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General Question

I am wondering, is it possible to use a 5.5 x 2.1 mm male plug inside a 5.5 x 2.5 mm jack for transferring 12 Vdc?

My thoughts are just that it may just be a tight fit on the inner conductor. It may also just not fit. I figured before spending money and waiting a few days to try it out, it was worth asking here.

My Use Case

I have an Intel NUC, which can be powered off 12 Vdc via a 5.5 mm x 2.5 mm jack:

The back panel DC connector is compatible with a 5.5 mm/OD (outer diameter) and 2.5 mm/ID (inner diameter) plug, where the inner contact is +12-19 (±10%) V DC and the shell is GND.

Source

And it seems the 5.5 x 2.1 mm plugs are much more common, so I would like to use that.

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    \$\begingroup\$ Not reliably. \$\endgroup\$
    – Hearth
    Oct 1, 2019 at 16:30
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    \$\begingroup\$ Without more specific information about the mechanical arrangement of the plug and jack, as well as their nominal dimensions and tolerances, I don't think anyone can give a better answer than the one you provided yourself: maybe, maybe not. \$\endgroup\$ Oct 1, 2019 at 16:32
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    \$\begingroup\$ In a 5.5x2.1 plug, 2.1mm is a hole diameter. Forcing a 2.5 dia centre pin into it .... no. Other way round you get a sloppy fit and unreliable contact. \$\endgroup\$
    – user16324
    Oct 1, 2019 at 18:35
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    \$\begingroup\$ You got the sex backwards: the plug is female (hole = vagina = female) and jack is male (pin = penis = male). \$\endgroup\$ Nov 6, 2022 at 18:07
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    \$\begingroup\$ @DavideAndrea Thanks for the straightforward anatomy lesson in this gender-confused world of coaxial power connectors :) \$\endgroup\$
    – user193130
    Dec 27, 2022 at 5:15

4 Answers 4

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This must be evaluated on a case by case basis, as sometimes the spring loaded tabs will allow for a few 0.1mm's of play. However, 0.4mm is most likely too far and a the right jack\plug combination with less than 0.4mm between the barrel and outer diameter of the plug needs to be found.

Whenever I do product testing with jacks\plugs, I usually buy several options and make sure that customers (boss, product development team) are happy with the plug\jack combination before I build a prototype.

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I have tried fitting a 2.1 mm barrel plug in a 2.5 mm jack (by accident or consciously) many times and it has never fit. Both sides are rigid and there is not enough clearance.

Size adapters are available; I recommend having some around for use when you have a device that doesn't come with a power adapter and an adapter with the right voltage. They do have the disadvantage of making the plug longer, hence more awkward and vulnerable to damage.

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    \$\begingroup\$ I seen instance in which they do fit and make an electrical connection, but it is extremely unreliable. \$\endgroup\$ Oct 1, 2019 at 16:54
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    \$\begingroup\$ @CalebReister Are you perhaps thinking of putting a 2.5 mm plug in a 2.1 mm jack? That is what I would expect to be an unreliable fit since the jack's pin is too small. \$\endgroup\$
    – Kevin Reid
    Oct 1, 2019 at 23:09
  • \$\begingroup\$ Okay I went ahead and bought some connectors, and also found everything was too rigid and it didn't fit. Thanks for your input @KevinReid! \$\endgroup\$ Oct 7, 2019 at 18:02
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Not all plugs are created equal. Some plugs have full, uniform tubes that have little tolerance, and a nominal 2.5mm center pin will not fit inside one designed for a 2.1mm pin. They look like this:

5.5x2.1mm plug (from Amazon.com listing)

Other plugs have a tube with an opening wide enough for a 2.5mm center pin and an inner contact shaped something like a tuning fork, so that it can make contact with a 2.1mm center pin and still accommodate a 2.5mm pin. See, for example, the plugs on this cable:

enter image description here

A quick look at the actual plug should give you an idea of which you are dealing with.

Also, you can get a wide variety of adapters that convert a 5.5x2.1mm plug to other sizes of plugs (and even other kinds of plugs, like USB), so for short-term use I use a variable power supply with a 5.5x2.1mm plug and converters to power anything I need while I wait for a dedicated power supply with the right tip to show up. Finding adapters for any other size plug is considerably harder.

Side note about gender of plugs and jacks

The term "plug" has been standardized for electronic connectors by widely recognized standard setting bodies such as IEEE, ASME, and several others, generally meaning the connector on the end of a cable. The thing the "plug" connects with, and more typically installed on a piece of equipment, rack, or wall, is referred to as a "jack" by IEEE and ASME (general electronics), a "socket" by USITT & TSDCA jointly for audio, and a "receptacle" by NEMA for mains power. (They all use "plug" for the other connector.) All of these standard setting bodies have entirely moved away from using "male" and "female" designations.

