# 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

The solid line is meant to show ground and the dashed line 12 V DC in your case. Usually these then go to a symbol that shows one being connected to the outside ring and the other to the center.

Much of the time though nothing tells you. There is no standard, so the only way to know for shure the polarity of a power supply is to measure it.

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I have never seen one where power was not the center ping, this stops you from touching it to a ground. – Kortuk Jun 15 '12 at 22:18
@Kortuk, if you've ever bought electric guitar effects pedals you would know the pain of having to deal with reverse polarity plugs. For some stupid reason they've adopted it almost universally. I have a hunch it's because they want you to buy their over-priced wall adapters. – Jon L Jun 15 '12 at 22:27
@Kortuk, even better, you can buy adapters (e.g. from RadioShack) where the tip is interchangeable and they are not keyed for orientation so you can make them whatever polarity you want. – vicatcu Jun 16 '12 at 2:10
@Kortuk - They exist. We got some FPGA boards from a distri which came with a power adapter, so I supposed it was the right thing and didn't check. It wasn't the right thing, and the board went kaputt. (Distri admitted the error and replaced boards and adapters) – stevenvh Jun 16 '12 at 3:35

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

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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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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