Simple Trick for Determining 2.1mm or 2.5mm Barrel Jacks

Here’s a tip I got recently from Donald Bell’s Maker Update. It’s from an old DigiKey “Teaching Moment.”

Many barrel jacks have a consistent 5.5mm outside diameter. But the inside diameter can vary, usually either 2.1mm or 2.5mm.

If you don’t have your digital calipers handy, you can determine this inside diameter with common objects: a toothpick, a ball point pen, or a test probe.

The brass housing on a ball point pen is 2.2mm. If it doesn’t fit inside the barrel of your jack, then the jack is 2.1mm. If the pen easily fits inside, the jack is 2.5mm.

A standard toothpick is 2.1mm. If it fits snugly in the jack, it’s a 2.1mm jack. Also a common multimeter probe needle with fit closely inside of a 2.1mm jack and be very sloppy inside of a 2.5mm jack.


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