CA1074025A - High gain sensing and switching means for smoke detectors - Google Patents
High gain sensing and switching means for smoke detectorsInfo
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- CA1074025A CA1074025A CA265,845A CA265845A CA1074025A CA 1074025 A CA1074025 A CA 1074025A CA 265845 A CA265845 A CA 265845A CA 1074025 A CA1074025 A CA 1074025A
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Abstract
ABSTRACT OF THE DISCLOSURE
A smoke detector of the ionization type is provided with high gain sensing and switching means for detecting the presence of airborne products of combustion in a measuring ionization chamber. Sensitive and stable operation is achieved through the use of a field effect transistor in combination with a plurality of cascaded switching element.
A smoke detector of the ionization type is provided with high gain sensing and switching means for detecting the presence of airborne products of combustion in a measuring ionization chamber. Sensitive and stable operation is achieved through the use of a field effect transistor in combination with a plurality of cascaded switching element.
Description
t~
This invention relates to smoke detectors of the ionization type and, more particularly, to high gain sensing and switching apparatu~ for sensing the presence of products of combustion and actuating an alarm when the sensed products of combustion exceed a predetermined level.
A smoke detector of the ionization type includes an alpha radiation source, such as a small quantity of Americium 241, in a measuring chamber having positive and negative electrodes. The measuring chamber is substantially freely accessible to the atmosphere, including airborne products of combustion. The alpha radiation in the measuring chamber ionizes the air between the electrodes, the result being the flow of a small electrical current when voltage is applied across the electrodes. When air-borne products of combustion (smoke) enter the measuring chamber, they reduce the mobility of the ions and thereby increase the resistance of the measuring chamber to the flow of current.
The resulting change in the electrical characteristics of ! the circuit containing the measuring chamber is sensed and used to trigger an alarm when the electrical change reaches a selected level representing a corresponding level of smoke or aerosols within the measuring chamber. The electrical characteristic normally sensed is the change in the voltage across the measuring chamber, the voltage change occurring as a result of the increased chamber resistance due to the presence of visible or invisible products of combustion in the measuring chamber. The sensing or detection apparatus senses this change in voltage and triggers the alarm when the voltage change reaches the selected level.
In modern smoke detectors of the ioni7ation type, low voltage batteries are often used as the power source for both the measuring chamber circuit and the cencing and alarm _ l _ ~)7'~32~ ~
triggering or switching apparatus. As a result, the voltage across the measuring chamber must also be relatively low at all times, and the sensing apparatus must be capable of sensing small changes in the voltage. It is, of course, essential that the sensing and switching apparatus be highly sensitive and reliable in operation so as to provide assurance that an alarm will be given whenever the level of smoke or other combustion aerosols reaches the selected level. In addition, the sensitivity and stability of the system must also minimize the possibility of the sensing and switching apparatus triggering a false alarm when the voltage signal has not reached the selected level. Heretofore, a number of electronic sensing and switching systems have been provided for sensing voltage changes across a measuring chamber and triggering an alarm when the voltage change reaches a selected level. The sensitivity and reliability of many of these systems in detecting smoke conditions without undue false alarming have not been entirely satisfactory.
In battery operated smoke detectors, it is essential that the battery have a relatively long service life, preferably at least one year. Since there must be a continual flow of current through the measuring chamber for operation of the detector, there is a corresponding continual current drain on the battery. If relatively long battery life is to be attained, the normal or continuous current flow through the remaining portions of the smoke detector, including the sensing and triggering apparatus, must be maintained at a relatively low level.
It is a primary object of this invention to provide highly sensitive and reliable sensing and witching apparatus for smoke detectors of the ionization type.
Another object of this invention ic to provide for 1~7~
ionization smoke detectors improved sensing and switching means for sensing small changes in measuring chamber voltage.
Still another object is to provide improved sensing and switching means capable of reliable operation without undue false alarming.
A further object of this invention is to provide improved sensing and switching means capable of highly sensitive and reliable operation in response to voltage signals indicative of the presence of products of combustion without being subject to undue false alarming.
Yet another object is to provide an improved sensing and switching means providing the foregoing objects while being suitable for long term battery operation.
Briefly stated, in carrying out the invention in one form in a smoke detector of the ionization type having an electric circuit including a measuring ionization chamber having an interior substantially freely accessible to air-borne products of combustion, a field effect transistor has its gate and source connected to the circuit for sensing the voltage across the mea~uring chamber when a predetermined substantiall~ fixed voltage is applied across the circuit.
The field effect transistor is selected such that it turns ON
only when the voltage across the measuring chamber is consistent with the presence of at least a predetermined minimum amount of airborne products of combustion within the measuring chamber. A high gain switching means is coupled to the field effect transistor, the switching means including a plurality of cascaded normally OFF
switching devices. The first switching device is coupled with the field effect transistor so as to be turned ON
whenever the field effect transistor is turned ON, and all others of the cascaded switching devices are coupled to the ~7~ ~r- 6D-4637 next preceding switching device 50 as to be turned O~ whenever the next preceding switching device is turned ON. An alarm means is connected in series with the last of the cascaded switching means to produce an alarm signal when the last switching means is turned ON. By turning OM the field effect transistor and the high gain cascaded switching means, the presence of a predetermined minimum amount of airborne products of combustion in the measuring chamber causes the production of an alarm signal.
By a further aspect of the invention, the cascaded switching devices are preferably all solid state elements.
Through appropriate selection of supporting circuit elements, the high gain switching means may comprise a Darlington amplifier or a series of silicon controlled rectifiers (SCR's). By a still further aspect of the invention, the field effect transistor is an enhancement mode MDSFET
transistor.
While the novel features of this invention are ~et forth with particularity in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings in which:
FIG~ 1 is a circuit diagram of a smoke detector in which the high gain switching means includes a pair of cascaded SCR's;
FIG. 2 is a graph illustrating the change in voltage across the measuring chamber of FIG, 1 upon the introduction of combustion products; -FIG, 3 is a c~rcuit diagram of a smoke detector in which the high gain switching means is a Darlington amplifier; and ~ 7~ 6D-4637 FIG. ~ is a circuit diagram of a smoke detector in which the high gain switching means includes a first stage transistor and a second stage SCR.
