HELP! AADE L/C meter

Field, specifically flux density.

As I said in an earlier reply, unbalanced flux will behave much the same as balanced flux -- if there's only one central gap, it can only go through the one gap.

Actually no, that's still silly. Because the flux will quite happily flow through the opposite leg in the wrong direction! Duh.

Suppose you have a center-gapped EE core. Turns around the center necessarily create flux that flows through both arms in the same direction, and crosses the gap.

Turns around both arms, same phase, do the same thing. Duh.

Turns around both arms, opposite phase, exclude the center post and gap. This mode has very high permeability, because the two arms are completely ungapped. They also have half the effective area (Ae), and twice the core length--BUT--because it's high permeability all the way, the reluctance of this path is considerably less than the reluctance of the normal mode (gapped) path!

A winding on just one arm, of course, is a superposition of both cases; the reluctance of the gap is high, so little flux flows there, while the majority of flux makes a complete circuit across both arms.

So it's actually quite a bit worse than I supposed before. Without flux balancing windings required for fractional turn construction, that one measly "half of a turn" experiences dramatically higher permeability and therefore inductivity, and much lower saturation current (which acts to saturate half the core!).

I still don't get why that would cause the wire near the gap to cook in Mikek's piezo resonator, but just half-assing it definitely is undesirable!

Experimental

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I have here a gapped EE16/14/5 core set. Probably standard mu_r ~ 2000 ferrite material.

Test winding: 5 turns #24, plus lead length. Est. stray: 0.195uH

With core, One leg only: 3.05uH Center peg: 1.51uH

So the A_L is 122 nH/t^2 per leg and 60.4 nH/t^2 in the center. Or a bit less if you subtract the test winding's self inductance.

The difference is probably not as exaggerated as I thought, because this core had a... ehm, disassembly malfunction. When you're taking apart finished ferrite transformers, your list of tools for disassembly generally includes superglue...

So for this core, 4 turns on the center would give 0.966uH.

4 turns and, I guess I'll call it a "naive half turn", should give about 1.082uH. But that's assuming it doesn't couple into the center peg, which it should. One should be able to churn these numbers I think, and come up with the right coupling coefficient, but I'm not going to right now.

5 turns would give 1.51uH, of course.

In any case, the conclusion is clear, even on an, er, poorly handled core such as this -- the leg-to-leg inductivity is much higher, and the area is much smaller, so it's prone to saturation at a lower current.

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com

Good info, Tim. I'll need time to digest that. I'd really like to follow you up with this but I have no cores. Maybe I'll buy some since I now have more instruments and knowledge than I had in '82.

Anyway, great input. Thanks.

John S

Magnetic field. Make a little search coil that's about the size of the core winding, connect it to a scope, and go sniffing near the core.

Joe Gwinn

I'll have to get a couple of cores to try it again after about 30 years of very fallible memory. But, I like to learn. Perhaps I'll do that.

Thanks. John S

Also, another corrolary: 300 is an interesting number because it's on the order of the permeability. So you would expect that the core-to-core part (full loop, leg to leg) has inductivity mu_r / mu_eff times that of the core-to-gap part. And throw in a factor of 2 or 4 because the path is narrower (~half Ae) and longer (twice l_e, or equivalently, about half mu_r).

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com

You mean sharp spike in amplitude. Hennyway, here is the latest update:

Xfmrs.TXT

Equipment: my factory-built "reliable" AADE L/C Meter IIB. Readings done as a set: xfmr T1, then xfmr T2; wait 1-4 hrs and repeat.

Xfmr | Pri mH | Sec mH | Series | Series | calc M | calc K | | | | aid mH | oppos | mH | | ------+--------+--------+--------+--------+--------+--------+ T1 | 20.00 | 20.00 | 101.6 | 20.37u | 25.39 | 1.27 | ------+--------+--------+--------+--------+--------+--------+ T2 | 19.05 | 21.06 | 70.71 | 2.688m | 17.00 | 0.849 | ======+========+========+========+========+========+========+ T1 | 26.82 | 26.81 | 136.2 | 27.21u | 34.04 | 1.27 | ------+--------+--------+--------+--------+--------+--------+ T2 | 25.54 | 28.23 | 94.79 | 3.594m | 22.80 | 0.849 | ======+========+========+========+========+========+========+ T1 | 26.42 | 26.43 | 134.2 | 26.96u | 33.54 | 1.27 | ------+--------+--------+--------+--------+--------+--------+ T2 | 25.15 | 27.81 | 93.39 | 3.527m | 22.47 | 0.849 | ------+--------+--------+--------+--------+--------+--------+

