In which I divulge the secrets of how to tell which wire is positive and which wire is negative. (I seem to get lots of hits from people searching for this sort of thing, so here you go.)
In particular, this page addresses two different situations: you have a battery or low-voltage power supply and you want to find out which wire or terminal is positive and which is negative, or you have a device that requires a battery or adapter and you need to figure out the polarity.
I make no claim that any of these methods is foolproof or even a good idea. In fact, poking around electronic circuits or devices, especially when you don't know what they are, is dangerous. Even if the power is off, you could break something, injure or kill yourself, or cause various things to explode. (Lots of devices have power-supply capacitors that stay charged after you turn them off and unplug them. I am not responsible for damage, injury, death, or apocalypse caused by following my advice.
Table of Contents
- Read the Label
- Color Conventions
- Use a Voltmeter
- Use an LED and resistor.
- Examine circuit board traces
- Using Water
- Polarity of components
- Wall Outlets/Plugs
[top] Read the Label
Sometimes you might see a label like this:
This one means "tip is positive." Note how the "+" symbol is connected to the little circle in the center (representing the tip) and the "-" symbol is connected to the ring on the outside, representing the outer conductor. If you see this on an adapter, it means that the tip conductor is positive; if you see it on a device, it means that you're supposed to use that kind of adapter.
[top] Color Conventions
If one wire is black and one wire is red, the red wire is most likely positive. In a computer power supply, black is ground, red is +5 volts and yellow is +12. In computer power supplies, these colors are standardized; in other applications they may not be.
In telephone wiring, the green wire should be "tip" and the red wire should be "ring." "Tip" is ground and "ring" is around -48V; the names come from back when switchboards had plugs with tip and ring conductors, like headphone plugs are today. If you have additional lines, their colors should be (black, yellow) and (blue, white). But this is of course not guaranteed; in fact, I've installed CAT3 phone wiring that didn't have red or green wires.
In eight-conductor twisted-pair wiring (like Ethernet cables), there are two standards, T568A and T568B, defined in TIA/EIA-568-B.
Home wiring is AC, so there's no positive or negative. Instead, there's "hot" and "neutral." In the US, white is generally reserved for neutral; red and black are usually hot, and green or bare wire may only be used for ground. However, you can mark any wire as "hot" by placing black tape at both ends; you can probably do the same thing for neutral or ground by using white or green tape. But don't take my word for any of this, as I am not a licensed electrician; if you're trying to figure out how the wiring in your home works by looking at the colors, you should probably call in a professional; not having your house burn down is probably worth the extra expense.
Outside the US, conventions are probably different. In the UK, for example, I think the "hot" (which is probably called "live") is red, neutral is black, and ground (or "earth") is green and yellow. Also note that I have never been to the UK and have no idea if any of this is true--I got the above information from this site.
[top] Use a Voltmeter
If you have a voltmeter, why are you asking me? First, plug the thing in. Then set the voltmeter to an appropriate range setting. If you don't know the voltage, set it to the highest setting first. Then, carefully, connect one of the test leads to one of the wires/contacts/whatever and connect the other lead to the other one. If the voltmeter says zero or something really small, maybe you have an AC-to-AC adapter (just a transformer); set the voltmeter to measure AC and try again. Or maybe your adapter/whatever is just broken.
[top] Use an LED and resistor.
First, you've got to figure out which wire from the LED is positive and which is negative. If it's in one of the standard dome-shaped packages, you can probably tell. Hold it with the wires toward you (so you're looking at the bottom) and see if it has a flat side. That side is (almost certainly) the negative side, or, as we say in EE-land, the "cathode." The positive lead is often longer, but this isn't always true; anyway, somebody could have cut the wires, just to confuse you.
Now you need to figure out what size resistor to use. For a conservative estimate, try using R = (V - 2) * 200. (The 2 should be in volts and the 200 in inverse amps, if you want the units to work out.) That assumes that you want a current of 5 mA and you have an LED with a forward voltage of 2 V. (Usually Vf = 1.8 V for red, 2.4 V for green, or 3.6 for blue, give or take a few.) So the LED won't be very bright, but you might be less likely to break it.
