GB1581246A

GB1581246A - Ionisation fire alarm

Info

Publication number: GB1581246A
Application number: GB33747/77A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1976-08-16
Filing date: 1977-08-11
Publication date: 1980-12-10
Also published as: FR2362454B3; FR2362454A1; CH617280A5; SE7708637L; SE421154B

Abstract

In the case of an ionisation fire detector having a measuring chamber (10), a reference chamber (12) connected in series with the latter, a first electronic circuit element (40) connected to the connecting point of the said reference chamber, a second electronic circuit element (50) connected downstream of the said first element, and a feedback resistor (30), the feedback resistor (30) lies between the reference chamber (12) and the detector terminal (32) connected to the latter. The main current route (E-C) of the second electronic circuit element (50) lies in a current path (E-C, 52, 36; E-C, 36) parallel to the feedback resistor (30). Such a fire detector can be produced with small dimensions, since the circuit elements can, at least for the most part, be fabricated using integrated circuit technology. <IMAGE>

Description

(54) IONISATION FIRE ALARM (71) I, HARTWIG BEYERSDORF of German Nationality of Konsulweg 29 D-2409 Scharbeutz, Germany, do hereby declare the invention for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:: The invention relates to an ionisation fire alarm having a measuring chamber accessible to the ambient atmosphere, a reference chamber connected in series therewith, a first electronic switching element which is connected by its control electrode to the connection point of the chambers, has a high input impedance, and at a predetermined threshold value of the potential of the connection point changes its state of conductivity - this switching element preferably being a field effect transistor which is nonconductive in the state of rest, a second electronic switching element, preferably a bipolar transistor, which is adapted to be set to the conductive state by the aforesaid first switching element on the change of the state of conductivity of the latter, a feedback resistor connected between the terminals of the alarm in series with the series connection of the chambers, and a resistor connected in parallel to the series connection of the chambers and forming a voltage divider with a feedback resistor, the second electronic switching element being coupled to the feedback resistor in such a manner that when the second electronic switching element becomes conductive the potential at the connection point of the chambers is displaced beyond the predetermined threshold value in the direction of positive feedback.

A fire alarm of this kind is known (DT-OS23 28 872). The feedback resistor is here situated between the measuring chamber and that terminal of the alarm that is connected to the said chamber, while the second electronic switching element in the form of a bipolar transistor is connected with its main current path, the emitter-collector path, parallel to the series connection of the chambers.When the second electronic switching element becomes conductive this electronic switching element and also the field effect transistor provided as first electronic switching element become self-holding because of the feedback, the voltage connected to the series connection of the chambers becomes approximately zero because the second electronic switching element short-circuits this series connection, and an alarm current which is increased in relation to the quiescent current of the alarm flows through the second electronic switching element and the feedback resistor.

In practice, in the construction of an ionisation fire alarm that one of the two electrodes provided in the measuring chamber which is not connected to the connection point of the chamber or does not form that connection point is generally situated on the outside of the alarm. This external electrode should be accessible for testing purposes, and for the purpose of screening the alarm against stray electric fields should be at a fixed potential, preferably frame or earth potential. In the known fire alarm mentioned above this, however, is not possible because the feedback resistor is situated between the external electrode and the alarm terminal associated with the latter, so that the external electrode has in any case a potential different from that of the terminal in question when the electronic switching element is conductive, and it must therefore be insulated against the outside.The insulation prevents the accessibility of the external electrode for testing purposes and entails corresponding constructional expense, while the fact that the potential of the external electrode is not fixed makes the alarm susceptible to interference.

Other disadvantages of the known fire alarm are due to the fact that when it responds, that is to say when the second electronic switching element becomes conductive, this element short-circuits the series connection of the chambers. The control current of the first electronic switching element must then flow through the parallel connection of the chambers (mainly through the reference chamber because of the higher resistance value of the measuring chamber in comparison with the reference chamber when smoke enters) to that terminal of the alarm which is connected to the reference chamber in the state of rest. This is possible only if the chambers, in particular the reference chamber, have a sufficiently low resistance value.However, in order to reduce the overall size of the alarm there is a tendency to give the chambers very small volumes and, in the interests of safety, to use for the ionisation of the chambers radioactive sources of very low activity and/or having a range shorter than the spacing of the electrodes of the chambers, whereby high resistance values are obtained for the chambers. Although in the state of rest these resistance values are lower by one or more orders of magnitude (powers of ten) than the input resistance of a field effect transistor of an equivalent integrated circuit utilisable as first electronic switching component, nevertheless when the second electronic switching element is con ductive they rise considerably because of the absence of the voltage driving the quiescent current.This is due to the fact that with very low voltages and currents the ions produced in the chambers travel very slowly towards the electrodes and during this long residence time are predominantly subject to recombi nation and/or become deposited on molecules of air or, where applicable, smoke, and therefore do not reach the electrodes.

The resistance values of the chambers when the alarm is in the responded state can there fore become so high that the control current of the first electronic switching element is no longer sufficient to keep it in the conductive state, or more generally in its modified con ductivity state. It is true that a remedy can then be achieved by utilising after the exam ple of US PS 3,676,680 a self-conducting field effect transistor which becomes conduc tive when the threshold value of the potential at the chamber connection point is exceeded and the resulting control current is zero.

Nevertheless if he selects a small-volume construction for the alarm and/or low radioactivity of the radioactive sources used, the specialist is then very restricted in his choice of utilisable types of first electronic switching elements. In particular, a p-channel enhancement field effect transis tor, which is self-blocking and which in itself is desirable because of its high input resis tance, cannot then be used. Furthermore, the use of a self-conducting field effect transistor makes it necessary for its source electrode to be connected to a potential which is equal to the threshold value of the potential at the chamber connection point, and in order to make this potential available additional circuit elements are required, for example an additional voltage divider connected between the terminals of the alarm.

