Why locker makers keep calling us about the same thing
Last month we had four separate inquiries from locker integrators, three in Europe and one in Korea, all asking for a “solenoid latch” without attaching a drawing. They meant different things. One wanted a fail-secure latch for a hospital medication cabinet. Another wanted a fail-safe release for a residential parcel locker. The component is the same family, but the spec is completely different. This article is the explainer I wish I could send them before the call.
What a solenoid latch actually does
A solenoid latch is an electromagnet with a moving plunger that engages or releases a mechanical catch. In a locker, the catch (usually a pawl or striker) holds the door closed. When the controller sends a voltage pulse, the plunger retracts, the catch releases, and a spring or the user pushes the door open. Power is only needed for the instant of release; the rest of the time the coil sits at zero current.
That last point is why locker designers like them. A maglock has to stay energized to stay locked, which means heat and a constant drain on the battery backup. A solenoid latch locks mechanically and only spends energy to unlock.
Fail-secure vs fail-safe, and why it decides your spec
This is the first question I ask a new locker client.
Fail-secure means power loss keeps the door locked. The solenoid holds the catch engaged with no power, and a spring or permanent magnet does the holding. You need a physical key or a supercapacitor backup to open it during an outage. Good for: medication cabinets, high-value parcel compartments, server rack locks.
Fail-safe means power loss opens the door. The solenoid has to stay energized to keep the catch engaged, so when power dies the door springs open. Good for: public parcel lockers where the user must retrieve their package during a blackout, and for fire-escape-adjacent cabinets.
Most parcel locker banks we ship use fail-secure latches on individual compartments (with a key override) and a fail-safe maglock on the technician service door. One battery pack covers both.
The numbers that actually matter on the BOM
| Parameter | Typical range we quote | Why it bites if you get it wrong |
|---|---|---|
| Voltage | 12 V DC or 24 V DC | Match the cabinet PSU exactly; 24 V coils run cooler |
| Stroke | 3 to 8 mm | Too short and the catch never clears; too long and the door gap looks wrong |
| Holding force | 0.5 to 5 kgf | Under-spec and the door drifts open in summer when the gap grows |
| Pull-in current | 0.3 to 1.2 A at 24 V | Drives your PSU sizing and your cable gauge |
| Response time | under 100 ms | Users perceive anything over 300 ms as “broken” |
| Duty cycle | 10% to 25% intermittent | Continuous-duty coils in a 30-bank will cook |
| Operating temp | −20 °C to +60 °C | Outdoor lockers in the Middle East hit the top of that range |
The air-gap trap
Holding force drops roughly 10–15% for every extra 0.1 mm of air gap between the plunger and the striker. In theory your door fits perfectly. In the field, every door fits slightly differently, the gasket compresses, the hinge wears. We spec the magnet with at least 30% force margin above the worst-case gap we can measure on a sample cabinet. If a client sends us the cabinet drawing up front, we model the tolerance stack-up instead of guessing.
Noise: the complaint nobody warns you about
A solenoid latch goes thunk. In a luxury apartment lobby that thunk annoys residents more than you’d expect, and we’ve had integrators ask us to “make it quieter” after installation. Three fixes, cheapest first: a soft-start driver that ramps current over 50 ms, a hydraulic damper on the plunger, or a brushless motor on conveyor-driven doors. The soft-start circuit is the one I recommend most; it costs cents and cuts the impact noise by maybe half.
When to step up to a bi-stable latch
If your locker is battery-powered or solar-powered, a standard solenoid latch still draws a pulse every unlock, and over 50,000 cycles that adds up. A bi-stable latch uses a permanent magnet for holding and needs only a short pulse to flip state in either direction. Zero standby current. We ran a 12-compartment solar cabinet in a desert deployment on a 20 Ah battery for 14 months with bi-stable latches; the previous design with ordinary solenoids lasted six weeks. If you’re off-grid, this is the part to specify.
A spec sheet checklist
When you email a solenoid supplier, include: voltage, stroke, required holding force with the worst-case gap, duty cycle, ambient temperature range, fail-safe or fail-secure, and whether you need a key override. Half the delays we see come from a missing one of those. If you’d rather not fill it all in, send us your cabinet drawing and we’ll work the numbers.
For background on where solenoids sit inside a full parcel locker system, see our deep dive on electromagnets in parcel lockers and vending machines.
Written by the engineering team at Cixin Solenoid. We manufacture custom solenoid latches, bi-stable solenoids, and tubular electromagnets for locker, medical, and industrial OEMs.
