A solenoid valve is a valve that uses **electrical signals to control the flow‑on/flow‑off of fluids (gas, water, oil)**. It mainly consists of two assemblies: an **electromagnetic coil**, an iron core (armature), and a valve body with spool. When energized, magnetic force drives the spool to shift and change fluid passages. When de‑energized, magnetic force disappears, and a return spring resets the spool to switch the operating state.
1. Basic Operation Logic
1. Energized: The coil receives power → generates electromagnetic attraction → pulls the iron core/spool to displace → opens or closes the fluid passage.
2. De‑energized: The coil loses power and magnetic force vanishes → the return spring pushes the spool back to its original position → restores the initial on‑off state.
2. Direct‑Acting Solenoid Valve (small port size, low pressure)
Energized: The coil lifts the spool directly, and the valve opens.
De‑energized: The spring presses the spool back, and the valve closes.
Features: Operates **without fluid pressure**. Relatively high power, generally small port diameter.
Application: Small‑bore pipelines, low‑pressure water and gas.
3. Pilot‑Operated Solenoid Valve (large port size, most widely used)
1. When the coil is energized, the small **pilot vent orifice** opens first.
2. A pressure difference forms across the main diaphragm. Fluid pressure pushes the main spool upward to open the main flow path.
3. When de‑energized, the pilot orifice closes, pressure re‑balances, and the main valve closes via spring and fluid pressure.
Features: Low coil power, available for large port sizes. **Requires minimum fluid pressure for switching; cannot work under zero pressure.
4. Common Types
① 2‑way 2‑position (most common on‑off valve)
Two ports, two operating states:
Normally Closed (NC): Closed when de‑energized, opens upon energization; automatically closes after power loss. Widely used in water supply and gas cut‑off.
Normally Open (NO): Open when de‑energized, closes upon energization; automatically opens after power loss.
② 3‑way 2‑position
Three ports, frequently used for pneumatic cylinder control: one inlet, one outlet, one exhaust port. Switches between output and exhaust when energized.
③ 5‑way 2‑position
Most popular for pneumatics, drives double‑acting cylinders for reciprocating motion. Five ports; shifts direction when energized and resets when de‑energized.
5. Important Notes
1. Common coil voltages: DC24V, AC220V, etc. Wrong supply voltage will burn out the coil.
2. Pilot‑operated valves require a minimum operating pressure differential. They fail to open at zero pressure.
3. The fluid medium must be clean. Contaminants may jam the spool, causing failure to open or tight shut‑off.
4. It only performs on‑off control; cannot adjust flow rate.