If on the other hand the chambers are so constructed that under all operating conditions they have low resistance values in comparison with the input resistance of the first electronic switching element, in the known fire alarm another difficulty arises because of the fact that when response occurs the control electrode of the field effect transistor provided as first electronic switching element is connected by way of the parallel connection of the chambers to that terminal of the alarm which in the state of rest is connected to the reference chamber, while the source electrode of the field effect transistor is connected to the alarm terminal associated with the measuring chamber, so that the control electrode-source voltage is approximately equal to the supply voltage of the alarm. This is generally between 10 V and 24V.So high an input voltage is however impermissible for many types of field effect transistors and similar switching elements, so that in this case also the choice of utilisable types of the first electronic switching element is once again considerably restricted.

An ionisation fire alarm is also known (DT-OS 24 13 162) in which for feedback purposes a controllable impedance formed by a bipolar transistor is connected between the reference chamber and the associated terminal of the alarm. The base of this transistor is connected to a voltage stabilisation circuit constructed with the aid of zener diodes, and in addition to the first electronic switching element and the second electronic switching element following it in the circuit there is provided a third electronic switching element, which is controlled by the said second electronic switching element and which is in the form of a thyristor which on becoming conductive when response occurs shortcircuits a zener diode of the voltage stabilisation circuit, whereby the conductivity of the transistor provided as controllable impedance is increased and positive feedback occurs. In this arrangement the great expense for circuitry necessary for the feedback is a disadvantage. Moreover, stabilisation of the voltage connected to the chambers by means of a stabilisation circuit containing zener diodes does not appear to be very advantageous for a fire alarm, since it may be exposed to different ambient temperatures and, in the case of fire, to high temperatures, so that an undesirable variation of sensitivity of response may occur because of the sensitivity to temperature of zener diodes.

In a fire alarm which is known from US PS 3,676,680, and which is similar to the fire alarm discussed above, a bipolar transistor is provided whose main current flow is in a parallel current path to the component resistor of a voltage divider to which the source electrode of the field effect transistor provided as first electronic switching element is connected. The base of the bipolar transistor in question is connected to another voltage divider which is connected in parallel to the feedback resistor.When the second electronic switching element becomes conductive, feedback consequently occurs not only through a variation of the potential at the chamber connection point, but also additionally through the fact that the field effect transistor source electrode, which in the state of rest is kept at a potential corresponding to the threshold value of the potential at the chamber connection point, is now connected to the potential of the alarm terminal associated with the measuring chamber. This step entails considerable additional expense for circuitry and can be applied only when the first electronic switching element used is a field effect transistor whose source electrode is connected to a voltage divider consisting of component resistors.

The problem underlying the invention is that of so constructing an ionisation fire alarm having only two feedback electronic switching elements in such a manner that, without incurring additional expense for construction, the measuring chamber electrode remote from the connection point of the chambers is at a fixed potential and that, even with an alarm construction of small volume, any desired types of field effect transistors or equivalent circuit elements can be used as first switching element.

According to the invention this problem is solved in an ionisation fire alarm of the kind first defined above in that the feedback resistor lies between the reference chamber and that terminal of the alarm which is connected to the reference chamber and that the main current flow of the second electronic switching element lies in a current path parallel to the feedback resistor.

In the fire alarm according to the invention that electrode of the reference chamber which is remote from the connection point of the chambers is connected through the feedback resistor to a terminal of the alarm. This electrode is generally situated inside the alarm, has an internal electrode protected against stray fields, and can therefore have varying potentials. On the other hand, the electrode of the measuring chamber which forms the external electrode and is remote from the connection point of the chambers receives the fixed potential of the alarm terminal associated with it, so that this electrode can be in the form of a Faraday's cage for the alarm in order to prevent interference, without such interference acting on the external electrode being able to influence the response behaviour of the alarm.The aforesaid electrode of the measuring chamber can in addition be arranged to be readily accessible for testing purposes and can even be left entirely without external insulation, which is advantageous in respect of low cost for construction and small overall size of the alarm. When response occurs the second electronic switching element, lying in the current path parallel to the feedback resistor, at least partly short-circuits the feedback resistor, so that the voltage over the series connection of the chambers is increased. The resistance of the chambers is not substantially modified thereby, so that for the first electronic switching element is it possible to use either a construction which is self-conducting and requires no control current for its operation, or a construction in which a control current is required in the responded state.Since on the other hand the feedback also does not substantially vary the voltage applied to the series connection of the chambers, the input voltage of the first electronic switching element consists only of voltages which are smaller than the supply voltage of the alarm, so that in this respect also no special requirements are imposed on the first electronic switching element.

Further developments of the invention are indicated in the sub-claims.

The invention is explained more fully below with reference to the drawings, which illustrate examples of embodiment and in which: Figure 1 shows an ionisation fire alarm according to the invention and, shown diagrammatically, the associated master station; Figure 2 is a graph explaining the mode of operation of the fire alarm of Figure 1, Figure 3 shows another example of a fire alarm according to the invention; Figure 4 is a graph explaining the functioning of the fire alarm of Figure 3, Figure 5 shows a modification of the fire alarm of Figure 3, Figure 6 is a graph explaining the functioning of the fire alarm according to Figure 5, and Figures 7 and 8 show further examples of fire alarms according to the invention.

