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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?

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

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?

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>

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

1. Most meters (VOMs, DMMs) measure the average value of an AC voltage and display the RMS value, which is why they're only accurate for measuring sine waves. Newer DMMs and higher-end DMMs are "true-RMS" meters, measuring the RMS value of the waveform and displaying the results in RMS volts. The old VTVMs from the EICO, Heathkit and Knight Kit VTVMs to the Hewlett-Packard 410 series, measured the PEAK value of the waveform and displayed the results in RMS volts. Again, these RMS reading were only accurate if the measured waveform was a sine wave. The older service-grade VTVMs also had a peak-to-peak display which was accurate for any waveform and was useful for TV technicians since their schematics usually had the P-P values listed.<p>2. Why are the leads color-coded? It's standard practice in case the end user wants to be careful of the "phasing" of the transformer. It may be that transformer secondaries were connected together for series-aiding and it was nice to have the phasing correct without experimenting.<p>3. The cheapest way of all to find the hot wire is to buy or build one of those inexpensive neon testers. The circuit is nothing more than an NE-2 neon lamp in series with a 270K ohm, 1/4-watt resistor. Poke the little circuit into an empty ball-point pen case with a window in the side to see the lamp and the leads (insulated) brought out one end with little test tips on the end. The tester is good for indicating the presence (but not the actual value) of any voltage, AC or DC, from around 90v up to 240v. If you hold one lead tip of this tester between your fingers and touch the other lead to the various wire, the hot wire(s) will cause the neon to glow while the ground and neutral wires will not. The tester will also indicate AC or DC. Both electrodes of the lamp glow on AC. The electrode that's connected to the negative side of a DC source will be the only one that glows.<p>Dean

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 !

Don't mean to beat up this topic, but....<p>Many respondents have told you true stuff, and Dean gave you lots of really practical advice. But I sense that your question may be a bit more fundamental. Please forgive me if I go back too far.<p>In household AC service, we talk about a "hot" wire. This isn't positive or negative. It's just not at ground potential. An ideally wired outlet has a ground wire (green or bare copper) and a neutral wire (white), both at zero volts potential to a true ground. Then there's the black "hot" lead. (N.B., more complex circuits, e.g. 3-way switches, will add additional colors, notably red.) The hot lead swings from positive to negative at about 120 volts RMS as referenced to neutral with the AC cycle.<p>Your transformer output is probably isolated from the ground at the mains, so to talk about "hot" is a bit of a strange concept. Neither output will necessarily have any particular relationship to ground if the secondary of that Xformer is isolated from the primary side. What you will see is a voltage differential between the leads of 6 or 9 or 12 or 24 or whatever AC volts that the transformer is designed to produce. For most purposes, that is sufficient.<p>[ April 09, 2004: Message edited by: Mr. K ]</p>