In industrial and automotive applications, it is frequently necessary that, when the pressure setting is reached, this is converted into an electrical signal that starts a machine cycle or indicates a certain situation (e.g. system leaks or alarms).
These operations are generally controlled by components, called PRESSURE SWITCHES, that are calibrated according to the specific application.
SPST pressure switches come in two types:
with NORMALLY OPEN (NO) (SPST) electrical contacts
with NORMALLY CLOSED (NC) (SPST) electrical contacts
Calibration is via an adjusting screw which, acting as a spring, determines its load. This spring counteracts the pressure of the fluid on the separating element (diaphragm or piston), thus enabling the electrical contact to close (or open) only when the pressure setting is reached.
In the NORMALLY OPEN (NO) version (Fig. 1), the contact is open, i.e. there is no flow of current in the absence of pressure. When the pressure setting is reached, the electrical contact closes.
The picture in Fig. 2 shows a pressure switch with NORMALLY CLOSED (NC) contacts in the absence of pressure. We can see that, in the absence of pressure, contacts are closed and the signal is present on the external contacts. When the pressure setting is reached, the electrical contact rises and interrupts the signal.