The ionisation fire alarm shown in Figure 1 comprises a measuring chamber 10 and a reference chamber 12 connected in series therewith. The outer electrode 16 of the measuring chamber 10, which electrode may form the outer housing of the alarm and which is remote from the connection point 14 of the chambers, is connected directly to the terminal 18 of the alarm, this terminal being at zero potential. The electrode 20 - connected to the connection point 14 - of the measuring chamber 10 and the electrode 22 connected to the connection point 14 - of the reference chamber 12 could be constructionally combined. Ambient air can penetrate into the measuring chamber 10, for example through perforations in the outer electrode 16. On the entry of products resulting from fire, particularly smoke, the measuring chamber 10 has a resistance value which is increased in relation to the state of rest.On the other hand the reference chamber 12 is more intensely closed in relation to ambient air and/or on the entry of products resulting from fire has a less greatly increased resistance value. Constructionally the reference chamber 12 in the alarm lies axially behind the measuring chamber 10, or the reference chamber 12 is surrounded by the measuring chamber 10, so that the inner electrode 24, remote from the connection point 14, of the reference chamber 12 is accommodated inside the alarm and protected against interference. In both chambers 10, 12 radioactive sources 26, 28 of low activity are provided, which produce ions in the chamber volume, so that an ion current can flow when voltage is applied.

The inner electrode 24 - remote from the connection point 14 - of the reference chamber 12 is connected via a feedback resistor 30 to a terminal 32 of the alarm, to which terminal a supply voltage of 20 V in relation to the connection 18 is applied. Between the connection point 34 of the reference chamber 12 and the feedback resistor 30, on the one hand, and the terminal 18 associated with the measuring chamber 10 on the other hand, and thus in parallel to the series connection of the chambers 10, 12, there is in addition connected a resistor which consists of the series connection of two component resistors 36, 38. Together with the feedback resistor 30 these component resistors 36, 38 form a voltage divider by which in the state of rest the series connection of the chambers 10, 12 is fed with the voltage falling at the partial resistors 36, 38.This voltage is selected so that in the measuring chamber 10 a value of the field strength between the electrodes 16 and 20 which is optimum for detecting products resulting from fire is obtained; in the example of embodiment the voltage at the series connection of the chambers 10,12 amounts to 12 V. If desired, this voltage may also be made variable by making the feedback resistor 30 adjustable.

A field effect transistor 40 is connected by its control electrode G to the connection point 14 of the chambers 10, 12 to serve as first electronic switching element. This is a self-conducting n-channel depletion MOS FET which has a high input resistance. The source electrode S of the field effect transistor 40 is connected to the connection point 42 of the two resistors 44, 46 which are connected as voltage divider between the terminals 32, 18 of the alarm. The drain electrode D of the field effect transistor is connected via a resistor 48 to that terminal 32 which is connected via the feedback resistor 30 to the reference chamber 12. In addition, the control electrode of a second electronic switching element, namely the base B of a bipolar transistor 50, is connected direct to the drain electrode D of the field effect transistor 40.

The emitter E of the transistor 50 is connected directly to the terminal 32, while its collector C is connected via a light emitting diode 52 to the connection point 53 of the partial resistors 36, 38 which together with the feedback resistor 30 form a voltage divider.

In Figure 2 the currents of the chambers 10,12 are plotted against the respective voltage applied to them. In the state of rest there flows in the measuring chamber 10 a current ilO.O which is the greater, the higher the voltage of the electrode 20 becomes in relation to the outer electrode 16 which is at zero potential. In addition, a current il2.0 flows through the measuring chamber 12 in the state of rest. As explained above, the inner electrode 24 of the measuring chamber 12 is at a voltage of 12 V in relation to the terminal 18, and therefore, starting from the abscissa value 12 V, the current il2.0 becomes the greater, the lower the potential of the electrode 22 falls in relation to the inner electrode 24.

Since the electrode 20 of the measuring chamber 10 and the electrode 22 of the reference chamber 12 are connected to the connection point 14, there is adjusted at the latter a voltage UO which is determined by the point of intersection of the curves ilo.o and it2.0. This voltage UO is lower in the state of rest than the threshold value Us of that voltage of the connection point 14 in relation to the terminal 18 of the alarm at which the alarm should respond.

If smoke enters the measuring chamber 10, its resistance value will rise and the current flowing through it decreases for a given voltage. When there is considerable entry of smoke, for example, the dependence of the current in the measuring chamber 10 on the voltage applied will be that described by the curve ilo.l. The potential at the connection point 14 is thereby modified; the point of intersection of the curves ill.1, it2.0 determines a new voltage Ul which is adjusted at the connection point 14 in relation to the connection 18 of the alarm. Even before U is reached, the threshold value Us, which is slightly lower than Ul and higher than the voltage UO, is exceeded.

The source electrode S of the field effect transistor 40 is kept, in relation to the terminal 18, at a voltage approximately equal to the threshold value Us, so that when the threshold value Us is reached the field effect transistor 40 becomes conductive. The base B of the transistor 50, which base is kept at a high potential by means of the resistor 48 in the state of rest, is now connected to a low potential, whereby the transistor 50 is made conductive.

When the transistor 50 becomes conductive, on the one hand an alarm current flows through the emitter-collector path of the transistor 50, the light emitting diode 52, and the component resistor 38. This alarm current can be given any desired value by suitable selection of the component resistor 38 as load resistor. On the other hand, when the transistor 50 is conductive the emittercollector path of the transistor 50, the light emitting diode 52, and the component resistor 36 form a current path parallel to the feedback resistor 30.Since the component resistors 36, 38 now no longer form a voltage divider together with the feed-back resistor 30, and since the resistance values of the feedback resistor 30 and and of the component resistor 36 are lower by at least one order of magnitude than the resistance values of the chambers 10,12, the voltage at the series connection of the chambers 10, 12, that is to say the voltage of the connection point 34 in relation to the terminal 18, rises approximately to the supply voltage of 20 V.

