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How bright would this be?

Discussion in 'Electronic Basics' started by [email protected], Aug 22, 2005.

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  1. Guest

    I am interested in high voltage and want to build a Marx Generator. the
    basic principle of a Marx Generator is that it cherges capacitors in
    parallel through high resistance resistors, once the capacitors are
    charged to there full potential they fire in series through spark gaps.
    If I built one using 5pf capacitors that output around 30kv, could I
    expect any visible sparks? I'm not sure if my math is right but based
    on a 3kv 5pf cap I get about 150uf. How high could I get the voltage at
    this amperage before it becomes lethal? thanks in advance!
  2. Put identical capacitors in series, and the capacitance of the series
    bank is the capacitance of one of them divided by the number of
    capacitors. If you have ten of these 5 pF capacitors, the total
    capacitance would be half a picofarad.

    You want more than that. Stray capacitances of wires, etc. are easily a
    few picofarads.

    The top of the "usual" small Van de Graaf generator available from a few
    scientific and educational equipment suppliers (for $200-$300 now I guess)
    is around 10-15 picofarads. That produces a spark that is easily visible,
    and a little bright - in good cases.
    At a voltage of 200 KV, this is 2-3 microcoulombs of charge.

    These have stored energy low enough to be considered at least somewhat
    safe as toys.

    Somewhat larger Van de Graaf generators in science museums have roughly
    20 pF of capacitance and reach a voltage around 300 KV, which indicates 6
    microcoulombs of charge. When discharging to a grounded sphere, sparks as
    short as 2 inches (150 KV between largish spheres, indicating about 3
    microcoulombs) appear a little bright.

    I know of one science museum taking precautions and requesting
    demonstration participants to cooperate with precautions against getting a
    full-force shock, although claiming that one should not die if shocked.

    I have seen what a 150 pF capacitor charged to 10 KV does. This is 1.5
    microcoulombs. It is a little bright.

    To get 3 microcoulombs at 30 KV, the capacitors need to have a
    capacitance of 100 picofarads.

    To get a really impressively bright spark, I would recommend 5-6
    microcoulombs. This requires 166-200 picofarads for 30 KV capacitors.

    I have seen what 100-200 microcoulombs does - that makes sparks that are
    very bright, almost or slightly able to put spots in your eyes. Assuming
    conductors involved have resistance no more than a few ohms, when the
    voltage is a few kilovolts.

    In a Marx generator, the output spark may have brightness impaired by
    resistance of the sparks in the spark gaps between capacitors. You may
    need a little more for really nice bright sparks.

    Now for shock hazard:

    I have heard that shocks with 10 joules or more of energy can be fatal
    and shocks of lesser energy are unlikely to be fatal.

    Please consider that whether or not electrocution occurs will depend on
    what point of your heartbeat cycle your heart is in, as well as random
    Electrocution is so unreliable that "The Electric Chair" uses enough
    current to either cook vital organs or paralyze breathing muscles so that
    the condemned's brain gets deprived of oxygen or gets impaired by blood
    acidity from CO2 buildup in the blood. Lack of electrocution I expect to
    be similarly unreliable. My father ordered me to not store more than 1
    joule in high voltage capacitors and high voltage capacitor banks when I
    was in my high-voltage spark-making younger days. I even remember one
    high-voltage-toymaking book titled "Electrostatics" advising to avoid
    getting shocked by more than 1/4 joule despite also saying that 10 joules
    is what it typically takes to have a major electrocution risk.


    Charge in coulombs is capacitance in farads times voltage in volts.

    Energy in joules is 1/2 times capacitance in farads times square of
    voltage in volts.

    Example: 100 picofarads at 150 KV:

    Charge is 15 microcoulombs. This will make an impressively bright
    spark, and between sharp points or ends of thin wires could do so over a
    distance around 20 cm or 8 inches. Between large spheres this only sparks
    through about 5 cm or 2 inches, and presumably with significantly more
    light per centimeter if the resistance and inductance of the conductors in
    the circuit is low to achieve peak current in the kilo-amps.

    Energy is 1.125 joules. It is easy enough to find sources indicating
    that this is safe to get shocked with, but I would not bet my life on it.
    I would get plenty of practice with quantities of stored energy less than
    1/4 joule until I have gone a few months without getting zapped. And with
    deadly or possibly deadly amounts of stored energy, I would use all
    recommended cautions and avoid taking chances. Keep handy some plastic
    sticks and some discharging tools.

    More shock hazard stuff:

    1. A capacitor usually does not get completely shorted by a spark.

    2. A capacitor that gets briefly completely shorted can have some charge
    afterwards. This phenomenon is called "dielectric absorption".

    3. Non-fatal shocks can still be dangerous. If you get startled or
    suffer involuntary muscle contractions, you could bump something or bump
    into something - possibly breaking or zapping your project, possibly
    getting injured by falling or hitting something or getting stabbed by
    something, and possibly from touching or getting within spark range of a
    source of a lethal shock.

    4. There is a bit of word out there that steady DC is the worst to get
    shocked by - NOT TRUE! AC and unsteady DC (including discharging high
    voltage capacitors with your body) are worse. Some horror stories of
    steady DC are out there because railroad workers lived to tell them after
    getting bigtime serious burns, including burning of internal tissues that
    take a long time to heal or never heal properly after getting cooked.
    I doubt you will cook any of your skeletal muscles from a Marx generator
    or a Van de Graaf or a neon sign transformer - the main worry here is
    shocking the heart into ventricular fibrillation.

    - Don Klipstein ()
  3. Jasen Betts

    Jasen Betts Guest

    30kV will ionise about 30mm of air so there's enough voltage there for a
    visible spark, how much current you get depends on the cyclic rate of the
    device - which depends on on your choice of charging resistors.

    I don't know where you're going to get resistors rated to withstand 15kV

    you can get about 30Kv from the high voltage circuit in a CRT based display,
    like a worn-out monitor... (dangerous though)
    do you mean uA ?
    I don't think less than 1mA has ever been fatal on the skin of a healthy
    person. if you limit the current the voltage will be restrained by the
    resustance of the load.
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