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'data over DC power lines'
1999\06\03@131437 by Michael Shiloh

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I've toyed with this idea and I don't know if it's practical.
Neither RF nor power transmission is my area of expertise, so
forgive me if I am asking stupid questions.

Suppose I have a situation where I want to send power and control
signals to a device. Suppose too that I want to do this all with
2 wires (power and return). Is it at all feasible to send the data
on top of the power line, and have it separated at the destination?

Kinda like X10 but this is DC.

Any other ideas along these lines?

Michael

1999\06\03@131643 by Harrison Cooper

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Modulate the DC maybe....FSK like ?

-----Original Message-----
From: Michael Shiloh [spam_OUTmichaelTakeThisOuTspamMAULER.SRL.ORG]
Sent: Thursday, June 03, 1999 11:23 AM
To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
Subject: data over DC power lines


I've toyed with this idea and I don't know if it's practical.
Neither RF nor power transmission is my area of expertise, so
forgive me if I am asking stupid questions.

Suppose I have a situation where I want to send power and control
signals to a device. Suppose too that I want to do this all with
2 wires (power and return). Is it at all feasible to send the data
on top of the power line, and have it separated at the destination?

Kinda like X10 but this is DC.

Any other ideas along these lines?

Michael

1999\06\03@141339 by Scott Dattalo

face
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On Thu, 3 Jun 1999, Michael Shiloh wrote:

> I've toyed with this idea and I don't know if it's practical.
> Neither RF nor power transmission is my area of expertise, so
> forgive me if I am asking stupid questions.
>
> Suppose I have a situation where I want to send power and control
> signals to a device. Suppose too that I want to do this all with
> 2 wires (power and return). Is it at all feasible to send the data
> on top of the power line, and have it separated at the destination?
>
> Kinda like X10 but this is DC.
>
> Any other ideas along these lines?

I haven't tried this, but it should work... You could have a 'high pass'
filter in parallel with a 'low pass' filter on both the transmit and
recieve sides of your 2-wire interface. The dc would flow through the low
pass filter while the communication signal, which is ac, would pass
through the high pass filter.

The filters do not need to be too sophisticated. The high pass filter
could be just a capacitor while the low pass filter could be just an
inductor.  The inductor needs to be chosen such that a) it doesn't
saturate when the full DC current pass through it b) the inductance is
large enough to keep the AC signal from being shorted. The capacitor is
less critical - the bigger the better (though I'd personally try to use a
ceramic cap [e.g. 0.1uF] and just look for the rising/falling edges
[assuming your transmitter is from a pic I/O pin]).

1999\06\03@151339 by Barry King

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I have seen several variants of this idea over the years, one current
one is Dallas Semi's "One-Wire" (actually, two wires, counting
ground) system.  They make a series of chips which are intended to be
daisy chained on a twisted pair, and powered by the head end
controller.

They make a bunch of security keys using the same tech called
"ibutton"s.

see http://www.ibutton.com for the one-wire stuff, it has a link back to
Dallas's main web pages.

------------
Barry King, KA1NLH
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Hinesburg, Vermont, USA
barryspamKILLspamnrgsystems.com
"The witty saying has been deleted due to limited EPROM space"

1999\06\03@153039 by William K. Borsum

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At 11:16 AM 6/3/99 -0600, you wrote:
>Modulate the DC maybe....FSK like ?
>
>Suppose I have a situation where I want to send power and control
>signals to a device. Suppose too that I want to do this all with
>2 wires (power and return). Is it at all feasible to send the data
>on top of the power line, and have it separated at the destination?

There are a number of companies doing this on top of 4-20 mA control and
signal loops.  There is even a standard to cover the protocol--uses
standard FSK signals on a high frequency carrier.  Simple capacitive
coupling at either end, and the frequency is high enough to not interfere
with the "DC" level process signals.

Check out the magazine "control engineering"--I think they have a web page
too.  The name "HART" comes to mind for the protocol, but I wouldn't swear
to it.


