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Electrostatic Current Monitor

Discussion in 'Electronic Design' started by Mike Monett, Sep 14, 2008.

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  1. Mike Monett

    Mike Monett Guest

    Monitoring the current in an elecrostatic air filter can be a
    problem.

    If you use an external dvm in series with the high voltage lead, you
    are exposed to a potential arcover that could give a very bad jolt,
    or even cost your life.

    If you put the dvm in the ground lead, accidentally removing one of
    the leads can place dangerous voltages on the remaining connections.

    US patent 7258729 has a neat solution. It uses a NE-2 bulb is in
    parallel with a capacitor, which goes in series with the high
    voltage. (It probably should go in the ground lead to help minimize
    corona from sharp points and small lead diameter on the
    connections.)

    The capacitor can be adjusted to suit the expected current.

    The patent uses a 0.047uF cap. Assuming the NE-2 fires at 90VDC, and
    the current drain is 1uA, the bulb would fire every

    dt = 0.047e-6 * 90 / 1e-6
    = 4.23 seconds

    which is fast enough to monitor visually.

    This is a very simple solution to what can be a difficult problem.

    Best Regards,

    Mike Monett
     
  2. Mike Monett

    Mike Monett Guest

    Sorry, typo. Should read

    dt = C * dv / I

    But you knew that already:)
     
  3. Mike Monett

    Mike Monett Guest

    Yes, I realized that just after posting. Searching google for the
    extinguishing voltage gives values all over the map. But I think the
    bulb will light at about 90V and go out around 60V. So the equation
    should be

    dt = C * dv / dt
    = 0.047e-6 * (90 - 60) / 1e-6
    = 1.41 seconds

    or thereabouts. The small voltage drop should have little effect on
    a 12KV electrostatic filter.
    I mentioned it should go in the ground side to prevent corona. The
    firing rate will vary with different bulbs, and probably other
    factors such as temperature. The cap will also have a tolerance. And
    you monitor the firing visually, so it's not going to be anywhere
    near the accuracy of a dvm.

    But it's just a monitor, basically to alert you to problems. The
    current drain of an electrostatic filter can vary by many orders of
    magnitude, depending on leakage on the insulators from dirt or
    contamination, ambient humidity, how much corona there is from sharp
    points, etc. So it's more than good enough for the application.

    My biggest problem is mounting the filter in the window and trying
    to keep the rain out. Raindrops hit the window ledge and make a fine
    mist. This gets sucked into the filter and shorts the high voltage
    to ground. I was trying to measure the voltage by holding a grounded
    lead near the electrode and estimating how much gap it would jump.
    But the loud bang made me nervous every time it arced over. The NE-2
    seems like a much better idea. Simple, reliable, and much quieter:)
    Mike Monett
     
  4. Guest

    just put the ne2 in series with the unit's psu and monitor with a
    photodiode, we used that technique to monitor currents down to 10 nA
    or lower in a electrostatics lab, when we needed a floating fiber
    optic current sensor. Or put a 100K ohm 1/4th watt resistor in
    series with the device and put a 10 nanofarad ceramic cap across it
    to protect the meter from the turnon surge. We also made floating
    meter assemblies in plastic boxes with the RC circuit.

    Steve Roberts
     
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