The curve i12.1 of the current of the reference chamber 12 is now offset to the right in relation to the quiescent current i12.0 in Figure 2; starting from the abscissa value 20 V i12.1 becomes the greater, the lower the potential of the electrode 28 falls in relation to the potential of the inner electrode 24. The new curve il2.1 intersects the curve ilo.l of the ion current of the measuring chamber 10 at a voltage U2, which is now adjusted at the connection point 14 in relation to the terminal 18.It can therefore be seen that positive feedback occurs in such a manner that after the threshold value Us is exceeded the voltage at the connection point 14 is further displaced in the same direction of variation, so that the field effect transistor 40 and the transistor 50 go over to self-holding and the alarm has a bistable behaviour. Even if in fact the ambient air becomes free from smoke again after a response and the measuring chamber 10 assumes its original resistance value again, the voltage of the connection point 14 will fall only to a value U3 which is obtained from the point of intersection of the curves ilO.O, i12.1 and which still lies above the threshold value Us.

The component resistor 36 connected between the connection point 34 of the reference chamber 12 and of the feedback resistor 30, on the one hand, and the connection point 53 on the other hand has two functions. On the one hand in the state of rest it forms in conjunction with the component resistor 38 the resistor 30. On the other hand, after the alarm has responded and accordingly the transistor 50 has become conductive, the component resistor 36 together with the emitter diode 52 forms a current path parallel to the feedback resistor 30, while an alarm current then flows through the resistor 50, the light emitting diode 52, and the component resistor 38 serving as load resistor. This state is indicated by the light emitting diode 52.With a relatively low value of the component resistor 38 serving as load resistor an alarm current which is relatively high compared with the quiescent current of the alarm can be achieved. The quiescent current is determined essentially by the series connection of the feedback resistor 30 and component resistor 36, which may have a resistance value substantially higher than the component resistor 38 serving as load resistor; the resistance value of the component resistor 36 should be at least one order of magnitude higher than that of the component resistor 38. Because the resistance values of the feedback resistor 30 and of the component resistor 36 are of the same order of magnitude, the situation is similar for the ratio between the resistance values of the feedback resistor.In an actual example the resistance value of the component resistor 38 amounted to 1 % of the resistance values of the component resistor 36 and feedback resistor 30.

Through the construction described above and the resistance ratios mentioned the effect is achieved that the alarm current can be selected independently of the voltage connected to the series connection of the chambers 10,12 and of the threshold value of the potential of the connection point 14 at which the alarm responds.Although the component of the resistor forming a voltage divider together with the feedback resistor 30, nevertheless the low resistance value of the component resistor 38 ensures that it has practically no influence on the voltage applied to the series connection of the chambers 10, 12 or on the response threshold; the doubling of the resistance value of the component resistor 38 in the example of embodiment described above would mean a variation of about 1 % of the voltage applied to the series connection of the chambers 10, 12 whereby because of the partial saturation of the chambers 10, 12, particularly of the reference chamber 12, a variation of the response threshold of the order of 0.5% would be obtained.

As further illustrated in Figure 1, the alarm is connected by two conductors 54, 56 of a line to a master station 58, where the conductors 54, 56 are supplied by a battery 60. Suitable means are provided for detect ing the alarm current; solely for the sake of simplicity a means of this kind is shown in the form of a relay 62 with a contact 64 adapted to be operated by it. In order to cancel the self-holding of the alarm after it has responded, a cut-out 66 is provided by means of which the alarm current is interrupted.

Figure 1 also shows a possible development which consists in that an additional connection 68, which is connected to the connection point 34 of the reference chamber 12 and of the feedback resistor 30, is provided in the alarm and is connected by way of another conductor 70 of the line and a push-button switch 72 is provided in the master station 58 to the positive pole of the battery 60. On operation of the push-button switch 72 the connection point 34 is thus connected to a potential which is at least approximately equal to the potential of the alarm tetminal 32 connected to the feedback resistor, so that in the same way as when the transistor 50 becomes conductive the potential of the connection point 14 of the chambers 10, 12 is increased and the alarm is operated and goes over to self-holding.

A plurality of alarms may be connected in parallel to one another to the conductors 54, 70, 56 of the line. In this case each alarm has a diode 74 which is connected between the additional connection 68 and the connection point 34 and which in relation to the current flowing when the push-button switch 72 is operated is forward-biased. If an alarm operates as the result of the penetration of smoke into its measuring chamber 10, the diode 74, which then receives current in the blocking direction, in this alarm prevents the transmission of the increased potential at the connection point 34 of this alarm to the other alarms and thus prevents their operation.

In the examples of embodiment illustrated in Figures 3, 5, 7 and 8, parts which are identical with those in the embodiment shown in Figure 1 or have the same function are given the same reference numerals.

The alarm shown in Figure 3 differs from that shown in Figure 1 primarily in the termi nal 32 associated with the reference chamber 12 is at a potential which is negative in relation to the terminal 18. The directions of conduction of the field effect transistor 40, of the transistor 50, and of the light emitting diode 52 are therefore reversed in relation to Figure 1.

Further important features of the alarm shown in Figure 3 consist in that the field effect transistor 40 is connected directly in parallel with its control electrode-source path of the measuring chamber 10 and that it is a self-blocking p-channel enhancement MOSFET. This field effect transistor has a very high input impedance and a relatively great capacitance between the control elec trode G and the source electrode S.

