A latching solenoid uses permanent magnets to hold the plunger in place after the coil is de-energized, reducing electricity consumption and preventing overheating. To turn it off, a reverse polarity current pulse is applied.
A latching solenoid is an electromagnetic device designed to provide actuating force as would a conventional solenoid, but then hold the solenoid in the actuated state with no electrical current applied to the coil. This is accomplished by installing a series of permanent magnets around the bottom of the coil core. Once the magnetic field generated by the energized solenoid coil has pulled the plunger into the center of the coil, power is cut off to the coil. Usually this would release the plunger, but, in the case of a latching solenoid, the magnets keep the solenoid actuated without any current draw from the power source. When it is necessary to deactivate the solenoid, a current pulse is applied to the coil in the opposite direction of the driving current which pushes the plunger, thus deactivating the solenoid.
Solenoids are one of the most common short stroke electromagnetic actuators in general use. They are typically very simple devices and consist of a coil of static hollow wire and a spring-loaded ferrous metal plunger. When an electric current is applied to the coil in the correct polarity orientation, a strong magnetic field is created around the coil. This pulls the plunger towards and into the coil core, providing the switching or actuating motion quickly in the process. Cutting power to the coil causes the magnetic field to collapse, releasing the plunger, which is returned to the neutral position by the spring.
When the solenoid is required to remain on for an extended period, power to the coil can be left on until the solenoid requests deactivation. While this works, it causes the coil to generate a lot of heat and waste electricity. The latching solenoid solves this problem with the inclusion of a series of permanent magnets arranged around the coil where the plunger stops at the end of its actuating stroke. These magnets are strong enough to hold the plunger in place, allowing the lockout solenoid to remain active even if power to the coil is interrupted. This prevents the coil from overheating and makes the solenoid more economical in terms of electrical consumption.
When the latching solenoid needs to be turned off, a short pulse of electric current is applied to the coil with a polarity orientation opposite to the turning on current. This creates a brief opposing magnetic field that pushes the plunger back far enough for the magnets to stop working, allowing the spring to return the plunger to its neutral position. This method of operation requires a slightly different type of solenoid controller capable of producing the required reverse polarity current pulse.
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