
31
PFP7/REV C/01-08
transistor TR2, which, in turn, switches off transistor TR1 which de-energises the coil of the
relay, breaks the relay contacts and removes the +12V supply from the solenoid, hence shutting
off any further gas flow. The inputs of IC1 are protected from the high voltage sparks generated
by the ignition discharge by diodes D1 and D2.
The reference level set by RV8 is factory set for optimum switching of the solenoid (on/off).
Adjustment of this may have a significant impact on the safe operation of the flame photometer
and should only be undertaken by trained engineers.
Back E.M.F.s generated by the coils in the relay and solenoid are dissipated by diodes D3 and
D4.
The collector of TR1 is taken to the main PCB to activate the FLM indicator when the flame has
been detected by the above circuitry.
Figure 8.4.1
: Power supply and flame failure detection printed circuit board layout.