A fail-safe sensor for flame detection

Abstract A fail-safe optical sensor for flame detection has been developed. The sensor consists of a hybrid circuit. Components are a silicon photodiode, a CMOS amplifier and a feedback resistor. The reasons for false behaviour of the sensor (faking a flame signal) are excessively high temperature, chip defects, environmentally induced effects and electromagnetic noise. To overcome this erroneous behaviour some useful methods are introduced. The temperature dependency of the sensor device is determined by the amplifier's offset-voltage temperature drift and the source resistance of the photodiode. A special offset-trimming method and selection concept provide a negative offset drift. The doping profile of the photodiode produces a large source resistance. The amplifier chip has been designed such that shorts and breaks in electrical connections on the chip result in a safe state of the circuit. The whole device is protected by a hermetic and metallic seal to reduce environmental and electromagnetic disturbances. Simulations and measurements confirm the proposed ideas.