Barrel jacks + plugs
A very wide variety of DC power adapters and plugs is used in consumer and business computers, IT equipment, and consumer electronic equipment. The most common connectors have a “barrel”-style plug on the cable, accepting a centre pin located on the receptacle (“jack”) mounted on the device. The sensible convention has the barrel at 0 V (“common”) and the centre pin positive (+ve).
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The most common (and sensible) connectors have a barrel 5.5 mm (OD) in diameter, accepting a 2.1 mm (ID) centre pin. On the (cable-mounted) plug, the barrel protrudes either 9.5 mm or 11 mm from the mating surface; to be conservative, choose 9.5 mm for a plug and 11 mm for a jack.
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Smaller equipment might have DC power provided through a 4.0 mm barrel with a 1.7 mm pin, again centre positive, and again either 9.5 mm or 11 mm deep.
There’s no standard for what DC voltage might be provided by an AC adapter or required (or tolerated) by a piece of equipment, but 12 V is very common. There’s no standard current capacity or requirement, but 12 V at about 1 A (ie, 12 W) is easily achieved by a wall-mounted AC adapter (“wall-wart”). A device requiring more than 12 or 15 watts would ordinarly use an AC adapter with an AC power cable.