GB910901A - Improvements in or relating to electric control circuitry - Google Patents

Abstract

910,901. Photo-electric control of burners. MINNEAPOLIS-HONEYWELL REGULATOR CO. Nov. 26, 1958 [Nov. 27, 1957], No. 38144/58. Class 40 (3). in a control system involving a non- self-quenching Geiger tube 16 as a flame detector interacting with a quenching circuit which passes on the resulting impulses to operate a " flame present " relay 37, the Geiger tube circuit is initially ineffective but the quenching circuit is actuated to energize the " flame present " relay when the system is switched on and that relay then energizes a " safe start " relay 42 which renders the Geiger tube effective and releases the " flame present " relay. Unless this sequence takes place, it is not possible to operate a main control relay 47 and in due course a thermal cut-out disconnects the power supply. Circuit operations, Fig. 1.-On operation of switch 59, the power circuit 57, 58 is connected to the supply transformer 33 and tube 19 fires after a delay to allow its cathode to heat up. The resulting voltage change at the control grids of tube 25 causes the operation of the " flame present " relay 37 in its anode circuit. Relay 37 energizes relay 42 which pulls up and locks, and connects the flame-detecting Geiger tube 16 to the supply source, at the same time removing a positive bias from tube 19. Tubes 19, 25 thereupon cease to conduct and relay 37 falls back. After (and only after) this sequence of operations the main control relay 47 can be operated over contact 60. Relay 47 locks, ensures that 42 shall be dependent on its locking circuit even after the operation of 37 and switches on the igniting circuit 15 and that of the pilot burner fuel valve 14. Assuming that the pilot flame is produced, tube 16 is ionized and interacts with its quenching tube 19 which passes on signals to operate relay 37. With 37 and 47 both operated, the valve 13 of the main fuel supply is turned on. If the pilot burner fails to light and 37 accordingly fails to operate, for a predetermined period, a thermal cut-out 54 actuated by a heater 53 switches off the mains supply and the system returns to its initial state. In case of a flame failure after normal working, 37 falls back and the thermal cut-out operates as before. When switch 60 is opened, relay 47 falls back and causes the fuel valves to be turned off. A circuit is also made for the heater of the thermal cut-out which operates if the absence of flame is not recognised by the Geiger tube arrangement and relay 37 accordingly fails to fall back. Modified circuits.-In one modification, Fig. 2 (not shown), the control grid of tube 19 is disconnected until the operation of 42 and in this condition the tube fires. Operation of 42 as before extinguishes the tube and prepares the Geiger tube for operation. In another modification, Fig. 3, 19 conducts prior to the operation of 42 and is subsequently quenched, except while the Geiger tube is ionized, by the voltage drop across resistor 114. Tube 105 is rendered insensitive to infrequent pulses due to the effect of background radiation on the Geiger tube by arranging for input signals to its two grids to be differently delayed and for the tube to respond only when both grids receive a signal. Specification 861,717 is referred to.