Re: Powering an In-wall Amp

["Robert Green" <ROBERT_GREEN1963@xxxxxxxxx>]

"Dan Lanciani" <ddl@danlan.*com> wrote in message

> | Might work, might not.  The voltage may drop too much after a long cable
run
> | to be useful (which is why AC and not DC is running throughout most of
the
> | world's homes - AC voltage doesn't drop the way DC does with long wire
> | runs).
>
> To a first approximation (good for anything you might be doing in a home
> with 50/60Hz AC and DC power distribution) AC voltage "drops" exactly the
> way DC does with long wire runs.  For power transmission on the level of
> grid interconnects the losses with DC are sufficiently _less_ than with
> AC that it becomes economical to use DC even after you factor in the cost
> and losses of conversion equipment on each end.

I thought the payback came from HVDC having a lower overall voltage compared
to AC's peak-to-peak and that allowed using existing transmission cables
designed for AC to transmit more power without overloading the wire
insulation and standoffs.

http://en.wikipedia.org/wiki/HVDC

says:  "Above a certain break-even distance (about 50 km for submarine
cables, and perhaps 600-800 km for overhead cables), the lower cost of the
HVDC electrical conductors outweighs the cost of the electronics."

HVDC has a lot of other important benefits that will insure its commercial
success.  It can tie two unsynchronized AC systems together and it's better
at powering long cable runs like undersea cables where the line capacitance
becomes a serious issue for AC power transmission.

I should have worded my comment about voltage dropoff differently.  AC has
been the traditional method of power distribution because when
Tesla/Westinghouse and Edison were fighting over AC v. DC for the national
power grid, there were no DC voltage convertors. AC at that time could be
easily transformed to higher voltages where the voltage drop is not as
significant.  Use of a higher voltage leads to more efficient transmission
of power which is one reason why large appliances are powered by 220VAC. The
same amount of power can be transmitted with a lower current by increasing
the voltage.  Edison power stations had to be local - the DC systems
couldn't power anything much beyond a mile from the source.  Westinghouse's
AC system could send HVAC for hundreds of miles and transform it down to a
safe voltage at local substations.

But this all has little to do with creating a home-brew 12VDC distribution
system.  My limited experience with running long DC wires is that the
voltage drop-off for 12VDC is greater than 110VAC simply because the voltage
is lower.  Some of the DC powered cameras I use come with variable voltage
PS's that can be spun up to about 18VDC.  By the time it reaches the camera,
it's back down to 12VDC.  Without that voltage boost the attenuation is so
great that the cameras won't operate reliably.  I suppose I could run 12/2
or heavier cable and cut down on the voltage drop, but that wouldn't be
practical.

Perhaps the EE's out there can tell us what the voltage drop would be for
both 110VAC and 12VDC over a 250' length of 12/2 wire.

All the above is of little consequence since as you point out, few wall wart
powered devices can be counted on to have no interaction with other devices
on the same power bus.  I suspect that electronics designed for auto or RV
use can be attached to a 12VDC bus without serious interaction because
that's how they are connected to the car/RV battery, but I haven't checked
to make sure.

--
Bobby G.