Conventional vs. Latching Solenoid
In a conventional
solenoid, valve components are in one position when the coil is de-energized. The valve switches positions when the coil is energized, then returns to its original position when the coil becomes de-energized again.
Some solenoid valves operate using a latching solenoid design. In this design, the valve is energized momentarily to change positions but then remains in the new state even when the coil is de-energized. The valve will return to its original state only when directed to with a second signal. Latching valves offer many benefits and are often used in applications where power is limited. They provide additional advantage in applications when it is beneficial for the valve to remain in its current state in the event of an electrical failure, or when the valve would otherwise be energized for a length of time that could cause the valve to overheat.
There are two common types of latching solenoids: those held in place by a mechanical feature and those that are magnetically latched. In a mechanically latched valve, the mechanical feature needs to release or be disabled in such a way to allow the valve to return to its original position (such as a manual reset feature).
A magnetically latched solenoid valve contains a permanent magnet with a fixed polarization. The solenoid is operated using a drive circuit that allows it to reverse polarity of the solenoid coil, which reverses both the current direction within the solenoid coil and the magnetic field. By reversing the magnetic field from positive to negative, the plunger will be attracted or repelled from the permanent magnet. After a momentary pulse of electrical current sufficient to change the magnetic field and the position of the valve, the current can be removed, and the magnet will hold the valve in its existing position.
It is also possible for a
solenoid valve to be designed with special internal features that allow residual magnetism from the voltage to provide sufficient latching force for the valve to remain in its current state even without a magnet. This scenario offers the same benefits as a magnetically latched design, without the need for the permanent magnet.
