Ah, took me a moment--for powdered iron cores, the
higher the permability, the more conductive the core
is. The core I'm playing with is ferrite though, and
I don't think they are conductive as a rule. Haven't
read up on their construction yet though. My core is
like mu r = 10,000, based upon some measurements.
I experimented around, and indeed I have not been able
to saturate my core, even at stupid high currents.
There is something that I need to go back and look at,
a pecularity that I couldn't quite figure out.
Basically, I figure, if the core saturates, I should
see flattopping in the current: flux doesn't increase
as fast as it should in saturation (core saturation is
sudden not gradual), and so current transfer on the
peaks should clip in a soft sort of way. Instead, it
looked like zero crossing error in the current. Could
be the driver.
Next week, I think I'll unhook the secondary and then
watch V vs I and see if I can detect a saturation
level, and then rehook the secondary and see if it
saturates at the same current. My understanding is
obviously wrong about this transformer, in terms of
saturation.
A fun side note: I attempted to make my own resistor
to measure current, using 23mm of 18g wire. 0.5
milliohm. I was able to get what I think was around
270App at 1kHz, not bad--but the shunt managed to
unsolder the scope probe at that power level!
[I had used RG-174 as a probe to the scope, that way I
could measure mV's. Works well. But the coax braid
went to the 3/8" braid I was using as a secondary, and
the shunt was connected to the DUT, and thus the
center conductor could get hot enough to unsolder
itself. It's a fun thing, to be able to smell your
circuit, and know it's working. Maybe I should go
work with some tubes in the future.]
Anyhow, I ran a bunch of tests, and then went back to
figure out if I could "trust" my shunt. Well, I
forget what inductance I calculated for 23mm of wire,
but in the end, at 5kHz, I found the wire to have 0.5
milliohm of inductive reactance. Not so good, means
the current waveform as measured isn't what I thought!
Not only that, but above 10kHz, the skin effect
starts to crop up too, increasing the resistance.
In the end, I'm shifting gears. I'm going to make a
push pull pair, using IRF510's, to drive the
transformer with a different set of windings (probably
6 or 7:1). OPA627's to drive the FET's, one as an
integrator and the other as an invertor, so as to
drive the gates out of phase. 0.1 ohm shunt, so as to
stay resistive past 100kHz, and some feedback so that
the response is flat.
It simulated well. Looked good, was about to build it
until I realized that the DUT is inductive. Adding in
the 60nH of inductance causes a nice oscillation, so I
haven't been able to build it yet. The slight phase
shift between the 60nH inductor and the large phase
shift from the gate capacitance (reacting with the
usual gate resistor) causes a 1.2MHz oscillation.
Drat, was so close! Haven't been able to get a lead
network to tame it yet either.
Shawn
--- John Popelish <jpopelish@...> wrote:
--- In Electronics_101@..., Shawn Upton
If you have several candidate cores on hand, you can
easily find the
high permeability ones that will provide highest
winding coupling by
touching them on two spots with your ohm meter
leads. The lower the
resistance reading, the higher the permeability, as
a general rule.
Shawn Upton, KB1CKT
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