[Bigdav160]

Since an orifice tube system is a fixed opening, it is common to have a low pressure switch that cuts the compressor off if the pressure (temp) drops too low that would cause the evaporator to freeze. It's a "cycling system.

Since an thermostatic expansion valve has a self-adjusting opening, it is common to have a high pressure switch to prevent compressor damage from high pressure if the expansion valve sticks/blocked closed. It's typically not a cycling system. The POA's job was to keep the evaporator's temp above freezing.