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1999\06\03@174212 by Jamil J. Weatherbee

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Kind of like a telephone??


On Thu, 3 Jun 1999, Michael Shiloh wrote:

{Quote hidden}

1999\06\03@175429 by tim

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what type of signals do you mean for a control?.......on/off/data
transmission?
please explain to e-mail........tim
{Original Message removed}

1999\06\03@180324 by Michael Shiloh

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Low data rate. An example would be a series of nodes, each one with a stepper
motor and a PIC based controller. Signal tells which stepper motor should
move by how much.

This sounds more and more like a network or bus. A reader suggested the CAn
bus. I really should look into that.


{Quote hidden}

1999\06\03@180731 by paulb

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Jamil J. Weatherbee wrote:

> Kind of like a telephone??

 Kind of *exactly* like a telephone!
--
 Cheers,
       Paul B.

1999\06\03@184310 by Bernhard Kraft

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I did such a PIC project once. It was a "key" for opening my garage door
;)

The key, as a tiny aluminium pipe with about 12mm
diameter and it was about 6cm long. Inside this pipe was a PIC 12C508 and
a little electronic stuff.
When I inserted the key into a slot next to the garage door a
communication started and the key sent out a 10byte code.
Before I explain how it worked I will draw the circuit of the Key :

        ---------------------------------
        |                               |
        |     D1                        |
+5V DC   |    |\ |                       |       -------------------------
---------O----| \|------O------------------------| Vdd                   |
        |    |/ |      |                |       |                       |
        |              |                --------| Inpin                 |
        |              |                        |                       |
         \  |          |     |-------|          |                       |
    T1     \|----------|-----|       |----------| Outpin                |
           /|          |     |-------|          |                       |
        |/_ |          |                        |                       |
        |           ------- +  C1               |                       |
        |           ======= -                   |                       |
        |              |                        |                       |
---------O--------------O------------------------| Vss                   |
GND                                              |                       |
                                                -------------------------

The capacitor C1 has to be big(a few yF) - I took a Tantal capacitor.
The power supply has to be an high Impedance supply (a few hundred Ohms) -
my supply had an impedance of 300 Ohm.
If the PIC wants to send data he just gives a short "HIGH" on the output
pin which switches the transistor (I took a off-the-shelf NPN transitor -
BC548) and shortens the power supply. The PIC will remain working during
the SHORT time where the power supply is shortened because he will be
supplied by the capacitor. So the longer the time when the PIC shortens
its "own" supply should be, so bigger must the capacitance of the
capacitor be.
The other side - which communicates with the PIC - is also able to pull
down the Power line for a short time. So the PIC could also receive data.

The only problem of this way of communication is you will have to play
around how long and how often you could pull down the power line (how fast
the communication could be) - My application transfered the 10 byte in a
few milliseconds ... ouhh .. I think it was 0,012 seconds ...
Its not fast but it's easy and you wont need a lot of electronic stuff for
such a way of communication.

Sorry for my bad english,
===================================================================
Kraft Bernhard aka. Krufti              /"\
ICQ# 3672982                            \ /  ASCII RIBBON CAMPAIGN
                                        X   AGAINST HTML MAIL
                                       / \
===================================================================

1999\06\03@185141 by Jamil J. Weatherbee

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Really lame idea:

Use an RS485 interface at the sending end, I think they can supply ~60ma
to the load.  Interface is differential, you have 2 wires A & B lets say.
Take A & B run them through a rectifier and big capacitor, and you have
relatively smooth power (might need to regulate).  Then use an RS485
receiver on the receiving end to get async serial signals at whatever
rate.  Best of all it would seem nearly impossible to violate the
common-mode grounding specs since the signal lines are your ground.

This is kind of unidirectional though unless someone has a bright idea
about that.

