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Re: Water heater eating X-10 signal



"Dan Lanciani" <ddl@danlan.*com> wrote in message
news:1338640@xxxxxxxxxxxxxxxxxxxxxx
> In article <9iqVh.322422$5j1.206155@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
> JeffVolp@xxxxxxx (Jeff Volp) writes:
>
> | I think the better approach is to provide best performance
> | for normal 120V loads, which are probably over 99% of X10 devices.  I
> don't
> | like the possiblity of a 240V resistive load possbily causing havoc with
> | signal levels.
>
> It is not at all clear that this is common occurrence, though.  If (as
> I suspect) it turns out that all commonly available repeaters drive the
> legs out of phase we would probably have to do a lot more investigation
> to determine why it (may have) happened in this case.  Also, keep in mind
> that driving the legs out of phase does not make 240V loads a special
> problem wrt reducing the signal; they will have approximately the same
> effect as two 120V loads of similar power.  If a 240V load causes a
> problem it may simply mean that the signal levels were already on the
> edge.  Driving the legs in phase mostly takes the 240V loads out of the
> picture, but not necessarily without a cost:

Many houses have 240V high-current resistive heating loads (ranges and
dryers).  The inductance of those circuits would be relatively low, so the
effect may be more significant than a variety of 120V loads that sum up to
the same current.

You are correct that 120KHz distribution over household electrical wiring is
a complex problem.  When I was working on the XTB-II output stage I built a
rudimentary simulation using 8 circuits with line inductance, a variety of
loads, and a few signal suckers.  It mirrors the decay of signal ampltude
that is normally seen as the distance from the panel increases.  Running a
frequency sweep shows peaks and nulls all over the place, along swith some
significant phase shift on some of the circuits.

> Consider that driving the legs in phase means that every 120V load will
> be coupling the carrier in-phase to the neutral, reducing the apparent
> leg-to-neutral signal available to all 120V modules.  In a distribution
> system where the 120V loads are reasonably well distributed between the
> legs the loss of the ability to cancel the carrier current in the neutral
> may dominate any advantage gained by taking 240V loads out of the picture.
> It would take a lot more analysis to convince me that driving the legs
> in-phase is in fact (at least on average) the way to provide the best
> performance for 120V loads.

At first I thought you had a point here, but remember that each neutral goes
back to the distribution panel common bus.  It doesn't make any difference
on each of the 120V circuits what the phase is because there is no summing
together of the return signals on a common netural.  Of course, that assumes
the transmitter is relatively close to the distrbution panel so the summed
return signals in its own neutral can be ignored.

> | If the installaton does include a 240V load that has a
> | problem with signal levels, it could be addressed by placing a .1uF
> | capacitor to neutral on one of the hot leads to imbalance the signal
> levels.
>
> You'd probably have to put the capacitor at the panel since pure 240V
> loads don't normally have a neutral available.  In addition to the code
> issues you wouldn't have the advantage of the series impedance of the
> line feeding the device to help drop the level, so you would be trying
> to unbalance the whole system.

That is true if there is no netural at the 240V X10 load.  Receptacles for
all our 240V appliances ARE wired with both neutral and ground.  I had to
buy a new dryer cord for our old Maytag dryer.  I was told that is a NEC
requirement now.

Jeff




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