Referring first to FIG. 1, a smoke detector 10 incorporating the present invention is illustrated. The smoke detector 10 includes a pair of ionization chambers 12 and 14 connected in series across a pair of terminals 16 and 18 to whieh a suitable source of direct current power may be connected. The particular circuit illustrated is designed to be connected to a direct eurrent battery having a voltage in the 10.5 to 12.5 volt range, the positive and negative terminals o the battery being eonnected to the terminals 16 and 18, respectively, as indicated. The chamber 12 is open to the atmosphere and its interior is thus freely accessible to air and airborne products of combustion or aerosols. The chamber 14 is substantially closed and its interior i5 thus not freely accessible to airborne products of combustion. For reasons which will become apparent as this description proceeds, the chamber 12 is a measuring ehamber and the ehamber 14 is a referenee ehamber.
As illustrated, the measuring ehamber12 ineludes a pair of spaeed-apart eleetrodes 20 and 22 and a source 24 of alpha radiation sueh as Amerieium 241 for ionizing the air in the interior spaee between the eleetrodes 20 and 22. As previously explained, an ion eurrent will flow between the eleetrodes 20 and 22 when a voltage is applied thereaeross. If aerosols or produets of eombustion enter the interior spaee of the ehamber 12, the eurrent flow will be redueed if the voltage aeross the eleetrodes is maintained ~-3Q eonstant. In other words, the introduetion of eombustion aerosols inereases the eleetrieal recistanee of the ehamber 12,the amount of resistanee ehange being indicative of the .
~ 6D-4637 amount of combustion products present in the chamber 12. For example, if a constant voltage Vl, as shown by FIG. 2, i9 applied across the measuring chamber 12, an ion current I
will flow when there is no smoke present in the chamber, and an ion current Il will flow when there is 2 percent smoke present in the chamber. The reference chamber 14 includes a pair of spaced-apart electrodes 26 and 28 and a ~ource 20 of alpha radiation such as Americium 241 for ionizing oxygen and nitrogen molecules in the interior space between the electrodes 26 and 28. Since products of com-bustion are effectively barred from entering the interior of the chamber 14, there is substantially only one possible ion current for each voltage applied across the terminals 26 and 28 (under constant ambient atmospheric conditions).
With reference to FIG. 2, it will be seen that the ion current through the reference chamber 14 will be Il if the voltage Vl is applied acro s the terminal~ 26 and 28 at the assumed ambient conditions.
Ionization chambers such as the chambers 12 and 14 have characteristic curves of the type illustrated by FIG. 2. The curve for each chamber has an initial generally linear slope in which there is a substantially direct relation-ship between applied voltage and ion current. When, however, the voltage exceeds a certain level, the chamber becomes saturated and will exhibit substantially constant current through a broad range of applied voltages. The actual configuration of the characteristic curve for a chamber depends on factors such as the voltage gradient within the chamber, the strength of the alpha radiation source, and the other physical characteristics of the chamber. The basic characteristics of the chambers 12 and 14 are illustrated by the curves of FIG. 2, the measuring chamber 12 being in its :
~ 6D-4637 essentially linear condition throughout the indicated voltage range and the reference chamber reaching saturation at relatively low voltages. It is desirable in the circuit arrangement of FIG. 1 that the measuring chamber 12 operate in its linear region and that the reference chamber 14 operate in its saturated region.
Still referring to FIGS. 1 and 2, the chambers 12 and 14 are connected in series across the terminals 16 and 18 such that the substantially fixed voltage VB of a battery connected to the terminals is applied across the circuit comprising the two chambers. Since the reference chamber 14 is intentionally designed to operate in its saturated range, it is clear that a substantially constant ion current Il flows through the chamber 14 at all times. Since the chambers 12 and 14 are connected in series, the same ion current Il will flow at all times through the measuring chamber 12. In the absence of smoke, the voltage drop across the chamber 12 will be V2. Similarly, the voltage drop across the chamber 12 will be V3 when 2 percent smoke is present between its electrodes, and the voltage across the chamber 12 will be V4 when 4 percent smoke is present. It will, of course, be obvious that the voltage across the reference chamber 14 i9 VB-V2 when no smoke is present, V~-V3 at 2 percent smoke, and VB-V4 at 4 percent smoke. It will thus be seen that the voltage at the junction 32 intermediate the chambers 12 and 14 is indicative of the level of airborne products of combustion within the chamber 12. ~he present invention provide~ improved means for sensing this voltage and producing an alarm signal when the voltage is consistent with the presence of a predetermined minimum amount of smoke or the like within the chamber 12. FIGS. 1, 3 and 4 disclose various embodiments of the high gain sensing and switching means of tne invention.
~ o~ .., 6D-4637 With reference to FIG. 1, a ~SFET field effect transistor 34 of the enhancement type has its gate coupled to the junction 32 intermediate th~ chambers 12 and 14. I~e source of the ~SFET 34 is connected to the positive terminal 16, and the drain of the ~)SFET is connected through series resistors 36 and 38 to the negative terminal 18. High gain switching means comprising a pair of cascaded SCR's are coupled to the ~SFET 34 by having the gate of the first SCR 40 connected to the junction 42 between the two series resistors 36 and 38. The cathode of the first SCR 40 is connected both to the gate of the second SCR 44 and through a resistor 46 to the negative terminal 18. The second SCR
is connected in series with a horn assembly 50 across the terminals 16 and 18. A resistor 52 is provided between the anode of the first SCR 40 and the horn assembly 50.
The horn assembly 50 includes a horn represented by a coil 54 in series with the SCR 44 and a pair of normally closed :
contact~ 56 mechanically connected to the horn mechanism for being rapidly opened and closed during sounding of the horn. A resistor 58 and a capacitor 60 are provided in series across the horn coil 54 to prevent large inductive spikes, which could damage other circuit components, from ~:
being generated by the coil when the horn is sounding~ A
capacitor 62 is provided across the terminals 16 and 18 to prevent rapid changes in supply voltage during sounding of the horn 50.
