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inrush current

Discussion in 'Electronic Repair' started by Wiebe Cazemier, Apr 10, 2006.

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  1. Hi,

    A friend of mine is trying to repair an active woofer of a speaker set, which
    started to blow out the primary fuse suddenly. The problem would appear to be
    in the torroidal transformer, because even when nothing is connected, the fuse
    blows. We decided to be stupid and put in a 2A instead of 1A fuse. It didn't
    blow, and the primary current was only 20 mA. All secondary voltages were
    normal. (two assymetric outputs, and one symmetric with center tap).

    So, it would appear the inrush current is very high. But, because the device
    worked normally in the past, it would appear that the inrush current suddenly
    increased. Is this possible? To be more precise, is this possible when all the
    secondary voltages are normal? If so, how?

    Thanks in advance.
     
  2. Guest

    Possibility: original fuse was 1A slow-blow. It died because of
    "normal" fatigue. Or of some temporary overload that cleared.

    Then it was replaced with a 1A fast-blow. This seems consistent with
    your description of the problem (especially as you leave out the very
    important facts of slow-blow vs fast-blow).

    Slow-blow fuses for capacitor-input DC filters are the norm, not the
    exception.

    Look carefully with the factory fuse or if not available the markings
    saying what sort of fuse is supposed to go there.

    Also, even though the average primary current may be 20mA, what
    actually matters to the fuse is something closer to the RMS current,
    and the current waveform is often wildly non-sinusoidal. A factor of 50
    to 100 between average and RMS seems implausable, though with no load
    (capacitor charging only on the very tip of waveform) it may actually
    be the ratio.

    Tim.
     
  3. Phil

    Phil Guest

    Apart from the slow/fast blow advice given elsewhere, check that the
    mounting bolt etc for the toroidal isn't causing a 'shorted turn' by
    touching the case on both sides.....if the case is metal then any bolt
    through the middle of the toroid must only connect with the case on one
    side.

    Phil.
     
  4. default

    default Guest

    Seems very unlikely inrush current increased. I use torroids in my
    amps and find they do use lots of inrush current (particularly with a
    stiff power supply) I fuse them with slow blow fuses and had to
    install two power switches - one for each pair of amps, or my clock
    and modem reset themselves when I turn on the stereo.

    Torroids have better regulation as a rule - so the current required to
    charge the supply caps is reflected in the inrush current to a greater
    degree than with E-I core transformers.

    Does it require a slow blow fuse?

    Sounds like you already eliminated the filter caps as a direct source
    of the problem?

    If you're using the correct fuse type and it still blows, and its been
    working all along prior to this, check the amp. Ideally, look at the
    current with a scope.. You may have a problem in the bias supply -
    coming up unbalanced or very high current due to a bad cap.

    A scope on the output terminals will show it coming on in an
    unbalanced condition (and a loud speaker thump) - but depending on the
    design, that could be normal.
     
  5. I have been thinking along these lines. But because we tried slow blows, the
    only thing I can imagine is that the original fuse is extra slow blow, but I
    don't know if those things even exist. I'll ask him if the transformer has any
    info written on it about special kinds of fuses.
    Perhaps, but there were no filtercaps connected. Going with the "extra slow
    blow" theory, perhaps this transformer has some kind of very deformed current
    waveform, which the original fuse was designed for.

    He doesn't have a scope, but I do. If we can't find the problem, we could bring
    it here, and put it on my scope. But I don't think I'm gonna measure the
    primary current with it, because I don't know enough about putting scopes on
    the mains.
     
  6. We/he tried with the tranny in the free. It was just lying there on the table.
    And, the metal plate to hold it in place in the (wooden) case, has a rubber
    sealing of course, so there cannot be any shorts.
     
  7. I have contructed a four channel power amp with a 600 VA torodial myself. I use
    two special NTC's in series with the primary coil to limit inrush current. Is
    that no solution for you?
    As I said, the fuse also blows with nothing connected to the transformer. It's
    tjust the tranny itself which is causing it to blow, not even filter caps.
    I thought speaker thumps were the result of the amp's inability to cope with
    low voltages. A trick in poweramp design is employing a constant current
    source to avoid that, if I'm not mistaken.
     
  8. default

    default Guest

    That would work, but sort of defeats the purpose of a super stiff
    supply. I have four 100 watt amps with banks of computer grade caps
    feeding photo flash low ESR caps in the amps. Four transformers, one
    per channel.

    I could use some NTC thermistors with time delay relays to short them
    after the supplies charged - just never got around to it. The two
    switch technique works well enough.

    It would be an excuse to add some more pilot lights and if I used a
    DPDT relays, I could shunt the caps to pairs of light bulbs to show
    the caps charge and discharge . . . "but that way insanity lies," and
    it wouldn't do anything for the sound.
    I still can't conceive of a way that a transformer can suddenly
    develop a problem with inrush current without increasing the
    excitation current. 20 milliamps sounds high for a torroid, but still
    reasonable. Do you have another torroid to compare it to? Leave it
    powered up and check the heat? Shorted turn perhaps?

    I have a small 120 VA supply with a torroidal transformer on my
    workbench. The inrush current is well over an amp and excitation
    current, with just the filter caps is ~ point four milliamps.

    My supply has a mil spec 1 amp magnetic circuit breaker on the 120
    input and it will pop that about half the time from a cold start -
    it has a variac input so if I turn it up from zero the circuit breaker
    holds. I'm sure a 1 amp slow blow would work since this magnetic
    breaker is very fast and holds only to 1.2 amps. 20 volt no load
    output with a 10,000 mfd cap.
    Constant current in the diff amp helps if that's the cause, but bias
    supplies and mismatched output transistors are another cause. With a
    one amp fuse in a subwoofer system I figure we're talking about a
    relatively inexpensive amp in something like a computer system.

