Published:2009/6/25 22:43:00 Author:May | From:SeekIC
The purpose of this circuit is to control the water temperature in a hot-water heating sys-tem. What it does is to lower the boiler tem-perature as the outside air temperature in-creases. The op amp is used as a comparator. Therrnistor TH2 and R2 form a voltage divider that supplies a reference voltage to the op-amp's inverting input. Thermistor TH2 is placed outdoors, and the values of TH2 and R2 should be chosen so that when the outside temperature is 25 °F, the resistance of the thermistor and resistor are equal. Resistor R1 and thermistor TH1 make up a voltage divider that supplies a voltage to the op amp's nonin-verting input. Thermistor TH1 is placed inside the boiler and the values of TH1 and RI should be chosen so that when the boiler's tempera-ture is 160 °F, their resistances are equal. The output of the op amp controls Q1, which is configured as a transistor switch. When the logic output of the op amp is high, Q1 is turned on, energizing relay RY1. The relay's contacts should be wired so that the boiler's heat supply is turned off (relay energized).
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