GB1598821A - Ionization detectors - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO IONIZATION DETECTORS (71) We, THE PLESSEY COMPANY LIMITED. a British Company of 2/60 Vicarage Lane, Ilford, Essex. do hereby declare the invention, for which we pray that a patent may be granted to us, and the method bv which it is to be performed, to be particularly described in and by the following statement: This invention relates to ionization smoke detectors.

Ionization smoke detectors include a detector having two chambers, i.e. a reference chamber and a sensing chamber. The sensing chamber is usually enclosed by metal gauze or a perforated metal wall to facilitate the ingress of smoke, whereas the reference chamber although normally vented to atmosphere is not vented so as to permit of ready ingress of the surrounding atmosphere. The chambers are arranged in series and separated by a conductive screen. When a potential is applied across the chambers a potential is measurable on the screen between the chambers which is determined largely by the atmosphere in the reference chamber as compared with the atmosphere in the sensing chamber since the potential on the screen is due to a current which flows through the serially connected chambers due to ion drift within them.

In these ionization smoke detectors it is desirable to provide a visual signal indicating that the relationship between the screen voltage between the chambers and the reference voltage is normal. This can be achieved bv means of a low frequency oscillator, the frequency of which is dependent upon the relationship between the voltage on the screen and the reference voltage, the oscillator being used to switch an LED to produce a light which flashes at the oscillator frequency. In this way changes in oscillator frequency indicative of the presence of smoke or circuit malfunction may be detected at a glance.

The present invention is directed to the provision of an ionization smoke detector embodying an oscillator which is more reliable in operation than certain known oscillators which under some operating conditions can cease to function as will hereinafter be explained.

According to the present invention an ionization smoke detector comprises a reference chamber and a sensing chamber having a conductive screen between them, and comparator means including an oscillator, said oscillator comprising a first capacitor arranged to be charged progressively towards a potential dependent upon the screen voltage through a resistor to a predetermined charge level at which first switch means operates to effect discharge of the capacitor, the predetermined level at which the first switch means operates being determined by a reference voltage applied to the switch means from reference voltage producing means and the capacitor having connected across it second switch means which is connected to receive a voltage developed across a load in series with said first switch means so that the second switch means conducts in dependence upon current through the load thereby to provide a further discharge path for the capacitor which is maintained after the first switch means has restored to a non-operated state, in which the load of the oscillator comprises alarm signal means operable in dependence upon the frequency of the oscillator.

The present invention will now be described with reference to the accompanying drawing in which: Figure I is a circuit diagram of a known form of relaxation oscillator using a programmable unijunction device; Figure 2 is a circuit diagram of another known form of relaxation oscillator using a PNP/NPN transistor configuration equivalent to the programmable unit junction device of Figure ]j;it diagram of Figure 3 is a circuit diagram of a relaxation oscillator of an ionization smoke detector according to the present invention; and, Figure 4 is a generally schematic blocks circuit diagram of the ionization smoke detector in which the oscillator of Figure 3 is embodied.

Referring now to Figure 1 which shows a programmable unij unction oscillator circuit of known form, the oscillator includes resistors Rl, R2, R3 and RL, a programmable unijunction device PUJ and a capacitor Cl.

In operation, the capacitor Cl charges through the resistor R1 until a voltage level is reached at which the programmable unijunction device conducts through resistor RL. The level at which conduction occurs is determined by the reference voltage VREF at the junction between the resistors R2 and R3. Figure 2 shows a similar circuit of known form wherein corresponding parts bear the same designations, but wherein a pair of complementary transistors TRl and TR2 are substituted for the programmable unijunction device PUJ. The circuits as shown in Figure 1 and Figure 2 are of great use where a very slow pulsing circuit is needed. In addition they start reliably even with very slow rates of rise of application of supply voltage where an astable multivibrator may sometimes fail to start.These circuits do however present problems with extreme combinations of circuit values in that if the charging current through R1 exceeds the holding current of the programmable unijunction or transistor combination TRI, TR2, then only the initial pulse occurs, after which the circuit stays in the 'bottomed' state and does not recover until switched off. The circuit as shown in Figure 2 is particularly prone to this problem.

In Figure 3 there is shown a relaxation oscillator which forms part of a relaxation oscillator which forms part of an ionization smoke detector which comprises additional circuit components included within the broken line 1. Where the circuit of Figure 3 is similar to the circuit of Figure 2 the same designations have been used. The additional circuit components comprise a transistor TR3 having a load R5. The base of the transistor TR3 is fed to a resistor capacitor combination R4, C2, the capacitor being charged via a diode D1 from the load of transistor TR2 which in this case is a light emitting diode 2.