Referring now to FIGS. 1 ~nd 2, the voltage across the measuring chamber 12 and across the source-to-gate of the M:)SFET 34 is V2 when there is no smoke or other airborne products of combustion within the measuring chamber 12, this voltage V2 being less than the threshold voltage of the M~SFET 34. Since the ~SFET 34 is of the enhancement .
, ~ r~ 6~-4637 type, this means that the MDSFET is essentially OFF (not conducting) under these conditions. Since the MDSFET 34 is essentially OFF, there is substantially no current flow ~hrough the resistors 36 and 38 and the junction 42 is maintained at a voltage substantially identical to that of the negative terminal 18. As a result, the first SCR 40 is also maintained in its OFF or non-conductive condition. Since the first SCR 40 is not conducting, the gate of the second SCR 44 is also maintained at the voltage of the negative terminal 18.
~his means that the SCR 44 remains non-conductive and the horn 50 is not sound. It should be noted that all elements of the sensing and switching means are turned OFF under these conditions and thus will place no substantial continuous current drain on a battery connected across the terminals 16 and 18.
If smoke or other combustion products enter the chamber 12, the voltage across the chamber 12 and the source-to-gate of the M~SFET 34 will increase and progre~sively turn on the MDSFET 34. As illustrated by FIG. 2, the voltage across the chamber 12 is V3 at 2 percent smoke. The elements may be selected and adjusted such that a preselected conduction level through the M~SFET 34 i9 reached when the voltage at junction 32 i9 consistent with the presence of at least 2 percent smoke in the chamber 12. In other words, the MDSFET
34 will conduct at the preselected level whenever the smoke concentration within the chamber has reached a preselected value. Through proper selection and adjustment of the components. me MDSFET 34 can be made to reach the preselected level of conduction at any desired minimum amount of smoke concentration. Once the M~SFET 34 reaches the preselected conduction level, current flow through the resistors 36 and 38 causes the voltage at junction 42 to increase sufficiently la ~ 6D-4637 to turn on the first SCR 40. Due to the current flow through the SCR 40 and the resistor 46, the voltage on the gate of the SCR 44 will be sufficient to turn on the SCR 44 and thus sound the horn 50. If the~noke level in the chamber 12 drops below the preselected trigger point, the voltage at the junction 32 will rise, and the source-to-gate voltage on the M~SFET 34 will therefore fall below the level required to maintain the preselected level of conduction through the MDSFET 34 and the resistors 36 and 38. This means that the voltage at junction 42 will also fall and the SCR 40 will turn OFF when its current falls below its holding level (due to periodic opening during horn operation of the normally closed contacts 56). This in turn will cause the second SCR 44 to turn OFF both itself and the horn 50. Although the MDSFET 34 may conduct slightly at smoke levels of less than 2 percent (or other selected trigger point level), the current flow will be too low to raise the voltage at junction 42 sufficiently to turn on the first SCR 40. Stated differently, the MDSFET 34 turns on sufficiently to turn on the SCR 40 and the SCR 44 only when the smoke concentration is at 2 percent (or other selected trigger point level). Accordingly, it may be said for the purposes of this description and the appended :~.
claims that the MDSFET 34 turns O~ only when the voltage it senses is consistent with the predetermined minimum amount of smoke in the chamber 12~ .
To assure long term stability of the solid state ~:
components and reliability of the smoke detection system, it is desirable that the current flow through the MDSFET 34 be kept as low as possible~ This can be accomplished by using :
large resistances 36 and 38 and using a high gain switching .
network in combination with the low current MDSFET 34~ Under ~
.
these circumstances, the irst SCR 40 should have a high gate sensitivity. To assure stability and to protect the SCR 40 against false triggering due to large rapid voltage variations generated by the horn 50 as it sounds, the resistance 52 connected to the anode of the SCR 40 is made relatively large. The SCR 44 is protected in this regard by the resistor 46 across its gate-to-cathode.
By way of example, a smoke detector having the circuitry of FIG. 1 has been built and successfully operated, the detector including a measuring chamber 12 having a 1 microcurie source of Americium 241 and a reference chamber 14 having a 2 microcurie source of Americium 241. The chambers were adjusted to provide a saturation current of 35 picoamperes (35 X 10 ampexes) and a voltage of approximately 3.3 volts across the measuring chamber 12 in the absence of smoke when a battery having a voltage range of 12.5 to 10.5 volts is connected to the terminal~
16 and 18. The actual battery used was a Mallory Model No. 304116 having an initial voltage of 12.3 volts. The chambers 12 and 14 were further adjusted to provide a voltage of 4.3 volts to the gate of the M~SFET 34 when the smoke level in the chamber reaches 2 percent smoke. The M~SFET 34 wa~ a 3 N 163, and the resistors 36 and 38 had resistance values of 27,000 and 15,000 ohms, respectively. The SCR 40 was a C 103 B and the SCR 44 was a C 103 B. The resistances of the resistors 46 and 52 were 1,000 and 6,800 ohms, respectively. The horn 50 included a commercially available horn Model 16003196, available from Delta Electric of Marion, Indiana, in parallel with a 0.01 microfarad capacitor 60 and a 200 ohm resistor 58. The capacitor 62 had a capacitance of 330 microfarads.
It will readily occur to those skilled in the art ~ 6D-4637 that the principles of this invention may be applied in slightly different forms or in similar form with different component values to accommodate different circumstances. For proper utilization of the present invention, however, it is essential that the enhancement mode MOSFET (or its electronic equivalent) and the cascaded switching devices coupled to the MOSFET be OFF except in the event of a smoke condition.
It is also essential that the MOSFET current be kept extremely low during smoke conditions and that the cascaded switching devices provide a high gain switching network. While the smoke detection system illustrated by FIG. 1 is a particularly desirable embodiment of the invention due to the extremely high gain switching characteristics of the SCR's, the invention can, as illustrated by FIGS. 3 and 4, take on different forms in its broader aspects.
