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How fast is an MOV? Really?

Discussion in 'Electronic Design' started by Tim Williams, Apr 5, 2009.

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  1. Tim Williams

    Tim Williams Guest

    This sounds like something John Larkin would do...

    Do MOVs respond instantaneously (i.e. as soon as the electric field
    appears across the faces of the metal-oxide stuff) or do they have
    some fundamental response time which is slower than the RLC of the
    geometry? Could you, for example, build a waveguide (matched to the
    MOV's dielectric, obviously) which is able to mostly block radar
    pulses for the radar reciever? (I know, they use different approaches
    to solve this problem, but just for an example anyway...)

    For that matter, avalanche diodes can, well, avalanche under peaky
    conditions (making some excellent steps, at that). Do MOVs have any
    negative resistance characteristic, or are they only plus-resistive?

    Tim
     
  2. Use a "Transzorb" instead.
     
  3. Eeyore

    Eeyore Guest

    Considered a couple of TVSs ?

    Graham
     
  4. Greegor

    Greegor Guest

    Archimedes' Lever >  Use a "Transzorb" instead.

    What's the response time, cost and lifespan?

    Googling transzorb was somewhat unsatisfactory.
     
  5. Tim Williams

    Tim Williams Guest

    Uh, it was a theoretical question.

    Tim
     
  6. I usually add a series inductor/ferrite bead. Also sometimes
    substitute a bidirectional TVS so as to prevent high currents if
    polarity reversed, with a series diode.


    -[polyfuse]-------[FB]-----|>|--------
    | |
    TVS ---
    | ---
    | |
    --------------------------------------

    The TVS limits an ESD event to a few tens of volts. The FB/cap
    flattens out the remainder.

    With a "mains filter" configuration of chokes and capacitors?


    John Devereux
     
  7. We used to put filter/trap beads right on the legs of the devices on
    some of our designs
     
  8. bud--

    bud-- Guest

    Measuring response time is a problem.
    A technical paper at:
    <http://www.eeel.nist.gov/817/pubs/spd-anthology/Lingering legacies.pdf>
    may be of interest.

    A major theme is that the current through the protective device will
    induce a current in adjacent circuit loops. One loop is even a small
    loop from unshielded scope leads to the device. Rather extreme measures
    to eliminate the loop were used, and the resultant scope trace shows no
    reaction time visible for a MOV with a trace of 2 us/div.

    The authors are interested largely in power circuits and extremely fast
    rise times are reduced by source wiring impedance. Pulses "have to
    travel only a few meters away from their origin to have their rise time
    and duration stretched into tens of nanoseconds or more". (But that may
    not help with nuclear EMP.)

    The same induction into a loop adjacent to the protective device would
    transfer energy into a downstream circuit unless departing leads were
    twisted.

    The paper also looks at lead length for power panel suppressors - lead
    length (primarily inductance) increases the effective clamp voltage.
     
  9. westom

    westom Guest

    Response time from an MOV is so fast that impedance even on its two
    inch wire leads causes it to perform slower. Impedance is also why
    effective protection is about connecting the MOVs as short as possible
    to earth. If two inch leads affect the response of an MOV, then
    better to keep any shunt mode protector connection to earth to less
    than single digit feet. So that its response time decreases and so
    that shunted transient currents away from electronics are maximized.

    Other transient protection devices have the same wire impedance
    problems when measuring their response times.
     

  10. Exactly. It is meant to 'fail' or 'short' enough to open the fuse.

    It isn't really meant to shunt an overvoltage. It is meant to cause a
    breaker to open because of its reaction to an overvoltage. In that
    sense, power sources feeding equipment failures from open fuses should be
    investigated where MOVs are used in the front ends of such devices. They
    are like alarms for your power condition.
     
  11. Rich Webb

    Rich Webb Guest

    Yup, I've had a similar incident. Came home and noticed that
    unmistakable odor of crisped electronics. Fortunately, the MOV in
    question was in a metal-cased power strip and the breaker tripped before
    anything nastier happened.

    Must have scared the cats out of a life or two, though ...
     
  12. MOVs do not have cumulative damage from absorbing energy, at least
    nowhere near linearly cumulative. An MOV rated xx joules can take xx
    joules in an event short enough to give it little chance to cool before it
    is done absorbing energy. That same MOV can take a lot more than xx
    joules in smaller events spaced far enough apart in time so as to not
    experience much temperature rise. It appears to me that MOVs fail from
    overheating, and that small events do not cause significant damage.
    Of course, MOVs should be deployed in a manner that allows them to blow
    up without starting a fire. A lightning strike or a transformer failure
    can deliver a lot of energy at voltage at which the MOV conducts.

