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Spark gaps etched into PCB

Discussion in 'Electronic Design' started by Stefan Heinzmann, Jan 7, 2004.

  1. Hi all,

    I have seen in several devices that the designers included spark gaps on
    the PCB near I/O connectors. The pattern was a pair of arrows facing
    each other, with the solder resist cleared around the tips.

    My question is how effective those are. I would have assumed that the
    voltage where arcing occurs depends a lot on environmental conditions,
    and on whatever stuff has been deposited over time on the PCB surface.

    Is there any design data available regarding such patterns, such as
    recommended arrow shape and distance, environmental influences etc.?

    Is this a generally accepted practice or is its merit controversial?

    Cheers
    Stefan
     
  2. henryf

    henryf Guest

    Sounds like the designers were concerned about the I/O pins
    getting zapped with static electricity. I think the decision
    to use spark gaps is based on a cost/benefits analysis. The
    breakdown voltage of DIY spark gaps is a crapshoot, but they
    are essentially free so they get designed in anyway.

    I vaguely recall reading a spec in some ancient edition of the
    Handbook of Physics and Chemistry that led me to believe that
    the voltage needed to jump an air gap was around 10 kV per
    inch at low altitudes. You may recall from field theory that
    the "sharper" the point of the arrow, -- that is, the smaller
    the radius of curvature -- the better the spark gap works.
    And if you dig around PCB design specs, you can find
    recommended PCB spacings to *avoid* arcing under various
    conditions.
     
  3. Capoot

    Capoot Guest

    Cirris manufactures cable and wire harness testers that include high pot
    test function. This is their High Voltage Arc Gap Calculator:
    http://www.cirris.com/testing/voltage/arc.html
    Note that the calculator starts at 400 volts DC with an arc thru air
    distance of .001". It takes at least 327VDC to start an arc thru air. Any
    insulating material between the points will increase the breakdown voltage
    dramatically. If you can produce clean .005" gaps, you could shunt 1KV
    charges.
    Both sides should be a point to facilitate arcing. If one side is a point
    and the other a plane,
    the arcing threshold will be different from one polarity to the other.
    Ray
     
  4. I read in sci.electronics.design that henryf <>
    Do a Google search for 'Paschen's Law'.
     
  5. Do I understand rightly that this assumes a spark in free air? Is that
    directly applicable to a spark along a PCB surface that may be
    contaminated with whatever (dust, moisture, oil...)? I guess that
    besides arcing, creepage may have to be taken into account, too.

    Calculating the arc voltage is one thing, but I hoped I could get some
    more experience data on how the thing performs in practice, taking
    environmental effects into account.

    What I was looking for was rules of thumb such as (I'm speculating wildly):
    o Don't make the gap smaller than 4 thou or creepage will be a problem
    o Use a tip angle of 30 degrees
    o Place the gap where you don't get dust buildup
    o Breakdown voltage will be 200-800 Volts per thou
    ....

    You get the point

    Cheers
    Stefan
     
  6. mike

    mike Guest

    Tektronix had some laser cut spark gaps on ceramic on the inputs of logic
    analyzer probes. The number stuck in my head is 75V breakdown, but
    that was 30 years ago. Most of the things in my head from that
    long ago are long gone.

    For most solid state stuff, you can't get low enough breakdown to do
    much good.
    I'd recommend other means to protect your inputs.
    One thing that wide spark gaps can do for you is define WHERE
    the spark goes. Doesn't help your input much, but does prevent the
    arc from jumping to nearby unprotected stuff.
    Don't get carried away with small spacing. If it's covered with solder
    mask, it won't arc. If it's not covered, it will eventually short out
    with accumulated crud and atmospheric contaminants.

    Wonder what might be done with buried vias when you just don't plate the
    hole?
    mike
    --
    Return address is VALID.
    Bunch of stuff For Sale and Wanted at the link below.
    Toshiba & Compaq LiIon Batteries, Test Equipment
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    Tektronix Concept Books, spot welding head...
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
  7. Rick

    Rick Guest

    Hmm, seems like some company could make little 0603 chips with a
    hermetically sealed spark gap etched onto some ceramic substrate with a very
    small gap (less than .001" for sure) that would withstand crud and
    contamination (because it is sealed).
     
  8. SMT spark gaps exist, at least from Japanese suppliers, maybe others.
    They are sealed tubes.

    Best regards,
    Spehro Pefhany
     
  9. And you would be right! The added "bonus" is that if they do arc, the PCB
    get scorched ....
    It is crap, IMO, at best useless.

    Tranzorbs, MOV's or, indeed, Gas Filled Discharge tubes are the way to go:
    These devices are stable, specified with regards to energy/voltage/response
    time and they can be readily purchased for any voltage between 5 V and up to
    100's of kV!
     
  10. N. Thornton

    N. Thornton Guest

    Well, its a standard protection technique on the phone networks here,
    each house incomer has spark gaps fitted, and it works. Gaps have to
    be supplemented with other passives to complete the picture, but with
    those it works.
    Yes, and even quarter inch gaps will short out if you have HT on them
    for several years: the dust is attracted and forms fingers across the
    gap. Look in old TVs to see this.
    Interesting. The trouble with including PCB material in the discharge
    path is it will char, leading to rapid loss of insulation. But in some
    cases thats not a worry.


    Regards, NT
     
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