Is there a standard (or common) spec on DC power jacks? - Page 1
Forum section: electronics / beginners

author: elroy
author_rank: Regular Contributor
timestamp: December 14, 2017, 08:09:15 pm
content: Recently I bought and assembled one of these simple function generator boards:https://www.amazon.com/gp/product/B01HM70CMYThe board has a jack for DC power in what appeared to me to be a size I'd seen fairly often. I figured it would be handy to have a supply of jacks and plugs so that I could swap around power supplies while working on various projects, so I bought this assortment:https://www.amazon.com/gp/product/B00N8C49OAUnfortunately, I found that some plugs and jacks that appear to me to be reasonable fits don't result in an electrical connection. I don't think any of them are necessarily defective, there just must be be slight size differences.So, if I wanted to stock up on some jacks and plugs that would run the best chances of fitting kits and so on, is there a particular size that tends to be used most frequently these days?

author: tooki
author_rank: Super Contributor
timestamp: December 14, 2017, 08:25:57 pm
content: The 5.5mm outer diameter, 2.1mm inner diameter is probably the most common, and 9V DC, centernegativepositive is probably the most common configuration. But these vary wildly.The most common almost-but-not-quite fit is the 5.5 outer/2.5 inner plug. It will fit a 5.5/2.1mm jack, but will make at best an intermittent connection.

author: james_s
author_rank: Super Contributor
timestamp: December 14, 2017, 08:42:00 pm
content: Unfortunately there is no standard, though the 2.5mm center pin is frequently used on adapters that put out AC voltage while 2.1mm is the most common with DC voltage but this is by no means absolute. Then occasionally you find something that uses the center pin as negative instead of positive. I have no idea why none of this was ever standardized.

author: Jeroen3
author_rank: Super Contributor
timestamp: December 14, 2017, 08:45:07 pm
content: No. There is no standard.You buy the plugs and sockets with the outer radius and the pin diameter.2.5mm/5.5mm is common though.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: December 14, 2017, 09:21:35 pm
content: Quote from: tooki on December 14, 2017, 08:25:57 pmDefinitelyQuote from: tooki on December 14, 2017, 08:25:57 pmI see far more 5 and 12, but 9 is somewhat commonQuote from: tooki on December 14, 2017, 08:25:57 pmNever in my life have I seen a barrel plug DC supply with negative on the center

author: james_s
author_rank: Super Contributor
timestamp: December 14, 2017, 09:28:28 pm
content: Quote from: suicidaleggroll on December 14, 2017, 09:21:35 pmIt's very common on musical gear. Samplers, keyboards, sequencers, etc. I also have a scanner radio that uses 9V center negative on a barrel connector. It's not nearly as common as center positive but I wouldn't call it rare either.

author: Kjelt
author_rank: Super Contributor
timestamp: December 14, 2017, 10:03:20 pm
content: Lots of stuff before 2000 had negative center.Luckily now it looks like + on center is standard, now only the pin diametersBTW I hate these connectors since they have no lock, every week or so a connector of my usb hubs or ip switches will be out of the chassis, i started to hot glue them stuck.

author: chrisl
author_rank: Regular Contributor
timestamp: December 14, 2017, 11:34:10 pm
content: I have seen a lot keyboards with negative center pin.

author: bitseeker
author_rank: Super Contributor
timestamp: December 15, 2017, 01:22:41 am
content: Center positive is much more common nowadays than it used to be. Center negative is often found on devices that use the jack as a switch (e.g., disconnect onboard battery when barrel plug is inserted).

author: tooki
author_rank: Super Contributor
timestamp: December 15, 2017, 06:10:28 pm
content: Ack, my brain meant to write center positive, but I wrote negative instead!

author: tooki
author_rank: Super Contributor
timestamp: December 15, 2017, 06:12:45 pm
content: Quote from: Jeroen3 on December 14, 2017, 08:45:07 pmErr, you mean outer diameter and inner (pin) diameter, don’t you? And that’d be 5.5/2.5mm, then.But the 2.1mm inner is actually more common, I think.

author: Gyro
author_rank: Super Contributor
timestamp: December 15, 2017, 07:10:37 pm
content: Quote from: Kjelt on December 14, 2017, 10:03:20 pmAlthough some connectors have smooth barrels and others have a reduced diameter 'ring' near the tip. These, in combination with the right socket, do provide a positive retention. It often doesn't work out in practice though (after-market adapters etc.).

author: BBBbbb
author_rank: Unknown
timestamp: December 15, 2017, 08:14:24 pm
content: 5.5/2.1mm with (+) in the middle is by far the most common.Something like this is useful to have on hand:https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F232022657600

author: Macbeth
author_rank: Super Contributor
timestamp: December 15, 2017, 10:23:03 pm
content: Quote from: suicidaleggroll on December 14, 2017, 09:21:35 pmYou haven't lived! Just recently I found a modernish Dymo label printer with its own custom wallwart running that same scam.But it appears there is a good reason behind '80s battery products with external power after all...

author: james_s
author_rank: Super Contributor
timestamp: December 15, 2017, 11:53:40 pm
content: Maybe I'm a bit tired but wouldn't the same thing work with center positive? You'd just disconnect the negative battery wire instead of the positive wire when external power is supplied. The battery is an isolated source anyway so that wouldn't make any difference.

author: Macbeth
author_rank: Super Contributor
timestamp: December 16, 2017, 12:02:20 am
content: Quote from: james_s on December 15, 2017, 11:53:40 pmBecause the DC jack socket doesn't have a SPDT switch to the centre, it does have one to the outside barrel - for obvious mechanical reasons. Think about it. @3:33

author: james_s
author_rank: Super Contributor
timestamp: December 16, 2017, 01:08:40 am
content: Quote from: Macbeth on December 16, 2017, 12:02:20 amSure but what difference does it make? Tie both the battery positive and the device positive to the center pin, when you plug in the external adapter the negative side of the battery is disconnected and negative is supplied by the external adapter instead. Sure convention is to switch the positive side but what difference does it make? The electrons don't care, old cars had positive ground systems, aside from convention it doesn't really matter.