Moving arc inside fluorescent tube? why?

Discussion in 'General Electronics Chat' started by rudyauction8, Jul 26, 2015.

  1. rudyauction8

    Thread Starter Member

    Jan 27, 2012
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    So I've been experimenting with creating high voltages and have a circuit generating 1kv ac, 20khz, about 40 watts. I hooked it to a 20w fluorescent tube, and it lights, but the arc inside moves in a spiral motion along about half of the tube. I know I'm pumping more power than its rated for, it was a burned out tube from my old kitchen counter lights so I don't care if it will be damaged. Note that it does not heat up much. It doesn't appear brighter than a new tube on a normal ballast. I'm just curious why the arc is moving like this? I have a video on youtube here: any ideas?
     
  2. rudyauction8

    Thread Starter Member

    Jan 27, 2012
    252
    2
    Also this effect seems to go away after a few minutes.
     
  3. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Short (i.e. qualitative) answer: The arc follows the path of least electrical resistance (technically; the path of most readily ionized fill)-- said path being determined by, among other factors, convective flow within the fill gas --- hence the observed phenomena are owed, primarily to thermodynamic effects - but 'recursive' in that, owing to corollary heating and charge distribution, the arc path is both its own 'cause and effect', if you will --- Add to this the fact that, at 20kHz on a linear tube, there will be significant capacitive coupling to nearby conductors and, to a lesser extent, radiation effects...

    The arc stabilizes as the fill gas and tube wall approach thermal equilibrium/mean uniformity...

    Hope you find this helpful:)
    Best regards
    HP

    PS: I probably don't need to explain this, however by 'radiation effects' I mean that the plasma (i.e. arc), in small measure, acts as an antenna --- No worries about ionizing radiation!:):D
     
    Last edited: Jul 26, 2015
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  4. rudyauction8

    Thread Starter Member

    Jan 27, 2012
    252
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    Thanks that makes sense I've never seen this in normal fixtures but I am dumping about 40 watts into a 20 watt tube which probably makes the arc hotter and therefore creates more movement inside the tube which would cause the arc to move around. It's kind of fun to watch too.

    I'm familiar with radio waves but the arc acting as an antenna never occurred to me. That would explain why the tube glows dimly with only one end connected to power.
     
  5. RichardO

    Well-Known Member

    May 4, 2013
    1,231
    382
    I have seen the glow flowing in random patterns in cold tubes such as in an unheated garage. HP's explanation probably works for that situation as well. :)
     
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  6. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
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    This is just a guess. It looks to me like the moving arc is due to local heating, which makes the gas easier to ionize. I've seen this happen in old, weak tubes. As the gas ionized near the electrode, it heats gas in the local vicinity, which in turn ionized, heating the gas in its local, and ionizing it, which in turn heats more local gas, etc., thus creating a 'heat wave' of ionized gas.
     
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