An additional caution I learned the hard way: if trying to fit an appropriate barrel plug to a barrel jack, **do not** get a barrel plug whose outer diameter is exactly as large as the jack's. If you do this, you may not get good connection with the center pin, because that is made by the jack's sprung outer contact pressing the (rigid) plug sideways against the center pin, which can't happen if the plastic shell holds the plug centered. (This is assuming a rigid center pin — some, but not all, jacks have split/sprung center pins.)