In FIG. 3, a single ionization cham~er 70 is provided in series with a resistor 72 across terminals 74 and 76 for connection to an appropriate source of direct current power. If products of combustion enter the measuring chamber 70,its resistance will increase, the result being both a reduction in the ion current flow through the circuit and an increase in the voltage across both the chamber 70 and the source-to-gate of a MOSFET 80. At a predetermined minimum level of smo~e in the chamber 70, the voltage at the junction 82 will drop sufficiently to turn ON the enhancement mode MDSFET 80. Conduction through the M~SFET
80 and its series resistor 82 will increase the voltage at junction 84 sufficiently to turn ON an NPN transistor 86 through an input resistance 88 connected to the gate of the transistor 86. The transistor 86 is coupled with a second NPN transistor 90 in an extremely high gain Darlington arrangement. Once the transistor 90 conducts, a horn 92 substantially identical to the horn assembly 50 - . . . ..
i V 7~ d 6 D--4 6 3 7 (FIG. 1) is turned ON to sound an alarm. Appropriate circuitry (not shown) may be provided to assure proper turn off of the transistors 86 and 90 and the horn 92 once the smoke level in the measuring chamber 70 drops below the predetermined minimum level. A capacitor 94 is provided across the terminals 74 and 76 to prevent rapid variations in supply voltage during horn operation.
In the arrangement of FIG. 3, it is essential that the Darlington arrangement of transistors 86 and 90 have a high gain characteristic in order to minimize the possibility that a borderline smoke condition will result in a partial turn ON of the transistor 90 and resulting current flow through the horn 92 and the transistor 90 at a level insufficient to actually sound the horn, but sufficient to rapidly deplete a battery connected to the terminals 74 and 76.
FIG. 4 discloses a smoke detector particularly suitable for use with a source of direct current having a voltage higher than desired for operation of the ionization chamber 102 and 104 and the MDSFET 106. This arrangement is particularly desirable for use with a source of direct current power derived from rectified alternating current. }n such a case, a voltage of approximately 20 volts might be applied to the terminals 108 and 110. A capacitor 130 across the terminals 108 and 110 smooths out the supply voltage, and a zener diode 116 is provided to maintain a desired lower substantially fixed voltage across the ionization circuit comprising the two chambers 102 and 104 and the circuit comprising the M~SFET 106 and the resistors 112 and 114. A resistor 118 limits the current flow through the zener diode 116. The smoke detector 100 operates in a manner similar to that of the smoke detector 10 of FIG. 1. At a predetermined minimum level of airborne products of combustion .
~07'~Z.i..~- 6D-4637 in the measuring chamber 102, the threshold voltage of the MDSFET 106 will be reached, and the MDSFET 106 will begin to conduct. The resulting increase in voltage at the junction 120 will turn ON the NPN transistor 122, which in turn will turn ON the SCR 124. Horn 126 connected in series with the SCR 124, which is substantially identical with the horn 50 of FIG. 1, Will sound when the SCR 124 conducts. Again, it is essential that the current through the MDSFET 106 be kept extremely low for reliable long term operation and that the switching means have a high gain characteristic. It is also essential that the various elements be stable in operation;
this can be attained through proper selection of the switching devices and the resistors 112, 114, 132, and 134.
In each of the described embodiments, a MDSFET of the enhancement type is utilized for sensing the voltage at a predetermined point in the ionization circuit. This type of element is particularly de~irable for use in the present invention in that its threshold voltage must be reached before it conducts. Those skilled in the art will appreciate that various components can be arranged to operate in the manner of an enhancement mode MDSFET. It is therefore not intended that the invention be restricted to the use of a MDSFET as the sensing element. It should also be recognized that any number of cascaded switching elements may be used in the high gain switching network, with each switching device turning ON in response to the next preceding switching device turning ON. ~t should also be appreciated that the alarm used may take on different forms, such as a horn, flashing lights, etc.
From the foregoing, it will be seen that this invention provides highly sensitive and reliable sensing and switching apparatus for smoke detectors of the ionization ,, ,. '' ' .. . ' '' ' ' ' ' :' ~ 2~ 6D-4637 type. This is accomplished by sensing small changes of measuring chamber voltage and sounding an alarm without undue false alarming due to system instability. The invention provides the foregoing while being suitable for l~ng term battery operation.
While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form, details, and application may be made therein without departing from the spirit and scope of the invention. Accordingly, it is intended that all such modifications and changes be included within the scope of the appended claims.
-.
' : :. , .
This invention relates to smoke detectors of the ionization type and, more particularly, to high gain sensing and switching apparatu~ for sensing the presence of products of combustion and actuating an alarm when the sensed products of combustion exceed a predetermined level.
A smoke detector of the ionization type includes an alpha radiation source, such as a small quantity of Americium 241, in a measuring chamber having positive and negative electrodes. The measuring chamber is substantially freely accessible to the atmosphere, including airborne products of combustion. The alpha radiation in the measuring chamber ionizes the air between the electrodes, the result being the flow of a small electrical current when voltage is applied across the electrodes. When air-borne products of combustion (smoke) enter the measuring chamber, they reduce the mobility of the ions and thereby increase the resistance of the measuring chamber to the flow of current.
The resulting change in the electrical characteristics of ! the circuit containing the measuring chamber is sensed and used to trigger an alarm when the electrical change reaches a selected level representing a corresponding level of smoke or aerosols within the measuring chamber. The electrical characteristic normally sensed is the change in the voltage across the measuring chamber, the voltage change occurring as a result of the increased chamber resistance due to the presence of visible or invisible products of combustion in the measuring chamber. The sensing or detection apparatus senses this change in voltage and triggers the alarm when the voltage change reaches the selected level.
