polarity of power cords

[kcthomas]

This question comes up because I lost my power cord to my xbox and so i am trying to manufacture one out of an N64 (or something similar) power cord. The XBOX cord is a figure 8 shape so it can go in either way ("right side up" or "upside down"). On the plug side it has 2 small plugs (you know the metal parts that go into the wall, so you can plugh the cord into the wall either way as well). The N64 cord only goes into the machine one way and into the wall one way (one of the metal plugs is bigger than the other).

Now i figure i can shape the N64 plug so that it fits into the xbox with no problem, but i was just curious what kinda of trickery they have in the xbox that allows the it to run with either positive or negative voltages applied.

 

[PowerEngineer]

An electrician who knows the National Electric Code can probably answer this better, but here goes...

The reason for different plugs has to do with safety, and how the electrical device you're plugging in might fail. The big danger is that you might come into contact with the "hot" conductor by touching the device if the insulation fails. Coming into contact with the neutral conductor isn't desireable either but will likely not hurt you.

When you have a polarized plug, it (should) force you to insert into the outlet so that the neutral (usually white) conductor is connected to the wide bladed side of the plug. That conductor inside the device is the closest one to the outside of the device (and therefore the one you would most likely come in contact with if its insulation fails). So, the polarized plug should make that much more likely that you'll contact the neutral rather than the "hot" conductor...provided that the plug and outlet haven't been tampered with and the house has been wired correctly.

Many devices are now "double insulated", which means they have equal amounts of insulation on both conductors inside the device. This means that either one could be "hot" and the insulation on either one could fail. These often have a grounding plug that ties to a shield on the inside of the device, so that if the insulation fails there will be a short through the shield back through the ground wire. This trips the breaker before you get zapped (hopefully). Again...provided that both plug and outlet have not been tampered with and that the house is wired correctly.

Edit: stupid typos

 

[pm]

Good description. Thanks, Power. I'd always wondered about that too. Ground I could understand, but I never understood polarized plugs - nor why the building code guy from the city checked all of my circuits for correct polarization when I added additions to the house. Now it all makes sense.

 

[DrPizza]

A really easy example to look at would be a typical lightbulb. Ideally, you would want the bottom part of the socket to be hot and the sides to be neutral - that way if you're standing in a puddle of water, screwing in the lightbulb and make contact with the side part of the metal on the bulb, you're not going to complete a circuit.

 

[kcthomas]

so what kind of circuitry is in the xbox that allows it to run at both positive and negative polarity?

 

[Mday]

Originally posted by: kcthomas
so what kind of circuitry is in the xbox that allows it to run at both positive and negative polarity?

it runs on DC as do all computers. AC-DC rectifiers generally don't have polarity.

 

[DrPizza]

Originally posted by: Mday

Originally posted by: kcthomas
so what kind of circuitry is in the xbox that allows it to run at both positive and negative polarity?

it runs on DC as do all computers. AC-DC rectifiers generally don't have polarity.

I think the problem is deeper than that. Does the OP realize that AC (the 110-120volt outlet you're plugging into) doesn't have a positive and a negative side? It *alternates* hence "alternating current."
Through the use of a couple of diodes, you can switch AC to DC, but not at a constant voltage. (not that AC is really constant either)

 

[silverpig]

Even simpler expanation:

hot prong : switch : toaster element : neutral prong

If you jam your knife into the toaster in this configuration when your toast is popped up but stuck, you won't get a shock.

hot prong : toaster element : switch : neutral prong

If you jam your knife in here, you'll get zapped.

Polarized plugs ensure the former scenario and preclude the latter.

AC - DC conversion is accomplished with a diode and a capacitor.

 

[FrankSchwab]

Originally posted by: DrPizza

I think the problem is deeper than that. Does the OP realize that AC (the 110-120volt outlet you're plugging into) doesn't have a positive and a negative side?

Actually, in the world of home wiring, there is a negative and a positive side (kinda...).

In the US, most houses have 240VAC coming into them on three wires. There are two "Hot" wires, and a neutral. The distribution is done such that there is 240VAC between the two hot wires, but only 120VAC between either hot wire and the Neutral. Roughly half of the lighting and outlets in your house get connected between one hot and neutral, the other half between the second hot and neutral.

At your house, the Neutral wire is connected to a ground (literally, THE ground), to all of the "ground" conductors on all of the outlets in your house, and to all of the "Neutral" conductors on all of the outlets in your house. The theory is that, inside your house, NONE of the ground wires carry any current at all except when something has gone wrong. ALL of the current supplied by the Hot wire is returned by the neutral wire.

So you can, in theory, grab two (Neutral and Ground) of the three wires feeding an outlet, and have no adverse effects. Alternatively, two (Neutral and Ground) of the three wires in your Blender, TV, drill, etc., could short to the metal case and you'd never notice. I'd call these "positive outcome" wires.

It's that third wire that causes the problem. I'd call it the "negative outcome" wire. If you grab it, the results depend on how well you are grounded. If you're in new, rubber-soled sneakers and aren't touching anything else, you probably wouldn't even notice. If you had one hand on the kitchen faucet and grabbed the wire (or something where the hot wire is shorted to the case) with your other hand, you'd probably die. If you were in bare feet on a concrete floor, you'd probably get zapped pretty good.

So, even though it's AC, it's AC referenced to Ground, and that can make all of the difference.

/frank

 

[Runamile]

Originally posted by: FrankSchwab

Originally posted by: DrPizza

I think the problem is deeper than that. Does the OP realize that AC (the 110-120volt outlet you're plugging into) doesn't have a positive and a negative side?