1999\06\03@185814 by Michael Shiloh

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>Use an RS485 interface at the sending end, I think they can supply ~60ma

I suppose I should add that the power I'm thinking of transmitting is
in the 40 volt 20 amp range. Data has to ride on top of this.

I suppose I could use AC instead of DC and have a rectifier/filter
at each node. Then use the X10 method. Any advantage to this?
The length of this system is not so great so the advantage of AC
over DC in terms of the costs of a thicker cable is not an issue.

Michael

1999\06\03@203228 by Donald Riedinger

picon face
Data over DC power lines?  How about parasite power derived from the
communication line?  It's easier to half wave rectify and filter data to
power a PIC than to drive data into a battery.  Maybe it's just
perspective or nomenclature.  Have you ever seen
http://www.dallassemiconductor.com and their 1-wire (tm) chips? Cool.  And,
"Remember: credit card orders are limited to 10 pieces or less except
for iButton products".  Can you believe that from a major chip manf.?

Don

1999\06\03@214132 by Richard Prosser

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Slightly OT.
       I remember from a long time ago, seeing an way of getting low power
from thin air. You need a circuit resonant at the frequency of your local AM
transmitter, an antenna and earth and a rectifier tapped into the coil.
Instant free energy. With the low power requirements of a PIC it just might
be possible!(When I tried it I found I needed a longish wire antenna - but I
also needed several milliamps)

Anyone else tried this?

Richard

> {Original Message removed}

1999\06\03@235801 by Graeme Smith

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Well...

if you send the data via an oscillating system. And send the power D.C.
you can split with a center-tapped transformer to separate the A.C. and
D.C. components.

You simply put the power to the center tap, and send the oscillating
signal via the outer end of the transformer. The voltage of the
Oscillating element will be added to the D.C. voltage, which will
be sent via the two wire system (you need a ground) at the other end, you
use another transformer to separate the two signals, and regulate the d.c.
that comes off the centertap.

Your mileage may vary ( Haven't tried it myself yet).



GRAEME SMITH                         email: EraseMEgrysmithspam_OUTspamTakeThisOuTfreenet.edmonton.ab.ca
YMCA Edmonton

Address has changed with little warning!
(I moved across the hall! :) )

Email will remain constant... at least for now.

1999\06\04@003050 by Donald Riedinger

picon face
In response to:

Slightly OT.
       I remember from a long time ago, seeing an way of getting low
power
from thin air. You need a circuit resonant at the frequency of your
local AM
transmitter, an antenna and earth and a rectifier tapped into the coil.
Instant free energy. With the low power requirements of a PIC it just
might
be possible!(When I tried it I found I needed a longish wire antenna -
but I
also needed several milliamps)

Anyone else tried this?

Richard

*******************************

I'm hip.  Only I think it was a really long time ago.  It was called the
Arc of the Covenant.  Gold antenna connected to a large capacitor.  The
movie didn't do it justice.  The blue arcs were OK but Harrison Ford in
the snake pit, ick.  Or maybe it was a crystal radio driving headphones.

Anyway.  Dallas Semiconductor communicates between devices such as
temperature sensors, RTC's,  4kEEEproms with 2 wires.  Ground and data.
Each chip on the 1-wire bus (not counting ground) get gets it's power by
half wave rectifying the data and storing energy when the data goes
high.
Check their weather station thing.

Serial and parallel infrared communication devices that don't use an
external power supply work the same way.  As do other "self powered"
RS-232 peripherals like serial mice.

If one PIC is powered (and the bus master) and it sends a constant
carrier signal (like a 50khz switching power supply square wave) but
periodically goes high impedance so the other PIC's can request service
but not so long that the capacitor runs down on the slave PIC, it
works.  Asynchronous bi-directional communication on 2 wires one of
which is ground with one or more slave devices operating on parasitic
power.  The Dallas parts have Time Slots and ID's.

The Dallas DS1820 is a sophisticated temperature sensor that looks like
a small plastic transistor.  The three leads are GND, VDD, and I/O.  In
parasitic power mode, VDD is connected to a capacitor which is charged
through diodes by the 1-wire bus signal.

