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Suppression of EMI from Spark Gap

Discussion in 'Electronic Basics' started by Vince, Oct 9, 2006.

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  1. Vince

    Vince Guest

    My heating system uses a constant ignition wiring configuration at the
    oil burner. This is causing an annoying high level noise in a HF
    receiver.

    I would like to kill the source by improving shielding if that is
    required. The electrodes are within a metallic housing (e.i., within
    the burner assembly).


    How can the spark gap EMI be suppressed? Would a connection from the
    burner housing to earth ground be helpful?

    Thanks.
     
  2. default

    default Guest

    The oil burner should already be grounded via the mains connection to
    it. But a ground if there isn't one should help.

    A differential and/or common mode filter will do wonders for noise.
    My Electric stove was triggering the "lightening detector" and causing
    my modem to drop out. I got a cheesy filter from some surplus company
    and ditched the toroid they had and rewound a pair of large ferrite
    cores with heavy wire to carry the 30 amps the stove can suck. No
    problems.

    I found my communications receiver worked much better with a dedicated
    ground (10' of 1/2" copper pipe) and not the building ground.. I was
    getting all kinds of switching transients in the building ground.
    Receiver ground is at the antenna and less than 5 feet from the
    receiver with nothing else on it. Building ground was a serpentine
    path through the attic to the outside service and over 60' of wire.

    Direct RFI radiation is seldom the problem - the oil burner ignition
    is not designed to be an efficient radiator - most of the problem is
    likely to be via the power lines. For that the source needs a filter
    and that filter has to be grounded.
     
  3. Make sure the spark transformer case is bonded to the
    firebox to provide an HF path for capacitive spark current
    back to the grounded center tap of the HV winding.

    Add an X rated (across the line AC rated) capacitor across
    the transformer primary and a ferrite bead over both primary
    leads, or add a line filter package to the transformer
    primary, if space permits.
     
  4. Vince

    Vince Guest

    When I disconnect the antenna lead from my transceiver, the
    interference is gone. Thus, I believe that the problem involves
    "radiation" across the RF spectrum and does not involve the AC mains.

    There is no center tap at this transformer that I know of. The
    secondary winding terminals in this application are actually spring
    terminals, that touch the electrodes used for the spark gap only when
    the (hinged) transformer housing is closed and resting upon the burner
    chassis (held down by two pressure screws).

    I wonder if the underside of the transformer housing flange needs to
    be scraped to remove paint... to make a better ground connection. I
    will check that out soon.
     
  5. Guest

    What if the mains are not conducting the noise to the transciever, but
    radiating it to the antenna?
    I have the same thing. You can check the center tap grounding with an
    ohm meter from each spring to case. Make sure you unplug the burner
    power before risking this. Mine is center grounded, to balance the
    high voltage between the two electrodes, to discourage an arc from
    either electrode to the surrounding burner housing.
    Mine has some screws on the sides. For this type, I would adsd a
    jumper wire from one of these to some bolt on the burner box, to
    guarantee a high frequency path around the painted hinges. There is
    also a slotted tab that gets clamped down, and a star washer might cut
    the paint and do the same job.
     
  6. JoeBloe

    JoeBloe Guest


    The spark itself emits. Radiates like a little antenna.
     
  7. JoeBloe

    JoeBloe Guest


    More likely that the spark itself is doing that. The radiation it
    produces is far more broad banded. For the mains to do that, high
    currents at those frequencies would need to be on them.
     
  8. JoeBloe

    JoeBloe Guest

    With gaps big enough to let out a LOT of HF noise. Certainly not a
    faraday cage. These sparks are pretty big spikes amplitude wise.
     
  9. But that spark is inside a metal fire box.
     
  10. I can really be sure because I can't see what he up against.

    Can you get a coffle can to go around it, to shield it for test?

    You can also use copper tape or any source of tin.
     
  11. Guest

    .. . .

    If it's radiating through space, then I would think you need a "Faraday
    cage". This doesn't necessarily have to be made out of solid metal. It
    can be made out of some sort of conductive mesh screening.

    A rule of thumb that I used to use is that the holes in the Faraday
    cage can be no larger than 1/8 wavelength. So, assuming a maximum
    frequency of 300 Mhz, for example, you can have holes as large as 4.68".
     
