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  #1  
Old 09-27-2009
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Determining wire polarity

I am attempting to install a stereo/cd player and wire it into the boats 12 volt system. I want to tap into the wires leading to a lamp that are right next to the stereo, but I can't tell which wire is positive and which is negative. The wires leading to the lamp are not properly color coded. Is there a test to determine which wire is positive and which is negative?
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Old 09-27-2009
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Get an analog volt meter (like a super cheapie from Radio Shack) and put the red (positive) test lead on one wire and the black (negative) lead on the other. If the needle moves to the right, then the positive lead is on the positive wire. Otherwise, its backwards.

Don't use a digital meter, as they will do you a favor and work either way.
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Old 09-27-2009
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Quote:
Originally Posted by jarcher View Post
...Don't use a digital meter, as they will do you a favor and work either way.

A GOOD digital meter will tell you if you have negative polarity.. positive polarity is assumed....
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... Or you can put one lead on the battery ground stud and one sucessively on each wire. The ground or negative wire will show close to zero and the +12 or positive will show close to 12 volts.

This assuming it's a 12 volt DC and negative ground system.
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Old 09-27-2009
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A GOOD digital meter will tell you if you have negative polarity.. positive polarity is assumed....
Yes, but given the question, my assumption is that the OP didn't have one. My Fluke does this well.
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Old 09-27-2009
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A digital multimeter can be had for $20-30 nowadays. My old one cost less than $30 CDN 7 or 8 years ago at Radio Shack. Still works well. I now have a good clamp meter but the old one is nice and small to fit in a pocket. If the probes are reversed the voltage shows as negative ( -12 ).
Brian
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Old 09-28-2009
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Let's get real, you can get a decent meter for around 7$, $3 on sale at places like Harbor Freight, sure its Chinese but so are our boats. I've been using them for years; home, hobby and boat(that seems redundant) actually I buy several rather than replace the batties or give them out.
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Old 09-28-2009
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Groundhog's idea is simple and pretty much fool proof. If your leads aren't long enough use a spare length of cable to extend it. Set the multi meter to DC volts( V followed by straight lines not wavy lines). A cheap meter will probably not be auto ranging so set it to the highest setting and switch it down till you get a reading other wise you risk blowing a fuse in the meter. If you are on shore power it might not be a bad idea to unplug it first. Its unlikely to be a mains voltage cable that you have but it never hurts to treat electricity with respect

Last edited by davewild; 09-28-2009 at 08:27 AM. Reason: clarify for safety
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Old 09-28-2009
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it's interesting that no one noted this, but tapping off existing wires, while common, isn't a very good way to get power to things. it's messy, difficult to debug later and you can easily exceed rated wire amperage.
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Old 10-02-2009
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If you want to make a 12 volt polarity checker for the cheap, you can use a light bulb and a diode, or LED, resister and diode. The current will flow
+ ----->------ to gnd, the stripe on the diode points to ground.
+--red-----light------[diode|]--------black-gnd
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