An ungapped ferrite core could be all over the place. 25% wouldn't
surprise me. They will vary with temperature, too. You can buy gapped
pot cores in tolerances around 2-5%, I think. Or use a pot core with a
slug adjuster if you need 1% or better. See the datasheets.
Powder-type cores can be bought with better tolerances.
The people who wind inductors commercially get the exact number of
turns every time.
John
It seems to me there are quite a few factors that could affect the
actual inductance achieved and perhaps the least of them is the accuracy
in counting the number of turns. I would expect there to be some
tolerance in the Al value of the ferrite, that its exact dimensions
would have an effect along with how neatly or otherwise the turns are
wound. I have absolutely no idea if these are the major factors nor of
the likely size of the actual major factors affecting the actual
inductance. I am just trying to get a feel for the likely tolerance of
ready made inductors.
The reason I ask is am am designing some passive audio filters and I
know exactly what tolerance of resistance and capacitance I can obtain
but not a clue about inductance. It is no good me using 1% capacitors
and resistors if inductors normally fail to achieve 5%.
Cheers
Ian