It's not easy.  But on the other hand, if the 2 bus wires are connected
to a battery to power all the PIC's directly,  you need to source or
sink current to change the voltage measured between the 2 wires for
communication.  That means running down or charging the battery a
little. Or something else has to give like internal battery resistance
or a series resistor at the battery.  You wouldn't want to have much
swing in the data signal.  Which brings on noise problems
(distinguishing signal from noise).  You would also need to sense small
voltage difference maybe with external comparator chips.

Let me know if it works. I might need to do a 2-wire soon.

Thanks,
Don

1999\06\04@015836 by Nigel Goodwin

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In message <199906031722.KAA10068spamspam_OUTmauler.srl.org>, Michael Shiloh
<@spam@michaelKILLspamspamMAULER.SRL.ORG> writes
>I've toyed with this idea and I don't know if it's practical.
>Neither RF nor power transmission is my area of expertise, so
>forgive me if I am asking stupid questions.
>
>Suppose I have a situation where I want to send power and control
>signals to a device. Suppose too that I want to do this all with
>2 wires (power and return). Is it at all feasible to send the data
>on top of the power line, and have it separated at the destination?

It's quite easy to do, it's been done with aerial amplifiers for years,
the power unit sends DC up the coax, and the amplifier sends RF back
down. You just split it at either end with a simple filter, a capacitor
feeding the AC signal (which blocks the DC), and an inductor feeding the
DC (which blocks the AC). Obviously the values required depend on the
frequencies used, and the amount of DC power you are drawing through the
cable.
--

Nigel.

       /--------------------------------------------------------------\
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       | Lower Pilsley   | Web Page : http://www.lpilsley.demon.co.uk |
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1999\06\04@015841 by Vic Lopez

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Dear Bern,
    Could you maybe send me an attachment of the circuit you drew?? It was
hard for me to tell components apart, and I'd like to see it more clearly.
Thanks!! Cheers. Vic
{Original Message removed}

1999\06\04@021950 by William Chops Westfield

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   I remember from a long time ago, seeing an way of getting low power
   from thin air. You need a circuit resonant at the frequency of your
   local AM transmitter, an antenna and earth and a rectifier tapped into
   the coil.

Yep.  This is exactly how a "crystal radio" (no batteries, no power
supply) works.  I have/had somewhere a book that used two tuners - one
tuned to a strong local station that acted as a power supply for a
more conventional amplified tuner...

BillW

1999\06\04@062755 by Marco DI LEO

picon face
Michael Shiloh wrote:
> Low data rate. An example would be a series of nodes, each one
> with a stepper motor and a PIC based controller. Signal tells which
> stepper motor should move by how much.
>
> This sounds more and more like a network or bus. A reader suggested
> the CAn bus. I really should look into that.

Take also a look at the Tiny Embedded Network. Is an open protocol that
should be easily implemented in microcontrollers with or without an
UART. It is based on a CSMA/CD bus and offers an interchangeable
physical layer. Currently backplane and twisted pairs are defined but
there are people working on a power+signal bus using coax or TP.

You can find more information at http://members.tripod.com/~mdileo/

Ciao
 Marco

1999\06\04@071941 by Nigel Orr

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At 16:07 03/06/99 -0700, you wrote:
>I suppose I should add that the power I'm thinking of transmitting is
>in the 40 volt 20 amp range. Data has to ride on top of this.

How noisy will the power supply be?  If you're driving 800W of load, I
would be surprised if it is a quiet 800W- unless it's a resistive heater,
there might be so much noise generated as to make comms very difficult

>I suppose I could use AC instead of DC and have a rectifier/filter
>at each node. Then use the X10 method. Any advantage to this?

No.

>The length of this system is not so great so the advantage of AC
>over DC in terms of the costs of a thicker cable is not an issue.

