Superimposing frequencies on mains AC

Discussion in 'General Electronics Chat' started by jaygatsby, Dec 29, 2011.

  1. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    I am reading through the excellent All About Circuits textbook, volume II. I see that sometimes other frequencies are put 'on top' of the mains 60hz, to convey information of interest to the power company across lines, or to carry data signals. I've heard of this being done before... but my question is, doesn't this affect the total power across the lines, or rms voltage? Won't this subject some household equipment to danger, due to this? And aside from this, don't some items rely on the frequency of the mains, as sort of a clock?

    Thank you
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    These added signals will be at low levels, and at frequencies very much higher than the 50Hz or 60Hz normally used to supply power. The net effect on the RMS voltage should be negligible.

    In normal operation of a mains supply system, there will always be some content at frequencies other than the fundamental, and so circuits using the AC as a clock may in any case incorporate some filtering to reject such interference. Timing devices based on synchronous motors are inherently resistant to high-frequency interference
     
  3. BillB3857

    Senior Member

    Feb 28, 2009
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    In addition, if you are talking about systems such as home automation, the signals typically take place very near the zero crossing point of house power. Do a search for X-10 signals, for example.
     
  4. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    Thanks. On a related note: could long mains runs make effective low frequency antennas for consumers? I mean for RX...
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    "Mains aerials" are not a great idea. There are safety issues, and while these can be minimised by suitable choice of coupling methods, the mains is full of nasty interference at the low frequencies for which a long antenna is appropriate.
     
  6. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    I understand there would be a large amount of interference around 60Hz, in VLF. You can see such interference from the mains while using a normal antenna. But wouldn't it exist only at harmonics? So one could filter out 60Hz and have a nice VLF antenna?

    I am speaking theoretically; don't hesitate to educate me based on trepidation regarding my intent. I have however used the cable (as in TV) system as a receive antenna, and found that I could receive signals over a fairly broad frequency band.
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    With all the horrid CFLs and switch-mode contraptions about, those nice big flat screen TVs, a computer in every bedroom in the land, the mains-borne signalling systems mentioned, the baby monitor next door... I would be surprised if you had much luck these days.

    Using the screen of a higher frequency coaxial system as an antenna is a different kind of proposition, I would think. I still wouldn't recommend it: it is probably not what the cable company would expect you to do, and it is potentially unsafe: a fault in a neighbour's equipment could give you a nasty surprise!
     
  8. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    Good points, thanks. I'll bet that the power company filters things so that the mains, outside of the customer premises, are not subject to all this noise. So my neighbor's contraptions shouldn't bother my house. But I'm assuming -- what do you think?
     
  9. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The only way to know for sure would be to try it, but I can't in all conscience recommend doing that. In the end however your safety is your own responsibility.

    I would be surprised if the power company provides much RF filtering at the customer's input, certainly I don't have it here, but then again this is an old house that hasn't been rewired in a long time. There is so much cr@p coming out of the wiring in this place that the best way of hearing European AM stations is to take a radio into the garden. :(
     
  10. jaygatsby

    Thread Starter New Member

    Nov 23, 2011
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    Haha. Thanks for the info and the laugh. If the power company isn't filtering, then it seems like any jerk could intentionally interfere with all of his neighbors' lines. There must be something in place to prevent this...
     
  11. crutschow

    Expert

    Mar 14, 2008
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    Dimmer switches are big offenders. They add a lot of high frequency harmonics on the power line. When I listed to AM radio I can always tell when a dimmer switch is turned on in my house.
     
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