Operation of the Latching Solenoid

Education
, ,
1

What is a latching solenoid?

A solenoid is an electromechanical mechanism that moves a plunger in response to an application of an electric current. A latching solenoid provides the same linear motion; however, it includes a mechanism to hold the plunger in a fixed position independent of electricity. The latch is formed from residual magnetism or, as featured in this article, permanent holding magnets. Energy savings and increased operational life are the primary advantage of the latching solenoid since power is not required to hold the solenoid in position. Note that monostable and bistable rotary solenoids are also available.

This engineering brief explores the operation of the Delta Electronics DSML-1153-24C as pictured in Figure 1. Note that the featured latching solenoid requires a forward polarity and reverse polarity to latch and unlatch. Please review this article for a detailed description of the pulsed electrical interlocking circuit as shown in Figure 2. This circuit is essential for solenoid operation, as it prevents the user or Programmable Logic Controller (PLC) control system from simultaneously applying forward and reverse polarity.

Figure 1: Picture of the Delta Electronics DSML-1153-24C latching pull solenoid featuring permanent magnets.
Figure 1: Picture of the Delta Electronics DSML-1153-24C latching pull solenoid featuring permanent magnets.1016×755 199 KB

Figure 1: Picture of the Delta Electronics DSML-1153-24C latching pull solenoid featuring permanent magnets.

Tech Tip: Solenoids are available for continuous or intermittent duty operations. Carefully review the datasheet to determine the conditions for intermittent operation. This will prevent coil overheating for long equipment life. Always remember the rule of 10 which implies that the life of a component will approximately double with every 10° C reduction in temperature.

Also, be sure to review the datasheet to determine the life of the solenoid in terms of mechanical cycling. Finally, consider how your load is applied to the solenoid. Off-axis loads can result in plunger binding and reduced equipment life.

Figure 2: Latching solenoid driven by an electric interlocking polarity reversing circuit as described in this article.
Figure 2: Latching solenoid driven by an electric interlocking polarity reversing circuit as described in this article.993×745 98.8 KB

Figure 2: Latching solenoid driven by an electric interlocking polarity reversing circuit as described in this article.

States of the unpowered latching solenoid

The latching solenoid is best understood in terms of states. The simple unpowered states are shown in Figure 3. On the left, we see the solenoid in the latched state, and on the right, the unlatched state. Both unpowered states are stable, as the plunger stays in its position. The solenoid will enter the latched state if the plunger is physically pushed into the solenoid body.

Note that the permanent magnets hold the plunger firmly when in the latched state. A screwdriver used as a prybar may be used to force the plunger out of the unpowered latched state. Once it has been moved approximately ¼ inch, the spring tension will keep the plunger in the unlatched state.

Figure 3: When unpowered, the latching solenoid may be latched (left) or unlatched (right). Permanent magnets firmly hold the plunger in the latched state.
Figure 3: When unpowered, the latching solenoid may be latched (left) or unlatched (right). Permanent magnets firmly hold the plunger in the latched state.946×696 181 KB

Figure 3: When unpowered, the latching solenoid may be latched (left) or unlatched (right). Permanent magnets firmly hold the plunger in the latched state.

Tech Tip: The solenoid’s coil acts as a large inductor. The associated flyback voltage at turn-off presents a problem, especially if the coil is driven by a semiconductor H-bridge. A simple 1N4004 diode will not work in this situation as the solenoid requires a bidirectional application of current. Instead, we must use four diodes within the H-bridge to mitigate the flyback voltage. The diodes are placed in parallel with each switch in the bridge.

Tech Tip: Consider adding a snubber circuit consisting of a series-connected resistor and capacitor in parallel with the coil. This will reduce the possibility of RF interference, especially if the solenoid is far from the control circuit. Recall that long wires can act like an antenna radiating RF energy.

Description of the magnetic fields within the latching relay

The latching solenoid featured in the article contains a pair of permanent magnets. The magnetic fields are shown in Figure 4 (left). Recall that magnetic fields travel in loops seeking the shortest possible path. In this example, the loops pass through the soft iron yoke. They also travel through the iron plunger.