Running an extra data pair would be the easy answer (as the distances are
small), but there's probably a good reason why you can't.  Otherwise, you
need to try to keep the data frequency away from the band of noise- either
lower frequency (better, as it avoids the inevitable noise harmonics) or
higher.

Nigel
--
Nigel Orr                  Research Associate   O   ______
       Underwater Acoustics Group,              o / o    \_/(
Dept of Electrical and Electronic Engineering     (_   <   _ (
    University of Newcastle Upon Tyne             \______/ \(

1999\06\04@080133 by paulb

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Michael Shiloh wrote:

> I suppose I should add that the power I'm thinking of transmitting is
> in the 40 volt 20 amp range. Data has to ride on top of this.

 Don't get confused.  Forget using AC power just so you can use X-10,
that's nonsense!  At each end of the power line you use a coupling
transformer, preferably balanced, in *series* with the line.  Let me try
and draw it:
                                .
                                .
                                .
                +-----------------------------------+
                |              Line                 |
                < ||            .                || <
                < ||            .                || <
                < ||            .                || <
                < ||            .                || <
DC IN  o-----+---+ || +--+       .          +---+ || +---+---o DC OUT
            |     || <  |       .          |   < ||     |
           === C  || <  |       .          |   < ||  C ===
            |     || <  +-o     .        o-+   < ||     |
DC IN  o-----+---+ || |   Sig    .       Sig    | || +---+---o DC OUT
                < || +----o     .        o-----+ || <
                < ||            .                || <
                < ||            .                || <
                < ||            .                || <
                |              Line                 |
                +-----------------------------------+
                                .
                                .
 Your signalling may be by modem, or a baseband protocol at whatever
baudrate you dare.  In either case, using the highest practical
frequency/ baudrate favours the lightest coupling transformer and fewest
turns on the windings and is likely to minimize interference from the
equipment supplied.

 We just had a thread on using transformers to isolate the ground on an
RS-485 link.  Virtually all the principles discussed apply to this case.
Read the archives; end of topic.
--
 Cheers,
       Paul B.

1999\06\04@105021 by Bernhard Kraft

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On Thu, 3 Jun 1999, Vic Lopez wrote:

>Dear Bern,
>     Could you maybe send me an attachment of the circuit you drew?? It was
>hard for me to tell components apart, and I'd like to see it more clearly.
>Thanks!! Cheers. Vic
>{Original Message removed}

1999\06\04@130411 by William K. Borsum

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At 12:57 PM 6/4/99 +1200, you wrote:
>Slightly OT.
>        I remember from a long time ago, seeing an way of getting low power
>from thin air. You need a circuit resonant at the frequency of your local AM
>transmitter, an antenna and earth and a rectifier tapped into the coil.
>Instant free energy. With the low power requirements of a PIC it just might
>be possible!(When I tried it I found I needed a longish wire antenna - but I
>also needed several milliamps)
>
>Anyone else tried this?

Wasn't there someone who put large helical coils under the high KVA power
transmission lines in the Nevada dessert and sucked off kilo-watts for a
while??  How about the ubiquitous 60 cycle stuff?

****************************************************************************
********
All legitimate attachments to this email will be clearly identified in the
text.
William K. Borsum, P.E.
OEM Dataloggers and Instrumentation Systems
<RemoveMEborsumTakeThisOuTspamdascor.com> & <http://www.dascor.com>

1999\06\04@140259 by Nigel Goodwin

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In message <spamBeGone199906032307.QAA12481spamBeGonespammauler.srl.org>, Michael Shiloh
<TakeThisOuTmichaelEraseMEspamspam_OUTMAULER.SRL.ORG> writes
>>Use an RS485 interface at the sending end, I think they can supply ~60ma
>
>I suppose I should add that the power I'm thinking of transmitting is
>in the 40 volt 20 amp range. Data has to ride on top of this.

