even in the presence of smoke. While'the foregoing difficulty is frequently encountered in the use of the junction type FET, it is generally not encountered with MOS field effect transistors.

With the smoke detector in accordance with this invention, the drain current of the FET 12 is limited so that it cannot exceed Idl. Through the utilization of the zener diode 24 in the source circuit of the FET 12, the drain current is limited, and accordingly it cannot adversely affect the gate leakage current. Since under normal conditions the ionization chamber 4 is free of smoke during the operating period, a constant ionization current will flow through chamber 4 as well as through chamber 2 so that thevoltage at junction 10 is stabilized. Under the foregoing conditions the drain current of the FET 12 will, at least, equal the leakage current of the zener diode 24'so that its value will be maintained slightly below Idl. Accordingly, the gate leakage current lgs will be maintained at a relatively low value and will not affect the impedance of the ionization chambers 2 and 4. Moreover, the small drain current cannot cause conduction of the SCR 30 so that an erroneousindication will not be produced. I

Inasmuch as the ionization current of the open ionization chamber 4 is relatively low, when smoke enters the ionization chamber 4, its impedance is increased with the result that the voltage at junction 10 is increased. This increases the gate voltage of the FET 12 and produces a reduction in the source-drain impedance of the FET 12 and thereby increases the voltage across the zener diode 24 to a point above the zener voltage. As a result, drain current will flow through the zener diode and will produce a relatively large voltage at the junction 26 of the resistors 20 and 22. This voltage exceeds the zener voltage of the zener diode 28, and a positive voltage is applied to the gate electrode 32 of the SCR 30. This voltage drives the SCR 30 into conduction which in turn shortcircuits the conductors 6 and 8 with a relatively low impedance so that a relatively large current signal is transmitted to the power supply. In the ionization smoke detector as shown in FIG. 1, the insertion of the zener diode 24 in the source circuit of the FET 12, will reduce the drain current during the operating period and of course the gate leakage current. With this arrangement erroneous indications can be completely prevented. Moreover, since the current consumed by the detector itself during the operating period is relatively small, it is possible to operate a number of detectors on a single power supply. Another advantage of the invention is that it is possible to utilize an inexpensive junction type FET as the FET 12 by limiting the drain current during the operating period. Furthermore, with the utilization of a junction type FET the cost of the detectors is not only reduced but there is no danger of damaging the FET should the mesh electrode 45 be inadvertently disconnected in the course of checking the equipment. With a MOS type FET,-it has a very low gate break-down voltage, and should the electrode 45 be disconnected, the gate electrode will be effectively floating and a relatively high voltage resulting from an electrostatic field will permanently damage the FET. Since the junction type FET has a high gate break-down voltage, it will not be damaged should the electrode 45 bedisconnected.

A modified embodiment of the invention is illustrated in FIG. 3 and corresponding components of FIGS. 1 and 3 are denoted by like numerals.

The drain electrode 16 in this embodiment of the invention is connected through a collector-emitter path of a transistor 40 to the positive conductor 6 of the power supply. A resistor 48 is connected between the base 46 of transistor 40 and the conductor 6 and a zener diode 50 is connected between the base and the source electrode 18 of the FET 12. The source electrode 18 is also connected through a resistor 52 to the negative conductor 8 and is further connected through zener diode 28 to the gate electrode 32 of the SCR 30. Normally the FET has a characteristic such that the gate leakage current Igs will vary as illustrated in FIG. 4 with an increase in the drain source voltage Vds. The drain source voltage increases sharply upon attaining a specific critical value Vsh which is generally referred to as the knee point voltage."

Assuming in the circuit of FIG. 3 that the transistor 40 is omitted and that the drain electrode 16 is connected directly to the conductor 6, if the drain source voltage Vds becomes greater than Vsh for any reason whatsoever, a large gate leakage current Igs will be produced and will affect the potential at the junction of the ionization chambers 2 and 4. Under these condictions, the FET 12 will not detect an impedance change of the open ionization chamber 4 faithfully and erroneous indications will be produced. This is especially characteristic of junction type FET's. With the ionization detector as indicated in FIG. 3, the drainsource voltage Vds of the FET 12 is always maintained below the knee point voltage Vsh during the operating time. In this way adverse affects on the gate leakage current are prevented, and this is attained through the utilization of the voltage limiting circuit which includes the transistor 40 and the zener diode 50 as described above.

