Electronics Forum / Basics / September 2008

Depends on the material. Anywhere from 20kHz - 1 GHz.

How would we know ? It might be metal powder. Post a pic of it.

Making it completely useless as a power tranformer.

But - for flyback, the biggie is the air gap. That's where the energy is
stored.

Graham

There is no industry wide standard for color code of toroids.

Your core could be solid ferrite (high permeability, low
saturation amp turns) or any one of a number of powdered
iron formulas.  Based on the heavy wire originally on the
core, I suspect it is powdered iron with lots of air gap
distributed all around the core between the iron grains.

As I said, different manufacturers use different color codes
for their materials.  For example:
http://www.elnamagnetics.com/library/catalogs/MHW/MH&W%20International%20Powder%
20Cores.pdf
http://www.uoguelph.ca/~antoon/circ/toroids.htm
http://zeus.cedcc.psu.edu/ind/irontoroid.html
http://www.qsl.net/ok1dxd/amidon.htm

A powdered iron core could work for a flyback design if the
permeability were high enough.  This doesn't explain your
zero output, though, unless you have a very low permeability
core.

Hardly critical if you swap the leads over.

Graham

It matters not a whit.  Only time you care is when the phasing is
critical to the operation - like the winding is a feedback to your
oscillator circuit to keep it running.

Then you just swap the "start" with the "finish" to change the phase -
no need to switch winding direction.

Without a scope you're handicapped.  Oops, visually challenged.  How
do you know you are driving the toroid with a signal?  You can do
things like monitor current or use capacitive coupling and a diode to
rectify the AC so your meter can read it, but a scope is lots better.

You also mentioned you only have 3 volts on the oscillator - you have
to drive the switching transistor on the toroid with enough voltage
and current to get it to saturate - you only need .7 volts with a
bipolar transistor, but with enough current to switch.  70 KHZ might
be too high for the circuit.  Tip31 only has a gain of 25 at one amp
collector current.

One easy circuit that almost always works is a blocking oscillator.
One transistor with a tapped primary winding.  Phase matters - the
feedback winding has to be out of phase with the collector.

See http://www.cappels.org/dproj/ledpage/leddrv.htm

It isn't a high power circuit - but it can be - lower the bias
resistance and use a cap around the resistor (drives it into
conduction faster and harder).  Start with the circuit shown until you
get it working then experiment. The frequency will be dependent on a
lot of factors.  In the 50's we used them to drive loudspeakers and
varied the frequency by putting the electrolytic feedback caps in
series with the bias resistor and using variable resistors to change
the pitch - I made a bike horn out of it and used it to look at
resonance in mercury vials.

I wind the toroids with a doubled over piece of magnet wire (bifilar
winding) then connect the start to the finish on one coil - this way I
may still miscount the turns but the center tap will always be in the
center.  With ~15 turns of wire and no load on the circuit - the
output can be as much as high as 90-100 - and it will fry a 2N4401
most of the time - so keep the led in the circuit; your TIP31 is only
good for 40 volts.

no need to remove the wire you already have on, just wind over it

Read some app notes. Google will help.

Graham

Flyback xfmrs need an airgap, you toroid probably doesn't have one.

It does not seem worthwhile to spend time and energy to do experiments on
unknown circuit components, especially without the necessary equipment,
like a scope or an LCR meter. Either of these can be obtained very
inexpensively, especially if you are willing to get an older model and
perhaps do a little repair work (which can be educational as well).

You can purchase toroidal cores with full specifications from DigiKey for a
few dollars, and then you can wind transformers that will perform
predictably. You can also probably get samples on-line.

For the test equipment and the cores, eBay can be your friend, and also
check out any local Hamfests that have electronic flea markets.

I would also suggest drawing up your circuit in LTspice, although
transformer modeling can be tricky. You could start with a sample circuit
for a flyback converter (5v 12v 20mA 1316.asc). It has a transformer model
that will work for what you are trying to do. It has a 40 uH primary and a
640 uH secondary, which is a ratio of 4:1.

Paul

On Wed, 27 Aug 2008 13:27:47 -0700, mrdarrett wrote:

I'd have to look it up; I'd probably put "flyback tutorial" or something
in google search.

Good Luck!
Rich