This is a circuit of room thermostat which is used to control heating or cooling system in air conditioning and heating system has been one of the common device used in residential and industrial buildings to control the temperature of a space in a warehouse, a room, a hall or an office. This is the figure of the circuit;
The circuit diagram shows the configuration of the Room thermostat. The LM358 Op Amp is used as a comparator to sense the inputs of the reference voltage (PIN 3) and room temperature (PIN 2). The thermistor used is a NTC (negative temperature coefficient) type where its resistance will drop when the temperature increases and vice versa. It has a resistance of of 20K ohm at 25 degree Celcius. When the room temperature increases, the thermistor resistance will drop and hence the output of the operational amplifier will be high. This cause transistor Q1 to turn ON and the relay is turn ON. The COM position of the relay contact will switch to NO and ON the chilled water valve which allow the cold water to pass through the coil. The cold air from the coil is transferred to the room by the fan until the temperature of the room goes down to its setpoint again.
The circuit diagram shows the configuration of the Room thermostat. The LM358 Op Amp is used as a comparator to sense the inputs of the reference voltage (PIN 3) and room temperature (PIN 2). The thermistor used is a NTC (negative temperature coefficient) type where its resistance will drop when the temperature increases and vice versa. It has a resistance of of 20K ohm at 25 degree Celcius. When the room temperature increases, the thermistor resistance will drop and hence the output of the operational amplifier will be high. This cause transistor Q1 to turn ON and the relay is turn ON. The COM position of the relay contact will switch to NO and ON the chilled water valve which allow the cold water to pass through the coil. The cold air from the coil is transferred to the room by the fan until the temperature of the room goes down to its setpoint again.
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