Pressure

Pressure switches are pressure actuated electrical switches. Wich simply means, that there is an electrical switch, which is forced open or closed by air pressure.
The air pressure at the inlet port acts on a flexible membrane. The pressure in transformed into a force. How much force depends on two things: the area (square mm's) of the membrane and the pressure (bar) of the compressed air. Since the area is constant, the force generated is directly proportional to the air pressure.
The other side of the membrane is connected to a spring and a lever that acts on the electrical switch. As long as the downward force of the spring is higher than the upward force of the compressed air, the membrane (and lever) stay down.

But when the pressure rises, there comes a point where the upward force of the compressed air is higher than the downward force of the spring. The membrane (and lever) will move upward at this point. When the membrane moves upward, so does the lever. The lever in turn opens the electrical contacts and the compressor stops.

Now this isn't a direct connection, but there is an ingenious mechanism inside, that makes the switch flip-over at once, at a certain pressure, and not slowly slowly ("snap-action"). This mechanism is also connected to the second set-screw which sets the differential pressure. For the inner workings of the pressure switch, this simply means that the switch closes again at a lower pressure than the pressure where the switch closes (built-in hysteresis).

Setting the pressure.

To be able to change the pressure setting, the downward force of the big spring can be adjusted .

The adjustment is done by compressing the spring using a long screw. When you turn the screw clockwise, the spring is compressed more. It will now generate a larger downward force on the membrane, so we need an higer air pressure to overcome this force. In other words: we raised the pressure setting.

When you turn the screw counter-clockwise, the spring becomes less compressed. It will now be easier for the compressed air to push against the spring. In other words: the pressure setting is lowered.

The off/on/auto switch

Some pressure switches have a switch or push button for on/off control. On some switches it says "ON / OFF", but the words "AUTO / OFF" would be more correct.

In the AUTO ('ON') position, the pressure switch works as expected. It opens and closed automatically according to the pressure. In the OFF position, the pressure settings are overruled and the electrical contacts are always open. The compressor is stopped.

(an ON postion wouldn't make much sense, as this would make the compressor run indefinately, which of course would result in a way too high pressure).

How does it work? The on/off buttons or switch are connected to a plasitc pin. When the pressure switch is set "OFF", the pin pushes down on a plastic block. The plastic block in turn opens the electrical contacts. The compressor will stop and stay stopted.