Patent GB-1463753-A (GB)
Title: Device for use in a monitoring system for ascertaining the presence of a flame in a burner
IPC: F23N5/08
Filed: 1974-05-14
Abstract: 1463753 Flame detectors SIE SOC ITALIANA ELETTRONICA SpA 14 May 1974 [22 May 1973] 21214/74 Heading G1A A monitoring device for use with a flamedetector head comprises a chain of stages connected in cascade and includes: a first stage for coupling to the detector head; a first one of the remaining stages being arranged to pass A.C. components but not D.C. components of the signals applied to it; a second remaining stage including energy storage means; a third remaining stage including first chopper means; and a fourth remaining stage including a threshold circuit and arranged to develop an output signal within a predetermined time after cessation of a flame-responsive signal. An ionization tube (A), Fig. 1 (not shown), positioned to respond to UV radiation from a furnace flame and supplied with high voltage from terminals c, d, Fig. 3, delivers in response to radiation a pulsating signal to a transformer T2 via a terminal e. Alternating signals induced in the secondary of T 2  charge, via a bridge rectifier 18-21, a capacitor 22 which discharges into the base of a transistor Q4 constituting a pulse shaper. A capacitor 25, responding to the alternating component in the Q4 output, charges via a diode pump 27-28 a capacitor 32 to a voltage which decays through a limiting diode 33 and resistor 34 in such a way that the mean voltage is logarithmically related to the pulse frequency. Variable resistors 30, 31 selectable by a switch S1 actuated by a relay S in response to a manual switch closing contacts h, i, provide adjustable sensitivity. Transistors Q2/Q3 regeneratively connected to a transformer T1 constitute a D.C./A.C. inverter generating squarewaves at about 1KHz which supply rectifier circuits providing 700 volts at terminals c-d for the ionization tube and - 15v for some stages of the electronic apparatus. Further square waves of  1.5 V, applied at terminals A-B as a cyclic test signal, cause a transistor Q5 to chop the voltage across the capacitor 32; the resulting pulses, if above a predetermined threshold, repetitively fire a Schmitt trigger Q6/Q7. A capacitor 40, responsive to the A.C. component of the Schmitt output, operates a diode pump 41/42 supplying current to one of two resistors 43, 44 selectable by a manual switch F to provide an adjustable discharge time constant in the event of any failure resulting in the cessation of pulses. The voltage across the resistor 43 or 44, chopped by a transistor Q8 fed with a further cyclic test signal at terminals C-D, is amplified by transistors Q9/Q10 supplying an A.C. signal to a transformer T3, from which a current after rectification is supplied to a relay K. If the apparatus correctly responds to the presence of a flame at every stage the contacts K1, K2 are held; but if the flame is extinguished, or if any stage of the apparatus fails to respond to A.C. components at its input, the current in the relay K will cease and so release the contacts. Analogue indication of the voltage across the capacitor 32 is provided by meters M1, M2; and a luminescent diode L indicates energization of the sensitivity selector relay S.