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