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Creating Electric Arcs?

  1. Oct 5, 2013 #1
    How can I safely produce an electric arc, preferably multiple centimetres long, the longer the better, out of some basic stuff like a transformer and a motorbike battery?

    I would like to produce an electric arc that is clearly visible, and that isn't dangerous, so the amperage should be quite low, as I'm not looking for heaps of power, but the length.

    Also, how do you calculate the amount of voltage required to produce an arc of a given length?
     
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  3. Oct 5, 2013 #2

    UltrafastPED

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    Last edited by a moderator: Sep 25, 2014
  4. Oct 5, 2013 #3
    In that experiment, are the plates capacitors? And if I am to run it from a battery, as using mains power seems pretty unsafe, would I then need to either convert it AC or make a square wave, for the transformer to work?

    So if I have a high enough voltage to breakdown 20cm of air, does the amount of current matter at all?
     
  5. Oct 5, 2013 #4

    UltrafastPED

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    The plates can be thought of as forming a capacitor (you can calculate the capacitance), or as electrodes.

    A spark gap system requires only voltage; the amount of current is determined by the energy stored in the system (e.g., the plates). You want to store a very small amount of energy ... think 1 joule max; use less if you can get it to work.

    You need DC, and you need high voltage. Safety first ... don't fool with this stuff if you don't know how to do it safely. If you are a beginner, find somebody local that you can work with.
     
  6. Oct 5, 2013 #5
    Why do I need DC, and how can I achieve high voltage with it?

    What is easier: creating a spark, or creating a consant arc, and what would you recommend?

    Does that mean that I could have hundreds of kV and a few nA and still have an arc?

    Thanks for your help.
     
  7. Oct 5, 2013 #6

    UltrafastPED

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    The spark is a current; it is the release of the stored energy after the voltage breaks down the atmosphere.

    I've created sparks ~ 3 cm long with 10 kV DC; I would take my photo-electron gun out of the vacuum chamber and power it up on the bench to see where in-vacuum breakdowns were occurring. The very first sparks would be from the defect or foreign matter, which could then be studied under a microscope and corrected or cleaned.

    I used a 30 kV power supply, and ramped up the voltage slowly.

    A TIG welder creates a constant arc; it also has a high current. The wall plug AC is converted to DC inside the welder.

    I don't have any suggestions for how to make a high voltage supply; there is lots of room for mistakes, and mistakes can result in fires, burns, explosions, and other bad things.
     
  8. Oct 5, 2013 #7
    So the spark needs to be DC, but for the transformer to work, it requires AC. So if I were to power it with DC, it would need to converted to AC, then back to DC.

    Could I then simplify it by starting with an AC source of power, possibly mains power? I know that mains power is dangerous if caution is not used. In Australia it is 240V, however I am looking for much higher voltage, so I would need to step it up with a transformer.

    Using a step up transformer of a turns ratio of ~50:1, connecting the mains power to the less turns side, would that cause lots of heat to build up? If so, could I just put a high power resistor in there to limit the amount of current going through it?

    That would put the voltage at ~12kV, and the amperage at < 0.2 without a resistor (I think the standard powerpoint supplies 10A). If there is still a worry of the hazard the spark could present due to the still lethal current, could I step it up again?

    Would 60kV at < 1mA be at all dangerous? Is there any affects of the high voltage itself?
     
  9. Oct 5, 2013 #8

    Bobbywhy

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    When you want to build a device like an electric arc generator it is important that you first understand how and why they form. One place to begin is to read, study and learn using Wikipedia:

    “An electric arc, or arc discharge, is an electrical breakdown of a gas that produces an ongoing plasma discharge, resulting from a current through normally nonconductive media such as air.”
    http://en.wikipedia.org/wiki/Electric_arc

    Don’t forget the “See Also” items and all the references.

    Here are a few basic facts: An electric arc can occur between two electrodes. It is an electrical current flow through the conductive ions of a very hot plasma. To begin this current flow there has to be enough initial voltage to jump over the gap. There also needs to be enough energy in this initial spark to initiate a plasma. Sometimes an arc is started by bringing the electrodes together and then drawing them apart as in arc welding. Otherwise the voltage for jumping an air gap is high - for example spark plug gaps need about 10,000V. This voltage may be a pulse from a high voltage transformer like an ignition coil. Once the arc has commenced there has to be enough continuous voltage to maintain the current flow. This is a lower voltage, typically < 100V. The current drawn has to be enough to maintain (sustain) the high temperature plasma. The current is normally controlled by a ballast resistor (for DC) or a ballast choke inductor (for AC). This will cause the voltage across the gap to adjust automatically, so long as the sustaining supply is suitably higher than the sustaining voltage.