Through the direct connection of the source electrode S of the field effect transistor 40 in Figure 3 the voltage divider consisting of the resistors 44, 46 in the example shown in Figure 1 is not required. The corresponding consumption of quiescent current is thereby also eliminated. Whereas in the embodiment shown in Figure 1 the control current of the transistor 50 had to flow through the resistor 46 of the voltage divider when the field effect transistor 40 was conductive, in the embodiment shown in Figure 3 the base B of the transistor 50 is connected directly to the terminal 18 associated with the measuring chamber 10 when the field effect transistor 40 is conductive.This is a circuit which has a substantially lower noise level and is less temperature dependent, and which requires substantially less current amplification of the transistor 50 in comparison with the embodiment shown in Figure 1; if in the embodiment shown in Figure 1 a noteworthy loss of power in the resistors 44, 46 of the voltage divider is to be avoided in the state of rest, the resistors 44, 46 must have high resistance values and the transistor 50 must have very great current amplification in order to become consuctive despite the resistor 46 incorporated in the control current path.Incidentally, it may be observed that both in the embodiment shown in Figure 1 and in that in Figure 3, in contrast to some ionisation fire alarms of the prior art, the base B of the transistor 50 is connected directly to the drain electrode D of the field effect transistor 40, so that an interposed resistor will not result in a loss of signal power. Furthermore, in all embodiments the light emitting diode 52 is not connected between the emitter E of the transistor 50 and the connection 52, where it might hinder the passing of the transistor 50 into the conductive state, but is placed between the collector C of the transistor 50 and the connection point 53 of the component resistor 36, 38.

The direct connection of the control electrode-source capaictance of the field effect transistor 40 in parallel to the measuring chamber 10 in the embodiment shown in Figure 3 has an advantageous effect when the supply voltage is switched on and when sudden voltage jumps occur. This will be explained below.

When the supply voltage is switched on the voltage at the connection point 34 in Figure 3 jumps immediately to the voltage value (negative in relation to the terminal 18) which results from the voltage divider ratio of the feedback resistor 30 on the one hand and of the component resistors 36, 38 on the other hand. Immediately after the supply voltage has been switched on the usual voltage corresponding to the state of the rest is thus applied to the series connection of the chambers 10, 12. On the other hand, the voltage which could be expected because of the resistance values of the chambers 10, 12 is not immediately obtained at the connection point 14 of the chambers 10,12. On the contrary, if a rapid variation of voltage occurs it can be seen that the chambers 10, 12 have considerable capacitance.The chambers 10, 12 therefore first act predominantly as a capacitive voltage divider, so that the voltage over the measuring chamber 20 in relation to the voltage over the reference chamber 12 is in inverse ratio to their capacitance values. With a high capacitance of the reference chamber 12 the potential of the connection point 14 of the chambers 10, 12 immediately after switching-on is therefore closer to the potential of the connection point 34 of the reference chamber 12 to the feedback resistor 30 than to the potential of the terminal 18. This could give rise to the danger that the threshold value of the potential at which the alarm goes over to selfholding could be exceeded, so that an alarm signal would be given incorrectly.Actually, however, it is permissible for the reference chamber 12 to have a higher capacitance than the measuring chamber 10, since the control electrode-source capacitance of the field effect transistor 40 is in parallel to the capacitance of the measuring chamber 10.

The action of the transistor 40 is the greater, the lower the capacitances of the chambers 10, 12, particularly that of the measuring chamber 10. It is expedient to endeavour through the shape and size of the electrodes 16, 20 and 22, 24 respectively to give the chambers 10, 12 respective capacitances which are of the order of magnitude of the control electrode-source capacitance of the field effect transistor 40.

The ratio of the voltage Ulo in the measuring chamber 10 to the voltage U34 of the connection point 34, in relation to the terminal 18, that is to say the voltage in the chambers 10, 12, is determined in the state of rest by the ratio of the resistance value Rlo of the measuring chamber 10 to the sum of this resistance value Rlo and of the resistance value Rl2 of the reference chamber 12: Uio Rio (1).

U34 Rio + Rl2 When the supply voltage is switched on a chronologically variable voltage UIOE exists in the measuring chamber 10, this voltage being determined by the capacitance values Cio, Cl2 of the measuring chamber 10 and reference chamber 12 respectively: UIOE C12 (2).

U34 Cio + Ci In order to ensure that after the supply voltage has been switched on the potential of the connection point 14 of the chambers 10, 12 will not exceed the threshold value Us at which the alarm goes over to self-holding, the following relationship must exist: I UIOE I S | UIO I (3).

By inserting the right hand sides of formulae 1 & 2 in equation 3 we obtain: Ri2c Ci2 < Cio (4).

Rlo The control electrode-source capacitance of the field effect transistor electrode-source capacitance of the field effect transistor 40 has here in the first instance not yet been taken into account. If this capacitance has a not negligible value, it is added to the capacitance Cio of the measuring chamber 10 as explained above, so that we obtain: Rl2 ~ C12 < Cio + CGs (4') Rio wherein Cos designates the control electrode-source capacitance.In order therefore to ensure that the alarm will not incorrectly go over to self-holding on switching on, the capacitance C12 of the reference chamber 12, multiplied by the inverse ratio of the resistance values Rlo, Rl2 of the measuring chamber 10 and the reference chamber 12, must be greater than the capacitance of the measuring chamber by an amount which at most is as great as the control electrodesource capacitance CGs of the field effect transistor 40. The reference chamber 12 is actually expediently given so high a capacitance that the left-hand term of the equation (4') is equal to the right-hand term of the equation (4') is equal to the right-hand side of this equation in order to assist to obtaining the most compact and least bulky construction possible for the alarm.

Because of the negative supply voltage of the alarm according to Figure 3, the dependence, shown in Figure 4, of the chamber current on the voltage existing in the chambers 10, 12 produces substantially mirror image symmetry of the i-axis in relation to Figure 2, in which the supply voltage according to Figure 1 is assumed to be positive. In Figure 4 it can be seen that the voltage existing in the series connection of the chambers 10, 12 in the embodiment shown in Figure 3 amounts to -10 V, which once again is achieved by suitable selection of the feedback resistor 30 and of the component resistors 36, 38 which together with it form a voltage divider. Otherwise the remarks made in connection with the selection of the feedback resistor 30 and component resistors 36, 38 in connection with Figure 1 also apply to the embodiment illustrated in Figure 3.In Figure 4 the curves and voltages shown also have the same meanings and properties in accordance with their references as those explained in connection with Figure 2.