Actually, in the world of home wiring, there is a negative and a positive side (kinda...).

In the US, most houses have 240VAC coming into them on three wires. There are two "Hot" wires, and a neutral. The distribution is done such that there is 240VAC between the two hot wires, but only 120VAC between either hot wire and the Neutral. Roughly half of the lighting and outlets in your house get connected between one hot and neutral, the other half between the second hot and neutral.

At your house, the Neutral wire is connected to a ground (literally, THE ground), to all of the "ground" conductors on all of the outlets in your house, and to all of the "Neutral" conductors on all of the outlets in your house. The theory is that, inside your house, NONE of the ground wires carry any current at all except when something has gone wrong. ALL of the current supplied by the Hot wire is returned by the neutral wire.

So you can, in theory, grab two (Neutral and Ground) of the three wires feeding an outlet, and have no adverse effects. Alternatively, two (Neutral and Ground) of the three wires in your Blender, TV, drill, etc., could short to the metal case and you'd never notice. I'd call these "positive outcome" wires.

It's that third wire that causes the problem. I'd call it the "negative outcome" wire. If you grab it, the results depend on how well you are grounded. If you're in new, rubber-soled sneakers and aren't touching anything else, you probably wouldn't even notice. If you had one hand on the kitchen faucet and grabbed the wire (or something where the hot wire is shorted to the case) with your other hand, you'd probably die. If you were in bare feet on a concrete floor, you'd probably get zapped pretty good.

So, even though it's AC, it's AC referenced to Ground, and that can make all of the difference.

/frank

Great post. But I wanted to clarify: In case you haven't noticed, when it comes to wiring in AC, there is no positive or negative. Only a hot wire(black, red) or cold (white, bare). And in many (older) setups the white and ground wires meet back up in the circuit breaker then go to a steak in the ground.

 

[uOpt]

Just to sneak this in:

a few my my friends bought houses. For some of them I was checking up after their electricians as good as I could.

A $5 tester from radioshack revealed that about 50% of the new outlets were not connected right with regards to polarity and would make this mechanism fail.

Totally unrelated a friend of mine a thousand miles away got a shot when getting a piece of bread out of the toaster. Obviously the same thing.

 

[FrankSchwab]

Sometimes, Typos can be pretty funny....

Originally posted by: MartinCracauer

Totally unrelated a friend of mine a thousand miles away got a shot when getting a piece of bread out of the toaster.

I'm trying to figure out whether this means the friend got shot (perhaps the toaster was in a married lady's house, and he wasn't her husband), or whether the friend really got a shot (a hair of the dog after a night's overindulgence, I guess).

Originally posted by: Runamile
And in many (older) setups the white and ground wires meet back up in the circuit breaker then go to a steak in the ground.

Adding the ever-popular "ground steak" to my list of favorite steaks - tube, New York Strip, and Filet Mignon. It's tough, generally gritty, and really hard to barbecue well....

/frank

p.s. Actually, I believe that all Modern (not older) wiring has the white and ground wires connected together in the ciruit breaker box, which are then connected to a ground (stake, concrete-encased wire, or other approved ground).

 

[Jeff7]

Originally posted by: MartinCracauer
Just to sneak this in:

a few my my friends bought houses. For some of them I was checking up after their electricians as good as I could.

A $5 tester from radioshack revealed that about 50% of the new outlets were not connected right with regards to polarity and would make this mechanism fail.

Totally unrelated a friend of mine a thousand miles away got a shot when getting a piece of bread out of the toaster. Obviously the same thing.

That would mean that the electrician that wired your house was suffering a hangover from the night before, and couldn't see clearly. Or else he was never taught to read enough English to understand the directions underneath the "DANGER" warning label.

Our house had a one reversed outlet like that, and a bunch with open grounds. If you have a computer that's not grounded, and then you stand barefoot on a concrete floor, and touch the case, you might get a mild shock. If the outlet is wired backwards too, you'll probably get something pretty serious.

 

[uOpt]

I meant "got a shock". It wasn't my house but the pattern is pretty clear here around Boston.

 

[Furbag]

Ok as previously stated in home wiring there are 2 hots and one common. The voltages between the hots is 240 and 120 from either one to the common. The ground and the common are tied together at the sevice for the location, but they serve different functions. The ground is for safety, it is supposed to provide an alternate return path for the current if there is a short,(the hot touching something condudtive that you can touch too) or an open in the common wire. Which still leaves the chance for you to compleate the circut. Both bad things.

The 2 hots are 180 degrees out of phase, if they are connected only to "resistive loads" and the loads are balanced ie 10 amps on the "A" phase and 10 amps on the "B" phase the phases will cancel and there will be no current on the common. This worked great in the old days when electricity only ran things like light bulbs, irons, and the occasional fan. Now we have lots of cool "electronics" which work on DC electricity we use a transformer coil to step it down to the voltages we want, some diodes to get it all on the positive or negative side, and some capacitors to smooth it out. The transformer causes nonliner loads in the circut. So now you can have 10 amps on "A" and 10 on "B" and have 20 amps the common, which is why polarity in ac circuts is important. As for the cord with the figure 8 shape they connect to power transformers since the diodes will sort out the polarity the device needs they do not need to be phased. Both blades on the plug the same size and the other end can plug either way too. And it is a specified use type of cord, but not a OEM only cord. You should be able to pick up a replacement pretty easy try radio shack.