In modern smoke detectors of the ioni7ation type, low voltage batteries are often used as the power source for both the measuring chamber circuit and the cencing and alarm _ l _ ~)7'~32~ ~
triggering or switching apparatus. As a result, the voltage across the measuring chamber must also be relatively low at all times, and the sensing apparatus must be capable of sensing small changes in the voltage. It is, of course, essential that the sensing and switching apparatus be highly sensitive and reliable in operation so as to provide assurance that an alarm will be given whenever the level of smoke or other combustion aerosols reaches the selected level. In addition, the sensitivity and stability of the system must also minimize the possibility of the sensing and switching apparatus triggering a false alarm when the voltage signal has not reached the selected level. Heretofore, a number of electronic sensing and switching systems have been provided for sensing voltage changes across a measuring chamber and triggering an alarm when the voltage change reaches a selected level. The sensitivity and reliability of many of these systems in detecting smoke conditions without undue false alarming have not been entirely satisfactory.
In battery operated smoke detectors, it is essential that the battery have a relatively long service life, preferably at least one year. Since there must be a continual flow of current through the measuring chamber for operation of the detector, there is a corresponding continual current drain on the battery. If relatively long battery life is to be attained, the normal or continuous current flow through the remaining portions of the smoke detector, including the sensing and triggering apparatus, must be maintained at a relatively low level.
It is a primary object of this invention to provide highly sensitive and reliable sensing and witching apparatus for smoke detectors of the ionization type.
Another object of this invention ic to provide for 1~7~
ionization smoke detectors improved sensing and switching means for sensing small changes in measuring chamber voltage.
Still another object is to provide improved sensing and switching means capable of reliable operation without undue false alarming.
A further object of this invention is to provide improved sensing and switching means capable of highly sensitive and reliable operation in response to voltage signals indicative of the presence of products of combustion without being subject to undue false alarming.
Yet another object is to provide an improved sensing and switching means providing the foregoing objects while being suitable for long term battery operation.
Briefly stated, in carrying out the invention in one form in a smoke detector of the ionization type having an electric circuit including a measuring ionization chamber having an interior substantially freely accessible to air-borne products of combustion, a field effect transistor has its gate and source connected to the circuit for sensing the voltage across the mea~uring chamber when a predetermined substantiall~ fixed voltage is applied across the circuit.
The field effect transistor is selected such that it turns ON
only when the voltage across the measuring chamber is consistent with the presence of at least a predetermined minimum amount of airborne products of combustion within the measuring chamber. A high gain switching means is coupled to the field effect transistor, the switching means including a plurality of cascaded normally OFF
switching devices. The first switching device is coupled with the field effect transistor so as to be turned ON
whenever the field effect transistor is turned ON, and all others of the cascaded switching devices are coupled to the ~7~ ~r- 6D-4637 next preceding switching device 50 as to be turned O~ whenever the next preceding switching device is turned ON. An alarm means is connected in series with the last of the cascaded switching means to produce an alarm signal when the last switching means is turned ON. By turning OM the field effect transistor and the high gain cascaded switching means, the presence of a predetermined minimum amount of airborne products of combustion in the measuring chamber causes the production of an alarm signal.
By a further aspect of the invention, the cascaded switching devices are preferably all solid state elements.
Through appropriate selection of supporting circuit elements, the high gain switching means may comprise a Darlington amplifier or a series of silicon controlled rectifiers (SCR's). By a still further aspect of the invention, the field effect transistor is an enhancement mode MDSFET
transistor.
While the novel features of this invention are ~et forth with particularity in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings in which:
FIG~ 1 is a circuit diagram of a smoke detector in which the high gain switching means includes a pair of cascaded SCR's;
FIG. 2 is a graph illustrating the change in voltage across the measuring chamber of FIG, 1 upon the introduction of combustion products; -FIG, 3 is a c~rcuit diagram of a smoke detector in which the high gain switching means is a Darlington amplifier; and ~ 7~ 6D-4637 FIG. ~ is a circuit diagram of a smoke detector in which the high gain switching means includes a first stage transistor and a second stage SCR.
Referring first to FIG. 1, a smoke detector 10 incorporating the present invention is illustrated. The smoke detector 10 includes a pair of ionization chambers 12 and 14 connected in series across a pair of terminals 16 and 18 to whieh a suitable source of direct current power may be connected. The particular circuit illustrated is designed to be connected to a direct eurrent battery having a voltage in the 10.5 to 12.5 volt range, the positive and negative terminals o the battery being eonnected to the terminals 16 and 18, respectively, as indicated. The chamber 12 is open to the atmosphere and its interior is thus freely accessible to air and airborne products of combustion or aerosols. The chamber 14 is substantially closed and its interior i5 thus not freely accessible to airborne products of combustion. For reasons which will become apparent as this description proceeds, the chamber 12 is a measuring ehamber and the ehamber 14 is a referenee ehamber.
As illustrated, the measuring ehamber12 ineludes a pair of spaeed-apart eleetrodes 20 and 22 and a source 24 of alpha radiation sueh as Amerieium 241 for ionizing the air in the interior spaee between the eleetrodes 20 and 22. As previously explained, an ion eurrent will flow between the eleetrodes 20 and 22 when a voltage is applied thereaeross. If aerosols or produets of eombustion enter the interior spaee of the ehamber 12, the eurrent flow will be redueed if the voltage aeross the eleetrodes is maintained ~-3Q eonstant. In other words, the introduetion of eombustion aerosols inereases the eleetrieal recistanee of the ehamber 12,the amount of resistanee ehange being indicative of the .
~ 6D-4637 amount of combustion products present in the chamber 12. For example, if a constant voltage Vl, as shown by FIG. 2, i9 applied across the measuring chamber 12, an ion current I
will flow when there is no smoke present in the chamber, and an ion current Il will flow when there is 2 percent smoke present in the chamber. The reference chamber 14 includes a pair of spaced-apart electrodes 26 and 28 and a ~ource 20 of alpha radiation such as Americium 241 for ionizing oxygen and nitrogen molecules in the interior space between the electrodes 26 and 28. Since products of com-bustion are effectively barred from entering the interior of the chamber 14, there is substantially only one possible ion current for each voltage applied across the terminals 26 and 28 (under constant ambient atmospheric conditions).