In Figure 5 are shown some possible modifications of the embodiment of Figure 3, which may be expedient when special requirements exist; for the sake of simplicity only these modifications will be described, while the other parts and their functions are unchanged unless otherwise stated.

A modification of the alarm shown in Figure 3 is illustrated in Figure 5 and consists in that an additional resistor 78 is connected in parallel to the series connection of the component resistors 36, 38 which together with the feedback resistor 30 form a voltage divider. The resistance value of this additional resistor is of the order of magnitude of the resistance values of the feedback resistor 30 and of the component resistor 36.

Through slightly modified proportioning of the last-mentioned resistors 30, 36 in comparison with Figure 3 it is possible to ensure that in the state of rest the voltage existing in the series connection of the chambers 10,12 will have a desired value of e.g. -10 V again.

In the state of rest and when products formed by fire enter the measuring chamber 10 the same effects - as illustrated in Figure 6 - will be achieved as with the fire alarm shown in Figure 3; the voltage Uo of the connection point 14 in relation to the terminal 18 is increased in amount to the voltage Ul and thus exceeds the threshold voltage Us. As in the embodiment shown in Figure 3, the resulting feedback and the fact that the transistor 50 becomes conductive result in the formation of a current path which is parallel to the feedback resistor 30 and which comprises a component resistor 36, the light emitting diode 52, and the emitter-collector path of the transistor 50.The parallel connection of the feedback resistor 30 and the current path mentioned now however forms a voltage divider together with the additional resistor 78, so that the voltage in the series connection of the chambers 10, 12 does not reach the value of the supply voltage; as shown in Figure 6, the resistance proportions may for example be so selected that the voltage of the series connection amounts to -15 V. As can also be seen in Figure 6, the voltage U3, which in this case is obtained at the connection point 14 in relation to the terminal 18 on the disappearance of the smoke in the measuring chamber 10, still has a higher value than the threshold value Us, so that in this case also the self-holding is maintained.

Through the relatively slight variation of the voltage in the measuring chamber 10 due to the feedback the effect is achieved that in the measuring chamber 10 an electric field prevails which is only slightly increased in relation to the state of rest, so that the tendency to deposit aerosols on the electrodes 16, 20 which at the low voltages used is in any case slight- is not appreciably increased. A similar slight action is moreover also obtained in all embodiments through the voltage drop of the order of 1V occurring at the light emitting diode 52 when the transistor 50 is conductive; instead of or additionally to the light emitting diode 52 it is also possible to connect between the transistor 50 and the connection point 53 other resistance elements, which expediently are non-linear but whose total resistance should not be greater than the resistance value of the load resistor 38.

Another development of the alarm of Figure 3, which is shown in Figure 5 and may also be applied independently of the other modifications shown in Figure 5, consists in that between the source electrode S of the field effect transistor 40 and the terminal 18 connected to the measuring chamber 10 there is connected a zener diode 80 which is connected in the blocking direction in relation to the source current of the field effect transistor 40. The threshold value Us (Figure 6) can thereby be raised and/or a field effect transistor 40 having a response voltage of a lower value may be used.

Another development which is shown in Figure 5, and which once again may be used independently of the other previously mentioned developments, consists in t hat a capacitor 82 is connected in parallel with the series connection of the chambers 10, 12.

The capacitor 82 is intended to prevent the potential of the connection point 14 when the supply voltage is switched on from exceeding the threshold value if the reference chamber 12 has a high capacitance in relation to the capacitance of the measuring chamber 10. For this purpose the capacitor 82 must have a sufficiently high capacitance for its charging current during its charging operation immediately after switching-on to produce an adequate additional voltage drop at the feedback resistor 30.

The possible developments of the alarm of Figure 3 which are illustrated in Figure 5 can also be applied to the alarm of Figure 1 and to other conceivable embodiments of the invention.

In the embodiments illustrated in Figures 7 and 8, as in the case of Figure 1, the cathode of the first electronic circuit element, namely the source electrode S of the field effect transistor, is connected by way of a resistor 46 to the terminal 18 of the alarm which is connected to the measuring chamber 10, this electrode 46 being part of the voltage divider which is connected between the terminals 18 and 32 and includes the additional resistor 44 and to whose connection point 42 the source electrode S is connected. As a modification of the preceding examples of embodiment, however, in Figures 7 and 8 there is connected in parallel to the resistor 46 the main current path of a third electronic switching element which can be set to the conductive state by the second electronic switching element, namely the bipolar transistor 50 when the latter becomes conductive.This will be explained in greater detail below.

If in Figure 7 the field effect transistor 40 and the transistor 50 become conductive when response occurs in consequence of the entry of products formed by fire into the measuring chamber 10, the third electronic switching element in the form of another bipolar transistor 84 is thereby made conductive in order to cancel the action of the resistor 46 and to increase the feedback. The other transistor 84 is in fact connected by its main current path, the emitter-collector path E' - C', in parallel to the resistor 46. The base B' of the additional transistor 84 is connected by a limiting resistor, which limits the base current, solely to the collector of the transistor 50.

In connection with Figure 7 it may be added that here again, as in Figures 3 and 5, a light emitting diode 52 is provided for the purpose of indicating the responded state, but that this diode is connected between the component resistor 38 serving as load resistor and that terminal 18 of the alarm to which the measuring chamber 10 is connected.

Thus the light emitting diode 52 itself forms a component resistor of that voltage divider which comprises the component resistors 36 and 38 and which is connected in parallel to the series connection of the chambers 10, 12.

