"Bill Jeffrey" wrote in message
...
Watson A.Name - "Watt Sun, the Dark Remover" wrote:
"Bill Jeffrey" wrote in message
...

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
====================


Okay, I have two identical adjustable core coils, one with the slug
all
the way in and the other all the way out. The Out one measures 100
uH
and the In one measures 180 uh. I put both into a box, each with
terminals to the outside, so that the physical coil can't be seen.
Then
I give them to you along with the inductance of each, and you tell
me
that, by your formulas, the Out one has a different number of turns
than
the In one????

No, I'm saying that you take the slug all the way out, and the bobbin
off the pot core/cup core, so you have an air core coil. Measure the
inductance and plug it into the formula. (You did say that it's wound
on a bobbin, which usually implies that you can get the bobbin off the
ferrite.)

There are many formulas for calculating inductance. All of them admit
to being approximations - but that's all you need. For example:

"For a coil of rectangular cross-section, of thickness t inches,
length
l inches and mean diameter (average of inside and outside) d inches,
Hazletine's formula is L = 0.8d^2N^2 /(12d + 36l + 40t) uH"

Now if your entire coil, including the ferrite, is potted in epoxy, it
is a different situation. But I don't see that in any of your posts.

Thanks for the info.

I guess I didn't say it outright, but I meant that the core was a bobbin
made of ferrite, so it can't be removed. And I did say that it could be
a toroid which is the same situation. Also, I didn't mention it but I
thought this could also be applied to a winding on a solenoid or motor.
In any case, removing the core from a bobbin in my experience is usually
destructive, or else the core won't go back in without having a major
problem, such as not fitting properly and hence the inductance won't be
the same as the original coil. This is because most ferrite bobbins
I've seen are glued to the core with epoxy or similar, and are a real
bitch to remove. So to me, that's not practical. Like someone
mentioned, it would be just as practical to cut the coil and count the
turns.



Bill