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Electrical wiring for dummies
March 8, 2010 4:58 PM   Subscribe

Blue and brown wires inside my PC's PSU....which is hot? Also, where can I learn some basic electric skills and knowledge so I'm not always deathly afraid.

I'm working on a project where I've ripped apart an old PSU from a PC. I'm trying to rig things so I can plug the PSU into the wall with the standard 3 prong cable, and run an extension cord off the inside of the unit.

Basically what I'm looking for is Wall outlet > old computer PSU > extension cord.

Thought this would be pretty easy, but when I opened up the PSU I have green/yellow, blue, and brown wires coming off the leads. How can I determine which of these is hot. Also, for fun: what would happen if I reversed the polarity between there and my extension cord?

I'm just kind of messing around with stupid, cheap projects for now, are there any other resources for someone looking to understand basic electric and wiring?
posted by pilibeen to Technology (34 answers total)
 
what would happen if I reversed the polarity between there and my extension cord?

People will send flowers to your funeral.

Don't do this. Find something else to play with.
posted by Chocolate Pickle at 5:03 PM on March 8, 2010 [1 favorite]


Brown is line, blue neutral. Do not reverse polarity. Test each lead to ground before doing anything.
posted by Raybun at 5:04 PM on March 8, 2010


A voltmeter is better than ask me for a question like this.
posted by hortense at 5:05 PM on March 8, 2010 [1 favorite]


Yeah, just to be clear...I'm NOT actually looking to experiment with wiring things the wrong way. That's why I'm asking. I'm just curious what would happen - I know it could kill me - but how?
posted by pilibeen at 5:06 PM on March 8, 2010


Also, I thought about getting a volt-meter...but still not entirely sure how I'd go about testing polarity with one. Step-by-step advice would be awesome.
posted by pilibeen at 5:08 PM on March 8, 2010


Yeah, I'm sort of confused as to what exactly you're trying to do with the project. Are you trying to extend one of the PSU outputs (+12V or +5V, for instance) through the extension cord? Or are you just wiring the AC input from the PSU plug to the extension cord? If the latter, what are you gaining by doing this inside a power supply, which has lots of wires and big capacitors to hurt you?

Seconding that this is not a great idea for a "just fiddling around" project. If you want to learn the fundamentals of electricity, you might want to buy a breadboard and some LEDs, maybe a breadboard power supply kit, to work at safer voltages. If you really want to do home wiring-type projects, maybe figure out how to replace a lightswitch or a power outlet (with the breaker off!) and go from there.

Feel free to tell us more about what you're trying to do- maybe I'm getting you wrong.
posted by aaronbeekay at 5:09 PM on March 8, 2010


Seconding Chocolate Pickle and aaronbeekay - "If you have to ask, you can't afford it" (where "it" is "the risk" in this case). I would strongly recommend doing projects involving lower voltages before trying this - if you screw up on a 9V battery you'll melt a wire or get a mild burn, if you screw up on a PSU you're liable to kill yourself or burn down your house.
posted by 0xFCAF at 5:11 PM on March 8, 2010


A multimeter, by the way, would help because it reads positive and negative voltages. Remember that voltage is a difference in electrical potential: imagine a cliff and a gorge. The cliff is one wire, the gorge another. Standing on the cliff, you have potential energy. Standing on the ground, you have less. If you were to step off the cliff, you would fall to the ground (current would flow).

When you connect the positive lead of the voltmeter to a positive wire, and the negative lead to a neutral/ground/whatever, it will register a positive voltage. If you switch it up, it'll be negative. (This applies to DC currents.) That's how you use a voltmeter to determine polarity.
posted by aaronbeekay at 5:13 PM on March 8, 2010


Buy a Radio Shack 50 in 1 kit and mess around with that.
posted by fixedgear at 5:17 PM on March 8, 2010


Yeah, I'm just trying to wire the AC input from the PSU plug to the extension cord. I'm making a display case out of an old tower.

I've already ripped apart the PSU. I waited a week or so for everything to discharge - so I made it out of that alive.

And yeah, this probably wasn't a great project to start with - but I'm working with what I've got for now.
posted by pilibeen at 5:17 PM on March 8, 2010


...where can I learn some basic electric skills and knowledge so I'm not always deathly afraid.

I'm working on a project where I've ripped apart an old PSU from a PC. I'm trying to rig things so I can plug the PSU into the wall with the standard 3 prong cable, and run an extension cord off the inside of the unit.


