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Pilot Light Flame Detector with Low-Battery Lockout using a Type R Thermocouple

Pilot Light Flame Detector with Low-Battery Lockout using a Type R Thermocouple

The LTC1541 combine a micropower amplifier, comparator and bandgap reference in an 8-pin package. The part operates from a single 2.5V to 12.6V or dual ±1.25V to ±6.3V supply with a typical supply current of 5µA. Both the op amp and comparator feature a common mode input voltage range that extends from the negative supply to within 1.3V of the positive supply. The op amp output stage swings from rail-to-rail. The comparator's inverting input is internally connected to the reference output. In the example, the amplifier “A”, running open loop, compares a small portion of the reference with the thermocouple generated voltage. When the thermocouple is hot, the amplifier’s output swings high, biasing Q1 on. Hysteresis, provided by the 10M resistor, ensures clean transitions, while the diodes clamp static generated voltages to the rails. The 100k–2.2mF RC filters the signal to the amplifier. The comparator “C” monitors the battery voltage via the 2M–1M divider and compares it to the 1.2V reference. A battery voltage above 3.6V holds C’s output high, biasing Q2 on and maintaining the small potential at A’s negative input. When the battery voltage drops too low, C goes low, signaling a low-battery condition. Simultaneously, Q2 goes off, causing A’s negative input to move to 1.2V. This biases A low, shutting off Q1. The low outputs alert downstream circuitry to shut down gas flow