The method I suggested using capacitors and inductors at each would
still work, BUT! would need inductors large enough to pass the full
current with reasonably small voltage drop. By modulating the control
signal on a fairly high frequency the value of the inductors could be
kept low, and could be hand wound using thick wire?.
--

Nigel.

       /--------------------------------------------------------------\
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       | Lower Pilsley   | Web Page : http://www.lpilsley.demon.co.uk |
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       \--------------------------------------------------------------/

1999\06\04@140308 by Nigel Goodwin

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In message <37575546.68457F0BEraseMEspam.....home.com>, Donald Riedinger
<EraseMEdrr9spamHOME.COM> writes
>In response to:
>
>Slightly OT.
>        I remember from a long time ago, seeing an way of getting low
>power
>from thin air. You need a circuit resonant at the frequency of your
>local AM
>transmitter, an antenna and earth and a rectifier tapped into the coil.
>Instant free energy. With the low power requirements of a PIC it just
>might
>be possible!(When I tried it I found I needed a longish wire antenna -
>but I
>also needed several milliamps)

There's always been a story going around about a LARGE! radio
transmitter in Germany many years ago - I can't vouch for it's accuracy,
but it's a nice story :-).

This very large transmitter was built, and one gentleman discovered that
he could build a resonant circuit that used the RF to heat his
greenhouse. Obviously all his neighbours were very impressed with this,
and he built them 'heaters' as well. According to the story, eventually
there were so many greenhouses close to the transmitter that it's
coverage began to be affected, and the authorities acted and ordered
them to be dismantled.

Don't know how true it is, but it's amusing anyway!.
--

Nigel.

       /--------------------------------------------------------------\
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1999\06\04@155508 by Jamil J. Weatherbee

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I've heard a story about a farmer who ran a cable paralell to the power
lines finding he could get all the power he needed that way.  When they
caught him his story was that the power was in the air so it would of just
been wasted anyway :)


On Fri, 4 Jun 1999, William K. Borsum wrote:

{Quote hidden}

1999\06\04@233620 by g.daniel.invent.design

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Jamil J. Weatherbee wrote:
>
> I've heard a story about a farmer who ran a cable paralell to the power
> lines finding he could get all the power he needed that way.  When they
> caught him his story was that the power was in the air so it would of just
> been wasted anyway :)
>

Nicola Tesla wanted to distribute power that way, he was an exceptional
inventor but lacked understanding of basic human nature.

--
Steam engines may be out of fashion, but when you consider that an
internal combustion engine would require recovery of waste heat by
transfer just before top dead centre then fashion becomes rather
redundant, USE STRATIFIED HEAT EXCHANGERS ! and external combustion.

You heard it first from: Graham Daniel, managing director of Electronic
Product Enhancements.
Phone NZ 04 387 4347, Fax NZ 04 3874348, Cellular NZ 021 954 196.

1999\06\05@093912 by Donald Riedinger

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Graham Daniel wrote:

> Nicola Tesla wanted to distribute power that way, he was an
> exceptional inventor but lacked understanding of basic human nature.

---------

While Edison wanted to continue with small (by today's standards) DC
generator plants powered by steam.  One small power plant for each city
block.  No wires over or under the streets.

The guys that finally won out, thought AC was the way to go.  They could
use Xfmr's and step up the voltage to avoid the geometrically
increasing  losses encountered when shoving more and more current
through a wire (I squared * R = P).  They could build bigger and bigger
power plants.

And maybe find "cleaner" and more efficient ways to produce mega power
in the future.  That way they could mail a bunch of power bills out once
of month and get a bunch of money back.  They could even tie all the
power plants together in a grid so there would never be any power
problems anywhere.  Computer communication seems to be a better
application of the principle.

The path was chosen over a 100 years ago.  It will be awhile before they
abandon all the power lines and the rest of the mega power plants.
There are some folks like http://www.homepower.com and http://www.nrel.gov who are
trying to figure out how to do it right.  It's a big job. They might not
get it done before it's too late.  They need help.

