Scratching my head again...

This is specific to the AADE unit.

I've got a coil of about 5mH. If I use the C function of the AADE I get
a reading in the range of 25 pf.

Am I seeing the actual interwinding capacitance here or is the
combination with inductance goofing on the measurement circuit that this
device utilizes?

If it helps anyone get a "feel" for the amount of capacitance of this
coil its about 175 turns, #37-ish loosely scatterwound in about 1.5"
diameter, 1/8" wide and high.

TIA,
Bill WX4A/KP4

I'd guess the effective parallel capacitance is quite a bit less than
that. Not sure just what's going on in the processor in the AADE to
figure out and display that number, but remember, the circuit works by
looking at the resonant frequency of a tank that includes your part,
and by asking it "capacitance" when you clearly put "inductance" across
the terminals is...um...asking for trouble or nonsense. You might pose
the question to AADE.

Cheers,
Tom

K7ITM wrote:


You might pose
the question to AADE.

Cheers,
Tom


I certainly will do so later. Their December vacation schedule
conflicts with mine

Tnx
-Bill

this is what happens when you make a test instrument do what it isn't
supposed to.
Yes, what you actually measure is the interwinding capacitance pf the
coil. the bigger coils have quite a lot of it. besides, the capacitance
is not a linear function of size. there are so may factors influencing
it that you can't realy calculate it. like they say, your shoes size
and underwear color go into it.
but it's OK: thi is the way it's supposed to work.
I compared it with the Autek analyzer I laso have
Saandy 4Z5KS

I don't believe that is correct.

What you are measuring is the net result of the interwinding
capacitance PLUS the reactance of the coil, but it is only correct at
the frequency of the test. Change the frequency and you will get a
different answer. You are NOT measuring the interwinding capacitance
alone.

73, Bill W6WRT


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ORIGINAL MESSAGE:

On 18 Dec 2005 00:13:14 -0800, "Saandy , 4Z5KS"
wrote:

Yes, what you actually measure is the interwinding capacitance pf the
coil.

Bill Turner wrote:

I don't believe that is correct.

What you are measuring is the net result of the interwinding
capacitance PLUS the reactance of the coil, but it is only correct at
the frequency of the test. Change the frequency and you will get a
different answer. You are NOT measuring the interwinding capacitance
alone.

73, Bill W6WRT

I partially agree, Bill. After looking at the schematic and crunching
some numbers I see that the added inductance at the 750 kc test freq
would have the equivalent 'shift' of about 20pf in this particular example.
So taking that into account there's no reason to believe that the
remainder of the reading is not the actual capacitance inherent in the coil.

-Bill

Bill,

A reliable way to find the effective shunt capacitance is to observe
the self-resonant frequency of the coil. If it's really about 25pF,
and the coil really is about 5mH, I'd expect self-resonance at about
450kHz. Can you make some simple measurement of the self resonance?
(e.g., square wave coupled loosely to the coil; scope monitoring the
ringing. Or signal generator swept across the suspected frequency
range, looking for a peak in the response. Key is to couple the
generator and the detector to the coil loosely.)

Cheers,
Tom