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Solenoid valves are manufactured to control the flow of air, water, inert gases, light oils, refrigerants, and other fluids. A large variety of valves is available to provide three-way or four-way control. Ill. 1 shows a diagram of a simple on/off pneumatic valve. This type of valve is called a three-way, two-position solenoid valve. The diagram for this type of valve is used in pneumatic or hydraulic diagrams. The symbol for the main valve body is a rectangle that's made by placing two boxes side by side. The ports for the valve are identified in this part of the symbol.
The small rectangle at the far left end with a slash line through it's the symbol for the solenoid coil.
The line at the far right end that looks like the symbol for a resistor is the symbol for a spring.
This symbol indicates that this valve uses a spring to return the valve to the de-energized position. P identifies the pressure port, and A and B identify the outlet ports.
The reason two boxes are used in the symbol for the valve is that the box that has the symbol of the coil attached indicates the position of the armature and the ports that are connected when the valve is energized. The box that has the spring symbol attached shows the position of the armature and the ports that are connected when the valve is de-energized. For instance, when the valve is de-energized, one would read the diagram in the box on the side toward the spring symbol, which shows port A is connected to pressure and port B is blocked. When the valve is energized, one would use the diagram of the ports that's in the box on the left showing that port A is blocked and port B is connected to pressure. Notice that the ports are only identified once in the diagram since this symbol represents the two positions of the ports of a single valve. Sometimes the symbol will have the ports identified in each part of the diagram.
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