If, as in Figure 7, the control electrode of the third electronic switching element is connected solely by way of a limiting resistor to a main electrode, of the second electronic switching element, this provides the advantage of a relatively simple circuit construction. On the other hand it is also possible for the control electrode of the third electronic switching element to be connected to a voltage divider which is connected between a main electrode of the second electronic switching element and that terminal of the alarm which is connected to the measuring chamber.If the alarm has a load resistor connected between a main electrode of the second electronic switching element and the alarm terminal just mentioned, this can be achieved in a simple manner by giving this load resistor the form of a voltage divider to which the control electrode of the third electronic switching element is connected. Thus, for example in the embodiment illustrated in Figure 7, it would be possible to vary the arrangement shown and to divide the component resistor 38 serving as load resistor into two additional ohmic resistors having a total resistance equal to that of the component resistor 38 and to connect the base B' of the transistor 84 to the connection point of the resistors mentioned. It would also be possible to replace the component resistor 38 by a potentiometer to whose tap the base B' of the transistor 84 is connected.

In some cases it is not possible or convenient to give the load resistor the form of a voltage divider. In such cases a separate voltage divider may be connected in parallel to the load resistor. A solution of this kind is shown in Figure 8. The example of construction of an ionisation fire alarm shown in Figure 7; identical parts are given the same references.

In the alarm shown in Figure 8 the load resistor is formed by the coil 38' of a relay which operates a potential-free make contact 88 when the alarm responds. In this case it is not possible in a simple manner to make the load resistor in the form of a voltage divider.

Instead, the limiting resistor 86 is connected in series with another resistor 90 between the connection point 53 and the terminal 18, so that these resistors 86, 90 form a voltage divider. The base B' of the transistor 84 is connected to the latter. Just as in the case of the other possible constructions of the voltage divider described in connection with Figure 7 it is thus ensured that on the response of the alarm the base-emitter voltage of the transistor 84 will assume a predetermined value, which amounts to only a fraction of the supply voltage, this fraction being determined by the voltage division ratio of the resistor 90 and limiting resistor 86.

In general it may be added that the ionisation fire alarm of the invention is distinguished by extreme simplicity. Since in addition, as has been indicated a number of times, it can be constructed with a very small volume, this alarm is an uncomplicated product which can be produced simply and inexpensively by mass production methods. In contrast to ionisation fire alarms of the prior art, in which it was endeavoured to achieve the greatest possible reliability of the fire alarm by additional circuitry measures, particularly by means of an additional fault signal in the event of slight variations of the resistance value of the measuring chamber, and/or of variations thereof extending over long periods of time, the fire alarm of the invention is produced in an expedient manner with its simple circuit construction described in the examples of embodiment.As an inexpensive mass-produced article the fire alarm can thus be regularly replaced by a new alarm before there is any danger that it will become unreliable because of aging or the deposition of dust. This method is substantially less expensive than the previously adopted method of on the one hand signalling faults by additional circuitry and on the other hand nevertheless having to call on the services of skilled labour in the event of a false alarm.

As another means of achieving the simplest and smallest possible construction of the alarm, provision can be made for the circuit elements of the alarm to be accommodated, at least to a predominant extent, as an integrated circuit on a common chip. For example, in the embodiment shown in Figure 3 the feedback resistor 30, the component resistor 36, the field effect transistor 40, the transistor 50, the resistor 48, and the diode 74 can be accommodated on a common chip as an integrated circuit, while the component resistor serving as load resistor should be disposed on the common chip only if good dissipation of heat from the chip is ensured.

Finally, it may further be pointed out that the steps by which the potential at the chamber connection point 14 is prevented from exceeding the predetermined threshold value after the supply voltage has been switched on, particularly the selection of the capacitance of the chambers 10, 12 explained in connection with Figure 3, can also advantageously be used for the same purpose in other ionisation fire alarms having measuring and reference chambers connected in series. This applies particularly to the alarms described in the preamble as prior art.

WHAT WE CLAIM IS: 1. An ionisation fire alarm having a measuring chamber accessible to the ambient atmosphere, a reference chamber connected in series therewith, a first electronic switching element which is connected by its control electrode to the connection point of the chambers, has a high input impe dance, and at a predetermined threshold value of the potential of the connection point changes its state of conductivity - this switching element preferably being a field effect transistor which is non-conductive in the state of rest, a second electronic switching element, preferably a bipolar transistor, which is adapted to be set to the conductive state by the aforesaid first switching element on the change of the state of conductivity of the latter, a feedback resistor connected between the terminals of the alarm in series with the series connection of the chambers, and a resistor connected in parallel to the series connection of the chambers and forming a voltage divider with the feedback resistor, the second electronic switching element being coupled to the feedback resistor in such a manner that when the second electronic switching element becomes conductive, the potential at the connection point of the chambers is displaced beyond the predetermined threshold value in the direction of positive feedback, characterized in that the feedback resistor lies between the reference chamber and that terminal of the alarm which is connected to the reference chamber, and that the main current flow of the second electronic switching element lies in a current path parallel to the feedback resistor.

2. A fire alarm according to Claim 1, characterized in that the second electronic switching element is connected, by way of a load resistor connected in series with it between the terminals of the alarm, to that terminal of the alarm which is connected to the measuring chamber, and that between the connection points of the reference chamber and of the feedback resistor on the one hand and a connection point situated between the second electronic switching element and the load resistor on the other hand a resistor is connected which has a higher resistance value than the load resistor.

3. A fire alarm according to Claim 2, characterized in that the resistance value of the resistor which is connected between the connection point of the reference chamber and of the feedback resistor, on the one hand, and the connection point situated between the second electronic switching element and the load resistor, on the other hand, is of an order of magnitude equal to that of the feedback resistor.

4. A fire alarm according to either of claims 2 and 3, characterized in that the resistance value of the feedback resistor is greater by at least one order of magnitude than that of the load resistor and lower by at least one order of magnitude than the lowest resistance values of the chambers.

