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Spark Gap Voltage transients...

Discussion in 'Electronic Design' started by booth, Jan 10, 2007.

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  1. booth

    booth Guest

    Dear All,
    We are discharging a High Voltage 26 kV 40 nF Capacitor through a
    Spark Gap. The device is used for medical purposes. The discharge is
    causing transients (ringing) on the supply voltage and this affects all
    of our Circuitry. ( Resets on Microcontrollers, sometimes eploding
    components).
    Any Idea how we can reduce the voltage transients? Also is there any
    way to prevent the transient reaching our circuitry. We tried all kinds
    of optical isolation. Unfortunately, as the transient is on the supply
    line all circuits are affected.
    Please please please help.
    Thanks in advance.
     
  2. There are perhaps, three main coupling paths. There is the
    magnetic field produced by the discharge current. There is
    the electric field change that takes place around the
    exposed voltage changes and there is a conducted path back
    through whatever supply charges the capacitor.

    You can minimize the magnetic field magnitude by keeping as
    much of the current coaxial as possible (current going one
    way concentric with current going the other way. If this is
    not possible, a twisted pair of paths is almost as good.
    Avoid any unnecessary open area loops that the discharge
    current must encircle.

    You minimize the electric field effects by enclosing as much
    of the high voltage system as possible with a shield that
    returns its capacitive current back to the source, the
    capacitor. If the case of the capacitor is grounded, the
    shield should also be grounded at the case.

    The best way to keep the discharge pulse from heading back
    down the supply wiring depends on the supply details, so I
    will have to let that go for now.
     
  3. default

    default Guest

    So you developed an EMP machine and wonder why it eats electronics?

    A lot will depend on the physical layout.

    Exploding components sounds like the brunt of the problem is in the
    supply electronics - you don't give any information on how the 26 KV
    is supplied, AC or DC etc..?????????

    In Tesla coils we deliberately discharge large HV caps into inductors
    to produce high frequency ringing in the inductor. To keep it out of
    the mains supply (where it will destroy other electronics) there's a
    pair of air core inductors and a "safety gap" just before the source.

    When the Tesla coil is in tune - no problem most of the energy is
    transferred into the load (sparks and heat in the air and on the
    output terminal or corona). When out of tune, the standing waves are
    reflected back to the source and that's when the damage occurs, or the
    safety gap starts firing.

    I'm guessing that you have the same system with no load - maximum
    energy reflected back to the source. A few chokes and filters and a
    safety gap should get it to the point where it won't eat anything you
    want to keep. You may still have EMI problems but less destructive
    ones.

    In Tesla coils I'd use an air core choke in preference to ferrite -
    because the ferrite ones would arc across the windings (through Teflon
    insulation) but your problem may be different in that respect.

    What would be nice to know is AC or DC? what is the source? what
    power level? How often does the gap fire? What is the goal?
    Diathermy, lasers, induction heating?

    You may not be intending to build an RF transmitter, but anytime you
    discharge high voltage from a capacitor into wires you have an RF
    transmitter. You have to start thinking like an RF engineer and
    concentrate on getting the power to dissipate in the load and not back
    at the source.
     
  4. booth

    booth Guest

    The capacitors are charged through a HV Power Supply, its Lambda EMI's
    102A. From looking to the schematics I see diode bridges on the TR's
    secondary, so its DC. An Inverter is present at the The primary. The AC
    Input is 220V AC.
    The Caps are discharged every 0.5 s. through a spark gap. All the HV
    section and the cable (which has to be 2 mts) are shielded.
    The application will be ESWL (Targeting Kidney stones.).
    Yes we do have EMI problems, but the shielding reduced them a lot, but
    the transients are really killing us.
    Any but any help will be appreciated...
     
  5. default

    default Guest

    The ideal situation for a run of 2 meters would probably be a shield
    over a twisted pair of wires carrying the HV and return out to the
    cap. The cap and spark gap should be close to the point of use
    (ideally) - that prevents that whole wire from becoming an EMI
    radiator. Is that practical?

