Solenoid valve

A solenoid valve controls compressed air electrically. When the coil is energised, the valve changes position and sends the air to another port. In pneumatics the valve usually controls a cylinder.

What 5/2 and 3/2 mean

The first number is the number of ports and the second is the number of positions. A 5/2 valve has five ports and two positions, and it is used to control a double-acting cylinder. A 3/2 valve has three ports: supply, working port and exhaust. It is used, for example, to control a single-acting cylinder. A normally closed 3/2 valve lets the pressure through only when the coil is energised.

Port numbers

• 1 compressed air supply (old marking P)
• 2 and 4 working ports, from which the air goes to the cylinder
• 3 and 5 exhausts to atmosphere
• 14 and 12 the controls, here the coils. The number tells which ports the control connects: 14 connects supply 1 to port 4, and 12 connects it to port 2. That is why the coil of a normally closed 3/2 valve is 12.

Letter markings vary from maker to maker, so rely on the numbers. In the SMC catalogue, for example, port 4 is A, 2 is B, 5 is EA and 3 is EB.

Monostable and bistable

A monostable (single-solenoid) valve has one coil and a spring return. The valve stays in the actuated position only as long as the coil has voltage, and the spring returns it to the rest position as soon as the voltage is removed. So in a power cut the cylinder returns to its rest position, as long as there is compressed air.

A bistable (double-solenoid) valve has two coils. A short pulse is enough, because the valve stays in its position until the other coil is energised. So the PLC has to control each direction with its own output. Do not energise both coils at the same time. On some models one coil then dominates, for example coil 14 on the Festo JMFDH valve.

A 5/3 valve also has a centre position, for example closed, open to exhaust or pressurised.

Coil and wiring

Control systems commonly use 24 V DC coils. The coil is wired between a PLC output and the 0V rail. Many valves also have a manual override for changing the valve position by hand without power.

Diagram symbol

Pneumatic diagram symbols are defined in ISO 1219-1. Each square is one valve position, and the arrows show the air flow. The ports are drawn on the square for the rest position. The coil is drawn next to the square as a rectangle with a diagonal line, and the spring as a zigzag.

Valves for liquids

For water and other liquids there are 2/2 solenoid valves: one inlet, one outlet and two positions, open or closed. A normally closed (NC) valve is closed when the coil has no voltage, and opens only when the coil is energised. A normally open (NO) valve works the other way round. A pilot-operated valve such as the Danfoss EV220B opens and stays open only when the pressure difference across it is at least 0.1–0.3 bar, depending on its size. So it does not work without pressure.