FB-CD Ionization
Part Number 7254-FBD-5226 & 7254-FBD-5227
The FB-CD Series® Ionization smoke detector is a good general purpose unit that responds well to fast burning, flaming fires.
The detector has a molded self-extinguishing white polycarbonate case with wind resistant smoke inlets. Nickel plated stainless steel wiper contacts connect the detector to the base. Inside the detector case a printed circuit board has the ionization chamber mounted on one side and the signal processing electronics onthe other.
The ionization chamber consists of a reference chamber contained inside as smoke chamber (See cross section below). The outer smoke chamber has inlet apertures fitted with insect resistant mesh. The radioactive source holder and smoke chamber form positive and negative electrodes respectively.
An Americium 241 radioactive source mounted within the reference chamber irradiates the air in both chambers, producing positive and negative ions. A voltage across the electrodes produces an electric field.
Ions are attracted to the electrode of the opposite sign to their own charge. Many recombine but a small electric current flows between the electrodes. At the junction between reference and smoke chambers the sensing electrode converts variations in chamber current into voltage changes.
When smoke particles enterthe ionization chamber ions become attached to them with the result that the current flowing through the chambers decreases. This effect is greater in the smoke chamber than in the reference chamber, and the imbalance causes the sensing electrode to become more positive. The voltage at the sensing electrode is fed to a comparator where it is compared with a factory-set clean air reference voltage. If the monitored voltage exceeds the reference voltage, the comparator switches the alarm latch on, increasing the current drawn from the supply from about 40μA to a maximum of 75mA. This fall in the impedance of the detector is recognized by the control panel as an alarm signal.
The alarm latch current alsoilluminates the detectorintegral LED. A remoteindicator connected betweenthe L1 IN terminal and the –Rterminal will have a voltageequal to the supply voltageless 1 volt across it and sowill illuminate.
To ensure correct operation ofthe detector the control panelmust be arranged to supply amaximum of 33 volts DC anda minimum of 9 volts DC innormal operation. The supply may fall to 6volts DC in alarm conditionsif a supply current of at least10mA is available at thisvoltage. To ensure effectiveillumination of the integral LEDand any remote indicator, thesupply to the detector shouldexceed 12 volts.
To restore the detector toquiescent condition, it isnecessary to expel any smokeand interrupt the electricalsupply to the detector for aminimum of one second.