solenoid valves are in principle can be divided into three categories:
Direct action solenoid valve
Principle: power is electromagnetic coil generates magnetic lift off from the valve seat, valve open; power, the electromagnetic force disappeared, spring pressure off at the valve seat, valve is closed.
Features: vacuum, vacuum, the zero-pressure can work, but size does not exceed 25mm.
Step by step direct acting solenoid valve
Principle: it is a combination of direct acting and pilot-principle, when the inlet and outlet when there is no pressure, power, the electro-magnetic drive guide small valve and main valve closes in turn lift the valve open. When the inlet and outlet when the starting pressure is reached, power, force pilot small valves, main pressure valve cavity, cavity pressure drop, which pushed up the main valve pressure drop; power, the pilot valve closed by spring force or media pressure to push, move down, the valve closes.
Features: at zero pressure or vacuum, high pressure can also be *, but more powerful, must be installed horizontally.
Pilot-type solenoid valve