Is white wire with grey stripes positive or negative wire?

Question

I want to re-wire an AC to DC power adapter for an ONT (Optical Network Terminal - AKA Fibre Box mounted on a wall) and replace the adapter with a standard DC plug. This is because I want to connect it to a UPS (uninterrupted Power Supply).
How do I tell which one of the two wires is the live or positive wire in this picture?

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I am from South Africa, if that is at all applicable. This is the cable connected to an ONT.
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This is the standard DC pin Jack I want to use:
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A photo of the plug blades may help. In US the smaller blade is hot — Kris, Jun 30 '22 at 12:31
What is a "standard DC plug"? Do you have DC wall service there? — isherwood, Jun 30 '22 at 12:37
@isherwood, no I am connecting this to a standard DC 10 V pin/plug that will plug inta a UPS (Uninterrupted Power Supply). — Romans, Jun 30 '22 at 14:53
@JACK I don't unfortunately. But even if I had I don't quite see how that would work. The cable in the image is the current power supply cable to an ONT (optical Network Terminal - AKA a fibre box mounted on a wall) — Romans, Jun 30 '22 at 14:54
Best bet is to get a multimeter that reads DC voltage. Simple touching of output pin will tell which is positive/negative. Will need the same test on the UPS. Hoping the pins are hooked up the same is foolish. — crip659, Jun 30 '22 at 15:26
@crip659 How would the multimeter work? This is a power supply? — Romans, Jun 30 '22 at 15:42
Why not simply plug the white "wall wort" (in pic #2) directly into your UPS? That's how all of my home electronics are protected by my UPSs, including my ONT, router, etc. — FreeMan, Jun 30 '22 at 15:53
Use the red and black leads, one on the outside, one on the inside. If put on the way, the meter reads positive number, if backwards it reads negative. — crip659, Jun 30 '22 at 16:04
@Romans You'd have to cut the wire to install your new jack. A DC meter has + and - test leads, red and black. You plug in the supply and connect a lead to each wire. If you get a correct secondary reading, the wire to the red lead is your positive. If you get a negative reading, then the wire to your black lead is positive. — JACK, Jun 30 '22 at 16:43
@Criggie, I found a PoE Splitter that takes a standard DC plug/pin as input and provides 12V DC as output over an ethernet cable. I am considering to hook up the PoE Splitter to my mini UPS and use the ethernet output from the PoE as power for the ONT. My question is: would the ONT be able to function with its main power supply left unplug and receive power through 1 ethernet cable while providing internet access through another? — Romans, Jul 02 '22 at 12:44
@romans I've added it as a separate question/answer https://diy.stackexchange.com/questions/252173/what-is-poe-power-over-etherent-and-how-does-it-help-the-diyer — Criggie, Jul 03 '22 at 01:27
https://electronics.stackexchange.com/questions/39450/what-do-solid-striped-lines-on-a-wire-indicate Perhaps this will help you find positive — user179320, Dec 14 '23 at 23:27

score 94 · Accepted Answer · edited Nov 25 '22 at 17:28

94

You don't have a voltmeter. Do you have a potato?

With the power adapter unplugged from your electrical outlet, cut the wires, strip a little insulation from the ends, twist the strands of each wire into a point. Do not allow the bare wires to touch each other from this point on.

Cut the potato in half. Take one half, and poke both wires into the cut face of the potato about 2 cm apart. Plug in the power adapter to the wall outlet. In a short time, the potato around one of the wires will turn green. That is the positive wire.

Unplug the adapter and clip off the ends of the wires so you have clean wire for soldering your new plug.

edited Nov 25 '22 at 17:28

Community

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answered Jun 30 '22 at 15:29

MTA

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Oh my I have never heard of this , I would put the wires in the potato before plugging in. And on an insulating surface, on dc I could see it work and since 10v is finger safe it would act like a plating process got to give a + lol – Ed Beal Jun 30 '22 at 16:21
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This works because the wires are copper, not (as I thought) because of the potato. Which is the case in this example, but that's not completely universal. – wizzwizz4 Jun 30 '22 at 21:12
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"Cut the potato in half. Poke both wires into the cut face of the potato about 2 cm apart." Note that this means, "both wires should be poked into the same half of the potato, not one into each half (with the halves held close together). – Joshua Taylor Jul 01 '22 at 15:11
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In the UK, in some towns from around 1900 to the 1930s or later, municipal authorities provided 110v or 220v DC, and the standard way of deciding polarity involved a potato. This was important for 'AC/DC' vacuum tube radios of the time. – Michael Harvey Jul 01 '22 at 19:27
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Is it safe to touch/handle the potato after plugging it in? Does the potato need to be placed on an insulator like a rubber sheet? – WackGet Nov 25 '22 at 04:20
Not that it's the most important information but... green is positive or negative? Other source says:
(...) What is happening? The electric current is causing electrolysis of the water in the potato, taking the water apart into hydrogen and oxygen. The hydrogen is released around the positive wire, forming bubbles if the potato is moist enough. The negative wire is producing oxygen, which combines with the copper of the wire, forming a green copper oxide. That is what colors the potato green.

