While resistive heating is not a primary concern, the environment the solenoid will be operating in is. Understanding the temperature of the environment as well as any potential temperature spikes is critical in the early feasibility stage of a solenoid’s design. For example, whether the solenoid will be located near a significant heat source may need to be taken into consideration. Permanent magnets can exhibit performance variations with temperature changes. They can also suffer permanent performance losses from overheating. Various magnet materials suffering that type of damage will perform differently and will also vary in terms of magnetic field consistency over a given temperature range. Understanding these factors is crucial when selecting magnet material.
The benefit of low power consumption does come with one challenge. A bistable latching solenoid will remain in the last commanded position in the event of power loss rather than return to the home position, as with a conventional on-off solenoid. A fail-safe release circuit must be part of the solenoid’s design if the solenoid needs to fail to a safe position in the event of power loss to protect vehicle systems and provide for passenger safety.
The fail-safe is simply an integrated capacitive discharge circuit. The capacitor is charged in parallel to the solenoid. Even though the solenoid only requires a short power pulse to change state, the capacitor continues to receive a constant voltage signal to maintain charge, but it draws nearly zero current. If power is lost to the capacitor, the capacitor discharges and moves the solenoid’s armature to the designated safe position.
There are two points that must be considered when using a fail-safe circuit. First, it adds cost and size to the solenoid that must be weighed against the other functional benefits. Second, discharged capacitors have very low electrical resistance, which means that the system will see a very high in-rush current that is not seen when using a stand-alone solenoid. In one case, an in-rush current of 24 amps was observed for a few milliseconds where 2 amps was the nominal peak current of the solenoid. This means measures do have to be taken for circuit protection and electromagnetic interference isolation where this could be a problem.
As OEMs design new vehicle systems, they need to choose components that maximize energy efficiency, perform consistently, and keep costs manageable. Latching solenoids are an ideal choice for applications where the solenoid must hold a position for an extended period. Because they only use a short power pulse to change state, they draw very little power, do not suffer the negative effects of resistive heating, and their smaller coil reduces the size of the overall package. The addition of a fail-safe circuit will add size and cost to the solenoid, but this functionality can be crucial for protecting vehicle systems and passengers.