There is an unseen magnetic air gap in Figure 4 emanating from the lower position of the plunger to the base of the yoke. There is a small force pulling the plunger into the solenoid body. However, it is balanced against the spring. Consequently, the plunger remains extended.
The stroke vs force relationship is shown in the lowest (continuous) curve of Figure 5. Like the magnets on a fridge, the force becomes stronger as the plunger is pulled into the solenoid body.

Figure 4: Image showing the magnetic flux lines for the permanent magnets and the electromagnets.
Figure 4: Image showing the magnetic flux lines for the permanent magnets and the electromagnets.1018×737 229 KB

Figure 4: Image showing the magnetic flux lines for the permanent magnets and the electromagnets. Note that the fields on the right assume a polarity where the permanent and electro fields oppose each other.

Figure 5: Datasheet description of the force as a function of stroke for the Delta Electronics DSML-1153-24C.
Figure 5: Datasheet description of the force as a function of stroke for the Delta Electronics DSML-1153-24C.975×496 34.6 KB

Figure 5: Datasheet description of the force as a function of stroke for the Delta Electronics DSML-1153-24C.

States of the powered latching solenoid

The featured solenoid is polarity sensitive. This critical statement leads us to a better understanding of the solenoid’s operation. From Figure 4 (right) observe that the magnetic fields from the permanent magnet and the electromagnetic interact. For clarity, we use the following terminology:

  • Forward polarity: The magnetic fields add together greatly increasing the force on the plunger. Another descriptive name for this condition is the pull state as the solenoid applies the maximum force pulling the plunger into the solenoid body.

  • Reverse polarity: The magnetic fields cancel each other reducing the force allowing the plunger to unlatch. This state is best described as the unlatching state as this is when the holding power of the permanent magnets is neutralized. It would be incorrect to call this a push state, as there is no magnetic force pushing on the plunger. Recall that spring tension alone extends the plunger.

Tech Tip: The featured solenoid is a pull type. The ability to unlatch is dependent on the spring tension. While the device can forcefully pull a load, it cannot push a load. As an experiment, you are encouraged to observe the solenoid’s operation without the spring installed.

Review of the solenoid states

We can summarize the solenoid states as follows:

  • Unpowered latched state: The plunger is stationary fully within the solenoid body and held in position by the permanent magnets. It will stay in this state until a large mechanical force is applied or until it is unlocked by application of a reverse polarity current.

  • Unpowered unlatched state: The plunger is extended. The solenoid will stay in this state until physically forced into the unpowered latch state or a forward polarity current is applied as described in the pulling state.

  • Pulling state: This is typically a temporary state where the plunger is being pulled inside the solenoid body in response to a forward polarity current. Once the plunger is pulled into position, power may be secured. The plunger will remain in position as described in the unpowered latched state.

  • Unlatching state: This is another temporary state where the plunger is in motion. It is moving from a latched state to an unlatched state. This state occurs when a reverse polarity current is applied to the solenoid.

  • Powered and latched: This state provides the highest holding power as the magnetic field caused by the forward polarity current aids the permanent magnet. The plunger will stay in position, even when power is removed as the system transitions into the unpowered latched state.

Confusion due to asymmetrical magnetic field

Before we conclude, we need to look at the asymmetry associated with the magnetic fields. We start by recognizing that the featured relay is designed for 24 VDC. In this application, the term is associated with a specific pull force at a specific distance as shown in Figure 5.

The coil is commonly assumed to have the same reverse polarity current. Although this assumption isn’t strictly false, it clouds our interpretation of the solenoid’s operation. This is especially true when we observe that the solenoid with a reverse polarity can be forced back into the latched position. Effectively, it looks like a powered latch operating despite reverse polarity. This is a confusing situation as the reverse current, designed to unlatch the solenoid, is holding it in place.

The key to understanding the solenoid is to recognize that the reverse polarity current should be less than the rated current. Empirical tests for the DSML-1153-24C show that the permanent and the electromagnetic fields cancel out when 12 VDC (reverse) is applied to the solenoid. When this is done, there is no apparent pull on the plunger when it is physically moved into the solenoid body. With the 12 VDC applied, the solenoid operates as if the permanent magnets were removed.

Best operating practices for the latching solenoid

Figure 6 presents one solution to the magnetic field asymmetry. The resistor R1 is chosen so that application of a reverse polarity will completely neutralize the magnetic field of the permanent magnets. The diode D5 ensures that full current will be applied when the forward polarity drive signal is applied.

