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Contact sparks

  1. Jul 15, 2012 #1
    I'm playing around with a circuit, I want to produce a spark reliably. If I take two wires and just barely touch them, then discharge a capacitor across the junction I get a nice spark. However this only happens once, maybe twice before the metals seem to fuse together at which point I cannot get a spark.

    I want to learn more about this, why is there a spark initially? Is it because the contact area is very small? And is there any way of building a junction that can produce this effect repeatedly?
     
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  3. Jul 15, 2012 #2

    Danger

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    Curl, my best suggestion without actually exerting myself if to suggest that you Google spark plugs, or at least spark gaps. I'm sure that anything arising from such searches will be easy to grasp. If further clarification is needed, check back with us.
     
  4. Jul 15, 2012 #3
    Those are inductor based which is not what I am interested in learning about.
     
  5. Jul 15, 2012 #4

    Danger

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    Since when?
     
  6. Jul 15, 2012 #5

    CWatters

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    Is it really a "spark" ?

    I suspect what you are seeing might be explosive melting of the point contact as excess current is passed through it. Bit like a fuse blowing. If the contact is improved it's ability to carry current improves to the point where it get's hot enough to weld the wire together but not hot enough to fully melt it.

    You need quite a high voltage to create a decent spark. If I remember correctly it's something like 10,000V to jump 3mm.
     
    Last edited: Jul 15, 2012
  7. Jul 15, 2012 #6

    sophiecentaur

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    Since IC (petrol) were invented - until 'capacitor discharge' systems were introduced just a few decades ago.
     
  8. Jul 15, 2012 #7
    There is a kind of contact spark that does not need very high voltage:
    When a metallic contact is opened (while current is flowing, of course) there is a spark. Designing (high power) switches has a lot to do with quenching this spark as fast as possible. There may be limits to this effect, but you don´t need more than a few Volts and a few tenths of an Ampere.
    AFAIK this spark starts as an arc (composed of ions of contact material and electrons) because of local heating when the contact area diminishes.
     
  9. Jul 15, 2012 #8

    Nugatory

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    That is a high-voltage spark - when the circuit opens there is a very large voltage spike when the current flow stops and the magnetic field around the wires collapses, and it is this voltage spike that drives the spark across the air gap of the open switch.
     
  10. Jul 15, 2012 #9

    sophiecentaur

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    When you put a spanner across the terminals of a car battery you can get a fat spark. The inductance of such a circuit it in the order of nH and the self resonance must be in the microwave region. That spark must certainly be caused by a high density current vaporising the metal just through heat. I really doubt that there can be a significantly high voltage generated in that case.
     
  11. Jul 15, 2012 #10

    Danger

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    I appear to have been unclear about what I meant. I wasn't referring to the cause of the electrical flow—just the material and construction of the area where the spark occurs. That's what I thought the question was about.
     
  12. Jul 15, 2012 #11
    Yes this is what I'm talking about. Is there any way to build something that can repeatedly produce such "sparks"?
     
  13. Jul 15, 2012 #12

    sophiecentaur

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    Can you separate the two issues validly?
     
  14. Jul 15, 2012 #13

    sophiecentaur

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    A large accumulator battery and some fat wire.
     
  15. Jul 15, 2012 #14

    Danger

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    Well, there's not a lot of difference between the spark plugs of a Model T Ford and those in an 8,000 horsepower top fuel motor with magnetos and capacitive discharge ignition.
     
  16. Jul 15, 2012 #15
    To make myself more clear, I want to build some junction where the spark occurs every time at the same spot, and I want to be able to trigger it by a switch. I built something like this on a breadboard with just 2 wires touching lightly, but the problem is after 2 sparks it stops working (because the wires get welded together and no longer spark).
     
  17. Jul 15, 2012 #16
    If you want to design a circuit in which the spark jumps a gap it has to be either alternating current or pulsed DC at very high voltages. Straight DC will not jump a gap, it has to be initiated with the conductors coming into contact.
     
  18. Jul 16, 2012 #17

    Bobbywhy

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    For spark plug design, see:
    http://www.madehow.com/Volume-1/Spark-Plug.html

    For High Voltage electrostatic spark gaps, see:
    “A spark gap will have a very repeatable breakdown voltages for a given atmospheric conditions. For mostly mechanical reasons, uniform field gaps (using, for example Rogowski or Bruce profile electrodes) are not used as much as sphere gaps where the spheres are quite a bit larger than the gap.”
    http://home.earthlink.net/~jimlux/hv/sphgap.htm
     
  19. Jul 16, 2012 #18

    sophiecentaur

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    As has been mentioned already, this sounds just like a petrol engine ignition system - using spark plugs (which last for millions of strikes). But if you want a low voltage arrangement, the problem is different.

    If you could cut off the current each time, before the contacts weld, that may achieve what you want. This wouldn't be too hard to achieve with a pulse from a fat power transistor in the circuit, to control the current. The pulse length would need to be increased gradually to maximise the energy supplied to each strike of the spark.

    Alternatively, if you look at the way the old carbon arc lamps (cine projectors) used to operate, you might get some ideas. The arc was struck by bringing the carbon electrodes together and then separating them until the current was what was required. There was then a servo'd system, involving a motor which slowly brought the carbon rods together at the appropriate rate to maintain that current as they eroded.

    Because the arc that you need is a non-linear resistance, I think you might consider controlling the separation of the electrodes in some way. This would not be difficult - just not what you had in mind - involving a solenoid, which could achieve the movement. If you don't actually separate the electrodes, once the arc has struck, you will risk the contact welding together permanently (for fat electrodes) or melting so that the spacing is too great (for a thin electrode) for another arc to strike. This is exactly what you do when you manually 'dab' two wires together to stimulate a spark.

    You will need to think 'laterally' for the best, suitable solution.
     
  20. Jul 16, 2012 #19
    I worked with a test rig for circuit breakers as an intern long ago. I don´t remember wether it was pneumaticlly driven or spring loaded; the electrical part was a capacitor discharge triggered by a SCR.
    You could scale that down for "household use". Triacs and SCRs have pretty good pulsed current ratings (rule of thumb: 10x continous current) and you can easily get 25A continous or higher rated types. Use some big relay and trigger the relay and the scr at the same time. If you start with a 30V 5000u capacitor you´ll probably see some effect.
    (You´ll want to add some charge control for the capacitor, a current limiting resistor and a few other things, but you get the idea).
    As has been mentioned, relay contacts are not designed for massive sparking/arcing. This is why the current and voltage rating for non-ohmic and DC loads are considerably lower than those for ohmic AC loads. (The zero crossing in AC will extinguish the spark). So you´d better expect frequent failure, unless you use a higher powered mechanical drive.
    I don´t see what you want this for. Is it for a millisceond spark or a continous arc? The ignition system in a car has become very much more reliable with the elimination of mechanical elements, so why not use a proven system?
     
  21. Jul 16, 2012 #20
    I want just a milisecond spark for an experiment I'm doing. I'm done trying the "car ignition" way because I don't want to carry around a 20lb ignition coil. I want something that's small and I just want some sort of spark, doesn't have to jump through air or anything it just needs to work repeatedly so I can run the experiment over and over.

    Any ideas on materials? If I build some triangle/cone type electrodes out say, brass, and touch the tips, then discharge a rather large capacitor on that. Would that work?
    I'm not sure how carbon electrodes would work in this case, it seems to me like they wouldn't make a spark at all. I'm more interested in what makes the "spark" in the first place, is that just hot metal being eroded out by the large current or what?
     
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