Sounds to me as if your problem is not being deathly afraid enough.

People who don't yet have basic electric skills and knowledge really, really need to stick to battery powered projects. I'm not trying to be a knowledge-elitist prick here, I'm looking out for you. Mains electricity kills ignorant people.
posted by flabdablet at 5:19 PM on March 8, 2010 [2 favorites]


That said: brown, blue and yellow/green are IEC standard wiring colors.
posted by flabdablet at 5:22 PM on March 8, 2010


A multimeter, by the way, would help because it reads positive and negative voltages.

Of course, when you're talking about AC, there is no such thing.
posted by Chocolate Pickle at 5:29 PM on March 8, 2010 [1 favorite]


Thanks everyone.

I work with some pretty handy folks so I think I'll defer to them for wiring help. This probably isn't the time to put my half-baked assumptions to the test.

Now once it's all wired up, I assume I'm going to want to cover up the exposed leads somehow. Would normal caulk work for this? Silicon sealant? Or is there some product specific for electrical projects?
posted by pilibeen at 5:52 PM on March 8, 2010


Jesus wept.

If you don't know enough to cover your solder joints with heat shrink insulation tubing, or if (worse still) you're not even making solder joints, please, please, please stop playing with mains wiring now.
posted by flabdablet at 5:55 PM on March 8, 2010 [2 favorites]


>: Thanks everyone.

I work with some pretty handy folks so I think I'll defer to them for wiring help. This probably isn't the time to put my half-baked assumptions to the test.

Now once it's all wired up, I assume I'm going to want to cover up the exposed leads somehow. Would normal caulk work for this? Silicon sealant? Or is there some product specific for electrical projects?


Hey, you know this thing that you're asking us how to do?

You really, really don't want to be doing it. Especially since you seem to have absolutely no knowlege about it whatsoever and are yet hell-bent on forging onward, to the point of flipping the polarity of a power cord1 (just to see what happens!) or using caulk to cover exposed (unsoldered?) leads. Stop it, now, lest you electrocute yourself or start a fire.

1. This can kill you, by the way. There's a very, VERY good reason all power cords are polarized and therefore in phase.
posted by dunkadunc at 6:23 PM on March 8, 2010 [1 favorite]


Really really DON'T DO THIS!!!
posted by Chocolate Pickle at 6:50 PM on March 8, 2010 [1 favorite]


Jeez, lay off the guy. He said he wanted to learn so he's not afraid, and throwing more fear at him isn't helpful.

I mean, house wiring all over the world is made without solder and the world hasn't burned down yet.

Now, that said, this isn't a great project. As a rule, wires aren't meant to come out of things, only go into them.

The reason the polarity is important is that many devices are built assuming that one leg is hot and one leg is common/neutral. The common leg is, in a perfect world, and assuming we are talking about standard US style electricity, much safer than the hot leg. So when polarized devices are manufactured, the neutral leg of the circuit has more "freedom" to be wired in a way that, if it were hot, would be unsafe. Like the location of fuses and such.

But all of this assumes that somewhere along the line, a failure occurs. If nothing is broken, reversing the polarity has almost no effect on anything. It has zero effect on appliances that have unpolarized plugs. So if something were plugged into your contraption, and short circuited, there's a chance it's chassis could become "hot", or it could be perfectly safe, but the chassis of your PSU could become hot.

If you must do this project, wire it up like older power supplies were and install one of these sockets into the power supply with a fuse.

Use electrical tape for all connections. All connections need to be mechanically connected somehow, either through solder, or wire nuts or some other way that stops them from coming apart. Then tape that connection up.
posted by gjc at 6:52 PM on March 8, 2010


I'm not talking about the solder joints - which I will definitely use heat shrink on. I really don't have the vocabulary to describe whatever I need. I'm going to find some live help.


And I had no intention of experimenting w/ the polarity of the wires - I was just hoping someone would explain exactly what would happen if it occurred.
posted by pilibeen at 6:55 PM on March 8, 2010


gjc, I'm all for learning by doing, but there are so many misconceptions and bad practices flying around simultaneously here that I've got to second those who say the OP should start with something less dangerous. pilibeen, I really think you ought to start with a battery-powered project and move on to something powered by the mains after you can trust your ability to connect two wires together safely. That said, since you seem hell-bent on proceeding despite our warnings, let's clear a few things up.

