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Question: I’ve been reviewing you website and find there are two styles of latching solenoids. Can you tell me what the difference is between the two as well as the advantages and disadvantages?

Answer: Both styles of latching solenoids have their own unique features and offer advantages and disadvantages solely based on the application. Both versions do offer a distinct advantage in that unlike constant current solenoids, they consume power only when actually moving and are held in position without power.

Permanent Magnet” (PM) latching solenoid actuators utilize permanent magnets in conjunction with the solenoid coil to maintain position of the plunger with no current applied. The permanent magnet generates a small magnetic flux in the magnetic circuit generating an attraction of the plunger and the fixed pole piece with no power applied. When a short pulse of electrical current is applied to the coil, the resulting electromagnetic flux generated by the coil can either add to or subtract from the permanent magnet flux depending on the polarity of the applied current. In applications where the load is acting to extend the plunger away from the fixed pole, latching solenoids can maintain the extended or retracted position without consuming continuous power. In applications where there is no load to act on the plunger, a spring can be used to hold the plunger in the extended position. In either case, a pulse of current is applied to generate magnetic flux to add to the permanent magnet and move the plunger to the fixed pole piece. When the current is removed, the plunger is held in the “latched” position by the permanent magnet. Conversely, applying a reverse polarity pulse will cancel the flux of the permanent magnet allowing the load or spring to release the plunger and move to the extended position.

“Residual Magnetism” (RM) latching solenoid actuators operate in much the same way as PM latching actuators do, but with some unique design differences. While both type of latching actuators maintain the latched position without electrical power, Residual Magnetism latches remain latched without the use of permanent magnets. RM latching actuators offer the same benefits as PM latching actuators by consuming no power, producing no heat and generating no electrical noise while in the latched position. RM latches utilize the inherent “residual magnetism” common to all DC actuators which has been enhanced through special internal design features to provide exceptional latching force without permanent magnets. Latching the RM latch is accomplished by providing a short pulse of electrical current of either polarity to pull-in and latch the plunger to the fixed pole piece. Unlatching the actuator is accomplished by applying a pulse of lower current in the opposite polarity of what was used to latch the actuator. Unlike the PM latching actuators which can be manually latched, RM latching cannot be reset after de-latching without applying a pulse of electrical current.