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Toroidal transformers vs laminated for power supplies (LM317,LM338)

Discussion in 'Electronic Basics' started by Imitheos, Aug 15, 2003.

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  1. Imitheos

    Imitheos Guest


    I have made some power supplies and when i went to buy a transformer i
    found both toroidal and regular(laminated) ones with not much difference
    in price.

    I didn't know anything about toroidal transformers so i visited this
    group's archives and read many opinions.
    Tha basics are that toroidal trafos have smaller size and very low leakage.

    I want to know if i can use a toroidal transformer in general use or it
    is for some special use ?

    I've also read that they have greater output voltage than laminated ones
    and that they have very large start current that will eventually blow
    fuses,capacitors etc. What does "greater output" mean ? if have a
    toroidal with specs 230V/2x12V won't i have 2x12V but something greater ?

    I would appreciate your opinion generally about toroidal transformers
    and if they are suitable for my power supply.
  2. Don Kelly

    Don Kelly Guest

    The output voltage (open circuit ) is dependent on the turns ratio (ie. as
    specified) rather than the core. Leakage inductance," if "lower, means that
    the voltage under load will be somewhat higher. However, the design rating
    will take this into account and it would not be so high that problems would
    occur (otherwise problems would occur with normal AC source voltage
    variations). You should have no problem.
  3. Imitheos

    Imitheos Guest

    Thank you all for answering.

    From your answers, i conclude to the fact that i can use a toroidal
    transformer without any problems, so i will get it.

    If you have any further comment to add about them i am glad to read it

    Ah, about the starting current and fuse thing, the solution is a
    slowburning fuse so that it can manage the start current for a while
    without burning ?

    Thank you again.
  4. John Jardine

    John Jardine Guest

    Yes a 'slow-blow' fuse will help.
  5. Imitheos

    Imitheos Guest

    Sorry i post again.

    I've just read that toroidal transformers "cannot handle dc well due to
    core saturation"

    Is that true ?

    Thank you
  6. John Jardine

    John Jardine Guest

    In normal use this does not matter.
    Both toroidal and laminated transformers do not particularly like DC being
    fed through them.
    Significant levels of DC current will add a static bias or offset to the
    core magnetism, sufficient that current peaks on one half of the waveform
    of the wanted AC signal will preferentially add on to this static
    magnetism and can push the core material (say 50 times per second) up into
    it's non-linear, 'saturated' region (fuses can blow).
    The laminated transformers do not provide a perfect path ('circuit') for
    their magnetic fields, as there are small air gaps in the cores mechanical
    assembly. As such, they can withstand DC+AC currents a little better than
    the toroidal core types, which allow a near perfect path for their magnetic
    The more, or wider, the air gaps in the magnetic loop path, then the better
    able the core can withstand DC current without saturating. ('fresh air'
    takes the strain)
    Both types of transformer, can and do work OK with some DC present but this
    is a secondary feature in a transformer selection process.
  7. They are good general purpose transformers with a price premium and a
    poor ability to be packed into cubic spaces.
    Since the toroid core has an almost nonexistent air gap (compared ot
    the unavoidable gaps in the flux path of stacked laminations) the core
    tends to retain the last magnetization field it experienced as it was
    being powered down. If the power up flux increases in the same
    direction as the stored flux, the core saturates, drawing a big pulse
    of primary current. Laminated transformers do this also, just not
    quite as bad. The primary fusing has to take this possible start up
    surge into account. I have had some success connecting an AC
    capacitor across the secondary, to make the transformer ring down to
    zero after being disconnected from the line, to help demagnetize the
    core to prepare for the next start.
    Toroidal transformers often have lower leakage inductance and better
    load regulation (less sag as load current increases, and more power
    out per pound that EI laminated core types. The lower leakage
    inductance also allows the energy storage caps to charge a bit faster
    on power up, which can add to the start up surge fusing problem. If
    you add any inductance in series with the primary as part of a line
    filter, it helps both the saturation and capacitor surges, but
    degrades the regulation a bit.
  8. Imitheos wrote:

    Toroidal transformers are smaller and lighter (for a given power) and
    slightly more expensive than the laminated ones. They produce less stray
    fields, which makes them ideal for HiFi-systems and the like.

    If you don't mind the extra cost, you can use them as general trafos.

    One thing is important though: Toroidals are mounted to the case with a
    bolt going through the axis. This bolt must be connected to the case
    only on one end, never ever on both as this would act as a
    sort-circuited single turn secondary winding. The result woould be quite

    Oh, and you need a slow fuse as toroidals have switch-on surges.
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