With reference to FIG. 2, it will be seen that the ion current through the reference chamber 14 will be Il if the voltage Vl is applied acro s the terminal~ 26 and 28 at the assumed ambient conditions.
Ionization chambers such as the chambers 12 and 14 have characteristic curves of the type illustrated by FIG. 2. The curve for each chamber has an initial generally linear slope in which there is a substantially direct relation-ship between applied voltage and ion current. When, however, the voltage exceeds a certain level, the chamber becomes saturated and will exhibit substantially constant current through a broad range of applied voltages. The actual configuration of the characteristic curve for a chamber depends on factors such as the voltage gradient within the chamber, the strength of the alpha radiation source, and the other physical characteristics of the chamber. The basic characteristics of the chambers 12 and 14 are illustrated by the curves of FIG. 2, the measuring chamber 12 being in its :
~ 6D-4637 essentially linear condition throughout the indicated voltage range and the reference chamber reaching saturation at relatively low voltages. It is desirable in the circuit arrangement of FIG. 1 that the measuring chamber 12 operate in its linear region and that the reference chamber 14 operate in its saturated region.
Still referring to FIGS. 1 and 2, the chambers 12 and 14 are connected in series across the terminals 16 and 18 such that the substantially fixed voltage VB of a battery connected to the terminals is applied across the circuit comprising the two chambers. Since the reference chamber 14 is intentionally designed to operate in its saturated range, it is clear that a substantially constant ion current Il flows through the chamber 14 at all times. Since the chambers 12 and 14 are connected in series, the same ion current Il will flow at all times through the measuring chamber 12. In the absence of smoke, the voltage drop across the chamber 12 will be V2. Similarly, the voltage drop across the chamber 12 will be V3 when 2 percent smoke is present between its electrodes, and the voltage across the chamber 12 will be V4 when 4 percent smoke is present. It will, of course, be obvious that the voltage across the reference chamber 14 i9 VB-V2 when no smoke is present, V~-V3 at 2 percent smoke, and VB-V4 at 4 percent smoke. It will thus be seen that the voltage at the junction 32 intermediate the chambers 12 and 14 is indicative of the level of airborne products of combustion within the chamber 12. ~he present invention provide~ improved means for sensing this voltage and producing an alarm signal when the voltage is consistent with the presence of a predetermined minimum amount of smoke or the like within the chamber 12. FIGS. 1, 3 and 4 disclose various embodiments of the high gain sensing and switching means of tne invention.
~ o~ .., 6D-4637 With reference to FIG. 1, a ~SFET field effect transistor 34 of the enhancement type has its gate coupled to the junction 32 intermediate th~ chambers 12 and 14. I~e source of the ~SFET 34 is connected to the positive terminal 16, and the drain of the ~)SFET is connected through series resistors 36 and 38 to the negative terminal 18. High gain switching means comprising a pair of cascaded SCR's are coupled to the ~SFET 34 by having the gate of the first SCR 40 connected to the junction 42 between the two series resistors 36 and 38. The cathode of the first SCR 40 is connected both to the gate of the second SCR 44 and through a resistor 46 to the negative terminal 18. The second SCR
is connected in series with a horn assembly 50 across the terminals 16 and 18. A resistor 52 is provided between the anode of the first SCR 40 and the horn assembly 50.
The horn assembly 50 includes a horn represented by a coil 54 in series with the SCR 44 and a pair of normally closed :
contact~ 56 mechanically connected to the horn mechanism for being rapidly opened and closed during sounding of the horn. A resistor 58 and a capacitor 60 are provided in series across the horn coil 54 to prevent large inductive spikes, which could damage other circuit components, from ~:
being generated by the coil when the horn is sounding~ A
capacitor 62 is provided across the terminals 16 and 18 to prevent rapid changes in supply voltage during sounding of the horn 50.
Referring now to FIGS. 1 ~nd 2, the voltage across the measuring chamber 12 and across the source-to-gate of the M:)SFET 34 is V2 when there is no smoke or other airborne products of combustion within the measuring chamber 12, this voltage V2 being less than the threshold voltage of the M~SFET 34. Since the ~SFET 34 is of the enhancement .
, ~ r~ 6~-4637 type, this means that the MDSFET is essentially OFF (not conducting) under these conditions. Since the MDSFET 34 is essentially OFF, there is substantially no current flow ~hrough the resistors 36 and 38 and the junction 42 is maintained at a voltage substantially identical to that of the negative terminal 18. As a result, the first SCR 40 is also maintained in its OFF or non-conductive condition. Since the first SCR 40 is not conducting, the gate of the second SCR 44 is also maintained at the voltage of the negative terminal 18.
~his means that the SCR 44 remains non-conductive and the horn 50 is not sound. It should be noted that all elements of the sensing and switching means are turned OFF under these conditions and thus will place no substantial continuous current drain on a battery connected across the terminals 16 and 18.
If smoke or other combustion products enter the chamber 12, the voltage across the chamber 12 and the source-to-gate of the M~SFET 34 will increase and progre~sively turn on the MDSFET 34. As illustrated by FIG. 2, the voltage across the chamber 12 is V3 at 2 percent smoke. The elements may be selected and adjusted such that a preselected conduction level through the M~SFET 34 i9 reached when the voltage at junction 32 i9 consistent with the presence of at least 2 percent smoke in the chamber 12. In other words, the MDSFET
34 will conduct at the preselected level whenever the smoke concentration within the chamber has reached a preselected value. Through proper selection and adjustment of the components. me MDSFET 34 can be made to reach the preselected level of conduction at any desired minimum amount of smoke concentration. Once the M~SFET 34 reaches the preselected conduction level, current flow through the resistors 36 and 38 causes the voltage at junction 42 to increase sufficiently la ~ 6D-4637 to turn on the first SCR 40. Due to the current flow through the SCR 40 and the resistor 46, the voltage on the gate of the SCR 44 will be sufficient to turn on the SCR 44 and thus sound the horn 50. If the~noke level in the chamber 12 drops below the preselected trigger point, the voltage at the junction 32 will rise, and the source-to-gate voltage on the M~SFET 34 will therefore fall below the level required to maintain the preselected level of conduction through the MDSFET 34 and the resistors 36 and 38. This means that the voltage at junction 42 will also fall and the SCR 40 will turn OFF when its current falls below its holding level (due to periodic opening during horn operation of the normally closed contacts 56). This in turn will cause the second SCR 44 to turn OFF both itself and the horn 50. Although the MDSFET 34 may conduct slightly at smoke levels of less than 2 percent (or other selected trigger point level), the current flow will be too low to raise the voltage at junction 42 sufficiently to turn on the first SCR 40. Stated differently, the MDSFET 34 turns on sufficiently to turn on the SCR 40 and the SCR 44 only when the smoke concentration is at 2 percent (or other selected trigger point level). Accordingly, it may be said for the purposes of this description and the appended :~.