    Just behind the cap the twisted pair (shield and all) should wind
    around a ferrite core a turn or two.

    If the cap has to be located at the machine - a ferrite or two at
    either end of the cable may help the EMI problem but may hurt the
    production of high energy shock waves.

    The shield should be grounded at one point only - at the machine -
    otherwise insulated from everything else

    The patient, tables, ultrasound wiring, etc. should be at ground (from
    a radiation point of view - safety is your problem)

    Another consideration with DC is that there be no exposed wires with
    sharp edges that can float ions into the surrounding air. Any
    conductive surface that is insulated from ground can build up quite a
    static charge - and that includes people. Build up a charge and walk
    over to a terminal or ground and you may damage some sensitive
    electronic gear. If the whole thing is shielded that shouldn't be a
    concern.

    So best idea is twisted pair with a shield and cap at the point of use
    and ferrite core just behind the supply cable as close to the cap and
    gap as possible. The twisted pair alone should make some difference.

    What does the spark gap look like? Is it also shielded?

    And for my own edification: How is the sound coupled into the patient?
    Plastic window? Is the sound focused?

    Can something like a barium titanate (ceramic piezoelectric) type
    transducer serve the same purpose? (at lower voltage with a SCR
    switch to fire it?)
     
  6. Dear booth,
    One path will be the switch that isolates the capacitor from the charge
    unit. There you have a parasitic capacitor even if it is switched off. only
    a few pF will be enough to transfer a lot of charge through it. Try to make
    2 switches with an aditional cap vs 1nF between:cap 1nF between them like
    this:

    |-----------switch----+-----switch----+------------|
    | | | |
    | | | °
    HV-Supply --- --- Gap
    | --- 1nF --- 40 nF ^
    | | | |
    | | | |
    |-----------switch----+-----switch----+------------|

    The switches in the ground may help too. All your wires have inductance. And
    if the capacitor is empty the current try to find a way to flow... may be
    not only through your 40 nF cap. If you use f.ex. only a diode as a switch
    to charge, this switch will be wide open for the negative swing of each
    ringing.

    Good luck

    Marte
     
  7. Michael

    Michael Guest

    Just for everybody's information. I measured (or rather estimated)
    dv/dt of the spark discharge.
    ~1us wide 10kV (measured by Tek P6015 probe) pulse is generated by
    small trigger transformer. The breakdown takes place somewhere around
    5kV (half way to the top).
    The falling edge was picked up by Tek P6139A probe sitting ~1" away.
    TDS3054B was used to record the data.
    dv/dt was ~2-3kV/ns (yes, kilovolt per nanosecond!).
    This will send 2-3A through 1pF capacitor!!
    I have a sneaky suspicion that actual dv/dt is higher as my
    measurements were almost at the limit of the instrumentation I used.
     
  8. booth

    booth Guest

    Dear All,
    Thank you very much for all the help. Apart from shielding, I wonder
    if I can Isolate the system from the HV Supply. What if I place a relay
    at the input of the HV Supply (220v) and break the phase and neutral
    just miliseconds before the HV Capacitor discharge happens. Do you
    believe this can prevent the transient to reach the system?


    Marte Schwarz yazdý:
     
  9. I doubt it. It will probably just cause an arc across the
    open contacts. I think you need to draw a full schematic of
    your system, with all wiring inductance and capacitance to
    all surrounding conductors added as explicit coupling
    components, so you can begin to unravel where energy is
    going and how you might detour it around the sensitive
    parts. Stabs in the dark are very unlikely to arrive at an
    optimal solution.
     
  10. LVMarc

    LVMarc Guest

    I have actually worked on 500KV 50 nS pulse width Pulsed power units for
    flash radiography. I am sure I can help your group.
    Contact me.

    .

    Marc Popek
     
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