As the most common external drive unit, solenoid valves are widely used in industry. So what is the principle of solenoid valves? What are the classifications? The solenoid valve is composed of a solenoid coil and a magnetic core, and contains a valve body with one or more holes. When the coil is energized or de-energized, the operation of the magnetic core will cause the fluid to pass through the valve body or be cut off to achieve the purpose of changing the direction of the fluid.