Ionization-type fire or smoke sensing system

Abstract

To render an ionization-type sensing element essentially immune to changes in ambient conditions, principally temperature or operating voltage, while still using low-current draining circuits to sense response of the ionization chamber of the sensor, the threshold response level of a field effect transistor (FET) is arranged to have approximately the same temperature response characteristic, within the range of ambient temperature considered, as the ionization cell so that the overall circuit or system combination of the cell and FET amplifier will have a response which is essentially independent of ambient temperature or similar conditions. The source path of the FET is connected to a voltage divider which is so dimensioned that the voltage division ratio (R 2 /R 1 ) is related to the temperature coefficient (α) of the base-emitter voltage of the FET and the temperature coefficient (β) of the measuring ionization cell chamber: R.sub.2 /R.sub.1 =(β/α)-1.

We claim: 
     
       1. Ionization-type fire or smoke sensing system having a source of electrical supply (U1, U2):   a measuring ionization chamber (MK);   a series resistance element (RK) connected in series with the measuring ionization chamber (MK);   a field effect transistor (FET) connected to the measuring ionization chamber, said FET having a conduction threshold voltage which is just above the level of the output voltage (U k ) of the measuring ionization chamber (MK) when smoke or fire aerosols are absent, so that, upon presence of smoke or fire aerosols, the output voltage of the chamber will rise and the FET will become conductive to provide an output signal;   and temperature control means (T1, R1, R2) to render the circuit combination of the ionization chamber (MK) and the FET essentially independent of temperature within a given range, comprising   a transistor (T1) having its collector-emitter path connected in the source supply path of the FET;   and a voltage divider (R1, R2) connected in parallel with the collector-emitter path of the transistor (T1), the tap point of the voltage divider being connected to the base of the transistor (T1);   to provide a temperature compensation characteristic which has the same relative control direction as the temperature characteristic of the measuring ionization chamber (MK) and dimensioned with respect to the FET to maintain .[.the conduction threshold thereof.]. .Iadd.the difference of said output voltage (U k ) and of the voltage (U s ) at the source of said FET .Iaddend.essentially independent of temperature within said range.   
     
     
       2. System according to claim 1, wherein at least one of the resistors of the voltage divider (R1, R2) is adjustable. 
     
     
       3. System according to claim 1, wherein at least one of the resistors of the voltage divider (R1, R2) is a temperature dependent resistor having a resistance value which depends on ambient temperature. 
     
     
       4. System according to claim 1, further comprising a further resistor (R3) connected in series with the measuring ionization chamber (MK) and the source voltage; the junction point between the further resistor (R3) and the chamber (MK) forming a test voltage terminal (U3).   
     
     
       5. System according to claim 1, wherein the series resistance element (RK) comprises a saturated reference ionization chamber. 
     
     
       6. System according to claim 1, wherein the transistor (T1) and the voltage divider (R1, R2) are selected to have a temperature characteristic such that the voltage drop of the network formed by the voltage divider (R1, R2) and the transistor (T1) has essentially the same temperature coefficient as the temperature coefficient of the voltage drop across the measuring ionization chamber (MK), so that the temperature characteristics of the source voltage applied to the FET will be similar to the temperature characteristics of the voltage drop across the measuring ionization chamber. 
     
     
       7. System according to claim 6, wherein the voltage division ratio (R 2  /R 1 ), the temperature coefficient (α) of the base-emitter voltage of the transistor (T1), and the temperature coefficient (β) of the measuring ionization chamber (MK) have at least approximately the following relationship:   R.sub.2 /R.sub.1 =(β/α)-1,     wherein R 2  /R 1  is the ratio of resistance values of the voltage divider; α is the temperature coefficient of the base-emitter voltage of the transistor (T1) and β is the temperature coefficient of the measuring ionization chamber (MK).   
     
     
       8. System according to claim 7, wherein the FET comprises a high gate resistance, MOS-type FET. 
     
     
       9. System according to claim 7, wherein the series resistance element (RK) comprises a saturated reference ionization chamber and wherein the FET is of the MOS type.