5. A fire alarm according to any one of Claims 2 to 4, characterized in that an additional ohmic resistor is connected in parallel to the series connection of the load resistor and that resistor which is connected between the connection point of the reference chamber and of the feedback resistor on the one hand, and the connection point lying between the second electronic switching element and the load resistor on the other hand.

6. A fire alarm according to any one of the preceding Claims, characterized in that the first electronic switching element is a field effect transistor connected by its control electrode-source path directly in parallel to the measuring chamber.

7. A fire alarm according to any one of Claims 1 to 5, characterized in that the first electronic switching element is a field effect transistor whose source is connected, by way of a zener diode connected in the blocking direction in relation to the current flowing through it when the field effect transistor is conductive, to that electrode of the measuring chamber which is remote from the connection point of the chambers.

8. A fire alarm according to any one of the preceding Claims, characterized in that the first electronic switching element is a self-blocking enhancement field effect transistor.

9. A fire alarm according to any one of the preceding Claims, characterized in that the control electrode of the second electronic

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. elements of the alarm to be accommodated, at least to a predominant extent, as an integrated circuit on a common chip. For example, in the embodiment shown in Figure 3 the feedback resistor 30, the component resistor 36, the field effect transistor 40, the transistor 50, the resistor 48, and the diode 74 can be accommodated on a common chip as an integrated circuit, while the component resistor serving as load resistor should be disposed on the common chip only if good dissipation of heat from the chip is ensured. Finally, it may further be pointed out that the steps by which the potential at the chamber connection point 14 is prevented from exceeding the predetermined threshold value after the supply voltage has been switched on, particularly the selection of the capacitance of the chambers 10, 12 explained in connection with Figure 3, can also advantageously be used for the same purpose in other ionisation fire alarms having measuring and reference chambers connected in series. This applies particularly to the alarms described in the preamble as prior art. WHAT WE CLAIM IS:

1. An ionisation fire alarm having a measuring chamber accessible to the ambient atmosphere, a reference chamber connected in series therewith, a first electronic switching element which is connected by its control electrode to the connection point of the chambers, has a high input impe dance, and at a predetermined threshold value of the potential of the connection point changes its state of conductivity - this switching element preferably being a field effect transistor which is non-conductive in the state of rest, a second electronic switching element, preferably a bipolar transistor, which is adapted to be set to the conductive state by the aforesaid first switching element on the change of the state of conductivity of the latter, a feedback resistor connected between the terminals of the alarm in series with the series connection of the chambers, and a resistor connected in parallel to the series connection of the chambers and forming a voltage divider with the feedback resistor, the second electronic switching element being coupled to the feedback resistor in such a manner that when the second electronic switching element becomes conductive, the potential at the connection point of the chambers is displaced beyond the predetermined threshold value in the direction of positive feedback, characterized in that the feedback resistor lies between the reference chamber and that terminal of the alarm which is connected to the reference chamber, and that the main current flow of the second electronic switching element lies in a current path parallel to the feedback resistor.

switching element is connected directly to a main electrode of the first electronic switching element.

10. A fire alarm according to any one of the preceding Claims, characterized in that the mutual spacings and the dimensions of the electrodes of the measuring chamber and of the reference chamber are in each case selected to be so large that the capacitances of the chambers are of the order of magnitude of the input capacitance of the first electronic switching element.

11. A fire alarm according to any one of the preceding Claims, characterized in that the mutual spacings and the dimensions of the electrodes of the measuring chamber and of the reference chamber are in each case selected to be so large that the capacitance of the reference chamber, mutlipled by the inverse ratio of the resistance values of the measuring chamber and of the reference chamber, is greater than the capacitance of the measuring chamber, preferably by an amount which is approximately as great as the input capacitance of the first electronic switching element.

12. A fire alarm according to any one of the preceding Claims, characterized in that a capacitor is connected in parallel to the series connection of the chambers.

13. A fire alarm according to any one of the preceding Claims, characterized in that an additional terminal connected to the connection point of the reference chamber and of the feedback resistor, is provided for the alarm and for the purpose of first switching element the said terminal is connected to a potential which is at least approximately equal to the potential of that terminal of the alarm which is connected to the feedback resistor.

14. A fire alarm according to Claim 13, characterized in that a diode is connected between the connection point of the reference chamber and feedback resistor on the one hand and the additional terminal of the alarm.

15. A fire alarm according to Claim 1 or according to Claim 1 and any one of Claims 2 to 5 or 8 to 14, characterized in that the cathode of the first electronic switching element is connected to the measuring chamber.

16. A fire alarm according to Claim 15 characterized in that parallel to the resistor connecting the cathode to the terminal connected to the measuring chamber there is connected the main current path of a third electronic switching element which is adapted to be set to the conductive state by the second electronic switching element when the latter becomes conductive.

17. A fire alarm according to Claim 16, characterized in that the third electronic switching element is a transistor, preferably a bipolar transistor.

18. A fire alarm according to either of Claims 16 and 17, characterized in that the control electrode of the third electronic switching element is connected by way of a limiting resistor solely to a main electrode of the second electronic switching element.

19. A fire alarm according to either of Claims 16 and 17, characterized in that the control electrode of the third electronic switching element is connected to a voltage divider which is connected between a main electrode of the second electronic switching element and that terminal of the alarm which is connected to the measuring chamber.

20. A fire alarm according to any of Claims 2 to 5 and according to Claim 19, characterized in that the load resistor is in the form of a voltage divider to which the control electrode of the third electronic switching element is connected.

21. A fire alarm according to any one of Claims 2 to 5 and according to Claim 19, wherein the load resistor is preferably a relay coil, characterized in that the voltage divider is connected in parallel to the load resistor.

22. A fire alarm substantially as hereinbefore described with reference to and as shown in the accompanying drawings.