Custom Bistable and Latching Solenoids

A conventional solenoid produces force only while current flows through its coil. A bistable solenoid adds a permanent magnet to the magnetic circuit. When the plunger is seated against the pole, the magnet's flux crosses a near-zero air gap and holds the plunger firmly with no current at all. That is the latched state.

To release it, the drive applies a short pulse that creates flux opposing the magnet. The holding force collapses, and a return spring, or in a two-sided design a second magnet at the opposite end of the stroke, carries the plunger to its other stable position. A pulse of the original polarity latches it again. Energy is consumed only during each switching pulse, which typically lasts from a few milliseconds to a few tens of milliseconds.

"Latching" covers several different mechanisms, and choosing the right one is the first design decision:

• Permanent-magnet latching at one end: a magnet holds the plunger at the end of stroke and a spring holds the opposite end. Simple, compact, and the most common construction.
• Two-sided magnetic latching: magnetic holding at both ends of the stroke, for mechanisms that need a defined holding force in both positions.
• Mechanical latching: a ratchet or cam, like a push-push mechanism, holds position so that successive pulses alternate the state. No permanent magnet, but more moving parts and more wear.
• Residual-magnetism latching: the core material's own remanence provides a light hold. Lower holding force, and more sensitive to material and process variation.

A single-coil bistable solenoid switches state by reversing the polarity of the pulse. It makes the best use of copper and space, but the drive needs an H-bridge or an equivalent circuit to reverse the current.

A dual-coil design carries separate set and reset windings. Each winding only ever sees one polarity, so the drive can be two simple one-direction switches, which helps when the controller is minimal or the design has to reuse existing electronics. The trade-off is a larger coil, because each winding gets only part of the available space. We choose the arrangement from your electronics, envelope, and cost targets rather than defaulting to one.

• Zero holding power: current flows only while switching, so standby consumption is effectively nil.
• Minimal heat: no continuous coil dissipation, which matters in sealed enclosures and next to heat-sensitive components.
• Long battery life: the energy budget is a series of short pulses instead of continuous current.
• Holds through power loss: the actuator keeps its last position when power is removed.
• Quiet, steady holding: no hum or current ripple while latched.

Latching solenoids typically fall within these ranges. Holding force drops as the stroke lengthens, so the extremes do not occur together. Every design we build is custom to your specification across this range.

• Holding force: about 0.2 N to 50 N.
• Stroke: about 2 mm to 25 mm.
• Supply: DC switching pulses, typically 6 V to 28 V, lasting milliseconds to a few tens of milliseconds.

A latching solenoid that works on the bench can still fail in the field. These are the details that decide reliability, and the ones we design and test for:

• Holding-force margin. The latch must resist shock, vibration, and external loads on the plunger without releasing. We size holding force against the real loads, not only the nominal one.
• Release-pulse window. Too little reverse energy and the plunger stays latched; too much and it can be pulled back into a latch or, at worst, partly demagnetize the magnet. We define a pulse range that works across supply voltage and temperature.
• Magnet behavior over temperature. Neodymium magnets are strong but lose strength as they heat up; samarium-cobalt holds up better at high temperature. Magnet grade and geometry are chosen for your operating range.
• Air-gap tolerance. Holding force is very sensitive to the residual gap when latched, so the tolerance stack-up of plunger, pole, and housing is controlled from the start.
• Known position after power loss. A bistable actuator stays wherever it last was, so some products need position sensing or a stored-energy reserve to drive it to a safe state. We design that in when the application calls for it.

A bistable solenoid is only as reliable as the pulse that drives it. Because Cerebrus develops embedded systems in-house, we design the drive alongside the actuator: H-bridge or dual-switch drive, pulse timing that compensates for supply voltage, capacitor-based energy storage that guarantees a switch on power loss, and Hall-effect or switch-based position feedback. You get a characterized actuation subsystem, not just a part and a datasheet.

Development starts from your force, stroke, holding, envelope, and power requirements. We model the magnetic circuit, then build prototypes in our own shop and characterize them on the bench: force versus stroke, holding force, switching energy, response time, and behavior over temperature. Prototypes are built in-house in two weeks, so iterations turn around fast, and once the design is approved we manufacture production units ourselves under our ISO 9001:2015 quality system.