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(Newbie) Fuses with MOV, thermal or PCB?

Discussion in 'Electronic Basics' started by Ant_Magma, Mar 7, 2006.

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  1. Ant_Magma

    Ant_Magma Guest

    I'm looking for a fuse to protect my MOV which is to be placed across
    the line voltage (240VAC, Live & Neutral). The MOV has a rating of
    around 375V-390V.

    L--|Fuse|---|-------| |
    | |MOV |Protected|
    | | |circuit |
    N-----------|-------| |

    Questions:
    1) Since one of my components requires a current of around 500mA, i was
    thinking a 1A fuse should work alright (or should i go for a 0.63mA
    fuse?).

    2) At Farnell, there are listed 2 types of fuses (among many others)
    PCB and thermal fuses. Since i have close to zero practical experience,
    which 1 should i be looking at?

    3) These fuses are packaged with leads, does it mean whenever the fuse
    blows i have to desolder and replace it?
     
  2. It is generally not practical to protect an MOV from damage, since
    this occurs during moments when the line voltage is way above normal,
    where the fuses are rated to open. What the fuse will do is isolate
    the burnt out (shorted) MOV from the power line, after the voltage
    surge that damaged it, has passed.
    You need to also be aware of any inrush current that charges up
    capacitors.
    It depends on the inrush situation. Thermal lag fuses tolerate a
    brief over current better (longer) than fast blow types do.
    Yes. You will also probably have to replace whatever else shorted and
    caused the fuse to blow (MOV, rectifiers, across the line capacitors,
    after rectifier capacitors, or other loads). Having an MOV lowers the
    likelihood that other things will be blown as a result of a line surge.
     
  3. Ant_Magma

    Ant_Magma Guest

    Thanks for your reply John.

    You mentioned that it's not practical to place a fuse before a MOV
    because by the time the fuse reacts and opens the circuit, the surge
    would have already destroyed the MOV correct?

    It's kinda confusing because in many papers they recommend using fuses
    together with MOVs. Then what do you propose?

    Or instead of placing the fuse before the MOV, i place the fuse after
    the MOV? As a last like of defence in case the MOV fails?

    How do i take into account the in rush current? Can you give some
    examples? Such as for A amount of current, what type of fuse should i
    use and what to do about the in rush current?

    Thank you
     
  4. Rich Grise

    Rich Grise Guest

    No, no! All he meant was that the fuse won't protect the MOV - it _does_,
    however protect the house from burning down when the MOV goes short.
    Use a fuse under any circumstances, whether you go with MOVs, transzorbs,
    spark gaps, or hash chokes - you _always_ need a fuse.

    Good Luck!
    Rich
     
  5. Read it again. I said no such thing. I said that you should not
    expect the fuse that is upstream of the MOV to protect it from damage
    during by a high voltage surge.

    But the fuse is needed to protect the wiring from the effect of a
    damaged MOV, after the surge has passed.
    Fuse upstream of the MOV, but don't expect it to save the MOV. It
    will prevent your house from burning down after the MOV partly shorts
    and sits there glowing red hot.
    The best way is to measure it, but it can be calculated or simulated,
    approximately, if you have an adequate model of the entire input
    circuit.

    There are also things you can do to reduce the inrush, including line
    inductors, selecting a higher leakage inductance transformer, adding
    one or more NTC thermistors in series, adding resistance in series,
    etc., as applicable. These remedial actions may allow a faster
    response fuse that will protect more components from damage. But the
    real purpose of a fuse is to limit damage after something has failed.

    The first step may be to measure what the surge current is for your
    present design, when the line is connected at the peak voltage point
    (worst for capacitive loads), and when turned on at the zero crossing,
    with the incoming half cycle in the same direction as the last half
    cycle before the line was previously disconnected (worst case for
    transformer core saturation). Of you make lots of random phase
    measurements and look for the worst one that happens by chance, and
    hope you have seen close to the worst that can happen.
     
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