Don

1999\06\05@130627 by Ernie Murphy

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I once saw a scheme for this work and work well. It provided
bi-dirrectional signaling. Even better, it was done over an internal AC
line and also provided 1,500 volts of isolation from both ends by using a
1:1 transformer built with 4 turns each tap on a ferrite bead.

What the circuit did was provide a 10 volt pulse wave (in the megahertz
range I dimly remember... to keep the transformer that small) to provide
power. This was caught by a diode and cap. The signals were all 5 volt
pulses so they could 'slide in" under the rectfier voltage and never
forward conduct it. Signals were interped by sending an ID pulse coded
train (this design had a driver side and 4 listeners), which send either
data or a command to an individual listener. An addressed listener then
could send back a pulse train before the next power pulse occured.



At 10:22 AM 6/3/99 -0700, you wrote:
{Quote hidden}

1999\06\05@130827 by Ernie Murphy

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I remember a story I heard about a farmer doing something like this, cept
he ran some cable for a few miles burried just beneath the surface for a
mile or two along the power lines cutting his property. He ran his whole
farm off it.

Too bad he was discovered and busted.

At 12:57 PM 6/4/99 +1200, you wrote:
{Quote hidden}

>> {Original Message removed}

1999\06\06@035041 by Graeme Smith

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GRAEME SMITH                         email: RemoveMEgrysmithTakeThisOuTspamspamfreenet.edmonton.ab.ca
YMCA Edmonton

Address has changed with little warning!
(I moved across the hall! :) )

Email will remain constant... at least for now.


On Fri, 4 Jun 1999, William K. Borsum wrote:

> Wasn't there someone who put large helical coils under the high KVA power
> transmission lines in the Nevada dessert and sucked off kilo-watts for a
> while??  How about the ubiquitous 60 cycle stuff?
>
I heard of that.... so called "FREE POWER"

turned out that the power company sued him, so it wasn't so "Free".

Apparrently, the power company can detect when it gets an unexpected load
and tries to track it down.... Something about power line safety or
something.... Anyway, they proved in court that the guy was "Stealing
Power"...

I gather that they, and probably the courts would take a dim view of big
helical coils under the power lines..... However, if the signal is in the
case already, who could complain about you sucking power off the
interference in your circuit...?

Using a LowPass filter to power a cap, at 60hz, will probably be very
practical, in most circuits. Who needs helical coils when your whole
circuit probably resonates at that frequency.

                               GREY

1999\06\07@170707 by Dave Vandenberg

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Elektor Electronics November 1994
has an article in it: "Phantom Power Supply for Guitars".
which showed how to supply DC power to an amplifier built in to a guitar via
the 2 audio wires.

I'm guessing it may not be too bad a starting point if you were sending DTMF
control tones as audio rather than digital, perhaps.
Then there's the cost of dtmf encoders & decoders...

And the bit of paper also has noted on it,
RS-232 Opto-isolator article December 1997 (Elektor Electronics),
PIP02 (David Tait) PIC Programmer, July\August 1998 (Elektor),
and part one of a series of PIC articles beginning July/Aug. 1994.

/David_v


'data over DC power lines'
1999\09\20@143050 by Larry G. Nelson Sr.
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I did that as a kid to light an LED. I had no problem but there was an AM
transmitter across the street from my house.


At 12:57 PM 6/4/99 +1200, you wrote:
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>> {Original Message removed}

1999\09\21@005352 by Gennette Bruce

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Uhm, the standard method for getting the audio signal from a two wire
electret microphone is to put a cap on the power line to extract it because,
you see, inside the microphone capsule the signal from the tiny fet
amplifier in there is passed to the positive power line via a capacitor.

The AC can pass through the caps while the DC can't. Now for digital data
maybe some form of level shifting could be used, or maybe bursts of a fixed
frequency AC can carry the pulses (sorta like in a TV remote control where
burst of 38kHz are puled to carry the data).

Bye.

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