Patent GB-1480659-A (GB)
Title: Circuit for a flame control device of a furnace
IPC: F23N5/20
Filed: 1975-10-09
Abstract: 1480659 Burner controls DANFOSS AS 9 Oct 1975 [9 Oct 1974] 41418/75 Heading F4T An electric circuit for a furnace flame control device comprises a photo resistance LD forming part of a voltage divider which consists of ohmic resistances R 5 , R 6 , and a tapping 6 for controlling an amplifier, i.e. transistors Tr 1 , Tr 2 , which controls a photo-relay including an electromagnetic coil F, which in turn controls switches F 1 , F 2 , the divider lying in parallel with a first storage condenser C 1 , chargeable to a predetermined low D.C. voltage through a rectifier D 1 , and a second storage condenser C 3 , chargeable through at least one sensitivity control terminal A or B that can be connected to receive the A.C. voltage by means of snapaction switch P 1  or P 2 , the second condenser being disposed in a branch circuit which is connected between a point of fixed voltage and a tapping 8 of the voltage divider. In operation, with switches P 1 , F 1 , on rest contacts a, boiler thermostat KT is closed, fan motor M is started, ignition transformer TT and heating coil P of a time relay are energized, terminal A is supplied with mains voltage and this provides for a preignition period during which ventilation of the combustion chamber and a check on a false light takes place. At the end of this period time relay switch P 1  moves to b, magnetic valve MV 1  is opened and oil flows to the pilot burner. Mains voltage is also supplied to terminal B through snap-action switches P 1 , P 2 , and F 2 , so that flame monitor FW becomes less sensitive than normal and will not respond even if a flame is formed at the pilot. At the end of pre-ignition, switch 2 opens, FW becomes more sensitive due to absence of voltage at B and snap-action switches F 1 , F 2 , move to operating contacts b and a mechanical lock FL locks switch P 1  in its operating position; heating coil P and ignition transformer TT are disconnected from supply 3 and a magnetic valve MV 2  is switched-on after cooling of the time relay and return of switch P 2  to its rest position, so that main oil flow commences. Normal operation is interrupted, e.g. when mains voltage fails, by opening boiler thermostat KT, and if the supply voltage fails magnetic valves MV 1 , MV 2 , close to shut off the oil supply and motor MV stops, the photo relay drops off, switches F 1 , F 2 , return to their illustrated positions and lock Fk is released. If no flame occurs during switch on, heating coil P on the time relay remains connected to supply through switch F 1 , until a manually reset safety switch P 3  responds and a fault is indicated at lamp L. After valves MV 1 , MV 2 , are switched off when the flame disappears, a renewed ignition attempt will determine whether the flame is extinguished permanently. If photo resistance LD is subjected to false light, switch F 1 , moves to b and is locked by FL. Coil P continues to be heated and after a safety period switching off is by switch P 3 . Fuel feed may be by valve MV 1 , and a post ignition period may be omitted, so that components P 2 , F 2 , MV 2 , D 5 , and R 7 , are not required. Rectifier D 2  may be dispensed with and the magnetic valves may be replaced by pumps or relays.