The operation of the circuit is as follows: At switch-on capacitor C1 charges up as a result of the current through resistor R1.

Initially transistor TR1 is cut off since its emitter potential is more negative than VREF which is the voltage which obtains at the junction of resistors R2 and R3. As the voltage on capacitor C1 rises past the level of VREF, the emitter of transistor TRl becomes positive with respect to VREF, and transistor TRl becomes conductive. Due to the cross coupling to transistor TR2, transistor TR2 also becomes conductive in a regenerative mode with transistor TR1, and capacitor C1 is rapidly discharged through the light emitting diode 2 to cause the diode to flash.If now the current through resistor R1 exceeds the current at which the current gain of the cross-coupled pair of transistors TRl and TR, is adequate to maintain the latched state of the circuit, then the operation will cease at that point. With good quality transistors the holding current needed to preserve the latch state may well be less than 1RA. The additional components diode Dl, capacitor C2, resistor R4, transistor TR3 and resistor R5 have the effect of extending the time of the discharge cycle sufficiently to further discharge capacitor C1 to a voltage below that at which the latch can remain on, that is to say a voltage lower than required to forward bias the base emitter junctions of transistors TR1 and TR2 to a sufficient state to maintain adequate conduction.

As transistors TR, and TR2 turn into a latched state the voltage across the LED 2 rises rapidly to some 2 volts (it may be higher if the load is resistive or inductive instead of an LED). This level of voltage causes the capacitor C2 to charge through diode Dl. The energy stored in this capacitor immediately starts to discharge through resistor R4 into the base-emitter circuit of TR3. As a result TR3 switches on and adds to the discharge of capacitor C1 by the current drain through resistor R5.

By choice of suitable values for the discharge time constant of capacitor C2 and resistor R4 the conduction can be maintained for a sufficient time to discharge component C1 below the switch-off voltage of the latch. The resistor R5 is necessary to prevent excessive current through transistor TR3 by-passing the discharge current from capacitor C1 into the load (LED). Diode Dl prevents capacitor C2 discharging through the load as the main discharge comes to an end and thus ensures that energy remains in capacitor C. to continue the discharge of capacitor C1 after the latch transistors TRI and TR2 start to switch off.

The percentage of energy taken from the load by the charging of capacitor C2 and the discharge of capacitor C1 through transistor TR3 can be kept quite small without impairing the performance of the circuit, and for any given set of values of capacitor C2, resistor R4 and resistor R5 the pulsing circuit will operate over a wide range of pulse repetition rates.

The application of this oscillator to an ionization smoke detector provides a very low consumption lamp flashing circuit which can provide a valuable indication of the active state of the detector circuit.

In such an ionization smoke detector the voltages can be arranged so that the circuit normally flashes slowly. Small quantities of smoke (or a circuit drift towards the alarm state) will cause the flash rate to speed up.

A drift of the circuit away from the normal operating point towards lower sensitivity will cause a slowing of the flash rate and eventually a cessation of flashing. A simple stop-watch test will therefore easily check detector condition without the need for access to the unit or any other form of test gear.

Referring now to Figure 4 this shows an ionization smoke detector in which the oscillator of Figure 3 is embodied for providing a flashing light the speed of flashing of which is indicative of the state of the detector. In the Figure the detector shown at 3 comprises a sensing chamber 4 to which the surrounding atmosphere is readily accessible through a perforated screen 5 and a reference chamber 6 which although vented to atmosphere does not readily permit of access to air in the surrounding atmosphere.

The dimensions of the chamber are arranged such that the inner reference chamber is saturated, that is to say, small voltage changes across it do not result in current changes through it, whereas the sensing chamber is constructed so that it is sensitive to changes in voltage across it.

Between the chambers there is a conductive screen 7. The voltage on the screen 7 is determined by the voltage between supply lines 8 and 9 and also by atmosphere in the sensing chamber 4.

In order to detect the presence of smoke in the sensing chamber 4 the voltage on the screen 7 is compared in a comparator or level detector 10 with a reference voltage on line 11. In the event of a difference between the reference voltage on the line 11 and the voltage on the screen 7 an alarm output is given on line 12. The voltage from the screen 7 is fed to the level detector via a field effect transistor 13 which feeds the level detector on line 14.

