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Fastest AC Electronic Fuse

Discussion in 'Electronic Design' started by D from BC, May 14, 2007.

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  1. D from BC

    D from BC Guest

    This is probably an oldie question on here...

    I'd like to break the AC "hot" if there's too much AC current.
    This would be due to a short or ground fault.
    Shunting the load to burn a fuse on the AC line would be ok too..

    I'm afraid that a fuse maybe too slow to protect the electronics from
    burn out..
    Let's say the electronics I'm protecting cannot withstand 1.5x

    What's faster than a fuse on a AC line?

    Load: The load on the line is not a transformer and is not inductive.
    Current draw: 2Amps
    Line voltage: 240Vrms, 60Hz
    Surge rate: guessing 45Amps in 1uS

    Rule: 1/4watt current sense resistor.

    The load circuit has a power bridge rectifier.
    Maybe I could modify the bridge with transistors to interrupt the
    surge current?
    D from BC
  2. Jim Thompson

    Jim Thompson Guest

    Years ago I used an SCR to blow fuses (on the load side). I think it
    was in the microsecond range. Warning, traces need to be fat,
    otherwise you blow traces instead of fuses... I had ~100A circulating
    momentarily to blow a 1 Amp fuse ;-)

    ...Jim Thompson
  3. John Larkin

    John Larkin Guest

    One of our customers recently asked us to do that. He needs,
    basically, an SSR that also acts as a programmable circuit breaker
    *and* a current limiter. He needs to simulate cable faults and switch
    power circuits without exploding anything. We've been scribbling
    circuits for days, and it's non-trivial. We just a few minutes ago
    finished whiteboarding the design of a breadboard pcb so we can test
    the dynamics.

  4. John Fields

    John Fields Guest

  5. There's some work already out there on IGBTs re shorts into hundreds
    of volts. IIRC, it's bad to try to turn it off too fast (or too slow,
    obviously), plus there are some other subtleties.

    Best regards,
    Spehro Pefhany
  6. Tim Williams

    Tim Williams Guest

    I drew a circuit here
    which someone built, and evidently works quite nicely. Maybe you could
    change the MOSFET to two back-to-back so they'll handle AC. Which I also
    had to draw for someone it seems: MOSFET Switch.gif

    Alternately, you could put a single FET across the DC terminals of a FWB.

    Obviously, the +12V supply needs to be floating. It could possibly be
    provided by a resistor+capacitor and diode from the AC terminals of the
    thing. Interfacing external control/feedback would probably be best done
    with optoisolators...

  7. D from BC

    D from BC Guest

    Wow!...peaking to 100A before blow out.....

    I guess it takes time for the fuse element temp to rise to the
    melting/vaporize point.
    D from BC
  8. D from BC

    D from BC Guest

    To break a DC circuit, a pass transistor can be used.. Easy...

    But breaking 240V AC...huhhh.. :(

    I might look at using a little transformer to sense a current surge
    and have that fire a shunt circuit until a fuse blows..

    D from BC
  9. D from BC

    D from BC Guest

    I might have to blow up some stuff to find out. :)

    I'm just interested in rough ideas for various levels of cut-out
    Doesn't matter what cost, complexity or parts count.

    At the moment, the faster ...the better..

    D from BC
  10. D from BC

    D from BC Guest

    mmm...makes sense to see transistors to shut down the mosfet..
    Op amp current sensing might be considered generally slower..
    The cct looks like it has a ~6A trip level....
    It's easy when a sense resistor has enough V to bias a transistor.

    Using a floating circuit to control a push pull pair of mosfets
    looking interesting..

    D from BC
  11. D from BC

    D from BC Guest

    I'm wondering what's more explosive...
    Slowly sneaking up the current through the fuse until it blows...
    suddenly connecting the fuse across a 300V charged 100uF low ESR

    Should I worry about fuse shrapnel and BANG! when it comes to fuse
    D from BC
  12. Jim Thompson

    Jim Thompson Guest

    This was a conventional fast-blow fuse. It flashed like a flash-bulb
    (but not as much light) and made a tiny "click" ;-)

    The filament was completely plated out onto the glass.

