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A 'solenoid valve' is an electromechanical valve for use with liquid or gas controlled by running or stopping an electrical current through a solenoid, which is a coil of wire, thus changing the state of the valve. The operation of a solenoid valve is similar to that of a light switch, but typically controls the flow of air or water, whereas a light switch typically controls the flow of electricity. Solenoid valves may have two or more ports: in the case of a two-port valve the flow is switched on or off; in the case of a three-port valve, the outflow is switched between the two outlet ports. Multiple solenoid valves can be placed together on a manifold

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How they work

How they work

A solenoid valve has two main parts: the solenoid and the valve. The solenoid converts electrical energy into mechanical energy which, in turn, opens or closes the valve mechanically. An excellent source of information on the different types of solenoid valve and how they work can be found at www.mmint.co.uk[1] .
Solenoid valves may use metal seals or rubber seals, and may also have electrical interfaces to allow for easy control. A spring may be used to hold the valve opened or closed while the valve is not activated.

In some solenoid valves the solenoid provides the full power for the operation of the main valve while there is a certain type where the solenoid, using very little power, controls a secondary pilot valve and it is the pressure of the fluid itself which provides the power for the actuation of the main valve. These types of valves are commonly used in washing machines, gardening and similar uses.
The diagram to the right shows the design of one such valve. If we look at the top figure we can see the valve in its closed state. The water under pressure enters at 'A'. 'B' is an elastic diaphragm and above it is a weak spring pushing it down. The function of this spring is irrelevant for now as the valve would stay closed even without it. The diaphragm has a pinhole through its center which allows a very small amount of water to flow through it. This water fills the cavity 'C' on the other side of the diaphragm so that pressure is equal on both sides of the diaphragm. While the pressure is the same on both sides of the diaphragm, the force is greater on the upper side which forces the valve shut against the incoming pressure. By looking at the figure we can see the surface being acted upon is greater on the upper side which results in greater force. On the upper side the pressure is acting on the entire surface of the diaphragm while on the lower side it is only acting on the incoming pipe. This results in the valve being securely shut to any flow and, the greater the input pressure, the greater the shutting force will be.
Now let us turn our attention to the small conduit 'D'. Until now it was blocked by a pin which is the armature of the solenoid 'E' and which is pushed down by a spring. If we now activate the solenoid, the water in chamber 'C' will flow through this conduit 'D' to the output side of the valve. The pressure in chamber 'C' will drop and the incoming pressure will lift the diaphragm thus opening the main valve. Water now flows directly from 'A' to 'F'.
When the solenoid is again deactivated and the conduit 'D' is closed again, the spring needs very little force to push the diaphragm down again and the main valve closes. In practice there is often no separate spring, the elastomer diaphragm is moulded so that it functions as its own spring, preferring to be in the closed shape.
From this explanation it can be seen that this type of valve relies on a differential of pressure between input and output as the pressure at the input must always be greater than the pressure at the output for it to work. Should the pressure at the output, for any reason, rise above that of the input then the valve would open regardless of the state of the solenoid and pilot valve.
A common use for 2 way solenoid valves is in central heating. The solenoid valves are controlled by an electrical signal from the thermostat to regulate the flow of heated water to the heating elements within the occupied space. Such valves are particularly useful when multiple heating zones are fed by a single heat source. Commercially available solenoid valves for this purpose are often referred to as ''Zone valves.''
Solenoid valves are also used in industry to control the flow of all sorts of fluids.
http://www.mmint.co.uk/solenoid/typical_apps.php]