With the oscillator of Figure 3 embodied in the detector the screen voltage on the screen 7 (line 14) may be applied to the terminal Vs in Figure 3 whilst the reference voltage on line 11 may be the voltage VREF in Figure 3.

In smoke detectors of the kind described one problem arises is that the voltage on the screen 7 can vary with temperature. In the detector shown, however, means are provided in a circuit affording the reference voltage which produce a corresponding change in the reference voltage with temperature so that false alarm signals due to temperature changes are obviated.

The circuit which produces the reference voltage comprises a potential divider in one arm of which is a resistor 15 and in the other arm of which there is a pair of diodes 16 and 17 connected in series with a resistor 18, the junction between the diodes and the resistors at the point 19 defining a bias potential for an FET 20 having similar characteristics to the FET 13. The FET 20 is serially connected with a variable resistor 21 which is adjustable to control the reference voltage on the line 11. The number and type of the diodes 16 and 17 are chosen so that they compensate for the voltage change on the screen 7 when subjected to similar temperatures to the sensing chamber 4 and reference chamber 6.The resistor 18 is chosen to have a value such that supply voltage variations are compensated for by changing the conductance of the FET 20 whereby the current through the resistor 15 is automatically adjusted so that the reference voltage 11 remains substantially constant even in the presence of small supply voltage variations.

Additional compensation is provided by arranging that the FET 13 and the FET 20 have similar characteristics whereby the temperature effects appertaining to the transistor characteristics are substantially mutually cancelling.

It will be appreciated that various modifications may be made to the arrangement shown. For example, the number of diodes may be changed according to the particular application the forward bias voltage change being normally approximately -2mV per "C per diode. Thus the number of diodes chosen will depend on the temperature coefficient of the reference chamber and other circuit parameters. The reference chamber does not have a perfect plateau characteristic and can be compared to a perfect current source in parallel with an effective shunt resistor. Similarly the current source represented by the resistor 18, the diodes 16 and 17, the FET 20 and the variable resistor 21, may be shunted by a resistor (not shown) to balance the effect of the imperfections of the reference chamber.

WHAT WE CLAIM IS: 1. An ionization smoke detector comprising a reference chamber and a sensing chamber having a conductive screen be

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

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. The percentage of energy taken from the load by the charging of capacitor C2 and the discharge of capacitor C1 through transistor TR3 can be kept quite small without impairing the performance of the circuit, and for any given set of values of capacitor C2, resistor R4 and resistor R5 the pulsing circuit will operate over a wide range of pulse repetition rates. The application of this oscillator to an ionization smoke detector provides a very low consumption lamp flashing circuit which can provide a valuable indication of the active state of the detector circuit. In such an ionization smoke detector the voltages can be arranged so that the circuit normally flashes slowly. Small quantities of smoke (or a circuit drift towards the alarm state) will cause the flash rate to speed up. A drift of the circuit away from the normal operating point towards lower sensitivity will cause a slowing of the flash rate and eventually a cessation of flashing. A simple stop-watch test will therefore easily check detector condition without the need for access to the unit or any other form of test gear. Referring now to Figure 4 this shows an ionization smoke detector in which the oscillator of Figure 3 is embodied for providing a flashing light the speed of flashing of which is indicative of the state of the detector. In the Figure the detector shown at 3 comprises a sensing chamber 4 to which the surrounding atmosphere is readily accessible through a perforated screen 5 and a reference chamber 6 which although vented to atmosphere does not readily permit of access to air in the surrounding atmosphere. The dimensions of the chamber are arranged such that the inner reference chamber is saturated, that is to say, small voltage changes across it do not result in current changes through it, whereas the sensing chamber is constructed so that it is sensitive to changes in voltage across it. Between the chambers there is a conductive screen 7. The voltage on the screen 7 is determined by the voltage between supply lines 8 and 9 and also by atmosphere in the sensing chamber 4. In order to detect the presence of smoke in the sensing chamber 4 the voltage on the screen 7 is compared in a comparator or level detector 10 with a reference voltage on line 11. In the event of a difference between the reference voltage on the line 11 and the voltage on the screen 7 an alarm output is given on line 12. The voltage from the screen 7 is fed to the level detector via a field effect transistor 13 which feeds the level detector on line 14. With the oscillator of Figure 3 embodied in the detector the screen voltage on the screen 7 (line 14) may be applied to the terminal Vs in Figure 3 whilst the reference voltage on line 11 may be the voltage VREF in Figure 3. In smoke detectors of the kind described one problem arises is that the voltage on the screen 7 can vary with temperature. In the detector shown, however, means are provided in a circuit affording the reference voltage which produce a corresponding change in the reference voltage with temperature so that false alarm signals due to temperature changes are obviated. The circuit which produces the reference voltage comprises a potential divider in one arm of which is a resistor 15 and in the other arm of which there is a pair of diodes 16 and 17 connected in series with a resistor 18, the junction between the diodes and the resistors at the point 19 defining a bias potential for an FET 20 having similar characteristics to the FET 13. The FET 20 is serially connected with a variable resistor 21 which is adjustable to control the reference voltage on the line 11. The number and type of the diodes 16 and 17 are chosen so that they compensate for the voltage change on the screen 7 when subjected to similar temperatures to the sensing chamber 4 and reference chamber 6.The resistor 18 is chosen to have a value such that supply voltage variations are compensated for by changing the conductance of the FET 20 whereby the current through the resistor 15 is automatically adjusted so that the reference voltage 11 remains substantially constant even in the presence of small supply voltage variations. Additional compensation is provided by arranging that the FET 13 and the FET 20 have similar characteristics whereby the temperature effects appertaining to the transistor characteristics are substantially mutually cancelling. It will be appreciated that various modifications may be made to the arrangement shown. For example, the number of diodes may be changed according to the particular application the forward bias voltage change being normally approximately -2mV per "C per diode. Thus the number of diodes chosen will depend on the temperature coefficient of the reference chamber and other circuit parameters. The reference chamber does not have a perfect plateau characteristic and can be compared to a perfect current source in parallel with an effective shunt resistor. Similarly the current source represented by the resistor 18, the diodes 16 and 17, the FET 20 and the variable resistor 21, may be shunted by a resistor (not shown) to balance the effect of the imperfections of the reference chamber. WHAT WE CLAIM IS:

