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A thermostat is a component of an HVAC control system which senses the temperature of a system so that the system's temperature is maintained near a desired ''setpoint''. The thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer fluid as needed, to maintain the correct temperature. A thermostat may be a control unit for a heating or cooling system or a component part of or air conditioner. Thermostats can be constructed in many ways and may use a variety of sensors to measure the temperature. The output of the sensor then controls the heating or cooling apparatus. A thermostat may switch on and off at temperatures either side of the ''setpoint''. This prevents equipment damage from frequent switching. Thermostats are also used in many household appliances. A thermostat is an instance of a "bang-bang controller" as the heating or cooling system output is not proportional to the difference between actual temperature and the set temperature; the equipment runs at full capacity until the set temperature is reached, then shuts off. Increasing the difference between the thermostat setting and the desired temperature therefore does not shorten the time to achieve the desired temperature. The term is derived from the Greek words θερμός ''thermos'', "hot" and στατός ''statos'', "standing, stationary". ==Sensor types== Early technologies included mercury thermometers with electrodes inserted directly through the glass, so that when a certain (fixed) temperature was reached the contacts would be closed by the mercury. These were accurate to within a degree of temperature. Common sensor technologies in use today include: * Bimetallic mechanical or electrical sensors. * Expanding wax pellets * Electronic thermistors and semiconductor devices * Electrical thermocouples These may then control the heating or cooling apparatus using: * Direct mechanical control * Electrical signals * Pneumatic signals 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「thermostat」の詳細全文を読む スポンサード リンク
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