Latching Solenoid Theory

Latching Solenoids utilize the electrical current pulse or internal permanent magnet material to maintain a set position without the constant application of an electrical current.

While the principal of operation is similar to all linear solenoids, latching solenoids are different in that the electrical polarity is important to obtain proper operation. As the current flows in one direction energizing the coil field in the solenoid, it adds to the pull of the permanent magnet. The armature is attracted to the stationary pole within the solenoid body. 

Once the armature has moved full travel and is in contact with the pole, it will remain in this position without any further electrical power input. The armature is held in this position by the permanent magnet. To release the solenoid from this hold position, the “holding” magnet’s attraction has to be cancelled by sending a current back through the coil field in the opposite direction.

Latching solenoids are most efficient when the pulse time is very short compared to the hold time. While a latching solenoid can be used in both short and long stroke applications, the solenoid stroke should be minimized to improve efficiency.

There are two main types of latching solenoids:

Permanent Magnet (PM) latching solenoid actuators utilize permanent magnets in conjunction with the solenoid coil to maintain the position of the armature with no current applied. The permanent magnet generates a small magnetic flux in the magnetic circuit generating an attraction of the armature 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 armature 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 armature, a spring can be used to hold the armature 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 armature to the fixed pole piece. When the current is removed, the armature 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 armature and move to the extended position.

Residual Magnetism (RM) latching solenoid actuators operate in much the same way as permanent magnet latching actuators do. However, there are some unique design differences. While both types of latching solenoid actuators maintain the latched position without electrical power, residual magnetism actuators 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 actuator is accomplished by providing a short pulse of electrical current of either polarity to pull-in and “latch” the armature 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 actuators cannot be reset after de-latching without applying a pulse of electrical current.

TLX has adapted latching technology for use in a host of applications including combining our latching technology with fluid control which allows valves for air, hydraulic and water control to be used in entirely new and different ways.

Have a technical question?

Ask our experts using the form below and we'll answer you promptly.