On Jun 29, 10:32 pm, "Paul E. Schoen" <pst...@[EMAIL PROTECTED]
> wrote:
> "Bob Eld" <nsmontas...@[EMAIL PROTECTED]
> wrote in message
>
> news:CrX9k.6561$L_.111@[EMAIL PROTECTED]
>
>
>
>
>
> > Phil above has it right. That gif is a flyback design. The primary
with
> > the
> > air gapped core forms an inductor designed to handle the DC component.
>
> > Interestingly some torroids can work in this mode, most cannot. The
ones
> > that can have a powdered iron core. The minute spacings between the
iron
> > grains form the equivalent of an air gap.
>
> > Again, you have to know what your core is to determine how to use it.
I
> > know
> > you got it from a power supply but that doesn't mean much without some
> > knowledge of the circuit.
>
> > It is possible to measure the core if you have a scope and a few
> > components
> > by winding a test winding on it then measuring the B-H curve. Then, by
> > applying DC, determine how the curve responds and what happens to the
> > saturation characteristics.
>
> > But, that's part of a course in magnetics and probably a bit beyond
the
> > scope of what you are trying to do.
>
> > I tell you to add a capacitor to the primary to keep DC out of the
core.
> > That is the safest thing to do, but your drive circuit has to both
source
> > and sink current. You can't do this with a single FET. You'll probably
> > have
> > to use a half bridge or what amounts to a two switch forward
> > configuration.
>
> > I think the turns ratio of 12:100 is correct. Remember you will have
+/-
> > 6
> > volts on the primary and +/- 50 volts center taped on the secondary.
> > That's
> > 6:50 or 12:100, same thing.
>
> This sounds a lot like the same project that was discussed by the OP at
> length March 22 ("Need a big transformer") and March 31 ("transformers
in
> parallel"). If the OP simply needs +/- 24 VDC at 2 (or even 5) amps,
there
> are easier and better ways to get that than tinkering with parts from an
> old PC power supply. Unless you need something with exotic
specifications,
> you can always buy it for less than you can build it (unless your time
is
> worth nothing or it is mostly for learning). And for learning, it is
best
> to build basic circuits until you understand them, and there is also a
lot
> you can do by just running simulations. Much better and safer than
blowing
> up parts in a breadboard.
>
> I bought a 200 watt dual output +/- 43V 2A switcher originally designed
for
> an audio amp for about $22 on eBay, and it is a well-made device that
will
> meet my needs if I ever decide to build a high-power op-amp or audio
> amplifier.
>
> Paul
Thanks for the info. That is a cheap find on eBay. I'll keep it in
mind, though I think +/- 43V would shorten the lifetime of an LM3886
(not to mention my brother-in-law's speakers).
I'll experiment on the lower power end to minimize possible damage
(and to avoid inadvertent in-house pyrotechnics).
Michael
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