LiftMaster K21-10340 Commercial Operator Transformer – 115V to 24VAC, 20VA

The LiftMaster K21-10340 is a replacement control transformer for select LiftMaster mechanical commercial door operators. It converts the operator’s 115-volt incoming supply to 24VAC control power for compatible relays, control stations and other low-voltage operator circuitry.

This is a voltage- and model-specific service component—not a universal garage-door opener transformer.

Specifications

• Manufacturer: LiftMaster

• Primary input: 115VAC

• Secondary output: 24VAC

• Power rating: 20VA

• Component type: Commercial operator control transformer

• Manufacturer part number: K21-10340

• Related part references: 21-10340, 215115LD and G215115LD

• Application: Select mechanical commercial door operators

• Quantity: One transformer

Compatibility

Use only when the operator’s parts diagram, electrical schematic or existing transformer specifies K21-10340, 21-10340, 2110340, 215115LD or a confirmed equivalent.

Before ordering, verify:

• Operator model and production revision

• Incoming line voltage

• Required 24VAC secondary output

• Required 20VA capacity

• Terminal configuration

• Mounting arrangement

• Existing transformer part number

Do not substitute this transformer for a 208/240V model based only on appearance. For operators requiring 208/240V input, see the LiftMaster 208/240V to 24VAC Transformer.

A LiftMaster 120/240V to 24VAC 20VA Transformer is also available for compatible Logic 5 operator applications.

Diagnostic Note

A failed transformer may be the result of a shorted accessory, damaged control wire, defective relay or overloaded low-voltage circuit. Diagnose and correct the underlying fault before energizing a replacement transformer.

Installation and Safety

Commercial operator electrical service should be performed by a qualified door technician or electrician. Disconnect and lock out the operator’s power supply before opening the electrical enclosure.

Confirm zero voltage before handling wiring. Follow the operator’s wiring diagram exactly, protect all conductors from abrasion and verify the correct primary and secondary connections before restoring power.