3.5mm / 1.1mm to 5.5mm / 2.1mm DC Jack Adapter
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2.1mm inner diameter (5.5mm outer diameter) is the most common type of DC plug, but you can use this adapter for anything else that has a 1.1mm ID (3.5 OD) DC jack. This adapter lets you convert something with a 3.5mm OD plug into something that will accept a 5.5mm OD plug.
Push the adapter in all the way, and you'll feel a satisfying click to know it's seated.