    I have also seen signs of massive line voltage surges a couple times
    with lots of energy from unknown cause at one location a few blocks from
    Philadelphia's very major 30th Street rail station (not known to be
    related to thye blowout events) - most of the fluorescent lamps (F40T12
    with 2-lamp rapid start ballasts) were blown by these events. Apparently
    the voltage was high enough long enough for arcs to form across the
    filaments of the lamps (takes about 12 volts to do that, normally present
    with F40 rapid start ballast is 8.5 volts). These blowout events had
    excessive voltage for enough time and with enough current to blow at
    least one filament in a couple dozen F40 lamps.

    An MOV should be deployed in a suitable metal enclosure, with a fuse or
    circuit breaker upstream of the MOV, maybe also a thermal fuse activated
    by the MOV overheating.

    If the MOV is fused independently of what it protects, then it can be a
    good idea to put an indicator lamp in parallel with the MOV to indicate
    that it is connected.

    - Don Klipstein ()
     
  13. westom

    westom Guest

    No, it is meant to shunt an overvoltage. MOVs must conduct current
    in microseconds. Circuit breakers and fuses take milliseconds to trip
    - one thousand times too long.

    A 15 amp circuit breaker must never trip due to MOV failure. MOVs
    must never fail by burning or shorting as some have described. MOVs
    threshold voltage must change no more than 5% and have no visual
    appearance changes. MOVs must never spit sparks and smoke.

    What happens when a grossly undersized (ineffective) protector fails
    catastrophically (in violation of MOV manufacturer Absolute Maximum
    Parameters)? It can only blow a tiny thermal fuse. That fuse
    disconnects the MOV and leaves appliances connected to the surge.
    However even that tiny fuse is sometimes insufficient to avoid a fire
    threat seen often by fire companies and as described by a NC fire
    marshal in:
    http://tinyurl.com/3x73ol
    entitled "Surge Suppressor Fires".

    An effective surge protector must even shunt a direct lightning
    strike (to earth) and remain functional. But no failure means the
    naive may not recommend it. So many are built to maximize profits.
    Use too few joules in and install that fuse to blow faster. That gets
    many to promote that protector when it really provided no protection
    (the same protection also listed in its numeric specs).

    An effective protector shunts current to earth, remains functional,
    and people never even knew the surge existed. A protector is to shunt
    surge current so short to earth that voltage is minimal. Lower
    voltage (due to more joules in a protector) means less energy is
    dissipated inside the protector or house AND more energy is safety
    dissipated in earth.

    A protector's purpose is not to stop or absorb surges. It is a
    shunt mode device. It connects the maximum transient current to earth
    with minimal energy dissipated in the protector. And so fast that no
    circuit breaker opens. If the MOV burns or vaporizes, then it has
    completely violates MOV manufacturer specs AND has created a dangerous
    and unnecessary human threat. Worse, many do so adjacent to a desktop
    full of papers or on a rug behind furniture.

    All reasons why effective MOVs are located in safer locations that
    are also closer to earth (low impedance connection) such as attached
    to a breaker box.
     
  14. I took a look now.

    This does confirm my previous response that damage is not linearly
    accumulated by absorption of energy, but does still state that degradation
    occurs. It states that a 20 mm MOV can be expected to fail from 1,000
    40-amp "10/1000 us pulses" (risetime 10 us from 10%-peak, falltime 1
    millisecond from peak to 50%), amounting to cumulative conduction of
    mostly somewhere between 20 and 40 amps for a second.

    - Don Klipstein ()
     

  15. Most of you sound like a female fart.
     
  16. bud--

    bud-- Guest

    A plug-in suppressor can wired with the protected load across the MOV,
    where it is disconnected when a failing MOV is disconnected (below), or
    the protected load can be connected across the incoming line. In the
    first case, the protected load is 'protected' even if the MOV fails.
    That is how a quality suppressor is likely connected.

    For w, all plug-in "protectors" are "grossly undersized" even if MOVs
    have ratings of 1000J.

    ---------------------------------
    Francois Martzloff was the NIST guru on surges, and has many published
    technical papers. One of them looks at a MOV on a branch circuit of
    10-50 meters with surges to the power service of 2,000-10,000A (the
    maximum with any reasonable probability of occurring, at least for a house).

