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'[OT] Transformers?'
1999\01\04@002805 by PJH

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Peter Hynes
PO Box 285, Burwood 3125, Victoria, AUSTRALIA
FAX: Int-613 9809 0604
EMAIL: spam_OUTelekTakeThisOuTspamnetstra.com.au

Hi all

I want to build an inverter. The design calls for a
ferrite toroid step-up transformer driven by a PIC.

OK, I know the permeablity and CSA of the toroid are
important factors in its design and I know what my
turns _ratio_ should be.

But I'm not too sure how to estimate the proper
_number_ of turns in the primary & secondary. I mean,
if I want a ratio of 1:5, how to choose between say
1 turn:5 turns or 100 turns:500 turns?

Anybody know of a website where transformer design is
discussed?

Many thnax in advance.

PJH

1999\01\04@013043 by Bob Blick

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>But I'm not too sure how to estimate the proper
>_number_ of turns in the primary & secondary. I mean,
>if I want a ratio of 1:5, how to choose between say
>1 turn:5 turns or 100 turns:500 turns?

If given those two options, I'd go with the latter :-)

How do you want your transformer discussion? Personally I hate the textbook
methods.

My rules of thumb on transformer fill more than a few hands, but I can
suggest that if you are using a push-pull(aka no DC) primary, you want the
inductance to be such that if unloaded(but at full duty cycle), the current
through the primary peaks at less than 30% of your anticipated full load
current. And really you want the current a lot less than that if you have
enough room. Especially if it's low frequency, but you said ferrite, so I'm
assuming high frequency.

Which leads me to another rule of thumb. If you size the wire correctly,
you have the right number of turns if the transformer is full of wire with
a primary and secondary. Typical wire sizing would be 400 to 700 circular
mils per ampere.

just my 2 cents(what's that in euro?).

-Bob

http://www.bobblick.com/

1999\01\05@091012 by Martin Nilsson

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I always find inductive components rather "magic." Here is an attempt
at giving some motivations for Bob's very good rules of thumb.

> From:    Bob Blick <.....bblickKILLspamspam@spam@SABER.NET>
>
> >But I'm not too sure how to estimate the proper
> >_number_ of turns in the primary & secondary. I mean,
> >if I want a ratio of 1:5, how to choose between say
> >1 turn:5 turns or 100 turns:500 turns?
>
> If given those two options, I'd go with the latter :-)

Motivation: The primary converts electrical energy to magnetic, and
the secondary back again. For a given current, the power transfer
capacity of a coil is proportional to its inductance, which grows as
the square of the number of turns. In order to transfer much power,
high inductance is desirable, i.e. many turns.

> How do you want your transformer discussion? Personally I hate the textbook
> methods.
>
> My rules of thumb on transformer fill more than a few hands, but I can
> suggest that if you are using a push-pull(aka no DC) primary, you want the
> inductance to be such that if unloaded(but at full duty cycle), the current
> through the primary peaks at less than 30% of your anticipated full load

Motivation: If the magnetic field becomes too strong, the core
saturates, and the field cannot increase anymore. This means a drop in
inductance. More power into the transformer will only become heat in
the winding. Margin is needed to be sure that the transformer doesn't
saturate.

> current. And really you want the current a lot less than that if you have
> enough room. Especially if it's low frequency, but you said ferrite, so I'm
> assuming high frequency.

Motivation: Even below saturation, core materials display internal
energy losses as the magnetic field changes (hysteresis). This effect
is pronounced for iron powder cores, used as low frequency core
material. In order to reduce these losses, the core shouldn't be
driven to the neigbourhood of saturation. Ferrite cores have less
hysteresis than iron cores.

> Which leads me to another rule of thumb. If you size the wire correctly,
> you have the right number of turns if the transformer is full of wire with
> a primary and secondary. Typical wire sizing would be 400 to 700 circular
> mils per ampere.

Motivation: Another disadvantage with too many turns is that the
series resistance becomes higher, and consequently causes losses for
high currents.  For this reason, wires with good conductivity should
be used, e.g. thick wire for low/medium frequencies, or Litz wire for
very high frequencies.

> just my 2 cents(what's that in euro?).

About 0.017.

-- Martin

1999\01\05@132343 by Eisermann, Phil

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> Hi all
>
> I want to build an inverter. The design calls for a
> ferrite toroid step-up transformer driven by a PIC.
>
> OK, I know the permeablity and CSA of the toroid are
> important factors in its design and I know what my
> turns _ratio_ should be.
>
> But I'm not too sure how to estimate the proper
> _number_ of turns in the primary & secondary. I mean,
> if I want a ratio of 1:5, how to choose between say
> 1 turn:5 turns or 100 turns:500 turns?
>
       you should probably start by calculating the
       required inductance of the primary. Once
       you have the inductance, you can figure out
       the number of turns required. This calculation
       is usually done at minimum input voltage and
       maximum duty cycle (the worst case).

       Bob Blick responded previously, giving a rule
       of thumb based on an unloaded secondary at
       peak duty cycle. Although I have never done
       this, it does make sense. Bob, if you're
       listening: I wonder why it is based it on an
       unloaded secondary? would it not make more
       sense to use the minimum load current? Then
       you could be sure that the transformer works
       under both min. and max. load conditions.
       (Unless the inverter has to work under no-load
       conditions, but is that realistic?)


> Anybody know of a website where transformer design is
> discussed?
>
       no, but perhaps if someone does, it could
       be posted to the list?

1999\01\05@144041 by Bob Blick

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>         Bob Blick responded previously, giving a rule
>         of thumb based on an unloaded secondary at
>         peak duty cycle. Although I have never done
>         this, it does make sense. Bob, if you're
>         listening: I wonder why it is based it on an
>         unloaded secondary? would it not make more
>         sense to use the minimum load current? Then
>         you could be sure that the transformer works
>         under both min. and max. load conditions.
>         (Unless the inverter has to work under no-load
>         conditions, but is that realistic?)

It's sort of a whole lot of stuff rolled into one rule of thumb. Hopefully
when you are running full load at full duty cycle, you'll get a change of
current that is still tolerable. It's easy to calculate the peak current
at zero load, based on inductance, and easy to look at and verify with a
scope. Zero power tests on the transformer you are in the process of
winding or testing a core are nice(when possible) because there's less to
worry about burning out. Also it's something you can do with just a
primary(remembering of course to catch the energy in a snubber or some
kind of load). Since the inductance changes so much on powdered cores,
checking with an L-meter isn't enough.

Cheers,
Bob

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