The working principle of the thermostat

What is the working principle of the thermostat? Today let us talk about it.

The thermostat is based on the principle of pressure to push the contacts on and off. Its structure consists of a bellows, a temperature sensor (test tube), an eccentric wheel, a micro switch, etc., a sealed induction system and a system for transmitting signal power. Control methods are generally divided into two types; one is controlled by the temperature change of the object to be cooled, and a vapor pressure temperature controller is mostly used; the other is controlled by the temperature difference of the object to be cooled, and the electronic temperature is mostly used Controller. Thermostats are divided into: Mechanical types are divided into: vapor pressure thermostats, liquid expansion thermostats, gas adsorption thermostats, metal expansion thermostats. Among them, the steam pressure type thermostat is divided into: aerated type, liquid-gas mixed type and liquid-filled type. This type of thermostat is mainly used in household air conditioners. Electronic type is divided into: resistance type thermostat and thermocouple type thermostat. The main components of the circuit system are: temperature controller, thermal protector, main control switch, running capacitor, and running capacity of the fan motor. The role of the temperature controller is only to control the start and stop of the compressor. Working principle The action of the steam pressure type bellows acts on the spring. The spring force of the spring is controlled by the knob on the control board. The capillary tube is placed in the refrigerator freezer and responds to the temperature of the indoor return air. When the temperature rises to the set temperature, the temperature-sensing agent gas in the capillary tube and the bellows expands, so that the bellows is extended and the switch contact is turned on against the elastic force of the spring. At this time, the compressor runs and the system cools until When the temperature is lowered to the set temperature, the temperature of the temperature-sensing bag shrinks, the bellows shrinks together with the spring, and the switch is placed in the off position to cut off the compressor's motor circuit. Repeat this action to achieve the purpose of controlling temperature. Electronic temperature controller The electronic temperature controller (resistive type) is measured by resistance temperature sensing method. Generally, platinum wire, copper wire, tungsten wire and semiconductor (thermistor, etc.) are used as temperature measurement resistance. These resistances Each has its advantages and disadvantages. The sensors of home air conditioners are mostly thermistor type. Thermistor thermostats are based on the Wheatstone bridge principle, the Wheatstone bridge. Connect the power source E to both ends of the BD. According to Kirchhoff's law, when the resistance of the bridge R1 × R4 = R2 × R3, the potentials of the two points A and C are equal, and no current flows between the output terminals A and C. The magnitude of the resistance R1 of the thermistor changes with the rise or fall of the surrounding temperature, so that the balance is destroyed, and there is an output current between AC. Therefore, when constructing a thermostat, you can easily change the temperature adjustment range and operating temperature by selecting an appropriate thermistor