  12. JoeBloe

    JoeBloe Guest


    Ok fine. Now think about how utterly stupid it would look to walk up
    to someone with a mesh box made from nearly 5 inch mesh, and say, look
    at my RF shielded cage!

    In fact, I cannot think of a single time I have ever seen such big
    gaps in a device, and have it still be called or considered to be
    "shielded".
     
  13. Guest

    Science and engineering are not always intuitive. Some people use 1/8
    wavelength and some use 1/10 wavelength as a rule of thumb. The theory
    is that it requires 1/4 wavelength to make an antenna and 1/8 and 1/10
    are significantly below that. Do you work as a compliance engineer?
    What sort of rule do you use?

    What aperture size would you allow for a microwave oven at 2450 Mhz?
    What aperture size would you allow for submarine ELV signals at 76 Hz?

    One reason electronic devices have smaller holes is that they are
    concerned with much higher frequencies.
     

  14. At 2.4 GHz I would prefer solid aluminum walls with the seams bonded,
    like an old portable shielded computer room. At the least, I would want
    a double shielded room made of fine woven bronze screening, and a double
    layer copper clad door with finger stock all around the seams and the
    two shields bonded only where the power comes in. I've worked in both
    types of shielded rooms. I designed a foil shielded room built on 2"
    foam panels and covered with wood paneling, but I haven't built it yet.
    I have to put a new roof on my shop, first.


    Do you know the wavelength of 76 Hz?


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  15. Guest

    If you were building a microwave oven, though, what is the largest
    aperture you would allow to seal the door or the window, for instance?

    If you think you might ever run a generator in your shop, by the way, I
    would put sheet rock on the ceiling (walls also) and fiberglass
    insulation.
    According to an online calculator the wavelength for 76 Hz is
    12,947,368.421 feet (2452 miles). The wavelength for 2450 Mhz,
    incidentally, is 4.82 inches. I've always wondered, incidentally, if my
    subwoofer would sound better if I built a room 1/4 wavelength long at
    it's lowest frequency. I haven't done it yet, though :>

    http://www.onlineconversion.com/frequency_wavelength.htm

    Me too--3 years--U.S. Army. I've lost everything else over the years,
    including all my year books, but I still have my DD214. I'm not sure
    why. Maybe subconciously I wanted to make sure they didn't call me up
    accidently :>
     

  16. No, the foam panels are quite rigid, and they carry the weight of
    each panel. Heavy aluminum foil is glued on each side, and over the edge
    where the panels join. You leave at least a half inch gap all the way
    around the edge to be sure they don't touch. Then you glue thin
    paneling or hardboard to each side to protect the foam and foil. The
    corners and edge joints are the secret, they hold everything together
    and connect the inside and outside foils into a pair of separate but
    continuous shields. Finally, you bring power in somewhere and bond the
    two shields. You can make a pair of holes for air conditioning, as long
    as you are careful to maintain the shielding with bronze screening. I'll
    put plenty of pictures on my website as I build it. You can build a 12'
    * 8' for well under a grand, as opposed to over 20 grand for a solid
    aluminum booth.

    Sheet rock if fairly heavy, so you paint the ceiling panels white, or
    another light color.



    A generator, as in Gasoline powered? It would be in a separate
    building, by the automatic transfer switch.


    A simple steel box will stop 76 Hz.


    It would cause a resonance over a narrow range of frequencies and
    sound like crap, unless all you want is a vibration that will shake the
    walls apart.



    You can get full copies of your military records by applying to the
    VA directly, or through your local Veterans Service Officer. A local
    DAV post should have at least one, and sometimes the county government
    has one, too. I have both, locally, and have had help from both at
    different times.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  17. Fred Abse

    Fred Abse Guest

    A scrap shipping container (welded steel) makes a good start for a
    shielded room, if you've got the real estate.
    My rule of thumb is: if the wavelength is farther than you'd want to walk,
    it's VLF :)
     

  18. I'm not zoned for it, or I would already have a couple of them. I would
    have to put them in my driveway and my neighbors would complain, loudly.
    :(


    Some days that would be up around 100 MHz.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  19. jasen

    jasen Guest

    how do you intend to bond the edges of the aluminium sheets ?

    do you have some sort of spot-welder?

    Bye.
    Jasen
     

  20. Aluminum tape with conductive glue.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
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