EEVblog #678 – What is a PCB Spark Gap?


In this tutorial Dave explains what a PCB spark gap is and how it can be a useful zero cost addition to your PCB layout to help prevent ESD damage.
He shows how to easily design them into your board and calculate the approximate voltage rating.
And of course has some fun applying 5kV to some gaps to show how them at work.
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    • Ivan Berton

      Nice video. It would be interesting to know how long it takes until it arks over. From the point where we reach the breakdown voltage until it begins to spark. Air between the sparking points need some time to ionize and be able to conduct current through it. If the time is too long and your device which you want to protect is very sensitive to over voltage…then, maybe we loose the goal.

      • nick

        For a data point, if you are testing to IEC61000-4-2 the rise time on the initial HVDC pulse is .7 to 1nS. That’s a problem for a lot of ESD protection ICs.

        • Ivan Berton

          Yep,this is why we should take care of a proper design. For eg. set the breakdown voltage of the protection circuitry a lot deeper than the breakdown voltage of the device to protect. Multiple protection mechanism which causes a lot of other problems such as impedance adaptation failure, noise figure increasing, losses where you don`t need it etc. etc…..mmmhh maybe I should become a cook

    • Sławek T.

      As said, it’s a good protection because it’s probably the cheapest possible solution to the ESD problem. Normally, you’d put simple TVS array or a couple of GDTs to do the job. I used these spark gaps in one of my projects, but obviously forgot to remove the soldermask… Now I’ve found my mistake, thanks, Dave 🙂
      Why do you always say thous instead of mils? Is it an Australian term?

      • tlhIngan

        A thou (thousandths of an inch) is the term used worldwide – mils are used primarily in the US only. The reason for this is “mils” is VERY close to “mill” which is short form for millimeter. So something that’s 3 mils may refer to 3 thousandths of an inch, or 3 millimeters.

        So most of the world use thou to avoid the confusion, while the US designs something that fails because of confusion over units.

        http://en.wikipedia.org/wiki/Thousandth_of_an_inch

        • Sławek T.

          Thanks for detailed explanation.

    • nick

      Continuing with the poor man’s input protection theme: How effective would it be to try to make the traces between the IC and the connector longer and more inductive by making them wavy or serpentine – in addition to having the spark gap? Generally speaking I try to stick a TVS to ground down at the connector and then add a resistor or inductor between the IC pin and the connector (depends on what type of signal is on that trace of course) so that when the TVS breaks down it will be a lower resistance path to ground + I’d get some voltage drop across the resistor – but that’s assuming a can spare a few cents for some extra parts!

    • pugglewuggle

      I’d really like to see a video or series of videos on input protection inputs just as you suggested. It’d be great to see this for both analog and digital (high speed and low speed, busses, etc).

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