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In what ways can a transformer fail....?

Discussion in 'Electronic Basics' started by royalmp2001, Feb 16, 2007.

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  1. royalmp2001

    royalmp2001 Guest

    Hi everyone,

    I am looking at safety issues concerning transformers.

    Basic question, I know, but other than an open circuit due to a
    winding burnout, or maybe a shorted turn, how else can a transformer

    Is it likely/possible that the primary voltage of a mains step-down
    transformer can appear on the secondary due to some fault?

    I have a transformer that has an oval core encapsulated in plasic with
    the primary on one side of the core and the secondary on the opposite
    side. Other than a simple open or shorted turn, the possibility of
    mains voltage getting to the secondary must be impossible due to the
    physical separation, right?

    Comments gratefully received with thanks.
  2. Phil Allison

    Phil Allison Guest

    ** Groper wanker alert

    ** Of course.

    Insulation can burn or just fail.

    ** Well, that ain't no mains transformer.

    You sure the two sides are not interconnected ?

    ** Not quite impossible.

    A secondary short might cause both windings to burn right down to the metal

    ........ Phil
  3. Yes, and either winding can short to the core.
  4. Don Bruder

    Don Bruder Guest


    Ferinstance, what happens if primary shorts to the core, and secondary
    is ALSO shorted to the core? without actually trying it, it seems fairly
    intuitive that the primary voltage *COULD* appear on the secondary. Not
    "will", but "could"...
  5. Ross Herbert

    Ross Herbert Guest

    Is this the type you mean?

    I have seen mains transformers of this type in only a few items of
    equipment. Usually more expensive than other more common types but P -
    S isolation is absolute and makes them very safe for medical
  6. Den Thu, 15 Feb 2007 21:23:50 -0800 skrev Don Bruder:
    I think it's easier to win in lotto, than to be in that scenario.
  7. Phil Allison

    Phil Allison Guest

    "Christian Treldal"

    ** Just love to have the same odds on lotto.

    Short the secondary winding and both it and the primary will heat to
    combustion temperatures - as they have very similar heat dissipations.

    Ordinary plastic or paper insulation used to separate them from the core
    fails when burned.

    Carbon soot conducts electricity.

    Humans die with as little as 35 mA flow between hands.

    Nothing a dumb kraut ham would comprehend.

    ........ Phil
  8. Phil Allison

    Phil Allison Guest

    "Ross Herbert" >>

    ** But *no way* are the primaries and secondaries on opposite sides of the
    R core !!!

    There are two of each, wound side by side & interconnected.

    ........ Phil
  9. Anything that CAN go wrong . . . . . .
  10. These appear to be C-cores, which are similar to toroids, having a
    tape-wound construction that is more efficient than E-I laminations. Unlike
    toroids, however, C-cores are generally cut so the winding bobbins can be
    installed, and then held back together, but this causes gaps in the
    laminations which cause higher impedance. Some high voltage transformers
    are made this way, with primary and secondary on opposite sides of the

    The safest transformer design I have seen is that used by Signal
    Transformer 241 series, Stancor SWC series, and and Triad F4-F8 series, as
    well as PC mounted styles like Tamura 3FS series. They are standard EI
    cores, but the bobbins are molded plastic with a divider, so the primary
    and secondary windings are physically isolated from each other.

    Usually a transformer winding will fail open in the hot spot deepest in the
    windings, or else where it is connected to the leads. I have seen most fail
    in the primary, where the insulation breaks down between two or more
    windings, causing a high current in that small area, which melts the wire.

    Some smaller transformers are impedance protected, or power limited, so
    they will not fail catastrophically with an overload or shorted secondary.
    I think this is how most wall-warts are designed. It is always good
    practice to ground the core of power transformers, so any primary to core
    short will not cause dangerous currents in the secondary. Larger
    transformers should be protected with fuses or circuit breakers rated
    according to normal current draw.

  11. john jardine

    john jardine Guest

    Just looked at the one I have. Yes, wound as you mention. The plastic cores
    are half shells that clip together and the whole thing looks much more
    effort to make than a normal toroid.
    Got mine (20Vac) from a cheapo mass produced hifi. Somehow just didn't seem
    right that it was in there. Couldn't bear to throw it away as the shaped
    tape core is a work of art.
  12. Phil Allison

    Phil Allison Guest

    "john jardine"

    ** It is actually FAR less effort to make an R-Core transformer than a

    Note how the bobbins have teeth around the perimeter so that can be spun in
    a winding machine.

