Spring cleaning, and I'm trying to get power supplies for all my devices with missing power supplies. They're all the typical barrel power connector, and I'm having a dickens of a time trying to figure out the pin/hole diameter.

DC connectorenter image description here

I ordered the power supplies I needed based on outside diameter (e.g., 5.5mm in my example below) and was surprised to discover that while the jack fit, the center pin did NOT. How do I prevent this from happening in the future? Do they even make calipers that can get into the hole to measure the pin diameter?

Radio Shack has their little keyring behind the counter with every known tip size, but all they can get from that is which stock number fits on their universal wall wart. Personally, I think that these types of "universal" kits are the worst thing to happen to electronics in, like, FOREVER. Too many parts to misplace and the tip-to-cable connector is almost always proprietary.

Connector kitenter image description here

If I try to pump them for information about what the outer and inner diameters are, they want to know if I'm happy with my current cellular provider. As you may surmise, I'm not a big fan of trusting my local Radio Shack for electronics guidance.

So...that leaves me with a bunch of power supplies that don't fit their devices, and me a little peeved that I have to deal with RMAs, return shipping, etc., especially when I really don't have a clue how to figure out what to order. That also begs the question about how to ensure that I buy the right jack when designing something that NEEDS wall wart power.

Where do I even start? Anyone have any ideas on how to finding the correct barrel & pin diameters when I don't have specs on the jack? Is it really trial and error? or is there some measurement device that's available to help?

  • 10
    \$\begingroup\$ I saw your comment below about the calipers measuring the I.D., another trick if you have a good range of drill bits is to use those. The shaft of a 2.5mm drill bit is a snug fit into a 2.5mm ID connector, and a 2mm bit into a 2.1mm connector has a little play. \$\endgroup\$
    – PeterJ
    Mar 3 '13 at 5:40
  • 4
    \$\begingroup\$ "a dickens of a time" +1 from me \$\endgroup\$
    – NickHalden
    Jul 18 '13 at 23:05
  • 4
    \$\begingroup\$ It's a shame that every company out there do what ever they want without any control for protection of the public, for the sake of money and over polluting nature. Talk to a handy friend that can commonise the manufactures BS. \$\endgroup\$
    – user32808
    Nov 16 '13 at 15:54
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    \$\begingroup\$ Why dont they just stamp the size on the plug for christ sake ! \$\endgroup\$
    – user63893
    Feb 1 '15 at 16:48
  • 3
    \$\begingroup\$ Alas, mourn the demise of Radio Shack. 😥 They had a rack of test connectors that allowed you to separately match the shell and the pin diameters and get the exact connector needed. \$\endgroup\$ Jun 9 '19 at 21:22

11 Answers 11


Those are barrel power connectors.

Looking at Digikey, it looks like common inner diameters with a 5.5mm outer diameter are 2mm, 2.1mm, and 2.5mm, but that doesn't mean that your target application doesn't have a custom size which doesn't match any of these.

The one I usually use for my projects is 2.1mm*5.5mm if I can, but as far as I know this is by no means a rule of thumb.

Knowing what the jack is being used for may help in identifying a correct size.

  • 5
    \$\begingroup\$ Barrel power connectors. Thanks. Title's been edited appropriately. I think I got a mess of 5.5mm x 2.1mm which led to this little predicament. Unfortunately, there are as many rules of thumb as there are thumbs, it seems. \$\endgroup\$
    – dwwilson66
    Mar 3 '13 at 4:22
  • 2
    \$\begingroup\$ what you can try is get a connector for each of the 3 common sizes and measure all the jacks you have (does it fit or not?). Keep a record either on the device itself or somewhere else (say, a spreadsheet) and you should be good to go. \$\endgroup\$ Mar 3 '13 at 5:07

Just look up a fractional inch to mm conversion chart. Then break out the drill bits.

5/64 inch = 1.9844 mm

3/32 inch = 2.3813 mm

7/64 inch = 2.7781 mm

a 5/64 bit will fit the 2.1mm barrel but not a 3/32

a 3/32 bit will fit the 2.5mm barrel but not a 7/64

  • 8
    \$\begingroup\$ My hat's off to the gentleman that suggested using a selection of drill bits - Simple and effective. Thank you Sir. \$\endgroup\$
    – user43497
    May 26 '14 at 18:28
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    \$\begingroup\$ The 5/64 drill bit fit and the 3/32 didn't ......... Brilliant, thanks for the drill bit idea. 2.1mm is my pin, Woo Hoo!!!!! Also thanks to Santa (my wife) for getting me the exact digital caliper pictured above for Xmas. I am doubly lucky man! \$\endgroup\$
    – user62441
    Jan 1 '15 at 0:34
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    \$\begingroup\$ This is the sort of great answer which comes both from experience and clever thinking! In contrast with many other "quick" or copy&paste answers. Why does it have so few up votes? \$\endgroup\$ Feb 4 '15 at 8:11

Vernier calipers can be used to measure both the inside diameter (i.d.) and outside diameter (o.d.). This works if you have only the device (receptacle) or the supply (plug).

enter image description here (Photo courtesy technologystudent.com.)

