Based on the manufacturing principle of thermostats, thermostats can be classified as follows:
1. Interrupting-type thermostats: All interrupting-type thermostats are collectively referred to as KSD. Common examples include KSD301, KSD302, etc. This type of thermostat is a new product of bimetallic strip thermostats. It is mainly used as an overheat protection device for various electric heating products. It is usually connected in series with a thermal fuse. The interrupting-type thermostat serves as the primary protection. The thermal fuse acts as the secondary protection when the interrupting-type thermostat fails or becomes ineffective, preventing the overheating of the electric heating element and thus avoiding fire accidents. [1]
2. Liquid expansion type thermostat: When the temperature of the controlled object changes, the substance (generally a liquid) in the temperature sensing part of the thermostat undergoes corresponding thermal expansion and contraction (volume change), and the diaphragm connected to the sensing part expands or contracts. Based on the lever principle, it drives the switch to open or close, achieving the purpose of maintaining a constant temperature. The liquid expansion type thermostat has the performance characteristics of accurate temperature control, stable and reliable operation, small temperature difference between start and stop, wide control temperature range, and large overload current. The liquid expansion type thermostat is mainly used in household appliance industries, electric heating equipment, and refrigeration industries for temperature control scenarios.
3. Pressure type thermostat: This thermostat converts the change in the controlled temperature into changes in pressure or volume through a temperature-sensitive working medium filled in the temperature bulb and capillary tube. When the temperature reaches the set value, through elastic elements and rapid instantaneous mechanisms, the contacts are automatically closed to achieve automatic temperature control. It consists of a sensing part, a temperature setting main body part, and a microswitch or automatic air valve for opening and closing. The pressure type thermostat is suitable for refrigeration equipment (such as refrigerators and freezers) and heating equipment, etc.
4. Electronic thermostat, electronic temperature controller (resistance type) measures using the resistance sensing method. Generally, platinum wire, copper wire, tungsten wire, and thermistors are used as temperature sensing resistors. These resistors have their own advantages and disadvantages. Most household air conditioners use thermistor type.
Steam type
Working principle: Steam pressure type
The action of the bellows acts on the spring. The spring force is controlled by the knob on the control board. The capillary tube is placed at the air intake of the air conditioner’s indoor unit to react to the temperature of the indoor circulating return air. When the room temperature rises to the set temperature, the sensing agent gas in the capillary tube and bellows expands, causing the bellows to elongate and overcome the spring force to connect the switch contacts. At this time, the compressor starts and the system cools down until the room temperature drops to the set temperature again. The sensing package gas contracts, the bellows contracts together with the spring, and the switch is placed in the off position, cutting off the electric motor circuit of the compressor. This repeated action is carried out to achieve the purpose of controlling the room temperature.
Electronic type
Electronic temperature controller (resistance type) measures using the resistance sensing method. Generally, platinum wire, copper wire, tungsten wire, and semiconductor (thermistors) are used as temperature sensing resistors. These resistors have their own advantages and disadvantages. The sensors of most household air conditioners are thermistor type.