Nevertheless, genders have been used for a long time, and continue to be used in many places. In the simplest case of connectors, such as a headphone jack and plug, the thing that goes inside the other thing is considered male, and what the male goes inside is considered female, by analogy to mammalian genitalia.

Most DC barrel connector pair consists of a jack that has a center pin that goes inside a tube in the plug, and a plug has a tube that then goes inside a bigger tube in the jack. So both parts have things projecting that fit inside other things.

Some DC barrel connector pair (such as pictured below) have the center pin on the plug, plug has a center pin that goes inside a socket in the jack, which tube goes inside a larger tube in the plug, which tube goes inside a still larger one in the jack.

Plug with center pin (from Amazon.com listing)

So the analogy to genitalia breaks down, leading to controversy.

Historically, the general rule has been that what are now defined as "plugs" have been called "male", because in most cases they followed the analogy, while jacks/sockets/receptacles were called "female" for the same reason. This led to most people calling DC barrel plugs "male" and jacks "female" despite the center pin in the jack.

Some have noted that DC barrel connectors share the same general cylinder-within-tube configuration as RF connectors do, and have been using the same gender convention. With the original "standard polarity" RF coax connectors, it became convention that the "male" was the one with the solid pin, the smallest thing that goes inside a larger thing. Some want to follow that convention with DC barrel connectors. However, that remains confusing to the general public for a few reasons.

First, it is at odds with historical usage with respect to DC barrel connectors, so adopting that convention means that what used to be called male in now called female and vice versa, which makes both designations useless at best. Second, RF connectors come in "reverse polarity" configurations, where the part with the center pin is called "female" and the part without is called "male", so although RF connector gender is well established, it is not so easy to analogize to DC barrel connectors. Third, while most DC barrel plugs do not have center pins, some (such as the one pictured above) do, and when both plug and jack have things going inside other things, people seem to find it more natural to assign gender based on plug and jack (perhaps due to historical convention and inertia) than based on which has a center pin.

The controversy and confusion is perhaps best demonstrated by how the terms are used by major e-commerce sites. Both of the plugs shown at the top of this answer are advertised as "male", the first on Amazon.com, the second on AliExpress.us. Newark.com, a distributor, and Switchcraft, a manufacturer, list the connectors' genders as "plug" and "jack" rather than "male" and "female". Mouser.com, a distributor, uses "jack" and "plug" as filters, but continues to list jacks as female (another example) and plugs as male, even plugs from Swithcraft. As far as I have seen, only DigiKey.com, a distributor, calls the standard plugs "female" and the jacks "male" .

Given these problems, and the societal move away from using genders overall (as indicated by the above-mentioned standards defining "plug"), it is best to simply avoid using genders where possible. Nevertheless, it remains important to note that when using genders, you still need to double-check which "standard" people are using.

To be safest, just refer to "jacks" and "plugs".

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  • \$\begingroup\$ @DavideAndrea - Hi, You edited someone else's answer which is allowed, but with several limitations. One allowed use is to "Clarify meaning without changing it". IMHO you seem to have changed the author's meaning. Perhaps I'm wrong, that it appears to have changed (but if I'm correct, someone can always rollback your edit to undo it, if wanted). However in any case, please be careful not to change the meaning of someone else's post, even if you think it's wrong, as it's their decision about what is right or wrong. If you think a post is wrong, ask them to explain in a comment below it. \$\endgroup\$
    – SamGibson
    Dec 7, 2023 at 1:57
  • \$\begingroup\$ @DavideAndrea - Also, you added a new image, without a link back to its source (saying "{Digikey}" is not enough). Please add a link to the specific source webpage for that added image. Thanks. \$\endgroup\$
    – SamGibson
    Dec 7, 2023 at 1:57
  • \$\begingroup\$ Hello Sam. You raise good points. I have a) updated my edits to remove any language contradicting the original intent of Sara's answer; b) moved the picture next to the Digikey part to indicate that that's what Digikey thinks, not what Sara thinks; and c) Added a link to Digikey's page for coaxial barrel connectors. I trust that these measures address your concerns and I thank you for expressing them. \$\endgroup\$ Dec 7, 2023 at 15:23
  • \$\begingroup\$ @DavideAndrea please stop editing my answer. If you disagree with me, write your own answer. I understand you are thoroughly convinced that everyone agrees about the "right" gender for these connectors, but I am thoroughly convinced you are wrong, so don't put words in my mouth. And DigiKey did not label that photo "male" and "female", you did, plus your link goes to a product category page, not a specific product with that image. See this photo from eBay making my point. \$\endgroup\$
    – Sara
    Dec 10, 2023 at 1:10
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I just drilled one out with a 3/32 bit (no drill, just pliers). I also cut with some thin wire snippers, then could force it on pretty easily. Once on far enough it works (but harder to get off than normal). If you have a 2.5mm drill bit try that (or, better of course, get the right connector).

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