On Fri, 4 Jun 2004 19:54:21 -0700, "Watson A.Name - \"Watt Sun, the
Dark Remover\"" wrote:

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".

That is a not-unworthy observation.

Given a common ferrite bobbin type inductor, you could apply a
reasonably high frequency sinewave to the inductor, then wind a
single-turn (or a few, maybe) sense winding over the existing winding,
then measure the voltage ratio (excitation/sense) to get the turns
ratio. This only works if the sense winding encompasses as much flux
as the main winding, which won't be entirely true for a bobbin with
air return path. It gets better if you can artificially close the gap
between the ends of the bobbin with some sort of ferrite or
transformer steel path, sort of a high-permeability c-clamp.

Hmmm... maybe it's better to apply an external magnetic field to the
thing to get the ratio. That may make it more likely that the sense
coil encompasses the same flux as the main coil. Probably so.

For a torroid of non-silly permeability, this voltage ratio thing just
works.

John