1）Direct actuated solenoid valve
Principle: when electricity is electrified, the electromagnetic force of the electromagnetic coil is brought up from the seat and the valve is opened; when the power is broken, the electromagnetic force is disappearing and the spring is pressed on the seat of the valve and the valve is closed.
Features: it can work normally in vacuum, negative pressure and zero pressure, but the diameter is generally not more than 25mm.
2) stepwise direct actuated solenoid valve:
Principle: it is a combination of direct and pilot, when there is no pressure difference between the entrance and exit, after electricity, the electromagnetic force directly brings the pilot valve and the main valve close up, and the valve opens. When the entrance and exit reach the start pressure difference, after the electricity, the electromagnetic force pilot valve, the main valve lower chamber pressure rises, the upper chamber pressure drops, thus using pressure difference to push the main valve upward; when the power is broken, the pilot valve uses spring force or medium pressure to push the closure, move down, and turn the valve down.
Features: it can also operate at zero pressure difference or vacuum and high pressure, but the power is larger, and the installation must be installed horizontally.