claims that the MDSFET 34 turns O~ only when the voltage it senses is consistent with the predetermined minimum amount of smoke in the chamber 12~ .
To assure long term stability of the solid state ~:
components and reliability of the smoke detection system, it is desirable that the current flow through the MDSFET 34 be kept as low as possible~ This can be accomplished by using :
large resistances 36 and 38 and using a high gain switching .
network in combination with the low current MDSFET 34~ Under ~
.
these circumstances, the irst SCR 40 should have a high gate sensitivity. To assure stability and to protect the SCR 40 against false triggering due to large rapid voltage variations generated by the horn 50 as it sounds, the resistance 52 connected to the anode of the SCR 40 is made relatively large. The SCR 44 is protected in this regard by the resistor 46 across its gate-to-cathode.
By way of example, a smoke detector having the circuitry of FIG. 1 has been built and successfully operated, the detector including a measuring chamber 12 having a 1 microcurie source of Americium 241 and a reference chamber 14 having a 2 microcurie source of Americium 241. The chambers were adjusted to provide a saturation current of 35 picoamperes (35 X 10 ampexes) and a voltage of approximately 3.3 volts across the measuring chamber 12 in the absence of smoke when a battery having a voltage range of 12.5 to 10.5 volts is connected to the terminal~
16 and 18. The actual battery used was a Mallory Model No. 304116 having an initial voltage of 12.3 volts. The chambers 12 and 14 were further adjusted to provide a voltage of 4.3 volts to the gate of the M~SFET 34 when the smoke level in the chamber reaches 2 percent smoke. The M~SFET 34 wa~ a 3 N 163, and the resistors 36 and 38 had resistance values of 27,000 and 15,000 ohms, respectively. The SCR 40 was a C 103 B and the SCR 44 was a C 103 B. The resistances of the resistors 46 and 52 were 1,000 and 6,800 ohms, respectively. The horn 50 included a commercially available horn Model 16003196, available from Delta Electric of Marion, Indiana, in parallel with a 0.01 microfarad capacitor 60 and a 200 ohm resistor 58. The capacitor 62 had a capacitance of 330 microfarads.
It will readily occur to those skilled in the art ~ 6D-4637 that the principles of this invention may be applied in slightly different forms or in similar form with different component values to accommodate different circumstances. For proper utilization of the present invention, however, it is essential that the enhancement mode MOSFET (or its electronic equivalent) and the cascaded switching devices coupled to the MOSFET be OFF except in the event of a smoke condition.
It is also essential that the MOSFET current be kept extremely low during smoke conditions and that the cascaded switching devices provide a high gain switching network. While the smoke detection system illustrated by FIG. 1 is a particularly desirable embodiment of the invention due to the extremely high gain switching characteristics of the SCR's, the invention can, as illustrated by FIGS. 3 and 4, take on different forms in its broader aspects.
In FIG. 3, a single ionization cham~er 70 is provided in series with a resistor 72 across terminals 74 and 76 for connection to an appropriate source of direct current power. If products of combustion enter the measuring chamber 70,its resistance will increase, the result being both a reduction in the ion current flow through the circuit and an increase in the voltage across both the chamber 70 and the source-to-gate of a MOSFET 80. At a predetermined minimum level of smo~e in the chamber 70, the voltage at the junction 82 will drop sufficiently to turn ON the enhancement mode MDSFET 80. Conduction through the M~SFET
80 and its series resistor 82 will increase the voltage at junction 84 sufficiently to turn ON an NPN transistor 86 through an input resistance 88 connected to the gate of the transistor 86. The transistor 86 is coupled with a second NPN transistor 90 in an extremely high gain Darlington arrangement. Once the transistor 90 conducts, a horn 92 substantially identical to the horn assembly 50 - . . . ..
i V 7~ d 6 D--4 6 3 7 (FIG. 1) is turned ON to sound an alarm. Appropriate circuitry (not shown) may be provided to assure proper turn off of the transistors 86 and 90 and the horn 92 once the smoke level in the measuring chamber 70 drops below the predetermined minimum level. A capacitor 94 is provided across the terminals 74 and 76 to prevent rapid variations in supply voltage during horn operation.
In the arrangement of FIG. 3, it is essential that the Darlington arrangement of transistors 86 and 90 have a high gain characteristic in order to minimize the possibility that a borderline smoke condition will result in a partial turn ON of the transistor 90 and resulting current flow through the horn 92 and the transistor 90 at a level insufficient to actually sound the horn, but sufficient to rapidly deplete a battery connected to the terminals 74 and 76.
FIG. 4 discloses a smoke detector particularly suitable for use with a source of direct current having a voltage higher than desired for operation of the ionization chamber 102 and 104 and the MDSFET 106. This arrangement is particularly desirable for use with a source of direct current power derived from rectified alternating current. }n such a case, a voltage of approximately 20 volts might be applied to the terminals 108 and 110. A capacitor 130 across the terminals 108 and 110 smooths out the supply voltage, and a zener diode 116 is provided to maintain a desired lower substantially fixed voltage across the ionization circuit comprising the two chambers 102 and 104 and the circuit comprising the M~SFET 106 and the resistors 112 and 114. A resistor 118 limits the current flow through the zener diode 116. The smoke detector 100 operates in a manner similar to that of the smoke detector 10 of FIG. 1. At a predetermined minimum level of airborne products of combustion .
