Re: Mains conditioners and power cables


"Roderick Stewart" <rjfs@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:VA.00000b86.00ceeebb@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> In article <8EYPe.314$n4.136@xxxxxxxxxxxxxxxxxxxx>, Arfa Daily wrote:
>> I agree with Don entirely here. His motorway analogy is exactly the same
>> as
>> my oval racetrack one except that his draws a better physical picture of
>> the
>> two wires side by side. You can couple the two sides of the motorway
>> together by exit roundabouts - you don't even have to go into town. You
>> could drive around this ' circuit ' all day, without ever engaging
>> reverse
>> gear, so you would have to agree that all cars are going in the same
>> circuit
>> direction. However, you would also have to agree that on the motorway
>> section, the cars are going in OPPOSITE directions ( some NORTH towards
>> Birmingham, and some SOUTH towards London ). They don't collide with each
>> other because they're on seperate carriageways ( wires ).
>>
>> I'm having great difficulty figuring why this seems to be such an alien
>> concept.
>
> It's not an alien concept, just an example of how the definition of the
> "direction" of something (cars on a road or current in a circuit) changes
> depending on what reference you use. Different references may be suitable
> for
> different purposes, but it is essential to state which one you are using.
> You
> may sit beside a closed circuit racetrack and say that some of the cars
> are
> moving left to right and some are moving right to left *relative to you*
> and
> consequently declare that some of the cars are moving in opposite
> directions,
> but if you are on the racetrack in one of the cars, you would say that all
> the
> cars are moving in the same direction around the track, and would be very
> alarmed if somebody told you that some of the cars were moving in the
> opposite
> direction..
>
> The same applies to currents in wires. You might find it convenient to
> regard
> the currents in a pair of long wires that form a balanced circuit as
> running in
> "opposite" directions, because you are considering the directions as benig
> relative to you, but when it comes to analysing the balanced circuit, it
> raises
> a problem.
>
> Consider a balanced audio circuit fed from an output transformer in the
> sending
> amplifier and loaded by the primary of an input transformer in the
> receiving
> amplifier. This circuit will work without an earth. Indeed it will work if
> the
> receiving amplifier is running from batteries and standing on a *** of
> glass,
> completely electrically isolated from everything except the two signal
> wires.
> What does it mean then to talk of individual signal voltages or currents
> in the
> two wires? Voltages relative to what? If you want to regard it as two
> current
> loops, one in each wire, then how do they find their way back if there is
> no
> earth connection? In this situation, regarding the two signal wires and
> the two
> transformer windings as a single closed circuit loop is the only way that
> makes
> sense, and the current *must* be flowing in the same direction all the way
> round it as it cannot oppose itself. The first thing you'll ever have been
> taught about an electric current is that in a simple closed loop it will
> have
> the same value everywhere in the loop. Kirchoff's laws become nonsense if
> you
> keep changing the sign that represents current direction.
>
> Make a circuit with a battery and some bulbs and see if it makes any
> difference
> where you break the circuit to connect an ammeter. Connect the meter with
> the
> same terminal towards the battery's positive terminal in the direction the
> wire
> travels, and see if you ever get an opposite polarity reading.
>
> Rod.
>

OK I give in. You're clearly never going to see it the same as the rest of
us.

I don't dispute anything you say about reference points, but you must
understand that the reference point for the interfering signal is GROUND. It
is a voltage induced in both wires, equally in amplitude and phase, as a
result of an electric field referenced to GROUND.

In the balanced circuit comprising legs A and B the reference is A or
indeed, B. There is a voltage on B with reference to A OR a voltage on A
with reference to B . This is a DIFFERENTIAL voltage and is what your
amplifier input, be it an op amp or a transistor long-tailed pair or a
transformer or a banana or a dustbin, responds to. The interference voltage
existing EQUALLY on A AND B does not represent a DIFFERENTIAL voltage,
and the banana or whatever will take no notice of it as if it DOESN'T EXIST.

Stop thinking about current flows and single wires and return paths and
everything else that everyone is muddying the waters with. It's a really
simple concept if you just free up your mind and let it take it in.

Somebody help me out here ? Am I deluding myself thinking that it's an easy
concept ?

Arfa


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