    Surprisingly, the maximum energy dissipated was 35 Joules. In 13 of 15
    cases it was 1 Joule or less. That is because at about 6,0000V there is
    arc-over from service hot bus to the enclosure. In US services, the
    enclosure is connected to the equipment ground wires, the neutral wires
    and the earthing system. Arc-over dumped most of the incoming energy to
    earth. In addition, the impedance of the branch circuit wiring greatly
    limits the current that can reach the MOV. Surges are very short
    duration, so the inductance of the wire is much more important than the
    resistance.

    The higher energies were for a 10M branch circuit and, even more
    surprising, the lower current surges below 5,000A. Contrary to
    intuition, at all branch circuit lengths the energy dissipation at the
    MOV was lower as the surge current went up. That was because the MOV
    acted to clamp the voltage at the service panel. With the short branch
    circuit and lowest surge currents, the MOV prevented arc-over. Higher
    current surges forced the voltage up faster, causing arc-over faster and
    more energy was dumped to earth.

    MOVs in this application do not protect by absorbing energy but absorb
    energy in the process of protecting.

    From the source: "More modern surge suppressors are manufactured with a
    Thermal Cut Out mounted near, or in contact with, the MOV that is
    intended shut the unit down overheating occurs [sic]."
    Plug-in suppressors have, since 1998, been required by UL to have a
    thermal disconnect to remove a MOV if it fails and overheats.

    As many are aware, w has a crusade against plug-in suppressors. He has
    never shown that UL listed suppressors made after 1998 are a problem.

    A thermal disconnect, which Don also refers to, is a good idea for
    general applications.

    I agree with w that fuses and circuit breakers are too slow to protect
    from a surge. They may be entirely adequate for overvoltage.
    w knows because he only buys cheap Chinese junk.
    So all talk of protection at devices is useless.
     
  17. bud--

    bud-- Guest

    It is w's religious belief (immune from challenge) that surge protection
    must directly use earthing. As probably everyone here knows, you can
    protect at a device with the protection clamping the voltage, limiting
    the voltage the device is exposed to.
     
  18. westom

    westom Guest

    "ka-blooie" teaches that the protector was ineffective. After 1000
    surges, an MOVs threshold voltage may change only 5%. It is
    degraded. Considered failed by manufacturer specs. Must never blow
    as others have exampled. Even a visual indication of degradation must
    not exist.

    But when undersizing a protector so that it blows, the naive then
    promote it and buy more. No protector should even fail as Joerg and
    Rich Webb have exampled. And yet, that human safety threat is too
    common with power strip protectors.

    More scary pictures of the human safety threat; including the NC
    fire marshal who explains why these ineffective protectors fail:
    http://www.hanford.gov/rl/?page=556&parent=554
    http://www.westwhitelandfire.com/Articles/Surge Protectors.pdf
    http://www.ddxg.net/old/surge_protectors.htm
    http://www.zerosurge.com/HTML/movs.html
    http://tinyurl.com/3x73ol
    http://www3.cw56.com/news/articles/local/BO63312/
    http://www.nmsu.edu/~safety/news/lesson-learned/surgeprotectorfire.htm

    A sales promoter will be along shortly to deny this reality and to
    insult me. It is his job. Last place you want these protectors is
    inside the house on a carpet or with a desktop full of papers. Even
    Norma described this problem with MOVs that did what every MOV
    manufacturer is completely unacceptable. On 27 Dec 2008 in
    alt.fiftyplus entitled "The Power Outage" also describes the danger of
    power strip protectors:
    Protectors that are properly designed do not create these human
    safety threats. That means earthing so that energy is harmlessly
    dissipated outside the building. Only more responsible manufacturers
    make these 'whole house' protectors. One protector to protect
    everything for tens or 100 times less money. General Electric, Cutler-
    Hammer, Intermatic, Keison, Square D, Siemens, and Leviton are just a
    few. Worry if your manufacturer is Belkin, APC, Tripplite, or Monster
    Cable.

    MOVs that fail "ka-blooie" provided no appliance protection, were
    grossly undersized, and were a threat to human life. The effective
    protector even earths direct lightning strikes AND remains
    functional. Worse that can happen - its threshold voltage must change
    by less than 5%.
     
  19. krw

    krw Guest

    ....said the turd.
     
  20. Jamie

    Jamie Guest

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