Once you've got all that straightened out, connect the LED and the resistor, in series, to the power source. The reverse it. If it only lights up one way, you've figured out the polarity. If it doesn't light up at all, you've either got a broken power supply, a broken LED, or too big a resistor. If it lights up both ways, your power supply is AC
[top] Examine circuit board traces
Usually, circuit boards have electrolytic capacitors near the power supply connector. These are little can-shaped things, often blue or black, that can only be connected one way. The negative side of an electrolytic capacitor is usually prominently marked with a stripe. If it's an axial-lead capacitor, which means that one wire comes out of each end, the stripe will have arrows pointing toward the negative side. Sometimes, the positive side is marked instead, with a "+" symbol. If you can figure out which power supply wire is connected to which side of an electrolytic capacitor, by following the traces on the circuit board, you can figure out the polarity. This is useful when you have a device and you don't know what kind of AC adapter to use, but following traces on a circuit board can be tricky. If you have an ohmmeter or continuity tester, you can poke around with it, but be careful: parts of the circuit might have fairly low resistance, especially when you're using a low voltage like that from a continuity tester or ohmmeter, so parts of the circuit that aren't directly connected can look like they are.
[top] Using Water
Note that this is not actually a good idea. It's one of those "if I were trapped on a desert island..." sort of things.
You can figure out the polarity of a DC power supply by using saltwater. Fill a cup with water and dissolve a good amount of salt. Then, using copper wires (other metals will probably work too; I'm not a chemist), connect the power supply to the saltwater. Hydrogen bubbles will form on the wire from the negative side as the water is electrolyzed into hydrogen and oxygen. The oxygen will (hopefully) react with the copper in the positive wire, turning it greenish and producing no bubbles.
Note that this is dangerous if you are using a power supply of more than a few volts, or you are not careful. Saltwater conducts electricity, so you have to be careful with it. Also this process produces hydrogen gas, which can explode. And it probably produces some chlorine gas, which is poisonous, and sodium hydroxide (lye), which can burn you.
If you see four big black diodes near the power supply connector, it's likely that the device has a rectifier, which means that the polarity doesn't matter. But don't be sure of this unless you can trace the circuit. Google "bridge rectifier" if you want to see what circuit you're looking for.
[top] Polarity of components
If you have a component such as a capacitor or a diode, you can often tell its polarity by looking at markings or other features:
Diodes generally have a stripe near the negative terminal (cathode).
Bridge rectifiers often have "~" symbols near the AC input leads and "+" and "-" symbols on the respective output leads.
Electrolytic capacitors are generally prominently marked with a stripe, almost always on the negative side. If the capacitor is axial-lead (one wire comes out of each end), the strip usually has an arrow that points toward the negative end. The positive wire is usually longer, but somebody could have cut the wires, so be careful.
If you put electrolytic capacitors in backward, and the voltage across the capacitor is low enough, they may work anyway. Otherwise, they swell up and explode in a rather exciting way.
Tantalum capacitors are also polarized, and most of these that I've seen have the positive lead marked with a "+" symbol. Other kinds of capacitors (ceramic, polyester/mylar, polystyrene, air, paper, etc.) are not polarized. The little round flat ones are usually ceramic.
LEDs usually follow the same convention as capacitors: the positive lead is longer. If you look at an LED with the wires toward you, the bottom usually has a flat side. (In other words, it is slightly D-shaped.) This is closer to the negative lead.
[top] Transistors, Triacs, etc.
There are several different conventions; the only way to tell for sure is to test the device with a tester or look up the part number.
[top] Wall Outlets/Plugs
In the US, where 125V outlets are the NEMA 5 type, the wider blade of the plug is the neutral and the narrow blade is the hot. The safety ground is the "D"-shaped third prong/hole. You can amuse yourself and others by sticking paper clips in the ground and (if you're feeling adventurous) neutral holes of wall outlets. You may get more amusement than you were expecting if the outlet is wired incorrectly or something else goes wrong.