Solenoids guide
Monostable vs Bistable Solenoids: How to Choose
Choosing between a monostable and a bistable solenoid is one of the first decisions in an actuator design, and it shapes the drive electronics, power budget, thermal design, and failure behavior of the whole product. This guide sets out the differences that matter and how we decide between them. It covers solenoid actuators; the same terms are used for solenoid valves, where the logic is similar but the hardware differs.
In short
A monostable solenoid has one stable position and needs continuous power to stay actuated, returning to rest by spring when power is removed. A bistable (latching) solenoid has two stable positions, switches on a short pulse, and holds either one with no power.
| Property | Monostable | Bistable (latching) |
|---|---|---|
| Stable positions | One (rest) | Two |
| Power to hold the actuated position | Continuous current | None: held by a magnet or latch |
| Energy use | Grows with time spent actuated | Only during each switching pulse |
| Coil heating | Continuous while actuated; limits duty cycle | Brief, only while switching |
| On loss of power | Spring returns it to rest (fail-safe) | Stays in its last position |
| Drive electronics | A single switch; peak-and-hold optional | Polarity reversal (H-bridge) or separate set and reset coils |
| Holding force set by | Holding current | Magnet strength and latched air gap |
| Best suited to | Safety defaults, short actuations, simple control | Battery power, long holds, sealed or heat-sensitive products |
The core difference
Both are electromagnetic actuators built from a coil, a magnetic frame, and a moving plunger. The difference is what holds the plunger when the coil is off. In a monostable solenoid, nothing does: a spring returns it to rest, so holding the actuated position takes continuous current. In a bistable solenoid, a permanent magnet or a mechanical latch holds the plunger at either end of its stroke, so current is needed only to switch.
Choose a monostable solenoid when
- The mechanism must reach a known position whenever power or control is lost.
- Actuations are brief, or power for holding is readily available.
- The control electronics should be as simple as possible.
- Holding times are short compared with the time spent idle.
Choose a bistable solenoid when
- The product runs from a battery or harvested energy.
- The actuator holds a position for long periods.
- Coil heat would be a problem in a sealed enclosure or near sensitive parts.
- The state should survive a power interruption.
- Holding must be silent and draw no current.
Hybrid approaches
The choice is not always binary. A bistable solenoid paired with a stored-energy reserve can drive itself to a safe position on power loss, combining zero holding power with fail-safe behavior. A monostable solenoid with peak-and-hold drive can cut its holding power enough to meet a budget that seemed to demand a latching design. We evaluate both before committing.
Questions that decide it
- What must the mechanism do when power fails: hold, return, or move to a safe state?
- How long does it stay actuated, compared with how long it sits idle?
- How much energy is available per actuation, and from what source?
- What envelope and thermal limits does the actuator have to fit within?
- What drive electronics exist, or can be added?
Solenoid valves versus solenoid actuators
Much of what is written about monostable and bistable devices concerns pneumatic and fluid valves, where a bistable valve keeps its spool in the last commanded position and a monostable valve springs back. The same logic applies to the plunger of a solenoid actuator. Cerebrus designs the actuator itself, including actuators that operate valves.
FAQ
Monostable vs Bistable: frequently asked questions
What is the difference between a monostable and a bistable solenoid?
A monostable solenoid has one stable position and needs continuous current to stay actuated; a spring returns it to rest when power is removed. A bistable, or latching, solenoid has two stable positions, switches on a short pulse, and holds either one with no power, usually using a permanent magnet.
Which uses less power?
For anything that holds a position for long, a bistable solenoid, because it draws current only while switching. If the actuator is energized only briefly and rarely, the difference can be small, since a bistable design needs a pulse to switch in each direction.
Which one is fail-safe?
A monostable solenoid is inherently fail-safe, because its spring returns it to rest on loss of power. A bistable solenoid stays in its last position, so fail-safe behavior has to be designed in, for example with a stored-energy reserve that drives it to a safe state.
Is a bistable solenoid more expensive?
Usually somewhat, because of the permanent magnet and the more capable drive electronics. At the product level, the savings from smaller batteries, lower power, or less heat often outweigh the difference.
Can a bistable solenoid be made fail-safe?
Yes. A capacitor or other stored-energy reserve can supply the switching pulse after power is lost, driving the actuator to a defined safe position. Position sensing can confirm it got there.
Are latching and bistable solenoids the same thing?
Generally, yes: "latching" describes how the solenoid holds position without power, and "bistable" describes its two stable states. Mechanical push-push latches are also called latching solenoids but use a ratchet rather than a magnet.
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Start a custom solenoid project
Send us what you know; rough numbers are fine. These are the requirements that define a solenoid design:
- Motion: linear push or pull, or rotary, plus the stroke or angle
- Force: required force at the start and end of the stroke, or the load curve
- Holding: monostable or bistable (latching), and any holding force needed with power off
- Electrical: supply voltage and source (battery, DC bus, AC), and the current or energy available per actuation
- Timing: response time, cycle rate, and how long it must stay actuated
- Envelope: maximum size, mounting, and how the load attaches
- Environment: temperature range, shock and vibration, moisture or washdown
- Life: required number of cycles
- Power loss: should it hold, return, or move to a safe position?
- Quantities: prototype quantity and expected annual volume
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