Thermostat

A thermostat is a component which senses the temperature of a system so that the system's temperature is maintained near a desired setpoint

A thermostat can often be the main control unit for a heating or cooling system, in applications ranging from ambient air control, to such as automotive coolant control, but is also used in many other applications, such as an electric clothes iron.

It is a "closed loop" control device, as it seeks to reduce the error between the desired and measured temperatures. Sometimes a thermostat combines both the sensing and control action elements of control system, such as in an automotive thermostat.

A thermostat exerts control by switching heating or cooling devices on or off, or by regulating the flow of a heat transfer fluid as needed, to maintain the correct temperature. Thermostats are used in any device or system that heats or cools to a set-point temperature, examples include building heating, central heating, air conditioners, HVAC systems, as well as kitchen equipment including ovens and refrigerators and medical and scientific incubators.

Thermostats can be constructed in many ways and may use a variety of sensors to measure the temperature, commonly a thermistor or bimetallic strip. The output of the sensor then controls the heating or cooling apparatus. A thermostat is most often an instance of a "bang-bang controller" as the heating or cooling equipment interface is not typically controlled in a proportional manner to the difference between actual temperature and the temperature setpoint. Instead, the heating or cooling 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. A thermostat may have a maximum switching frequency, or switch heating and cooling equipment on and off at temperatures either side of the setpoint. This reduces the risk of equipment damage from frequent switching.

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