    After we cracked a few, we went to the ceramic tube type.

    ...Jim Thompson
  13. Definitely the second. In the first case, the alloy just
    quietly melts. In the second, it vaporizes.
    If you exceed the fault current rating of the fuse, all bets
    for containment are off.
  14. Fred Bartoli

    Fred Bartoli Guest

    D from BC a écrit :
    How about an AC current limiter?

    | |
    | |
    | .---+---. |
    | | | | |
    .---+ | .-. .-. |
    | | | | | | | |
    | +-|| .----+ | | | | |
    - ->|| | | '-' '-' -
    ^ +-||--' >| | | ^
    | | |-+ +-|<-+---. |
    o-----+ +--. /| | | | | +-------o
    | .-. | | | | | | |
    - | | | ___ | |/ --- --- z -
    ^ | | '-|___|-+-| --- --- A ^
    | '-' |> | | | |
    AC in | | | | | | | AC out
    | |
    | |
  15. D from BC

    D from BC Guest

    At first I couldn't figure this cct out..

    The only way for current to pass is if the N Ch mos turns on.
    Otherwise the diodes block..
    I see the bridge rectifier in this circuit..

    I see the negative feedback loop from the Rsense through the NPN-PNP
    compound and back to the MOS that limits the AC current..

    Is the AZ a zener?

    The circuit looks fast..

    The only thing I don't like about this cct is that the current sensing
    resistor will be dissipating a nominal 1.2Watts in my app..
    0.6V*2Amps= 1.2Watts..
    To use a smaller current sense resistor, I could use an op amp, but
    I'll be trading off speed...
    However...I might be able to come up with a transistor arrangement
    that uses a smaller (cooler) sense resistor..

    Maybe I might try an SCR to shut down the mosfet...
    Or CMOS logic...
    It wouldn't be a limiter anymore but a breaker..


    Another circuit I'm looking at is,M1
    Triac blowing line fuse.
    I think this also has a hot current sense resistor (If I=2amps and
    Itrip = 3Amps).
    D from BC
  16. Fred Bartoli

    Fred Bartoli Guest

    D from BC a écrit :
    The positive FB loop between the NPN/PNP bjts make the 'limiter' trip.
    With the right time constants, the *small* bypass cap voltage gets down
    to the point where the 'SCR' self resets. This obviously has to happen
    within one line period (before injecting a new current pulse through the
    series RC).
    Yep. Limits the gate voltage and provides a path for the RC branch
    'negative half period' current.
    Should be.
    Can be fixed with just one more resistor.

    It is already. Well a periodic, self resetting one.
  17. Ian

    Ian Guest

    "Fred Bartoli"
    The power dissipated in the resistor is less than that, and as Fred says
    can be fixed easily (it isn't steady state, more like a sawtooth).

    I used something very similar as an inrush limiter on a SMPS a many
    years ago, and got bitten by ** changing the fab that supplied the
    Mosfet just after we went into production. I forget the details, but it
    was to do with wimpy metallisation to the Mosfet gate - to turn the
    Mosfet off fast you have to pull the gate charge back out, which
    takes current.

  18. Ian

    Ian Guest

    The fastest fuse in the world is an expensive microwave transistor.
    Works a treat to protect line fuses.

  19. Fred Bartoli

    Fred Bartoli Guest

    Ian a écrit :
    But you'll have to call your power company so that they speed up their
    generators quite a bit.

  20. I did tests, for example, on 5A 250V rated fuses. The 20mm glass
    variety would literally explode (a VERY loud bang) sending shrapnel
    everywhere, just on 240VAC/60Hz with an industrial source capable of
    enough source current.

    Best regards,
    Spehro Pefhany
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