    Note how the core itself can be easily clamped to a chassis.

    Note how it is easy to make class 2 ( double insulated ) examples.

    ** Agreed.

    ....... Phil
  13. jasen

    jasen Guest

  14. Ross Herbert

    Ross Herbert Guest

    Did you not see this paragraph on the link I gave.

    "The other unique feature is the use of Bobbins in two parts. The
    primary winding and secondary winding are done on separate bobbins
    thus ensuring complete isolation between the two windings. This allows
    meeting of any safety standards requirement."
  15. Ross Herbert

    Ross Herbert Guest

    I think I get what you are referring to Phil.

    You are correct in that the windings are distributed over both
    bobbins. The windings on an individual bobbin are separated by an
    interleaving barrier of the same material as the main bobbin.

    This page gives a cross-section of the winding pattern.
  16. Phil Allison

    Phil Allison Guest

    "Ross Herbert"

    ** The why the HECK snip the crucial line out of site ???

    " There are two of each, wound side by side & interconnected. "

    ** From the same site you gave:

    Under " Insulation " it says:

    " Double structure bobbins are used .... "

    ** Not all R- cores are like that example, which does not claim to be class
    2 compliant.

    The insulation material separating the primary and secondary must be very
    high temp and fireproof - eg Kapton - to meet the class 2 requirement.
    Easier if they use side by side - ie " split " bobbins.

    ....... Phil
  17. Ross Herbert

    Ross Herbert Guest

    That is NOT the page I gave though. You have gone to the comparisons
    page and I didn't give that reference. I also don't consider that the
    term "double-structure bobbin" conveys accurately to those not
    familiar with transformer winding how the separation is achieved. I
    gave the main page link so that anyone wanting to read further could
    do so, not to start a war on specific details.
  18. Phil Allison

    Phil Allison Guest

    "Ross Herbert"
    "Phil Allison"

    ** What did I write just above the link - Ross?

    ** It conveys the needed info on the point at issue - Ross.

    ( ie, the fact the P and S windings are duplicated and interconnected )

    ** Shame how the specific details of the winding arrangement IS the point
    at issue.

    Shame how YOU quoted a para on the main page that was ambiguous on the
    point at issue.

    Shame how YOU did not read the site further to resolve that ambiguity.

    Shame how YOU treated an ambiguous marketing blurb as technical fact.

    Shame how you did not have the sense to realise that any transformer with P
    and S winding on separate limbs of a core would have very poor voltage

    Shame, shame shame,

    ........ Phil

  19. Ross Herbert

    Ross Herbert Guest

    My original post was simply to ask the OP if he was in fact referring
    to an R-Core transformer. Somehow you seem to want to blow the whole
    thing into a discussion on the specific technicalities of how an
    R-Core transformer is constructed. While the page I gave didn't give
    all the details in this regard I didn't want to get into the specifics
    of the issue. Manufacturers will naturally slant advertising to list
    what they perceive as advantages for their products and I admit I
    didn't give too much thought to the content of the page to determine
    what it really said. And I admit that the ambiguity of the statement
    on the page did mis-lead me to some extent. However, I don't see it as
    any shame on myself for not delving into technical complexities
    considering that I was mainly asking the OP if he was referring to an
    R-Core in my first post.
  20. Phil Allison

    Phil Allison Guest

    "Ross Herbert"

    ** Bollocks.

    Your initial post contained a serious error that needed to be corrected

    So I politely did so.

    ** No "somehow" about it - see above.

    ** Naturally - in order to correct the error YOU posted then later tried
    to fallaciously prove was right.

    This one:

    " I have seen mains transformers of this type in only a few items of
    equipment. Usually more expensive than other more common types but P -
    S isolation is absolute and makes them very safe for medical

    Grow up - Ross.

    ** That is a damn lie.

    Grow up anytime - Ross.

    ** No sign of YOU thinking the matter through at all.

    Grow up anytime - Ross.

    ** And you posted that false impression in contradiction of my earnest
    attempts to correct your original error.

    ** Bollocks.

    You were wrong, stubborn and rude about it too.

    Grow up anytime & take responsibility for your actions.

    ** How typically dishonest of YOU to ignore and want others to ignore
    your posting of serious misinformation.

    Piss off and do something useful - Ross.

    Like checking out those illegal 2 core IEC leads that are in nearly every
    hobby shop in Perth and reporting them to that fellow in the dept.

    ....... Phil
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