To attain the foregoing objectives, a zener diode having a zener voltage below the knee point voltage Vsh is utilized as the zener diode 50. When the drain-source voltage Vds of the collector-emitter path of transistor 40 becomes high so that the drain-source voltage of the FET 12 will be limited to a point below the knee point voltage Vsh.

During the operating time and under the condition wherein the open ionization chamber 4 is free of smoke, an unsaturated constant ionization current will flow therethrough and a saturated constant ionization current will flow through the closed ionization chamber 2. Accordingly, the voltage at the junction 10 will be stabilized. Under this condition, while the drain-source voltage Vds has a tendency to increase, there is no possibility of a corresponding increase in the gate leakage current since the upper limit of the drain-source voltage is maintained below the knee point voltage Vsh by means of the transistor 40 and the zener diode 50. When smoke enters the ionization chamber 4, the impedance of that chamber is increased so that the voltage at the junction 10 will also increase. This voltage is applied to the gate of FET l2 and initiates the flow of drain current. When the voltage at the source electrode 18 exceeds the zener voltage of the zener diode 28 because of the drop across resistor 52, a triggering signal will be applied to the gate electrode 32 of the SCR 50 and drive it into conduction. This shortcircuits the conductors 6 and 8 of the power supply, and thus causes a large signal to be transmitted to the power supply. Inasmuch as the drain-source voltage Vds of the FET 12 is relatively low at this time, it will not affect the operation of the transistor 40 and the zener diode 50.

As described above, the smoke detector as illustrated in FIG. 3 is arranged to avoid undesirable effects of the gate leakage current during the operating period and therefore will operate in a stable and dependable manner and will not produce erroneous indications. Moreover, an inexpensive junction type FET can be utilized as in the case of the embodiment of the invention shown in FIG. 1.

While only certain embodiments of the invention have been illustrated and described, it is apparent from the foregoing description that alterations and modifications may be made without departing from the true spirit and scope thereof as defined by the appended claims.

What is claimed is:

1. An ionization smoke detector comprising a series circuit of a closed ionization chamber and an open ionization chamber each including a pair of electrodes and a radioactive source, the interconnection of said chambers one to the other forming a junction therebetween, a field effect transistor having the source-drain path connected to the power supply and the gate electrode connected to the junction of said both'ionization chambers, alarm activating means interconnected with said transistor, and a second circuit cluding the emitter-collector path of a transistor, the last said transistor having a base electrode and a zener diode connected between said base electrode and the source electrode to maintain the source drain current below said predetermined minimum value.

2. An ionization smoke detector according to claim 1 wherein said field effect transistor is a junction type field effect transistor.

t 1 i i t

Claims (2)

1. An ionization smoke detector comprising a series circuit of a closed ionization chamber and an open ionization chamber each including a pair of electrodes and a radioactive source, the interconnection of said chambers one to the other forming a junction therebetween, a field effect transistor having the source-drain path connected to the power supply and the gate electrode connected to the junction of said both ionization chambers, alarm activating means interconnected with said transistor, and a second circuit interconnected with said sourcedrain path of said field effect transistor to maintain the source-drain current below a predetermined minimum value during normal operation to minimize affects of the gate leakage current and maintain the total current drain of the detector at a minimum, said second circuit being responsive to a selected increase in gate voltage to cause said transistor to operate said alarm activating means, the drain electrode circuit of said field effect transistor including the emitter-collector path of a transistor, the last said transistor having a base electrode and a zener diode connected between said base electrode and the source electrode to maintain the source drain current below said predetermined minimum value.

2. An ionization smoke detector according to claim 1 wherein said field effect transistor is a junction type field effect transistor.

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