Latching Solenoid Drive Circuits

A bistable solenoid needs a pulse in one direction to set it and a pulse in the other direction, or on a second coil, to reset it. Between pulses, the drive must draw no current and must not let noise or a supply transient switch the solenoid by accident. Each pulse must carry enough energy to switch the solenoid at the lowest supply voltage and hottest coil, but the reset pulse must not be so strong that the reversed field pulls the plunger back into the latch, or in the extreme weakens the magnet.

A single-coil latching solenoid is reset by reversing the current through its one winding, which takes a full H-bridge: four switches arranged so either end of the coil can be driven high or low. Integrated H-bridge driver ICs, the same kind used for small brushed DC motors, do this in one package with built-in protection.

During a pulse, two diagonal switches conduct. When they turn off, the energy stored in the coil returns to the supply through the bridge's diodes, so no separate flyback diode is needed, but the supply must have enough capacitance to absorb it.

A dual-coil solenoid has separate set and reset windings, so each needs only a single switch, typically a low-side MOSFET, with a clamp across each winding to absorb the switch-off energy. The circuit is simpler, but each winding fills only part of the coil space, so a dual-coil solenoid is larger than a single-coil design with the same force. It suits products with minimal controllers or existing one-direction outputs.

The pulse must last long enough for the plunger to complete its travel and seat; cutting it short can leave the plunger mid-stroke, unlatched. The time needed grows as the supply voltage falls and as the coil heats, because both reduce current. Many designs therefore measure the supply voltage and stretch the pulse to deliver consistent energy rather than using one fixed width. Pulses typically last from a few milliseconds to a few tens of milliseconds.

• Verify switching at the lowest supply voltage and highest coil temperature.
• Verify the release pulse at the highest supply voltage, where the risk of over-driving is greatest.
• Interlock the inputs so set and reset can never be driven together.
• Limit how quickly pulses can repeat, so a fault cannot overheat the coil.

A latching solenoid may need more peak current than a small battery, such as a coin cell, can supply. Capacitor drive solves this: the battery slowly charges a storage capacitor, and the capacitor delivers the pulse, so the battery only ever sees a small charging current. The capacitor is sized to switch the solenoid at its lowest charge voltage, and the firmware confirms it is charged before firing.

The same stored energy can drive the solenoid to a safe position if main power is lost, which is how a latching design is made fail-safe.

A latching solenoid keeps its position through a power loss, but after a reset or a mechanical shock the controller may not know what that position is. Common ways to find out:

• A Hall-effect sensor detecting the plunger, or a magnet mounted on it.
• A mechanical or optical switch at one end of the stroke.
• Sensorless detection: the coil's inductance is higher when the plunger is seated, so the rate at which current rises during a brief, low-energy test pulse reveals the position.
• Driving to a known state at start-up, so the position is defined by design.

• Defined default states on the driver inputs, so they cannot float at power-up and fire a pulse.
• Enough supply capacitance to absorb the energy returned at the end of each pulse.
• Transient suppression on long coil leads.
• A hardware or firmware timeout, so a fault can never hold the coil on and overheat it.

Because the drive sets the voltage, current, and pulse the coil actually sees, we design the two together: coil resistance and turns matched to the driver and supply, pulse timing set from measurements on the prototype, and the complete actuator characterized across voltage and temperature. Cerebrus develops the embedded electronics in-house, and prototypes are built in-house in two weeks.