I'm sorry, but I have to ask the question.  When I was going to electronics
school they taught us that A/C stands for "Alternating Current".  This means
that the current starts out at zero (theoretically) goes positive to 120V,
back down to zero, and the negative to 120V.  So, regardless of which tine
plugs into which side of the socket, it will be positive on the up cycle,
and negative on the down cycle, in direct opposite proportion to the other
tine.

My question is this:  Why does polarity matter, and why is such a big deal
made out of plugs that supposedly force a particular polarity by allowing it
to be plugged in only one way?  Polarity IS a consideration with D/C power
("Direct Current") because there is a definite positive side and a definite
negative side (whether power flows from positive to negative, or negative to
positive, is a debate between the physicist and the electricians that will
continue for a long time).  The transformer doesn't care which way the plug
is put in because the diode bridge that converts the A/C to D/C is made
specifically to clip all the negative power out regardless of which side
it's flowing from at the time.

Devices that use A/C power only shouldn't care either.  Again, the negative
side is only negative half the time, and same with the positive.  For those
of you who wonder why they don't send D/C over regular power lines, the
answer is simple.  Latency.  It's hard enough to get 120V to your house as
it is (transformer stations are used to reboost the power for areas that are
a long ways from the power plant).  Getting D/C to your door would be nearly
impossible without a large number of repeater stations to maintain power
levels concurrently through the lines.

So, anybody know what I'm missing here to explain the "polarity" thing with
appliance plugs?  This thread has got me real curious.

Kyle Elmblade


From: "Kenneth J. Kovler" <[log in to unmask]>
Sent: Wednesday, August 23, 2000 3:46 AM
Subject: Re: [PCBUILD] A question for our resident EEs.


> Once upon a time on Tue, 22 Aug 2000,at 6:19pm Mark Rode wrote about:...
>
> > 120 >> 12 volt transformers for PC devices such as hubs, speakers and
> > modems never have polarized plugs.
> > However I have always made sure that the transformers are plugged in at
the
> > correct direction relative to the printing on the device so as to put
the
> > hot lead on the right. This is not always easy to arrange without
loosing
> > some outlets on a surge protector or UPS.
> >
> > Because the transformers never have polarized plugs I am not sure that
this
> > is in fact necessary ?
> > Is this a good idea or does it matter ?
> >
> > Thanks
> > Mark Rode
>
> It will not make any difference since you are connecting a transformer
> which will provide isolation between the device being powered and the 120
> volt circuit. Otherwise there would be a polarized plug and ground on
> these transformers.
>
>   ken
>
>           Kenneth J. Kovler

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