GB1468984A - Amplifier circuits for flame monitors - Google Patents

Abstract

1468984 Photo-electric flame monitors LGZ LANDIS & GYR ZUG AG 19 Nov 1974 [26 Nov 1973 50148/74 Heading G1A [Also in Division F4] A circuit for a flame monitor comprises a relay connected across the collector-emitter path of a transistor; a photo-resistor controlling the transistor base current; two resistors connected respectively between the collector and a first power-input terminal and between the emitter and a second power-input terminal; and a diode connected across, and poled in the opposite sense to, the base-emitter path. A transistor T Fig. 3, arranged to shunt a flame relay FR, has emitter and collector supply resistors R3, R2 arranged for direct connection across an A.C. supply R, Mp. A photo-resistor RF, in series with a chain including resistors R1, R5, controls the bias at a junction a applied to the base B in such a way that, during power half-cycles when R is positive to Mp, the sensing of a flame causes the base B to be cut-off, so open-circuiting the transistor T to allow current to energize the relay FR. A diode D 1 across the base-emitter path, but oppositely poled, ensures that when R goes negative to Mp, the collector-base junction in series with the diode D1 constitutes a low-resistance shunt across the relay FR, so retarding the decay of coil current and avoiding reverse-voltage surges across the baseemitter junction. A Zener diode ZD may be connected across the collector-base path to avoid excess collector voltages due, e.g. to power-line surges. If a flame is not sensed by the photo-resistor RF, the transistor T presents a low shunt resistance to the relay FR throughout the power cycle. A switch S arranged upon closure to turn on a fuel valve V also switches a resistance R4 across part of the bias chain to raise the threshold of illumination of the photo-resistor RF required to energize the relay coil FR. In a modification to avoid undesired switching that might be caused by stray capacitances C1, C3 in the photo-resistor leads, a balancing circuit comprises a resistor R6 inserted between Rl and the bias point a and a capacitor C2 shunting R1. In a further modification, the energizing of the flame relay FR may be delayed by a capacitor C4 across the base-emitter path, a diode D2 being included in series with C4 to avoid delay when the relay is to be switched off. A resistor R7 across the diode D2 provides a slow discharge path for C4.