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I'm working on an project which will be given to some friends and will be open-source too. I need to choose the power supply connector (5V or 12V @ 3A) and I'm thinking to barrel connectors. I found there're 2 version of them, the 2.1mm and 2.5mm for the internal pin.

I read also the 2.5mm male will fit the 2.1 female but connection is not stable, so it is not a reliable solution to choose the smaller one to accept both.

So, which is the most used? The goal is to have the maximum compatibility with existing PSU around the house.

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    \$\begingroup\$ Since both are about equally popular, you should make your own statistical analysis of the "existing PSU you have around the house". Because it will likely be different from what I have, or from what my neighbor has. \$\endgroup\$ – dim Nov 19 '18 at 12:37
  • \$\begingroup\$ Consider getting converters, cheap and useful. adafruit.com/product/2897 or use screw terminal barrel connector adapters. So you can cut the original connector off and use any adapter. \$\endgroup\$ – Passerby Nov 19 '18 at 15:42
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Alas, this is difficult to pin down. "Most used" depends on your industry, your region of the world and the year you ask the question.

For a long time every manufacturer would pick their own and supply the plug to suit. Consumers who tried to connect their own would quickly discover they'd fried the appliance by connecting the wrong voltage to it. So people learnt to limit the mixing and matching of supplies and devices, and manufacturers were motivated to pick uncommon sizes to prevent misconnection.

It's probably only since the Arduino that there's been much interest in supplying your own supply, and the question of compatibility became of general concern. The Arduino happens to use the 2.1mm barrel jack, and the Beaglebone followed suit.

So 2.1mm has become a de-facto standard in the maker community, which has influenced a lot of electronics. But this is neither official nor stable, so I'm afraid your goal of "maximum compatibility with existing PSU around the house" is probably largely unachievable in April of 2019.

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2.1mm is the most common used barrel jack. most open source boards usually comes with 2.1mm jack.

and even 2.5mm jack also has its market.

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    \$\begingroup\$ Can you cite any data to support your answer? Also, when you say "has its market", what do you mean? Is the smaller jack used twice as often as the larger jack or just 1% more? \$\endgroup\$ – Elliot Alderson Nov 19 '18 at 13:00
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    \$\begingroup\$ A quick check of jameco.com shows that for 12V AC-DC wall mounts the 2.1 : 2.5 ratio is 74:9. For table-top type supplies the ratio is 40:6. There doesn't seem to be any correlation to output current. If I were designing a product, I'd go with the 2.1mm. \$\endgroup\$ – isdi Nov 19 '18 at 14:42
  • \$\begingroup\$ Not really a defining answer, however I looked at Digi-Key this Morning of the 2,715 Normally stocking wall power supplies 1,378 used a form of 2.1mm and 638 used a 2.5x5.5 \$\endgroup\$ – Robert Fay Nov 22 '18 at 16:51
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A potential alternative solution for a standardised power connection that slowly becomes more common, is USB-C. A standard USB-C cable and charger will be able to deliver your requested 3A.

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