Flame detecting apparatus

Abstract

A flame detecting apparatus used in a combustion control apparatus for a water heater provided with a gas burner comprises a pressure switch serving as means for detecting presence or absence of flame and a voltage comparison circuit serving as means for determining the presence or absence of the flame on the basis of the input signal supplied by the pressure switch. The voltage comparison circuit has an offset voltage and has a non-inverting input terminal connected only to the ground. A first circuit including a first resistor and a diode connected in series to each other is connected in a loop together with the pressure switch, an impedance network and an AC voltage source so that an AC current which flows when the pressure switch is closed is rectified by the first circuit and a negative voltage obtained through the use of a capacitor or the like is applied to the inverting input terminal of the voltage comparison circuit to thereby detect the flame, while preventing erroneous operation of the voltage comparison circuit due to a short-circuit possibly produced across the resistor. By connecting a second resistor in parallel with at least the diode of the first circuit, erroneous detection of the flame ascribable to contact failure of the pressure switch as well as leakage is prevented. The capacitor may be replaced by a resistor to reduce expenditure.