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Short Circuit Protection

Discussion in 'Electronic Design' started by Rileyesi, Apr 27, 2004.

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  1. Rileyesi

    Rileyesi Guest

    I have a product that I want to add short circuit protection to. The system
    measures analog voltages from several sensors. Some of the sensors are self
    powered and some require external power. There is a PCB that provides the
    power to the sensors that need it and reads the signals from both styles of
    sensors. I am using ADC on a micro to read the signals.

    The power supply scheme I am using has one source to power the sensors, micro,
    and it also acts as the reference for the ADC. I did this to be sure that
    there was never a time when the signal is a greater value than the reference
    for the ADC.

    My world collapsed when one customer shorted the power supply driving the
    sensors. This cooked part of the PCB.

    I am looking for a scheme to provide short circuit protection without
    sacrificing the accuracy of the system. Ideally, I would like this to be
    external to the PCB so I can have all my customers retrofit the systems that
    are already in operation.

    I hope this is clear!

    Any ideas would be appreciated.


  2. Ah, now you start to understand (or your customers do) why commercial data
    acquisition boards cost so much.

    Is there a good reason, other than cost, not to use one? Because I can
    guarantee you that by the time you've solved all the same problems they
    have, your boards will cost even more.
  3. Rileyesi wrote...
    Any time a power-supply voltage is made available externally, there
    should be adequate short-circuit protection. For example, with an
    ordinary +5V power line, an easy way to obtain this is by adding a
    low-dropout-voltage (LDO) regulator IC in series, like an LM2940,
    etc. This will add a small extra voltage drop, e.g., 50mV for 50mA
    load, or 200mV for 300mA load, etc., which will acceptable because
    it's less than the poor tolerance of the 5V supply to begin with,
    but it'll provide you with a protective short-circuit current limit.

    If the sensor power provided must be precise, like for a ratiometric
    sensor such as a strain gauge, the LM2940-5.0 is accurate to 5%, but
    other parts are better, such as an LP2950A at 0.5%. However, to be
    safe you'll need to provide this with a higher input voltage (the 5V
    source might actually be lower than 5V, creating a problem), even as
    much as 12 or 15V if that's all that you have available. In this
    case, to keep the precision-regulator dissipation low, you may want
    to pre-regulate to say 6V with another IC like a 7806.

    - Win

    whill_at_picovolt-dot-com (use hill_at_rowland-dot-org for now)
  4. Rileyesi

    Rileyesi Guest

    Ah, now you start to understand (or your customers do) why commercial data

    There are some very "application specific" requirements and I could not find an
    off the shelf solution.

    Sometimes life works out that way!
  5. What do you mean 'cooked', blown traces? The simplest form of
    protection might be a glass fuse, or a more sophisticated
  6. Nice trick, to use a LDO regulator for that.
  7. mike

    mike Guest


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  8. Agreed. Just checking. Too often I start out thinking "oh, I can make that
    for a lot cheaper, I don't need all the bells and whistles" - but then
    people start asking for the bells and whistles, and I end up reengineering
    something that I could have bought off the shelf. But if what's available
    off the shelf doesn't meet the needs (as opposed to being overkill), then
    that's a different problem.

    In addition to the suggestions others have posted, you might be able to get
    some ideas from the published specs/schematics/block diagrams of
    commercially available data acquisition cards.
  9. I've seen 3-terminal regulators used as current
    limiters on a multi-output system. ie, They
    had a 24V regulated supply powering a set of
    24V regulators. In normal operation the regs
    ran bottomed, with just a small voltage drop
    across them, but with a fault current the reg's
    overcurrent/overtemperature mechanism did the
    req'd limiting.
  10. budgie

    budgie Guest

    Good scheme, but why not put the limiting reg UPSTREAM of the "regulating" reg.
    That way, you still get protection but without droop.
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