(1) You can absolutely get shocked to death by a power supply you've let "rest" for a week. Waiting assumes the presence of a bleed resistor across the capacitor, and while many designs have it, it's not always there (or not always functional). It is possible to discharge a large capacitor to 0V, leave it on a shelf for a year, and come back to find it charged to 100V. (for esoteric reasons I won't derail by going into here)

(2) Never, ever, but never work on any of this stuff while it's plugged in. I'm hoping that you've ripped all the guts out of the PSU by now; otherwise the warning in (1) goes double now that you're mucking around in there and have potentially wrecked the bleed resistor yourself.

(3) General electrical safety: Rubber-soled shoes, only put one hand in the box at a time (putting both hands in makes it possible that the best conductive path will be from your left hand to your right hand, with your heart in the middle). If that big cap is still in the PSU, you need eye protection too -- capacitors can explode when suddenly discharged, and will vent all sorts of hot nastiness when they do so.1

(4) Fires start at bad electrical connections. Any mains connections that are not soldered (more on that in a second) need to be connected by wire nuts or crimped terminal lugs. Electrical permit inspectors test wire nut splices by grabbing both wires and pulling as hard as they can; you should practice until you can pass this test.

(5) "Bad connection" also means "broken wire" -- the reason we use lugs and nuts instead of solder for home wiring is that soldering a stranded wire creates a sharp edge where the wire changes from being quite flexible to very stiff. That's where the wire will break, and where the fire will start.

(6) The quality of your splices matters more the more current you are trying to deliver. Running a desk lamp off this thing? Not super scary. Fog machine, microwave, small TIG welder, slide projector? Don't do that.

(7) The breakdown voltage of dry air is 1000V/mm. That means that for mains electricity (in the US, Vrms=110) you'll get a spark if the hot wire is within about .16mm of the neutral or ground. That number doubles in Europe and increases dramatically with humidity also. Once a spark starts, it doesn't stop until the circuit breaker trips.

Seriously, don't screw around with this stuff. We have no good physical intuition for the kind of energy delivered by "small" amount of electricity. If delivered efficiently, the energy in a D cell could lift your body over 200 feet into the air. There's a lot more oomph in the mains.

1This stuff is important! I spent last summer helping a group of students build an electric car with a 350V battery array -- this meant that every time they turned on another subsystem, I was standing in the shop, in full welding gear, with face shield, holding a long wooden pole to use to pry their convulsing hands off the car if something went wrong.
posted by range at 7:28 PM on March 8, 2010 [1 favorite]


What are you trying to accomplish with the power supply? I have a number of power supplies that I've removed the forest of cables from an turned them into dedicated supplies (as in 5 or 12 volt DC) for various purposes. (You can, of course, just take the DC power from the cables running out of the box, but I wanted to be rid of the other wires.)

If you flip the polarity of an AC cord, the odds are nothing will happen. Similarly if you use ungrounded outlets the odds are nothing will happen. Many people have bet with the odds and lost.

If you want to learn some electronics start with DC stuff in the 5-12 volt range. Lots and lots of stuff you can do with virtually no risk. For an absolute beginner I recommend Make: Electronics, after you get through that, Practical Electronics for Inventors, and when you want to go for that BS in electrical engineering level of things, The Art of Electronics.

Next, you are going to want some components - I was pretty please with the value packs that Futurelec sells. The warning: they ship things by boat and have a habit of waiting until they have everything in stock before they ship an order, so if you're in a hurry they're not a good choice, but if you want to get a bunch of cheap basic components to get you going and don't care if it takes a couple weeks, they've got you covered.

You're going to want a breadboard and jumper kit, and some tools, but the Make: Electronics book does a pretty good job of explaining all that.

Good luck.
posted by Kid Charlemagne at 9:51 PM on March 8, 2010 [3 favorites]


If nothing is broken, reversing the polarity has almost no effect on anything.

Except when it does.
posted by flabdablet at 11:25 PM on March 8, 2010 [1 favorite]


Also: electrical tape is a piss-poor choice for insulating connections. That's not what it's designed for. Electrical tape is for holding wire looms together. If you use it to insulate a connection, then in a year and a half when moisture and PVC creep make it fall off, you start a fire.

Use heat shrink tubing over soldered connections. Use at least two layers. You want at least as much thickness of heat shrink between the metal and the outside world as the original plastic insulation you stripped in order to make the joint.