https://thehappyscientist.com/content/potato-polarity
– ikari Nov 25 '22 at 11:16
@ikari The Happy Scientist reference is incorrect. Positive wire produces oxygen. For a full discussion, see https://en.wikipedia.org/wiki/Electrolysis_of_water – MTA Nov 25 '22 at 13:10
@WackGet UL 1310 allows that contact with DC voltages below 30 volts under wet conditions are safe, but you can still get a tingle. If you don't know what the DC voltage is, don't touch the potato. If you cut the potato on a dry wood or plastic cutting board and leave it on the board, that would make an adequate insulator. – MTA Nov 25 '22 at 17:50

score 6 · Answer 2 · answered Jul 01 '22 at 13:06

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From personal experience: don't rely on the wire marking in regard to polarity.

I had once the boring task of shortening some power brick wires from a single batch.

I ended up swapping some bricks vs some jacks and - surprise! - about 20% of them failed to work.

Luckilly, the devices were tolerant for wrong polarity so I only lost ~2 hour of work.

Since then, I always use voltmeter.

About 70% of these are marked wire negative, but the other 30% are marked wire positive.

answered Jul 01 '22 at 13:06

fraxinus

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Umm... if you were shortening the leads, wouldn't you have simply reconnected "marked" to "marked" and "unmarked" to "unmarked" and not cared one bit which was positive and which negative? – FreeMan Jul 01 '22 at 15:38
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Good... until you cut all of them to size first and then mix them later. – fraxinus Jul 01 '22 at 15:52
Ah! It was a process error, not necessarily a marking error. ;) – FreeMan Jul 01 '22 at 15:55

score 2 · Answer 3 · answered Jun 30 '22 at 12:34

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In North America the marked (striped or ribbed) wire is neutral (or negative, when it's actually DC). It also has a wider blade on the plug or connects to the outside of a barrel. The conductor sometimes has aluminum blended in with the copper.

I don't know South African standards, but I've never had a universal or international cord that was otherwise.

answered Jun 30 '22 at 12:34

isherwood

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Connects to the outside of a barrel. Not if it's a guitar pedal power supply, in which case it's center negative and I have no idea why
– Brydon Gibson Jun 30 '22 at 14:27
All of it is copper wire. There is no blend of aluminium. Also, in South Africa, we have poles, not blades. XD – Romans Jun 30 '22 at 15:16
@BrydonGibson I've played electric guitar for years and had no idea. But because of your last name...we'll take your word for it! (and an internet search confirmed it). – Steve Wellens Jun 30 '22 at 15:51
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Maybe it's been standardized, but I know that identifying the polarity of the jack is one of the critical steps in using "someone else's" wall wart for a new purpose. Has NA standardized on negative on the outside of the barrel? – FreeMan Jun 30 '22 at 15:54
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There is not a required standard for dc there are many positive barrel devices I have an older laptop that was positive barrel negative pin or center conductor – Ed Beal Jun 30 '22 at 16:27
Most power supplies do indeed specify the polarity of the barrel connector on the power supply itself. Just check that past you hasn't spliced in a new connector and done something backwards but not notated it – Brydon Gibson Jun 30 '22 at 17:02
@FreeMan - "the SNES uses a center negative pin which is the opposite of the de facto standard today." Most power supplies do indeed specify the polarity - even the knockoff ones Made in China : https://gaming.stackexchange.com/questions/397853/what-are-the-correct-specs-for-an-snes-power-supply#comment558793_397853 – Mazura Jun 30 '22 at 21:27
@BrydonGibson: Most likely, the first guitar pedal that used a barrel connector and became popular happened to use center negative, and other manufacturers then had a choice between making their pedal backward from other pedals, or from other devices. Having a mix of center-positive and center-negative pedals would have created a bigger risk of incorrect polarity than having pedals differ from other devices. – supercat Jun 30 '22 at 21:51
ALL my barrel jacks are centre positive EXCEPT for my son's guitar pedals. Fortunately, most pedal makers anticipated people like me, and made their pedals tolerate a reversed supply without blowing up, otherwise I'd be buying a shed load of new stuff for my son. – Neil_UK Jul 01 '22 at 13:46

Is white wire with grey stripes positive or negative wire?

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