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I am having trouble understanding barrel connectors. From what I read online either the inside or the outside can be positive or negative depending on the direction of the current.

So which end is the positive and which is the negative? Any useful advice or articles of this would be great.

This is the kind of connector I am talking about:

enter image description here

ocrdu
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Dan
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    It is whatever it is hooked up as. There is no standard, the tip (center) might be positive with respect to the ring (outside) or not. – R Drast Apr 29 '15 at 14:37
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    The end that measures positive with your multimeter is positive, and the end that measures negative is negative (assuming you have one at an actual wallwart) – PlasmaHH Apr 29 '15 at 14:38
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    @RDrast: In my experience the sleeve is the negative more than about 80% of the time. This seems to be some kind of rule, with exceptions (see comments): http://electronics.stackexchange.com/a/33969/5163 – 0x6d64 Apr 29 '15 at 14:54
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    It isn't a rule by any means. I am looking at three power supplies right now. Two the sleeve is positive, and one it is negative. Saying it is a 'Rule with exceptions' means that it isn't a rule. – R Drast Apr 29 '15 at 14:57
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    "either the inside or the outside can be positive or negative depending on the direction of the current." This doesn't make sense to me. The positive part is positive and the negative part is negative regardless of which direction the current flows. If current is flowing into the positive part then the device the plug is attached to is receiving power (would be a little against standard use of these kind of plugs). – Austin Apr 29 '15 at 15:25

3 Answers3

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The answer may be a bit unusual, but it's neither!

Barrel connectors don't have a defined polarity. Instead, what you'll have is the polarity set by the device that you want to use, if the device is using DC in the first place. Near the receptacle for the barrel connector you might see a sign with something like this:

Barrel connector input

Take note of the signs marked on your device where the + and - symbols are drawn.

On the example above, the + sign is connected to outer shell in this example, meaning that the outside of the connector is positive and the - sign is connected to the black dot in the middle, meaning that the inside connection is negative.

If the symbols are the other way round, the opposite is true. If the line touching the tip in the centre has a + (plus) symbol, this means the tip is positive, and the outer shell is negative.

In any case, you need to research this for your particular device. There is no general rule.

ocrdu
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AndrejaKo
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    Usually, negative/ground is outside. Probably because it is safer. – neverMind9 Oct 31 '18 at 14:35
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    While this answer is, in general true. It is far from true in the case of laptops and comparable devices (Arduino's, Raspberry Pis etc) where the pin (centre) is positive. How do I know this? Or what evidence is there for it? (a fair question given the market is large and diversity of equipment large). Two pieces of evidence. 1) Universal laptop supplies are easy to buy and I have some. They provide a variety of DC barrel connectors (different sizes) and always deliver pin positive (cannot be reversed) in my experience. 2) It's 2020 and de facto standardisation has progressed significanty IMO. – Bernd Wechner May 28 '20 at 00:37
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    @Bernd Wechner I feel the answer wasn't being too specific, but generic in the way that AndrejaKo mentions to look for the symbol. Was exactly what I needed just now and I was instantly able to work out the polarity – Jayden Lawson Mar 25 '21 at 05:37
  • True. Either I didn't read his intro well enough or it changed ;-). Regardless, you're right. The answer introduces well that there is no defined polarity. Flip side, every device I've ever checked form memory the pin in centre was +ve and the shell -ve. But that says more about my exposure to devices and memory than it does about any (non existent) published standard and the answer is strong: check! Easy to do with a multimeter if a sample power supply is on hand. – Bernd Wechner Mar 25 '21 at 08:18
  • @Bernd Wechner "every device I've ever checked" is exactly why I wrote that there is no defined polarity. For example, I often use portable shortwave radios. Back in the older times (when they were actually popular, and there was a large choice of new high-quality units), the unofficial power supply standard was center negative, but right now, there's a trend to move to center-positive and to USB. cont.. – AndrejaKo Mar 25 '21 at 09:37
  • @Bernd Wechner continued Those radios also tend to be manufactured for a long time (a model might be in production for 10+ years), and sometimes manufacturers will even switch from center negative to center positive as an internal revision of the product, which can cause issues, when searching for a new power supply, for example. Also, I've seen network equipment (from Netgear, I think) which uses AC power on barrel connectors, and the power supply is actually a transformer. – AndrejaKo Mar 25 '21 at 09:40
  • So that's why I always try to point out that the polarity needs to be checked. It's almost always center positive, except, when it isn't. – AndrejaKo Mar 25 '21 at 09:40
  • Not sure why you're explaining all that again. I already said in the previous comment that I clearly didn't read your answer well (in particular the intro) and that we agreed - there is no standard, check labels and/or test. – Bernd Wechner Mar 26 '21 at 08:58
  • I edited the answer to add more clarification based on the comments. Ideally 2 example images should be shown to showcase both options. – redfox05 May 11 '21 at 17:20
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TL;DR It's usually center positive.
Unless it's a laptop, a guitar effect pedal and/or stuff from SONY or Roland.

"Direction of current" is an horrible concept that you will eventually learn means nothing. (Pioneers thought electricity flowed from positive to negative, schematics kinda adhere to the pioneers' vision, physics showed it's all about electrons flowing in the opposite direction, so...) As such, it definitely doesn't influence barrel jack polarity.

Barrel jack polarity is pure convention.
And it mostly depends on what manufacturers feel like doing this month.
And that might seem especially true with 5.5 barrel jacks.

But once you start considering the voltage the barrel jack is delivering, patterns emerge. And manufacturers follow them. And this makes it easier to get readymade ultracheap power supplies for your projects.

In general, going positive center is the safe bet, but there's a big exception related to music gear that consumes 9v.

9v 5.5x2.1 plug center negative (typically .6 to 2A supplies) is a very widespread standard set by guitar pedals made by Boss/Roland. Many manufacturers copied and conformed to that, so finding a 9v device with that barrel size and center positive is super weird (it happens, but with really old gear).

12v 5.5x2.5 plug center positive, 1A or 2A, is another very widespread formula. A lot of musical keyboards, synthesizers, network gear and surveillance cameras go for it. Not entirely sure where it originated from, but Yamaha might be the culprit.

5v 5.5x2.1 plug center positive and 5v "type H" 3.5x13 plug center positive also seem to be quite widespread, with the latter one being pretty much ubiquitous with USB hubs. You can buy USB A to barrel jack cables without any polarity information advertised, and they will be center positive.

Then there's The Magic World of Laptops and their Infinite Nonsense. Their voltage typically spans from 18.5v to 24v, and it comes in all sizes and polarities. I strongly suggest everybody to stay away from that crazy bunch of connectors.

With voltages lower than 12, negative center is the odd one. And when you find it, it's usually related to Roland or SONY and their weird world domination schemes.

(SONY used preminently a 3v micro barrel jack center negative with walkmans and minidiscs -so, in the 80s and 90s- but I've never seen it catch on with other manufacturers)

ZJR
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  • FWIW the charger with modular plug I purchased for my ASUS laptop says 19V 2.37A 45W max. Plug diameter 4.0mm, length 10.0mm, center positive. (The charger that came with the laptop uses a USB-C connector. Which is nice except there isn't exactly a surplus of USB ports available.) – QTX Jul 31 '23 at 19:01
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The positive contact is the one that, when you connect the red probe of your meter (set for 20 Vdc) to it and the black probe to the other contact, shows a positive number on the meter.

Davide Andrea
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