Wire & connectorsRebuild shelfHand tools, works as-is

Barrel jack

Also called DC jack, 2.1 mm plug, 5.5 mm jack.

The round plug on routers, guitar pedals, and laptop supplies. The two common bores are 2.1 mm and 2.5 mm, both in a 5.5 mm shell, and they almost fit each other, which is how center pins get bent. Center-positive is usual and not universal.

If you are new to this category

Wire and plugs are how power and signals move. The insulation is the safety, not the copper. Ethernet cable is fine for small signals and low voltage. It is not a substitute for a power cord. A connector you can unplug is worth more than a wire you have to solder every time.

Match the plug that came with the brick. Center-positive is common and not universal. The drawing on the label is a map: the dot is the center, the ring is the outside.

Center pin and outside sleeve

The drawing on the brick is the polarity. A dot inside a C-shaped ring means the center is positive when the plus sign points at the dot. Measure anyway. The two common hole sizes, 2.1 mm and 2.5 mm, almost fit each other and that is how the center pin gets bent.

Specification. The right-hand column is what is typical. The label on the part in your hand wins if they disagree.
What to look for	Usual figure
Size	Measure the pin and the hole. Do not judge by eye.
Polarity	The symbol on the brick is a circle with a dot and a ring. Confirm with a meter.
Current	The connector is often rated around a few amps. A laptop tip that is proprietary is not this part; it is a model-specific plug.

Volts are electrical pressure, amps are how much current is flowing, and ohms are how hard the part resists that flow. A higher voltage than the part was built for is how salvage parts die on the bench. If a cell in the table says “read the stamp”, that stamp is the whole specification.

What people actually use it for

Power inlet on a project
Matching a brick you already labeled

Leave this one when

The sleeve is loose. Intermittent barrel jacks arc and heat.

Buying a new one is the better idea when

A bag of 5.5×2.1 mm jacks. Standardize on one size in your own projects.

How to tell if this one still works

Continuity from the plug's center to the correct pad, and sleeve to the other, while you flex the cable.

Tools you will want: Meter, Calipers if you have them.

Words on this page

Short meanings, so the rest of the entry is readable if you are not already doing this every weekend.

polarity: Which side is plus and which is minus. Reversed polarity ruins a lot of electronics. The symbol on a power brick is the map.
continuity: A meter mode that beeps when two points are connected. It is how you map a switch: beep, then click, then see if the beep changes.

Where people actually find it

2 notes, most useful first. “Leave it” means the part is in that machine and you should not take it: it is dangerous, worn out, or only useful if you already know that exact repair.

Harvest at a glance
From	Call	How many	How often	Where it sits
Router or modem	Worth it	1	always	Rear panel.
Wall adapter	Worth it	1	usually	The plug.

Worth pullingRouter or modem1 · always
In this particular machine
5.5 mm class. Confirm the bore.
Where to look
Rear panel.
How to get it out
Keep the jack with the brick.
The condition to expect
Loose sleeves are a reject.
Worth pullingWall adapter1 · usually
In this particular machine
Measure the bore. Confirm polarity.
Where to look
The plug.
How to get it out
Keep it on the brick.
The condition to expect
Bent center pins are scrap.