<mrdarrett@[EMAIL PROTECTED]
> wrote in message
news:bb24b823-bbb1-440b-ac7d-84ca0fc11e24@[EMAIL PROTECTED]
> On Jun 29, 2:27 pm, "Bob Eld" <nsmontas...@[EMAIL PROTECTED]
> wrote:
> > <mrdarr...@[EMAIL PROTECTED]
> wrote in message
> >
> >
news:bb341062-6a8f-4d0e-b4f9-7ee805e78362@[EMAIL PROTECTED]
> >
> >
> >
> > > I've got a toroidal core, and I'm planning on winding it with pulsed
> > > 12VDC on the primary, and want to get out 50V on the secondary,
center
> > > tapped so I can get +/- 25V.
> >
> > > The 12VDC will be provided from a 555 switching a power MOSFET at 10
> > > kHz, 50% duty cycle.
> >
> > > I'm hoping to get a maximum of 100W out of the secondary, so the
> > > secondary will be carrying 2 amps, and the primary will be carrying
> > > about 8 amps. I'll be using 22 gauge wire. (A little thin, I know,
> > > but it'll do at least for small scale tests at around 50W.)
> >
> > > 12V / sqrt(2) = 8.5 V RMS. (Does this apply for pulsed square
waves?)
> >
> > > 50V / 8.5 = 5.9, so if the primary is of length X, the center-tapped
> > > secondary should be of length 5.9X, right?
> >
> > > How many turns of primary are necessary for the transformer to work?
> > > In theory, only one turn of wire would be necessary (and then I'd
need
> > > 6 turns on the CT secondary), but I know this is hogwash. How do I
> > > know how many turns I will need on the primary?
> >
> > > Thanks,
> >
> > > Michael
> >
> > What's with the RMS? Your primary drive is 12 Volts peak to peak. Your
> > secondary is 100 Volts peak to peak. The turns ratio is 12:100. It's
not
a
> > matter of length but rather number of turns.
>
>
> secondary should be 50V p-p... or am I missing something here? +/-
> 25V after a bridge rectifier... as shown under "Power supply circuit
> diagram" here: http://www.redcircuits.com/Page100.htm,
to the right
> of the transformer
>
>
> > Be sure to drive the transformer through a capacitor to keep ALL DC
out
of
> > it.
>
>
> ok will do. But how do these guys get away without the cap?
>
> http://www.repairfaq.org/sam/smpsbd.gif
>
> from http://www.repairfaq.org/sam/smpsfaq.htm
>
>
> >
> > The number of turns depends on the area of the core and the maximum
flux
> > density, Bm for the particular core. The core material must not have
too
> > much loss at 10kHz.
> >
> > You need to know something about the core, it's size and material.
>
>
> What kind of cores are commonly found in pc power supplies? that's
> where I got it...
>
> thanks
>
> Michael
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.
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