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Keep On Arcing...

Discussion in 'Electronic Basics' started by Ron Hubbard, Jan 14, 2006.

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  1. Ron Hubbard

    Ron Hubbard Guest

    I am working on the strobe light found at and got it mostly finished when
    I decided to test it a bit. The strobe flashes okay, but the flashtube
    stays on, arcs, for a few seconds after each flash. I used a 400 V, 82
    uF cap for the main cap to limit the intensity down to fit the

    Does anybody know why the flashtube arcs like it does? Thanks?

  2. John Fields

    John Fields Guest


  3. Your selection for R3 is too low in resistance. After the main flash the
    voltage on capacitor C3 (which is also across the arc tube) might be
    something like ~50 volts. The voltage on the other side of R3 (which is on
    C1 and C2) is more like 320V. So, (320V-50V)/R3 is the DC current that can
    flow immediately after the flash.

    However, the arc tube can't reliably stop arcing if a large continuous
    current is available to it. If you increase the resistance of R3, this
    should make it more reliable. Unfortunately this will also mean maximum C3
    charging current will be reduced, and will mean flashes will have to be less
    frequent. Unfortunately that is the nature of that circuit as drawn.

    You can improve the performance by using an active switch to completely
    terminate C3 charging current immediately after a flash. This might be
    implemented by placing say an N-channel MOSFET of suitable current and
    voltage ratings (perhaps IRF720 or better) between the ground linkage in the
    schematic that goes from the negative side of C3 and the negative side of
    C2. You would want to drive it such that it is turned fully on between
    flashes but off for at least a couple of milliseconds immediately after each
    flash. With this approach you can shrink the resistance of R3
    substantially, thus resulting in very fast C3 refresh allowing very high
    flash rates without problems of arc tube quenching.
  4. Ron Hubbard

    Ron Hubbard Guest

    I had my doubts about the circuit design in the first place but I
    couldn't quite put my finger on it. Thanks for the tips . They are very
    much appreciated.

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