  • 7
    \$\begingroup\$ LOVE the animated caliper... :) I've got one that measures us to 2.5", but the jaws are still too beefy to get inside the hole to measure pin diameter. There's about 3-4mm before the pin is accessible; the slope of the jaws just don't do it. In most cases, the hole is easy with the internal jaws...the pin seems to be a bigger issue. Maybe I'll get a cheap Chinese caliper on eBay and spend an afternoon Dremel-ing the jaws... :) \$\endgroup\$
    – dwwilson66
    Mar 3 '13 at 4:44
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    \$\begingroup\$ I'd recommend using the internal jaws to estimate the o.d. of the pin. helloworld922 is spot on - there are only a few pin sizes per barrel o.d., so if you can estimate by eyeballing with your caliper, you can almost always pick the right one. \$\endgroup\$
    – HikeOnPast
    Mar 3 '13 at 5:02
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    \$\begingroup\$ I hate it when people call digital callipers vernier callipers. \$\endgroup\$ Dec 4 '13 at 0:02
  • 11
    \$\begingroup\$ Sorry, but this is Just Plain Wrong. A caliper will correctly measure the inside distance between two flat surfaces but will not correctly measure the inside diameter of a round hole. It will always give an undersized measurement. As suggested in other answers, PIN gauges, or faking a PIN gauge with drill bits, will give a much closer answer. If your hole is large enough you can use a "small hole gauge" (look that up too when you look up vernier) in conjunction with you caliper or micrometer, but you cannot expect a direct caliper measurement to be correct. \$\endgroup\$ May 5 '14 at 1:01
  • 1
    \$\begingroup\$ A caliper will tell the inside diameter close enough to determine pin size of a barrel jack. You rotate the caliper while pulling it apart until it stop growing. \$\endgroup\$
    – Joe
    Sep 13 '14 at 15:22

If you're using calipers, then use blue tack or something similar to make a mould of the internal dimensions of the plug or socket and measure the mold.

  • 3
    \$\begingroup\$ clever! just don't push the bluetack too deep into the hole :) \$\endgroup\$
    – bhillam
    Jul 19 '13 at 4:01

Source: https://www.accesscomms.com.au/reference/outputplugseiaj.htm

EIAJ RC-5320A Plugs

EIAJ is an acronym for Electronics Industries Association of Japan. EIAJ have now merged with the Japan Electronic Industry Development Association (JEIDA) to form the Japan Electronics and Information Technology Industries Association (JEITA). EIAJ developed a range of plugs and jacks for connection of low voltage power supplies to consumer equipment. EIAJ plugs have now been widely adopted by Japanese and other consumer product manufacturers. EIAJ Plugs are similar in design to more commonly used Concentric Barrel Plugs. They can usually be identified by a yellow coloured insulating ring at the tip. EIAJ RC-5320A plugs are available in sizes to suit five Voltage Classifications. EIAJ Type A (1, 2 and 3) have a hollow centre similar to the Concentric Barrel Plugs. EIAJ Type B (4 and 5) have an internal pin. The EIAJ standard requires that these plugs are always wired with the centre pin as positive (+) polarity.

enter image description here

  • \$\begingroup\$ Although just a link to outside source, this answer was actually the most helpful. \$\endgroup\$
    – Nakedible
    Jul 15 '14 at 6:33
  • \$\begingroup\$ @Nakedible, I agree. Didn't realize what I have isn't just a "concentric Barrel Plug". Also the yellow tipped connectors seem pretty common. \$\endgroup\$
    – freb
    Nov 25 '15 at 2:12

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 which is exactly as wide as the jack. If you do this, you will 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.

  • \$\begingroup\$ You answered the exact question I had in mind: what happens with those 5.5mm sleeve jacks? \$\endgroup\$
    – Erik Olson
    Nov 23 '19 at 22:44

Wikipedia has a conversion matrix with conversions between the radioshack letter codes and actual measurements found here this way you can use the radioshack keyring to find an appropriate size connector and then buy the correct adapter.


Differentiating between 2.5 and 2.1 mm plugs is very simple. Strip the insulation off of a #12 solid wire- used commonly to wire houses. It is 2.05 mm diameter and will be loose in a 2.5 mm plug. It will just fit in a 2.1 mm plug and can only be slightly wiggled.

  • \$\begingroup\$ Some variations on this are using toothpicks (~2.0mm), pen (2.25-2.35mm) and test lead (2.0mm) as described in forum.digikey.com/t/… \$\endgroup\$
    – Patanjali
    Feb 22 at 21:48

I found that the easiest, but not the cheapest, way to determine the inside diameter of a 'barrel' connector is to use a set of "pin gages" which are hardended and ground to precise external diameter steel 'rods' that come in sets. A lot of times sets are available cheaply at discount tool supply places, or if you have a machine tool supply store available, individual gage pins can be ordered. Then you just take a 'pin' and try its fit in the inside of the connector. Be careful if anything in the connector is gold plated (doubtful) as the ends of the 'pins' are capable of scratching the plating off their being 'as machined' and not debured.


I have not verified this, but a possible solution may be to use a spark plug gap tool. I have one that uses metal wire loops embedded in a plastic disk. I bought it years ago but I still see them around. Just snip the loops and bend them straight for an instant ID estimator. My disk has both metric and inch labeling. Hard to beat the price.


The radio shack plugs may not use a standard nomenclature, but it is also not arbitrary, here is the description for the size N connector:

This metal panel-mount coax-style DC power jack accepts 5.5mm O.D.x2.5mm I.D. plugs. Metal panel-mount coax-style DC power jack Tip size: 5.5mm O.D.x2.5mm I.D. Requires 7/16" mounting hole Nickel-plated, solder-type terminal


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