What am I missing here? If you know the inductance of the original
coil, there are formulas that will tell you the number of turns. Wind a
coil according to the formula, measure the inductance, and tweak the
number of turns to get as close as you need to be.

Bill
====================


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?