1. An ionization smoke detector comprising a reference chamber and a sensing chamber having a conductive screen be

tween them, and comparator means including an oscillator, said oscillator comprising a first capacitor arranged to be charged progressively towards a potential dependent upon the screen voltage through a resistor to a predetermined charge level at which first switch means operates to effect discharge of the capacitor, the predetermined level at which the first switch means operates being determined by a reference voltage applied to the switch means from reference voltage producing means and the capacitor having connected across it second switch means which is connected to receive a voltage developed across a load in series with said first switch means so that the second switch means conducts in dependence upon current through the load therebv to provide a further discharge path for the capacitor which is maintained after the first switch means has restored to a non-operated state, in which the load of the oscillator comprises alarm signal means operable in dependence upon the frequency of the oscillator.

2. An ionization smoke detector as claimed in claim 1, in which the second switch means of the oscillator comprises a transistor having its base arranged to receive the voltage from the load.

3. An ionization smoke detector as claimed in claim 2, in which the base of the transistor comprising the second switch means has coupled thereto a second capacitor which is arranged to be charged through a rectifier to a voltage developed across the load when the first switch means is operated whereby the transistor is biased to remain conducting after the first switch means has resumed a non-operated state.

4. An ionization smoke detector as claimed in any preceding claim, in which temperature sensitive means are provided for changing the reference voltage with temperature so as to compensate for changes in the voltage on the conductive screen of the detector with temperature.

5. An ionization smoke detector as claimed in any preceding claim, in which the screen voltage is fed to the gate of a field effect transistor connected through its source and drain terminals to feed the comparator means which take the form of a level detector responsive to a voltage dependent upon current through the field effect transistor for providing an alarm signal when a predetermined relationship exists between this voltage and the reference voltage.

6. An ionization smoke detector as claimed in claim 4 or claim 5, in which the reference voltage producing means includes a potential divider having in one arm a diode or diodes which are effective to compensate for temperature changes in the chambers of the detector when the diodes and chambers are exposed to the same temperature changes.

7. An ionization smoke detector as claimed in claim 6, in which for the purpose of compensating for variations in supply voltage the diodes are serially connected with a bias resistor to form a potential divider defining the bias for a transistor, current feeding the potential divider and the transistor being supplied through a further resistor across which the reference voltage is developed.

8. An ionization smoke detector as claimed in claim 7, in which a variable resistor is connected in series with the transistor whereby to adjust the reference voltage and in which a further field effect transistor having similar characteristics to the first mentioned field effect transistor is provided whereby variations in conductivity of the field effect transistors are mutually cancelling.

9. An ionization smoke detector as claimed in any preceding claim, in which the alarm signal means comprises a flashing light.

10. An ionization smoke detector substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawing.