Solenoid valves are not as simple as you may think. In fact, for the pneumatic display project (described elsewhere) my ignorance was plain to see.
It seems like a straight-forward concept. You run current through a coil, it creates a magnetic field and it moves a plunger which opens a valve, or changes the direction of flow. But this unfortunately, as I found out this week, does not adequately describe the operation of the solenoid valve shown above.
This is a very common 5/2 pneumatic valve, meaning 5 ports and 2 positions. One port is designated for the pump (ie incoming air pressure), two ports are used alternatively to source pressure to opposing sides of some type of actuator and the remaining two ports are exhaust. The way I hooked up the valve it would source, alternatively pressure and then vacuum, to an air chamber.
It didn’t work with my 12V power supply. After completely taking the thing apart, and being unable to discern its basic essence, I have come to some conclusions. The primary conclusion is that my low pressure Coleman mattress pump does not supply sufficient pressure to allow the plunger to activate. Of course I could have known that from the data sheet which shows a minimum 22psi. This is typical for many such valves. Go figure.
So what were my poorly conceived design considerations in purchasing a solenoid valve? There wasn’t too much there. I needed to make sure that rapid switching, up to say 5 Hz, would not create heat in the coil through repeated inrush current. That wasn’t an issue.
However it turns out that these valves use the operating pressure in the valve to aid in the movement of the plunger. This is called an air piloted system. The solenoid coil (far right in picture above) pulls in plunger, but only to create an opening for the pilot air to enter a very small area. Somehow the differential pressure (as the pilot air pressure drops), stimulates the movement of the valve between positions.
Apparently this type of solenoid is used because “direct-acting” solenoids require excessive amounts of power to move a plunger when subjected to high pressure.