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Travel lighter with Apple's power adapter
Authored by: donaciano on Jul 06, '06 02:44:55PM
That's a common misconception... there IS polarity in AC since one wire runs to the power companies generator and the other wire runs to the ground. Sure a power adaptor will convert the AC to DC and there shouldn't be a problem, but some are badly designed and really can fry it plugged in the wrong way. Here's an example... Zapped Laptop

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Travel lighter with Apple's power adapter
Authored by: dave_williams on Jul 06, '06 03:10:08PM

>That's a common misconception... there IS polarity in AC since one wire runs >tothe power companies generator and the other wire runs to the ground. Sure >a power adaptor will convert the AC to DC and there shouldn't be a problem, >but some are badly designed and really can fry it plugged in the wrong way. >Here's an example... Zapped Laptop

The guy zapped his laptop because the polarity of the DC voltage he put into it was the wrong way, not becaise of AC polarity. It's poorly explained- he talks about the polarity on the AC adapter, but it's obvious he's talking about the DC output voltage.

I'm not one to belittle the importance of being careful with mains AC power (especially the 240v variety we have here in the UK), but reversing the neutral and live connections doesn't make any difference on wholly insulated items such as power bricks. Check how many power bricks you have with figure of eight plug ( 8 ) input leads, which can be inserted either way round.



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Travel lighter with Apple's power adapter
Authored by: Bwana on Jul 06, '06 03:12:20PM

Plug direction won't make a difference - if you look at the European two prong plugs they can be plugged in either way - you'd be hard pressed to make sure it went in one way or the other.

The power that fried the guys laptop was DC.



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Travel lighter with Apple's power adapter
Authored by: Superboy on Jul 07, '06 12:17:46AM

Do you mean earth or neutral? If you mean earth, then that is incorrect. The blue/black/neutral wire does not lead to earth. The neutral wire leads back to the alternator in the power station, which produces AC right from the start. it doesn't matter what way you plug the thing in, as like I said, the polarity reverses every 50th or 60th of a second anyway. They use AC because AC is needed to make transformers work properly. They need a reverse in their magnetic field in one side of the transformer to induce a current in the other. The reverse in current reverses the magnetic field.



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Travel lighter with Apple's power adapter
Authored by: greed on Jul 08, '06 10:29:18AM

On North American AC wiring, the neutral wire leads to the centre-tap on the step-down transformer that provides 240V (nominal) house current. From the centre-tap to either hot wire gives you 120V, 'cause it's right in the centre. (The centre-tap is also grounded, but only at the transformer. Current flowing on the neutral return wire will produce a voltage compared to ground in your house wiring. The exact voltage depends on the current and wire gauge.)

The ONLY risk you run by reversing hot and neutral is having case components and cable shields float "hot". None of the electronic circuitry will be affected, UNLESS you connect two such devices together, in which case a current will flow on the cable shield connecting the two together.

This is a sign of an incredibly bad design, as it is possible to have hot-neutral reverse in a mis-wired receptecle, power-bar, extension cord, and so on. Relying on neutral to be the same as ground for safety applications is BAD. Either isolate mains from the case by a transformer, or don't connect the chassis to any mains wire at all. Use true ground (3rd pin) for chassis ground if needed. Heck, neutral is almost never true ground and you can get ground-loop currents on ungrounded devices that try and use neutral for shield or chassis ground.

You cannot identify a hot-neutral reverse without a ground reference. There is no way of having a fuse blow, or any sort of wiring fault indication, in a two-wire device. (Incidentally, if you have a "wiring fault" indicator on your power bar, that's what it is indicating--there is current flow from neutral to ground. A "wiring OK" indicator means that there is current from hot to ground.)

Lamps and devices like that are polarized so that the "hot" wire is always the little tiny one at the bottom of the lamp-socket, so that if you touch the lamp screw-threads while changing the bulb, you aren't going to get the full 120V to any ground you might be touching, just the 5-10V that tends to be the local difference between neutral and true ground (induced by current in the neutral cable in your house).

And yes, I have received electrical shocks from equipment with poor choices of chassis ground--and it isn't always because it's plugged into a mis-wired outlet. I can get a "tingle" on bare skin (it's summer, I'm wearing shorts) from the screws in the bottom of my iBook and the FireWire cable shield from my old Maxtor 80GB external drive, because the two devices use a different way to get chassis ground.



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