## How to test for hot (positive) wire?

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Sparky Williams
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### How to test for hot (positive) wire?

I have a step-down wall transformer that reduces household AC voltage to 12AC. It has a black wire and one that is black and white stripped. How can I test to see which wire is the positive one? <p>I put the red lead of my very inexpensive multimeter to the stripped wire and the black lead to the black wire and set the meter to AC voltage. I then reversed the leads. In both instances the readings did not indicate polarity -- just the voltage reading of 14.7. <p>Is there some other test I can perform?

haklesup
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### Re: How to test for hot (positive) wire?

That's what you should get.<p>There is no polarity because the voltage you are measuring is AC. Neither wire is positive. If you want a DC voltage, you need use a bridge rectifier also.<p>Your meter is either reading peak to peak voltage or more likly that the input voltage is a little high and that's why you are getting 14.7V RMS instead of 12VAC

Sparky Williams
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### Re: How to test for hot (positive) wire?

Haklesup,<p>Thanks for your reply. Maybe you can clear up some questions I have: 1) what is a "hot" wire?; and 2)why is one of the wires from the transformer black and white stripped and the other solid black?

Chris Smith
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### Re: How to test for hot (positive) wire?

Switch your meter to DC, the stripe wire is Pos [most of the time] and the other is NEG??? <p>You will not get a reading in AC, from a DC
voltage.<p>
If its actually AC, then each leg is positive 60 times a second, flip flopping from one to the other.<p>Alternately the wall power has one HOT, not to be mistaken for positive DC, and the other is NEUTRAL, or Ground like,... and to test which is hot, and which is ground or neutral, you hold one lead of the meter in your hand,[digital type meter only] or place it against a actual ground as in the earth or a water pipe, and the other lead is for testing,... and it will some what read a voltage from around 10 to 120 volts,[at 120 line volts] showing one as HOT, and the other wont read at all, or perhaps just a few milli volts static?<p>[ April 02, 2004: Message edited by: Chris Smith ]</p>

Mike
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### Re: How to test for hot (positive) wire?

Repeating what everbody else has said, a transformer puts out AC voltage.<p>To make it into DC voltage, purchase a bridge rectifier rated at enough voltage and amperage for that transformer, and connect the AC wires of it to the transformer's output (polarity doesn't matter). The remaining two wires on the rectifier are labeled + and -. Connect a capacitor or a few capacitors in parallel to make a capacitance of at least 2,000uf. More is better. This is neccesary to prevent AC voltages from reaching your equipment. Then, connect the equipment to the capacitor(s) or directly to the + and - wires on the rectifier.<p>Also, there really is no hot wire. AC is Alternating current. If you have an oscilliscope, connect that transformers outputs (with no rectifier) to the scope, plug in the transformer, and watch the scope. the line will go above and below the middle of the scope. Search for 'sine wave' for a picture if you dont have a scope. Anyway, there is no polarity regarding AC.<p>Does your transformer have three output wires? if so, the one with the line on it could be the center tap.<p>hope this helped,
-mike

Dean Huster
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### Re: How to test for hot (positive) wire?

Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

manuka
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### Re: How to test for hot (positive) wire?

Well DON'T try this with MAINS voltages, but for 12V or so there's a simple electrochemical way to tell polarity/ac/dc . Come on guys - H2O = high school chemistry ! This is not a joke, since I had to do this in an off shore boat once when uncoded battery leads came loose on a 2 way radio !<p>Just dip the exposed leads into water( made more conductive with lemon juice etc? )& -ve cathode should liberate considerable hydrogen bubbles, while +ve anode only a few oxygen (which is soluble in water as well. Of course AC will have equal gasing each electrode. Had a friend mention he can even "read" slow square wave duty cycle this way ! For elegant work a fruit/potato may suit- you may even see copper migration to the -ve cathode.
REMINDER - DON'T TRY THIS WITH MAINS VOLTAGES !

Mr. K
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### Re: How to test for hot (positive) wire?

In theory, there's no difference between theory and practice. In practice, there is.

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