    But you eliminated the amp already so that only leaves the
    transformer. Right?
     
  9. I.F.

    I.F. Guest

    There is in fact a commercially available chip that switches out an inrush
    limiting resistor/NTC after a delay.
     
  10. Scott Lane

    Scott Lane Guest

    Sounds like a shorted woofer coil. Sometimes they won't show a short until
    you 'push' on the cone, or until it moves when you apply signal to it.
    Scott
     
  11. Arfa Daily

    Arfa Daily Guest

    I rather think not, as if you read the whole thread, you will see that the
    OP has carefully stated, on more than one occasion, that the fuse is blowing
    with absolutely nothing connected, except the transformer primary ...

    Arfa
     
  12. Ross Herbert

    Ross Herbert Guest

    Depending upon the manufacturing method and materials used the inrush
    current can be as much as 50 times the normal primary current for 1/2
    cycle averaging 15 times for about 2 cycles.

    See this paper
    http://www.emeko.de/en/pdf/02-01-Introduction-reasons-for-transformer-inrush/02-01-10-01.pdf
     
  13. These NTC's get a very low resistance once they are warm. In believe it was in
    the order of 0.02 Ohms or something. The only thing I don't understand is,
    that they don't feel warm, even when the amp has been on for a few minutes.
    And, when the entire casing of the amp has warmed up, therefore the NTC's as
    well, and I turn it off and back on immediatly, there is nothing that
    indicates a high inrush current. Even my bench power supply has a higher
    inrush current, because sometimes one of my computer resets when I turn it on
    (and sometimes even when I turn it off...)

    This reasoning has me doubting a bit if the resistance really does get very
    low. Also because perhaps in most devices, a 600VA transformer normally has a
    higher minimal/standy current than what I'm using it for (the amp in idle only
    pulls about 25 watts of the mains).

    I'll see if I can measure the resitance when they're cold, and when the amp has
    been warmed up.
    Until someone who doesn't know how to operate it comes along...
    I don't have a torroidal to test with, but I'll ask if he does. I'll ask if he
    can measure the idle current, and check for heat on the "broken" one.
    0.4 mA? That is 0.0004 A. My DM can't even measure that, because you have to
    use the 10A unfused connection, which is only accurate to the mA.

    BTW, I got a reply to my question I asked him if the transformer said anything
    about what kind of fuse it required, other than the fact it needs to be slow
    blow. It doesn't say anything. But, he also mentioned that Amplimo
    (transformer manufacturer) requires fuses with a high I²t (I*i*t for those who
    cannot see the squared sign). And, also according to
    http://www.circuitprotection.ca/fuseology.html there are different kind of
    slow blows. He's asking the local electronics shop for advice, perhaps they
    indeed have a fuse for him with an extra high I²t.
     
  14. Do you also know it's typenumber? I think that would be a great solution.
     
  15. default

    default Guest

    I have to creep up with the voltage or it eats the fuse in my meter.
    Two of the supposed advantages of torroidal cores is high efficiency
    and good regulation - but all torroids aren't created equal, and
    industry can pervert anything to save a buck

    I can't see the label without disassembling the supply but I can see
    that it was made in the US. I have another torroidal in a DC supply
    that runs some fans - came out of a surplus motorized wheelchair
    charger. It sucks down 25 watts or so just sitting idle - guessing
    from the heat (idle current isn't a great predictor of power used
    since it is reactive)

    I prefer torroidal transformers for my tinkering - easy to add a buck
    or boost winding or whole new secondary to tweak it for a particular
    application.
    Interesting site. I didn't know about the time delay distinction
    and just lumped them with the slow blow types. Or at least didn't
    know about it in electronics applications - the motor starters I'm
    used to usually employ specific time delay fuses for their application
    and environment.
     
  16. The 25 watts I determined my poweramp uses, was determined with a special
    device, capable of keeping the angle between current and voltage into account,
    so I guess that's acurate. But, it's not the transformer alone which does
    that, it's the quiescent current of four amps.
    My electronics is mainly focussed on audio, and the small magnetic field of
    torroids is a good advantage for that. And, they are easily mountable.
    I guess there is a normal type of slow blow, because I hardly ever see/hear
    anybody talk about the I²t requirements of fuses.
     
  17. I have results. The NTC's are about 9 Ohms total when cold. And they don't
    decrease by any usable amount when warm (well, as far as they get warm)... On
    to find a better solution...

    About this. Ever since the beginning, there is a slight 50 Hz hum in the
    output, almost inaudible, you have to put your head against the speaker to
    hear it. There are no harmonics, it's just 50 Hz. Could this be caused by
    these NTC's, that the supply ripple is getting to high? How much ripple can an
    amp with good common mode rejection ratio handle?

    The filterbanks are 4x4700 µF, positive ripple is about 300 mV, negative 260.
    Transformer is 25-0-25, so DC is about +35 and -35. That ripple does seem
    quite high. The difference in ripple can be explained because the positive
    rail is used to drive 4 relays.

    The hum is not present when the amp is powered from my bench supply.
     
  18. I.F.

    I.F. Guest

    The NTC thermistors I've salvaged from a whole range of scrap PC monitors
    have ranged from a few hundred Ohms to as high as 12k at room temperature,
    the running resistance is usually 5 Ohms or less.
     
  19. I.F.

    I.F. Guest

    Its among 20 or so Gb of data sheets on my other PC, but next time I'm there
    I'll see if I can remember which folder to look in!
     
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