How to tell polarity expected of a DC barrel jack?

I'm sure this information is somewhere around the internet, but I can't find it.

I have a device that takes a DC barrel jack. I don't know the polarity it expects though. There is a marking though. It looks like this:

___
---  12VDC


I'm 99% sure it's a symbol to indicate the polarity, but which is it? Positive inside or positive outside?

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Have you measured it with a multimeter? –  Kortuk Jun 15 '12 at 21:35
I lost the original wall-wart for it @Kortuk –  Earlz Jun 16 '12 at 6:20
Ohm it out to ground on the device? –  kenny Jun 16 '12 at 14:55

Well I have a \$200 photo frame unit that needs a 12Vdc 2 amps power supply. The only clue I had was a tiny sticker that said 12vdc 2.0amps and the symbol __ and the ----- underneath.

So after searching the internet, and finding nothing simple like oh the __ is the outer shell and the ---- is the inner core, and the __ is a negative, and the ---- is positive.

I said to heck with it, and just powered it as I mentioned above, but I added 1.5 amp fast blow fuse just in case I was wrong, but alas, I was correct.

So to answer your question the __ upper line is the shell and is negative, the ----- is the core and its positive.

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Open up the device trace some circuit board traces and look for clues. For instance: polarity of electrolytic capacitors, or wiring of three-terminal voltage regulators, or the direction of diodes (but you have to understand the circuit for that one: sometimes diodes are reverse biased on purpose). If you see a diode which is connected squarely between the power rails, then it is reverse biased. The stripe end (cathode) of the diode is then on the positive rail. It provides a bypass path for incorrect polarity.

If there is any integrated circuit which is clearly marked, and you can find the data sheet, then you can determine which of its pins is ground. Then you can check for continuity between the tip or ring of the power connector and that pin, either with your multimeter, or by tracing the board and wiring.

There may be additional clues inside the device, like silk-screen markings on the circuit board such as VCC, GND, or other polarity indications. The circuit board might have an obvious ground plane, whose continuity can be traced to the connector.

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I suggest checking the polarity of the barrel conector before you overload your device & it blows up. Use a multimeter on ohms measurement. The negative terminal of the barrel connector will be shorted to ground plane on the PCB, or the chassis, and when you connect one test lead to the negative terminal of the barrel connector and the other to chassis/PCB ground, the multimeter will read 0 ohms. If you can't open the case and there is a battery compartment, you can place your test leads across the negative battery terminal and the barrel socket.

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Unless there is a figure like below, or some wording like "positive centre" then you can't tell.

or:

A supply can use positive or negative centre, as Olin says there is no standard. This is why you get the polarity switches on many of the universal DC supplies.

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I know how to read this, but this device doesn't have those markings and I lost the original wall-wart. I guess I'll just have to open it up and guess which one goes to the ground-plane –  Earlz Jun 16 '12 at 6:23
Before opening up, are there any other connectors (like headphones) of which you can be reasonable certain that it carries ground on one specific terminal? That way you don't have to open it, just measure it with an Ohm meter. –  jippie Jun 16 '12 at 8:03
With a 1/4" or 1/8" Tip Ring Sleeve jacks you can be 99% certain that the sleeve (the largest part nearest the cable) is ground. Again though, there are cases where it may not be the case (see the Wiki page for examples) –  Oli Glaser Jun 16 '12 at 8:37