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

Hello.

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.

 

----------
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.

 

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.

 

Yes a 'slow-blow' fuse will help.

 

Sorry i post again.

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

Is that true ?

Thank you

 

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
fields.
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.
regards
jhon

 

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.

 

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
spectacular!

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