Concentric Barrel Plug

These are the most commonly used output plugs for low voltage power supplies and are available in many different lengths and diameters. Although the length and outside diameter (OD) are easily measured, the dimension of the inside diameter (ID) is difficult to accurately measure and is critical to ensure good electrical contact and mechanical mating. The Concentric DC Barrel Plug has a insulating ring (usually black coloured) at the tip of the shaft to separate the inside and outside contacts. The inner contact is known as the TIP and the outside contact is known as the BARREL. A plug with an ID that is smaller than the jack will not fit, whist a plug with a larger ID than the jack will have intermittent electrical contact.  2.1mm ID and 2.5mm ID are particularly easily confused. The table below shows the most commonly used variants. 

Concentric Barrel Plug
A (OD) B (ID) C (L)

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6.5 3.0 8-14
5.5 2.5 8-14
5.5 2.1 8-14
5.0 2.5 8-14
5.0 2.1 8-14
3.8 1.35 8-14
3.8 1.05 8-14
3.5 1.35 7-14