So my first obvious question is, why would you care? If you want to
duplicate the inductor, you already know the inductance, and you can
measure saturation effects and even loss, with some ingenuity.

But playing along with your request, if you can wind turns around the
existing coil, you also have made a transformer. To the extent the
two windings share a common magnetic field, they will be coupled. You
can, in fact, measure the leakage inductances and come up with quite a
good model, and I suppose from that you can deduce the number of turns
fairly accurately, especially if the coupling is good (and the leakage
inductance small compared with the coupled inductance) as it would be
with a ferrite toroid or a pot core or such.

Cheers,
Tom

"Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote in message ...
Suppose that I have an inductor that's covered with epoxy or similar
that prevents me from seeing or finding out how many turns of wire are
on the core. The core is open, so that it's uncovered and most of the
magnetic field is outside outside of the inductor. Obviously it's a
bobbin type core.

I have measured the inductor with an inductance meter, so I know what
the inductance and other parameters are.

Suppose I take some wire, say roughly small if the inductor is small,
and wind it around the inductor, over the existing windings so that it's
within the magnetic field. I wind enough wire onto the inductor so that
I get about 1/9, or 1/16 or 1/25 the inductance in the new coil.

Since the inductance is the square of the turns, I can say that if I
have wound 10 turns and the inductance is 1/16th that of the original
coil, then the turns ratio is 4 to 1, so the original coil is about 40
turns.

Obviously the Real WOrld kicks in, and things may not always be exactly
as they should be. But I haven't tried this, and I'm wondering if any
other person has, and if it's a not unreasonably accurate[1] way to
guesstimate the turns, or if it is prone to a large amount of error. I
guess it would also apply to a toroid if there is enough room to loop
some wire thru the center hole, but this hole may be filled or covered
up.

So has anyone played around with this contrivance?

[1] A not uncommon journalistic contrivance nowadays; seems like these
authors just uncan stop not undoing this, and have unremembered to not
undo it the old fashioned way, and just say "common".

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