~07'~Z.i..~- 6D-4637 in the measuring chamber 102, the threshold voltage of the MDSFET 106 will be reached, and the MDSFET 106 will begin to conduct. The resulting increase in voltage at the junction 120 will turn ON the NPN transistor 122, which in turn will turn ON the SCR 124. Horn 126 connected in series with the SCR 124, which is substantially identical with the horn 50 of FIG. 1, Will sound when the SCR 124 conducts. Again, it is essential that the current through the MDSFET 106 be kept extremely low for reliable long term operation and that the switching means have a high gain characteristic. It is also essential that the various elements be stable in operation;
this can be attained through proper selection of the switching devices and the resistors 112, 114, 132, and 134.
In each of the described embodiments, a MDSFET of the enhancement type is utilized for sensing the voltage at a predetermined point in the ionization circuit. This type of element is particularly de~irable for use in the present invention in that its threshold voltage must be reached before it conducts. Those skilled in the art will appreciate that various components can be arranged to operate in the manner of an enhancement mode MDSFET. It is therefore not intended that the invention be restricted to the use of a MDSFET as the sensing element. It should also be recognized that any number of cascaded switching elements may be used in the high gain switching network, with each switching device turning ON in response to the next preceding switching device turning ON. ~t should also be appreciated that the alarm used may take on different forms, such as a horn, flashing lights, etc.
From the foregoing, it will be seen that this invention provides highly sensitive and reliable sensing and switching apparatus for smoke detectors of the ionization ,, ,. '' ' .. . ' '' ' ' ' ' :' ~ 2~ 6D-4637 type. This is accomplished by sensing small changes of measuring chamber voltage and sounding an alarm without undue false alarming due to system instability. The invention provides the foregoing while being suitable for l~ng term battery operation.
While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form, details, and application may be made therein without departing from the spirit and scope of the invention. Accordingly, it is intended that all such modifications and changes be included within the scope of the appended claims.
-.
' : :. , .
Claims (7)
1. In a smoke detector of the ionization type including a measuring chamber having an interior substantially freely accessible to airborne products of combustion, a reference chamber having an interior subject to gradual changes in ambient air conditions and substantially inaccessible to airborne products of combustion, circuit means connecting said measuring and reference chambers in series, and alarm means for producing an alarm signal; improved high gain sensing and switching means comprising:
a field effect transistor of the enhancement mode type having its gate and source connected across said measuring chamber for sensing the voltage across said measuring chamber when a predetermined substantially fixed voltage is applied across the circuit means, said field effect transistor turning ON only when the voltage sensed is in excess of the threshold voltage of said field effect transistor and is consistent with the presence of at least a predetermined minimum amount of airborne products of combustion within the measuring chamber, and a plurality of cascaded normally OFF switching devices, the first of said cascaded switching devices being coupled to said field effect transsistor so as to be turned ON
whenever said field effect transistor is turned ON, and all others of said cascaded switching devices being coupled to the next preceding switching device so as to be turned ON whenever said next preceding switching device is turned ON, said last of said cascaded switching devices being coupled to the alarm means to turn ON the alarm means to produce an alarm signal when said last switching device is turned ON, whereby an alarm signal may be produced in response to the presence within the measuring chamber of at least the predeter-mined minimum amount of airborne products of combustion.
a field effect transistor of the enhancement mode type having its gate and source connected across said measuring chamber for sensing the voltage across said measuring chamber when a predetermined substantially fixed voltage is applied across the circuit means, said field effect transistor turning ON only when the voltage sensed is in excess of the threshold voltage of said field effect transistor and is consistent with the presence of at least a predetermined minimum amount of airborne products of combustion within the measuring chamber, and a plurality of cascaded normally OFF switching devices, the first of said cascaded switching devices being coupled to said field effect transsistor so as to be turned ON
whenever said field effect transistor is turned ON, and all others of said cascaded switching devices being coupled to the next preceding switching device so as to be turned ON whenever said next preceding switching device is turned ON, said last of said cascaded switching devices being coupled to the alarm means to turn ON the alarm means to produce an alarm signal when said last switching device is turned ON, whereby an alarm signal may be produced in response to the presence within the measuring chamber of at least the predeter-mined minimum amount of airborne products of combustion.
2. High gain sensing and switching means for smoke detectors as defined by claim 1, in which said cascaded switching devices comprise only first and second transistors, said first transistor being coupled to said field effect transistor and said second transistor being connected in series with the alarm means.
3. High gain sensing and switching means for smoke detectors as defined by claim 1, in which said cascaded switching devices comprise only first and second SCR's, said first SCR
being coupled to said field effect transistor and said second SCR
being connected in series with the alarm means.
being coupled to said field effect transistor and said second SCR
being connected in series with the alarm means.
4. High gain sensing and switching means for smoke detectors as defined by claim 1 in which said last switching device is an SCR.
5. High gain sensing and switching means for smoke detectors as defined by claim 1 in which each of said switching devices is an SCR.
6. High gain sensing and switching means for smoke detectors as defined by claim 1 in which said plurality of cascaded switching devices comprises a Darlington amplifier.
7. High gain sensing and switching means for smoke detectors as defined by claim 1 in which said cascaded switching devices comprise a transistor and an SCR, said transistor being coupled to said field effect transistor and said SCR being connected in series with the alarm means.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA265,845A CA1074025A (en) | 1976-11-16 | 1976-11-16 | High gain sensing and switching means for smoke detectors |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA265,845A CA1074025A (en) | 1976-11-16 | 1976-11-16 | High gain sensing and switching means for smoke detectors |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1074025A true CA1074025A (en) | 1980-03-18 |
Family
ID=4107299
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA265,845A Expired CA1074025A (en) | 1976-11-16 | 1976-11-16 | High gain sensing and switching means for smoke detectors |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1074025A (en) |
-
1976
- 1976-11-16 CA CA265,845A patent/CA1074025A/en not_active Expired
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