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Overvoltage protection for sensitive device

Discussion in 'Electronic Design' started by Hans, Dec 11, 2008.

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  1. Hans

    Hans Guest

    Hello,

    We have got a device which we prepare for marketing but there was a
    problem during testing so it did not pass necessary laboratory test.
    Problem is voltage durability e.g. device has test inputs (it's purpose
    is acquiring data based on measuring of bio-potencial) which were tested
    on 4 kV in duration of 10 seconds. Device goes up in smoke so we search
    for adequate overvoltage protection to protect it's input circuits.

    Voltage should not exceed 20-30 Volts, so I am interested for a
    schematic that possibly includes standard spark gaps or varistors (not
    any exotic or expensive components) so we can achieve protection on 4-5
    kV in duration over 15 seconds.

    If anybody has a solution please send some links or ideas.

    Thanks,

    Hans
     
  2. Joerg

    Joerg Guest

    10-15sec is a long time. First, most resistors aren't spec'd for more
    than 150-200V although you can get higher voltage versions from
    companies like Vishay. Then, take a look at the dissipation. Proper
    protection requires clamping diodes after that resistor so lets say they
    clamp at 20V, then the other 3980V are dropped across the input series
    resistance. Let's say you have 10K in there: 3980V^2/10kohms = more than
    1.5 kilowatts ... one second ... two seconds ... phssst ... phooof ...
    *BANG*

    Somehow this calls for a totally isolated design.
     
  3. Hans

    Hans Guest

    Thanks, Joerg

    what about spark gaps ? I suppose all nominal spark gap voltages are too
    high ? Device which I am trying to protect is not my design so I
    mentioned 20-30 Volts just out of precaution. I suppose I can get to
    max. 200 V. Is there a solution with spark gap only ?

    I must mention that tests are for electrostatic charge, so they cannot
    provide such currents.
     
  4. Perhaps you could explain more about the test.

    An electrostatic discharge event is over in nanoseconds, so how do you
    get to 10 seconds? Or do you mean there are multiple discharges for 10
    seconds? Is this one of the a standard "human body model" or "machine
    model" tests?
     
  5. Joerg

    Joerg Guest

    Generally, no. Spark gap performance varies greatly with tolerances,
    humidity and so on.


    Well, that won't be 10-15sec. All you need is to survive a few dumps
    from a pre-charged (small) capacitor. You'd have to provide more info in
    order to help, maybe post a schematic.

    The usual method for inputs is a series resistor and clamp diodes to the
    rails, then another smaller resistor to the pin of the chip. Or a
    transient voltage suppressor (TVS). Outputs can really only be handled
    with TVS unless it's not a very low impedance output.

    However, you might want to gently mention to your bosses that in order
    to make an existing design not fail ESD a re-design is usually required.
    You can get there with rework but it'll be ugly and production will most
    definitely not like that.
     
  6. PeterD

    PeterD Guest

    4-5 KV for 15 seconds? SECONDS? That would require one hell of a high
    input inpedence to avoid letting a lot of magic smoke out.
     
  7. If you want a sensible answer you must provide a schematic of the test
    circuit (eg. 100pF charged to 4kV with 1K5 in series) and explain more
    about your input circuit and source characteristics so we can
    understand how much series and shunt impedance and leakage current you
    can tolerate. Otherwise we're just guessing.
     
  8. Hans

    Hans Guest

    That looks also absurd to me, testing inputs with 4 kV. What is more
    absurd, the device is robust in design and always worked flawlessly and
    now is threatened to be discarded by security and quality standards.

    I do not have any schematic here, I will contact engineers about that.

    Thanks.
     
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