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Carrier current communication on Low voltage/high current AC

Discussion in 'Electronic Design' started by Spehro Pefhany, Jun 8, 2005.

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  1. A little one-off (at the moment) G-job here:

    I'm interested in doing carrier current communication over 12VAC
    power. Total AC current will probably be in the 10-50A range, and I'd
    like to get 4800 or 9600 baud, but less would be acceptable. There
    could be a couple hundred feet of wire, and I don't control the source
    impedance.

    So, a fat choke (or the secondary of a transformer) in series with the
    line and couple the carrier in capacitively or inductively?

    Looks like there's some advantage in keeping the fundamental frequency
    low if this is ever to have to deal with FCC etc. standards.

    http://strategis.ic.gc.ca/epic/internet/insmt-gst.nsf/en/sf08216e.html

    Maybe the AM IF frequency of 455kHz, making a ceramic filter for the
    receiver easy?

    Any other suggestions?


    Best regards,
    Spehro Pefhany
     
  2. Joerg

    Joerg Guest

    Hello Spehro,
    A resonant circuit would make for a smaller device to isolate the RF
    path. At 50A a choke can become rather bulky.
    I would go much lower, below 150kHz, but avoid any Loran ranges. X10
    operates on 120kHz and AFAIK they blast several volts onto the mains line.
    Stay away from simple AM on-off protocols. That is one of the downsides
    of X10. It should at least be FM or something even more reliable. Your
    protocol may have to be fault tolerant against missed or misinterpreted
    bits. FEC and all that sort of things.

    As to receiver filtering you can't use crystals at 9600bps. But LC is
    pretty stable below 150kHz. Another upside of staying under 150kHz is
    the abundance of ferrites that are suitable.

    Using frequencies that others already have can yield FCC advantages
    similar to what FDA 510(k) does in medical: Prior art, less scrutiny.
    The downsides may be patent issues and the like.

    Regards, Joerg
     
  3. Take a look at how X10 stuff works (at the AC voltage zero crossings).
    You might be able to adapt some existing chipsets and other hardware.
     
  4. Joerg

    Joerg Guest

    Hello Paul,
    We have some X10 and I don't find this AM protocol all that reliable. It
    is also very slow.

    Regards, Joerg
     
  5. Rich Grise

    Rich Grise Guest

    Just OTTOMH - brute-force baseband (9600 HZ Manchester, DTMF, ?) through a
    current transformer?

    It's just I'm thinking "modulation", with a transmission line impedance of
    0.2 - 1 ohm, 60 HZ "carrier" - or just superimpose your data stream, coded
    at your leisure. What kind of noise and crap are we dealing with here?

    Good Luck!
    Rich
     
  6. John  Larkin

    John Larkin Guest


    Maybe pass the power wire through a low-mu core and inject the signal
    in series, rather than in parallel? Ditto receive end.

    If there are any bridge rectifiers in the system, the line parallel
    impedance will be modulated by diode conduction!

    John
     
  7. Maybe think about transmitting the comms as a common-mode
    voltage. The 24Vac system can be connected to Ground at
    60Hz (or have stray-C to ground), just devise something
    that makes that connection a high impedance at (and above)
    the comms carrier frequency.
     
  8. Fred Bloggs

    Fred Bloggs Guest

    That's one of three possibilities of selecting two of three terminals
    for the signal transmission , but for some reason it is forbidden for
    safety reasons. Also, the current carrier method of inducing a signal
    via transformer with secondary in series with line is also rejected-
    probably too much bulk required there.
     
  9. John Larkin

    John Larkin Guest


    "Is also rejected"? Some sort of Royal decree?

    John
     
  10. I like it better than that execrable term "deemed".


    Best regards,
    Spehro Pefhany
     
  11. John Larkin

    John Larkin Guest

    The one I hate is "good engineering practice" which usually means "I
    always do it that way but can't actually explain why."


    John
     
  12. Jim Thompson

    Jim Thompson Guest

    I use the term "good engineering practice" to indicate to the client
    that, if they'd done it my way, they wouldn't be in ka-ka-land right
    now ;-)

    ...Jim Thompson
     
  13. Jim Thompson

    Jim Thompson Guest

    I should have added... I told a client more than two weeks ago, "your
    design failed because....".

    They vehemently denied it could happen, that I was totally wrong, how
    could I be so wrong, all their simulations showed it was stable, etc.

    Guess who's kissing my ass today ?:)

    ...Jim Thompson
     
  14. Or they just don't like the looks of something innovative, therefore
    it doesn't conform to "good engineering practice".


    Best regards,
    Spehro Pefhany
     
  15. Mike Monett

    Mike Monett Guest

    Tony Williams wrote:

    [...]
    Thanks, Tony. I didn't see it was already mentioned.

    As far as isolation inductors, many power supplies have bypass caps from
    hot and neutral to case gnd, which is connected to the green wire. So
    there is a possible short for rf, which would negate the effect of the
    isolation inductors.

    If the driver can supply sufficient current, some voltage will be
    developed across the power cable connecting the device to the line and
    allow the signal to propagate. It's a bit like common-mode noise on a
    scope ground.

    In this case, perhaps the series inductors may not be needed. There are
    many examples of carrier current devices on the market, such as
    intercoms, baby-sitting units, and so on. I'm sure they don't use series
    inductors, which would be expensive and difficult to install.

    Mike Monett
     
  16. Mike Monett

    Mike Monett Guest

    You probably want to stay away from 455KHz - that could block every AM
    radio on your street:)

    Perhaps also try to avoid frequencies used by local low-power
    non-directional beacons (NDB) for aircraft navigation. These are often
    used to establish holding patterns for aircraft during storms.

    I'd look at some commercial products using carrier current, such as
    intercoms and baby-sitting devices, and figure out how they manage to
    work without series inductors.

    Here's one that simply injects the signal into the neutral, like my
    proposal in another post that was quickly shot down:)

    http://home.att.net/~weatheradio/carrier.htm

    Mike Monett
     
  17. Mike Monett

    Mike Monett Guest

    Mike Monett wrote:

    [...]

    and here's a complete transceiver at 135KHz

    http://www.redcircuits.com/Page56.htm

    Some signal processing may be needed to reduce interference from line
    noise, but it seems you can simply inject the signal wherever you feel
    like.

    Mike Monett
     
  18. Joerg

    Joerg Guest

    Hello Mike,
    Many PLC schemes simply blast RF onto the wires, with the driving
    impedance as low as can be. Or as low as the parts budget allows. A
    current coupling core would be way too expensive for most.

    In Spehro's case this might work well if the driving transformer is high
    enough in RF impedance. Most likely it is. A few ohms can be enough. So
    in a low voltage app it would boil down to a stiff driver that is
    capacitively coupled to the lines and protected against spikes by diodes
    or something.

    PLC such as X10 tend to fail when there is too much EMI filtering on the
    line, like all the X-caps and Y-caps. In LV gear that often isn't the
    case. IOW, know thy loads.

    Regards, Joerg
     
  19. Mike Monett

    Mike Monett Guest

  20. Mike Monett

    Mike Monett Guest

    Yes, that seems most reasonable. I listed urls for examples of three different
    injection methods in other posts.
    Perhaps the small inductance of a few feet of line cord is enough to allow the
    signal to propagate. Protection against spikes is definitely good. Also may be a
    good idea to think about transients due to nearby lightning stikes.
    Some carrier current transmitters may go up to 50 Watts. That might overcome the
    attenuation from a few line filters:)
    Mike Monett
     
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