Thermostats
The thermostat is the part of the system that tells the refrigerator whenever the temperature falls below or rises above a certain preset level, It then switches on an electrical circuit to power the compressor and required components. Likewise when the temperature is low enough it switches them off again.
There is usually a few degrees of temperature difference between the minimum and maximum temperatures, so that the compressor doesn’t repeatedly turn on and off.
Thermostats are made of three parts:
• sensor to detect temperature
• trigger that moves once the temperature is at the set point
• switch that opens or closes to control a component.
Using the float-valve analogy:The float is like the sensor of a thermostat – it detects the water level.The float arm is like the trigger – it moves with the float position. The control valve is like the switch – it opens and closes to control the make-up water flow.
* Bimetal Control
* Bellows Type
Bimetal Control
Inside this type of thermostat, a bi-metal coil contracts when cold and expands when warm. The sensor’s movement trips a switch to “On” or “Off.” The switch may be simply an open contact that closes or it may be a mercury-activated switch. With the latter, mercury in a small, glass vial completes or breaks an electrical circuit when the vial is tilted by the bi-metal coil.
Bellows Type
The bellows type of this type ofthermostat has gas filled bellows which expands or contracts on temperature changes. When the bellows have moved enough, it moves a mechanical contact arm up far enough so that it touches an electrical switch. This makes an electrical circuit which powers the compressor etc.