Liftmaster 21-16699 Transformer, 120/240V, 75VA, 24VAC
The LiftMaster 21-16699 replacement transformer provides low-voltage control power inside compatible commercial door operator electrical boxes. A failed transformer can cause symptoms such as no low-voltage controls, dead push-button stations, no board power, or intermittent control-circuit operation after the incoming line voltage has been confirmed. This transformer is rated for 120/240V primary input with a 24VAC, 75VA output and should be matched to the original wiring diagram before replacement. It is commonly used in LiftMaster Speed Jack and industrial-duty commercial door operator platforms where a larger 75VA control transformer is required. Because similar LiftMaster transformers can look alike but have different VA ratings, primary taps, and secondary outputs, the part number and wiring diagram should be checked before upload or installation.
Compatible With
Select LiftMaster Speed Jack commercial door operators; select LiftMaster Logic 4.0 and Logic 5.0 commercial operators; commonly associated with APT, H, DHJ, J, DJ, T, GH, GSD, GT, HJ, MGJ, MH, MHS, MJ, MT, and related industrial-duty operator families when the parts diagram lists 21-16699 or K21-16699.
Features and Specifications
| Manufacturer | LiftMaster / Chamberlain Group |
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| Product Type | Control transformer |
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| Part Number | 21-16699 |
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| Primary Voltage | 120/240V |
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| Secondary Voltage | 24VAC |
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| VA Rating | 75VA |
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| Application | Low-voltage control power for select commercial door operators |
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| Compatible Operator Type | Speed Jack, Logic 4.0, Logic 5.0, and related industrial-duty operators where specified |
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| Notes | Match transformer taps, VA rating, and wiring diagram before replacement. |
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Also Known As / Related Part Numbers
- 21-16699
- K21-16699
- 2116699
- K2116699
- LiftMaster 120/240V 75VA 24VAC transformer
Technician Note
For LiftMaster commercial operator service parts, the safest fitment method is to match the original component label and the model-specific parts diagram. Similar-looking electrical parts can have different ratings, wiring, or software versions.
Before Ordering Checklist
- Confirm the exact operator model and serial information when available.
- Match the original part number or service part number on the failed component.
- Verify voltage, phase, amp range, timing function, or control-circuit requirement before ordering.
- Compare connector layout, mounting style, terminal markings, and wiring diagram.
- Do not bypass safety devices, fire-door controls, or required monitored protection during troubleshooting.
Professional Advisory
This item may be part of a high-voltage commercial door or gate operator system. Electrical testing, motor replacement, transformer replacement, control-board service, overload adjustment, and fire-door operator repairs should be handled by a qualified technician. Never bypass required safety devices or fire-door controls to force an operator to run.
Questions and Answers
What does the 21-16699 transformer do?
It converts the operator line voltage to 24VAC control power for compatible LiftMaster commercial door operator control circuits.
Is 21-16699 the same as K21-16699?
K21-16699 is a common LiftMaster service-part reference for the 21-16699 transformer. Always match the label and parts diagram.
Can I use a lower VA transformer instead?
No. The VA rating must match the operator requirement. Using an undersized transformer can cause control problems or overheating.
Which operators commonly use this transformer?
It is commonly associated with select LiftMaster Speed Jack, Logic 4.0, and Logic 5.0 commercial operators, including APT, H, J, T, GH, GSD, GT, HJ, MJ, MT, and related models where specified.
Is this a high-voltage part?
Yes. The primary side connects to line voltage. Installation and testing should be performed by a qualified commercial door technician.
