In a message dated 08/24/2000 2:16:52 PM Eastern Daylight Time,
[log in to unmask] writes:

<< 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.>>

    This is only partly correct. One side of a 120 volt AC outlet does indeed
stay at "ground" potential (the "neutral" side...the one with the wider
blade). The power coming into the house from the pole outside is from a 220
volt "center tapped" winding in the transformer on the pole. If you look in
the breaker box in your house, you will see 2 rows of breakers (left side,
and right side), each side goes from the center tap wire (the "neutral) to
one "side" of the 220 (it is 110 from the center tap (neutral) to each side
of the 220).
 You can think of it as 3 wires A, B and C, with B in the middle being the
center tap (neutral). So from A to B you get 110 volts (for the left hand row
of breakers), from B to C you also get 110 volts (for the right hand row of
breakers). From A to C you get 220 volts (this is what gets used for any 220
volt outlets, like a dryer outlet, for instance).

                  (Neutral)
  A                   B                 C
   |                    |                   |
   |<--110volts-->|<--110volts-->|
   |                    |                   |
   |<------------220 volts----------->|

  It can be easier to "see" if you assume wire B stays at ground potential
(it does). Wire A goes up and down in voltage (AC volts) with respect to B.
This gives the 110 volts from A to B (for the left hand row of breakers).
  Wire C also goes up and down in voltage with respect to B (to give it 110
volts, for the right hand row of breakers)).
  But when wire A is going UP in voltage, wire C is going DOWN (so when A is
at +110 volts, C is at -110 volts, and vice-versa). This is why the two 110
voltages add to each other to give 220 volts from wire A to wire C.

 >>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.>>

  For the "power cube" transformer being talked about here, it does not make
any difference at all, hence the lack of polarizing blades on the power plug.
  But for a larger appliance, say a TV, it makes a BIG difference. A TV has a
chassis, normally metal, and this is connected to one side of the incoming
power. If the power plug is correctly inserted, then the chassis is connected
to the neutral (ground) side of the power line, and is at ground potential
(no shock hazard from the chassis, or any controls attached to it).
  If the power plug is inserted the wrong way 'round, the chassis (and any
attached controls) are connected to the line (hot) side of the AC outlet with
the full 120 volts on it. You don't have to be a rocket scientist to see the
hazards now present....

<< 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.>>

   There is no latency involved, the reason AC power is used is it allows the
voltage to be changed with transformers (you can't do that with DC).
  The reason this is important is as follows: any wire has a limit on the
amount of current it can carry (due to the resistance of the wire, thicker
wire has less resistance and can carry more current).
  With a given voltage, this means it can only carry a certain amount of
power (the voltage it is running at, times the current it is limited to). As
an example, if the wire can carry 100 amps of current, and was working at 120
volts (home voltage), it would be limited to 12,000 watts of power capacity
(100 amps x 120 volts = 12,000 watts)
  For DC power, if we wanted 120 volts at the house, then the power station
would have to put out 120 volts (actually more due to voltage drops in the
wiring between it and the house, but for simplicity's sake let's ignore
these).
  OK, now lets take the same wire in an AC system (100 amp limit). The power
station generates 250,000 volts (actually this is on the low side, many high
tension lines run well above this voltage). Now the SAME wire that could only
carry
12 kilowatts with DC can carry 25 MEGAwatts using AC (250,000 volts x 100
amps = 25,000,000 watts, ignoring AC power factor for simplicity's sake).
That's over two thousand times as much power. Instead of carrying enough
power for a house or two (on DC), it can now supply a small town.
  You don't want 250,000 volts at your house, but it's a simple matter to
step this voltage down with transformers, first to 12-50 thousand volts with
a transformer at a substation, then from that voltage to the 110-220 volts
the house uses at the transformer on the pole outside your house.

<< 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 >>


HTH,
Peter Hogan
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