If you must make non-soldered connections, use properly mounted terminal blocks or wire nuts that incorporate insulation into their structure. Don't use lug and bolt connectors unless those are securely mounted inside a double-insulated or properly earthed casing that won't be opened while the equipment has power applied. Don't use automotive-style spade lug or bullet connectors for mains wiring.

Mains electricity - even the feeble excuse for mains electricity you have in the US - deserves to be treated with far, far more respect than is evidenced in this question.
posted by flabdablet at 11:33 PM on March 8, 2010


Also: proper strain relief is vital. It should not be possible to break or even fatigue an electrical connection by picking a box up by its cord and swinging it around your head, because there should be a clamp on the cable that takes that physical strain rather than letting it through to the cable.

Also: the wires in any cable carrying mains current should be cut to length in such a way that if the strain relief does fail, the hot wire is the first one to break, followed by the neutral wire, followed by the earth. You never want to set up a situation where the hot wire is the only one connecting a piece of equipment to the wall.

Also: if you're putting switches in a circuit carrying mains current, use a double pole switch that breaks both the hot and neutral connections. For lighting circuits it's acceptable to break only the hot side. Never, never, never design a circuit where the switch or fuse is or could end up in the neutral leg. If a switch is off, or a fuse blows, you want it to disconnect the hot side.

All of the above principles follow from common sense applied to a basic attitude of respect for potentially lethal voltages.
posted by flabdablet at 12:04 AM on March 9, 2010


Blarg... there should be a clamp on the cable that takes that physical strain rather than letting it through to the cable connection.
posted by flabdablet at 12:06 AM on March 9, 2010


And look - the whole idea of designing a thing that has a live mains cable emerging from the inside of a PC power supply is just broken and wrong. Don't do that. If you want something that lets you do something like that, mount an IEC power outlet or a proper panel-mount three-pin mains outlet on the back of the power supply, and jumper it inside the supply to the IEC power inlet - making sure that you jumper live to live, neutral to neutral and earth to earth and don't get live and neutral swapped - and then use a plug-in power cable with a moulded plug and a moulded socket for your "extension cord". That way, when the five-year-old trips over the cord it just yanks out the plug rather than tearing apart your dodgy taped-up joints.
posted by flabdablet at 12:12 AM on March 9, 2010


And if you absolutely must run a mains cable out through a hole in a metal panel, use a proper clamping grommet.

Jesus, I'm getting sick with worry just trying to think of all the ways a gung-ho experimenter could come to grief by playing about casually with mains electricity.

Seriously. Do your learning on battery power.
posted by flabdablet at 12:15 AM on March 9, 2010


And, before someone comes in to claim that (US) house AC (110 Vrms) won't kill you, an electrician with 40+ years of experience died in Dayton OH while wiring up his wife's new mall store.

Under the right circumstances*, 110 is enough to screw with your muscles, including your heart.

Ironically, 110 is much deadlier than the higher European voltages. At 110 V, the heart defribrillates. At 220+ V, it's more likely to clamp shut, and will probably resume after the power is cut (if the power is cut quickly enough).
posted by IAmBroom at 9:31 AM on March 9, 2010


I think I just answered your other question - it kills by defibrillating or stopping the heart.
posted by IAmBroom at 9:32 AM on March 9, 2010


I like the idea, which was mentioned a couple of times, of just mounting an IEC outlet on the inside of the power supply. That would be great, I just want everything to look clean. Where can I find an outlet/adapter/plug/whatever which would be a good fit for the space in my old ATX psu?
posted by pilibeen at 3:01 PM on March 9, 2010


Digikey is good for this kind of stuff if you're in the US. Here are the cables that go with it.
posted by flabdablet at 5:02 PM on March 9, 2010


Whoops. Cable link fail. Here's one; searching Digikey for IEC cable C13 C14 will lead you to others.
posted by flabdablet at 5:09 PM on March 9, 2010


I can't plug a standard US extension cord or power strip into that female C13 end can I?
posted by pilibeen at 7:22 PM on March 9, 2010


No. You'd need a NEMA 5-15R connector for that.

I am still completely in the dark about where this design exercise is going. Does the old PSU still have any of the power conversion electronics in it? If so, how are you going to find room to incorporate an extra connector? If not, what's the point?
posted by flabdablet at 8:17 PM on March 9, 2010


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