A solenoid valve is flipped open and shut by an electromagnet instead of a handle. Watch the coil pull an iron plunger off its seat so fluid can pass, then let go and let a spring seal it again.
A solenoid valve is a valve that opens and closes itself when you switch an electric current on or off. The "solenoid" is the coil of wire that becomes a magnet when energised, and a small iron plunger inside it does the actual moving. No hand, no lever, no motor touches the fluid â the valve is controlled entirely by the wire carrying its current.
From the outside it is just a block of metal with a pipe in, a pipe out and two wires. Everything that decides whether fluid moves is hidden in the gap between the coil and the seat, so a cutaway is the only way to see how magnetism, a spring and a moving iron core open and close the passage.
With the coil switched off, the valve is at rest. The return spring holds the plunger firmly against the seat, so the passage through the body is blocked and whatever is in the supply pipe simply waits at the inlet. Nothing moves until current arrives.
When the coil is energised, current threads through hundreds of turns of copper wire around a core. Every turn adds a little magnetic field, and together they turn the coil into an electromagnet whose strength grows with the current and the number of turns. Nothing has moved yet â the valve is now primed to act.
The electromagnet pulls on the iron plunger, dragging it toward the coil. As the plunger slides, it compresses the return spring. This is the whole trick of the design: the magnetic force only ever has to beat the spring and the fluid pressure, and it is that straight pull that does the work.
Pulled off the seat, the plunger no longer seals the passage. Fluid can now run from the inlet, past the opened gap, and out through the outlet. The harder the coil is energised, the faster the plunger snaps open, so a solenoid valve responds almost instantly compared with a hand valve.
Switch off the current and the magnetic field collapses at once. With nothing holding it, the return spring pushes the plunger back onto the seat and the seal is restored, stopping the flow. That built-in fail-safe is why normally closed solenoid valves are trusted for safety shut-off.
An electric current through the coil turns it into an electromagnet. The magnet pulls an iron plunger toward it, the plunger lifts off the seat, and the passage between the inlet and the outlet opens so fluid can flow.
It describes the valve's resting state with no power applied. A normally closed valve is shut until you energise the coil; a normally open valve is open until you energise it, which then pulls the plunger down and closes the passage.
Enough to overcome the return spring and the fluid pressure pushing on the plunger. The force grows with the current and the number of coil turns, so a bigger bore or higher pressure needs a stronger, larger coil.
Holding the plunger open means current is flowing through the coil continuously, and a real coil has resistance, so it dissipates heat as I squared R losses. Valves that stay on for long periods are often rated for a lower holding voltage or use a more efficient design.
A plain solenoid valve is really just open or shut. To vary the flow you either pulse it rapidly (pulse-width modulation) or use a proportional solenoid whose plunger position follows the current, rather than slamming between two stops.
It snaps back to its resting state. In a normally closed valve that means the spring pushes the plunger onto the seat and the flow stops â which is exactly why this type is chosen for safety shut-off.