They are used to keep the pressure in a tank between a pre-set lower and upper pressure. When the tank pressure crosses the upper or lower limit, an electrical switch is operated. Normally Closed (NC) switch.
[pdf] These switches manage cut-in and cut-out pressure settings to maintain safe, consistent operation. We offer a variety of PSI ranges, port configurations, and voltage ratings to fit most compressor models, including many models from Square D, Worldwide Electric and Lefoo.
[pdf] If the capacity of the air compressor is really too low, check the following: Check differential pressure over oil separator. Replace separator when necessary. Check and replace compressed air filters (if installed). Compressor does not unload. Check if pressure switch is correctly set and working.
[pdf] 
