DC barrel jack and wall adapter

A 5.5 x 2.1 mm DC barrel jack with a wall adapter plugged into it: 12 V, 9 V or 5 V.

The adapters are 12 V 1 A, 12 V 2 A, 9 V 1 A and 5 V 2 A. Aliases: DC power jack, power adapter, wall wart, 12V adapter, 9V adapter, AC adapter.

The supply most Arduino projects end on. The adapter holds its voltage up to the current printed on its label; the jack brings it out on two pins, center pin positive and sleeve negative, the polarity nearly every adapter has.

Pins

Pin	What it does
V+	The center pin, positive.
V-	The sleeve, negative. Nothing connects it to ground but a wire.

Properties

adapter is 12V 1A, 12V 2A, 9V 1A or 5V 2A.

While it runs

The readout under the jack is the voltage across it.

What the model gets right

A regulated adapter is a supply with its knobs fixed. It is the bench supply's model under another name: the adapter's voltage behind 10 milliohm, with 10 uF across the output, until the load asks for more than the rated current. Into 24 ohm a 12 V adapter gives half an amp at 12 V. Into 6 ohm it would have to give 2 A: the 1 A adapter holds 1 A instead and the voltage falls to 6 V, and the 2 A adapter holds 12 V.

What it does not model

What a real adapter does past its rating varies: many fold back or hiccup, switching off and trying again, rather than holding the current, so the model's current limit is the gentlest version of it. Also left out: a switching adapter's ripple, the few percent its label allows, the cable's resistance, and the old unregulated transformer adapters, which read well above their label with nothing on them.

Common mistakes

Feeding 12 V to a board's 5 V pin. On an Uno, a 7 to 12 V adapter goes in the board's own barrel jack or on VIN, through the regulator; 5V is the regulator's output.

Running a motor or an LED strip from an adapter rated for less than it draws. Add up the currents first.

Forgetting to join V- to the board's ground when the adapter powers a module the board talks to.