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Can a loose connection lead to a blown mains fuse?

Discussion in 'Electronic Repair' started by N_Cook, May 19, 2013.

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  1. N_Cook

    N_Cook Guest

    800 watt toroidal transformer operating at say 400 watts. Secondaries lead
    to conventional bridge rectifier and smoothing caps for +/- rails. A break
    appears in the DC rails central 0V return to the transformer, could that
    lead to a blown mains fuse, via inductive action?
     
  2. Phil Allison

    Phil Allison Guest

    "Nutcase Kook "
    ** Absolutely..............

    And an eight legged Martian could suck you brains out you ear with a
    drinking straw.



    .... Phil
     
  3. c4urs11

    c4urs11 Guest

    Intermittent contacts within inductive circuits can be disastrous.
    Hail to the fuse for being fast enough.

    Cheers!
     
  4. N_Cook-

    It depends on the circuit. If the center tap opens, then voltage
    between each side and ground will be determined by the ratio of each
    side's resistance to ground. If one side's voltage gets so high that it
    causes components to break down, resulting high current might blow the
    mains fuse.

    An open center tap might upset circuit balance so that everything is
    turned on hard, resulting in too much current. On the other hand,
    balanced resistances between each side and ground might not cause any
    problem other than the circuit not working correctly.

    Fred
     
  5. N_Cook

    N_Cook Guest


    The power amps always survive . In the process of exploring suspect caps etc
    and turning over the pcb, the spade-connector for this 0V connection came
    off the pcb spade. Looking under x30 on the spade and what I can see of the
    line spade-socket, I can see no sputtering or smoke trails or anything
    suggestive of a problem there.
    I'm aware that a valve output matching transformer working hard and there is
    a break in the speaker wire/connection /voice coil then enough voltage can
    be induced in the primary side to knock out the output valves and weld turns
    on the primary. But I do not know if similar in a mains transformer would
    give enough current , from the energy stored in the inductance , to throw
    back enough current , at over the mains voltage , to knock out the mains
    fuse.
     
  6. N_Cook-

    For tubes, gain = Gm * R(load). A transformer coupled amplifier output
    stage has a very high voltage gain if the speaker is disconnected. With
    no load, tubes alternate between being saturated and cut off. When
    saturation current flowing in the output transformer primary is
    interrupted, inductance reacts to generate a voltage that attempts to
    keep the current flowing. Voltage = L * di/dt, where di/dt (change in
    current with respect to time) is almost infinite as current goes to
    zero. This is comparable to the points opening in an older auto
    ignition system, and voltage increases until insulation breaks down or a
    spark occurs.

    The reason this effect would not happen to your power transformer, is
    that the two sets of filter capacitors are in series across the full
    output voltage of the bridge rectifier. Current interrupted when the
    center tap is disconnected, will be supplied to or from the capacitors.

    Fred
     
  7. Guest

    It's like this. Every time you turn on the switch you are in effect "tightening" a connection. When it's a dirty lug or a shitty solder joint, the inrush of current happens many more times per hour or whatever. Understand if your mains run at 50 or 60 Hz it matters not, with a shitty connection it could be effecively running alot lower. The equipment can even work if there's enough filtering and regulation, but yes, because of having to recharge the main filter too many times a ten amp fuse could blow in a device that only pulls two amps.

    In the past there were devices that cheated on power consumption by doing that. No, they weren't on the shelves at Cosco or whatever. I designed a couple. With the mechanical electric meters, inertia is your friend :) I alsofound that back wayback if you need a lot of current you could half wave recify it and fool the electric meter. My cousin researched that alot more because he had an electric furnace.

    Anyway, the bottom line is the answer is yes. Think about it with basic math. You apply either 110 or 220 AC to it, the main filter cap has an ESR of less than an ohm, you might have a 2.2 ohm inrush limiter, figure out the current assuming a 0.6 volt drop across the recifiers, actually 1.2 volts. It's a hell of alot more than xis amps which might be the fuse rating. You know a 120V US market Sony TV or 32" pulls like 70 amps upon being turned on? Now THAT is not just the rectifiers, it's the degaussers, but the same thing applies. Your basic seven amp fuse will pass 100, but only for milliseconds.

    In fact this inrush current has spawned two design approacxhes in power supplies. One is using an extraa relay to jump the current limiting resistor so it no longer wastes power. The other is to use a zero crossing trigger tomake the relay contace right where the voltag5e is at or near zero so the peak current is not so high.

    Enough. Scuse my typos because my eyesight sucks and I REALLY cannot sit here and proof this.
     
  8. I've had amplifiers pop the breaker when turned on. (This is extremely rare.)
    And just the other day, the rear amps tripped their breaker when they were
    shut off.

    Can you say "statistical variation"? Sure you can.
     
  9. N_Cook

    N_Cook Guest


    You have a point there. This amp has an inrush limiter, the heatshrink over
    it looks fine. I don't know if a VDR variant or what but if it is the usual
    NTC thermistor then its cold resistance of 0.6 ohms looks suspiciously low.
    I'll have to strip off the heatshrink and try and find some numbers and data
    for it
     
  10. N_Cook

    N_Cook Guest

    Inrush limiter marked with logo something like 8 in a half circle
    VEI or UEI , 20SP, 0R7
    Having difficulty finding data on this but cold 0R7 agrees.
    In 240V land I usually find inrush limiters of cold value about 3 ohms. I'm
    wondering if as its a USA made amp then these limiters are used for all
    models and its not really high enough cold value for reliable 240V use. I'm
    thinking of odd magnetic domains building up in the core of torroids and
    occassionally reinforcing the inrush combined with an initially only
    marginal rating for 240V use.
     
  11. N_Cook

    N_Cook Guest

    looks as though its N20SP007 on
    http://www.uei.com.tw/psp.pdf
    looks too low a value of cold resistance for all eventualities 240V inrush
    current use
     
  12. N_Cook

    N_Cook Guest

    Its a bit much to see that UEI logo of an egg cup with a variant of the ON
    symbol inside instead of an egg, as the poor legibility 8 in a half circle
    here, on a roughish surface, but I'll accept it as so.

    6 amp rating for the international model, internal wiring set for USA is the
    correct component, for UK 3 amp is fine but I suspect the cold 0R7 is just
    too low. I'll replace with a higher initial ohmage NTC.
     
  13. whit3rd

    whit3rd Guest

    Yes, kinda. The trick is, an intermittent contact can accidentally
    rectify, and the primary (AC) current shoots through the saturated
    transformer. It'll more likely pop the small device fuse than the
    main breaker panel.

    It'd have to be a LOT of DC on the secondary to cause saturation.

    A bad connection could also generate heat at/near a fuse, and cause
    it to fail by heating from outside, but that's less likely.
     
  14. N_Cook

    N_Cook Guest


    A good introduction to this (I did only once) testing a 12V relay. 2 wires
    to the screw terminals of a power supply. Hold the relay at the pins , one
    finger holding one wire to one pin and make and break the other contact with
    another finger touching it. But that is high voltage , rather than high
    current.
     
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