GB1258327A - - Google Patents

Abstract

1,258,327. Controlling burners. MULLARD Ltd. 9 May, 1969, No. 23763/69. Heading F4T. Gas monitoring and controlling apparatus comprises a solenoid operable gas valve V which, when opened, allows a stream of gas to pass to a burner B, said solenoid being energizable by a current flowing through a semi-conductor switch SCR when the switch is triggered, ignition of the gas producing an ionization current which allows the switch SCR to be triggered, whilst in the absence of flame, after an initial finite time period to allow ignition of the gas, the switch is prevented from being triggered, thereby closing the valve V and cutting off the gas supply. The solenoid of valve V is energized if triggering of the switch SCR takes place before 90 degrees of each positive half-cycle when an A.C. voltage is connected across L and N, and a phase advance circuit formed by capacitor C3 and resistors R6 and R7 ensures that the current applied to the trigger electrode of switch SCR, which may be a thyristor or a triac, is 90 degrees advanced with respect to its anode voltage. Three npn transistors TR1, TR2, TR3 are connected in cascade such that when TR1 is conducting, TR2 and TR3 are respectively non- conducting and conducting, and vice versa, and SCR is only triggered when TR3 is non-conducting. On applying the A.C. voltage, SCR is triggered at the beginning of the first positive half-cycle, thus opening valve V and initiating gas flow. The presence of capacitor C1 renders TR1 initially non-conducting, but as C1 becomes charged, TR1 begins to conduct and thus divert current from the base of TR2, which therefore does not conduct until progressively later in each positive half-cycle. Thus SCR is not triggered until later in the half-cycle, allowing a capacitor C4 to first of all charge, and then discharge through SCR when this triggers. This discharge through a transformer T causes a spark between the ignition electrode I and the burner B. If the gas ignites, the resulting ionization current flows by way of a monitoring electrode M and resistor R3 to the base of TR1. This current is phase retarded by a capacitor C2 such that it opposes the bias current to TR1 during the first part of a positive half-cycle, thus allowing SCR to be triggered in this period. In this way, when the gas is ignited valve V is kept open and since capacitor C4 does not charge, no spark occurs. If the gas does not ignite, or if the flame is extinguished, C1 charges, rendering TR1 fully conducting and TR2 non-conducting. Thus, SCR fails to trigger, the valve V closes and the system becomes "locked out". The sequence may be re-started either by briefly disconnecting the A.C. supply, or by discharging C1 through a switch placed in parallel with it.