Thread: 12 volt DC transformer?

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I have a 12 volt DC transformer. The question I have is how can you tell which wire is + and which wire is -? There is a round plug on the end of the wire that was used to plug into what ever it was that it supplied power to.
Thanks.

Get yourself a cheap DMM (digital multimeter) at Harbor Freight or a similar place.
http://www.harborfreight.com/7-funct...ter-90899.html
Set the meter to read DC volts in the 0-20 volt range. Plug in the wart and touch the + and - probes to the plug, the "-" to the outer shell and the "+" to the inside of the plug. Check the voltage reading, if the display does not have a "-" in front of the numbers, it's center-positive.
The easiest way is to check the label on the wart, it should have a symbol that tells you if it's center-positive or center-negative. I still recommend getting a DMM, though. They're too useful not to have on the bench.