An alternative is a full semiconductor-based H-bridge with Pulse Width Modulation (PWM) control signals. This has the advantage of energy savings. However, the complexity may not be worth the trouble, as the on-time for a latching solenoid is low. After all, the latching mechanism is why we selected this technology in the first place.

Figure 6: Schematic showing the asymmetric current. The 80 Ω resistor and Diode D5 provide asymmetrical current.
Figure 6: Schematic showing the asymmetric current. The 80 Ω resistor and Diode D5 provide asymmetrical current.975×595 35.7 KB

Figure 6: The 80 Ω resistor and Diode D5 provide asymmetrical current. When reverse polarity is applied (S2 and S3 closed), the permanent and electromagnetic fields cancel. When forward polarity is applied (S1 and S4 closed), full current is applied through Diode D5.

Tech Tip: With regard to Figure 6, never simultaneously close the S1-S2 or the S3-S4 switch pairs. A destructive condition known as shoot-through will occur effectively shorting out the power supply.

Parting thoughts

As I was preparing this article, I had the opportunity to show the latching solenoid to my teammates in the DigiKey Application’s Engineering division. The mechanism is a curiosity, as it is just outside the norm causing every single person to stop and take a second look.

If you are an educator in electronics or physics, I highly recommend you add this device to your labs. The solenoid provides a unique and intriguing way to learn about magnetic fields including topics such as air gap, permanent magnets, and electromagnets. With a bit of additional work, you can even measure the force of the solenoid, for example, by pulling down against a pan-type balance scale.

Please add your comments and questions in the space below. Also, test your knowledge by answering the questions and critical thinking questions located at the end of this note.

Best wishes,

APDahlen

Helpful links

Please follow these links to related and useful information:

About this author

Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (partially interwoven with military experience). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing educational articles about electronics and automation.

Highlighted experience

Dahlen is an active contributor to DigiKey’s Technical Forum. At the time of this writing, he has created over 180 unique posts and provided an additional 580 forum posts. Dahlen shares his insights on a wide variety of topics including microcontrollers, FPGA programming in Verilog, and a large body of work on industrial controls.

Questions

The following questions will help reinforce the content of the article.

  1. What is meant by the term latching with respect to a latching solenoid?

  2. What are the advantages of a latching solenoid?

  3. Why is the featured solenoid polarity sensitive?

  4. What is meant by the term “pull type” as applied to the featured solenoid?

  5. Describe the interactions of the permanent and electromagnet fields for both forward and reverse polarity. For full credit, include a sketch of the magnetic fields within the solenoid.

  6. Describe the pull state.

  7. Why is it inappropriate to talk about a push state?

  8. Why is it inappropriate to use a 1N4004 diode in parallel with the solenoid’s coil?

  9. What is meant by the asymmetry in the magnetic field with regards to the pulling state and latch release state?

  10. Describe the operation of the solenoid when the spring is removed. Be sure to include an explanation for the powered latch and unlatched states. Also include a description of the dynamic action when powered for both polarities.

  11. Describe the experiment to determine when the permanent and electromagnetic fields cancel.

  12. What would happen if a moderate load held the latching relay in the latched position? Hint: Differentiate between a pull and push solenoid. Then explain what pushes against the load.

Critical thinking questions

These critical thinking questions expand the article’s content allowing you to develop a big picture understanding of the material and its relationship to adjacent topics. They are often open ended, require research, and are best answered in essay form.

  1. Research and describe the terms monostable and bistable in the context of latching relays.

  2. Research and then describe the operation of a latching rotary solenoid.

  3. Energy saving is an important consideration for selecting a latching solenoid. Calculate the yearly energy savings for the featured solenoid assuming it is active for 16 hours a day with an energy cost of $0.14 / kWh.

  4. Why is a semiconductor H-bridge considered unnecessary for the latching solenoid?

  5. Using technology of your choosing, design a circuit to detect when the solenoid fails to engage.

  6. How does the energy consumption of the latching solenoid compare to the continuous operation calculated in the previous question. Is this a good metric to use in a business case supporting use of the latching relay? What other consideration(s) enter the design decision?