Notes: T1,T2 wound on same type cores; it is believed that the same number of turns were used on each winding. Nov 30, 2014 * * * * * * * Second set of measurements, resonant amplitude across inductance adjusted to about 100mV pk-pk +/- 20% Dec 25, 2014 Capacitors added for different resonance: 50pF 5% meas 49.7pF 1uF 5% meas 1.09uF

HP 3312A Function generator (50 ohm src)

O---/\/\----+--*

See my just-posted added measurements. T1 is essentially perfect;

22mH / 88mH. Something slightly off for T2: pri->22mH, sec->23mH, aiding->84mH.

Other than that, low drive makes for better results.

Flux density, obviously.

Using your T1 measurements (Probe vs 50pF), I calculate that the distributed stray capacitance is 141pF. Wow!

Oops! My bad. Forgot about the secondary.

Your data indicates that T1 has about 70pF distributed C on each of the windings for a total of 140pF.

The primary at 86.7 kHz calculates to be about 20.4mH which agrees favorably with the inductance of 21mH calculated for 1054Hz and 1.09uF.

The SRF is calculated to be about 93.8kHz.

The real problem, I think, is with the value of K which is defined as 1 or less.

You have two windings either in parallel or in series with a distributed capacitance across them and a measurement system which changes frequency based on what it sees at its terminals. I do not think that the AADE is appropriate for the measurement of coupling coefficients under these circumstances (unless you can also supply the AADE's operating frequency at each point).

I can post an LTSpice listing showing the results of a frequency sweep, if you so desire. It is instructive, I think.

If you continue your sweep with only the probe attached, at what frequency do you see another (very sharp) maximum? Maybe above 1MHz.

Also, can you measure and post your pri-sec capacitance?

BINGO (at least for T1).

Added measurements:

Xfmrs.TXT

Equipment: my factory-built "reliable" AADE L/C Meter IIB. Readings done as a set: xfmr T1, then xfmr T2; wait 1-4 hrs and repeat.

Xfmr | Pri mH | Sec mH | Series | Series | calc M | calc K | | | | aid mH | oppos | mH | | ------+--------+--------+--------+--------+--------+--------+ T1 | 20.00 | 20.00 | 101.6 | 20.37u | 25.39 | 1.27 | ------+--------+--------+--------+--------+--------+--------+ T2 | 19.05 | 21.06 | 70.71 | 2.688m | 17.00 | 0.849 | ======+========+========+========+========+========+========+ T1 | 26.82 | 26.81 | 136.2 | 27.21u | 34.04 | 1.27 | ------+--------+--------+--------+--------+--------+--------+ T2 | 25.54 | 28.23 | 94.79 | 3.594m | 22.80 | 0.849 | ======+========+========+========+========+========+========+ T1 | 26.42 | 26.43 | 134.2 | 26.96u | 33.54 | 1.27 | ------+--------+--------+--------+--------+--------+--------+ T2 | 25.15 | 27.81 | 93.39 | 3.527m | 22.47 | 0.849 | ------+--------+--------+--------+--------+--------+--------+

Notes: T1,T2 wound on same type cores; it is believed that the same number of turns were used on each winding. Nov 30, 2014 * * * * * * * Second set of measurements, resonant amplitude across inductance adjusted to about 100mV pk-pk +/- 20% Dec 25, 2014 Capacitors added for different resonance: 50pF 5% meas 49.7pF 1.uF 5% meas 1.09uF

HP 3312A Function generator

O---/\/\----+--*

There are 4 ways to measure that: pri start to sec start, pri start to sec end, pri end to sec start, pri end to sec end; all being different. So i did a KISS. T1 pri start to sec start = 221pF; T2 pri start to sec end = 214pF. T2 pri start to sec start = 56pF; T2 pri start to sec end = 35pF. You can call these 5% at best, due to the variances mentioned.

Based on your latest data:

I am sure my model is not exact, however it indicates that T1's coefficient of coupling is about .9978 and T2's coefficient is about .96.

I always connect T1 pri start to T1 pri finish and T1 sec start to T1 sec finish. Then measure between the two shorted windings. But, no need to do over, I will use the averages. Thanks.

That sounds reasonable., as T1 IS bifilar and T2 has physically separate windings.

Oh. Will try that for ducks. Thanks.