    Now, you may use the Google search terms “electric arc generator” and find lots of do-it-yourself circuits to build. One favorite of mine that uses no battery or mains power is the “Kelvin Water-dropper”. Try it, it will amaze your friends!
     
  10. Oct 6, 2013 #9

    meBigGuy

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    The classic continuous, safe, long electric arc is the tesla coil. Of course even that can be unsafe if you overdo it or are careless.

    The simplest and cheapest arc generator would be with an automobile or motorcycle ignition coil driven by a 555/powerFet. That can be unsafe though, so be careful.

    The most dramatic (well the tesla coil can be dramatic) is the Jacob's ladder, but they are Very Dangerous.
     
  11. Oct 6, 2013 #10
    I'm not necessarily looking for something like a Tesla Coil or Jacob's Ladder - not yet anyway. I am simply trying to learn an easy way to create an electric arc (over 10cm would be really nice), that jumps across a gap, like a spark gap, and preferably doesn't cause fires... That Kelvin Water-Dropper seems like an intersting experiment.

    So I need a high voltage initially to break down the air, and the current of the arc will increase over time because of the ionisation of the air? And with the 30kV/cm value, does that mean that 30kV is required for 1cm, 60kV is required for a 2cm arc, etc...?
     
  12. Oct 6, 2013 #11

    meBigGuy

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    WOW --- never heard of the Kelvin Water Dropper. Amazing effect. Learn something every day here.
    Here is a video (see the last 6 minutes)
    http://www.youtube.com/watch?v=RQX8I9ZWtPQ&feature=relatedLecture

    The simplest arc would be with an old automobile ignition coil, resistor, and switch. When you open the switch you get a spark. Not sure it will reach 10cm though. Don't know much about modern ignition systems.
     
  13. Oct 6, 2013 #12

    mfb

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    Don't do it yourself. You clearly do not have the knowledge to do it safely.
     
  14. Oct 7, 2013 #13
    You're probably right, I shouldn't do it. So I won't at the moment.

    However, I would still like to learn more about high voltage, and electric arcs, both theoretically and practically. I have researched a bit on the internet, but haven't found many good sources of information. So could anyone please provide me with some knowledge on the subject, or resources that do? Information such as: producing high voltage, electric arcs, affects of high voltage on insulated wire, what equipment is used to handle it...

    Is high voltage covered at all in high school physics, or is it only taught in specific university courses?

    Thanks for you time.
     
  15. Oct 7, 2013 #14

    Baluncore

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    Any arc will generate intense UV radiation that can burn your skin and cause short and long term eye damage. This is especially true with a continuous arc such as electrical welding equipment.

    If you require only a momentary arc strike you should consider a Marx Impulse Generator. They can be built from simple parts. There are many descriptions on the internet. Start with;
    http://en.wikipedia.org/wiki/Marx_generator then follow the links such as this;
    http://www.electricstuff.co.uk/marxgen.htm
     
  16. Oct 7, 2013 #15

    meBigGuy

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    Is an old automobile ignition coil, a resistor and a switch considered too dangerous? It can easily do an inch or more. I thought it would be a good place to start.
     
  17. Oct 7, 2013 #16

    Bobbywhy

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    In my opinion, what you are describing here is not dangerous. When I was much younger we had a comparable device and I got shocked MANY times when playing around with it. So, one can assume that it's not life-threatening, at least.
     
  18. Oct 7, 2013 #17

    Baluncore

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    The voltage needed to strike a 1" arc with an automobile ignition coil is great enough to damage the coil internal resistance. The coil is designed to operate with a small plug gap that will clamp the OC voltage. They may be oil filled. Any spilled oil signals internal insulation problems.
     
  19. Oct 8, 2013 #18

    meBigGuy

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    Interesting, didn't know that. Guess that makes pulling the spark plug wire to check the spark a bad idea. I wasn't able to find any specifications for the secondary insulation breakdown voltage. The coils ranged from 30KV to 50 KV. Looks like they fire at around 15KV.

    There are numerous sites with impressive spark results from ignition coils though.
     
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