Keep Solenoid vs. Dual Bistable Solenoid: What's the Difference?

Latching solenoids are widely used in smart locks, industrial automation, medical devices, energy management systems, and access control equipment because they consume power only during switching. Among the most common types are the keep solenoid (single latching solenoid) and the dual bistable solenoid. Although both are designed to maintain position without continuous power, they differ significantly in magnetic structure, operating principle, performance, and cost.

Understanding these differences is essential when selecting the right actuator for your application. This guide compares keep solenoids and dual bistable solenoids in detail, including their working principles, structural design, advantages, disadvantages, and typical applications.

What Is a Keep Solenoid?

A keep solenoid, also known as a single latching solenoid, uses a permanent magnet to hold the plunger in one position after the coil is energized. Once the plunger reaches the holding position, the permanent magnet maintains the magnetic attraction without requiring continuous electrical power.

To release the plunger, the coil is energized with reverse polarity. The reverse magnetic field neutralizes the magnetic holding force generated by the permanent magnet, allowing the return spring or external mechanical force to move the plunger back to its original position.

Working Principle

1. Initial state: The plunger is released.
2. Forward current energizes the coil.
3. The electromagnetic force attracts the movable core.
4. The permanent magnet locks the plunger in position after power is removed.
5. Reverse current is applied to cancel the magnetic holding force.
6. The return spring or external mechanism pushes the plunger back.

Since only one stable holding position is created by the permanent magnet, this design is commonly referred to as a single latching or keep solenoid.

What Is a Dual Bistable Solenoid?

A dual bistable solenoid features a different magnetic architecture. Instead of relying on a single coil and one permanent magnetic holding position, it incorporates two independent coils with a permanent magnet positioned between them.

Each coil controls movement in one direction. When Coil A is energized, the plunger is attracted or pushed toward one end of the stroke. When Coil B is energized, the magnetic field reverses and drives the plunger toward the opposite position. Both end positions are maintained magnetically after power is removed, making the actuator truly bistable.

Working Principle

1. The permanent magnet is positioned between two electromagnetic coils.
2. Coil A is energized, pulling or pushing the armature to Position A.
3. The permanent magnet maintains the armature in Position A after power is removed.
4. Coil B is energized.
5. The magnetic field shifts, moving the armature to Position B.
6. The permanent magnet maintains the armature in Position B until the next switching operation.

Unlike a keep solenoid, both positions are magnetically stable, eliminating the need for a return spring in many applications.

Structural Comparison

Advantages of Keep Solenoids

• Simple mechanical structure
• Lower manufacturing cost
• Compact design
• Low power consumption
• Easy electronic control
• Suitable for high-volume production
• Reliable magnetic holding

Because only one coil is required, keep solenoids generally have fewer components, making them easier to manufacture and maintain. Their straightforward design also contributes to excellent long-term reliability.

Advantages of Dual Bistable Solenoids

• Two stable holding positions
• No continuous power required
• Fast switching performance
• Excellent positioning accuracy
• Suitable for complex automation systems
• Reduced dependence on return springs
• Greater design flexibility

Dual bistable solenoids are ideal for applications requiring reliable movement in both directions without relying on mechanical springs. Since both end positions are magnetically maintained, the actuator remains stable even during power loss.

Cost Comparison

Although both products are energy efficient, the manufacturing cost differs considerably.

For the same overall dimensions, a dual bistable solenoid generally costs more than a keep solenoid due to its more sophisticated magnetic circuit and additional components.

Main Reasons for Higher Cost

• Two precision coils instead of one
• More complex permanent magnet arrangement
• Additional magnetic components
• Higher assembly precision requirements
• More complicated manufacturing process
• Longer production time
• Additional electrical testing

In many cases, the price of a dual bistable solenoid may be 20% to 50% higher than a keep solenoid of comparable size, depending on the design complexity, production volume, and application requirements.

Performance Comparison

Typical Applications

Keep Solenoid Applications

• Electronic cabinet locks
• Smart door locks
• Parcel lockers
• Hotel lock systems
• Battery-powered devices
• Security equipment
• Medical locking mechanisms

Dual Bistable Solenoid Applications

• Industrial automation equipment
• Valve control systems
• Switching mechanisms
• Precision positioning equipment
• Medical automation devices
• Robotic systems
• Energy management equipment

How to Choose the Right Solenoid?

Choosing between a keep solenoid and a dual bistable solenoid depends on the functional requirements of your application.

• Select a keep solenoid if your system requires only one magnetically held position, uses a return spring or external mechanical force, and prioritizes lower cost and simplified control.
• Select a dual bistable solenoid if your application requires two stable positions, precise bidirectional movement, spring-free operation, or enhanced positioning performance.

An experienced custom solenoid manufacturer can evaluate factors such as stroke length, holding force, switching speed, duty cycle, operating voltage, installation space, and environmental conditions to recommend the most suitable solution.

Conclusion

Both keep solenoids and dual bistable solenoids provide significant energy savings by requiring power only during switching. However, their operating principles are fundamentally different.

A keep solenoid uses a single coil and a permanent magnet to maintain one holding position, while reverse current releases the armature with assistance from a return spring or external mechanism. In contrast, a dual bistable solenoid employs two independent coils and a centrally positioned permanent magnet to create two magnetically stable positions, allowing reliable bidirectional operation without continuous power.

Although dual bistable solenoids offer greater functionality and flexibility, their more complex design results in higher manufacturing costs. Selecting the appropriate solution depends on your application's motion requirements, control strategy, performance expectations, and budget.