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Adding Short Circuit Protection

Discussion in 'Electronic Design' started by [email protected], Mar 1, 2007.

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  1. Guest

    I have a circuit board that provides 5 VDC power to some sensors,
    reads the analog outputs of the sensors and converts that to an RS-232
    signal. The problem is that I did not design in any type of short
    circuit on the power supply output of my board. Anyone have a
    suggestion to add short circuit protection to the existing board
    already made? I'm adding the protection to my new boards, I am
    concerned about the ones I already have in stock.

  2. Isn't that a good thing?
  3. maxfoo

    maxfoo Guest

    If you're that worried about your design, add a over-voltage crowbar circuit.
    Zener,fuse,scr, and a few resistors...
  4. John Larkin

    John Larkin Guest

    Kluge in some polyfuses, if you can afford a small voltage drop.

  5. w_tom

    w_tom Guest

    What creates the five volts? Most power supply ICs include short
    circuit protection - called foldback current limiting. Most all power
    supplies also have that feature.

    Meanwhile, if your design has no short circuit protection, then an
    overvoltage crowbar recommendation would only make things worse.
  6. Guest

    The power supply chips using MOS pass devices tend to only have
    thermal shutdown since there is no SOA issue with the MOS device. If
    so, the output oscillates under short circuit.
  7. Tam/WB2TT

    Tam/WB2TT Guest

    If he has a regulator on board, he could put the polyfuse at the input of
    the regulator.

  8. Rich Grise

    Rich Grise Guest

    That's the second time someone has suggested an "overvoltage crowbar",
    which sounds like the exact opposite of what the OP was seeking - I
    seriously doubt that a short circuit condition would cause an overvoltage!

    What he needs is a _current sense_ that shuts down the supply over a
    certain limit. There are as many ways to do this as there are designers;
    I'd crank up the supply by, say, .7V, and put a current sense resistor
    and a couple of transistors. Of course, you take your voltage sense
    _after_ the current sense, and some kind of latching circuit to shut
    down the supply - maybe a LED to indicate "I've been shorted and shut
    myself down" or something.

    I once did a short-circuit protector in firmware for an SCR
    phase-controlled, 24V 40A battery charger thatused a 68HC11. It was pretty
    cool watching that sucker pumping almost a kilowatt into a load, and when
    you short it, the supply goes "Bup" and shuts down on the next half-cycle.

    Of course, we didn't actually short the battery terminals - that's a
    whole different ball game. ;-)

  9. w_tom

    w_tom Guest

    The second time an overvoltage crowbar was mentioned was to disparge
    the OVC recommendation. The second time OVC was mentioned was the
    complete opposite of a recommendation.

    Meanwhile, that current sense (which is standard in power supply
    designs) is also called foldback current limiting.

    Generally when a power supply oscillates under high load, then a
    power supply is unstable. Power supply designers learn closed loop
    theory, and the poles and zeroes in Nyquist stability criteria so that
    the supply does not oscillate.
  10. Fred Bloggs

    Fred Bloggs Guest

    When you figure how to add ss protection to the new boards, put that
    subcircuit on a daughter board and screw it onto the ones in stock.
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