Pressure switches and pressure sensors

Air compressors, hydraulic power units and vacuum grippers all need to know the pressure. A pressure switch gives an ON/OFF signal at a set pressure; a pressure transmitter gives the actual value.

Mechanical pressure switch

Pressure acts on a diaphragm, piston or bellows working against a spring. When the force exceeds the spring setting, a snap-action contact changes over. Turning the adjusting screw changes the setpoint. It needs no power and the contacts can switch a contactor coil directly.

Setpoint and differential (hysteresis)

A switch that turns off at exactly the same pressure it turns on would chatter. So it has a differential: for a compressor, for example, cut-out at 8 bar and cut-in at 6 bar. The motor stops at 8 bar and restarts when the pressure drops to 6 bar.

Electronic pressure switch

A pressure sensor element plus electronics and a display. You set the switch point (SP) and reset point (RP) on the display. It usually has one or two PNP/NPN outputs, and often an analog output and IO-Link as well. More accurate and easier to adjust than a mechanical switch.

Pressure transmitter (4–20 mA)

A transmitter sends the actual pressure as an analog signal: e.g. 0–10 bar = 4–20 mA. The PLC reads it on an analog input and the program decides when to act. This is how you show the pressure on an HMI or build a PID pressure control loop with a VFD-driven pump.

Typical uses

Wiring tips

• Use the NC contact for "pressure low" alarms so a broken wire also raises the alarm.
• Fit a snubber or gauge cock where pressure pulsates, to protect the sensor.
• Wire electronic switches like any 3-wire sensor: see PNP vs NPN.

The PLC Sim Lab Playground has a compressor, FRL unit, hydraulic power unit and cylinders, so you can build a complete air or oil system controlled by the PLC through a magnetic contactor.