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'[EE] burst digital data over an low voltage AC lin'
2009\01\26@223350 by alan smith

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I have a client who is wants to do a cheap..ok...inexpensive communications between two PICs that the only connection between them is a 12VAC set of wires.  They only need to transmit a few bytes now and then, and RF
is too expensive, and may not have range.  The 12VAC also is the supply for the end device.

The solutions I see are either imposing the data on the AC much like LONworks or X10 does, or actually kill the AC and then transmit the data over the pair.

So any thoughts? This is not a one off but something they want in small production...few hundred a year so it needs to be simple, easy to build, no custom components.

Ideas?


     

2009\01\26@225120 by Richard Pytelewski

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Alan:

Had a similar problem on a dc supply line.... if the 12v current demand is
stable (not changing), why not switch a load at the termination  point ...
similar to a 20 ma loop but using a resistive load to increase the current
demand  and then  use a resistor on the receiver end to measure the voltage
drop due to the higher current.  Not elegant or fast but it may work for
you.

Rich

{Original Message removed}

2009\01\26@232616 by Bob Ammerman

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Can you add a DC bias to the AC already on the line?

-- Bob Ammerman
RAm Systems

2009\01\27@040458 by apptech

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> I have a client who is wants to do a cheap..ok...inexpensive
> communications between two PICs that the only connection between them is a
> 12VAC set of wires.  They only need to transmit a few bytes now and then,
> and RF
> is too expensive, and may not have range.  The 12VAC also is the supply
> for the end device.

> The solutions I see are either imposing the data on the AC much like
> LONworks or X10 does, or actually kill the AC and then transmit the data
> over the pair.

What bit rate is acceptable?
What is the AC used to drive?
What voltage?
What frequency?
What power level?

Tone near zero crossing will probably not undergo as much loading as
elsewhere if AC line is rectifed to DC for power.

If you can add filters then tone + AC can coexist easily. At a high enough
frequency the tone filter can be small & cheap.

If AC frequency bit rate is OK then a bit per half cycle may be easy enough
by phase shifting the zero crossing point - eg adding or subtracting voltage
so zero crossing time is shifted. This is arguably equivalent to adding DC
as was suggested - ie you just add a small modulated low frequency signal to
the AC in phase with the AC. Mean DC level may have to be zero depending on
the AC load.


       Russell McMahon

2009\01\27@103611 by alan smith

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Possibly, I'd have to see what else the 12VAC is connected too, but that might work. Hmm...have an example of how thats done?


--- On Mon, 1/26/09, Bob Ammerman <spam_OUTrammermanTakeThisOuTspamverizon.net> wrote:

> From: Bob Ammerman <.....rammermanKILLspamspam@spam@verizon.net>
> Subject: Re: [EE] burst digital data over an low voltage AC line
> To: "Microcontroller discussion list - Public." <piclistspamKILLspammit.edu>
> Date: Monday, January 26, 2009, 8:25 PM
> Can you add a DC bias to the AC already on the line?
>
> -- Bob Ammerman
> RAm Systems
>
> --

2009\01\27@103858 by alan smith

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I can see that working on DC, pretty straight forward, but monitoring the AC for a change in the load isn't as simple...have to use either hall sensor (done that before) or current sense coil.  Gets a little complicated for a low cost device.  The one thought was to actually interupt the AC, use a supercap or something on the slave side to hold power and just burst the data up the line while the AC is off


--- On Mon, 1/26/09, Richard Pytelewski <.....rjp_psiKILLspamspam.....bellsouth.net> wrote:

{Quote hidden}

> {Original Message removed}

2009\01\27@151439 by Carl Denk

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Though I haven't tried it for data, X-10, and Insteon appaerently work
fairly well for data with the RS-232 adapters. These should be on the
same leg of the 120 VAC (Not on the other phase of the 220 VAC single
phase), but there are bridges available to get the signals on the other
leg. Also filters are available. Check out the FAQ's at:
> http://www.smarthome.com/_/index.aspx



alan smith wrote:
{Quote hidden}

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