Ionisation fire detector circuit - has fault monitor releasing alarm when supply voltage or isolation of chamber drops

摘要

The ionisation fire detector has a measurement chamber (MK) connected inseries with a reference chamber (RK). Smoke (62) and fault (46) alarm circuits are connected to the junction (38) between the two chambers. The fault alarm circuit releases an alarm signal when the isolation resistance of the measurement chamber drops below a given value. The fault alarm circuit is designed so that its input voltage between its input and one d.c. supply pole (28,30) is almost independent of the d.c. supply voltage. This means that the circuit also produces an alarm signal when any fault occurs that effects the voltage across the measurement chamber. An alarm is therefore released when the d.c. supply voltage drops below a given level. The fault alarm circuit comprises two bipolar transistors, one FET and five resistors.