LCN SEM7840 Magnetic Holder

The LCN SEM7840 is an electromagnetic door holder that mounts recessed in the wall and holds a fire door open until power is cut. The magnet body sits in the wall. An armature plate mounts on the door edge or face. When current flows through the magnet, the magnetic field pulls the armature and holds the door in the open position with 35 pounds of holding force. When the fire alarm activates and the circuit opens, the magnetic field collapses, the armature releases, and the door closer on the door takes over and shuts the door completely.

The SEM7840 does not have a door closer built in. It's a holder, not a closure. A compatible manual door closer mounted separately on the same door controls the closing action after the magnet releases. If there's no closure on the door, the door won't close after the magnet releases. Confirm a door closer is already specified or installed on the opening before specifying the SEM7840 as the hold-open component.

All SEM7800 Series magnets are UL listed for smoke barrier and labeled fire doors. ANSI A156.15 certified. This fire door listing is what separates SEM Series magnets from generic electromagnetic holders: the UL listing confirms performance under the specific conditions fire doors are tested for. For all LCN door closers, browse our door closers catalog. For all electromagnetic holders and electronic hardware, view our electronic hardware catalog.

Fail Safe Operation and Fire Alarm Integration

Fail safe means the door releases when power fails. The SEM7840 holds the door open as long as current flows. When the fire alarm panel cuts power to the magnet circuit, the hold releases immediately and the door closes shut. This is the required behavior for fire and smoke barrier doors: they must close automatically when the fire alarm activates, and the electromagnetic holder must release when that signal comes.

The wiring goes from the building's fire alarm panel output through the magnet circuit. When the panel sends the signal (typically cutting power to the held-open devices), the SEM7840 releases along with all other magnetically held fire doors in the system simultaneously. Note: the outlet box for the wall recess is not included with the SEM7840 and must be sourced and installed separately.

Tri-Voltage Design

The SEM7840 operates on 24V AC, 24V DC, 120V AC, or 120V DC without requiring a different model for each voltage. The tri-voltage design means the same unit works with both 24V access control systems and 120V building power, which simplifies specifying across different building electrical configurations.

LCN SEM7840 Specifications

• SKU: SEM7840

• Also known as: LCN SEM7840, SEM 7840, EM7840AL, SEM7840ALUM, SEM7840DKBRZ, cemd840, LCN magnetic door holder, lcn electromagnetic door holder, lcn mag hold open

• Series: LCN SEM 7800 Series, Sentronic

• Type: Electromagnetic door holder, recessed wall mount

• Hold force: 35 lbs

• Dimensions: 2" (51mm) x 4" (102mm) x 1-1/2" (38mm)

• Housing: Die cast

• Mounting: Low profile recessed wall mount

• Fail mode: Fail safe; releases when power is interrupted

• Voltage: Tri-voltage; 24V AC/DC or 120V AC/DC

• UL: Listed for smoke barrier and labeled fire doors

• ANSI: A156.15

• Includes: Die cast housing, cover, armature, WMS screw pack

• NOT included: Outlet box (must be sourced and installed separately)

• Closer requirement: Requires separate door closer for door closing function

• Finishes: Aluminum (BHMA 689), Dark Bronze (BHMA 695)

• Resources: SEM7800 Catalog Information in Resources section

SEM7840 vs Other SEM7800 Variants

The SEM7800 Series offers multiple mounting configurations. The SEM7840 is the recessed wall mount - the magnet body sits inside the wall with a low-profile appearance when installed. Other SEM7800 variants include floor mount and surface wall mount configurations for applications where wall recessing isn't practical. Confirm the mounting type required for the specific opening before ordering between SEM variants.